Arsenic strongly associates with ferrihydrite colloids formed in a soil effluent

Arsenic mobility may increase in liquid phase due to association with colloidal Fe oxides. We studied the association of As with Fe oxide colloids in the effluent from water-saturated soil columns run under anoxic conditions. Upon exfiltration, the solutions, which contained Fe2+, were re-aerated and ferrihydrite colloids precipitated. The entire amount of effluent As was associated with the ferrihydrite colloids, although PO43−, SiO44−, CO32− and dissolved organic matter were present in the effluent during ferrihydrite colloid formation. Furthermore, no subsequent release of As from the ferrihydrite colloids was observed despite the presence of these (in)organic species known to compete with As for adsorption on Fe oxides. Arsenic was bound via inner-sphere complexation on the ferrihydrite surface. FTIR spectroscopy also revealed adsorption of PO43− and polymerized silica. However, these species could not impede the quantitative association of As with colloidal ferrihydrite in the soil effluents.

► Ferrihydrite (Fh) colloids precipitated from a solution derived from soil.
► Arsenic that was discharged from soil entirely associated with these colloids.
► Arsenic was strongly bound to the Fh surface via inner-sphere complexation.
► A complexation of As by organic species discharged from soil was not detected.
► (In)organic solutes in the solution from soil could not impede the As–Fh association.