The azo dyes released into water from different industries are accumulating in the water bodies and bioaccumulating within living systems thereby affecting environmental health. This is a major concern in developing countries where stringent regulations are not followed for the discharge of industrial waste into water bodies. This has led to the accumulation of various pollutants including dyes. As these developing countries also face acute water shortages and due to the lack of cost-effective systems to remove these pollutants, it is essential to remove these toxic dyes from water bodies, eradicate dyes, or generate fewer toxic derivatives. The photocatalysis mechanism of degradation of azo dyes has gained importance due to its eco-friendly and non-toxic roles in the environment. The zinc nanoparticles act as photocatalysts in combination with plant extracts. Plant-based nanoparticles over the years have shown the potential to degrade dyes efficiently. This is carried out by adjusting the dye and nanoparticle concentrations and combinations of nanoparticles. Our review article considers increasing the efficiency of degradation of dyes using zinc oxide (ZnO) nanoparticles and understanding the photocatalytic mechanisms in the degradation of dyes and the toxic effects of these dyes and nanoparticles in different tropic levels.

  • The review focuses on green synthesis of metallic ZnO nanoparticles.

  • Dye degradation by greener and cost-effective approach is the need for many developing countries.

  • Green synthesised ZnO nanoparticle photocatalysis mechanism is reviewed comprehensively.

  • Toxicity of dyes and ZnO nanoparticles at tropic levels is highlighted.

The rapid increase of population affects the availability of pure and clean water. The pollutants and dyes become bioaccumulated in water bodies and the environment because of the release of pollutants into the nearby water bodies without any pre-processing and degradation. This has become a major concern for developing countries. The need to use safe and clean water leads us to approach methods of getting clean water (Brame et al. 2011; Jain et al. 2021).

Nanoparticles have many applications in the field of cosmetics, medicine, and wastewater management, etc. Nanoparticles can be synthesized from synthetic and biological sources with different elements such as silver, gold, copper, zinc, iron, etc. The chemical methods include two methods as: the bottom-up and top-down approaches which are costly and toxic. To cut down the expenses and reduce toxic effects, researchers have approached green synthesis methods (Agarwal et al. 2017). The biological sources for the generation of nanoparticles include the synthesis of nanoparticles from microorganisms, fungi, plants, etc. via phytochemicals. The phytochemicals consist of polyphenols, terpenoids, quinine, proteins, saponins, and alkaloids (Figure 1(a)). The phytochemicals are found in different parts of medicinal plants, fruits, nuts, cereals (Kurmukov 2013). These phytochemicals have many applications such as antibacterial, antiviral, catalytic properties, and many more. It also provides a route for the biosynthesis of nanoparticles through the green approach.

Figure 1

Phytochemicals present in medicinal plants and different plant parts used in the synthesis of nanoparticles.

Figure 1

Phytochemicals present in medicinal plants and different plant parts used in the synthesis of nanoparticles.

Azo dyes are toxic due to their functional groups, which can be degraded using different methods, which include physical, chemical, and biological methods. The physical techniques comprise adsorption, filtration, and ultraviolet (UV) light irradiation. The chemical methods include chemicals that degrade toxic dyes, and mechanisms that consist of oxidation processes with hydrogen peroxide and hypo chloride. The biological techniques involve microorganisms, fungi, algae, and plants (Saini 2017). As microorganisms are easy to handle and genetic manipulation can be done in species like Bacillus, these can be used to synthesize zinc oxide nanoparticles which can help in the degradation of textile dyes and exhibit antimicrobial activity (Rehman et al. 2019).

In this review article, we have focused on the extraction process of biosynthesized ZnO nanoparticles (NPs) using extracts of plants and microorganisms. As phytochemicals, microbial enzymes act as a reducing agent in the synthesis of nanoparticles. The review also focuses on the mechanism of photocatalytic dye degradation and how the adsorption of dyes is related to the degradation of dyes. It concentrates on the mechanism of toxicity of ZnO nanoparticles and their effects when they enters the different tropic levels like an aquatic ecosystem.

ZnO nanoparticles have a wide range of applications due to their semiconducting, piezoelectric, and pyroelectric properties. This is due to their large band gap and high binding energy and hence have been used in optics, electronics, biomedical, agricultural, and environmental remediation.

ZnO has optoelectronics applications and semiconductor applications due to its wide band gap Eg ∼3.3 eV at 300 K. The ZnO crystal exists as wurtzite, zinc blende, and rock salt. ZnO exists in ambient conditions on cubic substrates and at high pressures, as wurtzite, zinc-blende, and rock salt, respectively (Özgür et al. 2005). The electronic structure of the thermostable phase of ZnO in the wurtzite structure exists in an sp3 hybridization with each Zn atom surrounded by four neighboring O atoms at corners of a tetrahedron shape (Feng et al. 2013). ZnO nanoparticles act as semiconductors, photocatalysts due to their high energy gap and ferromagnetic properties (Garcia et al. 2007; Hong et al. 2009; Nair et al. 2011). ZnO nanoparticles can be doped with transition metals like manganese, nickel, and cobalt to generate different nanostructures like nanofilms, nanotubes, and nanobelts (Djurišić & Leung 2006). ZnO nanoparticles, which are generated by the CTAB-assisted hydrothermal process, exist as nanoflowers, nanoswords, and nanoneedle-like structures due to the interaction of cationic detergent and ZnO nuclei. At low temperatures (120 °C), the nanoparticles exist as nanorods, as temperature is increased from 120 °C to 160 °C, the shape changes from nanorods to nanoneedle-like structures and nanoflowers and nanoswords exist at low temperatures (Zhang et al. 2004).

The Nanoparticles can be synthesized by conventional and non-conventional methods. The Traditional methods can be expensive, toxic in nature, and non-ecofriendly. Whereas in the biological approach, the phytochemical acts as a reducing and capping agent, is more efficient, resulting in preservation of raw materials for a longer time, specifically plants consist of higher contents of phytochemicals which can inhibit the synthesis of nanoparticles (Singh et al. 2021). The synthesis of nanoparticles can be optimized by controlling the reaction time, pH, and temperature of the reaction. The extraction methods followed for the synthesis of nanoparticles use phytochemicals present inside plants and are depicted in Table 1. The nanoparticles can be synthesized using different plant parts like root, leaves, flower, stem, and barks (Figure 1(b)).

