Cactus cladodes as a renewable source of cellulose for the adsorption of Safranin O and Acid Blue 25 dyes: Optimization of extraction parameters, comprehensive experimental investigation, and mechanistic interpretation via Density Functional Theory (DFT)

Abstract

This study focuses on the extraction of cellulose from cactus cladodes in Las Palmas, Canary Islands, and its application for removing Safranin O and Acid Blue 25 dyes. The cellulose extraction process was optimized using Design-Expert software to maximize the BET surface area. The optimal conditions identified were 15 % NaOH, 98 degrees C, and 90 min, which resulted in a maximum surface area of 7.7890 m2/g. The adsorption of Safranin O and Acid Blue 25 was evaluated under varying conditions such as contact time, adsorbent mass, pH, initial dye concentration, and temperature. Safranin O exhibited a higher removal efficiency (91.98 %) compared to Acid Blue 25 (41.12 %). The adsorption kinetics followed the Pseudo-Second-Order model, indicating that chemisorption was the rate-limiting step. The Langmuir isotherm best described the adsorption behavior for both dyes, suggesting monolayer adsorption. Safranin O showed a significantly higher maximum adsorption capacity (124.38 mg/g) compared to Acid Blue 25 (11.07 mg/g). Thermodynamic studies revealed that Safranin O adsorption was exothermic and spontaneous, while Acid Blue 25 adsorption was non-spontaneous. Density Functional Theory (DFT) calculations provided further insights into the adsorption mechanism. This study demonstrates the potential of cactus-derived cellulose as an efficient and sustainable adsorbent for dye removal. The integration of experimental data and computational insights offers a comprehensive understanding of the adsorption mechanisms, which can guide the optimization of cellulose-based materials for wastewater treatment applications.