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In this research, zinc oxide (ZnO) nanoparticles synthesized using neem leaf (Azadirachta indica) extract were used as an adsorbent for removing two widely used pharmaceutical compounds acetaminophen (AMP) and sulfadiazine (SDZ). The synthesized ZnO nanoparticles were characterized using SEM-EDS, FTIR, TEM, BET, and XRD analysis. The synthesized ZnO nanoparticles were found to be in the size range of 10 nm with a surface area of 48.551 m2/g. The adsorptive performance of ZnO nanoparticles in both mono-component (MoS) and multi-component system (MuS) was investigated under various operational parameters viz. contact time, temperature, pH, concentration of pharmaceutical compound and ZnO nanoparticles dose. It was observed that the maximum adsorption capacity of ZnO nanoparticles was 7.87 mg/g and 7.77 mg/g for AMP and SDZ, respectively, under the optimum conditions of 7 pH and 2 g/L adsorbent dosage. The experimental data best-fitted with the pseudo-second-order model and Langmuir model, indicating monolayer chemisorption. Further investigation on removal of AMP and SDZ from multicomponent system was modelled using a Langmuir competitive model. The desorption study has shown 25.28% and 22.4% removal of AMP and SDZ from the surface of ZnO nanoparticles. In general, green synthesized ZnO nanoparticles can be utilized effectively as adsorbent for removal of pharmaceutically active compounds from wastewater.

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  • Zinc oxide nanoparticles were synthesized from Azadirachta indica leaf extract.

  • Removal of pharmaceutically active compounds sulfadiazine and acetaminophen using ZnO nanoparticles was investigated in a mono-component and multi-component system.

  • An adsorption study in a multi-component system was performed using Langmuir's competitive model and nature of adsorption was studied.

  • Recyclability of the adsorbent was studied.

Graphical Abstract

Graphical Abstract
Graphical Abstract
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acetaminophen, adsorption, competitive adsorption, green synthesis, sulfadiazine, zinc oxide nanoparticle
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Emerging contaminants (ECs) are the micro-pollutants present in the aqueous environment that do not have any discharge standards and are not monitored stringently (Taheran et al. 2018). The sources of ECs in the aqueous environment include the discharge of effluents from various industries such as pharmaceuticals, animal husbandry, poultry, aquaculture and wastewater from septic tanks of residential buildings (Thomaidis et al. 2012). The persistence and detrimental effects of ECs are recently a problem of rising concern across the world (Natarajan et al. 2021). These compounds are seldom removed in the conventional treatment process and require high end treatment method for their efficient removal (Gomes et al. 2020). As a result, they finally reach surface or groundwater bodies by infiltration, surface runoff etc. resulting in its contamination.

Pharmaceutically active compounds (PACs) such as hormones, stimulants, analgesic, antiepileptics, antibiotics, β-blockers etc., are an important category of ECs. The PACs from hospital effluent, animal husbandry, domestic sewage and pharmaceutical manufacturing industries are the main source of pharmaceutical contamination in aquatic ecosystem (Sim et al. 2011). Currently, more than hundreds of tonnes of PACs are produced annually and about 3,000 of them are used as medicines (Sim et al. 2011). Over the past few decades, the unrestricted use of PACs has led to the pollution of aquatic ecosystems. The consumption of PACs has increased by around 2.8 times over a span of 15 years resulting in an increase in revenue from US$390.2 billion (2001) to US$1,105.2 billion (2016).

Among the different ECs, sulfadiazine (SDZ) and acetaminophen (AMP) are the most extensively used antibacterial (Xu et al. 2013) and antipyretic drugs (Briones et al. 2014), respectively. In most of the developing countries, the intake of AMP is without any medical prescription and ∼58–68% of the AMP consumed finds its way into wastewater streams through urine and excreta (Ahmadzadeh & Dolatabadi 2018). In India AMP is identified as one of the most critical compounds for further treatment and monitoring (Chinnaiyan & Thampi 2018) and is largely detected in sewage treatment plants (Subedi et al. 2017), surface water bodies (Mutiyar et al. 2018; Kumar et al. 2019), etc. The presence of SDZ has also been reported in various environmental compartments like groundwater and river water, livestock farms, and wastewater treatment plant (Baran et al. 2011; Liu et al. 2018). Notably, AMP and SDZ can degrade and transform into different metabolites, of which some of them (e.g., benzoquinone and hydroquinone) are noxious to other life forms and humans (Yang et al. 2018; Phong Vo et al. 2019). It is essential to prevent these pharmaceutical compounds from reaching aqueous environments since they can lead to ecotoxicity through bio-accumulation and bio-magnification.

Various authors have investigated different methods for removing AMP and SDZ from aqueous media including physico-chemical and biological processes involving membrane technology, advanced oxidation, adsorption etc. (Phong Vo et al. 2019; Li et al. 2020; Yadav et al. 2022). In comparison to other technologies, adsorption is one of the effective and efficient technique for removal of ECs from wastewater owing to its low-cost installation, no sludge formation, simple operational design and easy maintenance (Liu et al. 2015). Activated carbon and its modified forms are the most commonly used adsorbent for removal of micropollutants (Phong Vo et al. 2019). Some examples of modified forms include granular type, chitosan-nano and phosphoric acid-modified activated carbon (Amouzgar et al. 2017; Wong et al. 2018; Yanyan et al. 2018). Lately, nano-particles are gaining wide popularity as adsorbents for removal of micro pollutants involving pharmaceutically active compounds from effluents owing to their higher surface area and excellent adsorption properties (He et al. 2021; Rathi & Kumar 2021).

Zinc oxide (ZnO) nanoparticles exhibit properties like thermal and chemical stability, high adsorptive nature towards inorganic and organic pollutants in the aqueous matrix, high surface area and photon absorption efficiency, non-toxicity, availability and cost effectiveness (Wong et al. 2018). Methods for synthesising ZnO nanoparticles includes sol-gel method (Al Abdullah et al. 2017), hydrothermal and (Aneesh et al. 2007) and precipitation method (Ghorbani et al. 2015). The major drawbacks associated with these methods are release of toxic chemicals (Sangeetha et al. 2011) and requirement of sophisticated conditions involving high temperature and pressure, resulting in increased energy consumption (Hassan et al. 2016).

Green synthesis of ZnO nanoparticles is gaining wide popularity nowadays due to its inherent advantages namely low cost, eco-friendly, and non-toxic chemicals (Moritz & Geszke-Moritz 2013). Several authors have synthesized ZnO nanoparticles using the extracts from different plants such as Aloe vera (Ali et al. 2016), Hibiscus subdariffa (Bala et al. 2014), Aloe barbadensis miller (Sangeetha et al. 2011), Azadirachta indica (Bhuyan et al. 2015; Madan et al. 2015; Mankad et al. 2016), Moringa olifera (Matinise et al. 2017) etc. The ZnO nanoparticles synthesized via green method (from different sources) can be used effectively for removing heavy metals (Angelin et al. 2015; Moustafa Ahmed P & Yousef 2015; Anusa et al. 2017; Azizi et al. 2017; Yuvaraja et al. 2018).

