Agricultural Bio-Waste-Derived KOH-Activated Carbon for Chromium(VI) Remediation: Reaction Pathways and Kinetic Behavior in Aqueous Media

This study examines the influence of the activation ratio on the properties of potassium hydroxide-activated carbon (KOH-AC) derived from safflower stems and its performance in hexavalent chromium Cr(VI) adsorption. The adsorbent was characterized using Brunauer–Emmett–Teller (BET) surface area analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction to evaluate porosity, morphology, and crystallinity. Key adsorption parameters—including pH, contact time, initial concentration, temperature, and adsorbent dosage—were systematically investigated. The optimal activation ratio (KOH-AC with 4 : 1 KOH to carbon; KAC-4) yielded a highly porous carbon with a BET surface area of 2089.5 m2/g and a maximum Cr(VI) adsorption capacity of 132.3 mg/g. The highest removal efficiency was achieved at pH 5.2. Kinetic analysis revealed that adsorption followed a pseudo-second-order model, indicating chemisorption, while thermodynamic results suggested an exothermic and non-spontaneous process with decreased adsorption at higher temperatures—implying a dominant physisorption mechanism governed by electrostatic interactions. The Freundlich isotherm provided the best fit, confirming multilayer adsorption on a heterogeneous surface. Overall, KOH-AC from safflower stems proved to be a promising, low-cost, and sustainable adsorbent for Cr(VI) removal, supporting agricultural waste valorization and circular economy initiatives.