Theme: 08. Metals

Topic Area: 8c. Precipitation and Stabilization of Metals

Formatted Title:

Chromium(VI) and Vanadium(V) in Groundwater: Investigating Bioremediation Solutions

Background/Objectives:

Chromium(VI) and vanadium(V) are emerging toxic and mobile contaminants in groundwater. The potential risks can be decreased by reduction to its less toxic counterparts; i.e., Cr(III) and V(IV/III). Those reduced forms typically precipitate and are therefore less mobile as well. The reduction can be facilitated by microorganisms via microbial reduction or indirectly via biologically produced reductants like iron sulfide. This mechanism was applied in a full-scale bioremediation project for a site contaminated with chlorinated solvents (>100 mg/L TCE) and Cr(VI) (>1000 mg/L). For a site contaminated with V(V) (>150 mg/L), a similar approach was tested at laboratory scale with the aim to investigate it as a feasible remediation technique and to gain more insights into the reduction processes.

Approach/Activities:

Biologically mediated reduction was applied to remediate the groundwater plume of chlorinated solvents and Cr(VI). To create anaerobic conditions, injections were carried out with an electron donor. The aim was to achieve (indirect) reduction of Cr(VI) to Cr(III) via biological processes. A laboratory study with sulfate-rich groundwater from a vanadium contaminated site was carried out to test if reduction of V(V) can be achieved. Sulfate reduction was monitored by measuring sulphate concentrations and the microbial community was assessed with Next Generation Sequencing (NGS) analyses. Electron donor was added to stimulate sulfate and vanadium reduction.

Results/Lessons Learned:

In a full-scale remediation project the required geochemical environment for Cr(VI) reduction was created and the biologically mediated process stimulated. The results showed a successful reduction of the toxic Cr(VI) contamination to Cr(III). The reduction of the mobile Cr(VI) fraction was > 99% within a year.

For vanadium, the results of an anaerobic lab test show that 64 to 84% of aqueous V(V) was reduced in the presence of an electron donor. To investigate the fate of the V removed from the dissolved phase, ICP-OES, XRD and SEM-EDX analyses were performed on the precipitate. The precipitate appeared to be reduced V in the form of different oxides with calcium, iron and aluminium. The V speciation analyses showed that most reduction of V(V) to V(IV) occurred during the first three weeks of the test when conditions were not fully anaerobic yet. According to the NGS, the aerobic bacterium Pseudomonas putida was the most abundant (8%) and likely candidate for vanadium reduction based on literature. Therefore, a second test was conducted under both aerobic and anaerobic conditions. In the aerobic conditions after 4 months, ± 5 % of the total V was present as V(IV) and 10 to 35 % remained present as V (V). 60 – 80% of total V likely precipitated or further reduced to V(III) or V(II). The aerobic samples all developed a bright yellow colouration which indicated the presence of predominantly V(V). In one anaerobic condition, more than 98% of V(V) was reduced and precipitated or further reduced to V(III) or V(II) in the water, since dissolved V(IV) could only account for 0.4%. The color change to petrol blue also indicated that dissolved V(III) was present.

Cr(VI) was successfully reduced to Cr(III) in a full-scale project within less than a year. V(V) was reduced to V(IV) under anaerobic and aerobic batch conditions in the lab. To better understand the reduction process, NGS were performed and the composition of the precipitate in the samples will be analysed with specialized geochemical analyses. Also long-term stabilization of the formed reduced vanadium will be investigated.

Sustainable biological treatment approaches of Cr(VI) in full-scale and of V(V) at laboratory scale showed promising results. Advanced geochemical and microbial analyses play a crucial role in understanding heavy metal reduction and determining a reliable full-scale approach.

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Michaela Bhend