The Significance of Hydrophobicity for the Water Retention Properties of Sand and Coal

Abstract

For mine wastes such as coal tailings, management of these materials requires complex geotechnical engineering that uses many soil properties, such as water retention. However, coal itself is chemically heterogeneous and often appears to be partially hydrophobic, which affects its water retention properties. This study aims to outline how hydrophobic soil particles and coal alter water retention curves compared to hydrophilic materials. The study involves four materials: sand, hydrophobized sand, crushed rock and crushed coal. Mixtures of sand with different proportions of hydrophobic particles had their water retention curves measured and compared, with the only variable being the particle surface characteristics. The rock and coal were separated into different particle size fractions and had their water retention curves measured and compared, with the only variable being particle hydrophobicity. A clear trend was observed for the sand mixtures: the degree of saturation at any suction was reduced when increasing the hydrophobicity of the material. This trend indicates the fundamental water retention behavior expected for soils more hydrophobic than is typical, which was not clearly demonstrated in previous studies. However, a similar trend was not seen when comparing the rock and otherwise identical hydrophobic coal samples, which actually appeared hydrophilic in terms of water retention. ESEM imaging shows a dual hydrophilic and hydrophobic behavior for coal which may explain the result. However, further research is required to understand the discrepancy, which appears to be caused by an unknown coal–water phenomena.

A Simple Water Retention Model Based on Grain Size Distribution

“…The contact angle for each material was measured using an approach based upon a sessile drop technique [65] and a sample preparation procedure specific to soils [66]; the same approach was used in previous research and the reader is directed to [37] for a detailed description. Note, only the receding contact angle was necessary for this study, as this angle is relevant for the drying path of the WRC.…”

“…However, because most soil minerals are hydrophilic with relatively low contact angle values (for example, as low as 17 • for quartz [35,36]), many GSD-WRC models assume a contact angle of zero and achieve good predictions for many soils [11][12][13][14][15][16][17]. In contrast, the materials of interest in this study include coal tailings that have significant coal content, and coal is known to be hydrophobic [37][38][39][40][41][42], which means that a contact angle of 0 • cannot be used nor justified.…”

“…There are a variety of existing WRC models that do consider the contact angle of soils [43][44][45], typically for the purpose of fitting wetting and drying paths to WRC data (i.e., capturing hysteretic behavior) where the advancing contact angle and receding contact angle are relevant to the wetting and drying path WRCs, respectively. In addition, it has been shown that many of the WRC models that assume a contact angle of zero during the model development can be easily altered to consider the contact angle of a soil [37]. Thus, consideration of the contact angle of a material is relatively simple to implement, at least for materials that have the same contact angle for every particle.…”

“…However, coal tailings contain a mixture of coal and mineral particles, and the coal particles can be more prevalent for particular size ranges, as indicated by the correlation between specific gravity (G s ) and particle size for one studied coal tailings [46]. No models have been found that can consider contact angle variations with particle size; however, for GSD-WRC models in particular, it would be simple to implement such the correction in [37], as a suction is associated with each particle size range.…”

“…In the proposed model, the contact angle changes with particle size when the material tested is coal tailings, which are comprised of coal (maceral) and inorganic mineral particles. Indeed, coal macerals are known to be more hydrophobic than minerals [37][38][39][40][41][42] and the contact angle of a given fraction depends on the relative proportion of macerals and minerals. In addition, it was found that those proportions vary with particle size [46].…”