Table 1

Methodology of synthesis of ZnO nanoparticles from plants

Common nameScientific nameExtraction methodPhytochemicalsReference
Red fruit passion flower Passiflora foetida 25 g of peel powder with 200 mL water in a beaker and stirred for 30 min at 70 °C. The solution is passed through Whatman filter paper no. 42 and centrifuged at 7,000 rpm. Saponins, tannins, steroids, terpenoids, flavonoids Khan et al. (2021), Siriwardhene (2013)  
Date palm Phoenix dactylifera Syrup washed to eliminate dirt room dried for 36 h, powdered and sieved through 100 mesh. Quinones, organic acids, and flavones Rambabu et al. (2021)  
Miracle grass Gynostemma pentaphyllum 5 g of dry plant powder was added to a conical flask with 100 mL distilled water. Then it was autoclaved for 40 min at 100 °C with high pressure. The autoclaved extract was filtered with Whatman No. 1 filter paper and centrifuged at 4,500 rpm at room temperature for 15 min to eliminate undesired components. Flavonoid, quercetin, alkaloids, rutin Park et al. (2021)  
Bush plum Carissa edulis 10 mg of extract in 20 mL of water mixed with 80 mL of 1 mM of zinc nitrate to synthesis ZnO NPs. Alkaloids, sterols, resin Fowsiya et al. (2016)  
Thick-leaf lavender Anisochilus carnosus Leaves extract allowed to boil using a stirrer heater. Then, 5 g of zinc nitrate was added to the above solution as the temperature reached 60 °C. This mixture was further boiled until its color changed into a dark yellow. Polyphenols, carboxylic acid, polysaccharides, amino acids, and proteins Anbuvannan et al. (2015)  
Redbush tea Aspalathus linearis The natural extract was used to reduce zinc-based salts including ZnNO3, and ZnCl2, as well as Zn-ammonium hydrate-based precursors. Flavones, flavanones and flavonols Diallo et al. (2015)  
Indian borage Plectranthus amboinicus 0.1 M zinc nitrate solution was prepared with 30 ml water. Then 10 ml P. amboinicus leaf extract was added to the above solution and kept under continuous stirring at 80 °C for 4 h. Saponins, polyuronides (pectins, mucilage, gums), tannins (gallic), reducing compounds, flavonoids Fu & Fu (2015)  
Maakada singi Caralluma fimbriata 1 mol% leaves extract, and an aqueous mixture of zinc nitrate was added to gadolinium nitrate solution with constant stirring to ensure uniform mixing. The Petri dish containing the redox mixture was placed into a muffle furnace maintained at 350 ± 10 °C. Alkaloids, flavonoids, carbohydrates, glycosides, sterols, saponins, oils and fats, tannins, phenolic compounds, proteins and amino acids, gums and mucilage Mishra et al. (2016), Packialakshmi & Naziya (2014)  
Autumn joy Sedum alfredii 30 g of Shoots of S. alfredii were mixed in 95% ethanol solution at 70 °C for 120 min after particles adhering to the surfaces of the shoots had been removed with water. The mixtures were filtered 3 times with filter paper. The residues and filtrates were collected, respectively. NaOH (10%) was added to the filtrates to adjust the pH values to 11. Chlorophyllin Wang et al. (2016)  
Pecan Carya illinoinensis 10 g of leaves are cut into pieces, ground into a paste, and soaked in 100 mL of deionized water in a 250 mL glass beaker. The solution was heated at 70 °C for 30 min using a magnetic stirrer until the color of the solution changed. The aqueous leaf extract was left to cool down at room temperature, filtered using Whatman No. 1 filter paper, and centrifuged at 7,000 rpm for 30 min. Total phenolics (TP), condensed tannin (CT) Ahmad et al. (2021), Villarreal-Lozoya et al. (2007)  
Olive Olea europaea 10 g of O. europaea leaves were mixed with 100 ml of deionized water. The mixture was heated at 60 °C for 30 min using a stirrer heater. The resulting product was filtered. Terpenoids, phenolic compounds, flavonoids, and alkaloids Hashemi et al. (2016)  
Thyme Thymus vulgaris 6 g in 100 mL distilled water (DW), heating at 80 °C for 1 h, and then filtered. Thymol, carvacrol Weldegebrieal (2020)  
Moringa Moringa oleifera 5 g of leaves were washed thoroughly with distilled water, and the surfaces of leaves were sterilized using alcohol. These leaves were heated for 40 min in 100 ml of distilled water at 50⁰ C. Then, the extract was filtered with Whatman No. 41 filter paper. Alkaloids, glycosides, phenols, saponins, tannins, volatile oils, and hydrolyzable tannins. Pal et al. (2018), Dahiru et al. (2006)  
Aloe vera Aloe barbadensis miller Small pieces of peel were cut and grounded with pestle mortar in distilled water to make an aqueous solution of peel extract. The aqueous solution was filtered with Whatman filter paper No. 1 to remove debris. Tannins, saponins, flavonoids, Weldegebrieal (2020)  
Okra Abelmoschus 10 mL of leaves mixed in 0.01 mol zinc acetate dihydrate was hydrolyzed with the 0.01 mol sodium hydroxide with the leaf extract, pH is adjusted to the basic at 9–11, then cool at room temperature. Centrifuge at 7,000 rpm for 10 min. Steroids, terpenes, alkaloids, flavones, lignins Mirgane et al. (2020), Chaudhary et al.(2019)  
Eucalyptus Eucalyptus globulus 20 g of leaf powder was added to 100 ml of deionized water and kept for boiling at 80 ⁰ C for about 1 h. The formed precipitate was filtered, and obtained supernatant was stored at 4 °C. Cuminic aldehyde Siripireddy & Mandal (2017)  
Neem Azadirachta indica 25 g of fresh leaves in 100 mL of double-distilled water (DDW), heating while stirring at 60 °C for 20′ and then filtered. Alkaloids, flavonoids, saponins, reducing sugars Weldegebrieal (2020)  
Coriander Coriandrum sativum 10 g Coriander leaf powder was dissolved in 100 ml of distilled water and stirred at 100 °C for 15 min. The solution was then filtered with a 1.5-micron Whatman filter paper No. 1. Flavonoids, saponins, carbohydrates, phenol Singh et al. (2019)  
Oak QuercusL. 20 g of fruit was added in 100 mL of distilled water and boiled for 5 min. after boiling, the color of the aqueous solution was dark brown, and the mixture was allowed to cool to room temperature. Steroids, terpenes, alkaloids, flavones, lignins Sorbiun et al. (2018)  
Mangosteen Garcinia mangostana 8 g of fruit pericarps in 100 mL water, heated at 70–80 °C for 20 min and then filtered. Phenolic acids, flavonoids, alkaloids, triterpenoids Weldegebrieal (2020)  
Dhobi tree Mussaenda frondosa Plant extract and distilled water in the ratio (1:10) were taken in a round-bottomed flask, and the extraction was carried out at 100 °C under reflux for 4 h. The extract was filtered through Whatman filter paper No. 1 and centrifuged to remove any undissolved debris Alkaloids, flavonoids, tannins, glycosides Jayappa et al. (2020), Pappachen & Sreelakshmi (2017)  
Dane wort Sambucus ebulus 2 mL of extract mixed with a solution for 2 h at 80 °C; the suspension was centrifuged for 15 min, and the precipitate was dried at 60 °C. Acetic acid, pentatonic acid, lignocaine, isovaleric acid Alamdari et al.(2020)  
Simple leaf chaste tree Vitex trifolia 40 g leaves were boiled with 200 ml of double-distilled water for 40 min at 60⁰ C. Mild yellow-colored solution is formed, once cooled at room temperature it was filtered with filter paper (Whatman No. 1). Alkaloids, saponins, tannin, phenols, terpenoids, flavonoids, and steroids Elumalai et al. (2015)  
Broccoli Brassica oleracea L. var. italica 8 g of the dried broccoli was weighed out, washed with double deionized water to eliminate superficial impurities. The pulverized broccoli was mixed with 80 mL of deionized water and heated at 70 ̊C for 20 min. Polyphenols and flavonoids Osuntokun et al. (2019)  
Pinwheel flower Tabernaemontana divaricata 60 mL of leaf extract was heated to 80 ̊C and kept stirring, and 6 g of zinc nitrate was added to this solution at 80 ̊C. This mixture was boiled until a yellow-colored paste was formed. Carbohydrates, glycosides, amino acids, flavonoids, tannins, alkaloids, and steroids, Raja et al. (2018), Jain et al. (2010)  
Hyacinth bean Dolichos lablab L. 20 g of leaves were weighed and heated in 100 ml of Milli-Q water at 70 °C for 30 min. The extract was allowed to cool and then filtered with Whatman No. 42 filter paper to produce greenish-yellow filtrate. Alkaloids, flavonoids, tannins, glycosides Kahsay et al. (2019)  
Sea buckthorn Hippophae rhamnoides 5 g of fruit mixed with 100 mL of water. Powdered fruit was treated using a high-pressure autoclave at 100 °C for 1 h. Autoclaved extracts were filtered using Whatman No. 1, 110 mm filter paper.  Rupa et al. (2019)  
Hemp Cannabis sativa 10 g of shade-dried leaves were crushed and added to 100 ml of deionized water in a 250 ml conical flask. The hot water bath was set at 60 °C for 12 h.  Chauhan et al. (2020)  
Pine spurge Euphorbia prolifera 50 g of dried leaves powdered was added to 250 mL double-distilled water in a 500 mL flask and mixed. The preparation of extract was using a magnetic heating stirrer at 70 °C for 30 min. Phenols Momeni et al. (2016)  
Piper betel Betel Water was added to the leaf extract in the 1:3 ratio and boiled at 800 °C for 45 min. The solution is cooled at room temperature for 6 h. Zinc acetate was taken (0.1 M) and added to the water. Alkaloids, tannins, phenolic compound, flavonoid, steroids glycosides, terpenes, anthraquinones Rajesh et al. (2016)  
Basil Ocimum basilicum Leaves of the plant were extracted with ethanol by maceration. Rosmarinic acid, flavionoids Fathiazad et al. (2012)  
Roselle Hibiscus sabdariffa An aqueous solution of flowers is prepared and left to stir for 2 h. The mashes were placed in a water bath at 60 °C for 1 h and filtered with Whatman No. 4 filters. Phenolic, flavonoids Soto-Robles et al. (2019)  
Chinaberry Melia azedarach 20 g of leaves in 125 mL of DDW subjected to Soxhlet extraction for 72 h and then filtered. Terpenoids, flavonoids, steroids, alkaloids Weldegebrieal (2020)  
Baikal skullcap Scutellaria baicalensis 5 ml of plant extract mixed with 95 ml of distilled water make up to 100 ml water. This combination was mixed with zinc nitrate heated to 75 °C for 1.5 h.  Chen et al. (2019)  
