title: Copper binding to soil fulvic and humic acids: NICA-Donnan modeling and conditional affinity spectra.
abstract: Binding of Cu(II) to soil fulvic acid (JGFA), soil humic acids (JGHA, JLHA), and lignite-based humic acid (PAHA) was investigated through NICA-Donnan modeling and conditional affinity spectrum (CAS). It is to extend the knowledge of copper binding by soil humic substances (HS) both in respect of enlarging the database of metal ion binding to HS and obtaining a good insight into Cu binding to the functional groups of FA and HA by using the NICA-Donnan model to unravel the intrinsic and conditional affinity spectra. Results showed that Cu binding to HS increased with increasing pH and decreasing ionic strength. The amount of Cu bound to the HAs was larger than the amount bound to JGFA. Milne's generic parameters did not provide satisfactory predictions for the present soil HS samples, while material-specific NICA-Donnan model parameters described and predicted Cu binding to the HS well. Both the 'low' and 'high' concentration fitting procedures indicated a substantial bidentate structure of the Cu complexes with HS. By means of CAS underlying NICA isotherm, which was scarcely used, the nature of the binding at different solution conditions for a given sample and the differences in binding mode were illustrated. It was indicated that carboxylic group played an indispensable role in Cu binding to HS in that the carboxylic CAS had stronger conditional affinity than the phenolic distribution due to its large degree of proton dissociation. The fact was especially true for JGFA and JLHA which contain much larger amount of carboxylic groups, and the occupation of phenolic sites by Cu was negligible. Comparable amounts of carboxylic and phenolic groups on PAHA and JGHA, increased the occupation of phenolic type sites by Cu. The binding strength of PAHA-Cu and JGHA-Cu was stronger than that of JGFA-Cu and JLHA-Cu. The presence of phenolic groups increased the chance of forming more stable complexes, such as the salicylate-Cu or catechol-Cu type structures.
authors.affiliation: State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and the Ministry of Water Resources, Yangling, Shaanxi Province 712100, PR China.
authors.forename: Jinling
authors.initials: J
authors.lastname: Xu
authors.affiliation: State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and the Ministry of Water Resources, Yangling, Shaanxi Province 712100, PR China; College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China.
authors.forename: Wenfeng
authors.initials: W
authors.lastname: Tan
authors.affiliation: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China.
authors.forename: Juan
authors.initials: J
authors.lastname: Xiong
authors.affiliation: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China.
authors.forename: Mingxia
authors.initials: M
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences and the Ministry of Water Resources, Yangling, Shaanxi Province 712100, PR China. Electronic address: flinc629@hotmail.com.
authors.forename: Linchuan
authors.initials: L
authors.lastname: Fang
authors.affiliation: College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, PR China; Physical Chemistry and Soft Matter, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands.
authors.forename: Luuk K
authors.initials: LK
authors.lastname: Koopal
keywords: Conditional affinity spectrum (CAS); Cu binding; Fulvic acid; Humic acid; Humic substance; NICA-Donnan model; Speciation