Sulfate radical-based degradation of polychlorinated biphenyls: Effects of chloride ion and reaction kinetics

Abstract

Advanced oxidation processes (AOPs) based on sulfate radical (SO₄⁻) have been recently used for soil and groundwater remediation. The presence of chloride ion in natural or wastewater decreases the reactivity of sulfate radical system, but explanations for this behavior were inconsistent, and the mechanisms are poorly understood. Therefore, in this paper we investigated the effect of chloride ion on the degradation of 2,4,4′-CB (PCB28) and biphenyl (BP) by persulfate, based on the produced SO₄⁻. The results showed that the presence of chloride ion greatly inhibited the transformation of PCB28 and BP. Transformation intermediates of BP were monitored, suggesting that the chloride ion can react with SO₄⁻ to produce chlorine radical, which reacts with BP to generate chlorinated compounds. To better understand the underlying mechanisms of these processes, a kinetic model was developed for predicting the effect of chloride ion on the types of radical species and their distributions. The results showed that chloride ion could influence the selectivity of radical species and their distribution, and increase the concentration of the sum of radical species. In addition, the second-order rate constants of sulfate radical with PCBs were determined, and quantum-chemical descriptors were introduced to predict the rate constants of other PCBs based on our experimental data.

Introduction

Polychlorinated biphenyls (PCBs) are a class of 209 congeners that were extensively used in industrial applications during 1929 to early 1970s due to their low flammability and electrical insulation characteristics [1]. Numerous water bodies in the world have been polluted by PCBs that pose long-term risks to public health and wildlife [2], [3]. Because of the magnitude of the problem, the remediation of PCBs contaminated sites is imperative. Conventional treatment methods, such as biological processes, are inefficient because of the high toxicity of PCBs to the microorganisms [4]. Certain technologies that demonstrated satisfactory results for the removal of PCBs from aqueous phase by adsorption to surfaces, such as activated carbon, require further post-processing treatment for the complete destruction of the adsorbate PCBs [5]. Other processes, such as incineration, require a large amount of fuel and can lead to the formation of highly toxic by-products, including polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-furans [6].

Recently, increasing attention has been paid to the sulfate radical (SO₄⁻) due to its high efficiency of mineralization of organic pollutants [7], [8], [9], [10], [11]. SO₄⁻ is a strong oxidant with a redox potential of 2.5–3.1 V [12], which is similar to the oxidizing power of hydroxyl radical (OH) with a redox potential of 1.8–2.7 V, depending on the solution pH [13]. The activation of persulfate (PS) and peroxymonosulfate (PMS) is commonly used to generate sulfate radical, and both of them can be activated by UV and transition metals [14]. Furthermore, it has been reported that the persulfate could be decomposed to generate sulfate radical without activators at ambient temperature, although its rate is relatively slow, compared to the activation of PS by transmit metals [15]. In contrast, PMS cannot generate sulfate radical species without activators [16]. Therefore, persulfate was used as oxidant in this study. In addition, persulfate has been applied to degrade various organic pollutants, including chlorinated ethylens [17], chlorophenols [7], [18], cyanotoxins [19], [20], polycyclic aromatic hydrocarbons [21] and numerous volatile organic compounds [22]. However, the degradation of PCBs by persulfate has not been extensively investigated, and until recently, little information has been available. The knowledge about second order rate constants of sulfate radical with PCBs would provide a powerful tool to optimize the degradation conditions. Unfortunately, only limited kinetic data are available for the oxidation of PCBs by sulfate radicals. Therefore, the intrinsic reaction kinetic constants of PCBs with sulfate radicals are required for further studies.

Chloride ion (Cl⁻) is ubiquitous in natural waters, and Table S1 showed Cl⁻ concentrations in different water matrices [23], [24]. In addition, Cl⁻ is a potential reaction product when chlorinated organic contaminants are oxidized by SO₄⁻, and the reactions of Cl⁻ with SO₄⁻ or OH radicals forming chlorine radicals with unique chemistry [18]. The mechanism of SO₄⁻ reaction with Cl⁻ is described in Eq. (1) [25]. The chlorine radical (Cl) can react with additional chloride (Cl⁻), forming the dichloride radical (Cl₂⁻) according to Eq. (2) [25].

SO₄⁻ + Cl⁻ → Cl + SO₄²⁻

Cl → Cl⁻ → Cl₂⁻

Therefore, Cl⁻ plays an important role in the degradation of pollutants in the processes that involve sulfate radicals. However, only few studies investigated the influence of Cl⁻ on the degradation of pollutants by sulfate radical. For example, Grebel et al. [24] reported the effect of halide ions on the degradation of organic contaminant by hydroxyl radical, and found that the hydroxyl radicals were scavenged by halides and converted to radical reactive halogen species (RHS) that participated in contaminant destruction.

Furthermore, the conclusions of several studies that reported the effect of Cl⁻ on the degradation varied, which led to some apparently conflicting findings about the effect of Cl⁻. For example, Chan et al. [26] studied the degradation of atrazine by the activation of peroxymonosulfate, and found that Cl⁻ exhibited inhibitory effects in the process; they deduced the reason was likely due to the scavenging of SO₄⁻ and the formation of weaker radical species such as Cl and Cl₂. In contrast, Wang et al. [27] found that the degradation of azo dyes by sulfate radical was significantly inhibited in the presence of Cl⁻ (0∼10 mM), while the degradation of azo dyes was enhanced at high Cl⁻ concentration (>100 mM). In addition, Liang et al. [28] used persulfate for the degradation of trichloroethylene (TCE) and found that the effect of Cl⁻ on the degradation of TCE was not significant when Cl⁻ concentration was below 200 mM. Therefore, to better understand these disputes, it is necessary to investigate the potential mechanisms of the effect of Cl⁻ on the degradation of pollutants in sulfate radical system.

In addition, the transformation products of PCBs by sulfate radical as well as the proposed transformation pathways were reported in our previous studies [29]. Therefore, the main objective of this study was to investigate the effect of Cl⁻ on the transformation of PCBs by persulfate based on sulfate radical. A kinetic model was used to predict the effect of Cl⁻ on the distribution of radicals and species at different pH, which is important for understanding the effect of Cl⁻ concentration on the degradation of the target pollutants. Electron paramagnetic resonance (EPR) technique was employed to confirm the kinetic model results. Furthermore, the second order rate constants of sulfate radical with PCBs were studied, and quantum-chemical descriptors were introduced to predict the rate constants of other PCBs.