Effect of soil fulvic and humic acid on binding of Pb to goethite–water interface: Linear additivity and volume fractions of HS in the Stern layer

The effects of soil fulvic (JGFA) and humic acid (JGHA) on Pb binding to goethite were investigated with batch experiments and modeling. The CD-MUSIC and NICA-Donnan model could describe the Pb binding to, respectively, the binary Pb–goethite and Pb–HS systems. The adsorption of humic substances (HS) on goethite strongly depended on pH and was promoted by Pb binding. The mass amount of adsorbed JGFA (mg/g) was smaller than that of JGHA, but when expressed in number of particles/nm2 the JGFA adsorption was higher. At low pH and low initial Pb concentration the linear additivity rule always underestimated Pb adsorption to goethite–HS complex, which was caused by the strong effect of adsorbed HS on the electrostatic potentials in the Stern layer region. At other conditions except the 450 mg/L JGHA in the 0.5 mmol/L Pb system the additivity rule predicted the experimental results reasonably well, and at high pH nearly all Pb is bound to goethite. At the same mass adsorbed, the effect of JGFA on Pb adsorption to goethite is stronger than that of JGHA, due to the fact that the JGFA particles were primarily adsorbed in the Stern layer, whereas JGHA particles were present in both Stern layer and diffuse layer. Therefore, the electrostatic potential profile of the goethite-JGFA complex is considerably different from that of goethite-JGHA complex and affects Pb binding strongly.

In the ternary system, the JGFA adsorbed by the goethite can be located in the Stern layer with strong conformation change and the JGHA is adsorbed in the Stern layer and diffuse layer. The Pb binding to the goethite–HS complex can be promoted by the presence of HS. At low pH, the amount of Pb binding to the goethite–HS complex is enhanced because the adsorbed HS by the goethite can decrease the electrostatic potentials in the Stern layer and at the Stern planes strongly. With the increasing of pH the amounts of HS adsorbed are smaller and smaller. It is noteworthy that the amount of JGHA adsorbed by the goethite is larger than that of JGFA due to the larger particles size. This lead to the amount of Pb binding to the goethite-JGHA complex is much higher than that of goethite-JGFA complex.Figure optionsDownload high-quality image (228 K)Download as PowerPoint slide