# The Significance of Hydrophobicity for the Water Retention Properties of Sand and Coal
## 1 Introduction  
The movement and retention of water in soils is fundamental to many important considerations, from ground stability to ecosystem sustainability. When mining for resources such as ore or coal, the extracted resources usually undergo a beneficiation process, which creates a fine-grained waste product of a high water content called tailings. Coal tailings are a form of artificial granular soil from coal mining that can have a coal content of 40% or higher [1, 2], with the rest being mineral particles coming from the overburden and underlying layers of soils and rocks. As for all wastes, tailings must be responsibly managed in the environment, and an understanding of their water retention properties is an essential consideration. Coal tailings are often disposed in dams, encapsulated in cells within waste rock dumps or mixed with coarse reject and co-disposed with waste rock in spoil piles. Tailings dams are usually capped to allow future usage of the land, and this capping is a non-trivial geotechnical operation requiring knowledge of the drying rate of the tailings which, in turn, requires an estimation of the water retention and permeability functions of the tailings. Previous research has shown that coal, and consequently tailings, are more hydrophobic than common soils [3, 4, 5, 6, 7], which can significantly alter their water retention properties and hydraulic behavior. However, most models for permeability functions and water retention curves (WRCs) developed in unsaturated soil mechanics pertain to hydrophilic soils. Thus, in the management of tailings, it is important to consider the significance of materials of higher hydrophobicity in the context of geotechnical and geoenvironnental engineering. The hydrophobicity of coal arises from the presence of various organic components called macroals that are predominantly composed of large aromatic hydrocarbon molecules, usually 150-250 carbon atoms. There are a variety of macroal types, and each varies in its degree of hydrophobicity [8]. Unlike the rock-forming minerals, macroals are not stoichiometrically predictable, so even a particular macroal type may exhibit variability in its degree of hydrophobicity. Coal itself is heterogeneous in its chemical composition [9; 10], and a single coal particle may contain many different macroals. The variability of macroal hydrophobicity causes local variation of hydrophobicity on individual coal particle surfaces [3]. Further complication arises when considering coal tailings, which contains heterogeneous coal particles as well as hydrophilic rock mineral particles. Few papers in the literature have considered how the hydrophobicity of soil affects water retention. One study investigated how the wettability (i.e., a measure of hydrophobicity) affects soil water drying curves [11]. This was done by comparing drying curves between three hydrophilic soils (sand, loam and silt loam) and then for the same three soils treated to become hydrophobic. Most notably, air entry was observed to occur at lower suctions for the hydrophobic soils. However, the study presented drying path retention curves in terms of the gravimetric water content, which may lead to an incorrect indication of air entry due to soil shrinkage. Drying curves expressed volumetrically would show when air entry occurs without any doubts of its reliability. Another study focused on the effect of the water-surface contact angle on WRCs [12]. This was achieved by measuring the pore size distributions and contact angles of two soils and estimating the WRCs from the pore size distributions using a capillarity model. Two WRCs were shown for each soil: one using a contact angle of zero and the other using the measured angle for that soil. As expected, increasing the contact angle caused a reduction of the predicted suction at any water content. However, the authors did not actually measure the WRCs, and this lack of experimental determination means the conclusions of the study may not represent reality.