(86b) Density Functional Theory Insights into Corrosion and By-Product Formation Caused By Drinking Water Disinfection
Currently, treatment plants most commonly disinfect drinking water with free chlorine to remove pathogens. Free chlorine, however, is prone to forming carcinogenic disinfection by-products from reactions with organic matter. Ill-considered application of disinfectants and complications from cleanup of their by-products, recently resulted in catastrophic corrosion and discharge of lead into the water supplies of Washington D.C. and Flint, Michigan. To avoid a recurrence of such mistakes, the strategy for drinking water disinfection in vulnerable, lead-containing supply networks was reconsidered. Specifically, we evaluated the merits and drawbacks of replacing free chlorine with chlorine dioxide in systems with lead infrastructure.
Free chlorine, the most commonly used disinfectant in drinking water treatment, is a powerful oxidant that readily reacts with viral and bacterial pathogens, but also with other water constituents, including dissolved organic matter and metallic infrastructure surfaces. Reactions with organic matter produce hazardous by-products, while reactions with infrastructure surfaces can increase the concentration of metals, including lead, in drinking water. Some water treatment facilities have substituted free chlorine with chloramines, an alternative disinfectant, to meet disinfection by-product regulations set by the Environmental Protection Agency. However, chloramines produce their own set of toxic by-products and have lower disinfection efficiency than free chlorine. Removal of disinfection by-products and the precursors to said by-products can exacerbate corrosion in the metal pipes of water distribution systems.
Unlike free chlorine, chlorine dioxide has a significantly lower propensity to form organic by-products, and it is as potent, and in some cases, more potent than free chlorine for the inactivation of bacteria and viruses. In a properly managed dose, chlorine dioxide can deliver effective disinfection of drinking water1. Chlorine dioxide is more expensive to use for drinking water treatment than free chlorine and it can decay in water to form chlorite and chlorate, two potentially hazardous inorganic by-products. It has been assumed that chlorine dioxide will behave similarly to free chlorine with regards to lead corrosion mechanisms because of their similar oxidation reduction potentials, but there have been few studies to confirm.
Using three different minerals, a series of experiments were performed to measure the kinetics of chlorine dioxide decay and by-product formation, considering chlorine dioxide concentration, and pH. In the presence of lead minerals, chlorine dioxide does not form chlorate as a by-product. These results are curious as copper minerals were shown to produce chlorite and chlorate through a disproportionation reaction. The lead mineral reaction was also significantly faster than the copper reaction and produced only chlorite.
To explain the disparity in by-product formation between copper and lead minerals, computational chemistry was employed. Density Functional Theory was used to calculate activation energy barriers and minimum energy structures for both lead and copper systems. These calculations provided insights into reactant, products, intermediates, and transition structures to explain experimental findings.
For experimental work, spectrophotometry was used to standardized chlorine dioxide concentrations at 359nm. Ion chromatography, by a Dionex 5000 Reagent-Free ion chromatograph, was used to measure by-products (chlorite and chlorate) and specified anions. All experiments were batch reactions, conducted in the dark under constant agitation of a stir plate. Computational studies were preformed using the Vienna Ab Simulation Package and Gaussian. Simulations were done on solvated surfaces and in the gas phase. The influencing factors on the kinetics of chlorine dioxide decay and by-product formation in the presence of lead minerals will be presented and discussed. Our data will be contrasted with the results of published results on other metals including nickel, copper, and iron.
Aging lead infrastructure will continue to be an issue in the United States. The replacement of distribution systems could take decades and at high environmental, economic, and social costs due to our poor understanding of corrosion and by-product formation mechanisms. There is a demand for alternative disinfectants that have the capability to protect society from waterborne diseases without polluting our environment with lead and toxic by-products. This work is intended to provide guidance, on the viability of chlorine dioxide as a disinfectant, to avoid costs and dangers associated with lead corrosion.
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