Azadirachta indica, popularly known as neem, is common to India and other southeast countries. It belongs to the Meliaceae family, and has been used widely in the Ayurvedic treatment for more than 4,000 years due to its excellent medicinal property (Pandey et al. 2014). Neem leaf extract was found to exhibit antimicrobial property against several fungal and bacterial strains (Ali et al. 2021). In this research, neem leaf is selected for synthesizing ZnO nanoparticles, as the neem leaf extract is rich in phytochemicals like alkaloids, flavonoids, steroids, carbohydrates, terpenoids and glycosides compounds (Bhuyan et al. 2015). These compounds function as bio-reductant which take part in the conversion of zinc ions to ZnO nanoparticles. Furthermore, the neem leaf extract also functions as capping and stabilizing agent, thereby eliminating the need for additional toxic chemicals during the synthesis process (Bhuyan et al. 2015).

Majority of previously reported studies have determined the adsorption of pharmaceutically active compounds from mono-component systems. Meanwhile, simultaneous removal of pharmaceutically active compounds from mono and multicomponent system has been less explored. The removal of AMP and SDZ from mono-component (MoS) and multi-component systems (MuS) using green synthesized ZnO nanoparticles has not been widely investigated. A proper knowledge of the effect of one PAC in the presence of another, i.e., cooperative adsorption (synergistic adsorption) or competitive adsorption (antagonistic adsorption) is not elucidated. It is also important to identify the antagonistic or/and synergistic effect of different PACs present in multicomponent systems. The equilibrium of multicomponent systems represented using multicomponent models is rarely addressed in literatures. Hence, it is intended to perform the sorptive removal of pharmaceutically active compounds of different classes from multi-component systems using green synthesized ZnO nanoparticles.

Green synthesized ZnO nanoparticles from neem leaf (A. indica) extract for removal of AMP and SDZ from the MoS (AMP/SDZ) and the MuS (AMP + SDZ) will be investigated in this research. The scope of this research includes (a) green synthesis of ZnO nanoparticles from the leaf extract of A. indica, characterizing using SEM-EDS, TEM, FTIR, BET, XRD and applying for removal of AMP and SDZ; (b) perform analysis of various kinetic and isotherm models to determine the rate constant, maximum adsorption capacity, and sorption mechanism; (c) assess the MuS with the help of competitive Langmuir model for examining the synergistic and antagonistic nature of adsorption for removal of SDZ and AMP; (d) perform a desorption study to check the reusability and recovery of ZnO nanoparticles and (e) performance evaluation of adsorption capacity of synthesized ZnO nanoparticles with other green synthesized ZnO nanoparticles from literature.

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Materials

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The fresh leaves of A. indica (neem tree) were collected from the campus of National Institute of Technology in Calicut, India for synthesizing the adsorbent. Zinc nitrate hexahydrate of analytical grade and 96% purity was purchased from Merck (Chemind, Calicut, India). AMP and SDZ standards of analytical grade and 99.9% purity was procured from Sigma Aldrich. Water, phosphate buffer, acetonitrile, and methanol used for the study were of analytical grade.

Synthesis of N-ZnO nanoparticles

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There are different methods for the synthesis of ZnO nanoparticles using A. indica (Mankad et al. 2016; Bhuyan et al. 2015; Madan et al. 2015). However, in this study, the synthesis of ZnO nanoparticles from A. indica leaf extract was performed by slightly modifying the procedure followed by Elumalai & Velmurugan (2015). The procedure for ZnO nanoparticles synthesis from neem leaf is depicted in Figure 1. Freshly collected neem leaves were washed using tap water initially, followed by distilled water to remove other impurities and dust. The washed leaves were kept for drying at room temperature and 20 g of neem leaf was allowed to boil in 100 mL distilled water for 10 minutes. The extract was filtered using Whatman 41 filter paper after cooling and 20 mL of the filtered extract was added with 2 g of zinc nitrate hexahydrate under continuous stirring at 1,500 rpm and heated to a temperature of around 60–80 °C until a thick yellow colour paste is obtained. The obtained paste was then collected to a crucible and was heated in a muffle furnace for 2 hours at 400 °C. A white-coloured powder obtained at the end is stored and used as an adsorbent ZnO nanoparticle for the removal of AMP and SDZ.
Figure 1
Green synthesis of ZnO nanoparticles adsorbent from neem leaf extract.
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Green synthesis of ZnO nanoparticles adsorbent from neem leaf extract.

Figure 1
Green synthesis of ZnO nanoparticles adsorbent from neem leaf extract.
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Green synthesis of ZnO nanoparticles adsorbent from neem leaf extract.

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Characterization of ZnO nanoparticles

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The synthesized ZnO nanoparticles was characterized using SEM, EDS, BET, FTIR and XRD techniques. Fourier-transform infrared spectroscopy (FTIR) analysis was performed to identify the functional groups present on the surface of ZnO nanoparticles (Jasco FTIR-4700, frequency range 400–8,000 cm−1). Meanwhile, the crystallinity of N-ZnO was examined using XRD analysis (X'Pert3 Powder, PANalytical). Subsequently, surface morphology and elemental composition was investigated by scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS) (FEI- Quanta 200, operating voltage 15 kV). Transmission electron microscopy (TEM) (JEOL JEM 2100, LaB6 Type, operating voltage 200 kV) analysis was performed to understand the size of the nanoparticles. The surface area of the samples was analyzed using Brunauer–Emmett–Teller (BET) analysis (Belsorp, passing nitrogen gas at 77 K).

Quantification of SDZ and AMP

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The AMP and SDZ procured from Sigma Aldrich is pharmaceutical secondary standard: certified reference material of 99.9% purity. Removal of AMP and SDZ was determined with the help of high-performance liquid chromatography (HPLC) (HPLCDGU-14A degasser model, Shimadzu LC-20AD pump, SPD-M20APDA detector) coupled with C18 analytical column at a flowrate of 1 mL/min. The concentration of AMP was determined using acetonitrile and phosphate buffer in the volume ratio of 40:60 at 254 nm. On the other hand, SDZ samples were quantified using water and methanol in the volume ratio of 40:60 at 263 nm. Since no recovery study was performed, to check the accuracy of the data, initially sample was run with HPLC water alone. This was followed by spiking water with 25 mg/L, 50 mg/L and 100 mg/L of AMP and SDZ respectively. For both AMP and SDZ no peak was detected for distilled water. However, sharp peak was detected at 3.4 minutes for AMP and at 4.2 minutes for SDZ respectively at all concentrations.

This was followed by preparing the stock solutions containing 1 g/L of either SDZ or AMP by solubilizing 1 g of AMP/SDZ in 1 L of distilled water. The prepared stock solution was diluted further to obtain the working solutions of different concentrations (5, 10, 25, 50, 75 and 100 mg/L). The prepared SDZ and AMP solutions were analyzed using HPLC. From the standard solutions of different concentration, a calibration curve was plotted for both AMP and SDZ. The R2 value was found to be 0.997 for AMP and 0.993 for SDZ respectively.

Experimental methodology

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Mono-component system (MoS)

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The adsorption experiments were carried out using an accurately weighed quantity of ZnO nanoparticles added to 25 mL AMP/SDZ solution in 100 mL conical flasks. This sorbent-sorbate mixture was kept at 200 rpm in a shaking incubator for 45 min at 30 °C. The experiments were performed by varying the solution pH (2–12), contact time (5–45 min), initial AMP/SDZ concentration (0.5–100 mg/L) and ZnO nanoparticles dosage (0.2–4 g/L). Subsequently, an aliquot of samples was collected at frequent time intervals and analyzed for determining the concentration of AMP and SDZ. The percentage removal of AMP and SDZ was calculated using Equation (1).
(1)
where, Ci and Ct indicates the concentration of target pollutants at beginning and at any time ‘t’, respectively.
The equilibrium adsorption capacity of green synthesized nanoparticles for removal of AMP and SDZ was determined using Equation (2).
(2)
where, m is the mass of the adsorbent (g) and V is the volume of pharmaceutical solution (L).

