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Abstract-Hexavalent chromium (Cr [VI]) is reduced by dissolved organic carbons (DOCs) such as humic substances, tannic acid (TA), and gallic acid (GA). The kinetic constants and the resulting chemical species after the reduction were compared with each other. The kinetic constants for GA and TA, which are model precursors of humic substances, were two to three orders of magnitude larger than those for the humic substances when these kinetic constants were expressed as a function of the molar concentration of the reductive functional group (F red ) in various DOCs. After the reduction of Cr(VI), the percentages of the species complexed with GA and TA were higher than those with the humic substances. This appears to be due to the formation of high molecular weight compounds by polymerization during the reduction of Cr(VI) and complexation of Cr(III) with the polymerized compounds. The UV-vis spectrophotometric data and gel permeation chromatography support this view.

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Chromium species after the reduction of Cr(VI) by humic acid (HA) were analyzed. Cr(VI) inorganic species were determined by a colorimetric method with diphenylcarbazide, and inorganic Cr(III) cationic and Cr(III)-HA complex species were determined by atomic absorption spectrometry after separation with a cation-exchanger. After the reduction of Cr(VI) by HA for 62 days, about 30 % of the total Cr existed in the Cr(III) complex bound with HA, and about 10 % of the total Cr was present as the inorganic Cr(III) form. The ratio of the concentration of Cr(III)-HA complex to reduced species of Cr(VI) increased with pH. It was found that the majority of chromium species after the reduction was present as complex form with HA. The results of the time-dependent speciation analysis showed that the concentration of inorganic Cr(III) cation dissociated from Cr(III)-HA complex increased with the reaction time in the acidified condition. The fate and hazardous of Cr species after the reduction were discussed by referring to the obtained kinetic results.