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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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Investigated soil properties with respect to the cation addition pH.(A) CECeff, (B) DOC, (C) CA, (D) T*, (E) T2,fast and (F) contribution of fast relaxing water molecules to the total T2 decay, (G) contribution of Lorentzian line to the 1H wideline and (H) Difference in the contribution of Lorentzian component after heating event. Dotted lines and dashed lines represents the average values observed for SP and SP-H samples, respectively.
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pone-0065359-g002: Investigated soil properties with respect to the cation addition pH.(A) CECeff, (B) DOC, (C) CA, (D) T*, (E) T2,fast and (F) contribution of fast relaxing water molecules to the total T2 decay, (G) contribution of Lorentzian line to the 1H wideline and (H) Difference in the contribution of Lorentzian component after heating event. Dotted lines and dashed lines represents the average values observed for SP and SP-H samples, respectively.

Mentions: Figure 2A shows CECeff for all treated samples in dependence of the cation addition pH and demonstrates that it is independent of cation addition pH. Thus, neither cation treatment (see Tables S1 and S2) nor cation addition pH affected CECeff, but the final pH (4.1in treated samples and 2.7 in untreated sample). This result can be explained with the higher degree of deprotonation of acidic functional groups at higher pH. This demonstrates that (1) 2 h of equilibration are sufficient to obtain a protonation/deprotonation equilibrium and (2) cation treatment did not affect CECeff.


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)

Investigated soil properties with respect to the cation addition pH.(A) CECeff, (B) DOC, (C) CA, (D) T*, (E) T2,fast and (F) contribution of fast relaxing water molecules to the total T2 decay, (G) contribution of Lorentzian line to the 1H wideline and (H) Difference in the contribution of Lorentzian component after heating event. Dotted lines and dashed lines represents the average values observed for SP and SP-H samples, respectively.
© Copyright Policy
Related In: Results  -  Collection

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

pone-0065359-g002: Investigated soil properties with respect to the cation addition pH.(A) CECeff, (B) DOC, (C) CA, (D) T*, (E) T2,fast and (F) contribution of fast relaxing water molecules to the total T2 decay, (G) contribution of Lorentzian line to the 1H wideline and (H) Difference in the contribution of Lorentzian component after heating event. Dotted lines and dashed lines represents the average values observed for SP and SP-H samples, respectively.
Mentions: Figure 2A shows CECeff for all treated samples in dependence of the cation addition pH and demonstrates that it is independent of cation addition pH. Thus, neither cation treatment (see Tables S1 and S2) nor cation addition pH affected CECeff, but the final pH (4.1in treated samples and 2.7 in untreated sample). This result can be explained with the higher degree of deprotonation of acidic functional groups at higher pH. This demonstrates that (1) 2 h of equilibration are sufficient to obtain a protonation/deprotonation equilibrium and (2) cation treatment did not affect CECeff.

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