Multi-component system (MuS)

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Similar to MoS, MuS adsorption experiments were performed in batch mode using a 100 mL Erlenmeyer flask and at a temperature of 30 °C. About 25 mL of the mixture of AMP + SDZ (mixed at different proportions) solution was kept at 200 rpm in a shaking incubator. The proportions of SDZ and AMP in the SDZ + AMP mixture were taken as 1 + 1, 5 + 5, 10 + 10, 25 + 25, 50 + 50 and 100 + 100 mg/L. The optimum dose obtained from the MoS study was added to the AMP + SDZ mixture. To study the adsorption in a MuS, the competitive Langmuir isotherm was adopted to model the AMP + SDZ mixture (Manjunath & Kumar 2018). Equations (3) and (4) represent the Langmuir model for MoS and MuS, respectively.
(3)
(4)
Where, qe and qm are equilibrium and maximum adsorption capacity, respectively (mg/g), KL indicates Langmuir equilibrium constant, and Ce indicates target pollutants concentration at equilibrium (mg/L).

Kinetics study

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A batch adsorption study was performed under a pH of 7 and an initial AMP/SDZ concentration of 5 mg/L for an optimum contact time. ZnO nanoparticles were added to the solution at a predetermined dose. The samples were collected at 5, 10, 15, 30, and 45 minutes. The experimental data was modelled using different kinetic models viz. pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion model (IDM), Elovich kinetic model (EKM), and liquid film diffusion model (LFD) to determine the kinetic constants (Akbour et al. 2020; Titchou et al. 2020, 2021).

Equilibrium study

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Equilibrium study was performed to check the pertinence of the isotherm models (Mall et al. 2006; Akbour et al. 2020; Gaayda et al. 2022), which include Freundlich (FI), Langmuir (LI), Dubinin–Radushkevich (DRI), Elovich (EI), and Temkin (TI), under specified conditions such as pH of 7, optimum contact time, ZnO nanoparticles dosage and AMP/SDZ concentration in the range of 0.5, 1, 5, 10, 25, 50, 75 and 100 mg/L at 30 °C. Table 1 presents the expressions of various kinetic and isotherm models adopted for the removal of AMP and SDZ using ZnO nanoparticles.

Table 1

Adsorption models for removal of AMP and SDZ using ZnO nanoparticles

TypeModelExpressionReferences
Kinetic Pseudo-first-order  Titchou et al. (2020)  
Pseudo-second-order  Akbour et al. (2020)  
Intra particle diffusion  Zhang et al. (2017)  
Liquid film diffusion  Manjunath et al. (2020)  
Elovich kinetic model  Yadav et al. (2022)  
Isotherm Langmuir  Gaayda et al. (2022)  
Freundlich  Manjunath et al. (2019)  
Temkin  Titchou et al. (2021)  
Dubinin – Radushkevich  Manjunath & Kumar (2018)  
Elovich  Yadav et al. (2022)  
TypeModelExpressionReferences
Kinetic Pseudo-first-order  Titchou et al. (2020)  
Pseudo-second-order  Akbour et al. (2020)  
Intra particle diffusion  Zhang et al. (2017)  
Liquid film diffusion  Manjunath et al. (2020)  
Elovich kinetic model  Yadav et al. (2022)  
Isotherm Langmuir  Gaayda et al. (2022)  
Freundlich  Manjunath et al. (2019)  
Temkin  Titchou et al. (2021)  
Dubinin – Radushkevich  Manjunath & Kumar (2018)  
Elovich  Yadav et al. (2022)  

Where,

Kinetic constants: qe is equilibrium adsorption capacity (mg/g); qt is adsorption capacity at time ‘t’ (mg/g); K1 is pseudo – first order rate constant (min – 1); K2 is pseudo – second order rate constant (g/mg/min); KID is instra – particle diffusion rate constant (mg/g/min0.5) and C is intercept; KFD is iLiquid film diffusion rate constant (1/min), A is Liquid file constant; α and β are Elovich kinetic model constants; Ko and α are Bangham constants; m is mass of adsorbent and V is the volume of adsorbate sample.

Isotherm constants: qe is equilibrium adsorption capacity (mg/g); qm is maximum adsorption capacity (mg/g); KL is Langmuir constant (L/mg); Ce is equilibrium concentration (mg/L); KF and n are Freundlich constants; AT is Temkin equilibrium constant (L/mg) and B is Temkin constant; KDR is mean free energy of adsorption (mol2/kJ2) and ε = RTln [1 + (1/Ce)] is Polyani potential; KE is Elovich equilibrium constant (L/mg).

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Thermodynamic study

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The nature of adsorption was evaluated from the thermodynamic viewpoint. This was performed by varying the temperature in the range of 20 °C, 25 °C, 30 °C and 35 °C respectively at optimum adsorbent dosage and contact time. The standard values of enthalpy (ΔH°, kJ/mol), Gibbs free energy (ΔG°, kJ/mol) and entropy (ΔS°, J/mol/K) from the Equation (5).
(5)
where, ΔG° represents the standard Gibbs free energy change, T is the absolute temperature (°K), R is the universal gas constant (8.314 J/mol K) and represents the distribution coefficient. The value of is determined from Equation (6) (Akbour et al. 2020).
(6)

The change in enthalpy and entropy was determined from the linear plot connecting ln Kd vs 1/T. (Titchou et al. 2021). The change in free energy, entropy and enthalpy helps to understand the spontaneity, randomness and heat change in the adsorption of AMP and SDZ onto the surface of the ZnO nanoparticles (Manjunath & Kumar 2018).

Regeneration and reusability of ZnO nanoparticles

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After the adsorption experiments, from the conical flasks containing the adsorbent ZnO nanoparticles, the supernatant AMP/SDZ solution was replaced with same volume of distilled water. The conical flasks with the ZnO nanoparticles-distilled water mixture is then mixed for 6 h at 30 °C in a shaking incubator. Samples were collected at an interval of 2 h. Adsorption-desorption studies were performed for three cycles to evaluate the percentage of sorbates desorbed from the surface of ZnO nanoparticles (Saranya et al. 2018). The desorption efficiency was determined using Equation (7).
(7)
where, is AMP/SDZ adsorbed on the ZnO nanoparticles at time of equilibrium and is AMP/SDZ desorbed from the surface of the ZnO nanoparticles ⁠.
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Characterization of green synthesized ZnO nanoparticles

