Thousands of sites across Europe are polluted with toxic metals and organic solvents; many more exist worldwide. As EU population grows, clean water will determine the quality of life and economic stability. Most sites remain contaminated because existing technology is costly and disruptive. Society needs an innovative way to decontaminate soil and groundwater directly underground. In METAL-AID, we develop new metal oxide reactants and implementation procedures for cost effective and sustainable, in situ, subsurface decontamination.
Two research projects of the MetalAid network dealing with in situ remediation of chromium contaminated sites by applying green rust sulfate, a layered Fe(II, III) hydroxide mineral. The research was carried out in strong cooperation of TU Berlin with Karlsruhe Institute of Technology, University of Copenhagen and Arcadis Germany GmbH.
In his PhD-project, Andrew Thomas examined the reduction of toxic hexavalent chromium to its more benign trivalent form by green rust sulfate. First, pure green rust sulfate was reacted with hexavalent chromium at a range of concentrations typical of groundwater contaminant plumes in order to determine the effect of chromium concentration on the eventual Cr carrier phase following the reaction. This study indicated that the primary Cr carrier phase in this study was Cr(III) hydroxide, which is unsuitable for in situ remediation because its solubility is higher than other Cr(III)-bearing phases (Thomas et al. 2018). Therefore, green rust sulfate samples with isomorphically-substituted Al, Mg and Zn were synthesized and reacted with identical chromate solutions. Furthermore, the lability of Cr in the reacted samples was assessed by reacting them with synthetic δ-MnO2 because oxidation of trivalent chromium in soils primarily occurs at the surface of biogenic manganese oxides. Aluminum and Mg substitution was found to substantially decrease the lability of Cr in the layered Fe(III) oxyhydroxysulfate product, a phenomenon attributed to the effect of these substitutions on electron conductivity in green rust (Thomas et al. 2020). In addition to the implication of these results for remediation, these experiments also provided valuable information on the mechanisms driving Cr redox behavior in soils, particularly the reduction by Cr by phase-bound Fe(II).
The mobility and reactivity of sulphate green rust (GRSO4) and sulphidized nanoscale zerovalent iron (S-nZVI) in porous media was investigated by Flavia Digiacomo in her PhD project. Overall, the study demonstrates the high potential for use of GRSO4 in Cr(VI) remediation applications, given the presence of sand matrices, dynamic flow conditions and changing pH conditions (Digiacomo et al. 2020). High immobilisation yields were reached that are considerable higher than what is observed for alternative reductants such as S-nZVI. Moreover, the data suggest that at much lower flow rates, Cr(VI) removal efficiencies by GRSO4 would likely be much higher. Thus, at flow rates closer to average groundwater flow (e.g., 6–220 cm/day), GRSO4 might achieve 100% removal efficiency because of higher contact time with Cr(VI). Overall, with this study we show that, under specific conditions, both GRSO4 and S-nZVI can successfully reduce and immobilise Cr(VI) in porous media. Therefore, these materials certainly warrant further study on how to apply them at larger scale.
Prof. Dr. Thomas Neumann
Coordination: Prof. Dr. Susan Stipp and Prof. Dr. Dominique Tobler (University of Copenhagen, Denmark)
Partners: Dr. Elisabeth Eiche (Karlsruhe Institute of Technology), Dr. Thomas Held (Arcadis Deutschland GmbH)
Dr. Andrew Thomas, Dr. Flavia Digiacomo
Digiacomo F., Tobler D.J., Held T. & Neumann T. (2020): Immobilization of Cr(VI) by sulphate green rust and sulphidized nanoscale zerovalent Iron in sand media: batch and column studies. – Geochemical Transactions 21:8 DOI: 10.1186/s12932-020-00073-9.
Thomas A.N., Eiche E., Göttlicher J., Steiniger R., Benning L.G., Freeman H.M., Dideriksen K. & Neumann T. (2018): Products of Hexavalent Chromium Reduction by Green Rust Sodium Sulfate and Associated Reaction mechanisms. - Soil Systems 2, 58, doi:10.3390/soilsystems2040058.
Thomas A.N., Eiche E., Göttlicher J., Steininger R., Benning L.G., Freeman H.M., Tobler D.J., Mangayamam M., Dideriksen K. & Neumann T. (2020): Effects of metal cation substitution of hexavalent chromium reduction by green rust. – Geochemical Transactions 21:2 DOI: 10.1186/s12932-020-00066-8.