Dissolution of mixed amorphous–crystalline Cd-containing Fe coprecipitates in the presence of common organic ligands

Abstract

Environmental context Common soil minerals, which often contain trace metals in contaminated environments, are thought to limit metal-related risks in the environment. We studied the stability of these contaminated minerals and found that the presence of plant-derived organic compounds can alter contaminant availability in such environments. Understanding how soluble organics may change trace metal availability helps to predict risk and potentially remediate such environments more efficiently. Abstract Trace metals are common impurities in Fe oxides in soils and can be liberated by organic compounds in the soil. Impurities can also alter and potentially destabilise mineral structures by increasing amorphous or nanocrystalline components. Two Fe oxides found in smelter-contaminated soils, goethite (α-FeOOH) and franklinite (ZnFe2O4), were synthesised as coprecipitates with Cd and subjected to dissolution using oxalic acid, citric acid, cysteine and histidine. Substantial quantities of amorphous- (10–60% of total) and surface-associated Cd (5–70% of total) were present in all coprecipitates. Histidine and oxalic acid consistently enhanced, while cysteine inhibited, Cd release from all coprecipitated minerals. Mixed amorphous–crystalline mineral aggregations are common in natural soils, and must be further studied to understand their role in contaminant availability. In addition, Fe oxides, often considered sinks for toxic metals, may be less effective at reducing contaminant mobility and bioavailability in biologically active soils.