Photocatalytic decolorization of cationic and anionic dyes over ZnO nanoparticle immobilized on natural Tunisian clay

Highlights

• ZnO/clay photocatalyst prepared using natural clay.

• ZnO has a promoting effect on photocatalytic activity of cationic and anionic dyes.

• Almost complete degradation using UV and solar irradiation

• Stability of the catalyst was confirmed after five consecutive reaction cycles.

Abstract

In the present work we describe a simple and low-cost method for the decolorization of textile dyeing and printing wastewaters, using ZnO as photocatalyst supported on natural Tunisian clay (ZnO/Clay). This composite ZnO/Clay material was synthesized through a sol–gel method. X-ray diffraction (XRD), Nitrogen Physisorption (BET), Infrared Spectroscopy (FTIR), Scanning Electronic Microscopy (SEM) and High-Resolution Transmission Electron Microscopy (HRTEM) linked with Energy Dispersive X-ray (EDX), were performed in order to explain the characteristics of the ZnO/Clay photocatalyst. The XRD patterns, pointing to the presence of ZnO of very small crystal sizes, i.e. highly dispersed on the clay surface. The photocatalytic activity of ZnO/Clay was assayed in the decolorization of a cationic dye (Malachite Green, MG) and anionic dyes (Red Congo, RC) in aqueous solution as models pollutants under UV irradiation. In addition, the effects of different parameters such as pH of the solution, catalyst dosage, concentration of the dyes, irradiation source, as well as the influence of the presence of inorganic ions were investigated. The ZnO/Clay photocatalyst exhibited high photocatalytic activity of MG and CR decolorization under simulated solar compared to UV irradiation. The recyclability of the ZnO/Clay photocatalyst was as well validated.

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⁻, SO₄^2 −, HCO₃^– and Cl⁻ on the photocatalytic performance of ZnO/clay was also investigated.