Abstract
The interaction of organic matter with mineral components of the solid phase of soils is the most important process that regulates the cycle and balance of carbon in the biosphere. The adsorption of humic acids on minerals is accompanied by their fractionation in size, composition, and amphiphilicity, thus decreasing their heterogeneity. Despite a strong interest in studying the regularities and mechanisms of the interaction between natural organic matter and layered aluminosilicates, it is necessary to take into account the natural diversity of soil organic matter, adsorption conditions, and mineral composition. This study was designed to investigate the adsorption regularities of fulvic acid (FA) and water-extractable organic matter (WEOM) isolated from horizon H of peaty-podzolic-gleyic soil on kaolinite and muscovite. Sorbates and sorbents were examined by the following methods: high-pressure size exclusion chromatography (HPSEC), high-performance liquid chromatography (HPLC), and potentiometric titration. The specific surface areas of the sorbents were determined by the sorption of N2 molecules. We found that hydrophobic components of FA and WEOM are mainly adsorbed on mineral surfaces. The adsorption of FA and WEOM on kaolinite and muscovite is followed by decreased hydrophobicity of organic matter and decreased heterogeneity of its amphiphilic properties in an equilibrium solution. At pH levels around 6, sorption of organic matter from FA solution containing 19% and 81% hydrophilic and hydrophobic components, respectively, onto kaolinite and muscovite occurs mainly due to hydrophobic components. Hydrophobic interactions on siloxane surfaces are the main mechanism to fix FA on both minerals. Kaolinite adsorbs slightly more organic carbon per unit area than muscovite. The adsorption of WEOM from a solution with 41% hydrophilic and 59% hydrophobic components results not only from hydrophobic and hydrophilic components but also from hydrophobic and electrostatic interactions and depends on pH. The most hydrophobic fractions of organic matter are adsorbed from the hydrophobic components on the surface of both minerals. Under conditions of the performed experiments at pH < 5, more WEOM is adsorbed on muscovite than on kaolinite.
Highlights
The interaction of organic matter with mineral components of the solid phase of soils is the most important process that regulates the cycle and balance of carbon in the biosphere
Humic acids with a large molecular size are adsorbed on the mineral surface, while fulvic acids with a smaller molecular size are adsorbed in the pores and reduce the mineral surface area [15]
Our research showed that adsorption of fulvic acid (FA) and water-extractable organic matter (WEOM) on kaolinite and muscovite is accompanied by decreasing hydrophobicity of organic matter and decreasing heterogeneity of its amphiphilic properties in an equilibrium solution
Summary
The interaction of organic matter with mineral components of the solid phase of soils is the most important process that regulates the cycle and balance of carbon in the biosphere. Humic acids attached to the surface considerably modify the surface of minerals [7] and change their adsorption properties [7,8,9,10]. The experiments on the adsorption of humic acids and water-soluble organic matter show that larger molecules are adsorbed to a greater extent than smaller ones [11,14,15,16]. Wang and Xing established that aliphatic fractions of humic acid were predominantly adsorbed on both kaolinite and montmorillonite, while aromatic structures primarily remained in the solution. Experiments with water-soluble organic matter from lake water revealed that in a wide pH range, large hydrophobic molecules had a greater affinity for kaolinite, while small molecules high in carboxyl functional groups had little or no adsorption on the mineral [14]. Organic substances can be fixed on the mineral matrix of soil through various mechanisms, such as electrostatic interactions, hydrophobic interactions, ligand exchange, cation bridging, as well as hydrogen and van der Waals bonds [15,18,19,20,21]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
