Excellent adsorption of Zn(II) using NaP zeolite adsorbent synthesized from coal fly ash via stage treatment

Herein, a novel method for the adsorption of metal ions from sewage by NaP zeolite adsorbent synthesized from stage-treated coal fly ash (CFA) was proposed. The pre-treatment of CFA and grading extraction of silicon and aluminum were explored, and the effects of crystallization parameters and sodium salts on the yield and purity of NaP zeolites were investigated. The characteristics of the zeolite product were measured by a series of characterization methods and the adsorption performance was also evaluated. Optimal treatment parameters were calcination at 800 °C for 3 h with m(CFA)/m(Na2CO3) = 1:1 and 3 mol/L HCl with m(CFA)/V(HCl) = 1 g: 10 mL, with high extraction efficiencies (Si: 91.40% and Al: 90.20%) through stage treatment method. NaP zeolite product exhibited higher purity (96.66%) and yield (94.24%) at 120 °C for 8 h with the addition of NaBr (entirely instead of NaOH, the purity was 90.57% and the yield was 87.82%). The effect of organic steric hindrance was connected with its molecular configuration. NaP zeolite with the addition of polyethylene glycol (PEG) at n(Al2O3)/n(PEG) = 1:5 had the highest single-grain dispersion with the smallest grain size (2.13 μm). However, NaP zeolite with the addition of cyclohexanol (CyOH) at n(Al2O3)/n(CyOH) = 1:3 had the largest surface area (80.4 m2/g). During the Zn(II) adsorption, the appropriate pH value was identified at 5, and the adsorption process followed the pseudo second-order kinetics and Langmuir isotherm adsorption equations. The thermodynamic parameters revealed that the adsorption process was spontaneous and endothermic. NaP zeolite adsorbent exhibited excellent adsorption performance with Zn(II) concentration less than 0.2 ppm in the absorption equilibrium. Moreover, NaP zeolite as an effective adsorbent possessed excellent regeneration.