Thiocyanate adsorption on ferrihydrite and its fate during ferrihydrite transformation to hematite and goethite

Abstract

Thiocyanate (SCN−) is a toxic contaminant produced by industrial processes such as gold ore cyanidation and coal coking. The potential for remediation by adsorption of SCN− on ferrihydrite, the influence of sulfate (SO42−) on SCN− adsorption, and the fate of adsorbed SCN− during ferrihydrite aging were studied using macroscopic techniques complemented with attenuated total reflectance–Fourier transform infrared analysis (ATR–FTIR), X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). Results showed that adsorption of SCN− was strongly affected by the concentration of electrolyte (NaNO3) and pH, with decreases in concentration of NaNO3 and pH leading to increased SCN− adsorption. The adsorption isotherms can be described by the Langmuir model. While at lower concentrations (0.52–1.04mM), the presence of SO42− had little impact on SCN− adsorption, at a higher concentration (2.08mM), SCN− adsorption was significantly inhibited. ATR–FTIR data confirmed that SCN− was bound as an outer-sphere complex on ferrihydrite, and this mechanism was not influenced by changes in pH or electrolyte concentration. XRD data showed that ferrihydrite transformed to a mixture of hematite and goethite at 75°C and pH 5 in the presence and absence of SCN−. Partitioning data revealed that during ferrihydrite transformation, all adsorbed SCN− was released into solution.