Fixed-bed column performance of methylene blue biosorption by Luffa cylindrica: statistical and mathematical modeling

Abstract

This study aimed to investigate the adsorption capacity of Iranian Luffa cylindrica as a natural lignocellulosic adsorbent for biosorption of methylene blue (MB) using a fixed-bed column. The response surface methodology based on central composite design was used to evaluate the interactive effects of three major operating parameters (inlet MB concentration, Luffa dosage, and feed flow rate) on the dye removal percentage (response variable). The significance of the proposed quadratic model was validated by a high coefficient of determination (R² = 0.995) and a low p value (<0.001). The optimum biosorption conditions were determined as inlet MB concentration 38.66 mg L⁻¹, Luffa dosage 2.78 g, and feed flow rate 5 mL min⁻¹, and the maximum MB removal efficiency was founded to be 51% (q_max = 21.4 mg g⁻¹) at optimum conditions. The breakthrough curves were predicted by the Adams–Bohart and Thomas models using nonlinear regression analysis, whereas the experimental data fitted well with the Thomas model (R² ≈ 0.96–0.98). Further surface modification studies enabled us to achieve the maximum MB removal efficiency of 77% (q_max = 46.58 mg g⁻¹) with NaOH-modified Luffa, which is attributed to the intensification of the negatively charged surface of the base-modified adsorbent with hydroxyl groups. The nature of biosorbent–dye interactions was also evaluated by FTIR analysis. The highest desorption performance of MB from Luffa fibers was obtained with HCl solution reaching a desorbed amount of q_des = 130.5 mg in the column studies. The ion exchange is introduced as the dominant biosorption mechanism of MB onto Luffa fibers in this study.

Keywords:

• Biosorption
• Column study
• Luffa cylindrica
• Methylene blue
• Response surface methodology
• Surface modification