Determination of Cr(III) and Cr(VI) species in natural waters by catalyticcathodic stripping voltammetry

A technique for determining the speciation of chromium in natural waters is presented to distinguish Cr(VI), active inorganic Cr(M) and non-active org anic Cr(IH) complexes. The technique is based on catalytic cathodic strippi ng voltammetry with adsorption of Cr(III)-DTPA complexes (CCSV-DTPA). Organ ic-Cr(III) complexes are not electrochemically active, whereas free aqua Cr 3+ and its hydroxyl ions active. According to different behaviors of Cr(VI) and active Cr(HI) in buck solution and at electrode interface, a new calib ration for [Cr(III)](active), and an entire protocol for Cr speciation are proposed. Peak currents for active Cr(III) decreased and could be fitted ex ponentially. Following the fitting curve for first 10-20 min, the extrapola ted peak current at time zero (i(pt0)) was used as a measure of [Cr(III)](a ctive) [Cr(Vl)] was measured 60 min after DTPA addition with a stable peak current when Cr(III)-DTPA complexes were all converted to a non-active spec ies. The operational conditions for CCSV-DTPA were optimized. The detection limit and the recovery of [Cr(Vl)] and [Cr(III)](active) were, respectivel y, 0.1 nM and (100 +/- 3)% at 10 nM level in the presence of organic ligand s. The speciation procedure was applied to natural samples collected from S wiss rivers, lakes, soil drainage, and landfill leachate. The [Cr](total) m easured by GF-AAS and by CCSV-DTPA is close in nanomolar. The results indic ate occurrence of non-active organic-Cr(M) complexes in natural waters. The fractions of [Cr(III)] were substantial in some samples from pollutant sma ll rivers and soil leachates. Further application of the technique with eva luation of species alteration would facilitate studies related to chromium transportation, transformation and remediation in natural and contaminated environments. (C) 2001 Elsevier Science B.V. All rights reserved.