Concurrent Sorption of Phosphate and Zinc, Cadmium, or Calcium by a Hydrous Ferric Oxide

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Abstract
Concurrent sorption of phosphate and Zn, Cd, or Ca by a hydrous Fe oxide sample was investigated by enclosing sparingly‐soluble Zn‐phosphate, Cd‐phosphate, or Ca‐phosphate in dialysis tubings and equilibrating with the oxide up to 2 yr in 0.1 mol L−1 NaNO3 solution at room temperature (296 ± 2 K). The ability of the oxide to concurrently sorb phosphate and Zn, Cd, or Ca was evident from their amounts present in the oxide, as well as from x‐ray analysis and microprobe mapping of the elements sorbed on Fe‐oxide particles. The selectivity of the oxide for the metals increased int he order Ca < Cd < Zn, and was apparently not affected by the absolute amounts of phosphate sorbed. No obvious complementary effect of metal sorption on phosphate sorption, or vice versa, was observed. Aging the oxide up to 1 or 2 yr in the presence of metal and phosphate adversely affected only the sorption of Ca but not the sorption of Zn, Cd, or phosphate. Therefore, it is likely that the affinity determines whether or not a sorbate, once sorbed, can be excluded from the oxide undergoing aging or changes in its crystallinity. Two simplified equations were tested for their ability to fit the sorption of the metals and phosphate by the oxide, with sorbate concentration, pH, and available sites as dependent variables. The good fit of Zn, Cd, and phosphate sorption data to the equations suggests that phosphate and the metal are being sorbed on specific sites.