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Restructuring of a peat in interaction with multivalent cations: effect of cation type and aging time.

Kunhi Mouvenchery Y, Jaeger A, Aquino AJ, Tunega D, Diehl D, Bertmer M, Schaumann GE - PLoS ONE (2013)

Bottom Line: The maximum uptake increased with pH of cation addition and decreased with increasing cation valency.Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged 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.

View Article: PubMed Central - PubMed

Affiliation: University of Koblenz-Landau, Institute for Environmental Sciences, Department of Environmental and Soil Chemistry, Landau, Germany.

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.

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Scheme for the chosen molecular model and calculated cluster models for the hydrated aluminum cation bridging two fatty acid chains.The scheme used for modeling (A) and optimized geometries of the model cluster at different R1 and R2 - (B), (C) and (D)- are shown, respectively. The spatially fixed carbon atoms are marked by black spheres in (A). For (B), R1 = 18.6 Å; R2 = 28.9 Å, (C) R1 = 25.8 Å; R2 = 36.0 Å, (D) R1 = 30.0 Å; R2 = 40.1 Å and (E) R1 = 31.9 Å; R2 = 42.1 Å.
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pone-0065359-g001: Scheme for the chosen molecular model and calculated cluster models for the hydrated aluminum cation bridging two fatty acid chains.The scheme used for modeling (A) and optimized geometries of the model cluster at different R1 and R2 - (B), (C) and (D)- are shown, respectively. The spatially fixed carbon atoms are marked by black spheres in (A). For (B), R1 = 18.6 Å; R2 = 28.9 Å, (C) R1 = 25.8 Å; R2 = 36.0 Å, (D) R1 = 30.0 Å; R2 = 40.1 Å and (E) R1 = 31.9 Å; R2 = 42.1 Å.

Mentions: In order to analyze the capacity of a hydrated multivalent cation cluster to hold two polar groups together which would be otherwise non-interacting, we conducted an exemplary molecular modeling study. This type of mechanism of cross-linking humic substance units has been intensively discussed in literature (see e.g. [8] and references therein). One main question concerns the spatial contribution to the binding power of the cation. To address it, a model has been constructed based on analogous previous work [1] where a cluster of Al3+ surrounded by thirty water molecules is placed between two fatty acid chains positioned parallel to each other (see Figure 1A for the scheme). They were simultaneously moved away from the aluminium cluster and at each selected distance a full geometry optimization of the obtained system was performed. In this procedure the two terminal carbon atoms and the two carbon atoms linked to the carboxyl groups were kept fixed. In total, seven structures with different interchain distances R1 and R2 were generated varying between ∼19-32 Å (R1) and 29-42 Å (R2), respectively. All calculations were performed by means of density functional theory (DFT) using the PBE functional. The TZVP [31] basis set was chosen for the carbon and oxygen atoms of the carboxylate groups and for Al+3. For all remaining atoms the SV(P) [32] basis set was employed. The calculations were carried out with the TURBOMOLE [33] program suite.


Restructuring of a peat in interaction with multivalent cations: effect of cation type and aging time.

Kunhi Mouvenchery Y, Jaeger A, Aquino AJ, Tunega D, Diehl D, Bertmer M, Schaumann GE - PLoS ONE (2013)

Scheme for the chosen molecular model and calculated cluster models for the hydrated aluminum cation bridging two fatty acid chains.The scheme used for modeling (A) and optimized geometries of the model cluster at different R1 and R2 - (B), (C) and (D)- are shown, respectively. The spatially fixed carbon atoms are marked by black spheres in (A). For (B), R1 = 18.6 Å; R2 = 28.9 Å, (C) R1 = 25.8 Å; R2 = 36.0 Å, (D) R1 = 30.0 Å; R2 = 40.1 Å and (E) R1 = 31.9 Å; R2 = 42.1 Å.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3672098&req=5

pone-0065359-g001: Scheme for the chosen molecular model and calculated cluster models for the hydrated aluminum cation bridging two fatty acid chains.The scheme used for modeling (A) and optimized geometries of the model cluster at different R1 and R2 - (B), (C) and (D)- are shown, respectively. The spatially fixed carbon atoms are marked by black spheres in (A). For (B), R1 = 18.6 Å; R2 = 28.9 Å, (C) R1 = 25.8 Å; R2 = 36.0 Å, (D) R1 = 30.0 Å; R2 = 40.1 Å and (E) R1 = 31.9 Å; R2 = 42.1 Å.
Mentions: In order to analyze the capacity of a hydrated multivalent cation cluster to hold two polar groups together which would be otherwise non-interacting, we conducted an exemplary molecular modeling study. This type of mechanism of cross-linking humic substance units has been intensively discussed in literature (see e.g. [8] and references therein). One main question concerns the spatial contribution to the binding power of the cation. To address it, a model has been constructed based on analogous previous work [1] where a cluster of Al3+ surrounded by thirty water molecules is placed between two fatty acid chains positioned parallel to each other (see Figure 1A for the scheme). They were simultaneously moved away from the aluminium cluster and at each selected distance a full geometry optimization of the obtained system was performed. In this procedure the two terminal carbon atoms and the two carbon atoms linked to the carboxyl groups were kept fixed. In total, seven structures with different interchain distances R1 and R2 were generated varying between ∼19-32 Å (R1) and 29-42 Å (R2), respectively. All calculations were performed by means of density functional theory (DFT) using the PBE functional. The TZVP [31] basis set was chosen for the carbon and oxygen atoms of the carboxylate groups and for Al+3. For all remaining atoms the SV(P) [32] basis set was employed. The calculations were carried out with the TURBOMOLE [33] program suite.

Bottom Line: The maximum uptake increased with pH of cation addition and decreased with increasing cation valency.Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged 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.

View Article: PubMed Central - PubMed

Affiliation: University of Koblenz-Landau, Institute for Environmental Sciences, Department of Environmental and Soil Chemistry, Landau, Germany.

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.

Show MeSH
Related in: MedlinePlus