Behavior of Chromium in Soils: V. Fate of Organically Complexed Cr(III) Added to Soil

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Abstract
Organically complexed Cr(III) added to soils may remain soluble, whereas the free Cr(III) metal ion would quickly become adsorbed and/or hydrolyzed and precipitated in the absence of soluble, complexing ligands. To separate these two phenomena, a laboratory procedure was developed to simulate the disposal of Cr(III)‐containing organic wastes in soils, and to investigate the solubilities of organic complexes of this heavy metal after addition to soil. The method involves adding Cr(NO3)3 to solutions of organic ligands adjusted to a pH between 3 and 8 to form inorganic hydrolyzed species and organic complexes at each pH. After measuring soluble Cr vs. pH, moist soil is added to bring the pH of each Cr‐ligand system to 6.5–7.0, thereby precipitating nncomplexed, inorganic Cr(III), and labile Cr(III)‐organic complexes. Subtracting Cr(VI) formed from total soluble Cr in the soil suspension gives a relative measure of organically complexed Cr(III) originally present in the aqueous systems at each pH.Citric acid, diethylenetriaminepentaacetic acid (DTPA), fulvic acids, and water‐soluble organic matter from air‐dried soil kept Cr(III) in solution above pH 5.5, and prevented its immediate removal by soil.Added Cr‐citrate remained soluble for at least 1 y in limed samples of an Ultic Hapludalf Ap horizon incubated at field capacity moisture. Because organic acids similar to those studied here may be found in sewage sludge, animal manures, and industrial wastewater, these results should be useful in predicting the fate of Cr(III) added to soils amended with such wastes.

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