Upcycling spent activated carbon blocks via chemical surface modification for enhanced hexavalent chromium removal from electroplating wastewater

Abstract

The widespread adoption of point-of-use water treatment technologies has resulted in significant waste accumulation from activated carbon block (ACB) filters. Although ACBs are effective at removing organic pollutants, their capacity to adsorb hexavalent chromium (Cr(VI)) remains limited. This study addresses this limitation by upcycling spent ACBs through chemical surface modifications to enhance Cr(VI) removal from the electroplating wastewater, which aligns with the circular economy principles. Five modified sorbents were developed using acid (r-PAC-HNO<inf>3</inf>), alkaline (r-PAC-NaOH), oxidation (r-PAC-H<inf>2</inf>O<inf>2</inf>), permanganate (r-PAC-KMnO<inf>4</inf>), and iron-oxide (r-PAC-Mag) treatments. Batch adsorption experiments demonstrated that r-PAC-H<inf>2</inf>O<inf>2</inf> exhibited the highest adsorption capacity (11.94 ± 0.03 mg/g), attributed to its increased surface area, larger pore volume, and abundant oxygen-containing functional groups. Optimal Cr(VI) removal was achieved at pH 3, with equilibrium reached within 360 min. The adsorption data conformed to the Freundlich isotherm and pseudo-second-order kinetic models, indicating a chemisorption-driven multilayer mechanism on heterogeneous surfaces. X-ray photoelectron spectroscopy confirmed the reduction of Cr(VI) to trivalent chromium, which subsequently complexed with the surface oxygenic functional groups, reinforcing a synergistic adsorption-reduction mechanism. The addition of a high sulfate and phosphate concentration at 0.1 M further enhanced Cr(VI) removal. Regeneration with 0.001 M NaOH preserved the adsorbent stability, maintaining Cr(VI) removal efficiency at approximately 55 %–85 % over three cycles. This study provides a practical and effective strategy for converting ACB waste into high-performance sorbents for Cr(VI) remediation, supporting sustainable wastewater treatment and resource recovery. The findings offer valuable insights for developing next-generation adsorbents suitable for industrial-scale applications.