COBALT, CADMIUM, AND LEAD SORPTION TO HYDROUS IRON-OXIDE - RESIDENCE TIME EFFECT

The adsorption-desorption of the divalent metal cations (Me(2+)) Co2+, Cd2+, and Pb2+ to hydrous ferric oxide (HFO) was investigated as a function of oxide aging and Me(2+)-oxide residence time. The HFO was produced and stored for up to 86 wk. Periodically, Me(2+) sorption was determined across the pH range of 2.5 to 12. In addition, the Me(2+) ions were contacted with freshly produced HFO and stored at a pH that dictated that 80 to 100% of the Me(2+) would be in the sorbed state; desorbability of the Me(2+) was determined as a function of Me(2+)-oxide residence time. The change in the crystallinity of the HFO as a function of time was also monitored. The HFO aged without the Me(2+) ions displayed no hysteresis between the adsorption-desorption curves and no substantial shifts in fractional Me(2+) adsorption were observed with pH throughout 21 wk of aging. The HFO aged with the Me(2+) 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 ph remains associated with the surface and excluded from incorporation.