Sustainable Doped and Co-Doped Carbon Cryogels for Competitive Heavy Metal Removal from Water

This research investigates sustainable water treatment by evaluating competitive adsorption of Zn, Cd, and Hg using nitrogen-doped and nitrogen co-doped carbon cryogels. The synthesized materials exhibited high specific surface areas (up to 1530 m2/g) and a maximum multicomponent adsorption capacity of 1.03 mmol/g for the CCNS3 sample. The practical applicability of these adsorbents was successfully validated in a real water sample, achieving up to 93% removal efficiency for the investigated metals despite the presence of background ions. Kinetic analysis confirmed that the adsorption process was predominantly controlled by the pseudo-second-order model while intraparticle diffusion modeling revealed a multi-stage adsorption process in which N, S co-doping significantly accelerated the initial mass transfer. The extended Sips isotherm best described multicomponent adsorption, indicating heterogeneous multi-site adsorption on sample surfaces. Concurrently, FTIR spectroscopy and DFT modeling confirmed a dual mode mechanism: while physisorption occurs on basal planes (Eads > −0.8 eV, no charge transfer), the adsorption energy increases significantly at structural defects (N- and S-defects: Eads < −2 eV, significant charge transfer). Excellent reusability properties of samples were confirmed over four consecutive adsorption–desorption cycles. These findings present a highly sustainable approach for competitive heavy metal removal establishing tailored doped and co-doped carbon cryogels as efficient, robust and reusable adsorbents.