Surface chemical properties of novel high surface area solids synthesized from coal fly ash

Abstract

The zeolite, Na-P1, was synthesized from fly ash samples originating from coal-fired power stations in South Africa by hydrothermal treatment of the raw ash with concentrated aqueous NaOH solutions. The zeolite was then further modified by acid leaching at elevated temperatures. This resulted in the formation of a novel high surface-area solid. Spectroscopic and potentiometric investigations into the surface properties of both solids indicate that acid-base properties are most likely due to the presence of =AlOH- and =SiOH-type surface groups. Surface protonation constants for the various solids (unmodified ash, base-modified ash, and acid-etched zeolitic product) are reported. Metal sorption studies were performed for cadmium and copper. It is suggested that the zeolitic product sorbs copper and cadmium by an ion exchange mechanism instead of a surface complexation mechanism, whereas the high surface-area solid, formed after acid etching, sorbs these metals via a surface complexation mechanism. Metal adsorption constants for the formation of =XOMOH species on the surface of the last mentioned solid for use in speciation models are reported.