Degradation of acetamiprid using graphene-oxide-based metal (Mn and Ni) ferrites as Fenton-like photocatalysts

Keeping in view the easy accessibility of clean water complying with WHO-standards, there is a strong need for developing effective water treatment technologies. Various physico-chemical and biological pre-treatments are currently used. Such treated water normally meets the legal requirements for discharge into water streams. Pesticides and drugs are among the major micro-pollutants found in urban secondary effluents (Al Aukidy et al. 2012; Bueno et al. 2012; Cabeza et al. 2012). Their low concentrations in aqueous streams threaten their inhabitants, owing to bio-accumulation in the muscular tissues of living creatures. Consequently, an effective, economical and eco-friendly treatment is now vital to cope with the recalcitrant chemicals. Pesticides are among the priority pollutants, specifically in countries that are developing agriculturally. Farmers usually spray pesticide formulations in excess onto the crops to raise the product quality and minimize the insect damage. The adsorbed pesticides become part of wastewater when fruits and vegetables are washed in the industry's food processing unit. Acetamiprid (ACTM) is a commonly used neonicotinoid insecticide, being sprayed on citrus fruit (Garcia-Reyes et al. 2008). ACTM is chronically as well as an acutely toxic and may cause cancer (Li et al. 2010).

The wastewater treatment processes such as adsorption, membrane filtration, biodegradation are usually inefficient for the complete removal/degradation of aquatic pollutants. Advanced oxidation processes (AOPs) have been proved as successful substitutes for these inefficient and costly techniques (Tabasum et al. 2019). In-situ generated highly reactive hydroxyl radicals (oxidation potential of 2.8 V), are non-selective oxidative species (Konstantinou et al. 2014). The photo-Fenton process is among the most effective pollutant-remediation AOPs (Klamerth et al. 2013). Generation of radicals and their attack on pollutants play a key role in the degradation process, and light assists the generation of electron/hole pairs as well as keeping photo-catalysts in cycle (active) (Pignatello et al. 2006; Pérez et al. 2018; Nadeem et al. 2020).

Different metal-based photocatalysts have been used, and the most widely studied is TiO₂. This study evaluates the catalytic power of manganese and nickel ferrites. Compared to TiO₂, these small sized metal ferrites are more easily separated from aqueous solutions, owing to their magnetic properties (Yu et al. 2019). For instance, manganese and nickel ferrites offer narrow band gaps and are fairly stable. But the usability of pure metal ferrites is hindered by the quick recombination of electron-hole pairs. The use of composites, with carbonaceous materials as a support, is an effective strategy to eradicate this problem.

Graphene is a two-dimensional assembly of sp² hybridized, covalently linked carbon atoms, with a larger surface area, electrically conductive properties, remarkable adsorptive nature, and excellent thermal stability. Graphene offers charge migration from one active site to another through its electrically conductive surface, which inhibits the electron-hole pair recombination and accelerates the process of catalysis (Zhang et al. 2010; Li et al. 2018).

In view of all of this, many attempts have been made to prepare hybrids with different materials to degrade pollutants (Xiang et al. 2012; Zahid et al. 2019). Composites of manganese and nickel ferrites with graphene oxide (GO) have been prepared and used extensively for adsorption of heavy metals (Kumar et al. 2014; Naushad et al. 2017). For example, the degradation of NH₃ using graphene manganese ferrite has been reported under visible light irradiations. The degradation efficiency of composite was found to be 92.0% using H₂O₂ as an oxidizing agent (Zhou et al. 2016). Further, the high reaction rate (0.01443 min⁻¹) of rGO-MnFe₂O₄ composites as compared to pristine MnFe₂O₄ (0.00248 min⁻¹) for photocatalytic degradation of methylene blue demonstrates the significant role of reduced graphene in photocatalytic activity enhancement (Huang et al. 2019).

This study focused on the simple approach to synthesizing magnetic metal (manganese and nickel) ferrites and their graphene-oxide-based hybrids. The prepared composites were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), and investigated for their photocatalytic potential in terms of degradation of ACTM in UV light. To study the effective role of various process parameters (such as pH, catalyst dose, pollutant concentration, oxidant dose and irradiation time), the process of ACTM's degradation was also evaluated using central composite design.

