Development of a Low-Cost Adsorbent for Removal of Excess Fluoride from Water in Mining Areas

Abstract

Fluoride is an essential mineral for the body as it helps strengthen the bones and teeth, but high amounts of fluoride exposure can have dangerous health effects. Along with the scarcity of water, fluoride contamination is recognized as a major problem worldwide. The World Health Organization has set the maximum permissible limit for fluoride at 1.5 mg/L. However, long-term exposure to excess fluoride has several side effects on human health, water bodies, environment, and agricultural fields. The source of fluoridation may include naturally occurring minerals, coal mines, or anthropogenic action. Several materials (biomass, rice husk ash, bone char, shale, and low-grade coal) and methods (adsorption, membrane separation, and column studies) have been in use for the defluoridation of water. Recent reports from the Central Ground Water Board (CGWB), Govt. of India, indicate an increase in the concentration of fluoride in both surface and ground waters in many industrial areas including mining locations. The mining areas such as Jharia and Bokaro (Jharkhand), Korba (Chhattisgarh), Jharsuguda and Talcher (Odisha), Nagpur (Maharashtra), Ledo (Assam), Neyveli (Tamil Nādu) are among the prominent places affected by a high concentration of fluoride in water. As per the records of CGWB, people in 27 out of the 30 districts of Odisha are affected by fluoride contamination. Drinking water in many villages of Angul, Khurda, Puri, Nayagarh, Boudh, Kandhamal, Bolangir, Bargarh, and Nuapada districts is contaminated with excessive quantities of fluoride. This attracts researchers to carry out further 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 were targeted to the huge quantities of water that are generated in coal mining areas. In this research, 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 Proximate analysis, XRD, SEM- EDS, FTIR, BET,and TGA. Two types of shale samples were collected, crushed, and used in the adsorption process in the laboratory using a synthetic fluoride solution. Key parameters such as particle size of shale, contact time, adsorbent dose, initial fluoride concentration, and pH were taken into consideration. The maximum efficiency of removal for type I (weathered) shale was 47.05% compared to type II (fresh) shale i.e. 40.02% for 3 ppm initial fluoride solution within 60 min in pH range 5–7 using batch adsorption process. The fluoride removal efficiency of the adsorbents was increased by using heat activation (100oC, 500oC, and 800oC) and chemical activation using different chemicals, viz. KOH, NaOH, H2SO4, and ZnCl2. Maximum removal of fluoride obtained was 85.01% with typeI heat activated 800oC and 90.2% using type I chemically activated KOH. The change in pH and total suspended solid present before and after the adsorption process were also taken into consideration. The efficiency of carbonaceous shale (CS I) collected from the Samleshwari opencast project was also compared with other carbonaceous shales (CS II) collected from the Basundhara opencast project and ferruginous shales collected from Guali (FS I) and Barsuan iron ore mines (FS II). Both Carbonaceous shale and Ferruginous shale have the potential to be used as an adsorbent for F- removal but a suitable adsorbent should not only have high F- adsorption capacity and cost-effectiveness but also be amenable to easy desorption of the adsorbed F- and capable of efficient regeneration for reuse. The desorption capacity obtained for carbonaceous shale 2M NaOH (24 h.) was obtained as 98.1%, whereas ferruginous shale desorption capacity was found to be 69%. The regeneration study was carried out up to five cycles using 1M HNO3 solution, kept for 2 h. In the fifth cycle, carbonaceous shale was able to remove 15.64% F- whereas ferruginous shale removed only 11.5% F- from the aqueous solution. The desorption and regeneration of ferruginous shale is less than carbonaceous shale making carbonaceous shale a better and suitable adsorbent for F- removal. The adsorption followed pseudo-second order 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 tests showed more clayey content in the weathered shale compared to fresh shale, making it more porous and suitable as a fluoride adsorbent. XRD analysis 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 that the fluoride is removed 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 an economical material to be used as an adsorbent for fluoride removal.