Jujube Ziziphus jujuba 100 ml jujube extract was drop-wisely added to 25 mL Zn(NO3)2.6H2O aqueous solution of 0.05 M, and the mixture was vigorously stirred at room temperature for 30 min. Triterpenic acids, cerebrosides, flavonoids, phenolic acids, amino acids, polysaccharides Golmohammadi et al. (2020)  
Red powder puff Calliandra haematocephala Air-dried leaves were mixed with water in a 1:20 weight proportion and were heated in a dry-bath at 80 °C for 15 min, to yield a thin pale-yellow soup of the leaf extract Caffeic acid and myricitrin Vinayagam et al. (2020)  
Desert horsepurslane Trianthema portulacastrum 10 ml of leaf extract was mixed with 25 mL of ZnSO4 in a 150 mL beaker at 25 °C.  Khan et al. (2019)  
Star fruit Averrhoe carrambola 0.5 M zinc nitrate solution was prepared with 50 mL distilled water. The plant extract was added to zinc nitrate in a ratio of 9:1 under continuous stirring. Oxalic acid Chakraborty et al. (2020)  
Teak Tectona grandis 20 g of leaves were collected, weighed, washed under tap water. Collected leaves were cut into fine fragments and placed into a round-bottomed flask with 100 ml of double deionized water. The whole reaction mixture was heated at 60 °C for 1 h and filtrate was obtained employing Whatman No. 1 filter paper. Alkaloids, flavonoids, carbohydrates, glycosides, steroids, and tannins Raizada et al. (2019)  
Chinese sweet-plum Sageretia thea The aqueous solution of leaves mixed with a zinc nitrate solution.  Talha Khalil et al. (2019)  
Rambutan Nephelium lappaceum L. Fresh peels were washed, dried at 50 °C in the oven, then 3 g in 40 mL DDW:20 mL EtOH solvent was heated at 80 °C for 10 min and then filtered. Polyphenols, flavonoids, alkaloids, tannins, saponins (EtOH extract) Weldegebrieal (2020)  
Golden shower Cassia fistula 1:10 proportion of the coarsely powdered plant material to water was taken in a round-bottomed flask, and the extraction was carried out at 100 °C with are flux arrangement for 5 h with constant stirring. The extract was filtered and centrifuged. Polyphenols (11%) and flavonoids (12.5%) Suresh et al. (2015)  
Jackfruit Artocarpus heterophyllus Zinc nitrate hexahydrate was added to the leaf extract and heated for about an hour to get a thick dark brown-colored liquid. Terpenoids, flavonoids, phenols, steroids, glycosides, carbohydrates, and saponins Vidya et al. (2016)  
Tomato Solanum lycopersicum Fresh tomatoes were washed, squeezed to get juice, dissolved in DDW by stirring at 30 °C for 30 min and then filtered. Flavonoids, phenolics, carotenoids, alkaloids Weldegebrieal (2020)  
Ginger Zingiber officinale Powdered leaves in 100 mL of DW was separately boiled at 60 °C for 1 h while stirring and then filtered. Terpenoids, phenolic acid, flavonoids, proteins Weldegebrieal (2020)  
Garlic Allium sativum Fresh and finely sliced bulbs were boiled at 70–80 °C for 20 min and then filtered. Flavonoids, anthocyanins, vitamins (B1, B2, B6, etc.) Weldegebrieal (2020)  
Flax Linum Usitatissimum 50 mL of distilled water has been added to 1 g of seeds and prepared mixture stirred for 2 h at 60 °C. The extract is filtered.  Alkasir et al. (2020)  
Alpine almond Hydnocarpus alpina 10 g of powder was taken to extract with an ethanol-water mixture (60:40) and ethanol (95%) separately.  Ganesh et al. (2019)  
Onion Allium cepa 5 g of dry brown outer onion peel were washed with tap water, followed by rinsing with distilled water and soaked in 50 mL of double-distilled water. The solution was boiled at 70 °C for 15 min. The peel broth was filtered through Whatman No. 1 paper. Phenolic compounds, proteins, and amino acids Rajkumar et al. (2019)  
Parsley Petroselinum crispum 20 g of fresh leaves of parsley were extracted in100 mL ultrapure water by refluxing for 60 min. Vitamins (beta-carotene, thiamin, riboflavin, and vitamins C and E), fatty acids, volatile oils Stan et al. (2015)  
Loquat Eriobotrya japonica 25 g of the seed powder was mixed with 100 mL deionized water. The mixture was then stirred on a magnetic hotplate stirrer at 40 °C for 60 min. Then, the supernatant was collected by Whatman No. 1 filter paper. Phenolics, alcohols, sugars, and proteins Shabaani et al. (2020)  
Malabar cardamom Amomum longiligulare 25 mg of powder were diluted in 100 ml of distilled water, and the suspension was autoclaved for 30 min at 100 °C to obtain an aqueous solution of extract. The extracts were centrifuged at 5,000 rpm for 10 min and filtered using Whatman No. 1 filter paper. Essential oil Liu et al. (2020)  
Saffron Crocus sativus 5 g of leaf powder was dissolved in 100 mL deionized water, blended for 60 min at 70 °C, and centrifuged at 6,000 rpm for 20 min. Then, the supernatant was collected by Whatman No. 1 filter paper. flavones, polyphenols, and terpenoids Rahaiee et al. (2020)  
Golden apple Aegle marmelos 2.974 g of Zn (NO3)2·6H2O was added to 10 ml of the as-prepared juice taken in silica crucibles and dissolved to get homogenous solutions.  Anupama et al. (2018)  
Guava Psidium guajava 1 M zinc acetate precursor to 100 ml leaf extract β-Carotene and meochrome Saha et al. (2018)  
Common nameScientific nameExtraction methodPhytochemicalsReference
Red fruit passion flower Passiflora foetida 25 g of peel powder with 200 mL water in a beaker and stirred for 30 min at 70 °C. The solution is passed through Whatman filter paper no. 42 and centrifuged at 7,000 rpm. Saponins, tannins, steroids, terpenoids, flavonoids Khan et al. (2021), Siriwardhene (2013)  
Date palm Phoenix dactylifera Syrup washed to eliminate dirt room dried for 36 h, powdered and sieved through 100 mesh. Quinones, organic acids, and flavones Rambabu et al. (2021)  
Miracle grass Gynostemma pentaphyllum 5 g of dry plant powder was added to a conical flask with 100 mL distilled water. Then it was autoclaved for 40 min at 100 °C with high pressure. The autoclaved extract was filtered with Whatman No. 1 filter paper and centrifuged at 4,500 rpm at room temperature for 15 min to eliminate undesired components. Flavonoid, quercetin, alkaloids, rutin Park et al. (2021)  
Bush plum Carissa edulis 10 mg of extract in 20 mL of water mixed with 80 mL of 1 mM of zinc nitrate to synthesis ZnO NPs. Alkaloids, sterols, resin Fowsiya et al. (2016)  
Thick-leaf lavender Anisochilus carnosus Leaves extract allowed to boil using a stirrer heater. Then, 5 g of zinc nitrate was added to the above solution as the temperature reached 60 °C. This mixture was further boiled until its color changed into a dark yellow. Polyphenols, carboxylic acid, polysaccharides, amino acids, and proteins Anbuvannan et al. (2015)  
Redbush tea Aspalathus linearis The natural extract was used to reduce zinc-based salts including ZnNO3, and ZnCl2, as well as Zn-ammonium hydrate-based precursors. Flavones, flavanones and flavonols Diallo et al. (2015)  
Indian borage Plectranthus amboinicus 0.1 M zinc nitrate solution was prepared with 30 ml water. Then 10 ml P. amboinicus leaf extract was added to the above solution and kept under continuous stirring at 80 °C for 4 h. Saponins, polyuronides (pectins, mucilage, gums), tannins (gallic), reducing compounds, flavonoids Fu & Fu (2015)  
Maakada singi Caralluma fimbriata 1 mol% leaves extract, and an aqueous mixture of zinc nitrate was added to gadolinium nitrate solution with constant stirring to ensure uniform mixing. The Petri dish containing the redox mixture was placed into a muffle furnace maintained at 350 ± 10 °C. Alkaloids, flavonoids, carbohydrates, glycosides, sterols, saponins, oils and fats, tannins, phenolic compounds, proteins and amino acids, gums and mucilage Mishra et al. (2016), Packialakshmi & Naziya (2014)  
Autumn joy Sedum alfredii 30 g of Shoots of S. alfredii were mixed in 95% ethanol solution at 70 °C for 120 min after particles adhering to the surfaces of the shoots had been removed with water. The mixtures were filtered 3 times with filter paper. The residues and filtrates were collected, respectively. NaOH (10%) was added to the filtrates to adjust the pH values to 11. Chlorophyllin Wang et al. (2016)  
Pecan Carya illinoinensis 10 g of leaves are cut into pieces, ground into a paste, and soaked in 100 mL of deionized water in a 250 mL glass beaker. The solution was heated at 70 °C for 30 min using a magnetic stirrer until the color of the solution changed. The aqueous leaf extract was left to cool down at room temperature, filtered using Whatman No. 1 filter paper, and centrifuged at 7,000 rpm for 30 min. Total phenolics (TP), condensed tannin (CT) Ahmad et al. (2021), Villarreal-Lozoya et al. (2007)  
Olive Olea europaea 10 g of O. europaea leaves were mixed with 100 ml of deionized water. The mixture was heated at 60 °C for 30 min using a stirrer heater. The resulting product was filtered. Terpenoids, phenolic compounds, flavonoids, and alkaloids Hashemi et al. (2016)  
Thyme Thymus vulgaris 6 g in 100 mL distilled water (DW), heating at 80 °C for 1 h, and then filtered. Thymol, carvacrol Weldegebrieal (2020)  
Moringa Moringa oleifera 5 g of leaves were washed thoroughly with distilled water, and the surfaces of leaves were sterilized using alcohol. These leaves were heated for 40 min in 100 ml of distilled water at 50⁰ C. Then, the extract was filtered with Whatman No. 41 filter paper. Alkaloids, glycosides, phenols, saponins, tannins, volatile oils, and hydrolyzable tannins. Pal et al. (2018), Dahiru et al. (2006)  
Aloe vera Aloe barbadensis miller Small pieces of peel were cut and grounded with pestle mortar in distilled water to make an aqueous solution of peel extract. The aqueous solution was filtered with Whatman filter paper No. 1 to remove debris. Tannins, saponins, flavonoids, Weldegebrieal (2020)  
Okra Abelmoschus 10 mL of leaves mixed in 0.01 mol zinc acetate dihydrate was hydrolyzed with the 0.01 mol sodium hydroxide with the leaf extract, pH is adjusted to the basic at 9–11, then cool at room temperature. Centrifuge at 7,000 rpm for 10 min. Steroids, terpenes, alkaloids, flavones, lignins Mirgane et al. (2020), Chaudhary et al.(2019)  
