Fe-modified biochar for aqueous Cu(II) capture and its reutilization in enhanced phase change materials

To address the dual challenges of developing high-performance phase change materials (PCMs) and heavy metal water pollution, this study proposes an eco-friendly integrated strategy coupling Cu(II) wastewater treatment with TES performance enhancement. Magnetically recoverable Fe-modified biochar (PFeC) was prepared from waste white pine via medium-temperature activation with ferric nitrate and was subsequently employed as an adsorbent for aqueous Cu(II) capture. After that, the Cu-loaded biochar was subjected to high-temperature carbothermal reduction. During this process, the adsorbed Cu(II) ions were converted in situ into uniformly distributed Cu phases, thereby forming a biochar framework with enhanced thermal conductivity (CuC). By loading paraffin wax (PW) into CuC via vacuum impregnation, a high thermal conductivity composite PCM (CuC/PW) was prepared. Characterization results indicate that the specific surface area of the FeC reaches 516.67 m²/g and the Cu(II) adsorption ability is about 70 mg/g. Thermal performance tests demonstrate that the thermal conductivity of CuC/PW reaches 0.987 W/(m·K), representing a 101.02% improvement over the traditional OC/PW. The composite also exhibited a latent heat of 149.4 J/g, a high encapsulation efficiency of 74.21%, and good stability over 100 thermal cycles. In addition, CuC/PW exhibits excellent light harvesting ability across the ultraviolet, visible, and near-infrared spectrum regions. In this work, magnetically recovered Cu(II)-laden biochar was used as a thermally conductive support for PW, thereby integrating aqueous-phase metal capture with the preparation and performance enhancement of PCMs. This capture–conversion–reuse pathway provides a sustainable approach for fabricating functional composite phase-change materials from biomass waste and recovered metal species.