Enhanced crystal violet removal via biopolymer-modified ZnFe layered double hydroxides: Adsorption mechanism and environmental impact analysis

Abstract

Water pollution by synthetic dyes like crystal violet (CV) poses severe ecological threats. This study introduces biopolymer-modified Zn–Fe layered double hydroxides (LDHs) as high-performance, sustainable adsorbents for the removal of CV. Three composites were synthesized via coprecipitation using chitosan, carrageenan, or cellulose and were comprehensively characterized. The properties of prepared materials were studied by SEM, EDX-Mapping, FT-IR, XRD, BET, and Rietveld refinement. Structural analysis revealed distinct morphologies: Zn-Fe LDH/chitosan showed aggregated nanosheets with the highest surface area (63.0 m²/g), while Zn-Fe LDH/carrageenan exhibited a rough surface with hydrogel nanoparticles and the largest pores (34.0 nm). Rietveld refinement confirmed that all composites retained the hydrotalcite-type LDH phase as the dominant crystalline phase with no detectable secondary oxide phases (ZnO, Fe₂O₃, Fe₃O₄, or Zn (OH)₂), thus verifying phase purity and confirming that Zn and Fe species are exclusively incorporated within the LDH framework. Adsorption studies identified Zn-Fe LDH/chitosan as the superior adsorbent, achieving a Langmuir capacity of 477.0 ± 9.0 mg/g and 84.51% removal at optimal conditions. Zn-Fe LDH/carrageenan also performed excellently at a lower dosage (458 ± 11 mg/g, 84.03%) at 298 K. Kinetics followed pseudo-second-order (R² > 0.99). Complete Langmuir isotherms were measured at four temperatures (298–328 K) for all adsorbents A comparative assessment highlighted chitosan's superiority due to its balance of high capacity, rapid kinetics, and cost-effectiveness (0.53 USD/g). The removal efficiency for CV still reached 70.50% after six cycles. Post-adsorption FTIR analysis revealed a broadening and red-shift of the O–H stretching band (~3400 cm⁻¹) for all composites after CV uptake, providing spectroscopic evidence for hydrogen bonding contributions to the adsorption mechanism alongside electrostatic attraction, π–π stacking, metal-site Lewis acid–base interactions, and ion exchange. The environmental sustainability of the composites was validated using AGREE (overall score 0.75) and MOGAPI metrics, confirming alignment with green chemistry principles. The thermodynamic analysis showed spontaneous CV adsorption using three tested materials. This work advances the design of biopolymer-LDH hybrids, offering an efficient, scalable, and eco-friendly solution for dye-contaminated water remediation.