Study On Adsorption Of Anionic Dye From Aqueous Solution By Calcined Layered Double Hydroxides

Al(NO₃)₃·6H₂O, Mg(NO₃)₂·6H₂O, NaOH and Na₂CO₃ being raw materials, nano magnesium aluminium layered double hydroxides (LDHs) with differrent Mg/Al mol rations were prepared by co-precipitation method and different magnesium aluminium calcined layered double hydroxides (CLDHs) were obtained by heating Mg/Al-LDHs at high temperature. Both LDHs and CLDHs were characterized by X-ray diffraction (XRD), TG-DTA, infra-red spectroscopy (FT-IR) and BET surface area. The ability of Mg/Al calcined layered double hydroxides to remove anion dye from aqueous solution was investigated. The effect of time, adsorbent dose, pH, ionic strength and temperature on the adsorption was carried out. The disposal mechanism was discussed. The experimental results showed that:(1) The optimum molar ratio of Mg/Al of LDH was 3:1.The maximal surface area of CLDH3 was 252.3m²/g by heating Mg/Al-LDHs at 500°C for 4 hours. So 500 °C is the optimum temperature. The pores in CLDH3 mainly are mesopores about 9nm at different temperature and independent of calcined temperature. The BET adsorption isotherms were similar and all attributed to IV type.(2) Effects of time, pH and adsorbent dose were studied. The maximum adsorption takes place within 1 h at pH 6.0. The percentage of adsorption increases with increase in the adsorbent dose. Adsorbent dose of activated red K-2BP and acid yellow G respectively were 0.02g and 0.04g for initiative concentration 40mg/L. The percentage of adsorption of CLDH3 was little affected by pH within the range pH 5-10.(3) The batch adsorption kinetics has been described by the Lagergren equation which is a second-order rate expression.(4) Isotherms for activated red K-2BP and acid yellow G by CLDH3 was well described using the Langmuir equation. Gibbs free energyâ–³G⁰<0, adsorption was spontaneous; â–³H>0, adsorption was exothermic reaction and higher temperature was propitious to adsorption reaction; â–³S>0, showed the increased randomness at the solid-solution interface during adsorption. (5) The 40mg/L initiative solution of activated red K-2BP and acid yellow G respectively had 1 adsorbed peak. After adsorption treatment, adsorbed peak disappeared entirely and no new peak appeared. This showed that dye decomposer didn't exit.(6) The removal process of activated red K-2BP and acid yellow G by CLDH3 using XRD was structural reconstruction. Only part of CLDH recovered LDH layer structure after adsorption. The FT-IR showed that red K-2BP and acid yellow G sorption by CLDH3-500 is arranged within the interlayer in the form of sulfonic anion.(7) The sorption capacity of the regenerated materials was unobvious. The first- regenerated materials were superior to second- regenerated materials and was especially fit to lower concentration. In the course of structural reconstruction, the decomposed compounds of activated red K-2BP and acid yellow G were probably incorporated into the mixing metal oxides after the first and second thermal regenerations.