Long-term reactions of Ni, Zn and Cd with iron oxyhydroxides depend on crystallinity and structure and on metal concentrations

Iron oxyhydroxides are long-term sinks for toxic trace metals and are suggested as factors to explain long-term metal immobilization in soil. It is unknown how crystallinity and crystal structure of iron (Fe) oxyhydroxides affect the long-term kinetics of trace metal adsorption. Adsorption kinetics of nickel (Ni), zinc (Zn) and cadmium (Cd) on to four synthetic Fe oxyhydroxides (hydrous ferric oxide (HFO), ferrihydrite (FH), goethite (GT), haematite (HT)) were characterized in suspensions during 70 days at different initial metal concentrations and various pH values (3.8–7.0) maintained within 0.2 units. The slow reactions, defined as those occurring beyond day 1 were most pronounced in GT and least in HT. The extent of the slow reaction was surprisingly unrelated to crystallinity or microporosity as determined by N₂ adsorption. Freundlich adsorption isotherms fitted to the data show that ageing between 1 and 70 days decreases metal concentrations in solution by factors of 2–30 (Ni), 1–20 (Zn) and 1–4 (Cd) depending on the type of oxyhydroxide. These factors become significantly larger at smaller concentrations for Ni and Zn in some oxyhydroxides and this concentration dependency suggests that slow reactions are not only related to diffusion. The short-term adsorption strength of carrier-free metal radio-isotopes was similar at 1 or 70 days after reaction with these iron oxyhydroxides and excludes the possibility that long-term reactions are related to solid-phase transformations or changes in solution properties. The extent of slow reactions in GT decreased with decreasing pH and was less than a factor 2 within 70 days at pH 3.7. The results confirm that slow reactions in Fe oxyhydroxides can explain lessened metal availability by ageing in pH neutral soils. The uncertainty of their actual structure and crystallinity in soils does not allow quantitative predictions based on the behaviour of synthetic oxyhydroxides.