Modified activated carbon loaded with bio-synthesized Ag/ZnO nanocomposite and its application for the removal of Cr (VI) ions from aqueous solution

Abstract

Chromium (VI) is recognized as a highly toxic contaminant metal ion, due to its potential toxic effect to human and animal health. From the most common processes applied for Cr (VI) ions removal is the sorption process hence it has been highly efficient and inexpensive technique. In the present work, a bio-synthesized Ag/ZnO nanocomposite loaded on activated carbon as a new eco-friendly and effective sorbent for the removal of Cr (VI) ions. The modification of activated carbon confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The optimum conditions for the uptake of Cr(VI) ions by Ag/ZnO-AC was found to be at pH 2.5, shaking time 60 h, Cr (VI) ions concentration of 40 ppm, and 0.4 g sorbent dose. The results indicated that sorption process optimally correlated with Freundlich isotherm and pseudo second order kinetic models. The values of ΔG° are negative at all studied temperatures (-0.55, -1.18, -2.43, -3.68, -4.93 and -7.43 kJ. mol⁻¹) suggesting that the sorption process is spontaneous. The positive values of ΔH° (36.7 kJ. mol⁻¹) and ΔS° (0.125 kJ. mol⁻¹) indicate the endothermic and physisorption nature of the sorption process. The results demonstrated that Ag/ZnO-AC can be used as a potential sorbent for the uptake of Cr (VI) ions.

Introduction

Water pollution becomes one of the most serious ecological concerns. The main cause of water pollution is the discharge of various pollutants into water bodies [1]. Among these pollutants, dyes, pharmaceuticals, pesticides, organic compounds, detergents and heavy metals [2], [3], [4], [5], [6], [7]. The existence of heavy metal ions such as Cr^3+/6+, Cd²⁺, Hg²⁺, Co²⁺ and Pb²⁺ with high concentration in both urban and industrial wastewaters, causes a significant environmental pollution [8]. Within these heavy metals chromium considered as the most toxic pollutant [9,10]. Chromium element and its compounds are extensively used in many manufacturing process such as coating, leather treatment, mining, photography and textile industries [11]. Usually Chromium presents as Cr^3+/6+in aqueous medium, hexavalent Cr ion is extremely toxic than Cr³⁺  [12]. Hence, its accumulation in the ecosystem leads to serious danger to the human health and  marine life the researchers devote a significant attention to the development of effective methods for the removal chromium (VI) ions from wastewater [13,14]. Among these methods, adsorption is in particular the main concern of scientific research due to their high  removal efficiency, easy handling and cost effectiveness [15], [16], [17], [18], [19], [20], [21]. Several sorbents have been reported for the uptake of chromium (VI) ions from waste water for example agricultural residue and bio materials, graphene nanosheets [22], graphene oxide–EDTA composite [23] graphene oxide/SiO₂@polyaniline microspheres composite [24], polypyrene/calcium rectorite composite sorbent [25], pine needles [26], activated carbon [27], [28], [29], [30], etc.

Activated carbon was demonstrated to be more effective sorbents owing to its high surface area, cost-effective, nontoxic and high efficiency. Thus, it is considered as example of the highly recommended sorbents for heavy metals removal from manufacturing wastewater. Modification and metal impregnation methods on the activated carbon sorbent were used to enhanced the sorption performance  and selectivity for the removal of inorganic pollutants [31]. Loading of metals such as Ag, Au, Zn and Fe on the activated carbon surface is known as impregnation, which can rise the capability of sorption for activated carbons. The nanoscale-based sorbents with active elements and high surface area were recognized to have high capacity for the removal of heavy metals pollutants. Using of nanocomposites photo-catalysts in the removal of environmental pollutants has attracted considerable attention now a days, specially ZnO it has a lot of consideration owing to its high chemical stability, non-toxicity, cost-effective, and high catalytic activity [32]. However, it has some drawbacks since it has very little quantum yield, instability, and the fast recombination rate of the photo-induced electron-hole pair, that limits its catalytic activity under regular conditions [33,34]. To solve these shortcomings many attempts have been made to prepare more effective photo-catalysts based on ZnO nanomaterials such as using metal doping of ZnO and surface modification with organic molecules [35,36]. Several studies have been made on metal and nonmetal elements, such as Ag, Cu, Fe, and N, doped into ZnO to improve it is photocatalytic activity. TiO₂/ZnO and ZnOCuO hybrid nanocomposites have been reported with very high catalytic behavior compared to the metal oxide alone [37,38]. ZnO/MgO nanocomposite has much higher photocatalytic activity when compared with the ZnO as reported by M. A. Karimi et al. [39]. The greatest applied  M/ZnO nanocomposite is Ag/ZnO nanocomposite [33,36,40,41]. Among these noble metals Ag was selected because it shows remarkable catalytic activity owing to its high surface area comparatively being economically effective and is well-known as the electron sinks.

This work will demonstrate the development and characterization of activated carbon loaded with bio-synthesized Ag/ZnO nanocomposite and its application for the uptake of hexavalent chromium ions from aqueous solution using direct UV–vis spectroscopy method as simple, stable, and effective method for environmental application. The effect of various variables on the sorption of Cr (VI) ions onto the modified sorbent were evaluated using batch experiments. Including sorbent dose, pH of solution, Cr (VI) ion initial concentration and contact time. Furthermore, the kinetics, equilibrium, and thermodynamics were examined to evaluate the adsorption process.