In a number of laboratory studies, sulfidated nanoscale zero-valent iron (S-nZVI) particles showed increased reactivity, reducing capacity, and electron selectivity for Cr(VI) removal from contaminated waters. In our study, core-shell S-nZVI particles were successfully injected into an aquifer contaminated with Cr(VI) at a former chrome plating facility. S-nZVI migrated towards monitoring wells, resulting in a rapid decrease in Cr(VI) and Cr_tot concentrations and a long-term decrease in groundwater redox potential observed even 35 m downstream the nearest injection well. Characterization of materials recovered from the injection and monitoring wells confirmed the presence of nZVI particles, together with iron corrosion products. Chromium was identified on the surface of the recovered iron particles as Cr(III), and its occurrence was linked to the formation of insoluble chromium-iron (oxyhydr)oxides such as Cr_xFe_(1−x)(OH)_3(s). Injected S-nZVI particles formed aggregates, which were slowly transformed into iron (oxyhydr)oxides and carbonate green rust. Elevated contents of Fe⁰ were detected even several months after injection, indicating good S-nZVI longevity. The sulfide shell was gradually disintegrated and/or dissolved. Geochemical modelling confirmed the overall stability of the resulting Cr(III) phase at field conditions. This study demonstrates the applicability of S-nZVI for the remediation of a Cr(VI)-contaminated aquifer.

Emissions of Ce from anthropogenic activities (anthropogenic Ce) into urban wastewater systems and the environment result from its widespread industrial use (abrasives, catalysts, nanotechnology). Because Ce in sewage sludge can also be of geogenic origin, the quantification of anthropogenic Ce in sewage sludge remains elusive. In this study, we evaluated the suitability of Ce oxidation state and rare earth element (REE) patterns for the quantification of anthropogenic Ce fractions in sewage sludge. A diverse set of soil samples served to gain baseline information on geogenic Ce. Geogenic Ce in the soils was characterized by high Ce(III) fractions (≥70%) and their REE patterns were comparable to the REE patterns of the upper continental crust. The sewage sludges contained on average ∼80% Ce(IV) (range 18–108%), pointing to the importance of anthropogenic inputs of Ce(IV). The quantification of the anthropogenic Ce fraction based on Ce oxidation state, however, was associated with considerable uncertainty because geogenic and anthropogenic Ce cannot exclusively be assigned to Ce(III) and Ce(IV), respectively. The REE patterns of most sewage sludges indicated a clear enrichment of Ce compared to heavier REE. Based on the assumption that the industrially used Ce is free of (most) other REE, we estimated the fraction of anthropogenic Ce in the sludges based on individual Ce/REE ratios. For the individual sludges the anthropogenic contributions were very variable (10–100%) but consistent fractions were obtained for individual sludges when calculated based on Ce/Dy (dysprosium), Ce/Er (erbium) and Ce/Eu (europium) ratios. Electron microscopy analysis of sludges dominated by anthropogenic Ce revealed that the Ce was mostly contained in nanoscale particles devoid of elements characteristic of Ce-bearing minerals. Thus, anthropogenic Ce contents derived from REE patterns may be used to validate current mass flow models for engineered CeO₂ nanoparticles.