Buffering agent for pH control during in situ bioremediation of chlorinated solvent source zones

Chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent contaminants in groundwater. Because of their low aqueous solubility, PCE and TCE are often present as dense non aqueous phase liquid (DNAPL) that can act as a long term source of groundwater contamination. In situ enhanced bioremediation is an attractive technology for removal of PCE and TCE. It relies on a process called dehalorespiration in which dehalogenating bacteria reduce PCE to ethene, which is a harmless compound for the environment. Enhanced bioremediation is achieved by stimulating the activity of these specialized bacteria through the addition of an electron donor. This method has been widely used for remediation of chlorinated ethene plumes for more than thirty years and recent studies have indicated that it is a promising technology for bioremediation of source zone. However, application of source zone bioremediation is still a significant technical challenge. One of the main issues is groundwater acidification due to dechlorination and fermentation processes thereby inhibiting activity of dehalogenating micro-organisms. Objective of this project is to develop an efficient buffering strategy for source zone remediation. Utilisation of silicate mineral as buffering agent has been investigated through geochemical modeling and batch experiment. A methodology to select appropriate silicate minerals for pH control based on their kinetics properties and field characteristics has also been developed. A geochemical model, done with the chemical speciation code Phreeqc, including transport phenomena has been implemented to assess the influence of silicate dissolution on acidity. Preliminary results have demonstrated that certain minerals belonging to aluminosilicate (nepheline) and magnesium iron silicate (olivine) present kinetics fast enough to neutralize acidity typically produced during bioremediation of DNAPL source zone. Experimental studies including mineral dissolution and reductive dechlorination by bacterial culture are currently performed to validate the results of this model.