Leachate treatment potential of nanomaterial based assemblies: a systematic review on recent development

One of the biggest challenges of the twenty-first century is municipal solid waste (MSW) management. Due to the low cost of investment and operation, landfilling has become a widely accepted and employed waste management system for MSW (Costa et al. 2016). MSW sites generate a huge amount of toxic gases, compounds, and chemicals. These materials are mixed with water and make a chemical soup called leachate (Chemlal et al. 2014). Leachates usually contain high concentrations of organics, ammonia nitrogen, heavy metals and hazardous chemicals that adversely affect the environment. The deposition depends on various parameters like hydrogeology, amount of rainfall, landfill method, age of landfill (He et al. 2017). Leachate is infiltrated with waste and reacts with chemicals and biological compounds to form complex organic compounds (Cai et al. 2014; Chemlal et al. 2014). Further, the seepage and percolation of leachates potentially causes the contamination of ground as well as surface water. Apart from this, it also deteriorates soil resources, natural flora and fauna, and adversely affects human health. Typically, the characteristics of the landfill leachate can be best represented by chemical oxygen demand (COD), total organic carbon (TOC), biological oxygen demand (BOD), BOD/COD ratio, pH, suspended solids (SS), ammonium nitrogen (NH₃-N), total Kjeldahl nitrogen (TKN), bacterial count, turbidity, and heavy metal content (Abd El-Salam & Abu-Zuid 2015; Baettker et al. 2020).

The water contaminants are generally classified as inorganic toxic elements, organic chemicals and microorganisms. The major group of pollutants present in the leachates is dissolved organic matters (DOM) (Lu et al. 2009; Wu et al. 2011) such as aldrin, chlordane, dichlorodiphenyl trichloroethane (DDT), dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene, polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and inorganic macro-components which are described in Figure 1 (Zhang et al. 2013a).

The typical organic pollutants in water include pharmaceuticals, personal care products, endocrine disruptors, pesticides, organic dyes, detergents, and common industrial organic wastes like phenolics, halogens, and aromatics. Others include heavy metals (Cd, Cr, Hg, Cu and Pb) (Fan et al. 2008; Feng et al. 2014), and xenobiotic organic compounds (dioxins and halogenated organics) (Zhao et al. 2012). In this regard, various physical, chemical, and biological approaches have been utilized for the treatment purpose. But a single approach, either biological or chemical, is not sufficient to efficiently remove pollutants from the leachates. The literature over the past ten years has shown that the major techniques involved in the treatment of leachates are adsorption, air stripping, fenton oxidation, biodegradation, coagulation, and precipitation. Figure 2 shows the database collected by Google scholar using key words ‘leachate treatment and adsorption’, ‘leachate treatment and Fenton oxidation’, ‘leachate treatment and biodegradation’.

Among them, the adsorption-based approach is widely utilized for treatment purposes. However, treatment of leachate by adsorption is well accepted, but due to the generation of sludge, saturated adsorbed media, and tedious regeneration practices for the adsorbent media, its efficiency and reusability are limited. Furthermore, the synthesis of a proper adsorbent material requires various pretreatment steps like thermal, acid, or alkali treatment (Reshadi et al. 2020).

In addition to adsorption, air stripping and fenton oxidation, options were also explored for the leachate treatment. However, these treatments require a combination of various chemical reactants as well as treatments like air strippers and tower columns for air stripping, which impact the high cost of leachate treatment and waste water treatment (Liu et al. 2015).

