Multifunctional chitosan tailored γ-aluminum oxy-hydroxide monolith aerogels for sustained environmental remediation†

Recently, improving the mechanical properties of monolithic silica and alumina aerogels by incorporating polymers for various applications, particularly for adsorption, has been the focus of research. In this study, we report the modification of the high surface area γ-aluminum oxy-hydroxide (γ-AlOOH) aerogel with biopolymer chitosan having nitrogen functional groups to improve the efficiency of adsorption. The chitosan + γ-AlOOH aerogel was synthesized using the sol–gel method via a supercritical drying process. The monolithic chitosan–γ-AlOOH aerogel was optimized at 1% (w/w) chitosan for all aluminum precursors used. The XRD pattern of the synthesized monolithic chitosan–γ-AlOOH aerogel shows an amorphous nature, while the FTIR peaks at 1065 cm⁻¹ and 1387 cm⁻¹ indicated the presence of –OH and –NH (amide) bonds. TEM images show the nanoneedle-shaped morphology of chitosan within the γ-AlOOH aerogel with specific surface area of 562.11 m² g⁻¹ and pore volume of 3.68 cc g⁻¹. The as-synthesized monolithic chitosan–γ-AlOOH aerogel was used for the removal of azo dyes, such as methylene blue (MB) and crystal violet (CV), heavy metal ions [Pb(II), As(III)], and rare earth metals [U(VI)]. The maximum adsorption capacities obtained from the adsorption isotherms are 167 ± 3 mg g⁻¹ for MB, 164 ± 2 mg g⁻¹ for CV, 644 ± 5 mg g⁻¹ for U(VI), and 102 ± 2 mg g⁻¹ for As(III) at pH 7. However, at pH 7 Pb(II) is found to precipitate; therefore, the adsorption capacity is obtained at pH 5 with 228 ± 1 mg g⁻¹. The recyclability of the monolithic chitosan–γ-AlOOH aerogel was 94.55% (MB), 94.48% (CV), 94.65% (Pb(II)), 94.8% (U(VI)), and 95.15% (As(III)). These results highlight the potential of heteroatom-rich biopolymers tailored to alumina aerogels for achieving superior multifunctional adsorption properties.