Chromium (III) interactions in seawater through its oxidation kinetics

M. Pettine, Frank J. Millero, T. La Noce

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46 Scopus citations

Abstract

The rates of oxidation of chromium(III) to chromium(VI) with H2O2 have been measured in NaCl and the major sea salts at pH 8 and 25°C. The effect of aging the chromium(III) solutions before the addition of H2O2 has also been investigated for the same solutions. Borate was the only major component of seawater found to affect the oxidation in seawater. Like OH-, an increase in the B(OH)4- concentration causes the rates of oxidation to increase. The rate-determining steps in seawater involve two chromium (III) species Cr(H2O)5OH2++H2O2 → KOH products Cr(H2O)4(OH) [B(OH)4] + + H2O2 → KB productsand the rate equation is given by dCr(III)/dt = -k3[H2O2] [OH-] [B(OH)4-]0.3 where k3=k1/[H2O2][OH-][B(OH)4-]0.3 (k1 is the pseudo-first-order rate constant). Aging of the solutions causes the rates of oxidation to decrease and is affected by the concentration of borate, carbonate and magnesium ions. Borate causes a reduction of the aging effect, while carbonate and magnesium produce an increase of the aging effect. The Mg2+ influence is much stronger than CO32- when these ions are present at their seawater levels in individual Na-Mg-Cl and Na-Cl-CO3 solutions. When CO32- and Mg2+ are present at the same time, the combined aging effect is less than that found in simple Na-Mg-Cl solutions. Since the aging effect is related to the precipitation of chromium(III), magnesium and carbonate ions will probably affect the solubility of chromium in natural waters.

Original languageEnglish (US)
Pages (from-to)29-46
Number of pages18
JournalMarine Chemistry
Volume34
Issue number1-2
DOIs
StatePublished - Sep 1991

ASJC Scopus subject areas

  • Oceanography
  • Chemistry(all)
  • Environmental Chemistry
  • Water Science and Technology

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