Use of white grape pomace for removal of cationic dyes from aqueous solution: kinetic, isotherm and thermodynamic characterization

In this study, grape pomace powder (GPP), a winemaking waste produced worldwide in large amounts, was evaluated as a potential biosorbent for dye removal from aqueous solutions. The GPP was characterized for its composition and structure using colorimetric, chromatographic and spectroscopic methods, Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry (XRD), field-emission gun scanning electron microscopy (FEG-SEM), and the point of zero charge (pHpzc). The analyses showed that GPP is a fibrous material rich in -OH groups, with an amorphous structure and an irregular, rough surface. The biosorption potential of GPP was evaluated using five cationic dyes: auramine O (AO), crystal violet (CV), malachite green (MG), methylene blue (MB), and safranin (SA) in aqueous solutions. The maximum adsorption capacities (qeq<overline>)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{{q }_{eq}})$$\end{document} were 75.96 mg/g for AO, 84.20 mg/g for SA, 174.62 mg/g for CV, and 183.96 mg/g for MB. MG had the highest affinity for GPP, with the maximum qeq<overline>\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\overline{{q }_{eq}}$$\end{document} of 313.08 mg/g and the shortest equilibrium time of 45 min. Isotherm results indicated that the Langmuir model was the most suitable for predicting AO and SA adsorption, while MB was better represented by the Freundlich model. CV was better described by the Sips model, and MG exhibited the most complex uptake, being described by both Langmuir and Freundlich models. The biosorption process was energetically favorable for all dyes tested, and the adsorption mechanism appears to involve hydrogen bonding, electron pair donation interactions, and electrostatic attractions. The results suggest GPP as a sustainable, cost-effective biosorbent for the removal of these dyes from aqueous solutions, particularly for MG.