CATALYTIC ADSORPTIVE STRIPPING VOLTAMMETRIC DETERMINATION OF CHROMIUM(VI) IN OVERLYING AND INTERSTITIAL WATERS ISOLATED FROM SEDIMENTS CONTAMINATED BY TANNERY WASTE

In the present paper, catalytic adsorptive stripping voltammetry (CAdSV) has been applied for the first time in a chromium speciation study conducted in pore sediments and overlying waters containing Cr(VI) in the presence of a great excess of Cr(III) and organic matter. The method is based on the adsorptive accumulation of the active Cr(III)-DTPA (diethylenetriammine-N, N, N', N '', N ''-pentaacetic acid) complex, which is formed instantaneously on the electrode surface as a result of Cr(VI) reduction to Cr(III), its complexation with DTPA, reduction to Cr(II)-DTPA, and its catalytic oxidation to Cr(III)-DTPA with nitrate. The main efforts were focused on the elimination of the influence of dissolved organic matter including organic surface-active substances (SAS) that adsorb at the electrode surface, and disturb the Cr(VI) determination or, in extreme cases, make it impossible. Of the three procedures applied for the elimination of this negative effect caused, namely, the matrix-exchange method, the removal of SAS by means of fumed silica, and their adsorption on the XAD-7 Amberlite resin present directly in voltammetric vessel, the last one proved to be the most useful. The matrix-exchange procedure also provides accurate and precise results but requires more reagents and time. The CAdSV method combined with the adsorption of surface-active substances on the XAD-7 Amberlite resin was successfully applied for the determination of the Cr(VI) in vertical profile in sediments from Dunajec River, which are severely polluted with chromium from tannery wastes.