About Chromium (VI) Extraction from Fertilizers and Soils

Extractions from fertilizer and soil samples were performed to yield the operationally defined fractions of “soluble” chromate (extractable with NH4NO3), “exchangeable” chromate (extractable with phosphate buffer pH 7.2), and these results were compared with the data obtained by extractions with ammonium sulfate, borate buffer pH 7.2, saturated borax pH 9.6, and polyphosphate (Graham’s salt). In order to maintain the pH of extractant solution about constant, the concentration of extractant buffer had to be raised to at least 0.5 M. The results strongly depended on the kind of extractant, and the solid: liquid ratio. For most of the samples investigated, the extraction efficiency increased in the order borate&#8211; sulfate &#8211; nitrate &#8211; phosphate. Whereas the recovery of K2CrO4 and CaCrO4 added to the samples of basic slags prior to the extraction was about complete, the recovery of added PbCrO4 was highly variable. In soil extracts, the color reaction was interfered from co-extracted humics, which react with the chromate in weak acid solution during the time period necessary for color reaction (1 hour). However, this problem can be overcome by standard addition and subtraction of the color of the extractant solution. In soil extract of about pH < 7, organic material reduced chromate during the extraction period also, and standard addition of soluble chromate is recommended to prove recovery and the stability of chromate in the samples. In admixtures of soils and basic slags, results for hexavalent chromium were lower than from the mere basic slags. This effect was more pronounced in phosphate than in nitrate extracts. As a proficiency test, samples low in organic carbon from contaminated sites in Hungary were tested. The results from NH4NO3 extracts satisfactorily matched the results of the Hungarian labs obtained from CalCl2 extractants.