Efficient Removal of Tannins from Vinasse with Citric Acid-Modified Sugarcane Bagasse: Process Optimization Via Response Surface Methodology, Kinetic and Equilibrium Studies

Abstract

This study evaluated the efficiency of tannin removal from vinasse using sugarcane bagasse as a biosorbent before and after citric acid modification. A central composite design optimized the modification process, considering bagasse mass, citric acid concentration, and reaction time as independent variables. The FTIR intensity ratio at 1,730/3,400 cm^-1 (I1730/I3400) was the response variable, where an increase indicated greater carbonyl incorporation, with 1,730 and 3,400 cm^-1 assigned to C=O and O–H stretching, respectively. We optimized the biosorption conditions using the same statistical design, with pH, bagasse mass, and vinasse concentration as the factors and tannin removal efficiency as the response. Both raw and modified bagasse samples were characterized by FTIR, SEM, pHpzc, and surface functional group analysis; the spent adsorbent was analyzed by FTIR and confocal microscopy. Optimal modification was achieved with 10% w/v citric acid, 1.0 g bagasse, and a 20-minute reaction, yielding an FTIR I1730/I3400 ratio of 1.458, indicating successful esterification. The optimal biosorption contact time was set at 120 minutes based on kinetic data showing rapid adsorption during the initial 30 minutes. Under these conditions, citric acid-modified bagasse outperformed raw bagasse in terms of both removal efficiency and pH independence. The kinetic data conformed to a pseudo-second-order model, indicating that intraparticle diffusion was not the sole adsorption mechanism. The equilibrium data fit the Freundlich isotherm, and the experimental adsorption capacity reached 64.84 mg g^-1 under optimal conditions. Evidence suggests that tannin adsorption onto citric acid-modified bagasse occurs via a mixed mechanism involving both physical and specific surface interactions. Confocal microscopy confirmed the presence of phenolic compounds on the bagasse surface. These results highlight the potential of citric acid-modified bagasse for treating tannin-rich effluents. Further research should address surface area, regeneration, and performance of diverse adsorbates to fully realize the potential of these sustainable materials.