Nowadays, the integration of biological and chemical methods for this purpose has attracted researchers. Bioremediation, biotransformation, and bio stimulation are the common biological approaches that, integrated with various physical and chemical treatments, were able to reduce the concentration of various pollutants in the leachate. Scientists are now looking into new possibilities such as the development of specific catalysts, membranes, and reactors. Nanomaterials such as nanoparticles (NPs), nanomembrane nanofilters, and nanotubes emerge as suitable candidates in these options. There is a database of the past ten years of research showing that nanomaterials have been continuously explored in the development of novel adsorbents (Yang et al. 2013), catalysts (Azadi et al. 2017), and filtration assemblies (Shafaei et al. 2016a), as immobilization support and entrapment matrix for different enzymes and microbes, and as reactive materials for the treatment of wastewater and leachate (see Figure 2). Further, the nanomaterial systems-based prototypes are also being scaled up for integration with already existing plants to improve their efficiency and pollutant degradation capacity (Ji et al. 2017). In the case of nanomaterial based assemblies, most of the research is focusing on the synthesis of metal oxide based NPs and exploring their use in leachate treatment. Apart from that, some of the works published are trying to integrate biological entities (microbes, enzymes, biopolymers) with metal oxide based NPs to enhance their pollutant degradation efficiencies. Leachate treatment and waste water treatment are two of the major issues, and the different types of complex pollutants are now aggravating the environment. Hence, continuous development and innovation are required in the existing treatment methods. Therefore, this review is designed in such a way that it will cover all the methods that are based on the use of NPs in leachate treatment. Therefore, keeping in mind the huge database available on the nanomaterial-based approaches for the treatment and removal of pollutants from wastewater, this review article provides comprehensive and systematic information about the recent and advanced development of nanomaterial-based approaches that will become useful for leachate treatment (see Figure 3). In addition to that, we are covering all the aspects in that area, i.e., the synthesis of NPs, their integration with biological entities or mechanisms of these NPs for the degradation of complex organic materials.

Nanomaterials are the materials that are sized (in at least one dimension) between 1 and 100 nm (Lau et al. 2017). These materials are already explored for their unique mechanical, electrical, optical, and magnetic properties, which make them significantly different from conventional materials (Bhagyaraj & Oluwafemi 2018). A wide range of nanomaterials have the characteristics of catalysis, adsorption, and high reactivity, which make them a suitable candidate for pollution remediation, especially in the case of water and industrial effluent. In particular, the application of nanomaterials in water and wastewater treatment has drawn wide attention to properties like surface area, fast kinetics, fast dissolution, high dispersion ability, adjustable pore size and surface chemistry, superparamagnetic for particle separation, and specific affinity towards certain contaminants (Zahrim et al. 2019). The NPs are also being stabilized via immobilization on different polymers (Sajjadi et al. 2020; Nasrollahzadeh et al. 2021), doping (Yuan et al. 2009), and conjugation (Hu et al. 2016) and may be used in multiple manners for the treatment of contaminants (see Table 1). In addition to that, metallic NPs have also been reported for the improvement of catalytic capability of different microbial enzymes (Hu et al. 2016).

Photocatalysis is considered as an auspicious approach for the deployment of photoenergy. In recent years, photocatalysis research has largely focused on water splitting, reduction of CO₂, synthesis or degradation of organic pollutants, air remediation, and self-decontaminating surfaces (da Silva et al. 2015) (see Figure 4). In the case of photocatalysis, the nanomaterials become a pronounced option due to the ability to adjust their properties, allowing highly specialized photocatalysis (Pascariu et al. 2018). In this regard, the research continued and led to the implementation of a system based on support material and metallic NPs as a photocatalyst material. In case of support, materials like silica, graphite, carbon, cellulose are used (Zahid et al. 2020) and for the metallic components Ti, Ag, Zn, Fe, Cd and Ni sulfides, mixed metal oxide and rare earth metal oxides are in use (Su et al. 2014). Some of the major nanomaterial based photocatalysts and their applications in the treatment of leachate and pollutant removal are as follows.

