Element fractionation by sequential extraction in a soil with high carbonate content.
The influence of carbonate and other buffering substances in soils on the results of a 3-step sequential extn. procedure (BCR) used for metal fractionation was investigated. Deviating from the original extn. scheme, where the exts. are analyzed only for a limited no. of metals, almost all elements in the soils were quantified by X-ray fluorescence spectroscopy, in the initial samples as well as in the residues of all extn. steps. Addnl., the mineral contents were detd. by X-ray diffractometry. Using this methodol., it was possible to correlate changes in soil compn. caused by the extn. procedure with the release of elements. Furthermore, the pH values of all exts. were monitored, and certain extn. steps were repeated until no significant pH-rise occurred. A soil with high dolomite content (27%) and a carbonate free soil were extd. Applying the original BCR-sequence to the calcareous soil, carbonate was found in the residues of the first two steps and ext. pH-values rose by around two units in the first and second step, caused mainly by carbonate dissoln. This led to wrong assignment of the carbonate elements Ca, Mg, Sr, Ba, and also to decreased desorption and increased re-adsorption of ions in those steps. After repetition of the acetic acid step until ext. pH remained low, the carbonate was completely destroyed and the distributions of the elements Ca, Mg, Sr, Ba as well as those of Co, Ni, Cu, Zn and Pb were found to be quite different to those detd. in the original extn. Furthermore, it could be shown that the effectiveness of the redn. process in step two was reduced by increasing pH: Fe oxides were not significantly attacked by the repeated acetic acid treatments, but a 10-fold amt. of Fe was mobilized by hydroxylamine hydrochloride after complete carbonate destruction. On the other hand, only small amts. of Fe were released anyway. Even repeated redn. steps did not destroy the amorphous Fe oxides completely, showing that 0.1 mol L-1 hydroxylamine hydrochloride was not strong enough to attack these oxides effectively. The extn. sequences were carried out not only on the soil samples, but also on their coarse and fine fractions (> or <2 mm). The fine fraction of the calcareous soil contained only 10% dolomite, but was enriched in org. matter and clay minerals, which also resulted in increased ext. pH-values during the sequential extn. Hence, the effects on ion release in the fine fraction were similar to those of the whole soil. Since the destruction of the org. matter was incomplete after regular oxidn., the H2O2-treatment of the fine fraction had to be repeated. The addn. of the extractable amts. of the two fractions showed good agreement to the results obtained for extn. of the whole soils. Likewise the pH-values of the carbonate-free soil exts. did not increase significantly, therefore it was concluded that repetitions of extn. steps for this soil were not necessary. Ext.-pHs should always be controlled so that extn. conditions are comparable; to be able to use the BCR extn. scheme or similar ones for carbonate- and org.-rich samples this is mandatory. Single extn. steps should be repeated if pH rises too much; addnl. the oxidizing step should be performed more than twice for samples rich in org. substances, depending upon the violence of the reaction with H2O2. If these precautions are neglected the validity of the extn. data is likely to be questionable.
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