An arsenate effect on ferrihydrite dissolution kinetics under acidic oxic conditions

The dissolution behaviour of ferrihydrite and ferrihydrite incorporating arsenate at different levels was investigated under low pH, oxic conditions. The dissolution of pure ferrihydrite was found to be well described by the cube root law for mineral dissolution. The rate of dissolution of ferrihydrite precipitated in the presence of selected levels of arsenate was strongly retarded by the presence of the arsenate. The surface coordinative chemical model of mineral dissolution was applied to interpret and quantify proton dissolution rates of ferrihydrite and the inhibitory effect of arsenate. The inhibitory effect of the arsenate appears to be related to its complexation with the ferrihydrite surface and the consequent involvement of the surface iron centres in competitive equilibria between protons and the surface-complexed arsenate. From the iron:arsenic ratios obtained during dissolution, it has been concluded that arsenic is incorporated into these precipitates by sorption onto existing surfaces of the ferrihydrite during precipitation, rather than by the formation of a solid solution or by co-deposition.