Immobilization of anionic dyes by intercalation into hydrotalcite

The uptake of the diazo dye Acid Blue 113 by hydrotalcite (HT) and calcined hydrotalcite (CHT) was studied as a function of reaction time and dye equilibrium concentration. Equilibration is attained in ca. 15h, irrespective of the sorbent. The kinetic profiles are however different; the uptake rate by HT is described by the pseudo-second order adsorption model, whereas sorption by CHT is accounted for by the Avrami's equation. Equilibrated sorption by HT and CHT complies, in both cases, with the Langmuir isotherm. Whereas the apparent affinity of CHT for the dye (K-L = 4.2 L mg(-1)) is about three times that of HT, its maximum sorption capacity is larger by a factor of 50; for CHT, q(m) is 2544 mg g(-1). The difference reflects the operation of different uptake mechanisms. XRD data and pH measurements demonstrate that sorption by CHT takes place by intercalation, whereas removal by HT is solely due to adsorption. The uptake by CHT is also described by classical anion-exchange formalisms. It is shown that the applicability of the Langmuir model is, in the case of CHT, fortuitous. The high dye removal efficiency of HT, and especially of CHT, makes these materials excellent candidates to substitute activated carbons. (C) 2008 Elsevier B.V. All rights reserved.