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Figure 2(a)–2(f) depicts the characterization of green synthesized ZnO nanoparticles. Figure 2(a) shows the SEM image of ZnO nanoparticles synthesized from neem leaf extract. The image shows that the ZnO nanoparticles are spherical and well distributed. From figure, the surface pores that helps in sorption of AMP and SDZ are visible. From the TEM image (Figure 2(b)), the average particle size was approximately 10 nm. However, aggregations of particles are observed, which may be attributed to the densification of particles leaving behind narrow space between them and the high surface energy that usually happens when nanoparticles are synthesized in an aqueous medium (Elumalai & Velmurugan 2015). Meanwhile, the elements present in the nanoparticle were examined using EDS analysis as shown in Figure 2(c). It depicts the presence of zinc (74.95 wt%) and oxygen (22.66 wt%). In addition to Zn and O, Ca (2.39 wt%) was also seen in the EDS spectrum. This is due to the element Ca present in the natural composition of A. indica (Sahito et al. 2003). FTIR helps in identifying the functional groups of the ZnO. Figure 2(d) represents the FTIR spectrum of the synthesized adsorbent. The peak between 400 and 600 cm−1 indicates the vibrations of the zinc and oxygen bond (Varadavenkatesan et al. 2019). The cliffs at 922, 1,393, and 3,451 cm−1 depicts O-H bonds presence (Sangeetha et al. 2011). The peaks at 2,987 cm−1 represent the stretching vibration of amide II, amide linkage between the amino acid residues present in the proteins, and the evidence of atmospheric CO2 (Bhuyan et al. 2015). However, on the surface of ZnO nanoparticles, some proteins, phenolic compounds, and terpenoids originating from plant extract can be identified. The carboxylic and free amino groups from these phytochemicals are responsible for the stability of the ZnO nanoparticles. The amide bands originating from proteins and the bonds -C-O-C-, -C = C- and -C-O- derived from heterocyclic compounds act as capping agents (Sangeetha et al. 2011). The XRD results of the ZnO nanoparticles (Figure 2(e)) marks the presence of well-defined diffraction peaks at 68.86°, 63.81°, 57.69°, 48.38°, 37.22°, 35.36°, and 32.72°, and denoting to (112), (103), (110), (102), (101), (002) and (100) respectively. This matches the standard JCPDS card No.89-0510 depicting ZnO nanoparticles' hexagonal wurtzite crystal structure (Varadavenkatesan et al. 2019). On the other hand, the BET adsorption-desorption isotherm for determining the surface area, pore size and pore volume of the ZnO nanoparticles is depicted in Figure 2(f). Since adsorption is a surface phenomenon, surface area of the adsorbent plays a critical role in the removal of target pollutants. The specific surface area obtained for ZnO nanoparticles was found to be 48.551 m2/g. The average pore diameter was found to be 46.843 nm, which indicates the presence of mesopores. Pore volume also impart a vital role in adsorption process. The pore volume for N-ZnO was found to be 0.5686 cm3/g. The specific surface area obtained from this study is higher when compared with other adsorbents synthesized using various sources as shown in Table 2. Furthermore, Figure 2(g) shows the pHpzc (pH at point of zero charge) plot of synthesized ZnO nanoparticles and the value of pHpzc of ZnO nanoparticles was observed to be 6.6. The pHsolution < pHpzc, specifies the surface of adsorbent N-ZnO as deprotonated (negatively charged) and pHsolution > pHpzc, indicates that the surface of ZnO nanoparticles are in protonated form (positively charged).
Table 2

Maximum BET area of green synthesized ZnO nanoparticles

Source of feedstockBET surface area (m2/g)References
Ocimum sanctum 32.72 Nayak et al. (2020)  
Apple-derived compounds 17.00 Soliman et al. (2020)  
Peltophorum pterocarpum 13.56 Pai et al. (2019)  
Calliandra haematocephala 9.18 Vinayagam et al. (2020)  
Peltophorum pterocarpum pod extract 19.61 Vinayagam et al. (2021)  
Cyanometra ramiflora 16.27 Varadavenkatesan et al. (2019)  
Ocimum tenuiflorum 21.20 Dayakar et al. (2017)  
Padina tetrastromatica 27.37 Pandimurugan & Thambidurai (2016)  
Azadirachta indica 48.55 Present study 
Source of feedstockBET surface area (m2/g)References
Ocimum sanctum 32.72 Nayak et al. (2020)  
Apple-derived compounds 17.00 Soliman et al. (2020)  
Peltophorum pterocarpum 13.56 Pai et al. (2019)  
Calliandra haematocephala 9.18 Vinayagam et al. (2020)  
Peltophorum pterocarpum pod extract 19.61 Vinayagam et al. (2021)  
Cyanometra ramiflora 16.27 Varadavenkatesan et al. (2019)  
Ocimum tenuiflorum 21.20 Dayakar et al. (2017)  
Padina tetrastromatica 27.37 Pandimurugan & Thambidurai (2016)  
Azadirachta indica 48.55 Present study 
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Figure 2
Characterization of ZnO nanoparticles: (a) SEM, (b) TEM, (c) EDS, (d) FTIR, (e) XRD, (f) BJH plot and (g) pHpzc analysis.
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Characterization of ZnO nanoparticles: (a) SEM, (b) TEM, (c) EDS, (d) FTIR, (e) XRD, (f) BJH plot and (g) pHpzc analysis.

Figure 2
Characterization of ZnO nanoparticles: (a) SEM, (b) TEM, (c) EDS, (d) FTIR, (e) XRD, (f) BJH plot and (g) pHpzc analysis.
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Characterization of ZnO nanoparticles: (a) SEM, (b) TEM, (c) EDS, (d) FTIR, (e) XRD, (f) BJH plot and (g) pHpzc analysis.

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Effect of operational parameters

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The effect of operation parameters viz. contact time, ZnO nanoparticles dose, AMP/SDZ concentration, solution pH and temperature on removal of AMP and SDZ have been investigated.

Effect of contact time

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The variation of removal with respect to contact time was studied at neutral pH for optimum adsorbent dose and target pollutants concentration 5 mg/L. Figure 3(a) shows the effect of contact time on removal of AMP and SDZ using N-ZnO nanoparticles. From figure it can be observed that the initial stages of adsorption till 5 min was found to be rapid, probably due to large concentration gradient between the solute adsorbed on the sorbent and the aqueous solute (Xu et al. 2013) along with the presence of a large number of active sites on the surface of ZnO nanoparticles (Wang et al. 2013). However, as time progresses, the rate of adsorption decreases as the vacant active sites get occupied by sorbate molecules and reaches equilibrium (Dutta et al. 2015). A maximum removal of 64.12% and 59.97% for AMP and SDZ, respectively, was observed at the end of 30 minutes which was taken as the equilibrium time. A similar trend in removal with respect to contact time was observed for the removal of AMP using double oxidized graphene oxide, tea waste-derived activated carbon, and nano-silica and for the adsorptive removal of SDZ using spent tea waste derived chemically modified activated carbon (Dutta et al. 2015; Moussavi et al. 2016; Eslami et al. 2020; Yadav et al. 2022).
Figure 3
Effect of operational parameters on removal efficiency (a) contact time (b) adsorbent dosage (c) Initial adsorbate concentration (d) pH (e) Temperature. Note: Experimental conditions: (a) t: 0–45 min, pH:7, ZnO nanoparticles dose: 2 g/L, SDZ/AMP conc: 5 mg/L, (b) N-ZnO dose (0.2–4 g/L), t: 30 min, pH:7, SDZ/AMP conc: 5 mg/L, (c) SDZ/AMP conc: 0.5–100 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min, pH:7, (d) pH:2–12, SDZ/AMP conc: 5 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min (e) Temp: 293–308 K, pH: 7, N-ZnO dose: 2 g/L, t: 30 min, SDZ/AMP conc: 5 mg/L.
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Effect of operational parameters on removal efficiency (a) contact time (b) adsorbent dosage (c) Initial adsorbate concentration (d) pH (e) Temperature. Note: Experimental conditions: (a) t: 0–45 min, pH:7, ZnO nanoparticles dose: 2 g/L, SDZ/AMP conc: 5 mg/L, (b) N-ZnO dose (0.2–4 g/L), t: 30 min, pH:7, SDZ/AMP conc: 5 mg/L, (c) SDZ/AMP conc: 0.5–100 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min, pH:7, (d) pH:2–12, SDZ/AMP conc: 5 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min (e) Temp: 293–308 K, pH: 7, N-ZnO dose: 2 g/L, t: 30 min, SDZ/AMP conc: 5 mg/L.

