Biochar-encapsulated Ca-alginate hydrogels for copper harvesting from water: scalable composite design and...

Copper released from industrial activities and urban surfaces poses persistent risks to aquatic systems, highlighting the need for scalable and deployable materials for the harvesting of copper from water for practical recovery and removal applications. In this study, biochar-encapsulated, calcium-crosslinked alginate (BECA) hydrogels are developed as a composite platform for Cu²⁺ removal from aqueous media. The system integrates the well-established ion-binding properties of alginate with the heterogeneous surface chemistry of algae-derived biochar within a mechanically stable hydrogel matrix, enabling a functional and deployable sorbent architecture rather than a standalone adsorbent. Batch adsorption experiments were conducted to evaluate Cu²⁺ uptake under varying initial concentrations and to assess the influence of hydrogel crosslinking conditions. The BECA system achieved removal efficiencies exceeding 98% at low Cu²⁺ loading and maintained removal above 87% at higher concentrations (up to 12.5 mM), with adsorption capacities reaching 69 ± 1 mg g^-1. FTIR analysis confirmed interactions between Cu²⁺ and carboxylate-functional groups within the composite structure. Density functional theory (DFT) calculations further supported favorable Cu²⁺ coordination within the alginate egg-box structure and stable binding at the alginate-biochar interface. Overall, the results demonstrate that BECA hydrogels function as scalable composite platforms for the harvesting of copper from water, combining adsorption functionality with structural stability and practical deployability. The novelty of this work lies in the integration of biochar into a hydrogel matrix to enable practical Cu²⁺ harvesting systems. Future work will further evaluate performance at environmentally relevant trace Cu concentrations and under dynamic flow conditions to extend applicability to real-world stormwater systems.