Eucalyptus Eucalyptus globulus 20 g of leaf powder was added to 100 ml of deionized water and kept for boiling at 80 ⁰ C for about 1 h. The formed precipitate was filtered, and obtained supernatant was stored at 4 °C. Cuminic aldehyde Siripireddy & Mandal (2017)  
Neem Azadirachta indica 25 g of fresh leaves in 100 mL of double-distilled water (DDW), heating while stirring at 60 °C for 20′ and then filtered. Alkaloids, flavonoids, saponins, reducing sugars Weldegebrieal (2020)  
Coriander Coriandrum sativum 10 g Coriander leaf powder was dissolved in 100 ml of distilled water and stirred at 100 °C for 15 min. The solution was then filtered with a 1.5-micron Whatman filter paper No. 1. Flavonoids, saponins, carbohydrates, phenol Singh et al. (2019)  
Oak QuercusL. 20 g of fruit was added in 100 mL of distilled water and boiled for 5 min. after boiling, the color of the aqueous solution was dark brown, and the mixture was allowed to cool to room temperature. Steroids, terpenes, alkaloids, flavones, lignins Sorbiun et al. (2018)  
Mangosteen Garcinia mangostana 8 g of fruit pericarps in 100 mL water, heated at 70–80 °C for 20 min and then filtered. Phenolic acids, flavonoids, alkaloids, triterpenoids Weldegebrieal (2020)  
Dhobi tree Mussaenda frondosa Plant extract and distilled water in the ratio (1:10) were taken in a round-bottomed flask, and the extraction was carried out at 100 °C under reflux for 4 h. The extract was filtered through Whatman filter paper No. 1 and centrifuged to remove any undissolved debris Alkaloids, flavonoids, tannins, glycosides Jayappa et al. (2020), Pappachen & Sreelakshmi (2017)  
Dane wort Sambucus ebulus 2 mL of extract mixed with a solution for 2 h at 80 °C; the suspension was centrifuged for 15 min, and the precipitate was dried at 60 °C. Acetic acid, pentatonic acid, lignocaine, isovaleric acid Alamdari et al.(2020)  
Simple leaf chaste tree Vitex trifolia 40 g leaves were boiled with 200 ml of double-distilled water for 40 min at 60⁰ C. Mild yellow-colored solution is formed, once cooled at room temperature it was filtered with filter paper (Whatman No. 1). Alkaloids, saponins, tannin, phenols, terpenoids, flavonoids, and steroids Elumalai et al. (2015)  
Broccoli Brassica oleracea L. var. italica 8 g of the dried broccoli was weighed out, washed with double deionized water to eliminate superficial impurities. The pulverized broccoli was mixed with 80 mL of deionized water and heated at 70 ̊C for 20 min. Polyphenols and flavonoids Osuntokun et al. (2019)  
Pinwheel flower Tabernaemontana divaricata 60 mL of leaf extract was heated to 80 ̊C and kept stirring, and 6 g of zinc nitrate was added to this solution at 80 ̊C. This mixture was boiled until a yellow-colored paste was formed. Carbohydrates, glycosides, amino acids, flavonoids, tannins, alkaloids, and steroids, Raja et al. (2018), Jain et al. (2010)  
Hyacinth bean Dolichos lablab L. 20 g of leaves were weighed and heated in 100 ml of Milli-Q water at 70 °C for 30 min. The extract was allowed to cool and then filtered with Whatman No. 42 filter paper to produce greenish-yellow filtrate. Alkaloids, flavonoids, tannins, glycosides Kahsay et al. (2019)  
Sea buckthorn Hippophae rhamnoides 5 g of fruit mixed with 100 mL of water. Powdered fruit was treated using a high-pressure autoclave at 100 °C for 1 h. Autoclaved extracts were filtered using Whatman No. 1, 110 mm filter paper.  Rupa et al. (2019)  
Hemp Cannabis sativa 10 g of shade-dried leaves were crushed and added to 100 ml of deionized water in a 250 ml conical flask. The hot water bath was set at 60 °C for 12 h.  Chauhan et al. (2020)  
Pine spurge Euphorbia prolifera 50 g of dried leaves powdered was added to 250 mL double-distilled water in a 500 mL flask and mixed. The preparation of extract was using a magnetic heating stirrer at 70 °C for 30 min. Phenols Momeni et al. (2016)  
Piper betel Betel Water was added to the leaf extract in the 1:3 ratio and boiled at 800 °C for 45 min. The solution is cooled at room temperature for 6 h. Zinc acetate was taken (0.1 M) and added to the water. Alkaloids, tannins, phenolic compound, flavonoid, steroids glycosides, terpenes, anthraquinones Rajesh et al. (2016)  
Basil Ocimum basilicum Leaves of the plant were extracted with ethanol by maceration. Rosmarinic acid, flavionoids Fathiazad et al. (2012)  
Roselle Hibiscus sabdariffa An aqueous solution of flowers is prepared and left to stir for 2 h. The mashes were placed in a water bath at 60 °C for 1 h and filtered with Whatman No. 4 filters. Phenolic, flavonoids Soto-Robles et al. (2019)  
Chinaberry Melia azedarach 20 g of leaves in 125 mL of DDW subjected to Soxhlet extraction for 72 h and then filtered. Terpenoids, flavonoids, steroids, alkaloids Weldegebrieal (2020)  
Baikal skullcap Scutellaria baicalensis 5 ml of plant extract mixed with 95 ml of distilled water make up to 100 ml water. This combination was mixed with zinc nitrate heated to 75 °C for 1.5 h.  Chen et al. (2019)  
Jujube Ziziphus jujuba 100 ml jujube extract was drop-wisely added to 25 mL Zn(NO3)2.6H2O aqueous solution of 0.05 M, and the mixture was vigorously stirred at room temperature for 30 min. Triterpenic acids, cerebrosides, flavonoids, phenolic acids, amino acids, polysaccharides Golmohammadi et al. (2020)  
Red powder puff Calliandra haematocephala Air-dried leaves were mixed with water in a 1:20 weight proportion and were heated in a dry-bath at 80 °C for 15 min, to yield a thin pale-yellow soup of the leaf extract Caffeic acid and myricitrin Vinayagam et al. (2020)  
Desert horsepurslane Trianthema portulacastrum 10 ml of leaf extract was mixed with 25 mL of ZnSO4 in a 150 mL beaker at 25 °C.  Khan et al. (2019)  
Star fruit Averrhoe carrambola 0.5 M zinc nitrate solution was prepared with 50 mL distilled water. The plant extract was added to zinc nitrate in a ratio of 9:1 under continuous stirring. Oxalic acid Chakraborty et al. (2020)  
Teak Tectona grandis 20 g of leaves were collected, weighed, washed under tap water. Collected leaves were cut into fine fragments and placed into a round-bottomed flask with 100 ml of double deionized water. The whole reaction mixture was heated at 60 °C for 1 h and filtrate was obtained employing Whatman No. 1 filter paper. Alkaloids, flavonoids, carbohydrates, glycosides, steroids, and tannins Raizada et al. (2019)  
Chinese sweet-plum Sageretia thea The aqueous solution of leaves mixed with a zinc nitrate solution.  Talha Khalil et al. (2019)  
Rambutan Nephelium lappaceum L. Fresh peels were washed, dried at 50 °C in the oven, then 3 g in 40 mL DDW:20 mL EtOH solvent was heated at 80 °C for 10 min and then filtered. Polyphenols, flavonoids, alkaloids, tannins, saponins (EtOH extract) Weldegebrieal (2020)  
Golden shower Cassia fistula 1:10 proportion of the coarsely powdered plant material to water was taken in a round-bottomed flask, and the extraction was carried out at 100 °C with are flux arrangement for 5 h with constant stirring. The extract was filtered and centrifuged. Polyphenols (11%) and flavonoids (12.5%) Suresh et al. (2015)  
Jackfruit Artocarpus heterophyllus Zinc nitrate hexahydrate was added to the leaf extract and heated for about an hour to get a thick dark brown-colored liquid. Terpenoids, flavonoids, phenols, steroids, glycosides, carbohydrates, and saponins Vidya et al. (2016)  
Tomato Solanum lycopersicum Fresh tomatoes were washed, squeezed to get juice, dissolved in DDW by stirring at 30 °C for 30 min and then filtered. Flavonoids, phenolics, carotenoids, alkaloids Weldegebrieal (2020)  
Ginger Zingiber officinale Powdered leaves in 100 mL of DW was separately boiled at 60 °C for 1 h while stirring and then filtered. Terpenoids, phenolic acid, flavonoids, proteins Weldegebrieal (2020)  
Garlic Allium sativum Fresh and finely sliced bulbs were boiled at 70–80 °C for 20 min and then filtered. Flavonoids, anthocyanins, vitamins (B1, B2, B6, etc.) Weldegebrieal (2020)  
Flax Linum Usitatissimum 50 mL of distilled water has been added to 1 g of seeds and prepared mixture stirred for 2 h at 60 °C. The extract is filtered.  Alkasir et al. (2020)  
Alpine almond Hydnocarpus alpina 10 g of powder was taken to extract with an ethanol-water mixture (60:40) and ethanol (95%) separately.  Ganesh et al. (2019)  
Onion Allium cepa 5 g of dry brown outer onion peel were washed with tap water, followed by rinsing with distilled water and soaked in 50 mL of double-distilled water. The solution was boiled at 70 °C for 15 min. The peel broth was filtered through Whatman No. 1 paper. Phenolic compounds, proteins, and amino acids Rajkumar et al. (2019)  
Parsley Petroselinum crispum 20 g of fresh leaves of parsley were extracted in100 mL ultrapure water by refluxing for 60 min. Vitamins (beta-carotene, thiamin, riboflavin, and vitamins C and E), fatty acids, volatile oils Stan et al. (2015)  
Loquat Eriobotrya japonica 25 g of the seed powder was mixed with 100 mL deionized water. The mixture was then stirred on a magnetic hotplate stirrer at 40 °C for 60 min. Then, the supernatant was collected by Whatman No. 1 filter paper. Phenolics, alcohols, sugars, and proteins Shabaani et al. (2020)  
Malabar cardamom Amomum longiligulare 25 mg of powder were diluted in 100 ml of distilled water, and the suspension was autoclaved for 30 min at 100 °C to obtain an aqueous solution of extract. The extracts were centrifuged at 5,000 rpm for 10 min and filtered using Whatman No. 1 filter paper. Essential oil Liu et al. (2020)  
Saffron Crocus sativus 5 g of leaf powder was dissolved in 100 mL deionized water, blended for 60 min at 70 °C, and centrifuged at 6,000 rpm for 20 min. Then, the supernatant was collected by Whatman No. 1 filter paper. flavones, polyphenols, and terpenoids Rahaiee et al. (2020)  
Golden apple Aegle marmelos 2.974 g of Zn (NO3)2·6H2O was added to 10 ml of the as-prepared juice taken in silica crucibles and dissolved to get homogenous solutions.  Anupama et al. (2018)  
Guava Psidium guajava 1 M zinc acetate precursor to 100 ml leaf extract β-Carotene and meochrome Saha et al. (2018)  