Figure 3
Effect of operational parameters on removal efficiency (a) contact time (b) adsorbent dosage (c) Initial adsorbate concentration (d) pH (e) Temperature. Note: Experimental conditions: (a) t: 0–45 min, pH:7, ZnO nanoparticles dose: 2 g/L, SDZ/AMP conc: 5 mg/L, (b) N-ZnO dose (0.2–4 g/L), t: 30 min, pH:7, SDZ/AMP conc: 5 mg/L, (c) SDZ/AMP conc: 0.5–100 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min, pH:7, (d) pH:2–12, SDZ/AMP conc: 5 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min (e) Temp: 293–308 K, pH: 7, N-ZnO dose: 2 g/L, t: 30 min, SDZ/AMP conc: 5 mg/L.
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Effect of operational parameters on removal efficiency (a) contact time (b) adsorbent dosage (c) Initial adsorbate concentration (d) pH (e) Temperature. Note: Experimental conditions: (a) t: 0–45 min, pH:7, ZnO nanoparticles dose: 2 g/L, SDZ/AMP conc: 5 mg/L, (b) N-ZnO dose (0.2–4 g/L), t: 30 min, pH:7, SDZ/AMP conc: 5 mg/L, (c) SDZ/AMP conc: 0.5–100 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min, pH:7, (d) pH:2–12, SDZ/AMP conc: 5 mg/L, ZnO nanoparticles dose: 2 g/L, t: 30 min (e) Temp: 293–308 K, pH: 7, N-ZnO dose: 2 g/L, t: 30 min, SDZ/AMP conc: 5 mg/L.

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Effect of adsorbent dosage

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The effect of ZnO nanoparticles dosage on the adsorption of AMP/SDZ was assessed at neutral pH, for an initial AMP/SDZ concentration of 5 mg/L and at contact 30 min. The result of the study is shown in Figure 3(b). From figure, it can be seen that increase in ZnO nanoparticles dose from 0.2 to 4 g/L showed increase in removal from 38.88% to 61.34% for SDZ and from 31.04% to 67.66% for AMP, respectively. This improvement in the removal with increase in ZnO nanoparticles dosage is mainly attributed to the increment in the number of available sites on the ZnO nanoparticles surface to contact with the fixed range of AMP/SDZ molecules (Moussavi et al. 2016). From the figure it can be seen that, for both SDZ and AMP, at a dose >2 g/L, there was no significant increase in the removal. This could be attributed to the fact that at a higher dosage, there is a reduction in the surface area of the adsorbent due to the particle agglomeration (Padmavathy et al. 2016). Therefore, the optimum dose of N-ZnO was considered as 2 g/L. Similar findings were reported by various researchers for the removal of SDZ using tea waste biochar, kaolinite etc. and for the removal of AMP using nano-silica adsorbent, coconut shell and activated carbon derived from animal hair (Liu et al. 2013; Xu et al. 2013; Nam et al. 2014; Eslami et al. 2020; He et al. 2021).

Effect of AMP/SDZ initial concentration

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The effect of initial concentration of the target pollutants on the removal was assessed at neutral pH, for an optimum N-ZnO dose and time. Figure 3(c) shows effect of AMP/SDZ initial concentration on removal. From figure, the removal of both the adsorbates was found to decrease with an increase in the initial concentration i.e., as the concentration of AMP and SDZ was increased from 0.5 mg/L to 100 mg/L, the removal was reduced from 70% to 37.45% for SDZ and 70.8% to 16.71% for AMP, respectively. This might be attributed to the fact that, at a higher AMP/SDZ concentration, the active sorption sites found on the surface of the ZnO nanoparticles would have saturated. This results in a reduction in the sorption of AMP/SDZ pollutants onto the adsorbent (Manjunath et al. 2020). It can be observed that the rate of reduction is less for SDZ than AMP at lower adsorbate concentrations (0.5 mg/L to 50 mg/L). However, at 5 mg/L adsorbate concentration, the percentage removal remains almost the same (∼64%) for both target pollutants.

Effect of pH

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The variation of pH was studied at an optimum ZnO nanoparticles dose and at optimum contact time. Effect of pH on removal of AMP and SDZ is depicted in Figure 3(d). From the figure, it can be inferred that the adsorption of AMP and SDZ onto ZnO nanoparticles is a pH-dependent process. A decreasing trend in removal efficiency was observed at a pH greater and less than the neutral range. The pHsolution was taken to be 6.8 ± 0.3 throughout experiment. The pKa value of AMP is 9.3, pKa1 and pKa2 of SDZ is 1.57 and 6.5, respectively and pHpzc of ZnO nanoparticles is 6.6, respectively. For pHpzc < pHsolution, the adsorbent surface carries negative charge, whereas SDZ and AMP present in solution appear as charged cations (SDZ-H+ and AMP-H+) signifying electrostatic attraction between positively charged ZnO nanoparticles and SDZ-H+ and AMP-H+. However, at higher pH, both adsorbent and adsorbate exist as negatively charged ions. Hence a reduction in removal efficiency at higher pH (10 and 12) is attributed to the repulsion between the like charges (Zyoud et al. 2020). AMP/SDZ molecules exist as neutral particles when pH is lower than pKa. Hence at lower pH (2 and 4), the active sites present on the N-ZnO surface are blocked by the formation of multilayers of contaminants due to strong electrostatic interaction (Basha et al. 2015). This results in a reduction in removal percentage at lower pH. However, at neutral pH, the hydrophobic interaction of target pollutants can be considered as the predominant mechanism involved in the adsorption process (Eslami et al. 2020). A similar trend was also observed in the removal of SDZ and AMP using other adsorbents such as spent tea leaf generated activated carbon, phosphonic chelating fiber, immobilized laccase, low-cost coconut waste shell, etc. (Ratanapongleka & Punbut 2018; Yanyan et al. 2018; Yang et al. 2019; Yadav et al. 2022).

Effect of temperature

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Figure 3(e) represents the variation in removal efficiency with an increase in temperature. The removal of both AMP and SDZ was found to increase with an increase in temperature from 293 to 308 K after a contact time of 30 minutes. The removal efficiency increased from 35.8% to 66% for AMP and 21% to 63% for SDZ. This increase is attributed to the rise in the diffusion rate of target pollutants to the internal and external pores of the ZnO nanoparticles with an increase in temperature. An increase in temperature is also responsible for increase in the total pore volume and porosity of the sorbent (Banerjee et al. 2016).

