Utilization of Silicate Minerals for pH Control during In Situ Bioremediation of Chlorinated Solvents

Background/Objectives. Chloroethenes such as tetrachloroethene (PCE) and trichloroethene (TCE) are among the most prevalent contaminants in groundwater due to their extensive use in industrial processes. In situ bioremediation (ISB) is an attractive technology for the removal of these compounds. It has been widely used for treatment of chloroethene plumes and recent studies have shown promising results for removal of dense non aqueous phase liquids (DNAPLs). However, application of source zone ISB is still a significant technical challenge. One of the main issues is groundwater acidification due to organohalide respiration and fermentation processes, which can inhibit the activity of dehalogenating micro-organisms. The main objective of this work was to develop an efficient pH control strategy for chloroethene ISB by using the acid neutralizing potential of silicate minerals. These minerals are of particular interest as their dissolution rate and their solubility is pH dependent with faster kinetics and higher solubility in the acidic range. In addition, they persist in the subsurface and are long term alkalinity sources. Their usage therefore potentially reduces the frequency of injection of buffering solution. Approach/Activities. To assess the potential of this technology, modeling and experimental approaches were combined. A geochemical model, implemented within PHREEQC, was developed to select appropriate buffer candidates and to help determine main parameters influencing mineral buffering capacity. The model included microbial degradation kinetics, mineral dissolution kinetics and chemical speciation. Anaerobic microcosm experiments were carried out to compare the buffering capacity of ten silicate minerals and to investigate interactions between minerals and dehalogenating bacteria, e.g., the potential inhibitory effect of minerals on the dehalogenating activity. Four sets of microcosms with four different consortia were conducted. Each microcosm was amended with either 5 mmol l-1 of PCE or or cis-DCE and a defined amount of mineral powder with grain sizes between 50 and 100 μm. Abiotic mineral dissolution experiments were also conducted on promising buffer candidates to give a precise estimation of keys parameters such as equilibrium and kinetics constant without the presence of bacteria. Results of these abiotic tests were used to validate and calibrate the geochemical model developed. Results/Lessons Learned. The results of model simulations confirmed that the efficiency of the system is dependent on mineral dissolution kinetic constants, equilibrium constants, temperature and reactive surface area. The geochemical model and literature parameter data were used to pre-select minerals with a buffering capacity sufficient to counterbalance acidity produced by dehalogenating bacteria at a rate of 4 mmol l-1 d-1 of chloride. From the 31 silicate minerals with published kinetic data, 10 suitable buffering candidates were identified. Results of microcosm experiments with silicate minerals and dehalogenating bacteria demonstrated that, under the selected conditions, 8 of the 10 minerals tested were able to maintain the pH in the appropriate range for PCE degradation, i.e between 5.5 and 6.5. However, the experimental results also revealed a mineral-induced potential inhibitory effect on dehalogenating activity for some minerals. It was shown that aluminum-containing mineral such as nepheline induced precipitation of phosphate which was detrimental to dehalogenating bacteria. Other sources of toxicity might have been the accumulation of toxic trace elements and the increase of the redox potential occurring during mineral dissolution. The bacterial species responsible for the transformation of DCE to ethene (Dehalococcoides spp.) appeared more sensitive than those responsible to reduce PCE to DCE. In conclusion, these study show that silicates mineral have a strong buffering potential, however, the choice of a mineral with a suitable composition is crucial to avoid inhibitory effect. Calcium and magnesium silicates appeared as interesting buffering agent.