title: Lead binding to soil fulvic and humic acids: NICA-Donnan modeling and XAFS spectroscopy.
abstract: Binding of lead (Pb) to soil fulvic acid (JGFA), soil humic acids (JGHA, JLHA), and lignite-based humic acid (PAHA) was investigated through binding isotherms and XAFS. Pb binding to humic substances (HS) increased with increasing pH and decreasing ionic strength. The NICA-Donnan model described Pb binding to the HS satisfactorily. The comparison of the model parameters showed substantial differences in median Pb affinity constants among JGFA, PAHA, and the soil HAs. Milne's "generic" parameters did not provide an adequate prediction for the soil samples. The Pb binding prediction with generic parameters for the soil HAs was improved significantly by using the value n(Pb1) = 0.92 instead of the generic value n(Pb1) = 0.60. The n(Pb1)/n(H1) ratios obtained were relatively high, indicating monodentate Pb binding to the carboxylic-type groups. The nPb2/nH2 ratios depended somewhat on the method of optimization, but the values were distinctly lower than the n(Pb1)/nH1 ratios, especially when the optimization was based on Pb bound vs log [Pb(2+)]. These low values indicate bidentate binding to the phenolic-type groups at high Pb concentration. The NICA-Donnan model does not consider bidentate binding of Pb to a carboxylic- and a phenolic-type group. The EXAFS results at high Pb loading testified that Pb was bound in bidentate complexes of one carboxylic and one phenolic group (salicylate-type) or two phenolic groups (catechol-type) in ortho position.
authors.affiliation: College of Resources and Environment, Huazhong Agricultural University , Wuhan 430070, P.R. China.
authors.forename: Juan
authors.initials: J
authors.lastname: Xiong
authors.forename: Luuk K
authors.initials: LK
authors.lastname: Koopal
authors.forename: WenFeng
authors.initials: W
authors.lastname: Tan
authors.forename: LinChuan
authors.initials: L
authors.lastname: Fang
authors.forename: MingXia
authors.initials: M
authors.lastname: Wang
authors.forename: Wei
authors.initials: W
authors.lastname: Zhao
authors.forename: Fan
authors.initials: F
authors.lastname: Liu
authors.forename: Jing
authors.initials: J
authors.lastname: Zhang
authors.forename: LiPing
authors.initials: L
authors.lastname: Weng
mesh_terms: D001578:Benzopyrans / Q000737:chemistry*; D006812:Humic Substances / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007854:Lead / Q000737:chemistry*; D008962:Models, Theoretical; D012987:Soil / Q000737:chemistry*; D013052:Spectrometry, X-Ray Emission; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
chemical_list: D001578:Benzopyrans; D006812:Humic Substances; D012987:Soil; D007854:Lead; C005023:fulvic acid