Simultaneous and synergistic effect of heavy metal adsorption on the enhanced photocatalytic performance of a visible-light-driven RS-TONR/TNT composite

Highlights

• Rh–Sb TONR/TNT composite was synthesized by Molten salt flux method.

• The composite removed heavy metals and degraded dye and BPA under visible light.

• Adsorption capacity for heavy metal ions was Pb(II) > Cd(II) > Cu(II) > Zn(II).

• The adsorbed metals enhanced photocatalytic degradation and Cu was most effective.

• Cu 2p_3/2 states support an active role of Cu⁺ in photocatalytic generation of ROS.

Abstract

A hydrothermally synthesized rhodium/antimony co-doped TiO₂ nanorod and titanate nanotube (RS-TONR/TNT) composite was prepared for removal of heavy metals and organic pollutants from water under visible light irradiation. The composite provides the dual function of simultaneous adsorption of heavy metal ions and enhanced degradation of dissolved organic compounds. Acid treatment transformed titanate nanotubes to irregular tubular structures distributed homogeneously over untransformed RS/TONRs. Synergistic removal and degradation was studied with various heavy metals, Orange (II) dye, and Bisphenol A. The adsorption capacity of the composite for heavy metal ions was Pb(II) > Cd(II) > Cu(II) > Zn(II). The adsorbed metals enhanced photocatalytic degradation of the organic pollutants, but Cu was most effective, with degradation exceeding 70% for the dye and 80% for Bisphenol A after 5 h of treatment. Photocatalytic activity was enhanced more by adsorption than photodeposition of Cu ions. A decrease in XRD rutile peak intensity with adsorbed metal indicates a change in crystallinity which may enhance photocatalytic activity. Thick and bulging nanostructures in FE-SEM images signify ion adsorption within titanate pores. BET analysis indicated titanate nanotubes with adsorbed metal are mesoporous but their tubular structure persists. XPS showed more active Cu 2p_3/2 states under light, supporting an active role of Cu⁺ in photocatalytic ROS generation. Detection of ROS and Cu species using methanol, EDTA, pCBA, and benzoic acid probes provided strong evidence for degradation via a charge transfer mechanism. Findings demonstrate the potential of the RS-TONR/TNT composite for simultaneous removal of heavy metals and degradation of organic pollutants.

Introduction

Removal of reactive elements from wastewater has attracted much attention, because of non-biodegradability, potential carcinogenicity and other health risks, as well as adverse ecological impacts (Garg et al., 2004; Konstantinou and Albanis, 2004; Nishiyama et al., 2016; Rafatullah et al., 2010; Wu and Tseng, 2008). The practice of discharging industrial effluent contaminates water with many hazardous substances, including heavy metal ions, radioactive ions, dyes, and other organic pollutants. The toxicity and persistence of heavy metals causes great environmental concern (Kabra et al., 2008) and one of the most common organic pollutants in the aquatic environment is dye. Heavy metals and dyes are often present together in wastewater and coexist in contaminated environments, posing threats to human health and the environment (Parshetti and Doong, 2011). Such co-contamination presents many challenges for effective waste management and remediation.

Recent studies have shown the utility of titanate nanomaterials for environmental remediation (Dhandole et al., 2016; Konstantinou and Albanis, 2004). Since the discovery of structurally modified titanium-based materials, 1D titanate nanotubes (TNTs) have been demonstrated to be a promising nanostructured absorbent for removing heavy metals and various toxic elements (Dhandole et al., 2018a; Nishiyama et al., 2016). The advantages of the large specific surface area and high pore volume of synthesized titanate nanotubes afford a simple, cost-effective, and environment-friendly technology (Chen et al., 2010; Xiong et al., 2011). These properties make titanate nanotubes (TNTs) useful not only as adsorbents to remove and recover heavy metals and toxic elements (Moon et al., 2000), but also for diverse applications such as in lithium ion batteries (Lan et al., 2005), hydrogen storage devices (Lim et al., 2005), ion exchange membranes (Feng et al., 2007; Sun and Li, 2003) and gas sensors (Han et al., 2007), as well as photocatalysts (Bavykin et al., 2006; Kim et al., 2018a).

Photocatalysis has emerged as an alternative technology to address pollution by organic compounds. Several studies have reported the use of modified TiO₂ nanomaterials in photocatalytic decomposition and degradation strategies for organic pollutants, including dyes (Aarthi et al., 2007; Doong and Tsai, 2015). However, the photocatalytic activity of TiO₂-based nanostructures is limited due to their wide optical bandgap energy that absorbs only in the short wavelength range of the UV spectrum, limiting the utility of TiO₂-based nanostructures for photocatalytic applications.

Previous reports have highlighted the effect of metal co-doping on reducing the optical bandgap energy of metal oxide semiconductors (e.g., TiO₂), which increases the charge separation of photogenerated electron-hole pairs and shifts the absorption wavelength toward the visible light region. However, reducing the bandgap energy is not sufficient for enhancing the performance of photocatalysts, due to the high recombination rate of photogenerated charge carriers (Dhandole et al., 2018b; Kim et al., 2018a, 2018b). Several studies have reported limiting the charge recombination rate by depositing metal, metal oxide, or metal sulfide nanoparticles on the photocatalyst (Hou et al., 2009; Wilson et al., 2012Yan et al., 2018; ). The deposited nanoparticles facilitate charge transfer, greatly enhancing photocatalytic performance for degradation of organic pollutants.

Mono-remediation approaches target either the recovery or degradation of pollutants in wastewater. Few reports are available on TiO₂-based dual remediation techniques for simultaneous removal of heavy metal ions and degradation of organic contaminants (Doong et al., 2012; Kyung et al., 2005). The challenge lies in the structural and/or optical properties of the nanomaterials that limit practical solutions for remediation of wastewater with multiple contaminants in an economic and environment-friendly way. To address these concerns, we previously designed an efficient TiO₂ nanorod and titanate nanotube (RS-TONR/TNT) photocatalyst for degradation of organic co-contaminants under visible light irradiation (Kim et al., 2018a, 2018b). However, bare RS-TONR/TNT alone did not show significant enhancement in photocatalytic activity. That was achieved by loading with copper ions, which enhances the catalytic reaction by improving charge transport in the composite. In the present study we eliminated the need for copper loading on RS-TONR/TNT by directly using the composite for simultaneous adsorption and degradation reactions.

The objective of the present study was to evaluate the capacity of the RS-TONR/TNT composite to remove various heavy metals and explore a synergistic effect of metal adsorption on the photocatalytic degradation of organic pollutants, using Orange (II) dye and Bisphenol A as test compounds. Because the photocatalytic performance of RS-TONR/TNT was maximized with adsorbed Cu, experiments were conducted with Cu to investigate associated mechanisms. Methanol (MeOH), EDTA, p-chlorobenzoic acid (pCBA), and benzoic acid (BA) were used as probes to detect reactive oxygen species (ROS) and Cu species, and a charge transfer mechanism was proposed.