Microbial strategies for the bioremediation of carbon tetrachloride and perchloroethene mixtures

Abstract

Soil and groundwater pollution by chlorinated aliphatic hydrocarbons (CAHs) poses a threat to environmental and human health. A large number of sites around the world are contaminated with the CAHs carbon tetrachloride (CT) and perchloroethene (PCE). The employment of bacteria to remediate pollution events, termed in situ bioremediation, is currently gaining momentum as an effective and environmentally friendly treatment technology. A class of anaerobic bacteria named organohalide respiring bacteria (ORB) are able to remove PCE directly by utilizing it as an electron acceptor for growth. Unfortunately, no such bacteria are known which can transform CT in the same manner. Furthermore, CT and its partly dechlorinated product chloroform (CF) negatively impact the enzymatic degradation of PCE by ORB, creating major challenges for the bioremediation of sites co-contaminated with CT and PCE. While it cannot be used as a growth substrate, CT can be transformed by certain reactive metabolites of anaerobic bacteria. In particular, sulphide (HS-) and ferrous iron (Fe(II)) generated by sulphate- and iron-reducing bacteria (SRB and IRB) can transform CT. In this work, experimental results are presented which support the exploitation of this CT transformation pathway combined with the activity of organohalide respiring bacteria in view of bioremediating mixed CT and PCE plumes. Firstly, experiments were carried out to investigate the solvent tolerance of anaerobic bacteria in general, and revealed that fermentative strains are more tolerant to CAHs than SRB and IRB. Avenues to take advantage of this finding are proposed. Secondly, IRB and SRB enriched from several environmental sources displayed the ability to survive and produce Fe(II) and HS- respectively in the presence of CT and PCE. Thirdly, the potential of naturally-occuring electron-shuttling compounds was tested for their enhancing effects on CT transformation by Fe(II) and HS-. Phenolic-rich solutions including tea and wine were found to significantly decrease the proportion of CF formed in CT dechlorination and in some cases also increased dechlorination rates. Finally, a proof-of-concept study incorporating HS- production by Desulfovibrio vulgaris and organohalide respiration by ORB pointed to the feasibility of combining these two biogeochemical processes to remediate CT and PCE mixtures.