Article

Studies on non-wetting sands. II. Hydrophobic properties and chemical characterisation of natural water repellent materials

Department of Soil Science, University of Adelaide, P.M.B.1, Glen Osmond, Adelaide, SA 5064, Australia
Journal of Hydrology (Impact Factor: 3.05). 05/2000; 231–232:47-58. DOI: 10.1016/S0022-1694(00)00182-7

ABSTRACT

Water-repellency in non-wetting sands is due to hydrophobic waxes present on the surface of sand grains and contained in particulate organic matter present in these sands. This study investigates the physico-chemical characteristics of these natural waxes and compares them to waxes extracted from potential original source materials.Non-polar and polar hydrophobic wax extracts were obtained from whole non-wetting sand, and its individual constituents, and associated organic matter. These included the sand fraction, the intrinsic particulate organic matter, tree litter, eucalyptus leaves, bark, lucerne and lupin plants, and fungi and actinomycetes isolated from these sands. Waxes were characterised for their hydrophobic properties and composition of their chemical constituents. The hydrophobicities of the waxes were assessed by measuring the water-repellency induced after treating acid washed sand with wax extracts.Non-polar and polar wax extracts of the tree litter displayed hydrophobic properties that were similar to the corresponding waxes isolated from non-wetting sand and intrinsic particulate organic matter. Unlike these plant-derived waxes, the microbial wax extracts possessed different hydrophobic properties.Characterisation of the components of the extracted waxes by gas chromatography-mass spectroscopy (GC-MS) analysis revealed a strong similarity in the composition of waxes isolated from non-wetting sand, tree litter and other plant material. The major components found were unbranched and branched C16 to C36 fatty acids and their esters, alkanes, phytanols, phytanes, and sterols. Some of these components were not detected in the microbial waxes.Unextracted samples, as well as wax extracts of non-wetting sand, intrinsic particulate organic matter, tree litter and fresh plant material were further analysed by solution and solid state Nuclear Magnetic Resonance spectroscopy which revealed the relative content of the different chemical species present.

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    ABSTRACT: The impact of non- and poorly wetting soils has become increasingly important, due to its direct influence on the water-limited potential yield of rain-fed grain crops at a time of enhanced global competition for fresh water. This study investigates the physical and compositional mechanisms underlying the influence of soil organic matter (SOM) on the wetting processes of model systems. These model systems are directly related to two sandy wheat-producing soils that have contrasting hydrophobicities. Atomic force microscopy (AFM), contact angle and Raman micro-spectroscopy measurements on model planar and particulate SOM-containing surfaces demonstrated the role of the hierarchical surface structure on the wetting dynamics of packed particulate beds. It was found that a nanoscale surface topology is superimposed over the microscale roughness of the packed particles, and this controls the extent of water ingress into particulate packed beds of these particles. Using two of the dominant component organic species found in the SOM of the two soils used in this study, it was found that the specific interactions taking place between the SOM components, rather than their absolute quantities, dictated the formation of highly hydrophobic surface nanotopologies. This hydrophobicity was demonstrated, using micro-Raman imaging, to arise from the surface being in a composite Cassie-Baxter wetting state. Raman imaging demonstrated that the particle surface nanotopography influenced the degree of air entrapment in the interstices within the particle bed. The influence of a conventional surfactant on the wetting kinetics of both the model planar surfaces and packed particulate beds was quantified in terms of their respective advancing contact angles and the capillary wetting force vector. The information obtained for all of the planar and particulate surfaces, together with that obtained for the two soils, allowed linear relationships to be obtained in plots of the contact angle data as a function of the wetting liquid surface tensions. These linear relationships were found to reflect the mechanisms underlying the surface energy parameter requirements for wetting. Copyright © 2015. Published by Elsevier Inc.
    Full-text · Article · Aug 2015 · Journal of Colloid and Interface Science
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    • "Soil wettability, usually associated with hydrophobicity and water repellency, has been studied mostly in soils of high organic matter content. The variation in soil wettability has frequently been explained by vegetation types, organic matter quality and quantity, influence of mycoflora or other microorganisms and their secretions, soil texture and acidity (Bachmann et al., 2008; Doerr et al., 2000; Feeney et al., 2006; Franco et al., 2000; Goebel et al., 2005; Hubbert et al., 2006; Hurrass and Schaumann, 2006). However, throughout the literature contradictory findings on their correlations with wettability are frequently reported. "
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    Full-text · Article · Jul 2015
    • "Soil wettability, usually associated with hydrophobicity and water repellency, has been studied mostly in soils of high organic matter content. The variation in soil wettability has frequently been explained by vegetation types, organic matter quality and quantity, influence of mycoflora or other microorganisms and their secretions, soil texture and acidity (Bachmann et al., 2008; Doerr et al., 2000; Feeney et al., 2006; Franco et al., 2000; Goebel et al., 2005; Hubbert et al., 2006; Hurrass and Schaumann, 2006). However, throughout the literature contradictory findings on their correlations with wettability are frequently reported. "
    [Show abstract] [Hide abstract]
    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.
    No preview · Article · Jul 2015
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