Competitive adsorption of binary dye from aqueous solutions using calcined layered double hydroxides

In the present paper, LDH hydrotalcite was synthesised via co-precipitation method. The LDO form was obtained by calcination of the LDH material at 500°C during 3 h. The as-prepared materials were characterised by TGA, XRD, N₂ adsorption-desorption isotherms, SEM, FT-IR, and pH _PZC techniques. Both solids were used as adsorbents of Indigo carmine (IC) and Congo red (CR) from simple and binary dyesolutions. The experimental results showed that the sorption equilibrium of each dye was reached within 100 min in the single and binary systems for both materials. The dynamical data were well fitted with the pseudo-second-order kinetic model. Based on the thermodynamic results,all the explored temperatures showed that the adsorption processes are endothermic and spontaneous. Based on the AICc and Wi values, it is evident that the adsorption isotherm data of IC onto LDH and LDO in a single solution were well fittedwith Freundlich isotherm and in the binary system were fitted more favorablywiththe Extended Freundlichmodel.For CR adsorption onto LDH and LDO in single solution,Sips model was the best.In the binary solution, the Extended Sips model was found as the best-fitting modelfor CR adsorption onto LDH. While for CR uptake onto LDO, the adsorption isothermdata werewell fittedwith the Extended Freundlichmore likely than another model.The competitive adsorption of IC and CR on LDH and LDO showed that LDO exhibited a greater affinity towards IC (168 mg/g) than that for the CR (80.94 mg/g). The adsorption mechanism was mainly monitored by electrostatic interactions between the positively charged LDH layer and the anionic dyes. However, for LDO the dyes molecules were effectively intercalated in the interfoliar space owing to the reconstruction of calcined LDH by the structural memory effect.