Utilization of fly ash for treatment of coal mines wastewater: Solubility controls on major inorganic contaminants

Abstract

Acid mine drainage (AMD) has been reacted with coal fly ash in a 24 h equilibration time using 1:3 and 1:1.5 FA:AMD ratios by weight to produce neutral and alkaline process waters. The capacity of the fly ash to remove the major inorganic contaminants was examined. The elemental concentration trends with time for the two ratios were used to discern which elements have solubility control in the neutralization process. The geochemical computer code PHREEQC and WATEQ4 database was used for geochemical modeling of the process water. The resulting solid residues (SR) were analyzed by X-ray diffraction (XRD), and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX) in an attempt to detect the minerals phases controlling the inorganic contaminants concentration in solution. The relative quantities of soluble bases (CaO, MgO) in fly ash and hydrolyzable constituents in AMD dictated whether the final solution at a given contact time will have a dominant acidic or basic character. Concentration of Fe, Al, B were observed to be controlled by mineral solubility for the entire reaction time while mineral solubility control for Ca, Na, Mg, Si and Mn concentrations developed after the initial rapid dissolution. Initial SO 4 2 - concentration was controlled by precipitation of gypsum and adsorption on iron-oxy-hydroxides at pH > 5.5. Increase of pH in solution with contact time caused the removal of the metal ions mainly by precipitation, co-precipitation and adsorption. Fe was mainly removed as Fe(OH) 3(a), goethite, Al as basaluminite, boehmite and alunite at pH 5.28–6.95 and as gibbsite and diaspore at pH 5.53–9.12. Cu and Zn were removed by adsorption onto the precipitating iron(oxy)-hydroxides and aluminum (oxy)-hydroxides. Si is released by dissolution of SiO 2(a) at pH < 5. Na was removed as Na-jarosite at pH 3.96–6.95 and Ca as gypsum and anhydrite. The treatment of AMD with fly ash was observed to be site-specific, i.e., the effectiveness of the treatment process will depend on the quality of the fly ash and the AMD. The product water meets the DWAF water quality limits for domestic use and irrigation at pH > 8 except for species Na, B, Mg, Ca, Sr and Ba which remain in solution. In addition B, Mg, Sr, Mo and Ba are released from dissolution of fly ash and will be of concern in the proposed treatment process.