Mesoporous rGO@ZnO composite: Facile synthesis and excellent water treatment performance by pesticide adsorption and catalytic oxidative dye degradation
Introduction
In recent years, there has been immoderate use of pesticides to meet the needs of the ever growing population. This has contaminated the water bodies. Organophosphorus pesticides bearing high toxicity are being over-utilized worldwide due to their capability to kill pests and weeds for the preservation of farm crops. The maximum permissible limit set by the European Union is 0.1 ppb for an individual pesticide (Garcia-Reyes et al., 2008). However, the indiscriminate use of these pesticides has contaminated the natural water resources and damaged the aquatic ecosystem (Alvarez et al., 2019) and caused food safety issues (Pagliuca et al., 2005) due to which these pesticides are given the status of priority chemicals in the EU Blacklist. Likewise, organic dyes are being redundantly used in the textile, food, tanning and cosmetics industries (Ma et al., 2019). Crystal violet (CV) is a triphenylmethane group based cationic dye commonly used as a colorant, veterinary medicine and biological stain and is under suspicion of being carcinogenic in nature (Zhang et al., 2014). To curtail the health hazards and ecological risks posed by these organic pollutants, it is necessary to develop efficient and viable methods of water decontamination and purification.
The fabrication of earth-abundant, cheap, environmentally benign, reusable, multifunctional and effective materials for the removal of pollutants has been the most imperative requirement in water remediation. In this regard, mesoporous materials have proved to be remarkable candidates with a display of promising results. They offer high surface interface and large space for the accommodation of guest molecules, which enables specific binding and enrichment of the target species with the adsorbent, and thus effective separation (Sharma and Kakkar, 2017, Sharma and Kakkar, 2018). These can be effectively applied as adsorbents and catalysts for the separation and removal of pollutants. The mesoporous materials, especially those which are fabricated from silica or carbon frameworks (Feng et al., 2018), have attracted a lot of attention globally. It is established that graphene, a carbon material, has an enormous specific surface area (Worsley et al., 2011), strong mechanical strength (Vadukumpully et al., 2011), and high electrical conductivity (Stoller et al., 2008).
However, graphene and its derivatives suffer various drawbacks like low adsorption capacity, aggregation of the sheets in solution and poor reusability (Zhou and Liu, 2010). Therefore, continuous efforts are being made on modification of graphene surfaces for improving their scope and applicability (Fakhri et al., 2017, Kyzas et al., 2018, Mandeep et al., 2018, Mandeep et al., 2019, Taghavi et al., 2017). The simultaneous reduction of graphene oxide (GO) sheets to reduced graphene oxide (rGO) and growth of metal oxide particles render steady binding of the composite on the rGO surface. Various synthesis routes have been used by researchers for the fabrication of the modified graphene composites (Boruah et al., 2017). Lately, the approach has shifted toward the green synthesis of the materials for the sake of the environment. This approach comprises of template-free or biotemplate assisted synthesis using non-hazardous, naturally occurring degradable solvents.
In this study, we have synthesized mesoporous rGO@ZnO composite by employing a glycerol-water mixture as the non toxic solvent without using any additional surfactant or template. The two constituents, rGO and ZnO, have gained formidable attention as gas sensors (Fu et al., 2018), photocatalysts (Ong et al., 2018), photodetectors (Liang et al., 2018), catalysts (Rajesh et al., 2014) and they also possess antibacterial properties (Azmy et al., 2017). However, the capabilities of rGO@ZnO for water remediation are still to be explored profoundly. We have tested the adsorption capabilities of the as-synthesised rGO@ZnO composite by selecting chlorpyrifos (CPF) as the model organophosphorus pesticide. The results shine light on the potential use of mesoporous rGO@ZnO as a reusable adsorbent for the removal of CPF. This is followed by the degradation of CV dye by an oxidation process dependent on the generation of sulfate radicals. A significant result is that the CV dye is completely degraded within 30 min without any external stimuli like microwave activation or light irradiation. To the best of our knowledge, no such work has been previously reported.
Section snippets
Materials
CPF was purchased from Sigma–Aldrich. All other chemicals were taken from Merck and used as received. All the solvents used were of HPLC grade and CPF as well as the CV dye were of analytical grade. Triply distilled water was used throughout the experiments.
Synthesis of mesoporous rGO@ZnO
GO was synthesized by a modified Hummer's method (Goncalves et al., 2009). The detailed procedure is reported in Supporting Information (SI). The rGO@ZnO composite was synthesized by following a hydrothermal route, as shown in SI Scheme 1.
Characterization of synthesized rGO@ZnO
Fig. 1(A) shows the PXRD pattern of the rGO@ZnO composite. There is only one sharp peak at 11.9° in the PXRD spectrum of GO (Fig. S1). In the rGO@ZnO composite, this peak at 11.9° disappears and a broad peak at 25° corresponding to the (002) hkl plane appears, indicating removal of the oxygen containing functional groups from the surface of GO, leading to the formation of rGO. Also, the peaks at 31.9°, 34.5°, 36.4°, 47.6°, 56.7°, 62.9°, 66.4°, 68°, 69.2° and 77.0° corresponding to (100), (002),
Conclusions
The as-synthesized rGO@ZnO exhibits commendable features, including facile and green synthesis, high surface area of 79.507 m2 g−1 and mesoporous nature, outstanding removal percentage of 95.4% for CPF within 70 min, insensitivity to pH, recyclability and reusability with no considerable loss in performance and it can be applied industrially for the purpose of water remediation owing to its high removal efficiency from contaminated tap water. Even as a catalyst, rGO@ZnO displays incredible removal
Supporting information
Procedure for synthesis of GO, synthesis scheme, PXRD of GO, pre-calcined product and ZnO, SEM and TEM images, EDX, change in zeta potential with pH, absorbance versus wavelength plot for different CPF concentrations, Langmuir and Freundlich plots, adsorption/desorption removal efficiency, SEM images of reproduced rGO@ZnO, treatment of CPF contaminated tap water.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
Archa Gulati acknowledges University Grants Commission (UGC), New Delhi, India for providing financial assistance in the form of Junior Research Fellowship (JRF) (UGC-Ref. No.- 111/(CSIR-UGC NET DEC. 2016)). Mandeep acknowledges Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing financial assistance in the form of a Senior Research Fellowship (SRF) (CSIR Award No.- 09/045(1341)/2014-EMR-I). We thank IITR for providing necessary facilities. The authors would
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