Nanoscale Observation of Nickel(II) Sequestration by Green Rust Sulfate

Green rust (GR) is a mixed-valent Fe-layered double hydroxide (Fe-(II)-Fe-(III)-LDH) mineral that is prevalent in reducing geochemical environments, where it exhibits high sorption and redox reactivity. Here, we examined the morphology and chemistry of individual GR particles with nanoscale resolution in the presence and absence of Ni-(II)_aq at reaction times between 1 h and 3 months using scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDXS). During the first day of reaction, sorbed Ni-(II) accumulated alongside Fe in ∼10 nm thick rims around the GR particle edges, consistent with the formation of mixed Ni-(II)/Fe-(II)-Fe-(III)-LDH. After 3 months, the rims were thinner and contained less Ni-(II) despite a doubling of the sorbed load, suggesting sequestration of Ni-(II) sorbates into the bulk during aging. Sequential extractions similarly provided evidence for declining levels of sorbed Ni-(II) at the GR surface with time, concurrent with increasing levels inside the mineral bulk. The combined STEM-EDXS and extraction results demonstrate chemical and structural variations across GR particle surfaces and redistribution of Ni-(II) sorbates during aging over time scales of days-weeks. These findings provide new mechanistic insights into the processes controlling the partitioning and mobility of trace metals in reducing geochemical systems.