BSA-modified g-C₃N₄ nanosheets as a sustainable dual-purpose adsorbent for efficient Pb²⁺ removal and CO₂ capture

The development of high-efficiency, selective, and regenerable adsorbents for simultaneous remediation of toxic heavy metals and greenhouse gases remains a growing challenge. This work reports the synthesis of a biofunctionalized graphitic carbon nitride composite (BSA-CN) through stepwise oxidation, epoxide activation, and covalent immobilization of bovine serum albumin (BSA) onto 2D g-C₃N₄ nanosheets. Comprehensive structural, chemical, and morphological analyses (FTIR, XRD, TGA, Raman, SEM-EDX, TEM, and BET) confirmed successful protein immobilization, enhanced surface roughness, increased porosity, and the introduction of protein-derived functional groups that significantly improve surface reactivity and adsorption capacity. The BSA-CN composite exhibited exceptional adsorption performance toward Pb²⁺ ions, achieving a high monolayer capacity (292.9 mg g^-1), rapid uptake within 30-60 min, and strong pH-dependent interactions governed by surface charge modulation. Kinetic modeling indicates that adsorption follows a mixed physicochemical adsorption process dominated by surface complexation, best described by the pseudo-second-order and Elovich models, while equilibrium behavior fits the Redlich-Peterson and Freundlich isotherms, indicating heterogeneity and multilayer sorption. Thermodynamic analysis revealed a spontaneous and endothermic adsorption process (ΔG° < 0, ΔH° = +45.03 kJ mol^-1), accompanied by increased interfacial entropy. The composite demonstrated excellent selectivity for Pb²⁺ over Cu²⁺, Cd²⁺ and Mn²⁺ and retained over 70% efficiency after five regeneration cycles, confirming strong operational durability. In addition, it was able to remove Pb²⁺ from real samples (lake water and surface water). Additionally, BSA-CN displayed measurable CO₂ adsorption capacity that increased at lower temperatures, attributed to enhanced interactions with its hierarchical porous architecture. Overall, this study highlights the potential of protein-functionalized g-C₃N₄ as a versatile, bio-derived, and dual-functional adsorbent for integrated heavy-metal remediation and low-temperature CO₂ capture.