Factors Affecting Oxidation of Cr(III) to Cr(VI) in Some Soils of North and North-West of Iran

The oxidation of Cr(III) to Cr(VI) in soils causes an increase in chromium mobility and subsequent environmental risks. In this research the influence of pH, Cr(III) concentration and the nature of background solution on chromium oxidation in some soils of north and north-west of Iran were investigated. The results showed that oxidation of Cr(III) to Cr(VI) in nitrate medium increased with increasing pH from 3 to 5, whereas such oxidation in chloride medium became greatest at pH=4. With increasing chromium concentration, percentage of Cr(VI) production decreased in chloride medium and was nearly constant in nitrate media. Oxidation of Cr(III) to Cr(VI) in chloride medium showed a rising increase during the experiment, whereas such oxidation reached a maximum during 30 minutes or earlier in nitrate medium. According to the results, the amount of Cr(VI) production in nitrate medium showed no relation with any general properties of the soils. However, the amount of Cr(VI) production in chloride medium was positively related to easily reducible manganese in the soils. Also, kinetics studies showed that parabolic diffusion equation was best fitted to the data of the oxidation of Cr(III) to Cr(VI).