Numerous plants and their phytochemicals have been utilized as reducing agents for the degradation of dyes (Table 2). The phytochemicals present in these plants can degrade azo dyes with an efficiency of 80%–90%. Moringa oleifera leaf extracts with ZnO have been utilized in the degradation of titan yellow with an efficiency of 96% (Pal et al. 2018). However, few plants are as efficient in degrading dyes. In leaf extracts of Sageretia, the synthesized ZnO nanoparticles degraded crystal violet under UV light, which was highest (40.65%), followed by in the dark (37%) and in light (36.45%) (Talha Khalil et al. 2019).

Table 2

Photocatalytic degradation of dyes by plant-based ZnO nanoparticles

Plant extractPlant partDyes degradedReferences
Passiflora foetida Peels Organic dye Khan et al. (2021)  
Phoenix dactylifera Pulp waste Degradation of hazardous dyes Rambabu et al. (2021)  
Gynostemma pentaphyllum Leaves Malachite green Park et al. (2021)  
Carissa edulis Fruits Congo red Fowsiya et al. (2016)  
Anisochilus carnosus Leaves Methylene blue Anbuvannan et al. (2015)  
Aspalathus linearis Flower Photoluminescence Diallo et al. (2015)  
Plectranthus amboinicus Leaves photocatalytic activity Fu & Fu (2015)  
Caralluma Fimbriata Aerial part Indigo carmine Mishra et al. (2016)  
Sedum alfredii Shoots 2-CP Wang et al. (2016)  
Carya illinoinensis Shells and kernels Rhodamine B Ahmad et al. (2021)  
Olea europaea Leaves Degradation of environmental pollutants. Hashemi et al. (2016)  
Camellia sinensis Leaves Methylene blue, malachite green Gonçalves et al. (2021)  
Cyanometra ramiflora Leaves Rhodamine B Varadavenkatesan et al. (2019)  
Thymus vulgaris Leaves Methylene blue Jaffri & Ahmad (2019)  
Moringa oleifera Leaves Titan yellow Pal et al. (2018)  
Aloe barbadensis Peel Remove organic compounds Lau et al. (2020)  
Abelmoschus Leaves Methylene blue and Methyl orange Mirgane et al. (2020)  
Eucalyptus globulus Leaves Methylene blue and Methyl orange Siripireddy & Mandal (2017)  
Azadirachta indica Leaves Methylene blue Fagier (2021)  
Coriandrum sativum Leaves Yellow 186 Singh et al. (2019)  
Quercus Fruit hull Basic violet 3 Sorbiun et al. (2018)  
Garcinia mangostana Fruit Malachite green Perera et al. (2020)  
Mussaenda frondosa Leaves/stem Methylene blue Jayappa et al. (2020)  
Sambucus ebulus Leaves Methylene blue Alamdari et al.(2020)  
Vitex trifolia Leaves Methylene blue Elumalai et al. (2015)  
Brassica oleracea L. var. italica Broccoli leaves Methylene blue/Phenol red Osuntokun et al. (2019)  
Tabernaemontana divaricate Leaves Methylene blue Raja et al. (2018)  
Dolichos lablab Leaves Methylene blue/Rhodamine B (RhB), orange II (OII) Kahsay et al. (2019)  
Hippophae rhamnoides Fruit Malachite green/Congo red/Methylene blue /Eosin Y Rupa et al. (2019)  
Cannabis sativa Leaves Congo red/Methyl orange Chauhan et al. (2020)  
Euphorbia prolifera Leaves Methylene blue/Congo red Momeni et al. (2016)  
Piper betel Leaves Methylene blue Rajesh et al. (2016)  
Psidium guajava Leaves Methylene blue Essawy (2018), Saha et al. (2018)  
Ocimum basilicum Leaves Malachite green Chijioke-Okere et al. (2019)  
Hibiscus sabdariffa Flower Methylene blue Soto-Robles et al. (2019)  
Melia azedarach Leaves Azo dyes Weldegebrieal (2020)  
Scutellaria baicalensis Root Methylene blue Chen et al. (2019)  
Ziziphus jujuba Fruit Methylene blue and Eriochrome black-T Golmohammadi et al. (2020)  
Calliandra haematocephala Leaves Methylene blue Vinayagam et al. (2020)  
Trianthema portulacastrum Leaves Synozol Navy Blue-KBF textile Khan et al. (2019)  
Averrhoe carrambola Fruit extract Congo red Chakraborty et al. (2020)  
Tectona Grandis Leaves Methylene blue Raizada et al. (2019)  
Sageretia thea Leaves Crystal violet Talha Khalil et al. (2019)  
Abelmoschus esculentus Leaves Methylene blue and Methyl orange Mirgane et al. (2020)  
Nephelium lappaceum Peel extract Methyl orange Karnan & Selvakumar (2016)  
Kalopanax septemlobus Bark extract Methylene blue Lu et al. (2019)  
Cassia fistula Plant extract Methylene blue Suresh et al. (2015)  
Artocarpus heterophyllus Leaves Rose Bengal Vidya et al. (2016)  
S. lycopersicum Leaves Congo Red Preethi et al. (2020)  
Leucaena leucocephala Leaves Gentian violet, Crystal violet Kanagamani et al. (2019)  
Garcinia xanthochymus Fruits Methylene blue Nethravathi et al. (2015)  
Zingiber officinale Root Methylene blue Haider et al. (2020)  
Allium sativum Root Methylene blue Haider et al. (2020)  
Prunus cerasifera Leaves Bromocresol green, bromophenol blue, methyl red, and methyl blue Jaffri & Ahmad (2019)  
Aerva lanata Flower Rhodamine B Duraimurugan et al. (2020)  
Aerva javanica Flower Rhodamine B Duraimurugan et al. (2020)  
Panax ginseng Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Acanthopanax senticosus Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Kalopanax septemlobus  Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Dendropanax morbifera Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Cocus nucifera Water Organic methylene blue Satheshkumar et al. (2020)  
Curry tree Leaves Organic methylene blue Satheshkumar et al. (2020)  
Rubus coreanus Fruit Malachite green Rupa et al. (2018)  
Vitex agnus-castus Leaves Methylene blue and Crystal violet Dobrucka (2019)  
Linum usitatissimum Seeds Methylene blue Alkasir et al. (2020)  
Cyadonia Seeds Methylene blue  Moghaddas et al. (2020)  
Hydnocarpus alpina Leaves Methylene blue Ganesh et al. (2019)  
Ipomoea pes-caprae Leaves Methylene blue Venkateasan et al. (2017)  
Monsonia burkeana Plant extract Methylene blue Ngoepe et al. (2018)  
Allium cepa Onion peels Methylene blue and Crystal violet Rajkumar et al. (2019)  
Rheum turketanicum Rhizome extract Methylene blue Nemati et al. (2019)  
Petroselinum crispum Leaves Methylene blue Stan et al. (2015)  
Seeds Methylene blue Shabaani et al. (2020)  
Amomum longiligulare Fruit extract Methylene blue and Malachite green Liu et al. (2020)  
Suaeda japonica Makino Plant extract Methylene blue Shim et al. (2019)  
Crocus sativus Leaves Methylene blue Rahaiee et al. (2020)  
Aegle marmelos Fruit pulp Methylene blue Anupama et al. (2018)  
Corriandrum sativum Leaves Anthracene Hassan et al. (2015)  
Plant extractPlant partDyes degradedReferences
Passiflora foetida Peels Organic dye Khan et al. (2021)  
Phoenix dactylifera Pulp waste Degradation of hazardous dyes Rambabu et al. (2021)  
Gynostemma pentaphyllum Leaves Malachite green Park et al. (2021)  
Carissa edulis Fruits Congo red Fowsiya et al. (2016)  
Anisochilus carnosus Leaves Methylene blue Anbuvannan et al. (2015)  
Aspalathus linearis Flower Photoluminescence Diallo et al. (2015)  
Plectranthus amboinicus Leaves photocatalytic activity Fu & Fu (2015)  
Caralluma Fimbriata Aerial part Indigo carmine Mishra et al. (2016)  
Sedum alfredii Shoots 2-CP Wang et al. (2016)  
Carya illinoinensis Shells and kernels Rhodamine B Ahmad et al. (2021)  
Olea europaea Leaves Degradation of environmental pollutants. Hashemi et al. (2016)  
Camellia sinensis Leaves Methylene blue, malachite green Gonçalves et al. (2021)  
Cyanometra ramiflora Leaves Rhodamine B Varadavenkatesan et al. (2019)  
Thymus vulgaris Leaves Methylene blue Jaffri & Ahmad (2019)  
Moringa oleifera Leaves Titan yellow Pal et al. (2018)  
Aloe barbadensis Peel Remove organic compounds Lau et al. (2020)  
Abelmoschus Leaves Methylene blue and Methyl orange Mirgane et al. (2020)  
Eucalyptus globulus Leaves Methylene blue and Methyl orange Siripireddy & Mandal (2017)  
Azadirachta indica Leaves Methylene blue Fagier (2021)  
Coriandrum sativum Leaves Yellow 186 Singh et al. (2019)  
Quercus Fruit hull Basic violet 3 Sorbiun et al. (2018)  
Garcinia mangostana Fruit Malachite green Perera et al. (2020)  
Mussaenda frondosa Leaves/stem Methylene blue Jayappa et al. (2020)  
Sambucus ebulus Leaves Methylene blue Alamdari et al.(2020)  
Vitex trifolia Leaves Methylene blue Elumalai et al. (2015)  
Brassica oleracea L. var. italica Broccoli leaves Methylene blue/Phenol red Osuntokun et al. (2019)  
Tabernaemontana divaricate Leaves Methylene blue Raja et al. (2018)  
Dolichos lablab Leaves Methylene blue/Rhodamine B (RhB), orange II (OII) Kahsay et al. (2019)  
Hippophae rhamnoides Fruit Malachite green/Congo red/Methylene blue /Eosin Y Rupa et al. (2019)  
Cannabis sativa Leaves Congo red/Methyl orange Chauhan et al. (2020)  
Euphorbia prolifera Leaves Methylene blue/Congo red Momeni et al. (2016)  
Piper betel Leaves Methylene blue Rajesh et al. (2016)  
Psidium guajava Leaves Methylene blue Essawy (2018), Saha et al. (2018)  
Ocimum basilicum Leaves Malachite green Chijioke-Okere et al. (2019)  
Hibiscus sabdariffa Flower Methylene blue Soto-Robles et al. (2019)  
Melia azedarach Leaves Azo dyes Weldegebrieal (2020)  
Scutellaria baicalensis Root Methylene blue Chen et al. (2019)  
Ziziphus jujuba Fruit Methylene blue and Eriochrome black-T Golmohammadi et al. (2020)  
Calliandra haematocephala Leaves Methylene blue Vinayagam et al. (2020)  
Trianthema portulacastrum Leaves Synozol Navy Blue-KBF textile Khan et al. (2019)  
Averrhoe carrambola Fruit extract Congo red Chakraborty et al. (2020)  
Tectona Grandis Leaves Methylene blue Raizada et al. (2019)  
Sageretia thea Leaves Crystal violet Talha Khalil et al. (2019)  
Abelmoschus esculentus Leaves Methylene blue and Methyl orange Mirgane et al. (2020)  
Nephelium lappaceum Peel extract Methyl orange Karnan & Selvakumar (2016)  
Kalopanax septemlobus Bark extract Methylene blue Lu et al. (2019)  
Cassia fistula Plant extract Methylene blue Suresh et al. (2015)  
Artocarpus heterophyllus Leaves Rose Bengal Vidya et al. (2016)  
S. lycopersicum Leaves Congo Red Preethi et al. (2020)  
Leucaena leucocephala Leaves Gentian violet, Crystal violet Kanagamani et al. (2019)  
Garcinia xanthochymus Fruits Methylene blue Nethravathi et al. (2015)  
Zingiber officinale Root Methylene blue Haider et al. (2020)  
Allium sativum Root Methylene blue Haider et al. (2020)  
Prunus cerasifera Leaves Bromocresol green, bromophenol blue, methyl red, and methyl blue Jaffri & Ahmad (2019)  
Aerva lanata Flower Rhodamine B Duraimurugan et al. (2020)  
Aerva javanica Flower Rhodamine B Duraimurugan et al. (2020)  
Panax ginseng Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Acanthopanax senticosus Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Kalopanax septemlobus  Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Dendropanax morbifera Flower Methylene blue, Eosin Y, and Malachite green Kaliraj et al. (2019)  
Cocus nucifera Water Organic methylene blue Satheshkumar et al. (2020)  
Curry tree Leaves Organic methylene blue Satheshkumar et al. (2020)  
Rubus coreanus Fruit Malachite green Rupa et al. (2018)  
Vitex agnus-castus Leaves Methylene blue and Crystal violet Dobrucka (2019)  
Linum usitatissimum Seeds Methylene blue Alkasir et al. (2020)  
Cyadonia Seeds Methylene blue  Moghaddas et al. (2020)  
Hydnocarpus alpina Leaves Methylene blue Ganesh et al. (2019)  
Ipomoea pes-caprae Leaves Methylene blue Venkateasan et al. (2017)  
Monsonia burkeana Plant extract Methylene blue Ngoepe et al. (2018)  
Allium cepa Onion peels Methylene blue and Crystal violet Rajkumar et al. (2019)  
Rheum turketanicum Rhizome extract Methylene blue Nemati et al. (2019)  
Petroselinum crispum Leaves Methylene blue Stan et al. (2015)  
Seeds Methylene blue Shabaani et al. (2020)  
Amomum longiligulare Fruit extract Methylene blue and Malachite green Liu et al. (2020)  
Suaeda japonica Makino Plant extract Methylene blue Shim et al. (2019)  
Crocus sativus Leaves Methylene blue Rahaiee et al. (2020)  
Aegle marmelos Fruit pulp Methylene blue Anupama et al. (2018)  
Corriandrum sativum Leaves Anthracene Hassan et al. (2015)  

The Nanoparticles are biosynthesized from microorganisms like algae, bacteria, and fungi. The bacterial, algal, and fungal enzymes help in the biosynthesis of nanoparticles and act as reducing agents. Algae like Sargassum, Cladophora, Chlamydomonas, and Garcilaria have been utilized for the synthesis of ZnO particles (Table 3). Bacteria like Arthrospira platensis, Lactobacillus sporogens, Pseudomonas aeruginosa, Bacillus haynesii, Bacillus licheniformis, Halomonas elongate, Aeromonas hydrophila, and Bacillussp. have been utilized. Penicillium chrysogenum, Trichoderma harzianum, marine yeast, Cochliobolus geniculatus, Aspergillus terreus, Phanerochaete chrysosporium, Periconiumsp., and Pichia kudriavzevii species of fungi also have been utilized for the synthesis of ZnO nanoparticles (Table 3).

