METAL-INDUCED SULFATE ADSORPTION BY SOILS .2. EFFECTS OF METAL TYPE, VALENCE, AND CONCENTRATION

Studies of the effect of the metal ion (K+, Cs+, Ca2+, Mg2+, Al3+, or In3+) on SO42- adsorption by four diverse acid soils, two from Iowa (d ominated with permanent charge), and one each. from Chile and Costa Ri ca (two highly weathered soils with variable charge) showed that SO42- adsorption was greatly affected by the concentration of the metal ion . For the same valence of metal ion, SO42- adsorption increased with i ncreasing ionic radius of the metal ion. When the metal was maintained at a high, constant concentration (12.00 mmol(c) L(-1)), SO42- adsorp tion was greater than when the SO42- and the associated metal ions wer e added at equivalent concentrations. For all the SO42- forms studied, the amounts of SO42- adsorbed by the two Iowa soils (low in hydrous A l and Fe oxides) mere considerably less than those adsorbed by the soi ls from Chile and Costa Rica (high in hydrous Al and Fe oxides). For t he two Iowa surface soils, the ratios of SO42-/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 Costs Rica than for the two Iowa soils, reflecting their greater contents of hydr ous Al and Fe oxides. The distribution coefficient (K-d) values for SO 42- were greater when Al3+ and In3+ were the metal ions than when K+, Cs+, Ca2+ and Mg2+ were the metal ions; the values increased with decr easing initial SO42- concentration, especially when trivalent metal io ns were the associated counter-ions. The results provide evidence that SO42- adsorption by soils is caused by more than one mechanism and th at the metal ion significantly affects SO42- adsorption, regardless of the mechanism involved.