Kinetic study

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Kinetic models aid in identifying the order and rate of the sorption process. The adsorption of AMP and SDZ onto the N-ZnO surface can be assumed to follow a series of diffusion processes. It starts with film diffusion, where the transfer of adsorbate mass happens from the bulk solution to the adsorbent surface, followed by particle diffusion, which takes place in different forms like pore and surface diffusion (Podder & Majumder 2017). LFD model, EKM and IPD model helps in understanding the diffusion mechanism of AMP and SDZ onto the ZnO nanoparticles surface. Table 3 represents the values of kinetic constants and R2 (linear regression coefficient) during the adsorption of AMP and SDZ onto the ZnO nanoparticles surface. From Table 3, it can be inferred that with respect to all kinetic models studied, the calculated R2 value is near to unity for PSO. Also, the equilibrium adsorption capacity calculated using the PSO model (1.82 mg/g and 1.55 mg/g for AMP and SDZ, respectively) was almost similar to the adsorption capacity calculated using the experiment. In LFD, if the plot of vs t passes through the origin, implies that the rate-limiting step of adsorption is film diffusion. However, in the LFD model, the curve is nonlinear and the value of A is non-zero. indicating that in the adsorptive removal of AMP and SDZ using N-ZnO nanoparticles, liquid film diffusion is not the only rate-limiting step (Manjunath et al. 2019). Similarly, EKM and IPD models were also used to predict if the adsorption mechanism is by pore diffusion. In IPD, the intercept value implies the thickness of the boundary layer (Mall et al. 2006). For the removal of both SDZ and AMP, the value of C was found to be high, specifying that the effect of boundary layer is more during adsorption process. In addition, the intercept of the line in IPD and EKM model specifies that pore diffusion solely is not the rate-limiting step in the adsorptive removal of SDZ and AMP by ZnO nanoparticles (Manjunath et al. 2019, 2020). Therefore, comparing all kinetic models, the experimental data best fitted PSO model indicating that the sorption of AMP and SDZ onto ZnO nanoparticles is chemisorption.

Table 3

Kinetic constants for the adsorption of AMP and SDZ using N-ZnO

Kinetic modelsConstantsAMPSDZ
qe,exp (mg/g)1.661.50
Pseudo-first-order qe (mg/g) 1.11 0.52 
K1 (min−1) 0.10 0.12 
R² 0.998 0.919 
Pseudo-second-order qe (mg/g) 1.82 1.55 
K2 (g/mg/min) 0.13 0.43 
R² 0.999 0.999 
Liquid-film-diffusion KFD 0.10 0.12 
 −0.40 −1.51 
R² 0.998 0.919 
Intra-particle diffusion  0.77 1.13 
KID (mg/g/min0.5) 0.14 0.06 
R² 0.921 0.873 
Elovich  1.82 330.13 
 3.30 7.63 
R2 0.978 0.935 
Kinetic modelsConstantsAMPSDZ
qe,exp (mg/g)1.661.50
Pseudo-first-order qe (mg/g) 1.11 0.52 
K1 (min−1) 0.10 0.12 
R² 0.998 0.919 
Pseudo-second-order qe (mg/g) 1.82 1.55 
K2 (g/mg/min) 0.13 0.43 
R² 0.999 0.999 
Liquid-film-diffusion KFD 0.10 0.12 
 −0.40 −1.51 
R² 0.998 0.919 
Intra-particle diffusion  0.77 1.13 
KID (mg/g/min0.5) 0.14 0.06 
R² 0.921 0.873 
Elovich  1.82 330.13 
 3.30 7.63 
R2 0.978 0.935 
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Equilibrium study

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The isotherm constants for the equilibrium study are shown in Table 4. Comparing the R2 value of all models it was found that for both SDZ and AMP, LI had a higher R2 value (R2 > 0.98). This indicates that the monolayer chemisorption happens on the homogeneous sites of ZnO nanoparticles and the adsorption of AMP and SDZ molecules onto one active site of the adsorbent is independent of other molecules present adjacent to it (Kumar et al. 2011). The maximum Langmuir adsorption capacity was observed as 7.87 mg/g for AMP and 7.77 mg/g for SDZ. The dimensionless equilibrium or separation factor parameter, ‘RL,’ (Equation (8)) is used to estimate the favorability of adsorption between the sorbent and the sorbate (Weber & Chakravorti 1974).
(8)
where, Co is the concentration of AMP/SDZ molecules at the beginning of the experiment and KL is the Langmuir constant. Based on the value of RL, it is possible to anticipate the adsorption nature, i.e. linear adsorption (RL > 1), favorable adsorption (0 > RL > 1) and irreversible adsorption (RL < 1) (Günay et al. 2007). From results, the value of RL was found to be in the range of 0.55 and 0.57 for AMP and SDZ, respectively, indicating that the adsorption process is favorable in nature. On the other hand, FI represents multilayer adsorption and the affinity towards adsorption is determined using ratio 1/n. Adsorption is found to be favorable if the value of 1/n lies between 0 and 1 (Manjunath et al. 2019). From Table 4, the value of 1/n was found to be 0.62 and 0.80 for AMP and SDZ, respectively, elucidating that adsorption is favorable in nature. Subsequently, TI is based on the assumption that in a layer, for all the molecules, there is a linear reduction (rather than logarithmic) in the heat of adsorption with respect to the coverage, and subsequently, adsorption is characterized by a uniform distribution of binding energy (Zhang et al. 2013). From Table 4, the Temkin adsorption constant AT was found to be 3.77 L/mmol and 6.52 L/mmol for AMP and SDZ, respectively, and Temkin constant B as 1.42 J/mol for both sorbates. The standard free enthalpy was calculated from Temkin model using Equation (9) (Zhang et al. 2013).
(9)
where, R and T are universal gas constant (8.314 J/mol K) and absoulte temperature (K), respectively The enthalpy was calculated from Equation (9), which was found to be 3.01 kJ/mol and 4.25 kJ/mol for AMP and SDZ, respectively. Furthermore, the DRI model aims to identify adsorption characteristics on heterogeneous and homogeneous surfaces. Meanwhile, the EI model assumes adsorption to be multilayer based on the presumption that with adsorption, the number of adsorption sites increases exponentially (Hamdaoui & Naffrechoux 2007). However, from the R2 value obtained from the study, it can be inferred that these two models are ineffective in predicting the adsorption of AMP and SDZ onto the ZnO nanoparticle surface.
Table 4

Equilibrium constants for the adsorption of AMP and SDZ using ZnO nanoparticles

Equilibrium modelsConstantsAMPSDZ
Langmuir qm 7.87 7.77 
KL 0.16 0.15 
R² 0.998 0.988 
Freundlich KF 0.80 0.87 
n 1.61 1.25 
R² 0.972 0.986 
Temkin AT 3.77 6.52 
B 1.42 1.42 
R² 0.903 0.858 
Dubinin-Radushkevich qm 4.83 6.68 
KDR 0.14 × 10−6 0.17 × 10−6 
R² 0.829 0.725 
Elovich qm 4.14 16.00 
KE 1.05 1.00 
R2 0.927 0.899 
Equilibrium modelsConstantsAMPSDZ
Langmuir qm 7.87 7.77 
KL 0.16 0.15 
R² 0.998 0.988 
Freundlich KF 0.80 0.87 
n 1.61 1.25 
R² 0.972 0.986 
Temkin AT 3.77 6.52 
B 1.42 1.42 
R² 0.903 0.858 
Dubinin-Radushkevich qm 4.83 6.68 
KDR 0.14 × 10−6 0.17 × 10−6 
R² 0.829 0.725 
Elovich qm 4.14 16.00 
KE 1.05 1.00 
R2 0.927 0.899 
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Thermodynamic study

