Inhibition and stimulation of two perchloroethene degrading bacterial cultures by nano- and micro-scaled zero-valent...

The pollutant perchloroethene (PCE) can often be found at urban contaminated sites. Thus in-situ clean-up methods, like remediation using zero valent iron (ZVI) or bacterial dechlorination, are preferred. During the remediation with ZVI particles anaerobic corrosion occurs as an unwanted, particle consuming side reaction with water. However, in this reaction H₂ is formed, which is usually scarce during anaerobic microbial dechlorination. Dehalococcoides needs H₂ for cell growth using it as an electron donor to dechlorinate chlorinated hydrocarbons. Combining application of ZVI with bacterial dechlorination can turn ZVI in a H₂ donor leading to a more controllable bacterial dechlorination, a smaller amount of ZVI suspension and decreased remediation costs. In this study nano- and micro scaled ZVI particles (nZVI, mZVI) were combined in microcosms with two dechlorinating bacterial cultures. The two cultures showed different dechlorination behaviors with ethene and cis-DCE as final products. Phospholipid fatty acids (PLFA) associated with Dehalococcoides (18:1w7, 18:1w7c, 10:Me16:0) and Geobacteriaceae (16,1w7c; 15:0; 16:0) have been found in both bacterial cultures, slight differences in their abundance could explain the different dechlorinating behaviors. The combination of both bacterial cultures with mZVI led to a stimulated dechlorination process leading to about two times higher k_obs for PCE dechlorination (0.01-0.05 h^-1). In the otherwise cis-DCE accumulating culture complete dechlorination to ethene was achieved. While addition of nZVI inhibited both cultures. Combined with nZVI the completely dechlorinating culture produced lower amounts of dechlorinated products (3.2 μmol) as compared to the single biotic treatment (5.1 μmol). Combining the incompletely dechlorinating culture with nZVI significantly reduced the k_obs,PCE (single: 8 × 10^-3 ± 3 × 10^-4 h^-1; combination: 5 × 10^-3 ± 2 × 10^-4 h^-1). H₂ produced by nZVI and mZVI was utilized by both bacterial cultures. The particle size, resulting specific surface areas, agglomeration tendencies and reactivity appears to be crucial for the effect on microbial cells.