Methodological drawbacks of the extraction of trace metals from Fe and Mn oxides in soil samples with oxalic acid-ammonium oxalate-ascorbic acid solution

Extractions with mixtures of oxalic acid, ammonium oxalate, and ascorbic acid have been used to analyze trace metals that are occluded in Fe and Mn oxides in soil samples. In our experiments, the amount of Fe extracted from two Andisols decreased with increased extraction time using a mixture of 0.1 mol L^-1 oxalic acid, 0.175 mol L^-1 ammonium oxalate, and 0.19 mol L^-1 ascorbic acid (OxAsc). Differential X-ray diffraction analysis showed the presence of Fe(II)C₂O₄·2H₂O in the residue after the extraction, and the amount of this precipitate increased with extraction time. These results indicate that the decrease in Fe extraction with time was caused by precipitation of Fe(II)C₂O₄·2H₂O. The amounts of some trace metals (Cd, Co, Mn, Ni, and Zn) extracted with OxAsc showed trends similar to that of Fe, whereas As, Cu, and Pb did not show this trend. A coprecipitation experiment showed that the concentrations of Cd, Co, Mn, Ni, and Zn markedly decreased after the precipitation of Fe(II)C₂O₄·2H₂O, whereas those of As, Cu, and Pb did not noticeably change. These results indicate that some trace metals coprecipitated with Fe(II)C₂O₄·2H₂O during extraction with OxAsc. This coprecipitation results in underestimation of oxide-occluded trace metals in soils; therefore, using mixtures of oxalic acid, ammonium oxalate, and ascorbic acid to extract some trace metals occluded in Fe and Mn oxides may yield erroneous data.