Preparation and characterization of P-type zeolite for adsorption of Cr, Ni, and Co.

Acid-washed coal fly ash (AW-CFA) was subjected to wet grinding activation followed by hydrothermal crystallization to synthesize P zeolite (FAZ-P). The FAZ-P obtained at 120 °C for 24 h exhibited a maximum relative crystallinity of 93.15% and was employed for the adsorption of Cr, Ni, and Co from aqueous solutions. The zeolitization of coal fly ash (CFA) leads to an increase in specific surface area to 44.00 m/g, resulting in the formation of nano-sized P zeolite crystals with uniformly narrow fissures and sizes within the range of 10-30 nm. Adsorption experimental results indicate that FAZ-P exhibits maximum adsorption capacities of 49.03 mg/g for Cr, 22.20 mg/g for Ni, and 27.25 mg/g for Co. The adsorption equilibrium data for both mixed and single-metal ion solutions conform to the Langmuir model, with the affinity sequence for heavy metal ions being Cr  > Co  > Ni. The pseudo-first-order and pseudo-second-order kinetic models effectively described the adsorption behavior of Cr, Ni, and Co. Increasing the initial pH value of the solution significantly enhanced the adsorption capacity of the adsorbent for heavy metal ions. The removal mechanism of metal ions involves both adsorption and ion exchange processes. The thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic.