The contact angle of wetting of the solid phase of soil before and after chemical modification

Abstract

Wettability of soil affects a wide variety of processes including infiltration, preferential flow and surface runoff. Wettability of surface is usually expressed in terms of contact angle (CA) measurement. If the CA between liquid and solid surface is less than 90°, the surface is called hydrophilic, otherwise the surface is called hydrophobic. If the CA of water droplet on hydrophilic surface is in a range of 0-30° this surface is called superhydrophilic. In case of superhydrophobic surfaces the CA exceeds 150° that means that these surfaces are extremely difficult to wet. CA of wetting of mineral soil particles depends on the overlying organic and iron compounds. The object of study is a sample of the humus-accumulative horizon of typical chernozem (Kursk, Russia) and two samples (horizons A1, B2) of red ferrallitic soils (Fr. Norfolk, NE Oceania). The soil samples were analyzed for organic carbon, forms of non-silicate iron and hydrophobic-hydrophilic composition of humic substances. CA of wetting was determined in the intact samples and after removal of organic matter (H₂O₂ treatment), amorphous and crystallized forms of iron. Static contact angles were determined with the sessile drop method using a digital goniometer (Drop Shape Analysis System, DSA100, Krüss GmbH, Hamburg, Germany). The contact angle was calculated by the Young–Laplace method (fitting of Young–Laplace equation to the drop shape). The measurements were repeated 10-15 times for every sample. Oxidation of organic matter (H₂O₂ treatment) causes an increase in the values of CA of wetting (in chernozem from 9.3 to 28,0-29.5º, in ferrallitic soil from 18.0 − 27.3 to 22.4 − 33.4º). CA remained constant for chernozem and slightly decreased in the case of ferrallitic soil, when the removal of amorphous and crystallized forms of iron was performed on samples pretreated with H₂O₂. CA increase occurs after successive removal of nonsilicate forms of iron from soil samples of chernozem (9.3 − 17,9 − 29.5º) and ferrallitic soils (27.3 − 30.6 − 33,4 and 18.0 − 29.0 − 29.2 º). Relative hydrophobicity of the soil solid phase surface after treatment by Mehra and Jackson (1957) occurs in parallel to the carbon content reduction. Loss of carbon in the samples after the extraction of iron is related to the solubility of the hydrophilic components of humic substances. These results indicate that the main factor, which determines the wettability of soil solid phase, is the organic substance.

Keywords

• soil solid phase, contact angle, organic matter, hydrophobic-hydrophilic humic substances, nonsilicate iron forms

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