KINETICS OF PHOSPHATE-INDUCED DESORPTION OF ARSENATE ADSORBED ON CRYSTALLINE AND AMORPHOUS ALUMINUM HYDROXIDES

Abstract

Research on arsenate adsorption by Al hydroxides is common; however, relatively little is known on the kinetics of arsenate desorption from these Al hydroxides. The batch method was used to compare the phosphate-induced desorption kinetics of preadsorbed arsenate from the crystalline and amorphous Al(OH) 3 at pH 5.0, background electrolyte of 0.01 M NaNO 3 at 298 and 318 K. The results showed that both the amount of arsenate adsorbed by the amorphous Al(OH) 3 and the mole fraction of arsenate remaining adsorbed on the amorphous Al(OH) 3 after desorption were substantially greater than those in the crystalline Al(OH) 3 system. The second-order rate equation was chosen to compare the rates of arsenate desorption. The rate constant of arsenate desorption from the crystalline Al(OH) 3 was 3.8 to 15.5 times greater than that from the amorphous Al(OH) 3 in the reaction period, suggesting that, compared with the crystalline Al(OH) 3 , the arsenate adsorbed on the amorphous Al(OH) 3 was much more difficult to be desorbed by phosphate. The rate constant of arsenate desorption increased with the increase of phosphate concentration from 0.1 to 1.0 mM and the increasing temperature. Although the activation energy for arsenate desorption from the crystalline Al(OH) 3 in the fast reaction was greater than that from the amorphous Al(OH) 3 , the much greater frequency factor for the desorption from the former resulted in a higher desorption rate of arsenate from the crystalline Al(OH) 3 . This information is of fundamental significance in understanding the dynamics of remobilization and fate of arsenate in soil and related environments.