Adsorption of SO42- and its associated metal ion by four soils from Io
wa, 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 u
sing 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 th
at 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 soi
ls containing large rather than small amounts of Fe and Al hydrous oxi
des, 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 o
f SO42-/metal (M) adsorbed (SO42-/M) by the four soils decreased sharp
ly with increasing pH up to 4.5 to 5.0, above which no significant cha
nge in these ratios occurred, For all soils, a relatively linear relat
ionship was obtained between log K-d (SO42- distribution coefficients)
and pH when the metal ions were monovalent, regardless of the ionic s
trength. But the K-d values for SO42- varied among the soils, dependin
g on the SO42- used,