Combined nano-biotechnology for in-situ remediation of mixed contamination of groundwater by hexavalent chromium and chlorinated solvents

Jan Nemecek, Petr Pokorný, Ondrej Lhotský, Vladislav Knytl, Petra Najmanová, Jana Steinová, Miroslav Cerník, Alena Filipová, Jan Filip, Tomáš Cajthaml

ENACON s.r.o., Krčská 16, CZ-140 00 Prague 4, Czech Republic.

Abstract

The present report describes a 13 month pilot remediation study that consists of a combination of Cr(VI) (4.4 to 57 mg/l) geofixation and dechlorination of chlorinated ethenes (400 to 6526 μg/l), achieved by the sequential use of nanoscale zerovalent iron (nZVI) particles and in situ biotic reduction supported by whey injection. The remediation process was monitored using numerous techniques, including physical–chemical analyses and molecular biology approaches which enabled both the characterization of the mechanisms involved in pollutant transformation and the description of the overall background processes of the treatment. The results revealed that nZVI was efficient toward Cr(VI) by itself and completely removed it from the groundwater (LOQ 0.05 mg/l) and the subsequent application of whey resulted in a high removal of chlorinated ethenes (97 to 99%). The persistence of the reducing conditions, even after the depletion of the organic substrates, indicated a complementarity between nZVI and the whey phases in the combined technology as the subsequent application of whey phase partially assisted the microbial regeneration of the spent nZVI by promoting its reduction into Fe(II), which further supported remediation conditions at the site. Illumina sequencing and the detection of functional vcrA and bvcA genes documented a development in the reducing microbes (iron-reducing, sulfate-reducing and chlororespiring bacteria) that benefited under the conditions of the site and that was probably responsible for the high dechlorination and/or Cr(VI) reduction. The results of this study demonstrate the feasibility and high efficiency of the combined nano-biotechnological approach of nZVI and whey application in-situ for the removal of Cr(VI) and chlorinated ethenes from the groundwater of the contaminated site.

Graphical abstract

Keywords: Chlorinated solvents; Hexavalent chromium; NZVI; Bioreduction; Dechlorination; Indigenous microorganisms; Co-mingled plume; PLFA.