Investigations on a dye desorption from modified biomass by using a low-cost eluent: hysteresis and mechanisms exploration

Methylene blue (MB) desorption from alkali-treated orange tree sawdust (ATOS) adsorbent was investigated under different experimental conditions. For an initial MB adsorbed amount of 63.78 mg g−1 and an initial aqueous pH of 6.0, the experimental data showed that MB desorption is time dependent and increases with NaCl concentration. A maximal desorption yield of 65% was reached after 120 min for a 1 M NaCl concentration. These experimental data were successfully fitted by the pseudo-second-order kinetic model (R2 ≈ 1 and APE < 1.155%), indicating that MB desorption mechanism involved mainly an ion exchange process between liquid and solid phases. Moreover, the effect of initially adsorbed dye concentration on MB desorption efficiency was apprehended for five successive desorption steps. The experimental results showed that MB release in the bulk solution followed a non-singular adsorption/desorption isotherm that become more intense with the increase in the initially retained dye amounts onto ATOS. The theoretical required number of MB desorption steps for a total regeneration of ATOS noticeably decreased from 575 when using distilled water to only 50 steps with NaCl aqueous solutions (0.5 M) as desorbing eluent. FTIR analysis showed the involvement of the hydroxyl and carboxylic functional groups in the sorption process. Ionic chromatography analysis presented a decrease in the aqueous solution concentration in Na+ and Cl− ions by 46% and 48%, respectively, which emphasizes on their migration onto the solid phase. These results confirmed that MB desorption is most likely driven by a cationic exchange mechanism between dye molecules and sodium ions.