The Effects of Modified Fly Ash on the Structure and Photocatalytic Performance of G-C3N4-based Materials

Regulating the polymerization of graphitic carbon nitride (g-C3N4) precursor is regarded as a desirable pathway to boost its photocatalytic activity. However, the current has been seldomly concerned in adjusting the polymerization of g-C3N4 precursor based on the structural characteristics of fly ash (FA). This study manipulates the polymerization reaction of dicyandiamide through alkali treatment of fly ash (KOH-FA, CaOH-FA and NaOH-FA), yielding three g-C3N4-based composite photocatalysts (CaFA-CN, KFA-CN, and NaFA-CN) with varying degrees of polymerization. Compared to the zeolite structures of fly ash treated with potassium hydroxide (KOH) and sodium hydroxide (NaOH), the fly ash treated with calcium hydroxide (Ca(OH)2) developed calcium silicate hydrate, leading to a more complex surface structure. Isotherm adsorption model analysis revealed that Ca(OH)2-FA's adsorption of methylene blue fits the Freundlich model, indicating uneven distribution of active sites. This unevenness results in an uneven collision probability during the high-temperature polymerization of dicyandiamide, leading to incomplete polymerization and the formation of the CaFA-CN catalyst with more amino groups and fewer carbon-nitrogen heterocycles. This incomplete polymerization enabled CaFA-CN to achieve a 55.3% degradation efficiency of methylene blue within 60 minutes. The photocatalytic activity of CaFA-CN is 4.03 times that of the original g-C3N4 (19.4%).