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.

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

Influence of pH on dehalogenating bacteria for bioremediation of chlorinated solvents

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

2011

Using Silicate Mineral Particles for pH Control during in situ Bioremediation of Chlorinated Ethene Source Zones

Elsa Marina Lacroix

Soil and groundwater pollution by chlorinated solvents such as tricholorethene (TCE) and tetrachloroethene (PCE) is a frequent problem in the industrialized world. Chlorinated solvents, characterized by a low solubility and a density greater than water, form dense non-aqueous phase liquids (DNAPLs) when released in the subsurface. DNAPLs accumulate along low permeability layers and slowly dissolve in groundwater acting as a long-term source of contamination that can last for decades. Remediation of chlorinated solvent DNAPLs is recognized as one of the most challenging problems in the field of environmental remediation. In situ bioremediation (ISB) is a promising and cost-effective technology for their removal that relies on the activity of specialized microorganisms able to transform chlorinated compounds to ethene (a non-toxic product) via a stepwise anaerobic process called organohalide respiration (OHR). ISB has been applied successfully for the treatment of dissolved phase plumes since the early 1980’s. However, its application for source zones, where contaminants are present as DNAPLs, is relatively recent and has only been developed in the last decade. One of the major issues limiting source zone ISB is the acidification of the groundwater due to the transformation of chlorinated compounds by organohalide-respiring bacteria (OHRB) and the production of organic acids by fermentative microbial populations. OHRB are inactivated when the pH is below 5-6 and therefore pH buffer amendments are required when the soil buffering capacity is insufficient. In field applications, the most common method used for pH adjustment is the injection of soluble buffers such as sodium bicarbonate. However, this method requires frequent injections and constant monitoring as alkalinity is rapidly consumed. Therefore, there is a need to develop more efficient and long-lasting buffering strategies. The objective of this thesis was to develop a novel method for long-term control of groundwater pH that relies on the use of ground silicate minerals. Silicate minerals may act as a long-term source of alkalinity release as i) they dissolve slowly compared to carbonates and ii) their dissolution rate and solubility is pH-dependent and increase with acidic pH. In addition, they are easily available at an affordable cost as a raw material or as a by-product of industrial processes. Silicate minerals are the most common rock forming mineral and constitute a very diverse group with highly variable dissolution rates, solubilities and compositions. Only a restricted numbers of these minerals present appropriate characteristics to act as buffering agents. A screening methodology, based on numerical simulations, thermodynamic and kinetic considerations, was developed to select potential candidates for pH control. A geochemical model including the main microbial processes driving groundwater acidification and silicate mineral dissolution was developed as well. This model provides a useful design tool to estimate the mineral requirement in the perspective of field applications. The results of numerical simulations showed that a dozen silicate minerals have the potential to act as buffering agents. Abiotic batch experiments were conducted with five silicate minerals (nepheline, fay alite, forsterite, diopside and andradite) to validate and improve the geochemical model. Abiotic experiments confirmed the buffering potential of these minerals and revealed the importance of secondary precipitation, a process not included in the original formulation of the model. Precipitation of secondary phases can decrease the reactivity of silicates, reduce the aquifer porosity and precipitate nutrients. Therefore, prediction of secondary precipitations was included in the model in order to predict this type of reaction. The influence of silicate mineral dissolution on OHRB and fermentative bacteria was investigated in batch cultures. As expected, the five silicate minerals (except nepheline) were able to maintain the pH in the tolerance range for the three microbial consortia tested. However, transformation of cA-DCE to ethene was completely inhibited in most of the experiments in the presence of minerals. These results showed that compatibility of silicate minerals with the bacterial community involved in in situ bioremediation has to be carefully evaluated prior to their use for pH control at a specific site. Subsequently, the long-term buffering potential of the most promising buffering agents (diopside, fayalite, forsterite) was tested in continuous-flow column studies simulating chloroethene source zone conditions for six and a half month and a half. In contrast to batch experiments, transformation of cis-DCE to ethene was not inhibited by mineral dissolution in continuous flow systems. Olivine minerals (such as fayalite and forsterite) appeared as suitable pH buffering agents. They successfully maintained the pH in the neutral range (7.5 for forsterite and 6.5 for fayalite) and sustained the activity of OHRB bacteria. In contrast, the buffering potential of diopside rapidly decreased due to the formation of a less-reactive cation-depleted leached layer at the mineral surface. This thesis demonstrated the potential of silicate minerals to act as a long-term source of alkalinity release for groundwater pH control. A global strategy for the selection of appropriate buffering agents based on site characteristics was developed. This methodology was applied to the particular case of chlorinated solvent ISB but can be extended to any groundwater remediation technology requiring close to neutral pH conditions.

EPFL2013

Using Silicate Mineral Particles for pH Control during in situ Bioremediation of Chlorinated Ethene Source Zones

Elsa Marina Lacroix

EPFL2013

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

Influence of pH on dehalogenating bacteria for bioremediation of chlorinated solvents

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

2011