Applying a modified Donnan model to describe the surface complexation of chromate to iron oxyhydroxide agglomerates with heteromorphous pores.

In this study, a modified Donnan model (mDM) is incorporated into surface complexation model (SCM) to better understand the physicochemical processes for adsorption of hexavalent chromium, Cr(VI), on porous iron oxyhydroxide agglomerates (IOAs). The mDM includes a chemical potential term μ to account for ionic transport and electrostatic interaction in micropores (d<2nm) which is usually neglected in typical Donnan model (tDM). To estimate the parameters of mDM in a simple and accurate way, a rigorous protocol was presented. First, the prepared IOAs was characterized with a heteromorphous pore structure (i.e., co-existing micropores and macropores, d>2nm) demonstrating high Cr(VI) adsorption in a broad range of ionic strengths. The batch data was then fitted with Donnan model in PHREEQC to obtain Stern (ψ) and Donnan (ψ) potentials used for μ calculation. The decreasing μ values with ionic strength indicated obstructing effect of electrolyte ions on Cr(VI) uptake in micropores. Finally, the ionic activity coefficients and reaction constants were corrected using Pitzer model due to the high level electrolytes accumulated in the Donnan layer through osmotic and electrostatic attraction. Results of this study have captured the effects of inner structure of IOAs on Cr(VI) uptake and quantitatively discerned the contribution of micropores and macropores for adsorption reactions at different ionic strengths.