A novel adsorbent for efficient metal recovery from aqueous solutions: Carbon-coated oil-treated plastic granules as a sustainable approach to resource conservation

This study investigates the development of carbon-coated oil-treated plastic granules (COTPG) by coating biochar derived from biomass onto granules made from end-of-life plastics, creating a novel adsorbent for metal recovery from aqueous solutions. We hypothesize that combining biochar’s functional surface chemistry with the structural advantages of plastic granules can result in a low-cost, selective, and scalable adsorbent capable of effectively removing transition metal ions such as Cu²⁺ and Fe²⁺ from water. Combining density functional theory (DFT) calculations, continuous-flow adsorption tests, and UV–Vis spectroscopy, we evaluated the adsorption performance and mechanisms of COTPG for copper (Cu²⁺) and iron (Fe²⁺) ions. Results demonstrate that COTPG exhibits a strong affinity for Cu²⁺, significantly outperforming traditional adsorbents such as sand and glass beads. Its adsorption capacity for Cu²⁺ was higher than for Fe²⁺, reflecting stronger interactions with functional groups on the biochar surface, particularly in the π-rich polyaromatic regions. Additionally, COTPG demonstrated enhanced Fe²⁺ adsorption compared to conventional materials, emphasizing its versatility in removing various metal ions. DFT analysis confirmed that oxygen- and nitrogen-containing functional groups on biochar facilitate complexation and ion exchange, while its polyaromatic structure contributes to cation-π interactions, reinforcing Cu²⁺ selectivity. This study not only demonstrates the effectiveness of COTPG as a promising adsorbent for metal recovery but also highlights its role in advancing resource conservation and recycling efforts.