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

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, N-2 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 nm(2) of the functionalised material giving a composition [(AlOOH)(230)center dot(H2O)(86)center dot(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 x 10(3) M-1 for AB9, 1.5 x 10(5) M-1 for AY23 and 8.4 x 10(4) 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.