Decolorization of Malachite green dye from wastewater by Populus deltoides: three-level Box–Behnken design optimization, equilibrium, and kinetic studies

Decolorization of Malachite green in aqueous solution by adsorption onto Populus deltoides sawdust (PSD) was optimized through a four-factor, three-level Box–Behnken design in response surface methodology. The influences of four independent variables such as initial pH of solution (3–7), dye concentration (50–300 mg/L), adsorbent dose (0.2–2 g/L), and temperature (23–50 °C) were studied to optimize the condition of dye removal. A natural log transformation was suggested by the Box–Cox plot in order to enhance the model significance. Regression analysis showed good fit of the experimental data to the second-order polynomial model with high coefficient of determination values ( R 2 = 0.996; R adj.2 = 0.9913; R pred.2 = 0.9769), F -value of 213.03, and p -value of <0.0001 ( α = 0.05). Under optimum values of all the four variables, viz., pH of 6.02, initial dye concentration of 262.6 mg/L, adsorbent dose of 0.23 g/L and temperature of 30.3 °C, the maximum uptake ( q e) was noted to be 920.9 mg/g. The experimental equilibrium adsorption data were fitted well to the Langmuir isotherm model ( R 2 = 0.9949). Kinetic studies revealed that adsorption followed pseudo-second order. It was found that PSD is suitable for reuse four times in successive adsorption-desorption cycles with loss of 25.2% in adsorption capacity.