Speciation Of Chromium In Sea Water Research Articles

Bringing part of the lab to the field: On-site chromium speciation in seawater by electrodeposition of Cr(III)/Cr(VI) on portable coiled-filament assemblies and measurement in the lab by electrothermal, near-torch vaporization sample introduction and inductively coupled plasma-atomic emission spectrometry

A field-deployable electrochemical approach to preconcentration, matrix clean up and selective electrodeposition of Cr(III) and Cr(III)+Cr(VI) in seawater is described. Using portable, battery-operated electrochemical instrumentation, Cr species in seawater were electrodeposited in the field on portable coiled-filament assemblies made from Re. Assemblies with dried residues of Cr(III) or Cr(III)+Cr(VI) on them were transported to the lab for concentration determination by electrothermal, near-torch vaporization (NTV) sample introduction and inductively coupled plasma-atomic emission spectrometry (ICP-AES).Electrodeposition offers selective species deposition, preconcentration and matrix clean up from seawater samples. For selective deposition, free Cr(VI) was electrodeposited at −0.3V and Cr(III)+Cr(VI) at −1.6V (both vs Ag/AgCl). Interestingly, at 0V (vs Ag/AgCl) and in the absence of an electrodeposition potential only Cr(VI) was spontaneously and selectively adsorbed on the coil and reasons for this are given. Due to preconcentration afforded by electrodeposition, the detection limits obtained after a 60s electrodeposition at the voltages stated above using buffered (pH=4.7) artificial seawater spiked with either Cr(III) or Cr(VI) were 20pg/mL for Cr(III) and 10pg/mL for Cr(VI). For comparison, the detection limit for Cr obtained by pipetting directly on the coil 5μL of diluted standard solution was 500pg/mL, thus it was concluded that electrodeposition offered 40 to 60 fold improvements. Matrix clean up is required due to the high salt content of seawater and this was addressed by simply rinsing the coil with 18.2MΩ water without any loss of Cr species. Reasons for this are provided. The method was validated in the lab using buffered artificial seawater and it was used in the field for the first time by sampling seawater, buffering it and immediately electrodepositing Cr species on portable assemblies on-site.Electrodeposition in the field addressed species transformation during sample pre-treatment. Such transformations occur due to sample acidification and may take place during transport and possibly storage prior to analysis. Thus, electrodeposition in the field is more reflective of Cr species concentration at the environmental conditions (e.g., temperature) at the time of sampling. It also opens up the possibility for shipping to the lab portable assemblies with Cr species on them rather than shipping large volumes of sample to the lab, thus also reducing shipping, handling and storage costs.

Speciation of Chromium in Seawater by ICP-AES with Dual Mini-columns Containing Chelating Resin

A method for the preconcentration and speciation of chromium in seawater was developed. On-line preconcentration and determination were carried out by using inductively coupled plasma atomic emission spectrometry (ICP-AES) with dual mini-columns containing a chelating resin. In this system, Cr(III) was collected on the first column. The effluent containing residual chromium from the first column was collected on the second column after passing through a reduction-switching unit, in which the reducing agent was introduced, or not, for the reduction of Cr(VI) to Cr(lII). Cr(VI) was determined as the difference between the concentration of pre-reduced Cr(VI) and Cr(III) in the effluent from the first column. The detection limits for Cr(III) and Cr(VI) were 0.04 and 0.09 microg l(-1), respectively.

The determination of the chromium speciation in sea water using catalytic cathodic stripping voltammetry

A voltammetric procedure to determine chromium in aqueous solution using cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complex species with diethylenetriaminepentaacetic acid (DTPA) on a hanging mercury drop electrode is optimized for sea water. The optimized conditions include a pH of 5.2 and a DTPA concentration of 2.5 mM. Comparison of the chemical speciation of DTPA in sea water and distilled water with the CSV sensitivity indicated that the adsorptive species is a complex of chromium(III_with H 2DTPA. The sensitivity of the CSV procedure for chromium(VI) in sea water is a decade lower than in distilled water due to major cation competition (of calcium and magnesium) for DTPA in the sea water. The limit of detection for chromium(VI) in sea water is 0.1 nM at a deposition time of 2 min. Chromium(VI) produces a stable peak using the optimized CSV procedures, whereas the peak due to chromium(III) is unstable due to probable conversion of the chromium(III) complex to an electrochemically inert complex over a period of ≈ 30 min. The different behaviours of chromium(VI) and –(III) were utilized to devise a procedure to determine reactive chromium(III) and chromium(VI) in sea water samples. Experiments with an added chelator indicated that the reactive chromium(III) concentration does not include complexes with organic material. The total dissolved chromium concentration was determined after UV-irradiation of the sample at its original (neutral) pH. Storage experiments indicated that neither the redox speciation of chromium nor its total dissolved concentration in sea water could be stored. The CSV method was applied succesfully to certified sea water samples and to the ship-board determination of chromium(VI) and –(III) in the water column of the N.W. Mediterranean.

Chromium speciation in seawater: The probable role of hydrogen peroxide

The rates of oxidation of Cr(III) to Cr(VI) with H2O2 were measured in NaClO4 solutions as a function of pH, temperature, ionic strength, and H2O2 concentration. The rate of oxidation of Cr(III) was found to be first order with respect to 1/[H+] or [OH−] (pH 7–9) and [H2O2] (112–2,621 µM). The rate equation is given by urn:x-wiley:00243590:media:lno19903530730:lno19903530730-math-0001 where the value of k2 = k1/[H2O2][OH−] = k1 [H+]/ [H2O2] Kw* (k1 is the pseudo‐first‐order rate constant and Kw* is the stoichiometric dissociation constant for the ionization of water). The values of k2 were independent of ionic strength and fit the equation urn:x-wiley:00243590:media:lno19903530730:lno19903530730-math-0002 with SD = 0.06 in log k2. The energy of activation for the reaction is 9.9±0.8 kcal mol−1. The rate constant was used to estimate the half‐time of Cr(III) in surface seawater. At a concentration of 0.1 µM H2O2, the lifetime of Cr(III) is controlled by H2O2.

Chromium speciation in sea water

Thermodynamic considerations suggest that the ions Cr(H 2O) 4(OH) 2 + and CrO 4 2− are the major forms of trivalent and hexavalent chromium in sea water. Cr (VI) should predominate in water in solubility equilibrium with the atmosphere but analysis shows that theoretically unstable Cr (III) species are often dominant.