Crystal violet removal from aqueous solution: adsorption studies, Box–Behnken optimization, safety, and reuse in methanol oxidation

Abstract

The discharge of toxic crystal violet (CV) dye poses significant environmental and health risks, necessitating sustainable treatment solutions. This study presents sulfuric acid-functionalized activated carbon derived from frankincense (SAC) as a high-performance adsorbent for CV removal. Characterization revealed a mesoporous structure with a high surface area of 842.97 m² g⁻¹, adorned with functional groups that facilitate CV binding. Process optimization via Box–Behnken design achieved an exceptional adsorption capacity of 638.51 mg g⁻¹ at pH 8, with a 0.075 g dose and 60 min contact time, following the Langmuir–Freundlich isotherm model. A pivotal finding from in vitro cytotoxicity assessment (HepG2 cells) was the effective detoxification of CV, where the half-maximal cytotoxic concentration (CC₅₀) increased dramatically from 84.7 µg mL⁻¹ (untreated CV) to >1000 µg mL⁻¹ (SAC-treated solution). Embodying circular economy principles, the spent adsorbent was successfully repurposed as an electrocatalyst for methanol oxidation, yielding a current density of 22.40 mA cm⁻². The process demonstrated economic viability at a low manufacturing cost of $0.06438 g⁻¹ SAC and excelled in green chemistry metrics (Eco-Scale: 90/100). This work underscores the dual utility of SAC as a sustainable, safe, and reusable material for advanced wastewater treatment and energy conversion applications.