METAL-INDUCED SULFATE ADSORPTION BY SOILS .1. EFFECT OF PH AND IONIC-STRENGTH

Adsorption of SO42- and its associated metal ion by four soils from Iowa, Chile, and Costa Rica was studied, The surface charge of each soil was characterized, and the relationship between SO42- adsorption and pH was constructed for pH ranging from 3.5 to 7 at 0.5 increments by using 1 mmol/L (2.0 mmol(c) L(-1), or 50.0 mmol(c) kg(-1) soil) each of seven SO42- forms [Cs2SO4, K2SO4, (NH4)(2)SO4, CaSO4, MgSO4, Al-2(SO4)(3), and In-2(SO4)(3)] made in 1 or 10 mmol/L NaCl. Results showed that SO42- adsorption was greatest at the lowest pH values (ca. 3.5) and decreased with increasing equilibrium solution pH and ionic strength. The amounts of SO42- adsorbed were three to four times greater in soils containing large rather than small amounts of Fe and Al hydrous oxides, regardless of the type and valence of the metal ion. The amounts of SO42- adsorbed from solutions containing different metal ions were affected by the charge of the metal ion and, in general, followed the order: In-2(SO4)(2) > Al-2(SO4)(3) > CaSO4 > MgSO4 > Cs2SO4 > K2SO4 > (NH4)(2)SO4. The effect of the metal ion was more pronounced at higher pH values (pH > 5), and Iowa soils (low in Fe and Al hydrous oxides) were more affected by the increase in the valence of the metal ion and ionic strength than the soils from Chile and Costs Rice, The ratios of SO42-/metal (M) adsorbed (SO42-/M) by the four soils decreased sharply with increasing pH up to 4.5 to 5.0, above which no significant change in these ratios occurred, For all soils, a relatively linear relationship was obtained between log K-d (SO42- distribution coefficients) and pH when the metal ions were monovalent, regardless of the ionic strength. But the K-d values for SO42- varied among the soils, depending on the SO42- used,