Studies of the molecular basis of soil water repellency

• Main Author: Llewellyn, Christopher Thomas
• Published: Swansea University 2005
• Subjects: 631.4
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637935

To identify the causes of water repellency in soils, a range of water repellent soils and wettable control soils, sampled from five countries (Australia, Greece, Portugal, The Netherlands, U.K.), were studied. Water repellency was assessed using the water drop penetration time (WDPT) test whilst total organic carbon (TOC) analysis and diffuse reflectance infrared spectroscopy (DRIFT) were used to measure the TOC and aliphatic content respectively in the soils. Water repellency correlated slightly better with aliphatic content than TOC content, although neither correlated well with water repellency. The efficiency of Soxhlet extractions with isopropanol: aqueous ammonia (7:3, v:v) in extracting compounds associated with water repellency was examined. Extraction efficiency was examined by determining: extract mass; severity of water repellency post-extraction; amounts of organic carbon and aliphatic C-H removed; and by assessing the ability of extracts to cause repellency in acid washed sand. Extraction removed repellency completely from 12 of 14 repellent samples and extracts from all soils (including the wettable control samples) were capable of inducing repellency. Samples were rendered wettable regardless of the mass extracted or the quantity of organic carbon removed, suggesting that provided there is some aliphatic material present, the amount is less important than its constitution and/or molecular arrangement. Low polarity solvents caused sample repellency to increase despite the removal of non-polar hydrocarbons, suggesting that compounds such as alkanes are not, in themselves, the main cause of repellency. Kinetics and selectivity of the extraction procedure (using isopropanol: aqueous ammonia) was also examined. Increasing extraction time caused an increase in extract mass and a decrease in soil water repellency. The same compound types were detected by GC-MS in all extracts, but their proportions varied with extraction time. In particular, the removal of alkanes from the soil sample was less rapid than that of more polar compounds.