Evaluation of silicate minerals for acid neutralization during bioremediation of high concentrations of chlorinated ethenes

Chloroethenes are the most prevalent groundwater contaminants in developed countries. They accumulate in the environment as a dense non aqueous phase liquid (DNAPL) which acts as a long term source of contamination. Enhanced in situ anaerobic bioremediation of source zones by reductive dechlorination is a promising technology. Nevertheless, it can be hindered by two major problems: (1) toxicity of high concentrations of chloroethenes in source zones towards organohalide respiring bacteria, (2) groundwater acidification due to dechlorination and fermentation processes. In a first time, the present work aims to deepen the knowledge on a consortium which can dechlorinate at high concentrations of PCE. Then, pursue the investigation of the use of silicate minerals in biotic batch experiments with organohalide respiring bacteria to observe the effect of dissolution on dechlorination activity. Once these data collected, it will allow to do biotic experiments on continuous-flow columns, in order to get closer to field conditions. To this end, a first estimation of the ratio of mineral dissolution kinetics between batch and columns experiments was also performed during this master thesis. Microcosms and biomolecular experiments were conducted to demonstrate the ability of Point Mugu (PM) culture to be used in contaminated source zones. Community pattern analysis showed that it is mainly constituted of Dehalococcoides spp. and Desulfitobacterium spp. Influence of the electron donor choice and the PCE concentration and the community’s ability to grow on zwitterionic buffer was evaluated in microcosms experiments. Butanol and lactate can both be provided as soluble electron donor. As the literature suggests, its tolerance to saturated aqueous concentrations (0.9 mM) of PCE was attested. Moreover, this consortium was able to grow on zwitterionic buffer. These three advantages make PM consortium a good candidate for future planned column experiments. Acidic conditions are a significant issue as low pH inhibits bacterial activity. It is therefore essential to develop appropriate buffering strategies in contaminated zones to allow the transformation of toxic chloroethenes into ethene. The use of silicate minerals as a buffer agent was investigated in microcosms with several consortia. With SL23-6 consortium, a complete dechlorination of 0.25 mmol of PCE to cis-DCE was performed in 50 mL batches containing 4 g of olivine, diopside and fayalite (fraction 50-100 μm) whereas the same amount of andradite and nepheline inhibited dehalogenating activity. Five minerals were tested on SL23-b4 consortium (diopside, fayalite, grossular, andradite and forsterite). They all presented the similar pattern of dechlorination with an inhibition characterized by an accumulation of cis-DCE and no production of ethene. All silicate minerals tested proved their ability to keep pH in optimal ranges if they are suitably dosed. Models were implemented from these experiments with the software PHREEQC to allow future predictions on buffering behaviors of these minerals during dechlorination process. Furthermore, abiotic experiment on fayalite anaerobic dissolution was performed in porous medium with a constant inflow medium at pH 4.5 and a hydraulic residence time of 2.3 days. In those conditions, mineral dissolution was 20 to 100 times slower in column than in head-over-end stirred reactor.