Zinc Sorption by Iron‐Oxide‐Coated Sand as a Function of pH

Abstract Mineralogy of Fe‐oxide coatings in soils may influence pH‐dependent sorption of heavy metals, but exchangeability of retained metals needs to be better understood to improve predictions of bioavailability. A laboratory study tested the effect of pH on the amount and form of Zn retained in exchangeable (extractable with 1 M KNO 3 ) or nonexchangeable forms on goethite‐ and hematite‐coated sand. Batch equilibrations with 250 and 2500 µmol Zn(NO 3 ) 2 /kg coated or uncoated sand were used to determine concentration effects on the form of Zn retained. Cation‐exchange capacity (CEC) of the goethite‐ or hematite‐coated sand increased with pH (4–7.6), but surface charge was a poor indicator of the amount and form of Zn retained. The goethite‐coated sand retained the Zn principally in a nonexchangeable form, while the hematite‐coated sand retained the Zn in both exchangeable and nonexchangeable forms. The distribution between the exchangeable and nonexchangeable Zn fractions changed from predominantly exchangeable to predominantly nonexchangeable Zn retention as pH increased in both oxide systems. The dominance of nonexchangeable‐Zn sorption by goethite was attributed to surface‐induced hydrolysis by OH groups, while O atoms of hydrated hematite and uncoated sand surfaces were hypothesized to be sorbing Zn in exchangeable forms.