Table 3

Synthesis of ZnO nanoparticle from microorganisms

Microorganism speciesGenusExtraction methodNanoparticle shapeNanoparticle sizeReferences
Algae 
Sargassum muticum Sargassum Dried algae powder (2 g) was mixed with 100 ml distilled water, heated to 100 °C, and filtered through Whatman No. 41 filter paper. Hexagonal structures 42 nm Azizi et al. (2014)  
Cladophora glomerata Cladophora Algae samples were washed with distilled water to remove the adhering particles. They were dried in the shaded place. The dried algae were powdered. Irregular shapes 14.39 nm to 37.85 nm Abdulwahid et al. (2019)  
Chlamydomonas reinhardtii Chlamydomonas 25 mL of algal extract was made up to 100 mL using deionized water; zinc acetate dehydrate was added to obtain a final concentration of 1 mM. Nanoflowers 40 nm  Parthasarathy & Narayanan (2014)  
Gracilaria edulis Gracilaria Fresh alga (10 g) was mixed in 50 mL of sterile distilled water and chopped into fine pieces of approximately 1 mm. The mixture was then boiled by microwave oven irradiation for 10 min. Then, the extract was filtered through Whatman No. 1 filter paper. Nanorods   Priyadharshini et al. (2014)  
Bacteria 
Arthrospira platensis Arthrospira  0.44 g of zinc nitrate was dissolved in 2 mL of distilled H2O. The 98 mL of biomass filtrated was added to get a final concentration of 2 mM. The mixture was incubated for 24 h at 30 °C ±2 °C and 150 rpm shaking conditions. Spheres 30.0 to 55.0 nm  El-Belely et al. (2021)  
Lactobacillus sporogenes Bacillus  10 mL of culture was doubled in volume by mixing an equal volume of sterile distilled water containing nutrients in five different hard glass test tubes. 20 mL of zinc chloride solution was added and heated on the steam bath up to 80 °C for 5 to 10 minutes. Hexagonal structures 11 nm Prasad & Jha (2009)  
Pseudomonas aeruginosa Pseudomonas The culture was centrifuged at 12,000 rpm for 15 min. The obtained supernatant was collected in the sterile separating funnel, acidified by 12 N HCl (pH 2.0) to precipitate, and incubated at 4 °C. Spheres 35 to 80 nm Singh et al. (2014)  
Bacillus haynesii Bacillus 100 mL of cell-free supernatant and 100 mL of zinc sulphate solution (1 mM) were taken, and the mixture was placed on a stirrer at room temperature for 24 h. Rods 50 ± 5 nm  Rehman et al. (2019)  
Bacillus licheniformis Bacillus Zinc acetate dihydrate was dissolved in 50 ml of deionized water in a flask and heated at 60 °C for 15 min followed by the addition of (0.6 M) sodium bicarbonate. Wet bacterial biomass (5 g) was then added to the mixture. The above solution was incubated under continuous stirring (200 rpm) for 48 h at 37 ± 1 °C. Nano flower 300 nm  Tripathi et al. (2014)  
Halomonas elongata Halomonas Taguchi method to obtain optimum conditions in zinc oxide nanoparticles (NPs) biosynthesis by Halomonas elongata IBRC-M 10214. Spheres 18.11 ± 8.93 nm  Taran et al. (2018)  
Aeromonas hydrophila Aeromonas The diluted culture solution was again allowed to grow for another 24 h. 20 mL of 0.1 g ZnO were added to the culture solution, and it was kept under shaking incubator at 120 rpm at 30 °C for 24 h until white deposition starts to appear at the bottom of the flask, indicating the initiation of transformation. Spheres 57.72 nm Jayaseelan et al. (2012)  
Bacillus sp. Bacillus The bacteria are mixed with alginate and used for experiments. Beads 2 mm Cai et al. (2020)  
Fungus 
Pichia kudriavzevii Candida  Fungal mycelia were separated from the culture medium through centrifugation and sterile water to remove any components of the medium. 20 g of biomass was resuspended in 100 mL of sterile deionized Milli-Q water. Hexagonal and irregular shapes 10–61 nm Moghaddam et al. (2017)  
Periconium sp. Periconia  20 g of zinc nitrate was dissolved in 100 ml of deionized water under constant stirring in a hot plate at 90 °C. 25 ml of fungal extract was added to the zinc nitrate solution, and the solution was evaporated to form a solution at pH 5, and it was kept as it is. The resultant sol was held in a hot air oven at 45 °C for 24 h. hexagonal wurtzite 40 nm Ganesan et al. (2020)  
Phanerochaete chrysosporium Phanerochaete  Fungal culture was inoculated in malt extract broth (150 ml) and incubated for 5–7 days, then filtered through Whatman filter paper No. 1 and added to zinc sulphate solution, followed by drop-wise addition of sodium hydroxide until the appearance of white suspended nanoparticles and again it was incubated for 24 h at 27 °C.  50 nm Sharma et al. (2020)  
Aspergillus terreus Aspergillus Fungal strains were taken to separate and purify the synthesized. The ultra-centrifugation collected NPs at 20,000 rpm (20 min), washed in deionized water with ethanol, and dried at 50 °C. Spheres with irregular margins 30.45 nm Mousa et al. (2021)  
Cochliobolus geniculatus Cochliobolus 10 g of fungal biomass was transferred to 100 ml of sterile ultrapure water and incubated for 72 h. The mycelial free filtrate was obtained by separating fungal mycelial biomass by centrifugation and combined with 1 mM of zinc acetate and maintained at 28 ± 1 °C for 72 h in an incubator shaker. Spheres 2–6 nm Kadam et al. (2019)  
Marine yeast  The cultured broth was collected after incubation and centrifuged at 8,000 rpm for 15 minutes. The supernatant was added with 1 mm of ZnO. Round 86.27 nm Aswathy et al. (2017)  
Trichoderma harzianum Trichoderma 50 ml of aqueous culture in flasks by stirring with zinc nitrate was added in culture to make a final concentration of 1–2 mM solution. The reaction was carried out in dark conditions at 45 °C, stirring vigorously. Spheres 87.5 nm Consolo et al. (2020)  
Penicillium chrysogenum Penicillium Fungal culture was grown up in a 100 mL fermentative broth medium for 7 days at pH 6.0, 30 °C, and shaking at 150 rpm, separated by Whatman filter paper No. 1. Hexagonal and spherical structures 9.0–35.0 nm Mohamed et al. (2020)  
Microorganism speciesGenusExtraction methodNanoparticle shapeNanoparticle sizeReferences
Algae 
Sargassum muticum Sargassum Dried algae powder (2 g) was mixed with 100 ml distilled water, heated to 100 °C, and filtered through Whatman No. 41 filter paper. Hexagonal structures 42 nm Azizi et al. (2014)  
Cladophora glomerata Cladophora Algae samples were washed with distilled water to remove the adhering particles. They were dried in the shaded place. The dried algae were powdered. Irregular shapes 14.39 nm to 37.85 nm Abdulwahid et al. (2019)  
Chlamydomonas reinhardtii Chlamydomonas 25 mL of algal extract was made up to 100 mL using deionized water; zinc acetate dehydrate was added to obtain a final concentration of 1 mM. Nanoflowers 40 nm  Parthasarathy & Narayanan (2014)  
Gracilaria edulis Gracilaria Fresh alga (10 g) was mixed in 50 mL of sterile distilled water and chopped into fine pieces of approximately 1 mm. The mixture was then boiled by microwave oven irradiation for 10 min. Then, the extract was filtered through Whatman No. 1 filter paper. Nanorods   Priyadharshini et al. (2014)  
Bacteria 
Arthrospira platensis Arthrospira  0.44 g of zinc nitrate was dissolved in 2 mL of distilled H2O. The 98 mL of biomass filtrated was added to get a final concentration of 2 mM. The mixture was incubated for 24 h at 30 °C ±2 °C and 150 rpm shaking conditions. Spheres 30.0 to 55.0 nm  El-Belely et al. (2021)  
Lactobacillus sporogenes Bacillus  10 mL of culture was doubled in volume by mixing an equal volume of sterile distilled water containing nutrients in five different hard glass test tubes. 20 mL of zinc chloride solution was added and heated on the steam bath up to 80 °C for 5 to 10 minutes. Hexagonal structures 11 nm Prasad & Jha (2009)  
Pseudomonas aeruginosa Pseudomonas The culture was centrifuged at 12,000 rpm for 15 min. The obtained supernatant was collected in the sterile separating funnel, acidified by 12 N HCl (pH 2.0) to precipitate, and incubated at 4 °C. Spheres 35 to 80 nm Singh et al. (2014)  
Bacillus haynesii Bacillus 100 mL of cell-free supernatant and 100 mL of zinc sulphate solution (1 mM) were taken, and the mixture was placed on a stirrer at room temperature for 24 h. Rods 50 ± 5 nm  Rehman et al. (2019)  
Bacillus licheniformis Bacillus Zinc acetate dihydrate was dissolved in 50 ml of deionized water in a flask and heated at 60 °C for 15 min followed by the addition of (0.6 M) sodium bicarbonate. Wet bacterial biomass (5 g) was then added to the mixture. The above solution was incubated under continuous stirring (200 rpm) for 48 h at 37 ± 1 °C. Nano flower 300 nm  Tripathi et al. (2014)  
Halomonas elongata Halomonas Taguchi method to obtain optimum conditions in zinc oxide nanoparticles (NPs) biosynthesis by Halomonas elongata IBRC-M 10214. Spheres 18.11 ± 8.93 nm  Taran et al. (2018)  
Aeromonas hydrophila Aeromonas The diluted culture solution was again allowed to grow for another 24 h. 20 mL of 0.1 g ZnO were added to the culture solution, and it was kept under shaking incubator at 120 rpm at 30 °C for 24 h until white deposition starts to appear at the bottom of the flask, indicating the initiation of transformation. Spheres 57.72 nm Jayaseelan et al. (2012)  
Bacillus sp. Bacillus The bacteria are mixed with alginate and used for experiments. Beads 2 mm Cai et al. (2020)  
Fungus 
Pichia kudriavzevii Candida  Fungal mycelia were separated from the culture medium through centrifugation and sterile water to remove any components of the medium. 20 g of biomass was resuspended in 100 mL of sterile deionized Milli-Q water. Hexagonal and irregular shapes 10–61 nm Moghaddam et al. (2017)  
Periconium sp. Periconia  20 g of zinc nitrate was dissolved in 100 ml of deionized water under constant stirring in a hot plate at 90 °C. 25 ml of fungal extract was added to the zinc nitrate solution, and the solution was evaporated to form a solution at pH 5, and it was kept as it is. The resultant sol was held in a hot air oven at 45 °C for 24 h. hexagonal wurtzite 40 nm Ganesan et al. (2020)  
Phanerochaete chrysosporium Phanerochaete  Fungal culture was inoculated in malt extract broth (150 ml) and incubated for 5–7 days, then filtered through Whatman filter paper No. 1 and added to zinc sulphate solution, followed by drop-wise addition of sodium hydroxide until the appearance of white suspended nanoparticles and again it was incubated for 24 h at 27 °C.  50 nm Sharma et al. (2020)  
Aspergillus terreus Aspergillus Fungal strains were taken to separate and purify the synthesized. The ultra-centrifugation collected NPs at 20,000 rpm (20 min), washed in deionized water with ethanol, and dried at 50 °C. Spheres with irregular margins 30.45 nm Mousa et al. (2021)  
Cochliobolus geniculatus Cochliobolus 10 g of fungal biomass was transferred to 100 ml of sterile ultrapure water and incubated for 72 h. The mycelial free filtrate was obtained by separating fungal mycelial biomass by centrifugation and combined with 1 mM of zinc acetate and maintained at 28 ± 1 °C for 72 h in an incubator shaker. Spheres 2–6 nm Kadam et al. (2019)  
Marine yeast  The cultured broth was collected after incubation and centrifuged at 8,000 rpm for 15 minutes. The supernatant was added with 1 mm of ZnO. Round 86.27 nm Aswathy et al. (2017)  
Trichoderma harzianum Trichoderma 50 ml of aqueous culture in flasks by stirring with zinc nitrate was added in culture to make a final concentration of 1–2 mM solution. The reaction was carried out in dark conditions at 45 °C, stirring vigorously. Spheres 87.5 nm Consolo et al. (2020)  
Penicillium chrysogenum Penicillium Fungal culture was grown up in a 100 mL fermentative broth medium for 7 days at pH 6.0, 30 °C, and shaking at 150 rpm, separated by Whatman filter paper No. 1. Hexagonal and spherical structures 9.0–35.0 nm Mohamed et al. (2020)  

Biodegradation of dyes using microorganisms is becoming increasingly popular due to the easy availability, cost-effective nature, and ease of growth under laboratory conditions. However, one of the significant reasons for the biological synthesis of ZnO nanoparticles is the existence of biologically active metabolites or enzymes that could be engaged as reducing agents during dye degradation (Khanna et al. 2019. Several dyes like methylene blue, methylene orange, have been degraded by ZnO nanoparticle synthesis using algae, bacteria, and fungi (Table 4).