Graphite powder (<45 μm, ≥99.99%), iron(III) chloride hexahydrate (99.0%) and manganese (II) chloride (99.0%) were purchased from Daejung Chemicals and Metals, Korea. Sodium nitrate (≥99.0%), sulphuric acid (clear, assay: 98.0%), hydrogen peroxide (30% w/w), iron (II) sulphate heptahydrate (≥98.0%) and sodium hydroxide (≥99.9%) were purchased from Sigma-Aldrich, USA). The technical grade (≥97%) pesticide sample, ACTM, was obtained from the local pesticide manufacturer in Pakistan. All chemicals were analytical grade and used as received without further purification. Distilled water was used throughout the study.

GO was synthesized by following the modified Hummer's method (Tabasum et al. 2019) using graphite powder. GO-MFe₂O₄ (where M = Mn, Ni) was prepared following the one-pot hydrothermal method in the presence of GO suspension (Yao et al. 2014). Dispersion of GO (10% of MFe₂O₄ mass used) was made using an ultrasonic bath in 10.0 mL of distilled water. Afterward, metal (Ni or Mn) chloride and FeCl₃.6H₂O, in a 1:2 mole ratio, were dissolved in 15 mL distilled water. Both the solutions were mixed under constant stirring. NaOH solution (0.1 molar) was carefully added to the mixed solution while keeping the reaction vessel on the magnetic stirrer until the pH reached 11 at ambient temperature. The final total volume of suspension was made up to 60 mL by adding the requisite amount of distilled water; it was then transferred into a 100 mL sealed Teflon-lined stainless-steel autoclave reactor and kept at 180 °C for 14 hours. GO-MFe₂O₄ particles were separated from the parent solution magnetically and rinsed with distilled water until the pH was neutral, and they were then washed thoroughly with ethanol. Finally, a magnetic powder was obtained after drying these magnetic particles at 65 °C. For comparative study, pure MFe₂O₄ particles were also synthesized separately via an analogous method without GO addition.

Morphological study and micro-structural analysis of the samples were performed using an SEM (JSM-5910 JEOL, Japan). Structural analysis of prepared metal ferrites and their composites with graphene oxide was done on a Bruker AXS D8-advance powder X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å), with the scan speed at 2⁰/min. Interaction among the various functional groups within the composite was determined by FTIR spectrometer (Thermo Nicolet). The surface chemical composition and elemental analysis of catalysts were conducted by XPS (Escalab 250 XPS system, Thermo Fisher Scientific UK).

Photocatalytic degradation of ACTM was investigated following similar protocols described previously (Tabasum et al. 2019). The homogeneity of the suspension was assured using an orbital shaker placed in appropriate light (in a UV chamber). The UV intensity was measure using a UV radiometer (UVX; UVP with 254 nm probe). All the tests were performed at ambient temperature (25 ± 2 °C). After the specific irradiation time, the catalysts were thoroughly washed with water, dried and kept for further investigation (relevant to its reusability potential). The residual ACTM in solution was estimated after measuring absorbance using a double beam spectrophotometer (Cecil 7200, UK) at 245 nm (λ_max for ACTM). The untreated ACTM solution was taken as the control, to determine the percentage degradation efficiency of the system. The effect of experimental parameters was studied and optimized systematically in different ranges of pH (2–8), oxidant/H₂O₂ dose (1.16–58 mM), catalyst dose (0–200 mg/L), ACTM conc. (2–16 ppm) and irradiation time (15–120 min).

The simultaneous interactions of influencing parameters were studied using central composite design (CCD) of response surface methodology (RSM). Three chosen factors for CCD were oxidant dose (X₁), catalyst dose (X₂) and ACTM concentration (X₃). Ranges of these independent variables along with their levels (three for each) are shown in Table 1.

Here Y is the predicted percentage degradation efficiency; β₀, β_i, β_ii and β_ij are designated to offset the term (linear/quadratic) and interaction coefficients for Xi and Xj as independent variables. Triplicate runs were performed for each experimental trial. Statistical analysis was performed using design expert 7.0.0 software and fitted to the polynomial regression model. The fitness of the model was tested using analysis of variance (ANOVA). Computation of F-value (at 0.05 probability value) gave a measure of the parameters' significance in terms of linear and interactive effects.

