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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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Related in: MedlinePlus

Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.
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pone-0065359-g005: Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.

Mentions: Sample storage for six months under defined conditions (T = 20°C, RH = 76%) resulted in significant changes of matrix rigidity, mobile and mobilisable water. T* increased significantly by 5–10°C and 4°C for the treated and untreated peat, respectively (Figure 5A). The increase was independent of cation type and loading (data not shown). The 1H wideline Lorentzian line intensity, indicating mobile water, decreased by 6–9% for pH 1.9 and by 8–12% for pH 4.1 samples, respectively (Figure 5B). The decrease was strongest for Na+-treated samples and weakest for Al3+-treated samples (Figure S5), but did not vary with cation loading within each set of cation treatment. Percentage of mobilisable water increased for cation addition at pH 4.1 in almost all samples (except the SP-Na sample with lowest loading), while for pH 1.9, mobilisable water of all Na+- treated samples increased, whereas it even decreased in some Ca2+- and Al3+- treated samples (3 out of 10 cases) (see Figure 5C and Figure S6 for details). This underlines again that the Na samples are subjected to strongest aging.


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)

Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0065359-g005: Aging effects on WaMB transition temperature (A), Lorentzian line fraction (B) and on the amount of mobilisable water (C), expressed by the difference of the respective parameters after and before aging.Positive values indicate an increase, and negative values indicate a decrease in the parameter, respectively. Dotted line along zero shown in (C) distinguishes the positive and negative effects.
Mentions: Sample storage for six months under defined conditions (T = 20°C, RH = 76%) resulted in significant changes of matrix rigidity, mobile and mobilisable water. T* increased significantly by 5–10°C and 4°C for the treated and untreated peat, respectively (Figure 5A). The increase was independent of cation type and loading (data not shown). The 1H wideline Lorentzian line intensity, indicating mobile water, decreased by 6–9% for pH 1.9 and by 8–12% for pH 4.1 samples, respectively (Figure 5B). The decrease was strongest for Na+-treated samples and weakest for Al3+-treated samples (Figure S5), but did not vary with cation loading within each set of cation treatment. Percentage of mobilisable water increased for cation addition at pH 4.1 in almost all samples (except the SP-Na sample with lowest loading), while for pH 1.9, mobilisable water of all Na+- treated samples increased, whereas it even decreased in some Ca2+- and Al3+- treated samples (3 out of 10 cases) (see Figure 5C and Figure S6 for details). This underlines again that the Na samples are subjected to strongest aging.

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