Adsorption and ion-exchange behavior of calcined Mg-Al layered double hy- droxide for high concentration Cl- from flue gas desulfurization wastewater

In order to obtian the magnesium aluminum hydrotalcite ( LDH) with a molar ratio of 4 ∶ 1 (magnesium: aluminum),the direct method of coprecipitation was used to calcine LDH at 450 ℃ for 3 h to obtain the calcined mag- nesium aluminum hydrotalcite (CLHD). The CLDH adsorbent was taken in a 250 mL Erlenmeyer flask. Under certain conditions,dechlorination adsorption experiments and CLDH regeneration and reusability experiments were carried out on 5 000-20 000 mg / L high concentration Cl- simulation solution and actual desulfurization wastewater. The effects of different reaction times,initial concentration,reaction temperature,initial pH of solution and dosage of CLDH on the removal of chloride ions were investigated. The adsorption mechanism was investigated by BET,XRD and FT-IR. The experimental results show that CLHD adsorbs Cl- due to interlayer anion exchangeability (“structural memory”),and Cl- adsorption conforms to the first-order kinetic model. The adsorption amount and dechlorination rate increase rapidly with the increase of reaction time. After the slow increase trend,the slow desorption reaction occurs after the adsorption equilibrium; the adsorption isotherm conforms to the Freundlich equation model,and the adsorption amount increases with the increase of the initial concentration of Cl- . The optimal dechlorination effect parameter is Cl- concentration 5 000 mg / L,then with the increase of chloride ion concentration,the dechlorination rate decreases due to the certain number of adsorption sites; the removal rate and adsorption capacity of CLDH for the adsorption of Cl- increase with the increase of temperature,when the reaction temperature is 65 ℃ . The dechlorination rate and the adsorption amount are the largest. When the temperature continues to increase,the adsorption effect of CLDH on Cl- is greatly reduced. When the pH value is changed,the adsorption effect of CLDH on Cl- is small,but when the pH is 8,the dechlorination rate and the maximum amount of adsorption are reached. With the increase of CLDH dosage,the dechlorination rate gradually becomes smaller,and the optimal dosage is 8 g / L. In the dechlorination experiment of actual desulfurization wastewater at 65 ℃ and pH = 8,the dechlorination rate of CLDH can reach 50. 90% ,the dechlorination rate of CLDH in one calcination is slightly decreased,and the dechlorination rate of CLDH in second calcination is reduced by about 50% . Desulfurization wastewater dechlorination has a reproducible reusability.