Continuous Adsorption of and Cr (VI) Using a KOH-activated Corn Cob Carbon-Coal Fly Ash Composite

Abstract

Industrial wastewater containing divalent cadmium (Cd^₂⁺) and hexavalent chromium (Cr (VI)) poses a serious environmental threat because of the toxicity, persistence, and bioaccumulative nature of these heavy metals. This study aimed to evaluate the performance of a composite adsorbent, prepared from KOH-activated corn cob carbon (CCAC) and calcined coal fly ash (FA) at a mass ratio of 5:1, for the continuous adsorption of Cd^₂⁺ and Cr (VI). The adsorbent was characterized using Fourier Transform Infrared (FTIR) spectroscopy, while continuous adsorption experiments were performed in an up-flow acrylic column at an initial metal concentration of 100 mg/L, evaluating the effect of flow rate (5–25 mL/min) on Cd^₂⁺ removal and comparing it with Cr (VI) removal from real industrial wastewater at 5 mL/min. FTIR analysis confirmed that KOH activation and calcination modified the surface functional groups of the CCAC and FA, respectively, increasing the availability of oxygen-containing active sites. The composite adsorbent achieved a high and consistent Cd^₂⁺ removal efficiency of 91.13–91.51%, with an adsorption capacity of 4.18–4.19 mg/g, across all tested flow rates. In contrast, Cr (VI) removal was very low (0.797%), attributed to electrostatic repulsion between the anionic Cr (VI) species and the negatively charged adsorbent surface at near-neutral pH, as well as competition from co-existing ions in the real wastewater matrix. These findings indicate that the FA/CCAC composite is a promising, low-cost, waste-derived adsorbent for continuous Cd^₂⁺ removal, while further surface modification or pH adjustment is required to enhance its performance for Cr (VI) removal.