Irreversible sorption of humic substances causes a decrease in wettability of clay surfaces as measured by a sessile drop contact angle method

• Published in: Journal of soils and sediments 2018-04, Vol.18 (4), p.1327-1334
• Main Authors: Kholodov, Vladimir A., Milanovskiy, Evgeny Y., Konstantinov, Andrey I., Tyugai, Zemfira N., Yaroslavtseva, Nadezhda V., Perminova, Irina V.
• Format: Article
• Language: English
• Subjects: Analytical methods; Aromatic compounds; Aromaticity; Calcium; Calcium ions; Clay; Clay minerals; Contact angle; Drying; Earth and Environmental Science; Environment; Environmental Physics; Function and Fate in the Environment; Humic substances; Hydrophobicity; Kaolin; Magnetic resonance spectroscopy; Molecular weight; Montmorillonite; Montmorillonites; Natural Organic Matter: Chemistry; NMR spectroscopy; Peat; Sessile drop method; Size exclusion chromatography; Sod; Soil; Soil Science & Conservation; Sorption; Thin films; Wettability
• ISSN: 1439-0108; 1614-7480
• DOI: 10.1007/s11368-016-1639-3

Purpose 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 13 C NMR spectroscopy. Both clay samples were saturated with Ca 2+ 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. Conclusions 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.