Cr(III) Oxidation by Soils with Fe-Mn Nodules as Affected by Various Factors.

Chromium(VI) is more hazardous and mobile than Cr(III) in soils. The Mn oxide minerals had been proven to be able to oxidize Cr(III) to Cr(VI). The study investigated the Cr(III) oxidation by natural soils which had high amounts of Fe-Mn nodules under various conditions. Three soils, Chuwei A and B, and Houhu, were used. The results showed that Chuwei A and B soils could oxidize 2.65% and 1.86% of added Cr(III) to Cr(VI) at the soil/water ratio of 1/75, however, no detectable Cr(VI) was found in Houhu soil. The results were mainly due to that the order of total soil Mn contents was Chuwei A > Chuwei B > Houhu. We also found that dissolved Mn (MnL) increased with time during the oxidation of Cr(III) by Chuwei A soil, suggesting that the Cr(VI) production is resulted from the reduction of soil Mn oxides. The mole ratio of Cr(VI) produced/soil Mn of the Chuwei A soil was 0.015-0.010, which was much smaller than theoretical value of mole ratio of 0.67 of Cr(VI) produced/soil Mn for Cr(III) to be oxidized to Cr(VI) by pure MnO2. The results of Cr(III) oxidation indicated that only a small portion of soil Mn was able to oxidize Cr(III). In order to mimic field conditions, the soil available Cr(VI) produced from the Cr(III) oxidation by the Cr(III)-spiked Chuwei B soils (0, 250, 500, and 1000 mg Cr(III) kg-1) incubated at field capacity as a function of time was extracted by KH2PO4 or DOWEX M4195 resins. The results showed that the amounts of soil available Cr(VI) increased rapidly in one day and reached the level of 24.8-29.7 mg kg-1 after 30 days no matter what the amounts of Cr(III) were spiked, suggesting that the main factor controlling the extent of Cr(VI) production is the contents of soil reducible-Mn for oxidizing Cr(III).