Impact of various inorganic oxyanions on the removal rates of hexavalent chromium mediated by zero-valent iron

Environmental context.Oxyanions in soil extract can interfere with the zero valent iron induced reduction of chromium(VI) to chromium(III). At pH 6, the reaction rate was decreased (2 to 6-fold) by an equivalent of arsenate, phosphate or silicate but was increased by sulfate and remained unchanged by borate or nitrate. At pH 2, not only was the rate of reaction dramatically increased (∼900-fold) but interferences from the major components of soil solution (nitrate, silicate and sulfate) were minimised. Abstract.The rate of zero-valent iron (ZVI) mediated reduction of CrVI was dependent on the condition of the ZVI surface, the pH of the medium and on the presence of inorganic oxyanions that can interfere with the process by competing for active sites on the ZVI surface. Whereas at pH 2, a single exponential decay provided an acceptable fit to the data, for pH 6 an appreciably better fit to the data was obtained with the sum of two exponential decays. The surface area normalised rate constant (kSA1) corresponding to the first decay was considered to model reactions at exposed active sites and kSA2, corresponding to the second decay, was considered to model decomposition kinetics through an intervening oxyhydroxide layer above the ZVI surface. The rate of CrVI reduction was decreased ∼900-fold when the pH was increased from 2 to 6 in the absence of competing ions. At pH 2, interferences from the major components of soil solution (nitrate, silicate and sulfate) were minimised.