Valorization of spent vanadium pentoxide catalyst from the sulfuric acid industry as an economical adsorbent for crystal violet removal.

Around 150 000-200 000 tons of spent vanadium pentoxide catalyst (SVC) are generated annually by sulfuric acid plants and are landfilled for $80-150 per ton. It is demonstrated here as an unmodified, zero-cost adsorbent achieving 94.5% ± 0.3% crystal violet (CV) removal at an adsorbent dose of 6.3 g L-1 and an initial concentration of 38.5 mg L-1, corresponding to a working capacity of 6.1 ± 0.2 mg g-1 with zero material cost and no pretreatment requirement. Physicochemical characterization by XRF, XRD, BET analysis, DLS, SEM-EDX, FTIR spectroscopy, TGA, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and zeta potential analysis showed that SVC contains amorphous diatomite SiO2 (52.4 wt%) and the monoclinic VO2 phase, with XPS confirming vanadium to be present predominantly as V4+ at the surface. The material has a BET surface area of 11.4 ± 0.3 m2 g-1 and a zeta potential of -9.58 ± 0.42 mV at pH 7. A 29-run Box-Behnken design identified adsorbent dose and contact time as the dominant influencing parameters. Nonlinear regression confirmed the Freundlich isotherm as the best fit and pseudo-first-order kinetics as the governing model. The D-R mean free energy E = 3.10 kJ mol-1 and the thermodynamic parameters confirm endothermic, entropy-driven physisorption through three parallel pathways: electrostatic attraction, hydrogen bonding, and a proposed vanadyl coordination contribution. XPS analysis shows the V 2p envelope to be essentially unchanged after adsorption, indicating that this interaction does not involve significant charge transfer. Ionic strength experiments confirmed the electrostatic mechanism. ICP-OES showed vanadium release of 12 ± 1 µg L-1 at a working pH of 7-9, below the California OEHHA notification level (15 µg L-1) but above the German guidance value (4 µg L-1), which increased to 340 ± 12 µg L-1 under the worst-case acidic conditions, restricting application to pH 6-9 with effluent monitoring. Acid-thermal regeneration (0.1 M HCl + 300 °C) retained 74.8% ± 1.1% capacity after 15 cycles. The preliminary unit treatment cost of $1.53 m-3 is 60-80% below commercial alternatives.