The microbiology of perchlorate reduction and its bioremediative application

Abstract

Microbial respiration of oxyanions of chlorine such as chlorate (C1O 3-) and perchlorate (C1O4-) [(per)chlorate] under anaerobic conditions has been known for more than half a century. 1 The high reduction potential of (per)chlorate (C10 4-/Cl- E0 = 1.287 V; C103XX G = 1.03 V) makes them ideal electron acceptors for microbial metaboism. 2 In general, chlorine oxyanions in the environment result from anthropogenic sources including disinfectants, bleaching agents, herbicide,3-5 and rnunition. 6,7 Ammonium perchlorate (NH4C104) represents approximately 90% of all perchlorate salts manufactured. 8 It is predominantly used by the munitions industry and the US Defense Department as an energetics booster or oxidant in solid rocket fuel. 6-10 Although a powerful oxidant, under most environmental conditions perchlorate is highly stable and non-reactive owing to the high energy of activation associated with its reduction. 9,10 Because of the large molecular volume and single anionic charge, perchlorate also has a low affinity for cations and as a result, perchlorate salts, such as ammonium perchlorate, are generally highly soluble and completely dissociate into NH4+ and C104- in aqueous solutions. Furthermore, perchlorate does not sorb to any significant extent to soils or sediments and, in the absence of any biological interactions, its mobility and fate are largely influenced by the hydrology of the environment. 12 Because of its unique chemical stability and high solubility, remediation efforts for perchlorate contamination have focused primarily on microbial processes 9and many novel bioremediative technologies are currently being developed. 13 Enhanced in situ bioremediation (EISB) of groundwater is increasingly being used as the commercial solution for the remediation of perchlorate and of the 65 different case studies of perchlorate-treatment technologies outlined in a 2001 report published by Ground Water Remediation Technologies Analysis Center, the majority (45 case studies) were either in-situ or ex-situ biological treatment technologies based on the unique ability of some microorganisms to reductively respire perchlorate completely to innocuous chloride in the absence of oxygen. 10 Given the large plume/source dimensions (i.e., width, depth) at many contaminated sites, EISB approaches often require extraction of impacted groundwater, amendment with soluble nutrients (electron donors), and reinjection of the nutrient-amended water into the aquifer to effectively mix and distribute the nutrients throughout the target treatment area. While a variety of delivery instrumentation has been used in EISB demonstrations (with varying levels of success), conventional injection wells are by far the most common tool used for nutrient delivery. Unfortunately, the repeated addition of electron donors such as acetate, ethanol, citrate, or more complex undefined substrates creates conditions within the injection well screen and the surrounding filter pack favoring the outgrowth of non-perchlorate reducing microbial communities. This results in ineffective treatment of the target contaminant, inefficient use of the added electron donor, plugging of the near-well aquifer matrix (biofouling), alteration of the physical-chemical nature of the aquifer matrix (mineral content, hydraulic conductivity, pH, redox etc.), and a further reduction in water quality through the direct or indirect release of undesirable end-products (Fe(II), HS-, CH4, mobilized heavy metals, etc.). The need to frequently rehabilitate nutrient delivery wells to overcome just one of these effects (biofouling) using conventional water well techniques (swab and purge) significantly diminishes the financial feasibility of EISB. As such, a better understanding of the microbiology involved in perchlorate reduction including the factors which control its bioremediative application and the issues associated with biofouling is warranted to allow the appropriate design of successful robust EISB remediation processes. © 2006 Springer Science+Business Media, Inc.