Application of soybean residue (okara) as a low-cost adsorbent for reactive dye removal from aqueous solution

This study investigated the feasibility of using okara as a low-cost adsorbent for the removal of Reactive Brilliant Blue KN-R (RBB), a typical reactive dye, from aqueous solution. Batch adsorption experiments were conducted to evaluate the effects of pH, adsorbent dosage, initial dye concentration, and temperature. It was proved that strong acidic condition was favorable for the adsorption process and the optimal pH was 2.0. Langmuir-Freundlich (L-F) isotherm fitted well with the equilibrium data. The maximum adsorption capacity determined from the L-F isotherm was 68.67, 159.60, and 402.58 mg/g at 20, 35, and 50 degrees C, respectively. Kinetic studies showed that adsorption of RBB followed the pseudo-second-order kinetic model, both boundary layer diffusion and intra-particle diffusion might affect the adsorption rate. Thermodynamic study demonstrated the spontaneous and endothermic natures of the adsorption process. Fourier transform infrared analysis revealed that chemical functional groups (e.g. amine, hydroxyl, carboxyl, phosphate, and ether) on okara would be the active binding sites for adsorption of RBB. The results showed that okara could be effectively used as a low-cost and alternative adsorbent for the removal of RBB from wastewater.