[Effects of Mn(III) on oxidation of Cr(III) with birnessites]

Cr(III) could be oxidized only by manganese oxide minerals as natural inorganic oxidants in nature, and so the rate and mechanism of interaction between manganese oxide minerals and Cr(III) were widely concerned. The effects of Mn(III) in birnessites, the most common Mn oxide mineral in the environment, on the rate of Cr(III) oxidation with birnessites and the kinetic characteristics were investigated through batch kinetic technique. The results show that Cr(III) oxidation rate follows a pseudo-first-order reaction, and the apparent rate constant K(obs), is 0.031 3 min(-1) when the average oxidation state (AOS) of Mn is about 3.50 in birnessite. When the birnessite is pretreated with Na4P2O7 solution, and the Mn(III) can be complexed out from the solid oxides. Therefore the content of Mn(III) in the birnessites decreases and the AOS of manganese increases. The AOSs of Mn for the pretreated birnessites increase from 3.50 to 3.63, 3.73 and 3.78 when the concentrations of Na4P2O7 are about 10, 20 and 50 mmol/L respectively. The Mn(III) content does not affect the initial oxidation rate of Cr(III) markedly, although oxidation amount of Cr(III) increases with the AOS of Mn. The apparent rate constants for the corresponding pretreated birnessites are 0.035 1, 0.032 5 and 0.0309 min(-1) respectively. The oxidation rate of Cr( III) is markedly influenced by the amount of Mn(III) produced in the transformation process of Mn(VN) --> Mn(III). The newly formed Mn(III) is complexed by Na4P2O7 and the oxidation rate decreases to 45%-88%. The lower content of Mn(III) in birnessites, the more Mn(III) newly formed from the transformation of Mn(IV) is complexed out from the minerals, and the greater amplitude in the decrease of Cr(III) oxidation rate. Thus the newly formed Mn(III) is highly active and possesses fast rate of electron transfer, however the rate of electron transfer in the transformation process of Mn(IV) --> Mn(III) is relatively slow. It could be deduced that the controlling step of initial oxidation rate of Cr(III) with birnessites may be the electron transfer process of Mn(IV) --> Mn(III).