Comparison of activity of three low-cost adsorbents for the removal of methylene blue dye - optimization using central composite design-response surface methodology

Three activated carbon was prepared from Prosopis cineraria (PCAC) barks, seeds of Ocimum basilicum (OBAC), and pods of Tecoma stans (TSAC) using phosphoric acid activation. Fourier-transform infrared spectroscopy (FTIR) analysis shows that all the activated carbon has a similar functional group and the FTIR spectrum of the activated carbon after adsorption indicates the effective removal of methylene blue (MB) by the activated carbon. Brunauer-Emmett-Teller analysis of the activated carbon shows its high surface area and pore size ideal for the removal of MB dye. Response surface methodology with the central composite design model was applied and the optimized condition for the removal of MB was obtained. Experimental optimized removal of 91.5% was obtained for PCAC at a concentration of 254 ppm, duration 33.3 min and temperature 326.4 K, 92% removal at a concentration of 233 ppm, duration 31.6 min and temperature of 326.4 K, for OBAC and 93.4% removal at a concentration of 255.9 ppm, duration 41.9 min and temperature 326.4 K for TSAC. The adsorption was found predominantly physical by all the activated carbon and endothermic. Freundlich isotherm was predominately followed for the removal of MB. Second-order kinetics and Webber-Morris model were found to be followed by all the carbon.