Iodinated X-ray contrast media (ICM) were investigated as a source of iodine in the formation of iodo-trihalomethane (iodo-THM) and iodo-acid disinfection byproducts (DBPs), both of which are highly genotoxic and/or cytotoxic in mammalian cells. ICM are widely used at medical centers to enable imaging of soft tissues (e.g., organs, veins, blood vessels) and are designed to be inert substances, with 95% eliminated in urine and feces unmetabolized within 24 h. ICM are not well removed in wastewater treatment plants, such that they have been found at elevated concentrations in rivers and streams (up to 100 μg/L). Naturally occurring iodide in source waters is believed to be a primary source of iodine in the formation of iodo-DBPs, but a previous 23-city iodo-DBP occurrence study also revealed appreciable levels of iodo-DBPs in some drinking waters that had very low or no detectable iodide in their source waters. When 10 of the original 23 cities' source waters were resampled, four ICM were found--iopamidol, iopromide, iohexol, and diatrizoate--with iopamidol most frequently detected, in 6 of the 10 plants sampled, with concentrations up to 2700 ng/L. Subsequent controlled laboratory reactions of iopamidol with aqueous chlorine and monochloramine in the absence of natural organic matter (NOM) produced only trace levels of iodo-DBPs; however, when reacted in real source waters (containing NOM), chlorine and monochloramine produced significant levels of iodo-THMs and iodo-acids, up to 212 nM for dichloroiodomethane and 3.0 nM for iodoacetic acid, respectively, for chlorination. The pH behavior was different for chlorine and monochloramine, such that iodo-DBP concentrations maximized at higher pH (8.5) for chlorine, but at lower pH (6.5) for monochloramine. Extracts from chloraminated source waters with and without iopamidol, as well as from chlorinated source waters with iopamidol, were the most cytotoxic samples in mammalian cells. Source waters with iopamidol but no disinfectant added were the least cytotoxic. While extracts from chlorinated and chloraminated source waters were genotoxic, the addition of iopamidol enhanced their genotoxicity. Therefore, while ICM are not toxic in themselves, their presence in source waters may be a source of concern because of the formation of highly toxic iodo-DBPs in chlorinated and chloraminated drinking water.
The transformation of the iodinated X-ray contrast media (ICM) iopamidol, iopromide, iohexol, iomeprol, and diatrizoate was examined in purified water over the pH range from 6.5 to 8.5 in the presence of sodium hypochlorite, monochloramine, and chlorine dioxide. In the presence of aqueous chlorine, only iopamidol was transformed. All other ICM did not show significant reactivity, regardless of the oxidant used. Chlorination of iopamidol followed a second order reaction, with an observed rate constant of up to 0.87 M(-1) s(-1) (±0.021 M(-1) s(-1)) at pH 8.5. The hypochlorite anion was identified to be the reactive chlorine species. Iodine was released during the transformation of iopamidol, and was mainly oxidized to iodate. Only a small percentage (less than 2% after 24 h) was transformed to known organic iodinated disinfection byproducts (DBPs) of low molecular weight. Some of the iodine was still present in high-molecular weight DBPs. The chemical structures of these DBPs were elucidated via MSn fragmentation and NMR. Side chain cleavage was observed as well as the exchange of iodine by chlorine. An overall transformation pathway was proposed for the degradation of iopamidol. CHO cell chronic cytotoxicity tests indicate that chlorination of iopamidol generates a toxic mixture of high molecular weight DBPs (LC50 332 ng/μL).
Chlorpyrifos (CP) was used as a model compound to develop experimental methods and prototype modeling tools to forecast the fate of organophosphate (OP) pesticides under drinking water treatment conditions. CP was found to rapidly oxidize to chlorpyrifos oxon (CPO) in the presence of free chlorine. The primary oxidant is hypochlorous acid (HOCl), kr = 1.72 (+/-0.68) x 10(6) M(-1)h(-1). Thus, oxidation is more rapid at lower pH (i.e., below the pKa of HOCl at 7.5). At elevated pH, both CP and CPO are susceptible to alkaline hydrolysis and degrade to 3,5,6-trichloro-2-pyridinol (TCP), a stable end product. Furthermore, hydrolysis of both CP and CPO to TCP was shown to be accelerated in the presence of free chlorine by OCl-, kOCl,CP = 990 (+/-200) M(-1)h(-1) and kOCl,CPO = 1340 (+/-110) M(-1)h(-1). These observations regarding oxidation and hydrolysis are relevant to common drinking water disinfection processes. In this work, intrinsic rate coefficients for these processes were determined, and a simple mechanistic model was developed that accurately predicts the temporal concentrations of CP, CPO, and TCP as a function of pH, chlorine dose, and CP concentration.
The speciation of aqueous free chlorine above pH 5 is a well-understood equilibrium of H2O + HOCl <==> OCl- + H3O+ with a pKa of 7.5. However, the identity of another very potent oxidant present at low pH (below 5) has been attributed by some researchers to Cl2(aq) and by others to H2OCl+. We have conducted a series of experiments designed to ascertain which of these two species is correct. First, using Raman spectroscopy, we found that an equilibrium of H2O + H2OCl+ <==> HOCl + H3O+ is unlikely because the "apparent pKa" increases monotonically from 1.25 to 2.11 as the analytical concentration is increased from 6.6 to 26.2 mM. Second, we found that significantly reducing the chloride ion concentration changed the Raman spectrum and also dramatically reduced the oxidation potency of the low-pH solution (as compared to solutions at the same pH that contained equimolar concentrations of Cl- and HOCl). The chloride ion concentration was not expected to impact an equilibrium of H2O + H2OCl+ <==> HOCl + H3O+, if it existed. These observations supported the following equilibrium as pH is decreased: Cl2(aq) + 2H2O <==> HOCl + Cl- + H3O+. The concentration-based equilibrium constant was estimated to be approximately 2.56 x 10(-4) M2 in solutions whose ionic strengths were approximately 0.01 M. The oxidative potency of the species in low pH solutions was investigated by monitoring the oxidation of secondary alcohols to ketones. These and other results reported here argue strongly that Cl2(aq) is the correct form of the potent low-pH oxidant in aqueous free-chlorine solutions.
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