A molecular-level perspective of soil water repellency in sand and clay

Mr Nicholas Daniel1, Assoc. Prof. David Henry1, Prof.  Richard Harper1

1Murdoch University, Swan View, Australia

Soil water repellency is estimated to affect over two million hectares of southern Australia,1 resulting in approximately $100 million in production losses.2 The cause is associated with soil organic matter, which can form a coating on the mineral grains.3 However, while speculative theories have been proposed to explain experimental data, little has been done to justify the theory at the molecular level. Thus, in this study, molecular dynamics simulations were carried out using mineral surface models of amorphous silica (weathered sand surface), kaolinite (Al-OH and Si-O) and quartz. The organo-mineral interactions between hexadecanoic acid on these surfaces was calculated using radial distribution functions, mean square displacements, torsion distributions, concentration profiles, equilibrium snapshots and interaction energies. Initial models were based on previous studies by Walden et al.,4 Uddin et al.,5  and Daniel et. al.,6 which excluded the presence of water; however, the current models now include the addition of water and charge effects. These more complex models show that both the organo-mineral interactions and structure/layering of the acid molecules on the weathered sand and kaolinite (Al-OH) surfaces differ, resulting in models that more accurately reflect experimental observations.

  1. Harper, R. J.; McKissock, I.; Gilkes, R. J.; Carter, D. J.; Blackwell, P. S. Journal of Hydrology. 2000, 231–232, 371–383.
  2. CSIRO Case Study: Water Repellent Soils. (accessed 29/03/2019).
  3. McGhie, D. A.; Posner, A. M. Aust. J. Soil Res. 1980, 18, 309 – 323.
  4. Walden, L. L.; Harper, R. J.; Mendham, D. S.; Henry, D. J.; Fontaine, J. B. Soil Research. 2015.
  5. Uddin, S. M. M.; Daniel, N. R. R.; Harper, R. J.; Henry, D. J. Biogeochemistry. 2017, 134 (1-2), 147-161.
  6. Daniel, N. R. R.; Uddin, S. M. M.; Harper, R. J.; Henry, D. J. Geoderma. 2019, 338, 56 – 66.

Biography: PhD student at Murdoch University, Western Australia. Studies involve the combination of computational chemistry with laboratory experiments to investigate the role of various organic compounds in inducing soil water repellency.

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