Abstract
We carried out Ca-Al exchange equilibrium experiments on a smectitic soil and calculated Gaines-Thomas selectivity coefficient. The natural logarithm of the Gaines-Thomas selectivity coefficient scattered around 4.6 with no noticeable trend. The natural logarithm of the thermodynamic equilibrium constant calculated with the aid of Rothmund-Kornfeld equation was 4.58 at 298 K and 1 atm, indicating that the soil strongly prefers Al3+ to Ca2+.
On the basis of the measured Al3+-selectivity we numerically examined the effect of cation exchange reactions involving A13+ during the acid extraction of Cd from soils. The results of computation showed that more Cd2+ is extracted when the extracting acid dissolves soil minerals and releases A13+. This is due to the higher preference of smectite for Al3+ than for H+.
