Synthesis and Properties of Zeolites from Coal Fly Ash

Abstract

Previous attempts to use fly ash as a soil amendment have had limited success because of its low nutrient value, low cation exchange capacity (CEC), and elevated levels of toxic trace elements. However, treating fly ash with NaOH or KOH at an elevated temperature converts the ash into zeolite minerals and solubilizes the toxic trace elements, which are removed in the base solution. The CEC of the untreated fly ash was <100 mmolC kg-1 but increased to over 3000 mmolC kg-1 when heated for 3 days at 100 °C in 3 M NaOH. The dominant zeolites formed at 100 °C in NaOH were zeolite Na-Pl and zeolite P-C, and at temperatures of 150−250 °C, the mineralogy changed to zeolite X and pectolite. In KOH at 100 °C, zeolite K-G (potassium chabazite) was formed. The fly ash zeolites had a high affinity for K+, Ca2+, and NH4+, although attempts to use the treated ash to remove NH4+ and heavy metals from wastewater and electroplating wastes were only partially successful. Potential uses of the treated ash were limited due to the high pH that resulted from the dissolution of the zeolite minerals. At pH 4 and pH 5, the rate of fly ash zeolite dissolution was 1000 times faster than most aluminosilicate minerals. Attempts to produce a zeolitic material with NH4OH or fluorides were not successful.