Formation of NaP zeolite from fused fly ash for the removal of Cu(ii) by an improved hydrothermal method

Abstract

Fly ash with a Si : Al ratio of 3.63 has been converted to a zeolite of high Cu²⁺ sorption capacity using a fusion conversion process and an improved hydrothermal method. Alkali fusion was applied to collapse the fly ash crystalline phases and release the Si content. The fused fly ash was then hydrothermally crystallized at low temperature to a Na–P zeolite. The zeolite was characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy and BET surface area. The effect on Cu²⁺ removal efficiency of the duration and temperature of the melt, the ratio of alkali to fly ash, initial alkali concentration, and hydrothermal temperature and time have been investigated. The results suggest that the improved hydrothermal process gives a significant improvement in zeolite yield and in the adsorption capacity of Cu²⁺. Fly ash fused at 550 °C for 2 h at a Na₂CO₃ : fly ash ratio of 1 : 1.2 and crystallized at 80 °C for 7 h with 2 M NaOH gave an optimal sorption capacity of 99 mg g⁻¹ Cu²⁺. The Cu²⁺ exchange capacity of the regenerated product was comparable to that of the untreated material.