Hydrophobic properties and chemical characterisation of natural water repellent materials in Australian sands

doi:10.1016/S0022-1694(00)00182-7

Journal of Hydrology

Volumes 231–232, 29 May 2000, Pages 47-58

https://doi.org/10.1016/S0022-1694(00)00182-7 Get rights and content

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.

Introduction

A research effort focussed on the amelioration of the problem of water repellency (Ma'shum et al., 1989, Ward and Oades, 1993, Franco et al., 1994, Franco et al., 1995) requires the detailed characterisation of the components that cause water repellency in non-wetting sand (NWS). A recent study of the role of intrinsic particulate organic matter (IPOM) in these sands described the contribution of these wax-containing particles, and that of a wax-coated sand surface, to the development of water repellency (Franco et al., 1995). In the current paper, we describe the physical and chemical properties of the hydrophobic waxes extracted from NWS and compare them to other materials that may be implicated in the development of water repellency in the field. These materials include the organic coating on sand particles (Roberts and Carbon, 1972), IPOM (Franco et al., 1995), waxes from microorganisms including basidiomycete fungi (Bond and Harris, 1964), fungal growth (Chan, 1992), plant materials (De Bano et al., 1970, McGhie and Posner, 1981) and eucalyptus tree litter (McGhie and Posner, 1980).

Ma'shum et al. (1988) studied the components of the hydrophobic extracts of non-wetting sands and reported the presence of both free and esterified long-chain, C₁₆ to C₃₂ fatty acids. Simulation of water repellency using these straight chain fatty acids and esters showed that although the addition of these compounds imparts hydrophobicity to wettable sand surfaces, the nature of the repellency is different to that found with the natural polar extract from NWS (Ward and Oades, 1993). Consequently, the hydrophobic waxes must also contain other chemical constituents which either have different hydrophobic properties or which influence the behaviour of the straight chain fatty acids. Our investigation involves the detailed characterisation of the waxes from NWS and correlates the findings with analysis of other related materials so as to establish the origin of these waxes.

The characterisation of organic material in complex mixtures found in soil and sediments have been the subjects of detailed analyses. The GC/MS technique requires the relatively volatile, extractable organic matter to be separated from the mineral component in sufficient quantity in order to identify the major components extracted from the soil and related samples. In addition to conventional MS techniques (e.g. GC/MS and MS/MS) that aid in analysis of the more volatile compounds, it is necessary to characterise the significant proportion of the organic matter that consists of high molecular weight non-volatile material and the more polar organic matter. This was accomplished using Solid State Cross Polarisation/Magic Angle Spinning-¹³C Nuclear Magnetic Resonance (CP/MAS-NMR) Spectroscopy to analyse the extracts and particulate organic matter.

These studies form an integral part of a broader strategy, which aims to ameliorate the problem of water repellency by biological remediation (Franco et al., 2000).

Section snippets

Soils and associated materials

Non-wetting sand (NWS) samples were collected from sites located at Western Flat and Coombe in the south–east of South Australia. Samples were taken from the top 15 cm of a homogenous siliceous sand horizon. The sand was air dried, sieved <2 mm and stored at room temperature. Samples were also collected from the area beneath the clumps of eucalyptus trees adjacent to the NWS sites that were rich in tree litter. These litter samples typically contain 55–65% organic matter (Franco et al., 1995)

Yield of the extracted waxes

Average extraction yields and percentage composition after fractionation are shown in Table 1. Non-wetting sands have a lower amount of total extractable material, as well as a lower content of non-polar material (5.75–6.6% of total waxes) compared to fresh tree litter, eucalyptus plant materials and microorganisms (11.95–20.85%). The polar wax fraction constituted between 41.7–51.75% of all the extracts analysed.

Water repellent properties of the extracts

The hydrophobic properties of AWS treated with the extracts (Fig. 1A) and extract

Conclusions

The physico-chemical analysis of water repellent sandy soils from the southeast of South Australia indicates that the major component that contributes to hydrophobicity is a polar wax. The behaviour of this polar wax, which is present on the surface of the sand grains and in the particulate organic matter (Franco et al., 1995), is mediated by the presence of low levels of a non-polar wax, that is highly hydrophobic, and the relatively high amounts of water soluble, hydrophilic components. The

Acknowledgements

This work was supported by the Australian Wool Research and Development Corporation and the Grains Research and Development Corporation. We are grateful to David Hein for his technical assistance, and to Yoji Hayakawa of the Australian Wine Research Institute for advice on the GC–MS analysis.

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Hydrophobic properties and chemical characterisation of natural water repellent materials in Australian sands

Abstract

Introduction

Section snippets

Soils and associated materials

Yield of the extracted waxes

Water repellent properties of the extracts

Conclusions

Acknowledgements

Soil Biol. Biochem.

Sci. Total Environ.

Org. Geochem.

Aspects of the chemical structure of soil organic materials as revealed by solid state 13C NMR spectroscopy

Biogeochem.

The influence of the microflora on physical properties of soils. I. Effects associated with filamentous algae and fungi

Aust. J. Soil Res.

Development of seasonal water repellency under direct drilling

Soil Sci. Soc. Am. J.

Translocation of hydrophobic substances into soil by burning organic litter

Soil Sci. Soc. Am. Proc.

How water moves in a water repellent sandy soil I. Potential and actual water repellency

Water Resour. Res.

Effect of drying temperature on the severity of water repellency

Soil Sci.

Aspects of the chemical structure of soil organic materials as revealed by solid state ¹³C NMR spectroscopy