Carbonate adsorption on ferrihydrite under atmospheric conditions: Experimental and modeling

Carbonate species are abundant in the environment, continuously interacting with other ions and minerals, and directly influencing their aq. equil. The ability of carbonate to strongly sorb onto minerals surfaces, such as iron oxides, affects the sorption of other oxyanions since carbonate species compete for binding sites of metal oxides. The increasing atm. world CO2 concn., besides contributing to global warming, has prominent implications on adsorption equil. as well. Although elucidating the carbonate adsorption on iron oxides under different atm. CO2 concns. is imperative, the development of an exptl. set-​up is a challenge in itself, and thus studies on CO2 adsorption are rather scarce. For ferrihydrite, the only carbonate adsorption data available come from batch expts. measured with 14C, which do not reflect the behavior under open atm. conditions. Recently, we have rebuilt the set - up previously used to quantify carbonate adsorption on goethite, by measuring the CO2 indirectly using an IR director. This set - up allows the simulation of both open (const. pressure) and closed (fixed amt. of total CO2) atm. conditions. In this study, we have quantified carbonate adsorption under open atm., and specifically for two PCO2, 400 and 600 ppm, at a pH range 4 - 11. The results showed continuous increase of carbonate adsorption with pH for both pressures. The data collected were used for the calibration of a surface complexation model of carbonate on ferrihydrite.