Geochemical Decoupling of Iron and Zinc during Transformation of Zn-Bearing Ferrihydrite in Reducing Sediments

The transformation of the mineral ferrihydrite in reducing environments, and its impact on the mobility of incorporated trace metals, has been investigated in model laboratory studies, but studies using complex soil or sediment matrices are lacking. Here, we studied the transformation of zinc (Zn)-bearing ferrihydrite labeled with 57Fe and mixed with natural sediments, incubated in reducing conditions for up to six months. We tracked the evolution of Fe and Zn speciation with 57Fe Mössbauer spectroscopy and with bulk and micro-X-ray absorption spectroscopy. We show that Fe was readily reduced and incorporated into a poorly crystalline mixed-valence Fe(II)–Fe(III) phase resembling green rust. In parallel, Zn was released in the surrounding porewater and scavenged by precipitation with available ligands, particularly as zinc sulfide (ZnS) or Zn-carbonates. Early in the mineral transformation process, the chemical behavior of Fe was decoupled from Zn, suppressing the impact of Zn on the rates and products of the ferrihydrite transformation. Our results underline the discrepancy between model experiments and complex field-like conditions and highlight the importance of sediment and soil geochemistry and ligand competition on the fate of divalent metal contaminants in the environment.

矿物水铁矿（ferrihydrite）在还原环境中的转化过程，及其对固载痕量金属迁移性的影响，已在模拟实验室研究中得到探究，但针对复杂土壤或沉积物基质的相关研究仍较为匮乏。本研究针对经57Fe标记、混合天然沉积物的含锌水铁矿展开探究，将其在还原条件下培养长达六个月。研究团队借助57Fe穆斯堡尔谱学，以及整体与微区X射线吸收光谱，追踪了铁（Fe）与锌（Zn）的形态演化过程。研究结果表明，铁可被快速还原，并固载至类绿锈的低结晶度混合价态Fe(II)-Fe(III)物相中。与此同时，锌被释放至周边孔隙水中，并通过与有效配体的沉淀作用被固持，主要产物为硫化锌（ZnS）或锌碳酸盐。在矿物转化过程初期，铁与锌的化学行为发生解耦，抑制了锌对水铁矿转化速率及产物的影响。本研究结果凸显了模拟实验与类复杂野外环境之间的差异，并强调了沉积物与土壤地球化学、配体竞争对环境中二价金属污染物归趋的重要性。