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 fu nction of oxide aging and Me(2+)-oxide residence time. The HFO was pro duced and stored for up to 86 wk. Periodically, Me(2+) sorption was de termined across the pH range of 2.5 to 12. In addition, the Me(2+) ion s were contacted with freshly produced HFO and stored at a pH that dic tated that 80 to 100% of the Me(2+) would be in the sorbed state; deso rbability of the Me(2+) was determined as a function of Me(2+)-oxide r esidence time. The change in the crystallinity of the HFO as a functio n of time was also monitored. The HFO aged without the Me(2+) ions dis played no hysteresis between the adsorption-desorption curves and no s ubstantial shifts in fractional Me(2+) adsorption were observed with p H throughout 21 wk of aging. The HFO aged with the Me(2+) ions display ed increasing desorption hysteresis with time for Co2+ and Cd2+, but n ot Pb2+. The magnitude of hysteresis followed the order Co > Cd > Pb, which is the inverse of the ionic radii of the metal sorbates. While o xalate-extractable Fe decreased with time during a 20-wk period, powde r x-ray diffraction was unchanged during the same period. The data pre sented here suggest that Co and Cd are being incorporated into the met al oxide structure via recrystallization, but ph remains associated wi th the surface and excluded from incorporation.