Titanium dioxide is most commonly used as a photocatalyst (Muraro et al. 2020). TiO₂ photocatalytic degradation has shown great potential as a low-cost, non-toxic, chemically stable, high-photoactivity, environmentally friendly, and sustainable treatment technique in the water and wastewater industry (Azadi et al. 2017). TiO₂ NPs are reported to trigger the generation of hydroxyl radicals that trigger the oxidation of organic contaminants (Niu et al. 2018). Most of the studies conducted on TiO₂ NPs showed semiconductor photocatalysts, but for better results, they must be irradiated with UV light to be excited and capable of photo-oxidation (Hu et al. 2016). When the TiO₂ adsorbs ultraviolet light with wavelength of between 200 and 400 nm, the electrons are photoexcited and move towards the conduction band, which leads to the formation of electron holes that may accelerate the oxidative-reductive reaction chain. Hence, the degradation of heavily decomposable substances can also be increased (Verma et al. 2020). TiO₂ based NPs are also used for killing pathogenic bacterial or viral components in waste water. A concentration of TiO₂ between 100 and 1,000 ppm is usually required to kill bacteria, depending on the size of the particles and the intensity and wavelength of the light used (Zhang et al. 2008). In the case of degradation leachate treatment, Jia et al. used TiO₂ NPs as a photocatalyst and observed that the initial COD 2,440 mg/L was reduced by up to 60% by 2 and 3 g/L of the optimal dose of the catalyst (Jia et al. 2011). Similarly, Chemlal et al. also investigated the TiO₂ and ultraviolet (UV) based heterogeneous photocatalytic processes for landfill leachate treatment and found that initial COD ranging from 26,000 to 30,000 mg/L, at the optimal pH of 5, mineralized up to 92% after 30 h (Chemlal et al. 2014). Similarly, Vineetha et al. investigated the performance of the TiO₂ photocatalyst in the treatment of concentrated wastewater with an initial COD of 500 mg/L. Their results indicated that the optimal pH for maximum COD and color removal was 6. In addition, they reported that in a 0.2 g/L dose of catalyst and pH of 6, COD and color removal efficiency were 32 and 84%, respectively (Vineetha et al. 2013). However, because UV light production is costly in long-term uses, this method cannot be practical. If the band gap width of TiO₂ NPs is somehow reduced, the application of this method will become quite affordable by using sunlight. For this reason, researchers offer various strategies to solve this problem, including morphological or chemical modifications (Pelaez et al. 2012). A common method to modify the band gap of TiO₂ NPs is doping the NPs with metal elements (Neville et al. 2013). In this field, various metals such as chrome, molybdenum, tungsten, and vanadium have been studied, but no studies on the use of doped NPs for leachate treatment under visible light irradiation have been completed this far.

TiO₂ is reported as a killing factor against viruses like Poliovirus (Zhang et al. 2008), Hepatitis B virus, Herpes simplex virus (MacLean et al. 1998), and MS2 bacteriophage (Hajkova et al. 2007). A concentration of TiO₂ between 100 and 1,000 ppm is usually required to kill bacteria depending on the size of the particles and the intensity and wavelength of the light used (Zhang et al. 2008). When the TiO₂ adsorbs ultraviolet light with wavelength of between 200 and 400 nm, the electrons are photoexcited and move towards the conduction band. As a result of this photoexcitation activity, electron holes are created, and the oxidative-reductive reaction chain also increases. Hence, in the initial step, biodegradation of heavily decomposable substances can also be increased. In 2005, Adams et al. developed a route for complete inactivation of fecal coliforms in 15 min at an initial concentration of 3,000 cfu/100 mL by exposing water in TiO_2. Exposing the coated plastic container to sunlight resulted in complete inactivation within 60 min. Here is the most important thing: the damaged bacteria do not self-repair. In the excitation of TiO₂, as a metal, silver has an advantage (Adams et al. 2006). Qilin Li, et al. showed that doping TiO₂ with silver gives improved photocatalytic inactivation of bacteria where 1% Ag with TiO₂ reduced the reaction time required for complete removal of 107 cfu/mL E. coli from 65 to 16 min in UV-A light (Li et al. 2008). Silver is also used in a surface coating of UV reactors, which increases the performance.

The nano sized ZnO has high UV adsorption efficiency and transparency to visible light (Franklin et al. 2007). ZnO NPs can inhibit the bacterial growth and also can cause disorganization of cell membrane. Zn⁺² ions will bind to the cell membrane and the lag phase of growth cycle will be prolonged (Atmaca et al. 1998). In 2007 Franklin et al. investigated whether the smaller ZnO particles are more toxic than the bigger ones, but no size related effect was found (Franklin et al. 2007). ZnO NPs are found for the treatment of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Bacillus subtilus (Azam et al. 2012).

The adsorption process is used as a stage of an integrated chemical-physical-biological process for landfill leachate treatment (Morawe et al. 1995). The most frequently used adsorbent is granular or powdered activated carbon. Carbon adsorption permits the 50–70% removal of both COD and ammonia nitrogen (Amokrane et al. 1997). Adsorption has emerged as the most effective and important method of removing pollutants from water without secondary potential pollution and at an acceptable cost. As a fast, efficient, and economical physicochemical method, adsorption technology has been extensively used in wastewater treatment for years. The adsorptive efficiency strongly depends on the type of adsorbate, adsorbent, and operating conditions. The adsorption mechanisms of organic pollutants are mainly attributed to electrostatic, p-p, hydrophobic, acid-base, H-bonding, and van der Waals interactions. Several adsorbents have been reported for the adsorption of organic pollutants such as pharmaceuticals in water, including activated carbons (AC), zeolites, clay, carbon nanomaterials (e.g. graphene (G), graphene oxide (GO), carbon nanotube (CNT)), and metal organic frameworks (MOFs).

