Surface complexation modeling of arsenate adsorption by akagenéite (β-FeOOH)-dominant granular ferric hydroxide

A surface complexation model has been set up for the first time with akaganéite (β-FeOOH), a major compound of granular ferric hydroxide (GFH) used in groundwater purification units worldwide for arsenic and other toxic pollutant removal. Unlike the situation for the α-FeOOH polymorph goethite, there is yet no published surface complexation model to predict competing oxyanion effects. This is due to a linked bulk-surface reactivity by which excess protons released in an unknown amount from tunnel sites hamper quantification of surface protonation by acid titration. The measured proton uptake thus exceeds what can be expected from OH groups active in surface protonation. However, chloride ions are co-sorbed in 1:1 stoichiometry and co-released with the protons from tunnel sites albeit with slow kinetics. Because of this kinetics, dynamic acid-base titrations using automatic titrators are not feasible for reliable estimation of surface charge vs. pH curves. We propose to precisely monitor the chloride exchange during batch titrations with at least one day batch equilibration times. The thus obtained chloride data were used to correct for the pH-dependent bulk proton exchange and to reconstruct the true surface potential vs. pH curves, and thus to reliably parameterize a CD-MUSIC surface complexation model for arsenate adsorption.