Geochemical and microbial reactions affecting the long-term performance of in situ ‘iron barriers’

Abstract

The in situ application of granular iron (Fe 0) has become popular for the destruction of halogenated organic compounds and for the immobilization of specific metals in groundwater. However, a knowledge gap exists concerning the long-term performance of the Fe 0-barriers. The corrosion of Fe 0 may produce mineral precipitates that alter the system’s hydraulic integrity. For example, data from existing barriers show varying trends in pH, alkalinity, mineral precipitation, and microbial activity. Although the chemical behaviors are site-specific, this paper discusses the concepts involved in developing a generic approach for predicting the trend of aqueous and surface speciation, and the resulting effects on Fe 0 treatment systems. Observations from existing Fe barriers are summarized, and the chemical and microbial processes that influence chemical speciation, both in water and on surfaces, are reviewed. A conceptual geochemical model is presented, which illustrates the factors that must be considered in developing a quantitative model that can be used to design monitoring plans for timely detection of clogging in Fe 0 reactive barriers. In order to develop quantitative predictive models, field and laboratory research should: (1) assess the extent and rates of media deterioration by analyzing coupled chemical and microbial reactions; and (2) identify the controlling mechanisms for hydraulic alteration within and around Fe 0 barriers.