Carbon nanotubes have a cylindrical nanostructure. The synthesis and application of CNTs have attracted extensive interest because they have unique and tunable physical, chemical, and electronic properties. They can be categorized as single-walled (SWCNTs) or multiwalled nanotubes (MWCNTs). Due to their hydrophobic structure, CNTs are stabilized in aqueous suspension to avoid aggregation that reduces the active surface. Due to their large specific surface area, high surface free energy, porosity and activity, CNTs have been widely used as effective adsorbents for the remediation of wastewater. Smith and Rodrigues have compared the four carbon-based nanomaterials: SWCNTs, MWCNTs, G, and GO, for sustainable water treatment (Smith & Rodrigues 2015). CNTs can be used for adsorption of persistent contaminants and detection of contaminants (Pan et al. 2008). By chemical bonding and electrostatic attraction, CNTs can adsorb metal ions (Lu et al. 2008) and also have antimicrobial activity by causing oxidative stress in bacteria and destroying cell membranes (Oviedo et al. 2022). With chemical oxidation, no chemical by products are produced, and it can be used in processes like chlorination and ozonation (Liu et al. 2013). Yan et al. developed plasma-modified ultralong CNTs that feature an ultrahigh specific adsorption capacity for salt that is two orders of magnitude higher when compared with conventional carbon-based water treatment systems (Yang et al. 2013). Next-generation potable water purification devices equipped with these novel CNTs are expected to have superior desalination, disinfection, and filtration properties. Recently, a team of US researchers developed a sponge made of pure CNTs with a dash of boron that shows a remarkable ability to absorb oil from water. The oil can be stored in the sponge for later retrieval or burned off so the sponge can be reused (Hashim et al. 2012).

Apart from carbon nanotubes, several other nano adsorbents were also reported for the treatment of leachates: nano-ionic copper doped oil palm frond activated carbon (n-OPFAC) (Adam et al. 2019), Saccharum officinarum leaves (SL) assisted nano-silica (NS) (Kaliannan et al. 2019), and others. Bentonite clay was modified with amino acids as an eco-friendly adsorbent (Hajjizadeh et al. 2020). Adam et al. prepared n-OPFAC. To test the potential of n-OPFAC through COD and color adsorption in semi aerobic leachate, researchers used orthophosphoric acid (H₃PO₄) impregnation and oxidation techniques. Activated carbon from oil palm fronds was saturated with H₃PO₄ for 1 hour in a 1:3 ratio. According to preliminary investigations, a maximum iodine number of 987 mg/g was achieved at a temperature of 400 °C and a heating period of 1 hour. A 1:2 ratio (w/w) of doped activated carbon with nano-ionic copper was achieved at the same heating temperature and time. Researchers conducted a batch adsorption experiment at a room temperature with several dosages of 0.8 g at 200 rpm for a 15 to 75 minute time period. The experiment resulted in the highest adsorption percentage of COD and color, which is 82% at 45 minutes and 57% at 60 minutes, respectively (Adam et al. 2019). Kaliannan et al. synthesized SL assisted NS as an adsorbent for the removal of Pb²⁺ and Zn²⁺ from aqueous solution. Characterization of nano-adsorbent was conducted by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The surface area and charge of NPs were also characterized by Brunauer–Emmett–Teller and zeta potential. As a result, the maximum adsorption capacity of Pb²⁺ and Zn²⁺ at room temperature (37 °C) was found to be 148 mg/g and 137 mg/g, respectively (Kaliannan et al. 2019). Hajjizadeh et al. used modified bentonite clay where amino acids were intercalated into the interlayer gap of bentonite. Researchers experimented with different dosages of L-glutamic acid added to bentonite and the effect was analyzed by contamination reduction in the landfill leachate. Batch studies were carried out to determine the parameters that influence leachate COD and turbidity removal efficiency. The effects of surfactant loading, contact time, pH, and adsorbent dose were investigated. Under optimal conditions, the maximum COD and turbidity removal for amino acid modified clay was 65.7% and 92.1%, respectively, under pH adjustment setups. Furthermore, the analysis showed that the adsorbent created under pH adjustment settings had a greater leachate treatment efficiency than the adsorbent synthesized under non-pH adjustment conditions (Hajjizadeh et al. 2020).

