Document Type

Article

Publication Date

Fall 9-29-2026

Abstract

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 mg L−1 at a working pH of 7–9, below the California OEHHA notification level (15 mgL−1 ) but above the German guidance value (4 mgL−1 ), which increased to 340 ± 12 mgL−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.

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