The Preparation And Application Of SO₄^2- Pillared Mg-Al Hydrotalcites Under Microwave Environment

Hydrotalcite(LDHs)laminates containing alkaline active edge can react with cations.Theinterlayer spacing and acid-base properties of surface are different for anions.Thereconstruction process of calcined MgAl-layered double hydroxides has been referred toas“structural memory effect".In this paper, SO42-type Mg-Al LDHs is prepared by the processof“calcination-reconstruction”of Mg-Al-CO3LDH to adsorb Pb2+and degrade NH4+.Microwave, because of its thermal effects and non-thermal effects,has been widely usedin the process of synthesis and modification of inorganic materials where shorttime-consuming,superior product performance are the main characteristics. Currently,microwave technology is used for crystallization of LDHs slurry in the process of LDHspreparation with the result that the product has regular structure and single-crystalline phase.Microwave techngoloy is used during the calcination-reconstruction preparation of SO42-pillared Mg-Al LDHs and degradation of NH4+in this article. oCThe optimal conditions of the preparation of SO42-pillared Mg-Al LDHs were as follows:the initial pH of3, the precursor LDHs with Mg/Al molar ratio of3, calcined hydrotalciteamount of0.1g, reaction temperature of60oC, reaction time of1.5h, reaction solution boiledfor10min and sealed for cooling, Under the optimal conditions, and relatively regular SO42-pillared Mg-Al LDHs was obtained (LDH-a); keeping the power of microwave500w and thesame reaction conditions，the reaction reached at optimal adsorption equilibrium after0.5h.As a result, relatively neat SO42-pillared Mg-Al LDHs was obtained (noted as LDH-m).BothLDH-a and LDH-m had been characterized by XRD, IR, SEM and EDS analyses. All thehydrotalcites possessed the hydrotalcite-like layered structure containing SO42-.The interlayerspacing increase, and the degree of regularity is over the LDH.The kinetic analysis of preparation of SO42-pillared Mg-Al LDHs showed that theLagergren first-order adsorption rate equation was more suitable to describing the process.The normal calculated Eavalue by Arrhenius equation was52.13kJ· mol-1, while theactivation energy of microwave enhanced is40.10kJ· mol-1, indicating that the process wasthe reaction control. Since the Ea value of microwave enhanced was lower than52.13kJ·mol-1by12.03kJ·mol-1, it showed that microwave can promote the preparation of SO42-pillaredMg-Al LDHs.The thermodynamic analysis of preparation suggestd that Langmuir isotherm model wasbetter for describing the process; the values of thermodynamic parameters were as follows:H of40.51kJ· mol-1, S of161.82J·(mol·K)-1. According to the values of enthalpychanges and entropy changes,the standard free energy ΔG at all different temperatures (30,40, 50and60oC) were calculated as follows:-8.55kJ·mol-1,-10.17kJ·mol-1,-11.78kJ·mol-1,-13.4kJ·mol-1, indicating that the reaction can be spontaneous.To Investigate SO42-pillared Mg-Al LDHs on degradation of NH4+. in the sameconditions, microwave-assisted LDH-m degradation of NH4+removal can be increase to rateof85%, more than the normal conditions LDH-m for NH4+biodegradation improve60%;Designing orthogonal experiment, optimized the reaction conditions of removel of NH4+, theresults indicate that the temperature is the main factors of removal of ammonia. The show isthat dosage of LDH-m is0.1g, reaction temperature designed in60oC is the optimaltemperature, and ultimately determined the stirring rate1400r/min is best. Under optimalconditions for Analyze the product (remember MLDH) used XRD, IR and SEM showed thatMLDH still has characteristic diffraction peaks of LDHs, and the more SO42-enter in LDHslayer room, particle size becomes smaller. The indicating that, the essence of LDH-m removelNH4+is generate H+when NH4+acidic hydrolysis process to modified LDHs laminatesprocess.the result of MLDH expose more active sites would improve performance cationadsorption, namely through LDH-m for NH4+removal while obtaining an efficient adsorptionof heavy metal cations agents, while maintaining MLDH slightly alkaline environment in theneutral pH of the solution.The products of above were used to adsorb Pb2+. Both SO42-and CO32-typeMgAl-LDHs are better on Pb2+adsorption (LDH:73.4%; LDH-a:88.6%; LDH-m:91.2%).SO42-pillared Mg-Al LDHs has higher removal rate than CO32-type Mg-Al LDHs forPb2+adsorption.Removal rate of Pb2+has an promotion of2.6%than that of LDH-a,whenLDH-m was used. When MLDH was used to adsorb Pb2+for2h, the removal rate reached90.4%, having promotion of26.2%compared with LDH-m (64.2%).