The interaction between organic acids and green rust-Co(II): Mineralogical changes of green rust and redistribution of Co(II)

Green rust (GR), as a vital intermediate product during the formation of various iron oxides, exists with organic matters and metals contaminants in natural environments. Understanding the effects of these natural factors on the transformation process of GR into iron oxides and the environmental behaviors of heavy metals and organic matters during process are critical for environmental quality management, but the fundamental identification of the interaction mechanisms between them and GR is still challenging. In this study, the transformation mechanisms of Co-bearing green rust (GR-Co) synthesized by co-precipitation, and the redistribution behaviors of Co(II) in an environment containing oxalic acid (OA) and citric acid (CA) were clarified. The findings indicated that OA promoted the Fe(II) dissolution and the transformation of GR-Co to goethite, while CA decreased the Fe(II) dissolution and the proportion of non-extractable Co. Furthermore, in the presence of CA, the transformation products of GR-Co were ferrihydrite, magnetite, lepidocrocite and goethite instead of only lepidocrocite and goethite. Meanwhile, CA prohibited ferrihydrite from transforming into more highly crystalline iron minerals. The finding of this study improves the understanding of the interaction mechanisms between GR-Co and organic matter, and the environmental geochemical behaviors of Co and organic carbon during the transformation processes in nature.