Utilization of Spent Yerba Mate as an Unconventional Sorbent for the Removal of Acid and Basic Dyes from Aqueous Solutions

This study investigated the potential use of Yerba Mate (YM) residue as an unconventional sorbent for removing acid dyes—Acid Red 18 (AR18) and Acid Yellow 23 (AY23)—and basic dyes—Basic Violet 10 (BV10) and Basic Red 46 (BR46)—from aqueous solutions. The research included characterization of YM (FTIR, BET specific surface area, porosity, pHPZC), examination of the effect of pH on dye sorption efficiency, analysis of sorption kinetics (pseudo-first-order and pseudo-second-order models, intraparticle diffusion model), and determination of maximum sorption capacity (Langmuir I and II models, and the Freundlich model). The sorption efficiency of the dyes onto YM was highest at pH 2 for AR18 and AY23, at pH 3 for BV10, and at pH 6 for BR46. The sorption equilibrium time for all dyes onto YM mainly depended on their initial concentration, ranging from 180 min (at 50 mg/L) to 210 min (at 500 mg/L). The kinetics of dye sorption were best described by the pseudo-second-order model. The maximum sorption capacity (Qmax) of YM for the acid dyes AR18 and AY23 was 24.95 mg/g and 22.86 mg/g, respectively. The sorption capacities of the tested sorbent for the basic dyes were higher, with Qmax = 46.24 mg/g for BV10 and Qmax = 60.54 mg/g for BR46.