Table 4

Photocatalytic degradation of dyes by microorganisms based ZnO nanoparticles

Microorganism speciesGenusDye degradedReferences
Algae 
Sargassum vulgare Sargassum Methylene blue Karkhane et al. (2020)  
Chlorella Chlorella Organosulfur pollutants Khalafi et al. (2019)  
Ulva lactuca Ulva Methylene blue Zhu et al. (2019)  
Chlamydomonas reinhardtii Chlamydomonas Methyl orange Parthasarathy & Narayanan (2014)  
Bacteria 
Arthrospira platensis Arthrospira Reactive red 120 Cardoso et al. (2012)  
Pseudomonas putida and Pseudomonas aureofaciens Pseudomonas Textile effluent from Rajkot, Gujrat Sur & Mukhopadhyay (2019)  
Bacillus licheniformis Bacillus Methylene blue Tripathi et al. (2014)  
Bacillus sp. Bacillus Reactive blue 19 Cai et al. (2020)  
Pseudochrobactrum sp.  Methanol blue and reactive black 5 Siddique et al. (2021)  
Fungus 
Phanerochaete chrysosporium Phanerochaete Reactive Black 5 and Bismark Brown R Kalpana et al. (2018)  
Aspergillus niger Aspergillus Bismark brown Enayatzamir et al. (2010)  
Penicillium corylophilum Penicillium Methylene blue Fouda et al. (2020)  
Microorganism speciesGenusDye degradedReferences
Algae 
Sargassum vulgare Sargassum Methylene blue Karkhane et al. (2020)  
Chlorella Chlorella Organosulfur pollutants Khalafi et al. (2019)  
Ulva lactuca Ulva Methylene blue Zhu et al. (2019)  
Chlamydomonas reinhardtii Chlamydomonas Methyl orange Parthasarathy & Narayanan (2014)  
Bacteria 
Arthrospira platensis Arthrospira Reactive red 120 Cardoso et al. (2012)  
Pseudomonas putida and Pseudomonas aureofaciens Pseudomonas Textile effluent from Rajkot, Gujrat Sur & Mukhopadhyay (2019)  
Bacillus licheniformis Bacillus Methylene blue Tripathi et al. (2014)  
Bacillus sp. Bacillus Reactive blue 19 Cai et al. (2020)  
Pseudochrobactrum sp.  Methanol blue and reactive black 5 Siddique et al. (2021)  
Fungus 
Phanerochaete chrysosporium Phanerochaete Reactive Black 5 and Bismark Brown R Kalpana et al. (2018)  
Aspergillus niger Aspergillus Bismark brown Enayatzamir et al. (2010)  
Penicillium corylophilum Penicillium Methylene blue Fouda et al. (2020)  

The azo dyes are synthetic organic dyes with an azo functional group. Azo dyes are grouped into different classes as reactive dyes, disperse dyes, acidic dyes, basic dyes, direct dyes, vat dyes, sulfur dyes, and solvent dyes (Saratale et al. 2011; Madhushika et al. 2020). The synthetic dyes consist of 70% azo dyes in the textile, food, cosmetics industries, etc. Azo dyes are toxic in nature, acting as carcinogens, genotoxins and mutagens (Ventura-camargo & Marin-morales 2013) (Table 5).

Table 5

List of azo dyes affecting water bodies

Dye NameDye iconicityToxicityReferences
Acid red Anionic Carcinogen Sudha et al. (2018)  
Acid violet Anionic Chromosomal aberration, lipid peroxidation Sudha et al. (2018)  
Acid yellow Anionic Carcinogen Swen et al. (2020)  
Allura red Anionic Genotoxic in vivo in mice and rats, hypersensitivity Kobylewski & Jacobson (2012)  
Amaranath Anionic Allergy, tumor, congenital disabilities, respiratory Problems Basu & Suresh Kumar (2014)  
Aniline yellow Nonionic Carcinogen Chung (2016)  
Bismarck brown R Nonionic Carcinogen Fatima et al. (2019)  
Bismarck brown Y Cationic Carcinogen Soriano et al. (2014)  
Direct blue 6 Anionic Carcinogen Lazear et al. (1979)  
Direct blue 15 Anionic Carcinogen Javaid & Qazi (2019)  
Direct brown Anionic Carcinogen Lazear et al. (1979)  
Direct red 28 Anionic Carcinogen Shetti et al. (2019)  
Direct orange Anionic Retards growth in plants Ventura-camargo & Marin-morales (2013)  
Disperse Orange 3 Nonionic Allergic Ventura-camargo & Marin-morales (2013)  
Disperse blue Cationic Mutagenic, cytotoxic Chequer et al. (2011)  
Disperse red Nonionic Mutagenic Kisku et al. (2015)  
Disperse Yellow 3 Nonionic Carcinogen Kisku et al. (2015)  
Methyl orange Anionic Mutagenic Freeman et al. (1996)  
Methyl yellow Nonionic Carcinogen Chung (2016)  
Reactive black 5 Anionic Carcinogen Sudha et al. (2018)  
Reactive brilliant red Anionic Inhibits functioning of human serum albumen Bafana et al. (2011)  
Sudan I Nonionic Carcinogen Stiborová et al. (2002)  
Sudan II Nonionic Mutagen Pan et al. (2012)  
Sudan III Nonionic Carcinogen Pietruk et al. (2019)  
Sudan IV Nonionic Carcinogen Pietruk et al. (2019)  
Dye NameDye iconicityToxicityReferences
Acid red Anionic Carcinogen Sudha et al. (2018)  
Acid violet Anionic Chromosomal aberration, lipid peroxidation Sudha et al. (2018)  
Acid yellow Anionic Carcinogen Swen et al. (2020)  
Allura red Anionic Genotoxic in vivo in mice and rats, hypersensitivity Kobylewski & Jacobson (2012)  
Amaranath Anionic Allergy, tumor, congenital disabilities, respiratory Problems Basu & Suresh Kumar (2014)  
Aniline yellow Nonionic Carcinogen Chung (2016)  
Bismarck brown R Nonionic Carcinogen Fatima et al. (2019)  
Bismarck brown Y Cationic Carcinogen Soriano et al. (2014)  
Direct blue 6 Anionic Carcinogen Lazear et al. (1979)  
Direct blue 15 Anionic Carcinogen Javaid & Qazi (2019)  
Direct brown Anionic Carcinogen Lazear et al. (1979)  
Direct red 28 Anionic Carcinogen Shetti et al. (2019)  
Direct orange Anionic Retards growth in plants Ventura-camargo & Marin-morales (2013)  
Disperse Orange 3 Nonionic Allergic Ventura-camargo & Marin-morales (2013)  
Disperse blue Cationic Mutagenic, cytotoxic Chequer et al. (2011)  
Disperse red Nonionic Mutagenic Kisku et al. (2015)  
Disperse Yellow 3 Nonionic Carcinogen Kisku et al. (2015)  
Methyl orange Anionic Mutagenic Freeman et al. (1996)  
Methyl yellow Nonionic Carcinogen Chung (2016)  
Reactive black 5 Anionic Carcinogen Sudha et al. (2018)  
Reactive brilliant red Anionic Inhibits functioning of human serum albumen Bafana et al. (2011)  
Sudan I Nonionic Carcinogen Stiborová et al. (2002)  
Sudan II Nonionic Mutagen Pan et al. (2012)  
Sudan III Nonionic Carcinogen Pietruk et al. (2019)  
Sudan IV Nonionic Carcinogen Pietruk et al. (2019)  

Textile dyes get mixed with industrial pollutants and are highly toxic and carcinogenic. These dyes are also toxic to the biological treatment units, thereby making the treatment of these dyes extremely complex (Tunçal & Kaygusuz 2014). The toxic dyes accumulate in sediments and soil, which then transport the water systems. The dyes assimilate in fish gills and accumulate in tissues. The azo dyes of chromium complexes damage the growth and development of plants (Lellis et al. 2019). The UV or chlorination process is efficient in degrading dyes from wastewater, but it has not been used in water bodies like ponds and lakes (Nikravesh et al. 2020). The toxic effects of dyes can be observed in aquatic animals like Xenopus laevis and Danio rerio (zebrafish) and can cause developmental stage and embryonic damage, respectively. Xenopus laevis tadpoles at the 46th stage of development were exposed to six textile dyes, mainly Astrazon Red, Astrazon Blue, Remazol Red, Remazol Turquoise Blue, Cibacron Red, and Cibacron Blue FN-R for 168 h in static conditions. The dyes caused oxidative stress, and the presence of organic pollutants caused increased levels of the glutathione S-transferase (GST) enzyme. The exposure of fishes to Metanil yellow causes increased GST enzyme activity in the liver and intestinal tissues (Güngördü et al. 2013). Zebrafish were exposed to textile dyes such as Maxilon blue 5G and Reactive blue 203 for 96 h, which caused acute toxicity and embryonic damage. The dyes have induced DNA damage and deformities in fish like curved body axis, tail malformation, and reproductive damage. Limited studies have been done on the mechanism of DNA damage caused by textile dyes (Köktürk et al. 2021). The toxic effects of textile dyes (Optilan yellow, Drimarene blue, and Lanasyn brown) were studied on Chlorella vulgaris, which caused the inhibition of growth, pigment, and elemental composition of the cells. After exposure for 96 h, there was complete suppression of growth of algal cells, reduction in protein synthesis, and decrease in chlorophyll a pigment density (Gita et al. 2019).

There are different types of synthetic textile dyes which include azo dyes, basic dyes, acidic dyes, nitro dye, disperse dye, vat dyes, direct dyes, mordant dye, reactive dye, solvent dye, reactive and sulfur dyes. The photocatalytic degradation mechanism is the same for all synthetic dyes, but they differ in the degraded product released. The zinc nanoparticles synthesized using medicinal plants are the mechanism responsible for degradation of azo dyes with the help of biosynthesized ZnO NPs under sunlight and UV irradiation. ZnO is a photocatalyst that helps in the degradation of dyes present in wastewater. ZnO nanoparticles are cost-efficient, and highly photoactive in the UV region. The plant extracted nanoparticles affect the morphology, and concentration of oxygen vacancies. The involvement of phytochemicals in nanoparticle synthesis increases the efficiency of ZnO NPs (Weldegebrieal 2020). Recent studies however showed that neither visible LEDs nor UV C irradiation resulted in photocatalytic bisphenol-A degradation. The inability of the amalgam UV C lamp to promote photocatalytic reactions in the vertical position could be attributed to either spatial photon emission caused by mercury gas settlement or photon–nanoparticle interaction at an incorrect collision angle. The vertically positioned UV LED-based illumination system, however, achieved a high energy consumption efficiency. Bisphenol-A was removed in the same manner as in the horizontally positioned reactor configuration, although the energy efficiency was almost unchanged (Tunçal 2020).

ZnO nanoparticles were dispersed in an azo dye mixture in water in a ratio of 1:10 (w/v pH = 6) under sonication for 20 min. The mixture was irradiated by sunlight, and absorption can be measured at regular intervals to observe the degradation of dyes. When light passes through the solution, electron–hole pair generation takes place (Equations (1) and (2)). The oxidation and reduction process creates free radicals and superoxides (Equation (3)) (Fageria et al. 2014). The OH functional group present in nanoparticles interacts with dye particles to generate OH free radicals (Equation (4)). The detailed mechanism of the photolytic dye degradation mechanism is shown below:
formula
(1)
formula
(2)
formula
(3)
formula
(4)
The azo dye solutions can be prepared by making a stock solution of the desired concentration. This dye mixture has varying concentrations of nanoparticles in normal pH and is exposed to light. The concentration reduction can be calculated using Equation (5):
formula
(5)

In the equation, A₀ depicts the initial concentration of the dye solution, and A depicts the final concentration of the dye solution after the photocatalytic process (Fageria et al. 2014).

