Metal-induced sulfate adsorption by soils

Abstract

Studies of the effect of the metal ion (K + , Cs + , Ca 2+ , Mg 2+ , Al 3+ , or In 3+ ) on SO 4 2- adsorption by four diverse acid soils, two from Iowa (dominated with permanent charge), and one each from Chile and Costa Rica (two highly weathered soils with variable charge) showed that SO 4 2- adsorption was greatly affected by the concentration of the metal ion. For the same valence of metal ion, SO 4 2- adsorption increased with increasing ionic radius of the metal ion. When the metal was maintained at a high, constant concentration (12.00 mmol c L -1 ), SO 4 2- adsorption was greater than when the SO 4 2- and the associated metal ions were added at equivalent concentrations. For all the SO 4 2- forms studied, the amounts of SO 4 2- adsorbed by the two Iowa soils (low in hydrous Al and Fe oxides) were considerably less than those adsorbed by the soils from Chile and Costa Rica (high in hydrous Al and Fe oxides). For the two Iowa surface soils, the ratios of SO 4 2- /metal adsorbed were low ; they ranged from 6.4 to 11.8 x 10 -2 . These ratios were several times greater for the two highly weathered soils from Chile and Costa Rica than for the two Iowa soils, reflecting their greater contents of hydrous Al and Fe oxides. The distribution coefficient (K d ) values for SO 4 2- were greater when Al 3+ and In 3+ were the metal ions than when K + , Cs + , Ca 2+ , and Mg 2+ were the metal ions ; the values increased with decreasing initial SO 4 2- concentration, especially when trivalent metal ions were the associated counter-ions. The results provide evidence that SO 4 2- adsorption by soils is caused by more than one mechanism and that the metal ion significantly affects SO 4 2- adsorption, regardless of the mechanism involved.