title: Restructuring of a peat in interaction with multivalent cations: effect of cation type and aging time.
abstract: It is assumed to be common knowledge that multivalent cations cross-link soil organic matter (SOM) molecules via cation bridges (CaB). The concept has not been explicitly demonstrated in solid SOM by targeted experiments, yet. Therefore, the requirements for and characteristics of CaB remain unidentified. In this study, a combined experimental and molecular modeling approach was adopted to investigate the interaction of cations on a peat OM from physicochemical perspective. Before treatment with salt solutions of Al(3+), Ca(2+) or Na(+), respectively, the original exchangeable cations were removed using cation exchange resin. Cation treatment was conducted at two different values of pH prior to adjusting pH to 4.1. Cation sorption is slower (>>2 h) than deprotonation of functional groups (<2 h) and was described by a Langmuir model. The maximum uptake increased with pH of cation addition and decreased with increasing cation valency. Sorption coefficients were similar for all cations and at both pH. This contradicts the general expectations for electrostatic interactions, suggesting that not only the interaction chemistry but also spatial distribution of functional groups in OM determines binding of cations in this peat. The reaction of contact angle, matrix rigidity due to water molecule bridges (WaMB) and molecular mobility of water (NMR analysis) suggested that cross-linking via CaB has low relevance in this peat. This unexpected finding is probably due to the low cation exchange capacity, resulting in low abundance of charged functionalities. Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged by CaB-WaMB associations. However, aging strongly increased matrix rigidity, suggesting successive increase of WaMB size to connect functionalities and thus increasing degree of cross-linking by CaB-WaMB associations. Results thus demonstrated that the physicochemical structure of OM is decisive for CaB and aging-induced structural reorganisation can enhance cross-link formation.
authors.affiliation: University of Koblenz-Landau, Institute for Environmental Sciences, Department of Environmental and Soil Chemistry, Landau, Germany.
authors.forename: Yamuna
authors.initials: Y
authors.lastname: Kunhi Mouvenchery
authors.forename: Alexander
authors.initials: A
authors.lastname: Jaeger
authors.forename: Adelia J A
authors.initials: AJ
authors.lastname: Aquino
authors.forename: Daniel
authors.initials: D
authors.lastname: Tunega
authors.forename: Dörte
authors.initials: D
authors.lastname: Diehl
authors.forename: Marko
authors.initials: M
authors.lastname: Bertmer
authors.forename: Gabriele Ellen
authors.initials: GE
authors.lastname: Schaumann
mesh_terms: D002412:Cations / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D008968:Molecular Conformation; D009930:Organic Chemicals / Q000737:chemistry; D012972:Sodium Hydroxide / Q000737:chemistry; D012987:Soil / Q000737:chemistry*; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000737:chemistry
chemical_list: D002412:Cations; D009930:Organic Chemicals; D012987:Soil; D014867:Water; D012972:Sodium Hydroxide