Utilization of silicate minerals for pH control during in situ bioremediation of chlorinated solvent

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 removal of these compounds. It relies on an anaerobic process in which specialized bacteria obtain energy for growth using chloroethenes as an electron acceptor via organohalide respiration. Engineered bioremediation is achieved by stimulating these microorganisms through the addition of electron donor in the subsurface. This technology has been widely used for bioremediation of chloroethene plumes and recent studies have indicated promising results for bioremediation of chlorinated solvent source zones. However, application of source zone ISB is still a significant technical challenge. One of the main issues is the 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. To do so, 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 chloroethene microbial degradation kinetics, mineral dissolution kinetics and chemical speciation. Second, anaerobic microcosm experiments were performed to determine the influence of pH on dehalogenation. These microcosms were inoculated with enriched consortia of dehalogenating bacteria and fed with PCE and hydrogen. Another set of microcosm experiments was 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. These microcosms were amended with 5 mmol l-1 of PCE and 4 g l-1 of mineral with grain sizes between 50 and 100 μm. The cultivation medium was modified such that the silicate mineral powder was the sole pH buffer present. Chloroethenes, pH and dissolved cation measurements were conducted to determine the system efficiency. Abiotic dissolution experiments were also performed to determine mineral dissolution rates in the absence of bacteria. The model confirmed that the efficiency of the system is dependent mainly on mineral dissolution kinetic constants, equilibrium constants 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-1of chloride. Of the 31 silicate minerals for which there were published kinetic data, 10 were identified as suitable candidates. The inhibitory pH for the dehalogenating consortia was found to vary between 5 and 6. The last steps of the dechlorination from DCE to ethene were more sensitive to pH than the first steps from PCE to DCE, as has been noted in other dechlorination studies. Results of microcosm experiments with silicate minerals demonstrated that, under the selected conditions, the pH control behavior and the impact on bacterial activity exhibited strong variations depending on the mineral. Of the ten minerals tested experimentally, three (olivine, fayalite and diopside) maintained the pH in the appropriate range, i.e., between 5.5 and 6.5 and led to complete transformation of PCE. For the other minerals tested, either the acid neutralization capacity was insufficient due to slow dissolution kinetics (glaucophane and staurolite) or dechlorination activity was inhibited by (unmeasured) compounds released during mineral dissolution. Both modeling and experimental results demonstrated the feasibility of using selected silicate minerals as a buffering agent during ISB. However, the experimental results also revealed a mineral-induced potential inhibitory effect that should be investigated prior to application at a contaminated site.