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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

Effect of sample treatment by exchange resin followed by addition of specific cations on the major cation content with respect to different charging pH, shown for highest cation loading.Also the values for untreated (SP) and exchange resin-treated samples (SP-H) are shown.
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pone-0065359-g003: Effect of sample treatment by exchange resin followed by addition of specific cations on the major cation content with respect to different charging pH, shown for highest cation loading.Also the values for untreated (SP) and exchange resin-treated samples (SP-H) are shown.

Mentions: Figure 3 shows the cation composition of peat – without any treatment, after removal of cations and after addition of cations. Data for samples with highest cation loading are shown, exemplarily. The untreated peat contained mainly calcium (45.3±0.5 mmolckg−1), magnesium (21.6±0.3 mmolckg−1), iron (16.9±0.1 mmolckg−1), aluminium (11.1±3.2 mmolckg−1), and sodium (7.8±0.3 mmolckg−1). Cation removal by exchange resin (sample SP-H in Figure 3A) resulted in removal of 84% of initially present cations, retaining 5.7±0.4 mmolckg−1 calcium, 2.1±0.2 mmolckg−1 magnesium, 6.7±0.1 mmolckg−1 iron, 1.3±0.4 mmolckg−1 aluminium and 0.21±0.03 mmolckg−1 sodium. As expected, higher sodium content (5.0±1.0 mmolckg−1) was observed in exchange resin-treated samples of the experiment where pH was later adjusted to 4.1 using NaOH. Iron and calcium were comparably less affected by treatment with exchange resin: 40% and 15% of their initial amounts were, respectively, retained. The incomplete removal of these cations during resin treatment indicates that they are only partly bound via cation exchange, and will be discussed further below.


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)

Effect of sample treatment by exchange resin followed by addition of specific cations on the major cation content with respect to different charging pH, shown for highest cation loading.Also the values for untreated (SP) and exchange resin-treated samples (SP-H) are shown.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0065359-g003: Effect of sample treatment by exchange resin followed by addition of specific cations on the major cation content with respect to different charging pH, shown for highest cation loading.Also the values for untreated (SP) and exchange resin-treated samples (SP-H) are shown.
Mentions: Figure 3 shows the cation composition of peat – without any treatment, after removal of cations and after addition of cations. Data for samples with highest cation loading are shown, exemplarily. The untreated peat contained mainly calcium (45.3±0.5 mmolckg−1), magnesium (21.6±0.3 mmolckg−1), iron (16.9±0.1 mmolckg−1), aluminium (11.1±3.2 mmolckg−1), and sodium (7.8±0.3 mmolckg−1). Cation removal by exchange resin (sample SP-H in Figure 3A) resulted in removal of 84% of initially present cations, retaining 5.7±0.4 mmolckg−1 calcium, 2.1±0.2 mmolckg−1 magnesium, 6.7±0.1 mmolckg−1 iron, 1.3±0.4 mmolckg−1 aluminium and 0.21±0.03 mmolckg−1 sodium. As expected, higher sodium content (5.0±1.0 mmolckg−1) was observed in exchange resin-treated samples of the experiment where pH was later adjusted to 4.1 using NaOH. Iron and calcium were comparably less affected by treatment with exchange resin: 40% and 15% of their initial amounts were, respectively, retained. The incomplete removal of these cations during resin treatment indicates that they are only partly bound via cation exchange, and will be discussed further below.

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