RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The adsorption-desorption of the divalent metal cations (Me²⁺) Co²⁺, Cd²⁺, and Pb²⁺ to hydrous ferric oxide (HFO) was investigated as a function of oxide aging and Me²⁺-oxide residence time. The HFO was produced and stored for up to 86 wk. Periodically, Me²⁺ sorption was determined across the pH range of 2.5 to 12. In addition, the Me²⁺ ions were contacted with freshly produced HFO and stored at a pH that dictated that 80 to 100% of the Me²⁺ would be in the sorbed state; desorbability of the Me2+ was determined as a function of Me²⁺-oxide residence time. The change in the crystallinity of the HFO as a function of time was also monitored. The HFO aged without the Me²⁺ ions displayed no hysteresis between the adsorption-desorption curves and no substantial shifts in fractional Me²⁺ adsorption were observed with pH throughout 21 wk of aging. The HFO aged with the Me²⁺ ions displayed increasing desorption hysteresis with time for Co²⁺ and Cd²⁺, but not Pb²⁺. The magnitude of hysteresis followed the order Co > Cd > Pb, which is the inverse of the ionic radii of the metal sorbates. While oxalate-extractable Fe decreased with time during a 20-wk period, powder x-ray diffraction was unchanged during the same period. The data presented here suggest that Co and Cd are being incorporated into the metal oxide structure via recrystallization, but Pb remains associated with the surface and excluded from incorporation.