Response surface methodology approach for optimization of color removal and COD reduction of methylene blue using microwave-induced NaOH activated carbon from biomass waste

Coconut leaves (Cocos nucifera. L) biomass waste were utilized for the preparation of activated carbon by microwave-induced NaOH activation. The surface characteristics of the coconut leaves activated carbon (CLs-AC) were determined by scanning electron microscopy, Fourier transform infrared spectroscopy and pHPZC. Subsequently, the CLs-AC was applied for the color removal and chemical oxygen demand (COD) reduction of methylene blue (MB) from aqueous solutions. The face-centered composite design (FCCD) and response surface methodology were used to investigate the effects of main operating variables such as initial solution pH (3-8), temperature (298-323 K), adsorbent dosage (0.2-1.50 g/L), and contact time (15-90 min), while the initial MB concentration was fixed at 100 mg/L throughout the optimization process. Maximum color removal (99.37%) and COD reduction (98.27%) for MB can be achieved by simultaneous interaction between temperature with time and adsorbent dosage with contact time. The optimum pH, temperature, adsorbent dosage and contact time were found to be 8.00, 323.00 K, 1.50 g/L and 90 min, respectively. Under optimal conditions, the adsorption equilibrium data was well fitted with the Langmuir adsorption isotherm with R-2 of 0.9907 and q(max) at 87.72 mg/g. The adsorption kinetics was found to follow pseudo-second-order model.