Optimization of porous volcanic ash-based geopolymer for crystal violet adsorption using the Box-Behnken design

Abstract

Volcanic ash was used as a precursor for the synthesis of a geopolymer activated by sodium hydroxide and using hydrogen peroxide as a pore-forming agent. Factors controlling geopolymer synthesis such as sodium hydroxide concentration (6–12 mol/L), liquid/solid mass ratio (0.3–0.5), and H 2 O 2 mass concentration (0%–2%) were optimized using the Box-Behnken design method. The chosen process variables were optimized to enhance both the geopolymer's porosity and its effectiveness in removing crystal violet. Sodium hydroxide concentration and H 2 O 2 mass concentration had a significant effect on both responses. Under optimal conditions of 6 mol/L NaOH concentration, a 0.3 liquid/solid ratio, and 2% H 2 O 2 mass concentration, the model-predicted and experimental values for both responses were highly comparable. Additionally, response surface methodology was used to assess the removal of crystal violet from an aqueous solution, employing the geopolymer produced under these optimal conditions as the adsorbent. Experiments were carried out according to the Box-Behnken statistical surface design with four input parameters, namely, contact time (A: 10–120 min), initial crystal violet concentration (B: 20–100 mg/L), adsorbent dose (C: 0.1–0.6 g), and pH (D: 3–9). Regression analysis indicated a strong fit of the experimental data to the second-order polynomial model, with a coefficient of determination ( R 2 ) of 0.9864 and a Fisher's F value of 61.97. Optimization of the parameters A (35.415 mg/L), B (98.184 min), C (0.359 g), and pH (6.950) achieved a maximum crystal violet removal of 98.413% by the geopolymer.