Time-Dependent Iodate and Iodide Adsorption to Fe Oxides

Abstract

Aqueous iodine removal via adsorption onto Fe oxides could provide an efficient remedial pathway for the vadose zone and groundwater contamination. We conducted a series of macroscopic batch experiments to determine the extent of the time-dependent iodate (IO3–) and iodide (I–) adsorption onto four Fe oxides (i.e., ferrihydrite, goethite, magnetite, and hematite) at different pH values and solution ionic strengths (IS). The results showed that the IO3– adsorption extent [in terms of the average distribution coefficient (Kd) after 2 days of reacting time] followed the order: ferrihydrite (927.5 mL/g) > goethite (84.9 mL/g) > magnetite (23.8 mL/g) > hematite (9.5 mL/g). However, the range of specific surface area (SSA)-normalized Kd values was narrow (2–4.6 mL/m2), suggesting SSA control over the adsorption extent. The adsorption extent was correlated negatively with both pH and IS, implying IO3– outer-sphere adsorption. The adsorption extent increased or decreased with time (up to ∼48%) after 200 days, at relatively high or low I concentration ranges, respectively, likely because of multiple geochemical reactions, including interparticle diffusion, mineral transformation, and I speciation changes. I– adsorption was insignificant for all Fe oxides. Because of its large SSA, ferrihydrite could be efficient at removing aqueous iodate, potentially decreasing the time of groundwater plume spreading.