Research paperPhotocatalytic decolorization of cationic and anionic dyes over ZnO nanoparticle immobilized on natural Tunisian clay
Introduction
Azo and triphenylmethane dyes, Malachite Green and Red Congo, are two of the most widely used colorants in various industries such as the textile, cosmetic, food, printing, paper and leather industries (Forgacs et al., 2004). Congo Red (CR), an anionic diazo dye, can be metabolized into benzidine, a well-known human carcinogen (Ong et al., 2016). Malachite Green (MG), a cationic triphenylmethane dye, is resistant to fading on exposure; when this dye is discharged into water it affects the aquatic life and can cause detrimental effects in the gills, intestine, liver, kidney and gonadotrophic cells (Bel Hadjltaief et al., 2013, Saikia et al., 2015). These pollutants are quite refractory to both aerobic and anaerobic digestions, and stable to light, heat, and moderate oxidizing agents, being thus difficult to remove (Bel Hadjltaief et al., 2013, Saikia et al., 2015). Therefore, the elimination of these pollutants from wastewater remains a very challenging task.
A wide range of methods have been developed aiming to the removal of Malachite Green and Red Congo dyes from the wastewaters such as adsorption, membrane separation, ion exchange, photocatalytic degradation and biological treatments (Bel Hadjltaief et al., 2013). Among these methods, semiconductor photocatalytic processes, show important removal efficiencies and have an important potential for industrial application. Photocatalytic proceses moreover allow full water decontamination under relative mild operation conditions (Bel Hadjltaief et al., 2014b).
Zinc oxide as a semiconductor, (ZnO, 3.37 eV), has been historically used for the removal of CR or MG from water due to its high photo sensitivity, large band-gap, stability and relatively low toxicity (Saikia et al., 2015, Balcha et al., 2016, Ong et al., 2016). Aiming to its practical utilization, ZnO has been immobilized on various supports such as zeolites (Nezamzadeh-Ejhieh and Khorsandi, 2014), activated carbon (Muthirulan et al., 2013, Soltani et al., 2014) and clay (Fatimah et al., 2011, Motshekga et al., 2013, Li et al., 2014, Ye et al., 2015, Zhou et al., 2015, Xu et al., 2015).
In particular, clay-based catalysts, i.e. metal oxide supported clays, have been frequently used in heterogenous photocatalytic applications (Bel Hadjltaief et al., 2014b, Bel Hadjltaief et al., 2016). These catalysts have been employed in the photocatalytic degradation of organic pollutants such as phenol and of some phenolic derivates (Zhou et al., 2015, Ye et al., 2015), organic dyes (Motshekga et al., 2013, Li et al., 2014, Xu et al., 2014, Xu et al., 2015) and other persistent compounds (Dhakshinamoorthy et al., 2011, Abedi et al., 2015). For example, Xu and co-workers reported the preparation of a composite material consisting in ZnO loaded onto the surface of commercial Bentonite following an in situ sol–gel technique. Compared with the original raw clay and pure ZnO, the composite material exhibited considerably higher photocatalytic activity in the photodecomposition of Acid Red 35 in water solution under ultraviolet irradiation (Xu et al., 2015). In another study, ZnO particle supported on commercial Rectorite (ZnO/rectorite) was prepared through sol-gel synthesis using zinc acetate and lithium hydroxide as raw materials and Rectorite as support (Li et al., 2014). The photo-catalytic performance of such ZnO/rectorite was investigated in the removal of Methylene Blue under simulated solar irradiation. So far, to the best of our knowledge and according to the literature research, using ZnO supported on natural Tunisian clays has never been considered and used as a photocatalyst in the decolorization of organic dye-containing waters, under solar and UV irradiation.
In the present work, natural Tunisian clay was therefore used as a support for the immobilization of ZnO following a sol-gel method. The photocatalytic activity of this clay-supported ZnO was assayed in the decolorization of water containing both cationinc (Malachite Green, MG) and anionic dyes (Red Congo, RC). The influence of key operational parameters, such as pH, catalyst dosage, initial dye concentration, has been considered. In addition, the potential interference of inorganic ions such as NO3−, SO42 −, HCO3– and Cl− on the photocatalytic performance of ZnO/clay was also investigated.
Section snippets
Materials and chemicals
The natural red clay (NC) used in this study was sampled in Jebel Tejera-Esghira deposits located in the Southeast of Tunisia from the area of Medenine (Bel Hadjltaief et al., 2017). From geological point of view, those clays were attributed to the Lower Triassic. It is a very thick series with dominant of sandstone alternating with red clays and with some silty intercalations. The outcrops extended from the Beni Kheddache cliff to the J. Tebaga of Medenine; it occupies the anticline of the J.
XRD studies
Mineralogical analysis of the natural clay sample was identified by XRD measurements; evidenced the characteristic diffraction peaks of quartz (26.7), kaolinite (22.8) and illite (12.6) (Bel Hadjltaief et al., 2016). From the XRD spectrum of ZnO–Clay, shown in Fig. 1, diffraction peaks appeared at 2θ equal to 31.8°, 34.4°, 36.2°, 47.5°, 55.6°, 62.8°, corresponding to hexagonal crystal lattices (100), (002), (101), (102), (110) and (103), respectively. ZnO presents the typical XRD diffraction
Conclusion
A ZnO/Clay composite, i.e. ZnO immobilized on Tunisian clay, was prepared by sol-gel method and characterized in order to gain more insight about its structure, morphology and physico-chemical properties. The XRD patterns evidenced the presence of ZnO clusters at small sizes, i.e. from 9 to 15 nm, pointing to its high dispersion on the clay surface. The presence of ZnO was further confirmed by TEM, SEM and FTIR analyses. The ZnO/Clay composite was used in the photocatalytic degradation of Red
Acknowledgments
The authors thank the University of Sfax and UPMC Sorbonne University for allowing access to their respective technical facilities.
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