Optimization of anionic dye adsorption onto Melia azedarach sawdust in aqueous solutions: effect of calcium cations

Adsorption of RR141 (an anionic dye) was carried out onto Melia Azaderach sawdust (MAS) in solutions containing low concentrations of calcium ions. The effect of dye concentration, solution pH, Ca2+ concentration, adsorbent dose, and contact time was investigated by response surface methodology. The results showed that by adding 50 mg L-1 Ca2+ ions, the decolorization rate increased by 3 fold. Cation bridging between anions of RR141 and negatively charged sites of MAS was recognized as the main mechanism for the enhanced dye removal. Higher removal efficiencies were obtained at higher calcium concentrations. Solution pH (4-10) had a negligible effect on the process in presence of bridging Ca2+ ions. Initial dye concentration and adsorbent dose were the most influential factors in the process. The equilibrium time was found to be 46 min, which was unchanged at different dye concentrations (10-100 mg L-1). The dye uptake obeyed the non-linear equations of pseudo-second order kinetic model and Freundlich/Brunauer-Emmett-Teller (BET) isotherm models. The maximum adsorption capacity of 12.92 mg g(-1) was calculated from the Langmuir model. Based on the obtained results, adsorptive properties of Melia Azaderach sawdust can be greatly enhanced for uptake of anionic dyes in calcium-containing solutions like natural waters. (C) 2015 Curtin University of Technology and John Wiley & Sons, Ltd.