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Engineered in situ biogeochemical transformation as a secondary treatment following ISCO – A field test

Němeček, Jan, Nechanická, Magda, Špánek, Roman, Eichler, František, Zeman, Josef, Černík, Miroslav
Chemosphere 2019 v.237 pp. 124460
Dehalobacter, Dehalococcoides mccartyi, Desulfitobacterium, bacteria, bioaugmentation, biogeochemistry, chlorine, dechlorination, ethylene, groundwater, hydrochemistry, hydrogen sulfide, iron, magnetite, microbial activity, models, neutralization, sodium, solvents, starch, sulfates, tetrachloroethylene
ISCO using activated sodium persulphate is a widely used technology for treating chlorinated solvent source zones. In sensitive areas, however, high groundwater sulphate concentrations following treatment may be a drawback. In situ biogeochemical transformation, a technology that degrades contaminants via reduced iron minerals formed by microbial activity, offers a potential solution for such sites, the bioreduction of sulphate and production of iron sulphides that abiotically degrade chlorinated ethenes acting as a secondary technology following ISCO. This study assesses this approach in the field using hydrochemical and molecular tools, solid phase analysis and geochemical modelling. Following a neutralisation and bioaugmentation, favourable conditions for iron- and sulphate-reducers were created, resulting in a remarkable increase in their relative abundance. The abundance of dechlorinating bacteria (Dehalococcoides mccartyi, Dehalobacter sp. and Desulfitobacterium spp.) remained low throughout this process. The activity of iron- and sulphate-reducers was further stimulated through application of magnetite plus starch and microiron plus starch, resulting in an increase in ferrous iron concentration (from <LOQ to 337 mg/l), a decrease in sulphate concentration by 74–95% and production of hydrogen sulphide (from <LOQ to 25.9 mg/l). At the same time, a gradual revival of dechlorinators and an increase in ethene concentration was also observed. Tetrachloroethene and trichloroethene concentrations decreased by 98.5–99.98% and 75.4–98.5%, respectively. A decline in chlorine number indicated that biological dechlorination contributed to CVOC removal. This study brings new insights into biogeochemical processes that, when properly engineered, could provide a viable solution for secondary treatment.