Iron NPs are an attractive component for nano-remediation. Iron at the nanoscale was synthesized from Fe (II) and Fe (III), using borohydride as the reductant. The diameter of nanoscale zero-valent iron particles (NZVI) ranges from 10 to 100 nm. They have the classic core shell construction. The core is predominantly made up of zero-valent or metallic iron, with the mixed valent [i.e., Fe (II) and Fe (III)] oxide shell forming as the metallic iron oxidizes (Li et al. 2006). Because NPs have a larger surface area and a greater number of reactive sites than micro-sized particles, they are often selected for nano-remediation (Mortazavian et al. 2018) and they possess dual properties of adsorption and reduction; as a result, they can be utilized to remediate a wide spectrum of toxins that are present in the environment. Furthermore, when zero-valent iron was given more access to the contamination site, it was discovered that it produced less hazardous waste during the treatment process (Nadagouda & Varma 2009). Nano-remediation has become a major focus of study and development in recent years. This method has a lot of potential for cleaning up contaminated places and protecting the environment from pollution. Nanomaterials are best suited for in situ applications due to their unique characteristics. When compared to large-sized particles, their modest size and modified surface allow them to spread further and wider (Mortazavian et al. 2018). Using a magnetic field, the super-paramagnetic property of iron NPs was regulated and they were retrieved without the risk of being released into the environment. Iron oxide NPs with a diameter of 12 nm have been shown in laboratory experiments to remove more than 99% of arsenic from water samples (Rajan 2011). A modified surface of zerovalent iron NPs containing an oil-liquid membrane was developed for the treatment of trichloroethane (TCE). This oil-liquid membrane is hydrophobic and creates an emulsion with ZVI. It is made up of food-grade surfactant, biodegradable oil, and water. Emulsified zero-valent iron (EZVI) is the name given to this type of iron (Quinn et al. 2005). The hydrophobic membrane surrounding the NZVI protects it from other groundwater constituents, such as some inorganic compounds, that might otherwise react with the NZVI, reducing its capacity or passivating the iron, which could be a potential benefit of EZVI over NZVI for environmental applications (Hara et al. 2006). Kashitarash et al. studied the determination and efficiency of iron NPs for the removal of COD, BOD and the color of Hamadan City landfill leachate. Where they performed in the batch reactor process, the main affecting factors were pH, reaction time, and concentration was iron NPs. The removal efficiency was 47.94% with a fast procedure of 10 minutes and an optimal condition of around pH 6.5. The iron NP concentration was 2,500 mg/L. High efficiency and rapid action make this method effective (Kashitarash et al. 2012).

In recent years, a number of technologies have been developed to increase efficiency of bioremediation techniques. Bioremediation techniques are new and effective method for the removal of pollutants in wastewater treatment. The use of nanomaterials combined with microorganisms is an innovative technique for enhancing the efficiency of bioremediation process beyond the limits. This integrated or combined technique is also cost-effective for treating pollutants in groundwater and wastewater.

