Functionalised pseudo-boehmite nanoparticles as an excellent adsorbent material for anionic dyes

Abstract

Pseudo-boehmite has been functionalised with L-lysine by refluxing an aqueous solution containing these two reactants overnight. The resulting nanosized (<10 nm) product is insoluble in water and has been characterised by solid-state NMR spectroscopy, powder X-ray diffraction analysis, N₂ adsorption–desorption analysis and zeta potential measurements. The affinity of this new nanostructured organic–inorganic hybrid material for anionic dyes has been quantified using UV-vis spectrophotometry and by constructing the adsorption isotherms for Acid Blue 9 (AB9), Acid Yellow 23 (AY23), and Acid Red 37 (AR37). Elemental/micro analyses indicate that one lysine molecule is covalently bonded to every 8 nm² of the functionalised material giving a composition [(AlOOH)₂₃₀·(H₂O)₈₆·(C₆N₂O₂H₁₅)]. The introduction of the positively charged amino groups resulted in a tremendous increase in dye affinity in contrast to the unfunctionalised material. The adsorption isotherms of the functionalised pseudo-boehmite were fitted to the Langmuir model and yielded equilibrium binding constants (K_a) of 2.6 × 10³ M⁻¹ for AB9, 1.5 × 10⁵ M⁻¹ for AY23 and 8.4 × 10⁴ M⁻¹ for AR37. AR37 gave a higher monolayer coverage (C_m) value of 0.13 mmol g⁻¹ than AB9 (0.085 mmol g⁻¹) and AY23 (0.081 mmol g⁻¹). Dye adsorption is correlated with surface coverage of L-lysine and, in the case of AR37, two dye molecules are concluded to be adsorbed per L-lysine while for AY23 a multi-point interaction is proposed to result in a lower dye capacity and a relatively higher affinity of this dye for FPB when compared with AR37.