The morphology of graphene oxide and its composites with metal ferrites is shown in Figure 1. The SEM images of graphene oxide show the high surface area, which contributes in its pronounced adsorptive property for oxidants as well as for pesticide molecules. Ferrite particles firmly anchored onto GO sheets and contributed towards enhanced durability, magnetic separation, and reduced catalyst leaching during the degradation process. The SEM images clearly show how the ferrite molecules were fabricated onto GO. The composite formation not only immobilized the magnetic ferrite particles but also assisted in preventing their agglomeration.

The structural analysis of ferrite samples was done by XRD; the patterns were analyzed and indexed using powder X software (Bai et al. 2012). Results of XRD analysis of GO, MnFe₂O_4, NiFe₂O₄, GO-MnFe₂O₄ and GO-NiFe₂O₄ are shown in Figure 2(a) and 2(b). The XRD patterns of GO showed a strong peak at 2Ѳ = 10.7°, corresponding to the (002) inter-planar spacing. No further peaks indicated that no unreacted or unoxidized graphene was present in the sample.

As both the ferrites are mixed oxides of transition metals (Mn and Ni), occupying tetrahedral or octahedral positions in the crystal lattice, different diffraction peaks are assigned to each ferrite, which reflects various crystal planes i.e. diffraction peaks at 2ϴ equals to 29.7°, 34.9°, and 61.6° could be indexed to pure phase spinel NiFe₂O₄. The XRD results confirm the successful synthesis of composites (GO-MnFe₂O₄ and GO-NiFe₂O₄) using the one-pot hydrothermal method (Zhou et al. 2010).

Electronic absorption by composites and their constituent metal ferrites in the infra-red region are shown in Figure 3(a) and 3(b). FTIR spectra help in elucidating the chemical bonding and interactions when composites are being formed. Spectra of both the composites show the differences in bands throughout the region of 3,500–900 cm⁻¹. The two absorption bands that appear below 900 cm⁻¹ justify the formation and presence of metal ferrites, as they are characteristic of ferrites. For instance, the band at 582 cm⁻¹ is attributed to Fe-O stretching vibrations in metal ferrites (Qi et al. 2019). In another way, these bands appear because of lattice vibrations of oxide ions against their respective cations, present in a ferrite crystal. In ferrite samples, the absorptions near 1,642 cm⁻¹ and 3,392 cm⁻¹ are attributed to vibrations of bonded hydroxyl groups as well as deformation of water molecules (Yu et al. 2016). The three peaks at 3,424 cm⁻¹, 1,642 cm^−1, and 1,037 cm⁻¹ show the presence of GO. Since the spectra of both composites reveal that their compositional GO gave a broad band near 3,424 cm⁻¹, attributed to stretching vibrations of GO structural hydroxyl groups. The band near 1,642 suggests C = O stretching vibrations in attached carboxylic groups (Liu et al. 2017). The sharpness, intensity and red-shifted positions of bonds reflect strong chemical interaction among Fe-O-C bonds, i.e. C = O stretching vibration peak (of the carboxylic group) near 1,642 cm⁻¹ and 3,392 cm⁻¹, attributed to the complex linkage of COO- with iron in Fe₃O₄. Formation of Fe-O-C bond confirmed the proper crystal growth, nucleation, and impregnation of magnetite on active sites of GO. Thus, from FTIR results it can be inferred that ferrite particles were firmly anchored on the GO surface.

The surface chemical composition of the graphene-based photocatalyst has a vital role as most of the reaction occurs over the surface of the composites (Jilani et al. 2017, 2018a). The surface compositional analysis of the prepared ferrite samples was investigated through XPS. Figure 4(a) shows the surveyed surface composition of NiFe₂O₃ and GO-NiFe₂O₃ while Figure 4(b) shows the elements detected and their respective atomic percentages. The surface compositions revealed the presence of O 1s, Fe 2p3, Ni 2p, and C 1s, which shows the simplicity and accuracy of the synthesis method. The NiFe₂O₄ has 76.7% of O 1s and 21.5% and 1.8% of Fe 2p3 and Ni 2p respectively. Moreover, the presence of C 1s (21%) was observed in the case of GO-NiF₂O₄, which indicates the successful incorporation of GO into NiF₂O₄. However, GO has affected the elemental composition of the NiFe₂O₄ as shown in Figure 4(b). This change in the composition can enhance the photocatalytic activity of GO composite materials (Jilani et al. 2018a, 2018b).

