Chlorination decreases acute toxicity of iodophenols through the formation of iodate and chlorinated aliphatic disinfection byproducts

Highlights

• HOCl was reactive with 2-iodophenol, 4-iodophenol and 2,4,6-triiodophenol

• Acute toxicity of iodophenols could be largely eliminated after chlorination

• Iodophenols were transformed into chlorinated aliphatic DBPs, iodinated-THMs and iodate

• The transformation products were verified with chemical standards

Abstract

Highly toxic iodinated phenolic by-products were frequently detected in the oxidative treatment and disinfection of iodine-containing water. Herein, it was found that three model iodinated phenolic disinfection byproducts (DBPs), 2-iodophenol, 4-iodophenol and 2,4,6-triiodophenol, were reactive with HOCl, and the reaction rate constants (at pH 7.0 and 25℃) were 1.86 ×10², 1.62 ×10² and 7.5 ×10¹ M⁻¹s⁻¹, respectively. When HOCl was in excess (HOCl/iodophenol = 40/1, [iodophenol]₀ = 20 μM), acute toxicity of water sample containing iodophenols could be largely eliminated (> 85%), with the conversion of iodophenols into stable and non-toxic iodate (IO₃⁻) and iodinated and chlorinated aliphatic DBPs. Besides IO₃⁻, seven kinds of aromatic intermediate products including iodophenols, chloroiodophenols, iodoquinones, chloroiodoquinones, chloroquinones, chlorophenols, and coupling products were detected. C-I bond of iodophenols was cleaved in the reaction and the resulted aromatic products were further transformed into chlorinated aliphatic DBPs [trichloromethane (TCM), trichloroacetic acid (TCAA), dichloroacetic acid (DCAA), and chloral hydrate (CH)] (mg/L level) and iodinated trihalomethanes (μg/L level). HOCl was effective for converting iodophenols into IO₃⁻ and less toxic chlorinated aliphatic DBPs. Considering that chlorine was widely used as disinfectant, transformation and toxicity alteration of emerging DBPs during chlorination/booster chlorination warrant further investigations.

Introduction

Disinfection is always the last defense for controlling waterborne diseases. Although other methods such as ozone oxidation and ultraviolet irradiation were effective for water disinfection, chlorination was cheap and easy-to-use (Cantor 1994, McGuire 2006). More importantly, the residual chlorine existing in treated water would maintain disinfection effect during water distribution process. A main shortage of chlorine disinfection/pre-oxidation is the formation of disinfection by-products (DBPs) (Dong et al. 2019, Liu et al. 2020a). HOCl is an electrophilic reagent and reactive with organics containing electron-rich moieties. As natural organic matter (NOM) ubiquitously existed in natural waters, formation of chlorinated DBPs raised public concerns due to their toxicity and potential health impacts (Allard et al. 2015, Ashbolt, 2004).

Accompanied with global warming and the rising of sea level, saline water intrusion may greatly impact quality of source water in coastal area (Ferguson and Gleeson 2012). Background halogen ions such as bromide and iodide would react with HOCl to generate active halogen species, which could further react with NOM with the formation of halogenated DBPs (, Criquet et al. 2012, Liu et al. 2018). Iodide exists in natural waters with concentration ranging from 0.4 µg/L to 100 µg/L (Plewa et al. 2008, Schwehr and Santschi 2003). In the oxidative treatment of iodide-containing water, iodide would be oxidized into active iodine (i.e. HOI and I₂). The active iodine could further react with NOM and other dissolved organics with the formation of iodinated DBPs (Bichsel and von Gunten 1999, 2000, Criquet et al. 2012, Liu et al. 2021, Sayess et al. 2020, Ye et al. 2013). Recent study showed that odorous iodinated by-products (μg/L level) even formed in the coagulation of surface water in Yangtze River delta, China, as FeCl₃ (coagulant) oxidized iodide into active iodine (HOI) and the HOI further reacted with NOM in raw water with the formation of iodinated aliphatic and phenolic by-products (Ding et al. 2019).

