PROTON BINDING AND CADMIUM COMPLEXATION CONSTANTS FOR A SOIL HUMIC-ACID USING A QUASI-PARTICLE MODEL

In order to better understand the fate of Cd in soils it is important to ascertain the influence of soil humic substances on the retention o f Cd in soils, In this study the acid-base and Cd complexation propert ies of a humic acid fraction extracted from a soil were investigated u sing potentiometric titrations. A number of electrostatic and nonelect rostatic equilibrium models were considered in order to obtain the sim plest model that would best describe the titration data. All proton bi nding and Cd complexation constants were determined using the optimiza tion procedure in the computer program FITEQL. The model that best fit the experimental data was a nonelectrostatic model, which included tw o diprotic acids, H(2)A and H2B. The dissociation constants for H(2)A were calculated to be log beta-1,0,1,0 = -4.00 +/- 0.02 and log beta(- 2,0,1,0) = -9.32 +/- 0.03 and for H2B were calculated to be log beta(- 1,0,0,1) = -7.43 +/- 0.24 and log beta(-2,0,0,1) = -16.66 +/- 0.31. Th e model indicated the presence of four Cd complexes, CdHA(+), CdA(o), CdH(-1)A(-) and CdHB+, with complexation constants calculated to be lo g beta(-1,1,1,0) = -1.29 +/- 0.09, log beta(-2,1,1,0) = -5.92 +/- 0.07 , log beta(-3,1,1,0) = -14.39 +/- 0.09, and log beta(-1,1,0,1) = -3.72 +/- 0.18.