Combined abiotic and biotic in-situ reduction of hexavalent chromium in groundwater using nZVI and whey: A remedial pilot test

•In-situ whey application after nZVI supported microbes to reduce efficiently Cr(VI).•Reduction was mediated mainly by iron and sulphate reducing bacteria.•Bacteria recovered previously spent nZVI into Fe(II).•Recycled nZVI and natural iron in Fe(II) form enabled further Cr(VI) reduction.•Efficient Cr(VI) removal was observed for 10 months. The paper describes a pilot remediation test combining two Cr(VI) geofixation methods – chemical reduction by nanoscale zero-valent iron (nZVI) and subsequent biotic reduction supported by whey. Combination of the methods exploited the advantages of both – a rapid decrease in Cr(VI) concentrations by nZVI, which prevented further spreading of the contamination and facilitated subsequent use of the cheaper biological method. Successive application of whey as an organic substrate to promote biotic reduction of Cr(VI) after application of nZVI resulted in a further and long-term decrease in the Cr(VI) contents in the groundwater. The effect of biotic reduction was observed even in a monitoring well located at a distance of 22m from the substrate injection wells after 10 months. The results indicated a reciprocal effect of both the phases – nZVI oxidized to Fe(III) during the abiotic phase was microbially reduced back to Fe(II) and acted as a reducing agent for Cr(VI) even when the microbial density was already low due to the consumed substrate. Community analysis with pyrosequencing of the 16S rRNA genes further confirmed partial recycling of nZVI in the form of Fe(II), where the results showed that the Cr(VI) reducing process was mediated mainly by iron-reducing and sulfate-reducing bacteria.