Layered double hydroxides (LDHs) have received considerable attentions in recent years owing to their unique layered structures. Their general formula is [M_1-x²⁺M_x³⁺(OH)₂](A^n-)_x/n·mH₂O, where M is metal cation and A is an exchangeable anion. LDHs are layered materials with hydroxide sheets, where a net positive charge is developed on the layer due to partial substitution of trivalent for divalent cations, balanced by exchangeable charge compensating anions and water molecules, that are present in the interlayer space. The adsorption property and mechanism of Acid Blue 7 (AB7) on MgAl Layered double oxides with 3∶1 Mg/Al molar ratio were investigated. The adsorption of AB7 on MgAl-LDO was carried out using the batch method. We changed the dosage of adsorbent, pH value and initial dye concentration to study the influences of various factors and then calculated the maximum capacity of AB7 adsorbed at equilibrium (Q_e) and percentage of adsorption (η%) respectively. The results of adsorption experiments indicated that the maximum capacity of AB7 at equilibrium (Q_e) and percentage of adsorption (η%) with a fixed adsorbent dose of 0.33 g·L^-1 were found to be 446.9 mg·g^-1 and 99.31%, when AB7 concentration, temperature, and pH were 150 mg·L^-1, 288 K and 6.74, respectively. Adsorption isotherms showed that the adsorption process was consistent with both Langmuir and Freundlich equations. It was also spontaneous and exothermic. The Gibbs free energy was calculated to be 8 to 10 kJ·mol^-1 because of hydrogen bonds between the layers and dye molecular. According to the calculation of Materials Studio5.5, a periodical model of MgAl-LDHs was established in hexagonal (2H) stacking sequence, containing MgAl-OH as host layer and CO₃^2- as guest anion, that is the host-guest interaction model with in the position of hcp-Al. The mechanism of adsorption process was that the majority of AB7 were adsorbed on the surface of MgAl-LDO. The regeneration for re-use of LDO after AB7 sorption was feasible with four cycles.

Key words: calcined Mg/Al layered double hydroxides (MgAl-LDO), adsorption, mechanism, Acid Blue 7