Calcium and zinc adsorption by iron hydrous oxide gel

In the presence of 1M NaNO/sub 3/, freshly precipitated iron hydrous oxide gel suspensions (0.093M Fe) selectively adsorbed Ca and Zn over a wide concentration range. Adsorption was highly pH dependent, especially for Zn. For Ca, the pH at which 50% adsorption occurred, pH/sub 50/, varied from pH 7.15 at 2 x 10/sup -6/M Ca (total) to pH 9.55 at 10/sup -2/M Ca, whereas for Zn, the pH/sub 50/ values varied from pH 5.00 at approx. 10/sup -7/M Zn to pH 6.25 at 10/sup -2/M Zn. Adsorption isotherms for Ca (pH 8.0) and Zn (pH 5.5 and 6.5) showed no adsorption maxima. With high Zn concentrations at pH 6.5, Zn precipitated as a separate, visible zinc hydroxide (or hydroxycarbonate) phase. At an equilibrium concentration of 10/sup -7/M Zn, Fe gel adsorbed approximately 5 ..mu..g Zn/g at pH 5.5 and an estimated 240 ..mu..g Zn/g at pH 6.5 indicating that amorphous iron oxides retain small, but perhaps significant, quantities of Zn at realistic soil solution concentrations