Influence of pH on dehalogenating bacteria for bioremediation of chlorinated solvents

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

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix

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.

2011

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

David Andrew Barry, Christof Holliger, Elsa Marina Lacroix

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.

2010

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

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix

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.

2012

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

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix

2011

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

David Andrew Barry, Christof Holliger, Elsa Marina Lacroix

2010

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

David Andrew Barry, Alessandro Brovelli, Christof Holliger, Elsa Marina Lacroix

2012