Effect of low molecular organic acids on the transformation of Ni-green rust: Mineral phase transformation and elemental fate

Green Rust (GR) significantly affects the geochemical cycle of heavy metals and organic carbon, but a comprehensive knowledge of the underlying mechanisms is still lacking. To explore the impact of low molecular organic acids on the transformation of GR and the cycle of elements, Nickel (Ni(II)), oxalic acid (OA), and tartaric acid (TA) were selected in this study. Due to the partial occupation of Fe(II) by Ni(II), the obtained GR containing Ni(II) (Ni-GR) exhibited higher crystallinity and thinner layer than pure GR. The main conversion products of pure GR were magnetite and goethite after 24 h transformation, while Ni-GR was mainly converted into goethite and lepidocrocite due to the rapid hydrolysis of Ni(II). Since complexation and reducibility slowed down the dissolution and oxidation of Fe(II) in GR, the inhibition effect of TA on Ni-GR transformation was greater than OA. OA and Ni(II) easily formed a soluble complex, thus promoting the dissolution of Ni(II) in Ni-GR. However, TA showed potent inhibition on Ni-GR transformation and low complexation for Ni(II). Therefore, compared to OA, TA was more easily immobilized by GR and its conversion products, suggesting that carbon sequestration closely related to the structure of organic matter and characteristics of iron oxides.