Phenolic groups and aromatic rings are the basic structure of NOM, and iodinated phenolic DBPs were more and more frequently detected during chlorination or chloramination of source water and wastewater (Gong et al. 2017, Liberatore et al. 2017, Liu et al. 2020b, Liu and Zhang 2014, Pan et al. 2016b, W. Parsons 1983, Yang and Zhang 2013, Yang et al. 2019). For example, 2,6-diiodo-4-nitrophenol (up to 3.9 ng/L) and 2,4,6-triiodophenol (up to 0.43 ng/L) were detected in tap water which used HOCl as disinfectant in Yangtze River delta in 2016 (Pan et al. 2016a). 4-Iodophenol (up to 0.1 nM) was detected in the chloramination of iodide-containing source water (Gong et al. 2017). Both in the chloramination of iodide-containing source water and in the chlorinated saline secondary sewage effluent, 2,4,6-triiodophenol was detected with up to 330.3 ng/L and 0.4 ng/L of concentration, respectively (Gong et al. 2017, Yang and Zhang 2013). Moreover, production of gas and oil through hydrofracturing raised concern about the pollution of source water with the subsurface injection of “produced water” containing halides. Enhanced formation of halogenated DBPs was observed in downstream drinking water treatment plants (Burgos et al. 2017, Harkness et al. 2015), and dozens of iodinated phenolic DBPs including 2-iodophenol, 4-iodophenol, 2,4,6-triiodophenol and iodomethylphenols were detected in chloraminated oil and gas wastewaters (Liberatore et al. 2017).

I-DBPs were more genotoxic and cytotoxic than their chlorinated and brominated analogues (Richardson et al. 2008), while iodinated phenolic DBPs were more toxic than iodinated aliphatic DBPs (Hu et al. 2018, Liu and Zhang 2014, Pan et al. 2016b). Cytotoxicity of 2,4,6-triiodophenol to human hepatoma cells HepG2 was 10.7 times higher than that of triiodomethane (TIM) (Hu et al. 2018). Developmental toxicity of iodinated phenolic DBPs to embryos of a polychaete P. dumerilii was 50 - 200 times higher than that of iodinated aliphatic DBPs (Pan et al. 2016b). Toxicity of 2,4,6-triiodophenol and 4-iodophenol to a marine alga was as much as 76.3 and 16.2 times higher than that of iodoacetic acid, respectively (Liu and Zhang 2014). Since the detection frequency of iodinated phenolic DBPs increased in tap water and wastewater, finding methods to eliminate their toxicity is important.

Previous studies reported the transformation of brominated and chlorinated phenolic DBPs during chlorination and the transformation of iodinated phenolic DBPs during chloramination (Gong et al. 2017, Hu et al. 2018, Jiang et al. 2020, Jiang et al. 2017, Nunez-Gaytan et al. 2010, Zhai and Zhang 2011). Chlorinated and brominated phenolic DBPs formed in the chlorination of source water and these phenolic DBPs acted as intermediate compounds to further react with HOCl to generate trihalomethanes and haloacetic acids (Jiang et al. 2020, Jiang et al. 2017, Nunez-Gaytan et al. 2010, Zhai and Zhang 2011). In order to control the formation of regulated DBPs, chloramine was used as disinfectant instead of HOCl. However, study reported that 2,4,6-triiodophenol was finally converted into iodinated aliphatic DBPs during chloramination treatment, and cytotoxicity of the water sample increased significantly after chloramination (Hu et al. 2018). In the chlorination of wastewater and source water, iodinated phenolic DBPs were often detected. So far, transformation of iodinated phenolic DBPs during chlorination including products, reaction pathways and the variation of toxicity had not been revealed. Exploring the underlying mechanism could provide useful information for optimizing chlorine disinfection and controlling its negative impacts. 2,4,6-Triiodophenol was one of the most frequently detected iodinated phenolic DBPs and 2-iodophenol and 4-iodophenol may experience electrophilic substitution by active iodine (HOI) to be converted into 2,4-diiodophenol and 2,4,6-triiodophenol. Meanwhile, 2-iodophenol and 4-iodophenol were also frequently detected iodinated phenolic DBPs (Gong et al. 2017, Liberatore et al. 2017). Herein, 2-iodophenol, 4-iodophenol and 2,4,6-triiodophenol were selected as model compounds to study their transformation mechanism in the reaction with HOCl in terms of reaction kinetics, transformation pathways, and variation of acute toxicity.