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The thermodynamic parameters ΔGo, ΔSo, and ΔHo was determined by performing the adsorption of AMP and SDZ on to ZnO nanoparticles at different temperatures. The result of the study is presented in Table 5. From the study it can be inferred that, as temperature increased from 293 to 308 K, the value of ΔGo decreased from 3.11 kJ/mol to 0.08 kJ/mol for AMP and 4.96 kJ/mol to 0.44 kJ/mol for SDZ, respectively. Meanwhile, the positive values of ΔGo for removal of both AMP and SDZ elucidates that the non-spontaneous nature of the adsorption system and higher temperature enhances the favorability of the adsorption process. (Manjunath et al. 2017). The positive value of ΔHo (64.83 kJ/mol for AMP and 94.43 kJ/mol for SDZ) indicates that the sorption process is endothermic where the process reaches equilibrium by consuming energy from the system considered (Rashtbari et al. 2022). The adsorption process in the solid-liquid interface is comprised of two stages, the first stage being the desorption of water molecules (previously adsorbed) from ZnO nanoparticles surface and the sorption of AMP and SDZ species onto the sorbent surface in second stage. For the sorption of AMP and SDZ on the ZnO nanoparticles surface, AMP and SDZ ions have to replace more water molecules which results in endothermic adsorption process. In an endothermic process, the energy is attained from the surrounding environment/molecule in the form of heat. This clearly depicts the adsorption to be chemisorption (Manjunath & Kumar 2018). Furthermore, positive value of ΔSo (0.211 kJ/mol for AMP and 0.307 kJ/mol for SDZ) implies that a higher translational entropy was found for the displaced water molecules than that of AMP and SDZ ions (Banerjee et al. 2016). This causes an increase in randomness at the solid/liquid interface during fixation of AMP and SDZ on active sites of ZnO nanoparticles during the adsorption process (Gaayda et al. 2022).

Table 5

Thermodynamic parameters for the adsorption of AMP and SDZ onto ZnO nanoparticles

AdsorbateT (K)ln ΔGo (kJ/mol)ΔHo (kJ/mol)ΔSo (kJ/mol K)R2
AMP 293 −1.28 3.11 64.83 0.211 0.93 
298 −0.68 1.68 
303 −0.11 0.28 
308 −0.03 0.08 
SDZ 293 −2.04 4.96 94.43 0.307 0.91 
298 −0.97 2.4 
303 −0.29 0.72 
308 −0.17 0.44 
AdsorbateT (K)ln ΔGo (kJ/mol)ΔHo (kJ/mol)ΔSo (kJ/mol K)R2
AMP 293 −1.28 3.11 64.83 0.211 0.93 
298 −0.68 1.68 
303 −0.11 0.28 
308 −0.03 0.08 
SDZ 293 −2.04 4.96 94.43 0.307 0.91 
298 −0.97 2.4 
303 −0.29 0.72 
308 −0.17 0.44 
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Competitive adsorption of AMP and SDZ in AMP + SDZ mixture

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Competitive Langmuir isotherm model was used to analyze the competitive sorption of SDZ and AMP in MuS. Equations (3) and (4) shows the corresponding Langmuir model for MoS and MuS. In addition, the model's linearized form is represented in Equations (10) and (11). This model helps to explicate the synergistic/antagonistic nature of one sorbate in the presence of another in a MuS (i.e., the effect of AMP on SDZ in AMP + SDZ and the effect of SDZ on AMP in AMP + SDZ).
(10)
(11)
where, and indicate equilibrium concentration of AMP and SDZ, respectively, in AMP + SDZ mixture; and represent equilibrium adsorption capacity of AMP and SDZ respectively, in AMP + SDZ mixture; and depict the capacity of adsorption (maximum) of target pollutants in AMP + SDZ solution and and indicate Langmuir constants AMP and SDZ, respectively. The results obtained from the model are presented in Table 6. From table, it can be seen that in MoS, the adsorption capacity of N-ZnO nanoparticle for SDZ was higher when compared to that of AMP. Meanwhile, in a MuS, the adsorption capacity was found to be ∼0.88 times and ∼1.79 times for AMP and SDZ, respectively in comparison to MoS. In a MuS, the synergistic/antagonistic nature was investigated using the ratio ⁠. Where, and denotes the adsorption capacity (maximum) of the SDZ and AMP in MoS and MuS. Based on the value of three possible interactions can be predicted i.e., (a) = 1, no interaction, i.e. the sorbates in AMP + SDZ, shows no prominent effect on individual sorbates in the ⁠, (b) < 1 represents antagonistic nature, i.e. sorption of one sorbate is lowered by the presence of other in pollutant mixture and (c) > 1, represents synergistic interaction, i.e. sorption of one sorbent is enhanced by the presence of other in AMP + SDZ solution.

For AMP, the ratios were found to be less than 1. This indicates an antagonistic effect of AMP in a MuS. i.e., the adsorption of AMP was suppressed in presence of SDZ, indicating a competitive adsorption. However, for SDZ, the ratios were found to be greater than 1. This implies a synergistic effect of SDZ in a ⁠. i.e. the adsorption of SDZ was enhanced by the presence AMP in the AMP + SDZ system, indicating a cooperative adsorption. Luo et al. 2015; Tovar-Gómez et al. 2015; Goze et al. 2016; Manjunath et al. 2019; Yadav et al. 2022, also studied the synergistic/antagonistic nature of different adsorbents and adsorbates in a MuS and reported similar findings.

Table 6

Langmuir competitive model for sorption of AMP and SDZ in AMP + SDZ system

SystemAdsorbateQmKLR2
AMP + SDZ AMP 6.94 0.12 0.88 0.992 
SDZ 13.89 0.04 1.79 0.996 
SystemAdsorbateQmKLR2
AMP + SDZ AMP 6.94 0.12 0.88 0.992 
SDZ 13.89 0.04 1.79 0.996 
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Desorption studies

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Figure 4 represents the percentage desorption of the target pollutants from the surface of ZnO nanoparticles. During the first cycle, the percentage desorption was found to be 43.03% and 35.6% for AMP and SDZ, respectively. In the second cycle, the desorption was reduced to 31.7% and 24.1% for AMP and SDZ. However, at the end of the third cycle, it was reduced to 25.28% and 22.4% for AMP and SDZ, respectively. It can be inferred that the desorption was higher during the first cycle of adsorption-desorption for both pollutants, which gradually reduced at the end of the third cycle. A lower percentage of desorption indicates a higher affinity of pollutants towards the adsorbent. Hence a strong bond exists between AMP/SDZ and N-ZnO nanoparticle and adsorption is irreversible (Yadav et al. 2022).
Figure 4
Desorption of AMP and SDZ from ZnO nanoparticles surface.
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Desorption of AMP and SDZ from ZnO nanoparticles surface.

Figure 4
Desorption of AMP and SDZ from ZnO nanoparticles surface.
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Desorption of AMP and SDZ from ZnO nanoparticles surface.

