Binding characteristics of Pb²⁺ to natural fulvic acid extracted from the sediments in Lake Wuliangsuhai, Inner Mongolia plateau, P. R. China

Humic substance (HS) is the determining factor in controlling the geochemistry behavior and progress of heavy metals. Using fulvic acid (FA) extracted from natural sediment as sorbent, this work studied binding characteristics between FA and Pb²⁺ considering the influences of temperature, ionic strength and FA concentrations. The results showed the amount of bound-Pb increased with increasing initial Pb²⁺ concentrations and FA concentrations, but decreased with increasing ionic strengths. Correspondingly, the logarithm conditional distribution coefficients (log K _D) of bound-Pb sharply declined as the increase of initial Pb²⁺, and log K _D increase accompanied the increase of FA concentrations and decrease with the ionic strengths. The two-site ligand binding model fit all the binding experimental data very well and proved that two classes of binding sites were involved in the interaction of Pb²⁺ with the extracted FA. The results clearly confirmed the chemical heterogeneity of the binding sites present in the FA extracted from the lake sediment. Fourier transform infrared spectra indicated that two types of functional groups exist in the extracted FA and participated in the interaction of FA and Pb²⁺, corresponding to the two-site ligand binding model in description of the characteristics of Pb²⁺–FA complex. The apparent stability constant (K _S) values calculated for the strong binding sites (S₁) were higher than for the weak sites (S₂) by 1–2 orders of magnitude; however, S₁ sites were limited in numbers, as evidenced by lowering the maximum binding capability (B _max) values, which were 3–40 times smaller than those for S₂ sites. The bound process of Pb²⁺ to FA is spontaneous and exothermic from the result of the thermodynamic parameters. This work is important to reveal the mechanism of binding characteristics between Pb²⁺ and FA. It also gives a theoretical and practical direction to understanding the environmental effect of HS, environmental geochemical behavior and bio-availability of heavy metals in lakes.