The survey of the surface composition of MnF₂O₄ and GO-MnFe₂O₄ (Figure 4(c)) also confirmed the presence of GO with C 1s (29.7%). The Fe 2p3 was about 15.0% and 15.3% for MnFe₂O₄ and GO-MnFe₂O₄ respectively. The O 1s was about 80.7% and 51.1% for MnFe₂O₄ and GO-MnFe₂O₄ respectively. The complete composition of detected elements in the case of MnFe₂O₄ and GO-MnFe₂O₄ is concisely shown in Figure 4(d).

As heterogeneous photocatalysis is a surface linked phenomenon and the pH of the solution greatly affects superficial charge distribution on the catalyst's surface, it is important to investigate the effect of varying the pH of the solution on catalytic potential. The pH range of 2 to 8 was investigated in the heterogeneous photo-Fenton process for ACTM's degradation and the results are shown in Figure 5. It is evident from Figure 5(a) that degradation efficiencies (for all catalysts, ferrites and their GO composites) were pH dependent. Acidic pH (pH = 3) favored the degradation of ACTM and degradation rate gradually decreased as the conditions were shifted to alkaline pH. This is because pH determines the charge properties of the catalyst's surface. In acidic conditions, the catalyst's surface became protonated and the ⁻OH ions, generated in an aqueous system, could accelerate oxidant decomposition. The decomposition of oxidant molecules generates hydroxyl radicals, which non-selectively attack pollutant molecules. On the other hand, in alkaline conditions, the surface of the catalyst became deprotonated and therefore negatively charged, with no enhanced production of reactive hydroxyl radicals, resulting in decreased ACTM degradation (Ren et al. 2012).

Furthermore, in an alkaline solution of ACTM, there was a greater likelihood of the reaction between ferrous ions and hydroxyl ions (present in excess), which would account for the lower degradation rate. Consequently, brownish sludge formed on the surface of catalysts, which not only blocked active sites but also hindered light penetration (Aleksić et al. 2010). The solubility product constant for Fe(OH)₂ is 1.8 × 10¹⁵, thus fewer hydroxyl radicals and iron ions were available for the Fenton reaction (Pignatello et al. 2006; Soto et al. 2018).

Oxidant dose is also an important parameter for the efficient pollutant degradation process in the Fenton-based advanced oxidation reaction. A catalyst's enhanced efficiency lies in an appropriate catalyst to oxidant ratio. The impact of varying concentrations of hydrogen peroxide, as the oxidant, was studied in the range of 1.16–58 mM and the results are shown in Figure 5(b). Oxidant concentration on very high or low values normally disturbed the balance of the Fenton process, as evident from Figure 5(b). It is clear from the experimental results that the addition of 21.75 mM (in the case of MnFe₂O₄) and 36.25 mM (for NiFe₂O₄) was enough to enhance photocatalytic activity to 60–80%.

Beyond this value up to 58 mM, excess hydrogen peroxide molecules would not participate in the oxidative degradation of pollutants. At higher H₂O₂ concentrations, these molecules react with each other to form another oxidative radical (the peroxyl radical). The oxidative potential of peroxyl radicals is lower than that of hydroxyl radicals. An increase in the oxidant dose up to the optimum value enhanced pesticide degradation because it was able to activate the surface-bound active sites. On the other hand, H₂O₂ in excess scavenged the produced hydroxyl radicals, making them unavailable for the oxidative reaction, because the hydroxyl radicals are transformed into hydro-peroxyl radicals (HOO^̇) (Yu et al. 2016). Moreover, H₂O₂ molecules also compete with the ACTM molecules for adsorption (catalytic) sites bound to the surface of catalyst (Titouhi & Belgaied 2016).

The surface of a catalyst provides reaction sites to degrade pollutant molecules so varying catalyst concentration greatly affects the overall efficiency of the degradation process. The effect of varying catalyst dose was investigated in the range of 0–200 mg/L and the results are shown in Figure 5(c). It is evident from the figure that the maximum catalytic degradation was found at 100 mg/L of catalyst concentration. The higher the amount of catalyst, the more active sites will be provided. These active sites facilitate the enhanced production of activators (hydroxyl radicals) as well as the adsorption of the pollutant molecules.