The degradation process can be affected by many factors, like size, pH, temperature, aeration, catalysts, dopants, and oxidation. The surface area plays an important role in catalysis reaction, and a bigger particle size inhibits the reaction (Talebian et al. 2013). The photodegradation observed in nano-size particles was more efficient than micro-size particles because of the high surface area and more availability of active sites (Ateeq 2012). The effect of pH on photodegradation was measured in the ranges 4, 7, and 10. The methyl orange dye was removed efficiently at pH = 7, and pollutants were efficiently removed at pH = 4 (Abbasi & Hasanpour 2017). ZnO at higher pH efficiently degrades anionic dyes such as Congo red. Photodegradation was better at pH = 10 than pH = 7 due to the high concentration of hydroxyl ions (Adam et al. 2018). The temperature varies for each dye to be degraded. The degradation of direct red-23 dye at different annealing temperatures showed different efficiencies of degradation, which include temperatures in the range of 400 °C, 500 °C, 700 °C, and 800 °C, and photodegradation efficiencies were 88.48%, 95.49%, 92.63%, and 86.40%, respectively. It was observed that at 600 °C annealing temperature, complete degradation was observed (Umar et al. 2015). The synthesis of ZnO in the low-temperature reaction was efficient in Congo red degradation (Ong et al. 2016). The Reactive green 19 dye showed 77% degradation under 7 h in the presence of aeration, but in the absence of aeration, it resulted in the extension of reaction time to 8 h and a decrease in degradation efficiency to 56% (Lee et al. 2017). The ZnO nanoparticles extracted from Prosopis juliflora leaf extract degraded methylene blue with an efficiency of 99% under UV illumination and continuous aeration (Sheik Mydeen et al. 2020). The efficiency of degradation was directly proportional to the catalyst concentration. The reaction could not be proceeded due to the low catalyst surface. The degradation efficiency of decomposing methyl orange gradually increased from 50, 200, and 1,000 nm ZnO photocatalysts at pH = 10 (Wang et al. 2007). With increased catalyst concentration, it opens up more active sites for interaction with the dye solution. The methylene blue degradation was higher at a high concentration of catalyst (de Moraes et al. 2018). The concentration of dopants was inversely proportional to degradation efficiency. Metals like Ni, Co, and Ti act as doping metals and affect degradation efficiency (Mohseni-Salehi et al. 2018). Silver-doped (2%) ZnO nanoparticles used to degrade Brilliant green dye resulted in higher photocatalytic efficiency than alone (Gnanaprakasam et al. 2016). The photolytic degradation of Brilliant green was 99% in the presence of TiO₂ (Munusamy et al. 2013). The photocatalytic degradation of Acid orange 7 dye follows first-order kinetics under the presence of hydrogen peroxide and sodium periodate (Sadik 2007). The degradation efficiency of dye decreased with an increase in dye concentration, and the rate increased with Coxidant/Cdye ratio (Madhavan et al. 2006). The behaviour of ZnO suspensions was investigated at pH values ranging from 3 to 11 in order to investigate pH variation, the effect of dissolution on the zeta potential, and aggregate size stability. For all three constituents, the most stable pH region achieved in 1 hour corresponds to an initial pH of 7.7 (Fatehah et al. 2014).

The adsorption of dyes is essential to the efficient degradation of dyes. Several workers suggested that there was no relationship between adsorption of dyes and degradation in which they have used both anionic and cationic dyes (Liu et al. 2013). However, ZnO nanospheres were prepared by using a hydrothermal method and used efficient azo dye (Bismarck brown) in which different parameters affected the adsorption of dye and degradation by ZnO (Zaidi et al. 2019). Malachite green, Alexa fluor, and Congo red adsorbed a maximum of 2,963, 3,307, and 1,554 mg/g, respectively, on ZnO nanoparticles. The temperature and pH play an essential role in the adsorption process. The adsorption process was maximized by chemical precipitation, electrostatic attraction, and hydrogen bonding between the ZnO nanoparticle and different dyes (Zhang et al. 2016). ZnO supported with activated carbon or brick grain particles using the simple co-precipitation method resulted in a higher adsorption capacity for Malachite green and Congo red dyes (Raizada et al. 2014).

Adsorption and photocatalytic activity of the ZnO nanoparticles showed a higher efficiency to degrade the dyes. ZnO/Ag-montmorillonite nanoparticles with Urtica dioica leaf extract increased the discoloration of methylene blue from 38.95 to 91.95% (Sohrabnezhad & Seifi 2016). ZnO–CuO thin films were prepared by carbothermal evaporation with ZnO and Cu, and photocatalysis of methyl orange and methylene blue was observed in visible and UV light (Kuriakose et al. 2015). ZnO–graphene nanocomposites using grape and Eichhornia crassipes leaf extract degraded Rhodamine B dye efficiently with 70.0% and 97.5% degradation rate (Ramanathan et al. 2019). ZnO nanospheres generated by the hydrothermal method followed the pseudo-first-order rate reaction, degraded Bismarck brown dye with an efficiency of 94% after 2 hours of exposure (Zaidi et al. 2019). ZnO-graphene obtained by the hydrothermal process degraded Azure B dye 99% within 20 minutes of exposure under UV illumination (Rabieh et al. 2016). Strobilanthes crispus (B.) leaf extract fabricated with La2CuO4-decorated ZnO were capable of degrading Malachite green dye following pseudo-first-order kinetics (Yulizar et al. 2020).

Zebrafish share 70% of their genes with human beings. ZnO nanoparticles damage neural and vascular systems. Dissolved Zn causes less damage to the nervous system than chemically synthesized ZnO particles. Zebrafish when exposed to dissolved oxygen matter (DOM) water with ZnO nanoparticles, it converted to zinc ions that are toxic. ZnO damages the hatching rates and degrades the DOM (Kteeba et al. 2018). The ZnO nanoparticles synthesized from plant extracts are more efficient and eco-friendlier than chemically synthesized nanoparticles. Plant extracts were used as a reducing and stabilizing agent, and zinc nitrate can be used as a zinc precursor. ZnO nanoparticles are exposed to sunlight and electrons become excited from the valence band to conduction band resulting in the formation of superoxides and hydrogen oxide radicals which are potent reducing agents that are capable of degrading dyes, resulting in a less toxic degradation products such as carbon dioxide and water (Sharma et al. 2021).

In today's world, we use nanoparticles in all fields that lead to the accumulation of ZnO nanoparticles. When nanoparticles reach the water bodies, it leads to aggregation of nanoparticles that in suspension form, converted to zinc ions that induce toxicity (Beegam et al. 2016). ZnO nanoparticles enter the soil, and are converted to Zn2+ in soil and plants. The soluble Zn was more toxic than ZnO nanoparticles (Wang et al. 2013).

ZnO nanoparticles affect soil, water bodies, the environment, and human health. Zn deficiency in humans leads to severe anemia, weakening of the immune system, inflammation, and lung toxicity due to inhalation of ZnO nanoparticles (Beegam et al. 2016; Rajput et al. 2018). The toxicity of ZnO nanoparticles can be observed in mammalian cells, bacteria, and zebrafish. In bronchial epithelial cell lines such as BEAS-2B and A549 cells, ZnO nanoparticles induce cytotoxicity and mitochondrial dysfunction. ZnO had both short-term and long-term effects on mammalian cells, and short-term effects included apoptosis, whereas long-term effects included increased ROS generation, and decreased mitochondrial activity (Vandebriel & De Jong 2012). The nanoparticles are toxic to both Gram-positive and Gram-negative bacteria as nanoparticles are bactericidal at the log phase of bacterial growth, and cell viability/membrane integrity was lost after 15 h of exposure. (Reddy et al. 2007). When ZnO nanoparticles enter the marine environment this leads to ROS production, toxicity in zebrafish embryos, and damage in their hatching enzyme due to hypoxia caused by ZnO nanoparticles (Bai et al. 2010) (Yung et al. 2014).

The highly toxic deposition of dyes in water bodies stops the oxygenation capacity of water and blocks sunlight from penetrating inside, affecting the biological activity of aquatic life and the photosynthesis process of aquatic plants. The dyes keep on accumulating in the sediments, in fishes, or other marine organisms. Decomposition of dyes into water bodies are carcinogenic or mutagenic compounds that cause allergies, skin irritation, or different tissue changes. The toxicity was observed in mammalian cells, which mimics human cell conditions in which briefly the authors have reported that ZnO induces cytotoxicity but not Zn2+. To understand the mechanism, they performed in vitro studies on bronchial epithelial cell lines, dermal cell lines, colon cell lines, and immune cells (RAW264.7). The ZnO nanoparticles dissolved the extracellular membranes of the cells and were converted to Zn2+ that leads to lysosome destabilization. As zinc ion concentration increases inside the cells, it leads to a decrease in enzyme and transcription factors. Proteins such as bovine serum albumin adsorb to the surface of ZnO. ZnO damages the cells, which leads to calcium flux, ROS generation, membrane damage, and mitochondrial dysfunction (Vandebriel & De Jong 2012). Nanoparticles induce toxicity in both Gram-positive and Gram-negative bacteria and caused cell proliferation in cancer cells. The ultra-sonication of HL60 cancerous cell lines induced lipid peroxidation, which leads to enhancement of the mechanism in the presence of ZnO nanoparticles (Premanathan et al. 2011). E. coli is the most common pathogen that can be used to see toxicity mechanisms. The ZnO nanoparticles dissolve in medium and release Zn ions. High concentrations of ZnO nanoparticles damage physiological features, decreases toxicity tolerance levels, and causes deformation in the cell membrane, and leaking out of intercellular substances. The release of zinc ions leads to maximum toxicity (Li et al. 2011) (Figure 2).

Figure 2

Mechanism of toxicity due to ZnO nanoparticles.

Figure 2

Mechanism of toxicity due to ZnO nanoparticles.

Organic pollutants such as azo dyes are common environmental pollutants, and considered hazardous materials for human health. Photocatalytic activity plays a significant role in the degradation of organic derivatives by ZnO and nanoparticles. ZnO nanoparticles have high thermal conductivity that helps in the removal of azo dye pollutants. Researchers have efficiently degraded azo dyes using plant-based nanoparticles. The photocatalytic degradation is enhanced by electron–hole pair generation. The nanoparticles generated from plant extracts help to increase the efficiency of degradation of dyes. Recently, several investigations have shown that plant extract nanomaterials have highly improved the photocatalytic efficiency compared to nanoparticles alone. To make nanoparticles, an appropriate compound, and an exclusive photocatalyst, researchers have tried different phytochemical concentrations to optimize the size of particles and surface area. Research has been carried out on numerous kinds of plant extract on ZnO as a photocatalyst by several approaches and the application in photodegradation of organic pollutants. The mechanism of toxicity is understood because of the interaction between the ZnO nanoparticles and different surfaces like bacteria, aquatic animals, and human cells. The deposition of nanoparticles damages cells, resulting in oxidative stress and is lethal to the environment and living organisms. The exact mechanisms of toxicity have still not been explored. With these pollutants either dyes or nanoparticles accumulate in water bodies and animal systems and this could pose a severe threat to health, especially in developing countries where water is consumed without much analysis or testing. Hence future posts should address new challenges for the application of ZnO in various emerging fields as well as well the identification of effects of these pollutants at the DNA level and how mutagenic they can be. However, these ZnO nanoparticles have been approved by the US FDA and are generally recognized as safe (GRAS), these nanoparticles can have huge prospects in the future towards biomedical, agricultural, and environmental remediation fields.

All relevant data are included in the paper or its Supplementary Information.

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