Mild sulfonation process of food grade algae for removal of methyl violet and methylene blue toxic dyes: Statistical modelling and desirability function optimization

In this study, food-grade algae (FGA) underwent a mild sulfonation process via hydrothermal process-assisted activation in 1 M sulfuric acid (H₂SO₄) for 4 h at 100 °C. The sulfonated FGA (S-FGA) was applied to be a potential biosorbent for removal of two structural types of cationic dyes, namely, methyl violet (MV) and methylene blue (MB) from aqueous media. Thus, the physicochemical properties of S-FGA biosorbent were investigated using several analytical methods including XRD, FTIR, SEM-EDX, and pH_pzc. Moreover, the adsorptive performance of S-FGA towards MV and MB dyes was statistically optimized using the Box-Benken design (BBD) with desirability functions. In this process, the key biosorption parameters including S-FGA biosorbent dose (A: 0.02–0.1 g/100 mL), pH of the dye's solution (B: 4–10), and the contact time (C: 10–120 min) were optimized. Thus, the desirability function results indicate the best removal of MV (94.9 %) was achieved using 0.09 g/100 mL of S-FGA dose, pH of 8.2, and contact time of 119 min, while MB removal (74 %) was reached using 0.09 g/100 mL of the S-FGA, pH of 9.5, and 109 min of contact time. The kinetic and isotherm profiles of the biosorption process for MV and MB dyes onto the S-FGA surface followed the pseudo-second order (PSO) kinetic and Freundlich isotherm models, respectively. The estimated biosorption dye capacity (Langmuir model) for MV and MB dyes by S-FGA was 224.6 mg/g and 406.1 mg/g, respectively. The thermodynamic analysis indicates a spontaneous and exothermic-driven biosorption process with decreased randomness at the solid-liquid interface (ΔS° < 0). The biosorption of MV and MB dyes by S-FGA was likely governed by contributions due to hydrogen bonding, Yoshida-H bonding, and electrostatic attractions. Hence, the sulfonation of food algae biomass-based biosorbent shows promising potential for the effective removal of these toxic cationic dyes from contaminated water.