The kinetics of reduction and oxidation of soluble chromium in various soils (peat, clay, sand, and luvi-chromic cambisols) were studied at constant soil moisture and constant temperature. A total of 1500 g of each soil was incubated with different concentrations of aqueous solutions of K 2 Cr 2 O 7 or CrCl 3 [1-1000 µg (g of dry soil) -1 ]. The total exchangeable chromium and Cr(VI) were analyzed in soil extracts over a period of up to 10 days. The parameters influencing the reduction and oxidation of soluble chromium were studied. Reduction of soluble chromium was observed in all the soils examined. It depends mostly on the content of organic matter, on the concentration of the added chromium, and on the pH of the soil. Up to 3 days of the experiment, the reaction was found to be firstorder with respect to Cr(VI). Oxidation of soluble chromium was observed especially in soils high in manganese(IV) oxides and low in organic matter. The oxidation of added soluble chromium did not occur in peat soil. The results indicate that the oxidation and reduction of soluble chromium added to soils depends on the soil structure and on the reduction/oxidation conditions in the particular soil.

Three different analytical techniques [Ir5-diphenylcarbazide spectrophotometry, chelating ion-exchange chromatography (Chelex-IOO), and ion-pairing reversed-phase high-performance liquid chromatography (RP-HPLC) combined with electrothermal atomic absorption spectrometry (ETAAS)] were critically evaluated for the determination of CrV1 in soil extracts. Spectrophotometry was not applicable t o the analysis of most soil extract samples owing t o its high limit of detection (LOD = 30 ng cm-3), and the possibility of the instantaneous reduction of Crvl under the acidic conditions employed. A Chelex 100 column, although adequately sensitive (LOD = 1.5 ng cm-3), is inclined t o give higher results as inert and moderately labile Cr"l complexes partially passed through the resin together with CrVt. In addition, very small particles (€0.45 pm) carrying chromium can produce severe positive systematic errors. In order t o avoid this, filtration employing a 0.1 p m filter is recommended. Ion-pairing RP-HPLC was found t o be the most sensitive technique (LOD = 0.3 ng cm-3). It might also give high chromate results if negatively charged Cr1I1 complexes form ion pairs with tetrabutylammonium phosphate and their elution partially coincided with that of CrV1. Fulvate ligands showed this type o f interference. Reversed-phase HPLC is not suitable for analysis of extracts obtained from soils with freshly added tannery waste owing t o the effects of the undestroyed tannery waste matrix. This study showed that each method investigated was vulnerable t o some type of interference.

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