Lead Binding to Soil Fulvic and Humic Acids: NICA-Donnan Modeling and XAFS Spectroscopy

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• 作者：Juan Xiong ; Luuk K. Koopal ; WenFeng Tan ; LinChuan Fang ; MingXia Wang ; Wei Zhao ; Fan Liu ; Jing Zhang ; LiPing Weng
• 刊名：Environmental Science & Technology
• 出版年：2013
• 出版时间：October 15, 2013
• 年：2013
• 卷：47
• 期：20
• 页码：11634-11642
• 全文大小：429K
• 年卷期：v.47,no.20(October 15, 2013)
• ISSN：1520-5851

文摘

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鈥檚 鈥済eneric鈥?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 n_Pb2/n_H2 ratios depended somewhat on the method of optimization, but the values were distinctly lower than the n_Pb1/n_H1 ratios, especially when the optimization was based on Pb bound vs log [Pb²⁺]. 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.