Wettability characteristics of a sample of minerals: influence of mineral type, particle size and chemical weathering

Abstract

Characterization of mineral properties has been thoroughly studied since they have numerous applications in several industrial fields, from mining, to optics and material science. Nevertheless, there is a lack of information about the wettability of minerals as a surface property. This property has a great importance in terms of Earth’s shallow surface processes such as water infiltration, soil water retention, erosion or landslides. This dissertation sets out a research proposal to examine the wettability of twenty one minerals and draw conclusions from this study. Moreover, this research pretends to assess the effects of the environmental conditions such as chemical weathering, mineral type and particle size in the mineral surfaces, and consequently, in their wetting behaviour. The water wettability of the minerals was determined by measuring the contact angle (CA) by means of the sessile drop method (SDM), and water drop penetration time (WDPT). The first study revealed that the galena, pyrite, malachite, sandstone (rock) and sphalerite presented some repellent behaviour. Afterwards, a second study was carried out with these minerals in order to study the influence of the size particle. The results reflected that the smaller the particle size, the larger the contact angle. Finally, a simulation of acidic rainwater was passed through the minerals and the experiment indicated the increase of the wettability after the chemical weathering. The results clearly revealed a wide range of contact angles for the freshly crushed surfaces. A mechanism based on surface charge (cations and anions) is proposed. The least wettable minerals have metal in their composition, which become neutral towards the exterior and lose the property to attract water i.e. they become water repellent. By contrast, the surfaces of the minerals after the chemical weathering were protonated and they gain the ability to attract water i.e. they become wettable. As far as the particle size is concerned, combination of surface area increase and surface roughness decrease may have contributed to the increase of contact angles. Understanding the controls of the extreme water reppellency of some of these minerals (e.g. malachite) may have applications well beyond mineralogy, in particular it may aid the development of new hydrophobic materials.