Contents lists available at ScienceDirect Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv Understanding the coagulant activity of zirconium oxychloride to control THMs formation using response surface methodology Tanwi Priya, Prem Prakash, B.K. Mishra ⁎ Department of Environmental Sciences and Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand 826004, India ARTICLE INFO Keywords: Absorbance slope Index Cancer Risk Assessment Natural Organic Matter Response Surface Methodology Trihalomethanes Zirconium oxychloride ABSTRACT In the present study, impact of coagulant activity of zirconium oxychloride and aluminium sulphate on the kinetics of chlorine consumption and trihalomethanes (THMs) formation has been delineated. Zirconium Oxychloride showed rapid chlorine decay within the first 30 min, which further achieved steady rate after 60 min, but in case of aluminium sulphate chlorine consumption has been increased drastically throughout the chlorine decay. Zirconium oxychloride has effectively reduced significant amount of slow reducing agents (SRA) as well as fast reducing agents (FRA), which correspond to the rate of reduction in phenolic groups from water enriched with Natural Organic Matter (NOM) which eventually decreased trihalomethane mediated cancer risk by ~ 2.3 times among adults as compared to aluminium sulphate. Result depicts the outstanding coagulant activity of zirconium oxychloride as it tends to surpass aluminium sulphate in reducing NOM “measured as Absorbance Slope Index (ASI)” and phenol by 57.98% and 49.02% respectively from NOM enriched chlorinated water, which also resembles the THMs removal trend observed during cancer risk assessment. 1. Introduction Over the past years, the chlorine-based disinfection process has been the most effective public measure for the control of microbial con- taminants in drinking water across the world (Xie, 2016; How et al., 2017; Priya and Mishra, 2017). However, the reactivity of chlorine towards aromatic moieties of NOM has elicited deleterious environ- mental impact due to the formation of carcinogenic chlorinated by products such as THMs and their associated cancer risks (Priya and Mishra, 2017). In surface water, THMs is the most dominant chlorinated by product formed due to abundance of NOM deposited through various anthro- pogenic activities (Chu and Li, 2002). Chloroform, Dibromo- chloromethane (DBCM), Bromodichloromethane (BDCM) and Bromo- form are four components of THMs group but chloroform is the most dominant species formed in surface water (Uyak and Toroz, 2005). USEPA (1999) has classified chloroform, BDCM and bromoform as probable human carcinogen group, which has also been validated by several researchers. Literature suggests that THMs mediated cancer risk has surpassed USEPA reference limit by 10–100 times in many South East Asian countries (Priya and Mishra, 2017). During disinfection, chlorine exists either as acids like hypochlorous acid (HOCl) or anions (OCl - ), which tend to react with aromatic moi- eties of NOM to form THMs. However, the second order reaction between chlorine and NOM gets catalysed in the presence of stronger nucleophiles such as hydroxyl and amino groups (Minear and Amy, 1996; Cowman and Singer, 1996). During chlorination, chlorine gets hydrolysed to form HOCl (Eq. (1)) which further dissociates to form hydrogen and hypochlorite ions (Eq. (2)): + ⇆ + + + − Cl HO HOCl H Cl 2 2 (1) ⇆ + + − HOCl H O Cl (2) Several researchers have demonstrated the role of substitutions re- action and oxidation (carbon bonds) while delineating the reaction pathway between halogens and NOM (Westerhoff et al., 2004). The kinetics of chlorine consumption by aromatic moieties of NOM to form THMs depend upon the availability of FRA and SRA in surface water (Zhan et al., 2010). A chlorine decay model depicted occurrence of two prominent phenomena, namely Initial Rapid Decay and Slow Continuing Decay due to the presence of FRA and SRA respectively during chlorine consumption by reactive sites of NOM in water as shown in (Eqs. (3) and (4)) (Zhan et al., 2010). + → + Cl FRA Inert products THMs 2 (3) + → + Cl SRA Inert products THMs 2 (4) Where Cl 2 and SRA represent free chlorine available in water. https://doi.org/10.1016/j.ecoenv.2018.04.036 Received 29 November 2017; Received in revised form 14 April 2018; Accepted 18 April 2018 ⁎ Corresponding author. E-mail address: brijesh@iitism.ac.in (B.K. Mishra). Ecotoxicology and Environmental Safety 159 (2018) 28–37 0147-6513/ © 2018 Elsevier Inc. All rights reserved. T