Novel green synthesized multifunctional and multilayered nZVI–Chitosan–CQDs nanocomposite: simultaneous remediation of microbial and chemically diverse contaminants

Abstract

The increasing presence of emerging pollutants (EPs), including chemical and resilient microbial contaminants, in aquatic environments demands multifunctional, sustainable treatment solutions. In this study, a green-synthesized, multifunctional, and multilayered carbon quantum dots (CQDs)-embedded chitosan-doped nZVI nanocomposite (nZVI–Chitosan–CQDs) was developed using waste-derived precursors to achieve synergistic microbial inhibition and chemical adsorption. Response surface methodology (RSM) was employed to optimize synthesis parameters, thereby achieving the maximum yield. Comprehensive characterization using FTIR and XRD for surface functionalization and crystallinity, SEM-TEM imaging for uniform multilayer encapsulation, BET analysis for high mesoporosity, XPS for Fe⁰/Fe²⁺/Fe³⁺ chemical states and strong interfacial bonding, and Tauc analysis demonstrating reduced bandgap and enhanced reactivity confirmed the successful integration of CQDs, chitosan, and nZVI to form a stable core–shell–shell structure. The developed nanocomposite completely inhibited the growth of Escherichia coli, Staphylococcus aureus, and Enterobacter sp., and disrupted the structure and EPS matrix of natural multispecies marine biofilms at 250–500 μg mL⁻¹. High removal efficiencies of 92% for Cr(VI), 89% for drugs, and 94% for dyes were demonstrated by the nanocomposite, which followed Langmuir isotherm behaviour and pseudo-second-order kinetics. The removal mechanism involved Langmuir-type monolayer chemisorption, electrostatic interactions, π–π stacking, surface complexation, and reductive reduction of Cr(VI) by the nZVI core. The antibacterial action was attributed to ROS production, which caused membrane damage and disruption of the EPS matrix, leading to biofilm destabilisation. The adsorption reaction was endothermic (ΔH° > 0) and spontaneous (ΔG° < 0), according to thermodynamic analysis. After multiple regeneration cycles, the nanocomposite maintained over 85% of its efficiency and was biocompatible. Overall, this green engineered nanocomposite presents an efficient, eco-friendly, and sustainable platform for simultaneously removing chemical and microbiological pollutants from contaminated water.