Effectiveness of stimulating PCE reductive dechlorination: A step-wise approach

Highlights

• A step-wise strategy proved to be effective to enhance PCE dechlorination.

• Pre-treatment with 75 μmol electrons/g dry soil stimulated PCE dechlorination.

• Both ascorbic acid and lactate successfully lowered redox potential to − 450 mV.

• The impact of ascorbic acid and lactate on redox potential was not specific.

Abstract

Reductive dechlorination of tetrachloroethene (PCE) and its daughter products in aquifers is often hampered by Fe(III) reducing conditions. Rigorous treatment to adjust the redox potential and stimulate dechlorination may be costly and potentially have negative effects on other aquifer functions. A step-wise experimental strategy was applied to investigate the effectiveness of various adjustment scenarios. Batch experiments with ascorbic acid (AA) and sodium lactate (SL) showed that 75 μmol electron equivalents per gram dry mass of aquifer material was required to reach a sufficiently low redox potential for the onset of PCE dechlorination. Similar effects of either AA or SL on the measured redox potential suggest electron donors are not specific. However, the relative rates of Fe(III) and sulphate reduction appeared to be specific to the electron donor applied. While redox potential stabilised around − 450 mV after titration and sulphate was reduced to zero in both treatments, in the AA treatment a faster production of Fe^2 + was observed with a final concentration of 0.46 mM compared to only 0.07 mM in the SL treatment. In subsequent batch experiments with aquifer material that was pre-treated with AA or SL, PCE reductive dechlorination occurred within 30 days. Further stimulation tests with extra electron donor or inoculum revealed that adding electron donor can accelerate the initiation of PCE biodegradation. However, bioaugmentation with dechlorinating bacteria is required to achieve complete reductive dechlorination to ethene. The findings from step-wise approaches are relevant for improving the cost-effectiveness of the design and operation of in-situ bioremediation at initially unfavourable environmental conditions.