Fungus and fungal enzymes exhibit a remarkable potential for the complete degradation of various complex organic compounds (Zhang et al. 2013b). Bioremediation based studies suggest that the fungal biomass is also able to accumulate various metal ions such as Cu, Cd, Hr, Sn (Gaur et al. 2014). Apart from that, the fungal biomass may also adsorb different heavy metals and toxic pollutants (see Figure 5). Therefore, the use of fungal landfill leachate treatment has also been widely investigated (Nadagouda & Varma 2009; Mortazavian et al. 2018). Initially, the fungus was explored for the reduction of organic materials in industrial wastewater. Fungi showed excellent degradability of recalcitrant compounds and good removal of COD and color. After that, fungi, especially white-rot fungi (WRF), have been applied in leachate treatment. Research indicates that the WRF have evolved some intrinsic nonspecific mechanisms to degrade an extremely diverse range of very persistent or toxic environmental pollutants (Huang et al. 2018). In another way, we may conclude that WRF have considerable potential for utilization in toxic and hazardous pollutant removal (Kalčíková et al. 2014). In this regard, WRF are able to secrete various extracellular enzymes (e.g. laccase (Lac), lignin peroxidase (LiP), and manganese peroxidase (MnP)), which are competent for biodegradation of natural lignocellulosic substrates, and are also efficient in removing a wide range of organic pollutants such as phenols, dyes, pesticides, polychlorinated biphenyls, and so on (Brijwani et al. 2010). Nowadays, fungus based treatment has been coupled with NPs, which may be able to enhance the degradation capacity of both fungus and NPs. The research conducted by Liang Hu et al. investigated the technical applicability of a combination of Phanerochaete chrysosporium (P. chrysosporium) with photocatalyst graphitic carbon nitride (g-C₃N₄) for organic matter removal from landfill leachate under visible light irradiation. Photocatalyst g-C₃N₄ was well immobilized on the hyphae surface of P. chrysosporium by calcium alginate. The typical absorption edge in the visible light region for g-C₃N₄ was at about 460 nm, and the optical absorption bandgap of g-C₃N₄ was estimated to be 2.70 eV, demonstrating the great photoresponsive ability of g-C₃N₄. An optimized g-C₃N₄ content of 0.10 g in immobilized P. chrysosporium and an optimized immobilized P. chrysosporium dosage of 1.0 g were suitable for organic matter removal. The removal efficiency of TOC reached 74.99% in 72 h with the initial TOC concentration of 100 mg L⁻¹. In addition, the gas chromatography coupled with mass spectrometry (GC-MS) measurements showed that immobilized P. chrysosporium presented an outstanding removal performance for almost all organic compounds in landfill leachate, especially for the volatile fatty acids and long-chain hydrocarbons. The overall results indicate that the combination of P. chrysosporium with photocatalyst g-C₃N₄ for organic matter removal from landfill leachate may provide a more comprehensive potential for landfill leachate treatment (Hu et al. 2016).

Cherni Yasmin et al. investigated the technical applicability of a combination of a photocatalytic process with TiO₂/Ag nanocomposite and a biological treatment using Candida tropicalis strain for pollutant removal from mature Jebel Chakir landfill leachate under visible light irradiation. They used nanocomposite treatment as a first stage under the optimum conditions and achieved removal of 70, 71.25, and 49.1% for COD, TOC, and NH₃-N, respectively. Biological treatment of pretreated landfill leachate for enhancement was performed as a second treatment, and about 90% of COD, 84.61% of TOC, and 75% of NH₃-N were removed after the combined process. Furthermore, heavy metals, especially, Fe, Zn, Cu, Cd, and Pb, were significantly reduced with the average removal rates of 50, 63.8, 83, 95 and 95%, respectively (Yasmin et al. 2020).

WRF can efficiently degrade a broad range of recalcitrant micropollutants such as endocrine disruptor chemicals (EDCs), pharmaceutically active chemicals (PhACs), hormones, and pesticides via one or more extracellular enzymes (see Figure 5). WRF and their lignin modifying enzymes (LMEs) have been successfully used for the treatment of several xenobiotics. They have been applied in bioremediation technologies for the treatment of dyes, polycyclic aromatic hydrocarbons (PAHs), PCBs, phenols, pesticides and industrial wastes in different environments (Cabana et al. 2007). LMEs have low substrate specificity, which allows them to degrade a wide range of xenobiotics and also secrete low molecular weight mediators which enlarge the spectrum of compounds that they are able to oxidize (Pointing 2001).

Enzymatic bioconversion technologies have attracted growing interest in the field of wastewater treatment. Their advantages include a high activity on a broad range of substrates at mild conditions, a high specificity that allows elimination of the selected and specific compounds, as well as less toxic biodegradation of products (Kim et al. 2006).

