Photocatalytic degradation and rate constant prediction of chlorophenols and bisphenols by H₃PW₁₂O₄₀/GR/TiO₂ composite membrane

Highlights

• The HPW/GR/TiO₂ composite with excellent photocatalytic activity was synthesized.

• The removal rates of chlorophenols and bisphenols were improved.

• The HPW/GR/TiO₂ membrane exhibited good stability and reusability.

• The QSAR model was successfully used to predicted the k values of phenolics.

Abstract

Photocatalysis is a promising approach to remove highly toxic and refractory aromatics pollutants. However, developing highly active photocatalyst is a long-standing challenge for pollutant degradation. This study addressed this challenge by developing GR (graphene)/TiO₂ and HPW (H₃PW₁₂O₄₀)/GR/TiO₂ membranes by sol-gel method. The removal efficiencies of HPW/GR/TiO₂ (the doping of 1.0% HPW) membrane for chlorophenols (including o-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol) and bisphenols (such as Bisphenol A, Bisphenol AP, Bisphenol AF, and Bisphenol S) were up to 97.02–82.71% and 93.28–68.63% with simulated sunlight radiation for 5 h, respectively. Compared with GR/TiO₂ composite membrane, HPW/GR/TiO₂ remarkably accelerated the formation rates of O₂⁻ and OH, due to the simultaneous transfer of photo-generated electrons (generated by TiO₂) to GR and HPW. In addition, the activity of the HPW/GR/TiO₂ membrane did not decline noticeably after 10-time recycle. Furthermore, the photocatalytic degradation reaction rate constants (k) of phenols by HPW/GR/TiO₂ membrane were calculated, and those for other chlorophenols and bisphenols were predicted using a quantitative structure-activity relationship model. The HPW/GR/TiO₂ membrane developed in this study poses high potential as an ideal photocatalyst for removal of phenolic pollutants in wastewater.

Introduction

Chlorophenols (CPs) are extensively used chemicals in daily life and their amounts present in wastewater are considerable (Prashanthakumar et al., 2018). CPs can severely affect human nervous and respiratory systems and cause serious hazards to human health. Odorous CPs in the environment have the persistence, toxicity, and carcinogenic characteristics (Fang et al., 2019; Yan et al., 2018a, 2018b). Bisphenols (BPs), a type of endocrine disrupting chemicals (EDCs), have attracted wide concern in the last few decades owing to its widespread application in industry (Romena et al., 2020). In recent years, BPs have caused various environmental problems including indoor dust, surface water, sediment, and wastewater (Shun et al., 2019). Moreover, the treated effluent from sewage treatment plants is a major source of BPs entering the aquatic environment (Salgueiro et al., 2019).

Titanium dioxide (TiO₂) has been used as a photocatalyst, due to its non-toxicity, high chemical activity, long-term stability, and relatively low cost (Fujishima et al., 2000; Yu et al., 2018). However, the rapid recombination of e-h⁺ pairs is one key factor limiting the photocatalytic activity of TiO₂. An effective way to avoid the recombination of the e⁻-h⁺ pair is to couple TiO₂ with other materials (Sánchez-Rodríguez et al., 2018; Khaki et al., 2017). Graphene (GR) could increase the separation efficiency of e⁻-h⁺ pair as well as expand the optical absorption range of TiO₂ to the visible light region (Perera et al., 2012; Wang et al., 2013; Ma et al., 2016; Naderi et al., 2017; Martins et al., 2018; Ton et al., 2018; Xu et al., 2018), and enhance the photocatalytic activity of GR/TiO₂. Furthermore, POM (Polyoxometalate) has unique nanoscale and environmentally friendly transition metal-oxygen cluster, and exhibits superior photochemistry and reversible redox properties (Zheng et al., 2018). POM is a promising building block in the photocatalytic system and photovoltaic devices owing to the electronic transmission and storage. As a typical POM, H₃PW₁₂O₄₀ (HPW) could be an excellent candidate for delaying the e⁻-h⁺ recombination on the surface of TiO₂ and improve the photocatalytic activity due to its intrinsic electronic attribution (Niu et al., 2018; Liu and Qu, 2017). Based on the structures and properties of TiO₂, GR, and H₃PW₁₂O₄₀, a HPW/GR/TiO₂ composite could be developed for advanced photocatalytic activity.

Due to the continuous emergence of new pollutants, it is not realistic to investigate their degradation individually by using photocatalysis. Therefore, we predict the photocatalytic degradation of pollutants by models. Quantitative structure-activity relationship (QSAR) is efficient at correlating molecular structures of organic compounds with their “reactivity”. However, QSAR has been limited to predict the toxicity and environmental process parameters of organic pollutants (Peric et al., 2015; Wang et al., 2015; Heo et al., 2019), while the prediction of the photocatalytic degradation reaction rate constant (k) values of pollutants has rarely been reported. Therefore, using the QSAR model established by the k values of pollutants to predict the k values of homologous pollutants is more practical.

In this work, GR was prepared using grass according to our previous study (Ma et al., 2016). GR/TiO₂ membranes were fabricated by sol-gel method that mixed the GR into TiO₂ solution. HPW/GR/TiO₂ membranes were prepared by doping HPW on GR/TiO₂ and loaded on the quartz plate. The photocatalytic degradations of GR/TiO₂ and HPW/GR/TiO₂ membranes for CPs (including O-chlorophenol, 2,4-dichlorophenol, 2,4,6- trichlorophenol, and pentachlorophenol) and BPs (such as Bisphenol A, Bisphenol AP, Bisphenol AF, and Bisphenol S) were also investigated. In addition, the mechanism of the photocatalytic degradation of phenolic pollutants using HPW/GR/TiO₂ was explored. Moreover, a QSAR model was improved to calculate the photocatalytic degradation reaction rate constants of HPW/GR/TiO₂ membranes for phenolic pollutants and predict the characteristic of photocatalytic degradation of it for other CPs and BPs.