Effects of nano-scale zero-valent iron particles on a mixed culture dechlorinating trichloroethylene

Nano-scale zero-valent iron particles (NZVI) are increasingly being used to treat sites contaminated with chlorinated solvents. This study investigated the effect of NZVI on dechlorinating microorganisms that participate in the anaerobic bioremediation of such sites. NZVI can have a biostimulatory effect associated with water-derived cathodic H2 production during its anaerobic corrosion (730 ± 30 μmol H2 was produced in 166 h in abiotic controls with 1 g/L NZVI) or an inhibitory effect upon contact with cell surfaces (assessed by transmission electron microscopy). Methanogens, which are known to compete for H2 with dechlorinators, were significantly biostimulated by NZVI and methane production increased relative to NZVI-free controls from 58 ± 5 to 275 ± 2 μmol. In contrast, bacteria dechlorinating TCE were inhibited by NZVI, and the first-order degradation rate coefficient decreased from 0.115 ± 0.005 h−1 (R2 = 0.99) for controls to 0.053 ± 0.003 h−1 (R2 = 0.98) for treatments with 1 g/L NZVI. Ethene production from TCE was initially inhibited by NZVI, but after 331 h increased to levels observed for an NZVI-free system (7.6 ± 0.3 μmol ethene produced in 502 h compared to 11.6 ± 0.5 mmol in the NZVI-free system and 3.8 ± 0.3 μmol ethene for NZVI alone). Apparently, cathodic H2 was utilized as electron donor by dechlorinating bacteria, which recovered following the partial oxidation and presumably passivation of the NZVI. Overall, these results suggest that reductive treatment of chlorinated solvent sites with NZVI might be enhanced by the concurrent or subsequent participation of bacteria that exploit cathodic depolarization and reductive dechlorination as metabolic niches.