Unexpected Byproducts Formed During Oxidative Water Treatment of Organic Contaminants

Oxidative water treatment can be used to transform organic contaminants present in trace concentrations in industrial wastewater, municipal wastewater undergoing advanced treatment prior to its use as drinking water, and soil and groundwater impacted by hazardous waste sites. As part of oxidative water treatment applications, which are referred to as advanced oxidation processes (AOPs) for the treatment of industrial wastewater or recycled water, and in situ chemical oxidation (ISCO) for groundwater remediation, chemical oxidants such as hydrogen peroxide (H2O2) and peroxydisulfate (S2O82–) produce HO• and SO4•–, respectively when exposed to ultraviolet light, heat, catalysts, or transition metals. These radical species are non- selective and highly reactive with organic compounds found in contaminated water, such as petroleum hydrocarbons, organic solvents, pesticides, and pharmaceuticals. When oxidative reactions do not result in complete mineralization to CO2 and H2O, the transformation products of organic contaminants include stable compounds of greater polarity, mobility in groundwater, and even toxicity than their parent compounds. Incomplete oxidation of organic contaminants is a concern for drinking water from a public health perspective and poses challenges for the further use of chemical oxidation, an increasingly popular technology, for water treatment.Although oxidation of aromatic compounds by HO• and SO4•– has been studied for decades, the carbon mass of the reactant that has been lost in the initial phase of radical attack is not entirely accounted for by quantification of the typically monitored intermediates, such as oxygenated aromatic compounds (e.g., phenols and quinones) and organic acids (e.g. formic acid). Using mass spectrometry and nuclear magnetic resonance spectroscopy, we identified classes of stable transformation products (i.e., α,β-unsaturated aldehydes and organosulfates) that are not widely recognized. Their chemical properties make them difficult to measure with conventional analytical techniques. Although they are produced in low yields under conditions relevant to water treatment, these oxidation products have the potential to impact human health. Analysis of the initial products of HO• and SO4•– oxidation of benzene, toluene, ethylbenzene, and xylene isomers (BTEX) yielded a suite of ring cleavage products, including acetaldehyde, formic acid, 6-, 7- or 8-carbon oxoenals and oxodials, comprising approximately 2 to 10% of the reacted carbon mass, while ring-retaining hydroxylation products and compounds produced from oxidation of the alkyl substituent accounted for an additional 15 to 40% of the product yield. The identification of multiple α,β-unsaturated aldehydes, a class of compounds with concerning toxicity due to their electrophilic reactivity with biomolecules, and observation of the formation of numerous unidentified carbonyls, raise concerns about the formation of toxic ring cleavage products during the initial stage of oxidation whenever HO• or SO4•– are used for treatment of water containing benzene or alkylbenzenes. Unexpectedly, when SO4•– was employed for BTEX oxidation, aromatic organosulfates were also formed. Although one-electron transfer is considered to be the dominant reaction mechanism for SO4•– oxidation of aromatic compounds, the pathway for radical addition that involves the addition of a sulfate moiety onto the ring followed by termination of the radical chain before the substituent can be transformed by additional reactions has been considered insignificant. To assess these reactions, experiments on organosulfate formation were conducted by using SO4•– to oxidize a suite of 28 model organic compounds representative of a range of contaminants found at hazardous waste sites. Organosulfate formation was nearly ubiquitous, with the identification of one stable sulfate-containing product for 25 of the compounds, including aromatic compounds with a variety of substituents and certain aliphatic compounds. Although much is still unknown about the toxicity of organosulfates, phenyl sulfate and p-cresyl sulfate (the respective products of benzene and toluene oxidation by SO4•–) are uremic toxins that have been implicated in kidney disease and renal cancer. The assessment of the human health risks associated with the consumption of treated water includes consideration of the likelihood that oxidation products will be more mobile in the environment than their precursors (e.g., sulfate-containing compounds exhibit a low affinity for soil and aquifer solids). In addition, the compounds could undergo further transform reactions in natural systems (e.g., biodegradation of carbonyls by microbes in aquifers) or drinking water distribution systems (e.g., transformation of carbonyl compounds by residual disinfectants). Experiments conducted on the chlorination of three representative α,β-unsaturated carbonyls (i.e., acrolein, crotonaldehyde, and methyl vinyl ketone) under conditions relevant to drinking water distribution systems were used to assess the reaction pathway and identify other transformation products. All three carbonyl compounds reacted with hypochlorite to form stable intermediates such as β-diketones, substituted alcohols, and carboxylic acid derivatives such as acyl chlorides and hypochlorites. These compounds are likely to undergo further transformation reactions to produce carboxylic acids and polyhalogenated disinfection byproducts.

Advisor: Sedlak, David L