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 (Ka) 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 (Cm) 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.