Removal of 4-chlorophenol, bisphenol A and nonylphenol mixtures by aqueous chlorination and formation of coupling products

Highlights

• Transformation of mixtures of 4-CP, BPA and NP by chlorination was investigated.

• Effects of free chlorine dosage, solution pH and humic acid were examined.

• Large abundance of cross-coupling products generated in mixture solutions.

• Theoretical analysis of DFT calculations rationalized the reaction mechanism.

Abstract

Phenolic endocrine disrupting chemicals (EDCs) such as bisphenol A (BPA), 4-chlorophenol (4-CP) and nonylphenol (NP) are the most commonly detected pollutants in natural environments. The transformation of mixtures of the three phenolic pollutants during chlorination process was studied for the first time in this work. Single phenol can be effectively removed by free chlorine via electrophilic substitution and electron transfer reactions, with the generation of chlorinated phenols and self-coupling products. In mixture solution, the apparent second-order rate constants (k_app) of 4-CP, BPA and NP at pH 8.0 was increased by 8.6%, 25% and 16%, respectively, as compared to the degradation of single-compound. According to the products identification and density functional theory calculations, the cross-coupling process can occur more easily than the self-coupling reaction, which might accelerate the removal of phenolic mixtures. In addition, humic acid (HA) exerts some inhibitory effect on the degradation of phenolic compounds, due to the competition for chlorine consumption and the reduction of phenoxy radicals to parent phenols. These findings shed light on the environmental fate of phenolic mixtures during aqueous chlorination process, which provide new information for the application of chlorination in water and wastewater treatment.

Introduction

With the development of industrialization in recent years, large amounts of endocrine disrupting chemicals (EDCs) were discharged into the environment, which has received extensive attention. Among these contaminants, 4-chlorophenol (4-CP), bisphenol A (BPA) and nonylphenol (NP) are typical phenolic environmental hormones that have been widely used in chemical synthesis [1], [2], [3], [4]. 4-CP is commonly found in industrial wastewater effluents from the production of pesticides, paper and pulp, and petrochemical products [5], [6]. BPA is mainly incorporated into polycarbonate, epoxy resin, polysulfone resin and many other polymer materials, with an annual production volume of more than six billion pounds before prohibition [7]. NP is widely used as a raw material for the manufacture of nonionic surfactants and lubricant additives, and its major source in aquatic environment includes the oxidation of nonylphenol ethoxylates (NPE) which have widespread applications in the textile industry for printing, dyeing and cleaning [3], [8]. These compounds are persistent, bioaccumulative and can mimic the effects of estrogen, thus causing female precocity, decreased sperm count and prostate growth even at extremely low concentrations [9]. Currently, the environmental presence of these chemicals is even more concerning, because they mostly appear as a complex mixture, which usually lead to synergistic toxic effects [10].

Chlorine is the most commonly used reagent in drinking water disinfection and municipal wastewater treatment. A wide range of organic chemicals including phenolic compounds could be degraded by free chlorine [11]. For instance, Zheng et al. [12] have found that bisphenol F (BPF) could react with chlorine at a pseudo-first-order rate constant (k_obs) of 0.2162 min⁻¹ under pH 8.0 and 25 °C and the chlorination reaction is dependent on free available chlorine. The apparent second-order rate constants (k_app) for chlorination between phenol and aqueous chlorine vary between 0.02 and 0.52 M⁻¹·s⁻¹, for the reaction phenolate and HOCl between 8.46 × 101 and 2.71 × 10 4 M⁻¹·s⁻¹ [13]. Compared with traditional disinfectants such as chlorine gas and chlorine dioxide, sodium hypochlorite has gradually caused global attention as it is more efficient and generates less toxic by-products in the process. To date, large efforts have been devoted to the chlorination of single pollutants during water and wastewater treatment, leaving the simultaneous transformation of multi-component pollutants less studied [14]. In fact, the phenolic substances do coexist in aquatic environments. It was reported that the average concentration of NP and BPA in the Tuojiang River of China is 33.1 ng/L and 47.2 ng/L, respectively [15]. Thus, it is of great significance to investigate the reaction behaviors of phenolic mixtures with free chlorine.

The primary objective of this research was to investigate the chlorination process of 4-CP, BPA and NP mixture during water treatment, with special emphasis on the cross-coupling reaction mechanisms. First, the reaction kinetics of single compound were studied over a pH range of 5.0–11.0 with different concentrations of free chlorine (10.0–60.0 μM) to determine the species-specific second-order rate constants. Second, the transformation of three phenols in mixture solutions was evaluated to explore the effect of co-existing pollutants, and the reaction rate constants in presence of dissolved organic matter (DOM) were also compared. Third, the transformation products generated during the reaction were determined by high-resolution mass spectrometry, and the reaction pathways were then tentatively proposed. Finally, the reaction mechanisms were elucidated by quantum chemical computations (atom partial charges, spin densities, Gibbs free energy change, transition states, and activation energies). This work would shed light on the behaviors of multi-component phenolic pollutants during aqueous chlorination process.