Examining the Kinetics and Equilibrium of Crystal Violet Dye Removal Using Olive Pomace as a Function of Time

Abstract

Synthetic dyes released into water bodies represent a growing environmental problem, since many resist natural breakdown and persist in aquatic systems for long periods. Crystal violet, a cationic dye of the triphenylmethane family, is widely used in textiles, printing inks, and biomedical staining, and it can pose ecological and health hazards even at very low concentrations. Developing inexpensive, renewable, and effective materials for removing it from water has therefore become a research priority. This study examines how effectively untreated dry olive pomace — a widely available Mediterranean agro-industrial by-product -removes crystal violet from aqueous solution under simple batch conditions. In each experiment, 1 g of dry olive pomace was mixed with 20 mL of a 1 × 10⁻⁵ M crystal violet solution and held at 23 ± 2 °C for contact periods reaching 144 h; the dye remaining in solution was measured spectrophotometrically at 557.3 nm, its wavelength of maximum absorbance. Equilibrium behaviour was examined using the linear forms of the Langmuir and Freundlich isotherms, and the uptake rate was modelled with the pseudo-first-order (PFO) and pseudo-second-order (PSO) equations. Of the two isotherms, the Langmuir model gave the closer fit (R² = 0.9981) with a favorable separation factor (RL = 0.3135), consistent with monolayer coverage on a surface of largely uniform energy, while the Freundlich model also described the data reasonably well (R² = 0.9955). For the kinetics, the pseudo-second-order equation reproduced the experimental trend more closely (R² = 0.9998) than the pseudo-first-order equation (R² = 0.9938), pointing to chemisorption — bonding between dye molecules and functional groups on the pomace surface — as the step controlling the overall rate. Overall, these results show that raw, unmodified olive pomace performs well as a cheap and eco-friendly biosorbent for crystal violet, and converting this residue into a wastewater-treatment material offers a practical way to combine agricultural-waste valorization with the goals of a circular bioeconomy.