A novel technique by Ji et al. directly immobilized the in-house crude enzyme extracts from Pleurotus ostreatus (P. ostreatus) onto the functionalized TiO₂ NP surface. Comprehensive investigations were carried out to understand the interactions between complex crude enzyme extracts and the immobilization support. By simple dilution of the crude enzyme extract, the immobilization efficiency can be significantly improved. The resultant biocatalytic NPs had comparable performance to the immobilized purified commercial enzymes. Finally, the micropollutant degradation capability of the biocatalytic NPs was demonstrated with two micro-pollutants, namely, bisphenol-A and carbamazepine, commonly detected in sewage. The efficient extraction and immobilization of laccase on biocatalytic NPs shows great promise as a cost-effective alternative to conventional wastewater treatment processes for recalcitrant micropollutants (Ji et al. 2017).

Bardi et al. compared the effectiveness of seven redox-mediating compounds, namely, 1-hydroxybenzotriazole (HBT), N-hydroxyphthalimide (HPI), 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), violuric acid (VA), syringaldehyde (SA), vanillin (VA), and 2,20-azino-bis (3- ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), which follow distinct oxidation pathways, for the degradation of trace organic contaminants (TrOCs). These redox-mediators were investigated for improved degradation of four TrOCs showing resistance to degradation by crude laccase from the WRF P. ostreatus (Bardi et al. 2017).

For the treatment of leachates and other major pollutants, the use of bacteria with the combination of NPs has been reported in some articles. In this regard, Kumari et al. reported the treatment of leachate using a synergetic effect of microalgae and bacteria on leachate collected from landfills in Northern India. Leachate samples have the presence of various toxic materials, and in metals they have a high concentration of Zn, Cr, Fe, Ni and Pb. The combination of bacteria and algae co-cultures was used, where the Paenibacillus bacterial strain was used for the treatment. The bacterial and algal strains were isolated from the Vasant Kunj, New Delhi landfill site. For the treatment, the shake flask method was used with 20% leachate sample. Results showed that this synergic treatment is efficient for the removal of toxic organic contaminants and heavy metals (Li et al. 2006). In another study, Wang et al. reported the degradation of di-n-butyl phthalate (DBP), one of the most widely used plasticizers, which has been listed as a priority pollutant because of its toxicity to both humans and animals. Pseudomonas sp. W1 was discovered to be capable of decomposing 99.88% of DBP (1,000 mg/L) in just 8 days after being isolated from activated sludge in their study. They used Fe₃O₄ iron NPs (IONPs) coated with poly-dopamine (PDA) to immobilize the W1 strain and then tested its DBP degrading efficiency. Immobilization increased W1's DBP degradation performance, with 99.69% DBP degradation efficacy (Wang et al. 2020). Similarly, Bai et al. also developed a polydopamine (PD)-coated- Fe₃O₄ iron NPs system for the immobilization of Sphingomonas sp. strain Y2. This immobilized system does not affect the activity of Sphingomonas sp. and exhibits 79.5% and 99.9% of nonylphenol polyethoxylates (NPEOs) (500 ppm) degradation efficiency on days 1 and 2 (Bai et al. 2018).

In addition to the biological methods, the use of NPs has been reported for the leachate treatment in different ways, such as for the disinfection of coliforms, hybrid ultrafiltration with activated charcoal, and in the reactive barrier systems. In this regard, Sepehry Atefe et al. did a modified oxidation disinfection study by investigating the removal rate of fecal coliforms in leachate. In their experimental setup, they took an ozone concentration of 0 to 5 g h⁻¹ for 0, 20, 40, 60, 80 min, and the copper NP concentration was 0/01, 0/03, 0/05 g L⁻¹. They observed that the copper concentrations of 0/01 g L⁻¹ were able to remove 66% of coliforms from leachate, but when this concentration was associated with a concentration of 5 g h⁻¹ ozone, up to 91% of coliforms were removed. The maximum removal rate was experienced with 5 g h⁻¹ ozone concentration and 0.05 g L⁻¹ copper NPs for 80 min, which removed 98% of coliform from the sample. Increasing the process time leads to an increase in the percentage of coliform bacteria elimination (Sepehri et al. 2016).

Furthermore, Pirbazari et al. reported a hybrid ultrafiltration method based on the use of a biologically activated carbon membrane for landfill leachate treatment. Biologically activated membrane combines adsorption, biodegradation and membrane filtration in the treatment process. In their experimental set up, they worked on the tubular cross-flow ultrafiltration membrane modules made from cellulosic material, which were 21 cm in length and 1 cm in diameter, with a 65 cm² operative surface and a pore size of 0.2 μm. The process efficiency for leachate was 95–98% in terms of TOC removal (Pirbazari et al. 1996).

