Combined effects of adsorption and photocatalysis by hybrid TiO₂/ZnO-calcium alginate beads for the removal of copper

Abstract

The use of nanosized titanium dioxide (TiO₂) and zinc oxide (ZnO) in the suspension form during treatment makes the recovering and recycling of photocatalysts difficult. Hence, supported photocatalysts are preferred for practical water treatment applications. This study was conducted to investigate the efficiency of calcium alginate (CaAlg) beads that were immobilized with hybrid photocatalysts, TiO₂/ZnO to form TiO₂/ZnO–CaAlg. These immobilized beads, with three different mass ratios of TiO₂:ZnO (1:1, 1:2, and 2:1) were used to remove Cu(II) in aqueous solutions in the presence of ultraviolet light. These beads were subjected to three cycles of photocatalytic treatment with different initial Cu(II) concentrations (10–80 ppm). EDX spectra have confirmed the inclusion of Ti and Zn on the surface of the CaAlg beads. Meanwhile, the surface morphology of the beads as determined using SEM, has indicated differences of before and after the photocatalytic treatment of Cu(II). Among all three, the equivalent mass ratio TiO₂/ZnO–CaAlg beads have shown the best performance in removing Cu(II) during all three recycling experiments. Those TiO₂/ZnO–CaAlg beads have also shown consistent removal of Cu, ranging from 7.14–62.0 ppm (first cycle) for initial concentrations of 10–80 ppm. In comparison, bare CaAlg was only able to remove 6.9–48 ppm of similar initial Cu concentrations. Thus, the potential use of TiO₂/ZnO–CaAlg beads as environmentally friendly composite material can be further extended for heavy metal removal from contaminated water.

Introduction

Heavy metal-laden wastewater has gained considerable attention due to its potential threat to both humans and the environment. Various efforts have been devoted towards removing heavy metals from wastewater. The non-degradable nature and persistency of heavy metal often render physical and chemical approaches ineffective, thus leading to the attenuation of heavy metals in various compartments of the environment. Prevailing wastewater treatment techniques, which include chemical precipitation, coagulation, chlorination, membrane filtration, adsorption, ion-exchange, and electrochemical treatment technologies has somehow been ineffective to completely remove heavy metals from wastewater, which causes secondary issues (Chong et al., 2010). For example, chlorination produces hazardous by-products, while chemical precipitation generates large amount of sludge and its disposal causes long-term environmental nuisance (Aziz et al., 2008). Hence, a simplistic alternative is necessary to either remove or convert heavy metal cations into non-toxic forms, mainly to combat their ceaseless accumulation in the environment.

Photocatalysis employs semiconductors (or photocatalysts), such as titanium dioxide (TiO₂) and zinc oxide (ZnO) for the oxidation of various organic compounds. These photocatalysts are able to destroy and mineralize compounds, such as pesticides, phenols, chlorophenols, dyes, and pharmaceuticals to water, CO₂, and minerals (Zhou et al., 2011, Fenoll et al., 2012, Kanakaraju et al., 2015a, Zhang et al., 2016). The application of photocatalysis for heavy metals or inorganic species is based on reduction. Metal removal has been reported to be dependent on the standard reduction potential of the metals for the reduction to take place (Kabra et al., 2004). The properties accounting for the wide use of TiO₂ are its low cost, photostability, commercial availability and low toxicity (Carp et al., 2004). The band gap energy of anatase TiO₂ is 3.2 eV (Bhatkhande et al., 2001). ZnO has a comparable band gap to TiO₂, which makes it an equally efficient photocatalyst. However, it is viewed less favourably compared to TiO₂. Indeed, studies have demonstrated ZnO as being a better photocatalyst in photocatalytic degradation compared to TiO₂ due to its broader absorption within the solar spectrum (Sakthivel et al., 2003, Elmolla and Chaudhuri, 2010).

Numerous studies have reported the application of TiO₂ photocatalysis for the removal of metals from contaminated water (Chen and Ray, 2001, Wang et al., 2004, López-Muñoz et al., 2011, Satyro et al., 2014). However, studies on ZnO-mediated photocatalysis for heavy metal removal are rather limited (Qamar et al., 2011, Singh et al., 2013). Application of TiO₂ and ZnO in the suspension form is efficient for photocatalytic removal of heavy metal cations. Such an approach however, requires a post recovery method at the end of the treatment process. Post-filtration process and catalyst recycling can be expensive and time consuming. Therefore, supported photocatalysts are more desirable and practical to remove heavy metals as they can be easily separated from the solution after the treatment. Among the solid supports reported thus far include zeolites, chitosan, silica, quartz, activated carbon, fibreglass, and polymer (Kanakaraju et al., 2015b).

Alginate (Alg), the salt of alginic acid is a natural polymer, which is extracted from brown seaweeds and widely used as a component for entrapment. It is cheap, non-toxic, efficient and often used as a polymeric matrix (Kimling and Caruso, 2012). The most abundant functional group found in the alginate polymer is the carboxylic groups, which enhances the adsorption of metal ions (Fourest and Volesky, 1995). Alg is capable of forming stable hydrogels, which can be used as a catalyst support (Papageorgiou et al., 2012).

Calcium alginate (CaAlg) gel has been chosen in this study because it is effective, low cost, can easily be prepared and has an open lattice structure for the fast diffusion of metal ions (Gopalakannan and Viswanathan, 2015, Kimling and Caruso, 2012). The feasibility of using CaAlg as the support in the removal of heavy metals and dyes has been reported. A study by Showky (2011) has revealed that a composite of Alg-montmorillonite (clay) material has demonstrated high removal efficiency of Pb. CaAlg encapsulated iron-based nanoparticles (CaAlg–Ni/Fe beads) have increased the removal efficiency of Cu(II) from 83.9% to 86.7% compared to bare Ni/Fe (Kuang et al., 2015). Additionally, TiO₂-immobilized Ca–Alg beads were investigated for the degradation of methylene blue (Albarelli et al., 2009). Their study has revealed that recycled TiO₂–CalAlg beads were able to enhance the degradation process, due to the increased roughness of the beads upon repeated usage. Another study has revealed that CaAlg polymer fibres have demonstrated high efficiency in removing methyl orange from water (Papageorgiou et al., 2012). This present study aims to shed new light on the immobilization of hybrid photocatalysts, TiO₂ and ZnO on CaAlg gel for the removal of copper (Cu), the chosen heavy metal cation. Numerous studies have been developing nanocomposite materials as aforementioned. However, to the best of our knowledge, no other studies have dealt with CaAlg beads immobilized with TiO₂/ZnO mixture for pollutant removal.

Cu has been chosen as the heavy metal cation in the present study owing to several reasons. Cu(II) is one of the major metal cations that can be found in industrial wastewater. Major industrial activities that contributes to the release of Cu(II) include electroplating, paints and dyes, fertilizers, petroleum refining, and chlor-alkali (Shrivastava, 2009). Cu usually exists in the environment as Cu(II) but can also be found in the form of Cu(I). Cu can cause serious illnesses, such as hematemesis, convulsions, Wilson's disease, and even fatality (Fu and Wang, 2011).

This study aims to investigate the efficiency of TiO₂/ZnO–CaAlg beads for the removal of Cu(II) in aqueous solution. The effects of initial Cu(II) concentration, molar ratios of TiO₂/ZnO, initial solution pH, and the recyclability of the TiO₂/ZnO–CaAlg beads on the amount of Cu(II) removal were investigated.