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Fluoride Removal from Calcium-Rich Mine Water by Steel Slag Adsorption

The objective of this work was to design a process to remove fluoride from a mining wastewater stream and reduce the concentrations of fluoride below the Environmental Protection Agency’s (EPA) secondary drinking water standard of 2 mg/L fluoride. The effectiveness of electric arc furnace (EAF) steel slag as a method for fluoride removal was investigated. The synthetic mine water contained a range of ortho-phosphates, 1.7 g/L gypsum, and 22.1 mg/L sodium fluoride. Batch testing using glass beakers to test various conditions of the slag were conducted. Two columns in series filled with EAF slag were bottom fed with the synthetic mining water for continuous testing. The best fluoride removal occurred when the phosphates to sulfates ratio was 3:1, at 89.6% removed. Fluoride concentrations were tested using a fluoride ion selective electrode (ISE). The best results occurred at a neutral to slightly basic pH. From testing, it was concluded there are three reaction mechanisms removing the fluoride. The main mechanism through the formation of fluorapatite, Ca5(PO4)3F. The second mechanism uses competitive adsorption between hydroxides and metal ions in solution, formed from the steel slag, to form a flocculant with the fluoride ion. The third happens from the formation of calcium fluoride. The desorption of fluoride from the surface of the slag was explored, but could not be tested due to laboratory restrictions.