Chromium(VI) reduction by sulphur(IV) in aqueous solutions期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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where k (M^− 2 s^− 1) can be determined from the

in the pH range 2 to 5, from 5 to 40 °C and 0.01 to 1 M.The effect of pH and ionic strength on the reaction suggest that the rates are due to

where H₂A = H₂CrO₄, HA = HCrO₄⁻, H₂B = H₂SO₃ and HB = HSO₃⁻. The overall rate expression over the investigated pH range can be determined from

k=k_{H₂A–H₂B}(α_H₂A)(α_H₂B)²+k_HA–H₂B(α_HA)(α_H₂B)²+k_H₂A–HB(α_H₂A)(α_HB)²

with k_{H₂A−H₂B} = 5.0 × 10⁷, k_HA–H₂B = 1.5 × 10⁶ and k_H₂A–HB = 6.7 × 10⁷.Fe(III) in the range 1.5 to 20 μM exerts a small catalytic effect on the reaction and significantly lowers the initial concentration of Cr(VI) compared to the nominal value. Contrary to Fe(III), formaldehyde (20 to 200 μM) reacts with S(IV) to form the hydroxymethanesulfonate adduct (CH₂OHSO₃⁻), which does not react with Cr(VI). Major cations Mg²⁺ and some minor elements such as Ba²⁺ and Cu²⁺ did not affect the rates. The application of this rate law to environmental conditions suggest that this reaction may have a role in acidic solutions (aerosols and fog droplets). This reaction becomes more important in the presence of high Fe(III) and low HMS concentrations, contributing to affect the atmospheric transport of chromium species and the distribution of redox species of chromium, which reach surface water from atmospheric depositions.

Chromium(VI) reduction by sulphur(IV) in aqueous solutions

Authors:	Maurizio Pettine Daniele Tonnina Frank J Millero

Institution:	^aWater Research Institute, Consiglio Nazionale delle Ricerche, via Reno 1, Rome 00198, Italy;^bRosenstiel School of Marine and Atmospheric Chemistry, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA

Abstract:	The rates of the reduction of Cr(VI) with S(IV) were measured in deaerated NaCl solution as a function of pH, temperature and ionic strength. The rates of the reaction were found to be first order with respect to Cr(VI) and second order with respect to S(IV), in agreement with previous results obtained at concentrations two order higher than the present study. The reaction also showed a first-order dependence of the rates on the concentration of the proton and a small influence of temperature with an apparent energy of activation ΔH_app of 22.8 ± 3.4 kJ/mol. The rates were independent of ionic strength from 0.01 to 1 M. The rate of Cr(VI) reduction is described by the general expression −dCr(VI)]/dt=kCr(VI)]S(IV)]²

Keywords:	Chromate Sulphite Rate expression Cr(VI) half life
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