Effect of Sulfate on Cu(II) Adsorption/Desorption Behavior on Ferrihydrite Colloids and Cu Speciation

Iron oxide colloids have great adsorption capacity of heavy metals due to their high specific surface area. Inorganic oxyanion can cause colloid aggregation and further affect its reactivity and mobility. However, the effect of sulfate on the metal sequestration by iron oxide colloids is still unknown. In this study, using aggregation, batch adsorption/desorption, and settling experiments combined with XPS and TEM, we elucidated the sorption behavior of Cu(II) on ferrihydrite colloids and the Cu speciation change at various sulfate concentrations. Our results showed that the Cu(II) adsorption on ferrihydrite colloids was improved by adding sulfate, but the enhancement extent obviously decreased at high sulfate concentrations because of the drastic diminishment of available sites. Further desorption experiments indicated that the extent of Cu(II) desorption on ferrihydrite colloids was smaller than that on ferrihydrite particles under the same conditions because of the physical constraints created by aggregation. Moreover, Cu speciation (dissolved, colloidal, and particulate) was controlled by sulfate concentration, with no colloidal Cu found when the concentration exceeded the critical coagulation concentration (63 μM). The study of the multiple interactions between ferrihydrite colloids, Cu(II), and sulfate provided the basis for a comprehensive understanding of the transport and geochemical fate of heavy metals.