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Adsorption mechanism

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As observed from the equilibrium study, the removal of AMP and SDZ using ZnO nanoparticles followed the LI model. It indicates that the sorption of both target pollutants on the sorbent is monolayer, and chemisorption is the major mechanism involved in the sorption process. The pore size determined using porosimetry analysis confirms the formation of mesopores (46.843 nm) on the surface of ZnO nanoparticles generated from A. indica extract. During the adsorption process, the target pollutants are found to adsorb onto these pores. However, ℼ-ℼ interaction, hydrogen bond formation and formation of electron complex are generally considered as the plausible mechanism of AMP adsorption onto sorbent. In the case of SDZ, sulfonamide groups, amino groups, and aromatic rings act as ℼ electron acceptors (Peiris et al. 2017). Oxygen, in the carbonyl group of AMP molecules, has electron lone pairs acting as acceptors of hydrogen bonds and -OH bond present on the surface of ZnO nanoparticles acts as a donor (Quesada et al. 2019). Even though hydrogen bonding is the key mechanism observed in the adsorption of AMP onto the ZnO nanoparticles surface, other interactions like electrostatic forces, covalent bonding and Van der Waals forces are induced between the complex functional groups present on the ZnO nanoparticles surface and AMP molecules (Suriyanon et al. 2015). Desorption study also helps to predict the mechanism of adsorption. A lower percentage of desorption in all three cycles indicates the presence of a strong bond (due to chemisorption) between target pollutants and ZnO nanoparticles (Manjunath et al. 2019).

Apart from this, the relationship between pHpzc and pH was also used to explain the adsorption mechanism. The pHpzc of ZnO nanoparticles was found to be 6.6. When pHpzc < pHsolution the adsorbent surface is negatively charged, whereas SDZ and AMP present in solution occurs as charged cations (SDZ-H+ and AMP-H+) signifying electrostatic attraction between positively charged ZnO nanoparticles and SDZ-H+ and AMP-H+, respectively. at higher pH, both sorbent and sorbate carry negative charge. Hence a reduction in removal efficiency at higher pH is attributed to the repulsion between the like charges (Zyoud et al. 2020). AMP/SDZ molecules exist as neutral particles when pH is lower than pKa. Hence at lower pH (2 and 4), the active sites present on the ZnO nanoparticles surface are blocked due to the formation of multilayers of contaminants due to strong electrostatic interaction (Basha et al. 2015). This results in a reduction in removal percentage at lower pH. However, at neutral pH, the hydrophobic interaction of target pollutants can be considered the predominant mechanism involved in the adsorption process (Eslami et al. 2020).

Performance evaluation of ZnO nanoparticles with other sorbents

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The adsorption capacity of ZnO nanoparticles and other adsorbents are compared and represented in Table 7. The maximum adsorption capacity of ZnO nanoparticles used in this study is comparable with other sorbents presented. Whereas the removal of both SDZ and AMP from a MuS is not reported by other researchers as seen from literature. The qm of ZnO nanoparticles in a multi-component system is higher than that of other sorbents in an MoS.

Table 7

Operational conditions and adsorption capacity of different adsorbents for the removal of AMP and SDZ

AdsorbateAdsorbentOperational Conditions
qm (mg/g)Isotherm modelReferences
Time (hrs)Co (mg/L)Dose (mg/L)
SDZ Expanded graphite 12 5–80 1,000 16.59 LI Zhang et al. (2017)  
High silica ZSM-5 48 0.5–20 200 6.21 FI Zuo et al. (2020)  
Phosphonic chelating cellulos 72 0.3–2 100 0.22 FI Yang et al. (2019)  
Zeolite 1.67 0.5–20 1,000 2.57 LI Liu et al. (2018)  
ZnO nanoparticle (MoS-component) 0.5 0.5–100 2,000 7.77 LI Present Study 
ZnO nanoparticle (MuS-component) 0.5 1 + 1–100 + 100 2,000 13.89 LI Present Study 
AMP Amine hexagonal mesoporous silica 10 0.04–0.3 2,000 0.153 LI Suriyanon et al. (2015)  
Mercapto- functionalized hexagonal mesoporous silica 10 0.04–0.3 2,000 0.05 LI Suriyanon et al. (2015)  
Hexagonal mesoporous silica 10 0.04–0.3 2,000 0.03 LI Suriyanon et al. (2015)  
Powdered activated carbon 10 0.04–0.3 2,000 0.50 LI Suriyanon et al. (2015)  
ZnO nanoparticle (MoS-component) 0.5 0.5–100 2,000 7.87 LI Present Study 
ZnO nanoparticle (MuS-component) 0.5 1 + 1–100 + 100 2,000 6.94 LI Present Study 
AdsorbateAdsorbentOperational Conditions
qm (mg/g)Isotherm modelReferences
Time (hrs)Co (mg/L)Dose (mg/L)
SDZ Expanded graphite 12 5–80 1,000 16.59 LI Zhang et al. (2017)  
High silica ZSM-5 48 0.5–20 200 6.21 FI Zuo et al. (2020)  
Phosphonic chelating cellulos 72 0.3–2 100 0.22 FI Yang et al. (2019)  
Zeolite 1.67 0.5–20 1,000 2.57 LI Liu et al. (2018)  
ZnO nanoparticle (MoS-component) 0.5 0.5–100 2,000 7.77 LI Present Study 
ZnO nanoparticle (MuS-component) 0.5 1 + 1–100 + 100 2,000 13.89 LI Present Study 
AMP Amine hexagonal mesoporous silica 10 0.04–0.3 2,000 0.153 LI Suriyanon et al. (2015)  
Mercapto- functionalized hexagonal mesoporous silica 10 0.04–0.3 2,000 0.05 LI Suriyanon et al. (2015)  
Hexagonal mesoporous silica 10 0.04–0.3 2,000 0.03 LI Suriyanon et al. (2015)  
Powdered activated carbon 10 0.04–0.3 2,000 0.50 LI Suriyanon et al. (2015)  
ZnO nanoparticle (MoS-component) 0.5 0.5–100 2,000 7.87 LI Present Study 
ZnO nanoparticle (MuS-component) 0.5 1 + 1–100 + 100 2,000 6.94 LI Present Study 
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Synthesis of ZnO nanoparticles were performed using neem (A. indica) leaf extract. SEM-EDS, TEM, FTIR, BET and XRD analysis were performed for the characterization of ZnO nanoparticles. ZnO nanoparticles having a surface area of 48.551 m2/g and an average particle size of 10 nm were obtained at the end of the synthesis. ZnO nanoparticles was found to be effective in removing pharmaceutically active compounds like AMP and SDZ from both MoS and MuS. The equilibrium time, optimum dosage and pH from adsorption study was found to be 30 min, 2 g/L and 7, respectively. The kinetic study best fitted PSO reaction for both AMP and SDZ with an R2 value nearly unity. The adsorption was monolayer and maximum adsorption capacity was found to be 7.87 mg/g and 7.77 mg/g for AMP and SDZ, respectively. In MuS, from Langmuir competitive model, AMP exhibited antagonistic nature and SDZ exhibited synergistic nature respectively. Percentage desorption obtained for 3 cycles was found to be ∼43.03%, ∼31.7% and ∼25.28% for AMP and ∼35.6%, ∼24.1% and ∼22.4% for SDZ respectively. This lower desorption percentage indicate a stronger bond between the ZnO nanoparticle and AMP/SDZ molecules. From this study it can be concluded that ZnO nanoparticles can be effectively used as a nano-adsorbent for the treatment of pharmaceutically active compounds.

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Not Applicable.

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Not Applicable.

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Included in the manuscript.

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All authors contributed to the study's conception and design.

Aswathy Erat Valsan: Supervision and proof reading.

Nayanathara O Sanjeev: Experimentation and Characterization, analysis and manuscript preparation.

Manjunath Singanodi Vallabha: Modeling, Manuscript preparation and proof reading.

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Not Applicable.

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

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The authors declare there is no conflict.

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