Complexation of cadmium (Ⅱ) with humic materials from different origins

Abstract

Investigation was carried out using three humic acids (HAs) of differing origins and one fulvic acid (FA) to compare their complexation behaviours with $Cd^{2+}$ ions in aquatic media. Aldrich-HA is the commercial available material and is taken as the reference material and Gohy-573-HA is aquatic HA extracted from Gorleben underground aquifer system. T3-HA and T3-FA are the HA and FA, extracted from soil samples from the Kuye San region of the Okchun Metamorphic Belt. These materials are all extracted and purified according to the recommended methods. Therefore, the variability of humic substances results from their source materials and origins can be investigated by this comparitive study. The complexation of $Cd^{2+}$ ion with three HAs and one FA was investigated in 0.1 M $NaCIO_4$ at different pH values (5.0, 5.5 and 6.0)by ultrafiltration method. The experiment results verified that $Cd^{2+}$ ions are extensively bound to humate and fulvate as 1:1 complexes at low Cd/humus ratios. In this investigation, the maximum binding ability (MBA) of three HAs and one FA for $Cd^{2+}$ ion were determined as function of pH (5.0, 5.5 and 6.0). Their respective MBA values and the total humus concnetrations ([HA]$_t$ or [FA]$_t$) were used to obtain the total effective polyelectrolyte ligand cocentrations ([e-HA]$_t$ or [e-FA]$_t$). The same experimental procedure used in all these investigations and the conditional stability constant calcuated using the simplified equation where $\beta$ = [CdHA]/[$Cd^{2+}$]$_f$[e-HA]$_f$. The average values of the conditional stability constant of HAs and FA with $Cd^{2+}$ ion are found to be consistently clustered around $\log\,\beta\,\approx\,4\,mol^{-1}\,L$, demonstrating that these values are independent of pH of the aquifer systems and origins of humic substances. However, the cation binding ability humic substances vary significantly for one humic substance to another and also on pH of the quifer systems, demonstrating that the t...