Self-Assembly of Poly(ethyleneimine)-Modified g-C₃N₄ Nanosheets with Lysozyme Fibrils for Chromium Detoxification

We disclose a straightforward approach to fabricate nanocomposites for efficient capture of Cr(VI) from an aqueous solution through the self-assembly of poly(ethyleneimine)-modified graphitic carbon nitride nanosheets (PEI-g-C₃N₄ NSs) and lysozyme fibrils (LFs). The as-made PEI-g-C₃N₄ NSs@LFs exhibited mesoporous structures with a high specific surface area of 39.6 m² g^-1, a large pore volume of 0.25 cm³ g^-1, several functional groups (e.g., -N, -NH, -NH₂, and -COOH), and a zero-point charge at pH 9.1. These merits allow the PEI-g-C₃N₄ NSs@LFs to further enhance their physical adsorption and electrostatic attraction with the negatively charged Cr(VI) species of HCrO₄^- and CrO₄^2-, which is beneficial for the uptake of Cr(VI), >80%, from an aqueous solution in a wide pH range. Interestingly, X-ray photoelectron spectra indicate that the PEI-g-C₃N₄ NSs@LFs converted Cr(VI) to Cr(III) through visible-light-induced photoreduction. The adsorption of Cr(VI) on the surface of PEI-g-C₃N₄ NSs@LFs was found to obey the Freundlich isotherm model, signifying that they have a heterogeneous surface for the multilayer uptake of Cr(VI). In contrast, the PEI-g-C₃N₄ NSs and LFs as Cr(VI) adsorbents followed the Langmuir isotherm model. Adsorption kinetic studies showed that the uptake of Cr(VI) through the PEI-g-C₃N₄ NSs@LFs was highly correlated with a pseudo-first-order model, suggesting that physisorption dominates the interaction of Cr(VI) and the PEI-g-C₃N₄ NSs@LFs. In real-life applications, the PEI-g-C₃N₄ NSs@LFs were used for the detoxification of the total chromium in the industrial effluent and sludge samples.