Hydrophobic compounds in sands in New Zealand—extraction, characterisation and proposed mechanisms for repellency expression

Organic compounds were extracted from samples of water repellent sandy soils from two development sequences found on the south-west coast of the North Island, New Zealand. A number of extraction procedures and solvent mixtures are effective at reducing the repellency of these soil samples as measured by the Molarity of Ethanol Droplet (MED) test. The extracted compounds were partitioned into lipid and water-soluble fractions. The lipid fraction includes neutral lipids (predominantly alkane hydrocarbons and triglycerides), acidic lipids (mainly long-chain fatty acids), and polar lipids. The water-soluble fraction exhibits amphipathic behaviour, and shares some similarities with the hymetamelonic fraction of the humic acid pool. When added to soil, this material binds very successfully and its effect on MED depends on the type of solvent in which it is added. There is no good correlation between repellency (MED value) and total carbon content, the quantity of water-soluble material, the quantities of total lipid extract or any of the lipid fractions. The repellency of soil samples can be modified in vitro by the addition of soil lipid extract (increases MED values), or by the addition of soil water-soluble extract (decreases MED values): this is true, regardless of whether the lipid and water-soluble extract have been sourced from repellent or wettable soil. When a sample of repellent Himatangi sand is serially extracted by fresh isopropanol:ammonia (7:3 v:v) mixtures interspersed with extraction by fresh solutions of ammonia, the MED value fluctuates markedly. The former solvent mixture removes lipid compounds and lowers the MED value; ammonia removes polar and amphipathic material and increases the MED value. The results of this study are best explained by the hypothesis that water repellency is determined by the composition or nature of the outermost layer of organic material, particularly amphipathic compounds, rather than the characteristics of the bulk of the organic matter. Four mechanisms, which are consistent with the results reported here, are proposed to explain the expression of repellency in both laboratory studies and in the field.