Low-cost adsorbent prepared from soybean hulls residues as potential alternative for cationic dyes removal

Neutral red (NR) and safranin-O (SO) dyes are used in industrial processes, which their inappropriate disposal in surface waters causes negative impacts. Alternative techniques have been investigated for the complete removal of dyes, in which the adsorption presents itself as a feasible option, especially when low-cost adsorbents obtained from industrial waste are used, such as soybean hulls (SBH). This study proposed the use of SBH for the adsorption of NR and SO dyes. The characterization showed heterogeneous and porous morphology with predominantly negative surface charges, which favored the fixation of the cationic dyes. The comparative study of SO and NR adsorption onto SBH and commercial granular activated carbon confirmed the good performance of the biosorbent material. The maximum adsorption capacities were 287.30 and 221.74 mg g(-1) for NR and SO onto SBH, respectively. The equilibrium time was reached in 90 minutes for both dyes. The experimental data was adjusted to the Langmuir and pseudo-second-order models. Thermodynamics inferred endothermic, spontaneous and reversible characteristics to adsorptive processes. The main proposed adsorption mechanisms were hydrogen bonds, pi-interactions and electrostatic interactions. The regeneration study showed the possibility of reusing SBH for 6 cycles, and the effect of ionic strength found that the adsorption efficiency was not reduced. Therefore, SBH demonstrated versatility and efficiency in the removal of cationic dyes from wastewater, evidencing its industrial applicability.