Coal mine waste characterization and defluoridation property

Fluoride contamination has been addressed as a major problem worldwide, along with the scarcity of water. This attracts researchers to do more research in the field of fluoride contamination and mitigation of high fluoride content in water. There are various studies, that used a variety of adsorbents for the removal of fluoride from contaminated water. But none of them utilized the vast number of waste, that is generated from the coal mining industries. In this paper, Shale, which is a common coal mine waste, has been used as an adsorbent for fluoride removal from aqueous solution. Along with the defluoridation property of shale, the effect of weathering on the adsorption property of shale was also analyzed using different characterization tests, such as XRD (X-ray powder diffraction), SEM (Scanning Electron Microscope), and FTIR (Fourier Transform Infrared spectroscopy). Two types of shale samples were collected, crushed, and used in the adsorption process in the lab using synthetic fluoride solution. The maximum efficiency of type I (weathered) shale was obtained as 47.05% compared to type II (fresh) shale as 40.02% for 3 ppm initial fluoride solution for 60 min of contact time in pH range 5–7 using batch adsorption process. Other parameters such as shale particle size, pH, and total suspended solid present before and after the adsorption process were also taken into consideration. The adsorption followed pseudo-second kinetics and Freundlich isotherm with an adsorption capacity of 23.66 mg/g, and 21.33 mg/g for weathered and fresh shale respectively. The characterization test showed more clayey content in the weathered shale compared to fresh shale, making it more porous and suitable as a fluoride adsorbent. XRD showed the F(−) containing minerals such as Villiaumite (5.1%) and Fluorite (4.3%) in F(−) loaded weathered shale, confirming the F(−) adsorption onto the shale surface. The major advantage of shale over other existing adsorbents is, it removes the fluoride without significant change in pH, and there are no or very less suspended ions that can be found in treated water. This means the water may not need any secondary treatment after the adsorption process. Shale is a very common and readily available mine waste, that is used for the ceramic industry, building materials, and road construction, making it a no-cost material to be used as an adsorbent for fluoride removal.