Sulphates removal from AMD using CFA hydrothermally treated zeolites in column studies

Abstract

Hydrothermally treated zeolites were synthesized from a coal combustion waste by-product, coal fly ash. Synthesis of this material involved dissolution of the coal fly ash matrix followed by crystallization of the slurry. Characterization of coal fly ash using BET indicated that the coal fly ash had a surface area of 3.141 m2 g−1. The XRD patterns on coal fly ash showed dominance of the mullite and quartz peaks and was observed to be amorphous. The optimized parameters for dissolution of the coal fly ash matrix was a concentration of 3 M NaOH aged for 24 h at a solid/ liquid ratio of 15 g/120 mL. The second stage was focused on optimization of crystallization conditions of the CFA/NaOH slurries for synthesis of zeolite. The effect of variation of water content, crystallization time and crystallization temperature were studied where the optimized conditions obtained were a H2O/SiO2 ratio of 0.53, crystallized at 140 °C for a duration of 24 h. Powder x-ray diffraction studies indicated successful conversion of coal fly ash into zeolites as the dominant coal fly ash minerals such as mullite and quartz diminished while the zeolitic hydroxysodalite phase was formed. Fourier transform infra-red studies showed formation of zeolitic bands at 948 cm−1 due to Al–O and Si–O asymmetric stretching, indicating conversion of the CFA aluminosilicate phase. Thereafter, the adsorption performance of hydrothermally treated zeolites for SO42 was evaluated. The parameters evaluated included, zeolite bed height, flow rate and concentration where the optimized conditions were 4.0 cm, 0.45 mL min−1 and 5.0 mg L−1 respectively. Application studies on AMD derived water from Fleurhof dam indicated an anion selectivity with an increasing order of Cl− < NO3– < Br− < NO2– < SO42−.