Optimisation using central composite design (CCD) and the desirability function for sorption of methylene blue from aqueous solution onto Lemna major

Water pollution due to contamination of dye containing effluents is a great threat to water body. A study on the biosorption of methylene blue (MB) onto low-cost Lemna major biomass was conducted and the process parameters were optimized by response surface methodology (RSM). A two-level, four-factor central composite design (CCD) has been employed to determine the effect of various process parameters namely initial concentration (600–1000 mg L^‑1), bioadsorbent dose (0.20–1.50 g/100 mL), pH (5–12) and stirring rate (250–800 rpm) on MB uptake from aqueous solution. By using this design a total of 30 biosorption experimental data were fitted. The regression analysis showed good fit of the experimental data to the second-order polynomial model with coefficient of determination (R²) value of 0.9978 and model F-value of 953.48. The optimum conditions of initial concentration (1000 mg L^‑1), adsorbent dose (0.2 g), pH (5) and stirring rate (251.51 rpm) were recorded from desirability function. The adsorption isotherm data were best described by both Freundlich and Langmuir models with a maximum adsorption capacity of 488 mg MB g^‑1L. major biomass at 30 °C which is higher than that available with adsorbents used by past researchers. Finally the pseudo second order kinetic model described the MB biosorption process with a good fitting (R² = 0.999). The adsorbent was characterised by scanning electron micrograph (SEM) and Fourier transform infrared spectroscopy (FTIR).