Selectivity reversal and dimerization of chromate in the exchanger Amberlite IRA-400

Chromate sorption on the strong basic anion exchanger, Amberlite IRA-400 is studied as function of pH (2-11), temperature (20-40 degrees C) and different initial concentrations (1.030-15.449 mmol/l). The sorption increases with the decrease in pH and increase in temperature. The ratios of chloride released/chromate sorbed are determined, which are found to decrease with the decrease in pH of the solution, while the effect of temperature on mole ratios is negligible. It is suggested that the chromate sorption takes place initially as CrO42- followed by HCrO4-. The differential isosteric heats of adsorption, <Delta(H)over bar>, are also computed which show the process to be endothermic in nature. IR spectra of chromate sorption on the chloride form of the resins have been studied as a function of pH at 20 degrees C. These spectra reveal the dimerization of HCrO4- to Cr2O72- at lower pH inside the exchanger.