The effects of selenate on goethite synthesis and selenate sorption kinetics onto a goethite surface - A three-step process with an unexpected desorption phase

Iron oxyhydroxides are widely spread in soils and sediments. They can considerably affect the migration of selenate in the environment, due to their high affinity towards oxyanions. To assess the sorptive properties of one of the most common oxyhydroxides, the selenate sorption onto goethite and its effects on goethite's structural properties during its crystallization were investigated. The sorption kinetics of selenate onto goethite at different pH revealed the presence of a desorption phase that had manifested extensively, especially at higher pH values. Also, an isotherm study indicated that the selenate sorption onto goethite is multi-layered at higher concentrations. Phosphate, which is one of the common naturally occurring oxyanions, affected selenate sorption onto goethite significantly, while chloride and sulfate influenced sorption only marginally. The Mossbauer spectrometry showed no structural changes after iron oxyhydroxides crystallization in the presence of selenate. However, changes outside the goethite lattice were manifested by the alterations in distributions of hyperfine magnetic fields obtained from Mossbauer spectra analyses, as well as by the changes in values of the selenate-treated samples' surface charge.