Chromium mobilisation by ligand complexation in oxic and anoxic environments and the isotopic fingerprint

Chromium (Cr) and its isotopic composition (δ⁵³Cr) are emerging as valuable proxies in both environmental and paleoenvironmental studies, yet their redox behaviour is complicated by the presence of organic ligands. This study performed laboratory-controlled batch dissolution and flow-through column experiments on soils/sediments to examine the effects of low-molecular-weight organic acids on Cr mobility and isotopic fractionation. Our results reveal that citric and oxalic acids can enhance the dissolution of Cr(III) under strongly reducing conditions where relatively high levels of ferrous iron are present, highlighting an overlooked pathway of Cr mobilisation. The Cr isotope variability (δ⁵³Cr = −1.39‰ to +0.11‰) in batch leachates and column effluents is not directly driven by redox changes; the very light δ⁵³Cr signatures likely reflect effects of incongruent dissolution and kinetic isotope fractionation. Ligand-bound Cr(III) can potentially be transported across environmental interfaces with distinct isotopic signature, complementing conventional views of Cr and Cr isotope biogeochemical cycling.