Removing Esoteric Contaminants from Drinking Waters: Impacts of Treatment Implementation

Abstract

At first blush, the production and distribution of drinking water seems to be a very straightforward process. There is a need to remove microbial agents and any anthropogenic or autochthonous contaminants that may be a health concern. A disinfectant is often added to maintain a microbiologically-safe water throughout the distribution system. However, for those that design or operate drinking water facilities or conduct drinking water research, there are many subtle issues that arise in both individual processes and in the combined treatment train. To elucidate these issues, evaluation of the removal of a specific contaminant in a controlled setting can shed a great deal of light on treatment and what the implementation of treatment entails for residual and distribution system plans. A valuable contaminant to investigate in this light would be one that has chemical properties that differ from those of other contaminants for which treatments are already in place. In recent years, due to more sensitive analytics, the contaminant perchlorate has been found in a number of natural waters USEPA 2004a,b . Regardless of how perchlorate ended up in the environment or its regulatory status, the unique chemistry of perchlorate offers an opportunity to look at the holistic approach to drinking water treatment for a class of contaminants generally not studied. In natural water, under typical pH conditions, perchlorate ClO4 exists as a monovalent anion dissociated from perchloric acid HClO4 . It has a unique molecular structure: a chloride atom in the center of a tetrahedral grouping of four oxygen atoms; the negative charge is evenly dispersed over the four oxygen atoms. This structure is similar to that of the methane molecule. In addition, perchlorate molecular weight of 99.46 is larger than a chloride molecule, but smaller than sulfate. The perchlorate ion is unreactive as a ligand and its salts are extremely soluble, even in organic solvents. Due to these properties, it does not adsorb well on most surfaces. Despite its strength as an oxidizing agent, perchlorate is very slow to react in dilute solutions because of the high activation energy necessary for perchlorate reduction. Likewise, common reducing agents do not react with perchlorate. Perchlorate also has a relatively low charge density; consequently, it does not generally form complexes with metals the way other anions do. Therefore, common cations do not precipitate it. Perchlorate is so nonreactive that it has often been used in thermodynamic, kinetic, and other solution chemical studies as an inert supporting electrolyte Espenson 2000 . Because of its fully oxidized status, perchlorate would not be subject to aerobic biological treatment. These chemical characteristics suggest unique solutions to treatments that then have unusual implications upon implementation.