However, the degradation efficiency declined at a higher concentration of catalysts because of the catalyst's agglomeration. Resultantly, catalyst particles stack over each other, leaving active sites unavailable for the Fenton reaction to proceed (Hoque & Guzman 2018). Also, at higher catalyst doses, light is unable to penetrate the solution for the photo-Fenton reaction, which decreases the photo-catalytic potential. Thus, the rate of production of hydroxyl radicals is decreased. Besides this, scavenging of produced –OH radicals by excess catalyst species occurs (in case of higher catalyst doses) and thus terminates the degradation process. Figure 5(c) also shows that the photo-Fenton efficiency would not be disturbed even at a high dosage of catalyst in the presence of GO-ferrite composites. The GO not only facilitated the ferrites with conductive support but also prevented the catalysts from agglomerating (Mirzaei et al. 2017).

The effect of pesticide concentration was studied in a range from 2 to 16 ppm. The results are presented in Figure 5(d) and show that pesticide is efficiently degraded (more than 85%) in the case of GO-ferrite composites for up to 10 ppm pesticide concentration. However, with further increases in pesticide concentration, the degradation efficiency is continuously decreased. It is justified by the fact that at this stage (ACTM ≤ 10 ppm) limited target molecules are being adsorbed and catalyzed by more available active sites, so the catalytic potential of the system is remarkable. Further addition of pollutant (ACTM > 10 ppm) lowers the catalytic degradation efficiency by making the ACTM molecules stay in the solution (due to lack of available active sites). This gives no space for the reactions responsible for the formation of surface-bound reactive oxygen species (Tabasum et al. 2019).

Another reason is that the high concentration of pesticide molecules absorbs more photons, which in turn decreases the number of photons available for photocatalytic reaction. In other words, the transparency of the solution is greatly affected. Moreover, with the increase in pesticide load, the oxidant to pollutant ratio is reduced, which has a negative impact on photocatalytic degradation (Luo et al. 2010).

The irradiation time for UV light (λ = 254 nm) on the degradation of ACTM solution at optimized conditions was studied for continuous monitoring of degradation efficiency for 120 min. The present study is advantageous as compared to previously reported research using similar material (Yamaguchi et al. 2017), that 60 min of reaction time (under UV irradiations) was enough for almost complete degradation of ACTM. Further, UV irradiation was found to be the most important factor along with prepared material for the efficient degradation of pesticides. The results show (Figure 5(e)) that pesticide degradation increased sharply during the first 60 min. Over time, the rate of the ACTM degradation reaction slowed. The active sites located on the catalyst's surface need a specific period to degrade the adsorbed pollutant molecules. Hydroxyl ions are furnished from the aqueous pesticide solution, affecting the pH directly and the rate of catalysis indirectly. These generated -OH ions also reacted with bivalent ions and hence suppressed the Fenton reaction. Moreover, with time, hydrogen peroxide became the limiting reactant because the recommended stoichiometric amount of hydrogen peroxide was depleted (Velichkova et al. 2013).

Under optimized conditions, the stability of the catalysts was verified by conducting successive trials of the repeatedly used catalysts (see Table 2). To do so, used the catalysts were separated from the treated ACTM solution by applying an external magnetic field, then rinsed three times with distilled water, dried in the oven at 60 °C, weighed and investigated for their catalytic potential against the pesticide solution. All the reusability trials were run at the optimized conditions, i.e. the pH of reaction media was 3, the H₂O₂ concentration was 21.75 mM (for Mn based catalysts) and 36.25 mM (for Ni based catalysts), the catalyst dose was 100 mgL⁻¹, the ACTM concentration was 10 ppm and the temperature was 298 K. There was no significant loss in catalytic activity after five consecutive runs; approximately 5–10% decrease in catalytic activity was observed.

The values of iron leaching after each run were measured using an atomic absorption spectrometer. Iron leaching was also far below the range suggested by European Union directives (2.0 mg/L). The concept of heterogeneous catalysis has evolved owing to heavy iron leaching during the homogeneous catalytic process. Feasible application of heterogeneous Fenton catalysts in wastewater treatment requires that they should be stable in terms of metal leaching from the solid to the liquid phases. Gradual and continuous metal leaching leads to the deactivation of catalysts with further pollution of water. Leaching studies give us a better insight into the elucidation of degradation mechanisms. The interionic forces of attraction are weaker in the case of ferrous and oxide ions in the lattice, so monitoring the stability of Fe²⁺ is more important. The reduction of ferric ions to ferrous ions is the time when there are more chances of iron leaching (Xu et al. 2007). From Table 2, it can be seen that iron leaching is quite low after five consecutive cycles, indicating the mechanism to be heterogeneous.

