Behavior of Chromium in Soils: VII. Adsorption and Reduction of Hexavalent Forms

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Abstract
Based on adsorption studies with Fe(OH)3, Cr(OH)3, and soils, 10 mM K‐phosphate buffered at pH 7.2 was found to be adequate for characterizing exchangeable Cr(VI). The quantity of Cr(VI) removed from solution by 19 A and 19 B horizon soils from the northeastern United States was partitioned into two fractions, based on the amount of soil‐Cr extracted with the phosphate buffer. The fraction removed by the buffer was termed “exchangeable;” that retained by the soil was termed “nonexchangeable.” The nonexchangeable Cr(VI) not recovered by phosphate had been either reduced to an insoluble form of Cr(III), or had been precipitated or very tightly adsorbed by the soil as anionic Cr(VI).Of 20 µmol/100 g soil (23 kg/ha) Cr(VI) added, an average of 7.2 was removed by the unlimed 38 soils (mean pH 5.4). Of this quantity, 4.8 µmol were exchangeable by phosphate. Surface soils generally removed less Cr(VI) than B horizons (5.8 vs. 8.6 µmol/100 g), and the proportion of Cr(Vi) removed in phosphate‐exchangeable form also was lower. Liming to pH 7 decreased the average quantity of Cr(VI) removed by the soils to 2.6, with exchangeable and nonexchangeable forms each accounting for about 50% of the total.Sulfate and phosphate added with Cr(VI) to two A and two B horizon soils decreased Cr(VI) removal by the soil, with phosphate having a greater effect than sulfate. Although liming decreased Cr(VI) removal by the soils, even less Cr(VI) was held in exchangeable or nonexchangeable form in limed treatments containing the competing anions.Gallic acid rapidly reduced Cr(VI) held by soil colloids in exchangeable form except in limed samples of spodic horizons and a Hydric Dystrandept Ap, all high in amorphous, organically complexed Al and Fe sesquioxides.

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