Irreversible sorption of humic substances causes a decrease in wettability of clay surfaces as measured by a sessile drop contact angle method
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The objective of the study was to obtain quantitative assessments of the hydrophobic impact of irreversible sorption of humic substances (HSs) onto clay mineral surfaces using a sessile drop contact angle method.
Materials and methods
Two clays (kaolin and montmorillonite) were modified with four humic materials: (1) sod podzolic soil, (2) chernozem, (3) peat, and (4) coal (leonardite). The humic materials were characterized using elemental analysis, size exclusion chromatography, and 13C NMR spectroscopy. Both clay samples were saturated with Ca2+ prior to modification with HS using a sorption isotherm technique. Contact angles (CAs) of the obtained HS-clay complexes were determined using a static sessile drop method after drying the obtained HS-clay complexes in the form of a thin film.
Results and discussion
HS modification rendered both clays under study—kaolin and montmorillonite—more hydrophobic. In case of Ca-kaolin, the CA values increased from 27° (Ca-kaolin) up to 31°–32° (all HS-kaolin complexes) with no significant difference among the HS types used for modification. In the case of Ca-montmorillonite, the CA values increased from 41° (Ca-montmorillonite) up to 51°–83° with the following ascending trend for the humic types investigated: chernozem HS < coal HS < peat HS < sod-podzolic HS. This trend is in reverse to the degree of aromaticity of the HS, expressed as the content of aromatic carbon, and it is directly proportional to the molecular weight of each HS.
Application of a sessile drop method showed increased surface hydrophobicity of HS-modified clays. Much more substantial hydrophobization was observed for montmorillonite as compared to kaolin, which was explained by the differences in the sorption mechanism.
KeywordsContact angle Humic substances Hydrophobization Kaolin Montmorillonite Sorption Sessile drop method
This work was supported by the Russian Science Foundation (grant no. 14-26-00079 for the portion of the working involving preparation of soil-HS complexes and using a sessile drop method for contact angle measurements). Grant no. 16-14-00167 supported the portion of the study contributing to HS characterization and the data interpretation that determined the hydrophobicity assessment.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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