Removal/degradation of ACTM from the aqueous system (either through adsorption or during oxidative degradation) was studied using different catalysts and methodologies. The results are shown in Figure 6, which shows that adsorption of pesticide onto the catalyst's surface could only remove 17% of the ACTM's initial concentration (C_o) after 60 min of contact time. Oxidant dose (36.25 mM) and UV irradiation (for 60 min) alone were also not able to achieve satisfactory degradation of ACTM (less than 15%).

In contrast to the individual use of oxidants, catalysts and UV light, their combination synergistically shifts the degradation reaction from about 15% to 62–68% (roughly seven times the individual effect of oxidant and UV light). An extremely positive effect in the ACTM's removal is noticeable (% degradation = 90) when metal ferrites are used in combination with GO. The present study also validates the findings of Fe₃O₄ and GO-Fe₃O₄ described elsewhere (Tabasum et al. 2019). The successful utilization of metal (Mn, Ni) ferrite and their GO-based composites for effective degradation of pesticides suggests that these catalysts are as effective as control samples (Fe₃O₄ and GO-Fe₃O₄). ACTM depletion (under the same processes conditions) showed the enhancement of the oxidation reaction by graphene oxide's catalytic support in the GO-ferrite composites. So the catalysts provide a greater number of adsorption sites along with the decrease in electron/hole pair recombination. The generation of more hydroxyl radicals is superficially linked to composites in the presence of GO. Resultantly, the composite's surface becomes more reactive for the degradation of ACTM molecules.

The analysis of the above polynomial equation reveals that all three factors have a certain influence on degradation reaction, but an appropriate balance should be there between the two catalytic reagents (oxidant concentration and catalyst dose).

The response surfaces showing the interaction of two dependent variables (keeping the third factor fixed at the center level) in terms of the degradation of ACTM is shown in Figure S-1 (Supplementary Material). It has been observed that in the case of oxidant concentration and catalyst dose, the impact of increasing catalyst concentration is more pronounced at higher concentrations because the optimum balance in the ratio between the two variables is significant. In the case of the interaction of oxidant concentration with ACTM load, it has been demonstrated that the degradation efficiency decreases above certain oxidant doses due to a number of factors, such as self-decomposition, scavenging, back reactions, etc. The response surfaces showed the variables are interacting significantly with each other. Hence, the optimum combination of the influencing factors is vital to maintaining the efficient degradation of ACTM.

This study deals with the facile synthesis of manganese and nickel ferrite composites using graphene oxide. The impregnation of the metal ferrites onto the GO was successfully validated through SEM, XRD, XPS and FTIR analyses. Composites showed efficient oxidative degradation of ACTM under UV light. The catalytic activity of metal ferrites increased in the presence of graphene oxide, as GO supported the material to enhance durability and reusability of composite with concomitant ease in electron transportation from one active site to another. The combination of metal ferrites with graphene oxide improved catalytic performance of pure metal ferrites by electron/hole pair recombination. Also, it made the catalyst more stable than pure ferrite. The catalysts were reused for five successive runs without significant loss of degradation efficiency. Negligible iron leaching was found, which indicates the stability of the catalysts towards heterogeneous catalytic reaction.

The direct correlation between the ACTM removal with oxidant dose, catalyst dosage, pollutant load, and pH were investigated. The optimum operational conditions for achieving maximum catalytic efficiency (∼90%) were pH 3, catalyst dose of 100 mg/L, initial ACTM concentration of 10 mg/L and irradiation time 60 min. The results also reveal that CCD coupled with RSM is a reliable and valuable tool, that helps to optimization of different parameters influencing the rate of pollutant degradation. High catalytic power, strong magnetism for easy separation, extraordinary durability, low iron leaching, and efficient reusability are the salient advantages of GO-based manganese and nickel ferrites.

The authors would like to acknowledge the Higher Education Commission, Pakistan, for financial support to successfully perform this work.

There is no conflict of interest among the authors in relation to this study.

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.098.