Similarly, Nader Shafaei et al. worked on the development of a membrane for leachate treatment. They reported a photocatalytic self-cleaning polysulfone (PSF) membrane fabricated by adding different concentrations of WO₃ NPs (0 to 2 wt%) via the phase inversion method. In this study, they carried out filtration studies with and without UV irradiation for efficient removal. They found that the addition of WO₃ improves pore size, porosity, and hydrophilicity of the PSF membrane. The flux of the nanocomposite membrane after irradiation by UV light rose in comparison with the same membrane without UV light. The flux of the membrane with 2 wt% of WO₃ shows the self-cleaning property. By further increasing the WO₃, NPs' removal of COD was also improved. With 2 wt %WO₃, COD removal was increased from 54.91% to 77.45% with UV irradiation (Shafaei et al. 2016b).

Although there have been significant breakthroughs in the use of nanomaterials for wastewater treatment, the health and environmental hazards connected with nanomaterials are causing substantial concern. Some of their distinctive properties give them ecotoxicological repercussions. In this regard, Colvin (2003) reported that due to their nanosize, these materials have the potential to cross the cell membrane and interfere with the cellular functions. Similarly, the generation of reactive oxygen species due to UV-irradiation is one of the risks associated with the nanomaterials (Hund-Rinke & Simon 2006). In another study, Zhu et al., observed that extending the exposure length of the acute test from 48 to 72 hours enhanced the toxicity of nano-TiO₂ to Daphnia magna (Zhu et al. 2010). In the case of plant toxicity, most of the literature showed that nanomaterials can accumulate in the roots and shoot biomass during germination, affecting plant growth (Colvin 2003; Hund-Rinke & Simon 2006; Song et al. 2012). Kerstin Hund-Rinke et al. studied the impact of silver NPs (Ag-NPs) on soil microorganisms and fish embryos and found that aging of NPs during the process in the WWTP induces toxicity in the fish cells (Hund-Rinke et al. 2021). Similarly, ecotoxicological tests on human cell lines also suggest that the nanomaterials may induce oxidative stress (Sharma et al. 2009), alteration in antioxidant enzyme levels (Gornati et al. 2016), DNA damage (Ng et al. 2011), decrease in cell viability, and changes in cell morphology (Dwivedi et al. 2014). However, the data available on the nanomaterials' toxicity is still not sufficient to conclude the exact impact of nanomaterials on human health and the environment. Furthermore, research into the relationship of toxicity with various nanomaterials' properties such as morphologic properties, composition, size, or synthesis method, as well as receptor behavior, may provide insight into the fate of nanomaterials in the environment.

This review aims to summarize the current status of research on the advancement of nano-biotechnology for treatment and management of landfill leachate with the association of microbes, adsorbents, and photocatalysts. The NPs immobilized in bio sorbents show superior ability in the treatment of leachate. As support or synergistic agents, NPs can enhance the stability and bioremediation performance of wastewater treatment. In the case of the photocatalysts, the NPs of TiO₂ and ZnO were investigated by various research groups. Photocatalyst based NPs improve the photocatalytic degradation of organic pollutants. Further, to stimulate the efficiency of the photocatalytic degradation, various modified or doped NPs may also be explored. In the case of fungi based biodegradation, the WRF is reported as a suitable fungus and it is also conjugated with the NPs for the leachate treatment. Furthermore, in that process, the problem related to the extraction of NPs from WRF mycelia or media is not well investigated. In order to avoid such problems, enzyme linked NPs can be synthesized that can increase the efficiency of the leachate treatment. In addition, membrane based filters in association with microbes and NPs can also be used for the bioremediation of the pollutants. Scale up of the membrane based filter will serve as an alternative for the efficient degradation of organic matter from the landfill leachate.

In this regard, the approaches that have been used for the separation of NPs lead to changes in the structure and properties of NPs. For further investigation, a similar system may be applied to other pollutants that coexist in wastewater and hinder the treatment process.

All authors are agreed that for the submission there is no conflict of interest.

This work was supported by Research Program supported by the Gujarat State Biotechnology Mission for the Grant. (No: GSBTM L1Y5SU) INDIA.

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