The chemistry of perchlorate in the environment

Abstract

Current interest in the chemistry of perchlorate is primarily due to its presence as a contaminant in groundwater and drinking water. The negatively charged perchlorate ion is composed of one chlorine atom surrounded by four oxygen atoms arranged in tetrahedral geometry. The perchlorate anion (ClO 4-) commonly originates as a contaminant in the environment from the improper disposal of solid salts of ammonium, potassium, or sodium perchlorate'. These salts are very soluble in water, and the perchlorate ion is kinetically inert to reduction and has little tendency to adsorb to mineral or organic surfaces. Therefore the perchlorate anion persists in groundwater, and its mobility in surface or groundwater is so high that perchlorate essentially moves with the flow of water (diffusion and convection controlled movement). Compounds containing perchlorate include the oxidant in solid rocket fuel as well as that in fireworks, military ordinance, flares, airbags, and other applications where an energetic oxidant is required. Ammonium perchlorate is among the most important propellants because it has a high oxygen content and decomposes to the gaseous phases products water, HCI, N 2, and O2, leaving no residue. Salts of perchlorate do not function well in solid-fueled rockets after the salts adsorb too much water, and improper disposal has led to environmental contamination. A report on the occurrence of perchlorate in drinking water by the American Water Works Association 2indicates that perchlorate contamination is a national problem with significant concentrations being found in 26 states and Puerto Rico. The presence of perchlorate in groundwater and drinking water is a potential health concern because perchlorate can impair proper functioning of the thyroid gland. 3 The thyroid gland produces hormones that are maintained within narrow concentration limits by an efficient regulatory mechanism. Hormones required for normal development of the central nervous system of fetuses and infants are secreted by the thyroid gland. These hormones are required for normal skeletal growth and development. In both infants and adults, thyroid gland hormones determine metabolic activity and affect many organ systems. Iodine is a component of the thyroid hormones, and one of the functions of the thyroid gland is to control iodide levels in the bloodstream. Perchlorate competitively inhibits the uptake of iodide ions by the thyroid; in fact, potassium perchlorate has been used to treat hyperthyroidism. In the discussion of the physical properties of the perchlorate ion that follow, it will be seen that the competitive inhibition is a consequence of the hydration energies of the two ions. A recent report by the National Research council 3 concluded that the levels of perchlorate that have been found in groundwater are unlikely to affect a healthy adult. However, the situation may be different for infants, children, pregnant women, and people with pre-existing thyroid disorders. The National Research Council suggested a reference dose of 0.7 μg/kg of body weight, which is an order of magnitude lower than the dose with no observed effect on iodine uptake by the thyroid. The committee concluded that this reference dose would protect the health of even the most sensitive populations. This reference dose is elevated from the provisional reference dose of 0.1 μg/kg suggested by the U.S. EPA in 2002. This latter value prompted the State of California to set a maximum contaminant level (MCL) of 6 μg/L for perchlorate in drinking water. The higher reference dose translates to an MCL for perchlorate in drinking water of 24.5 μg/L. The environmental chemistry community has an interest in the chemical and physical properties of perchlorate for the purpose of developing analytical methods, separation schemes, and destruction technologies. A number of review4,5,6,7 of perchlorate chemistry from this perspective have been published in recent years as well as the proceedings of a symposium. 8 A recent review paper9 discusses bioremediation approaches to perchlorate contaminated waters. This chapter will not discuss bioremediation other than to comment on the mechanism of reduction. © 2006 Springer Science+Business Media, Inc.