Preparation Of Hydrotalcite-like Materials And Their Adsorption Performance For Chromium Ion

Layered Double Hydroxides（LDHs） is a typical anionic laminar compound. LDHs possess several advantages, such as simple synthesis process, high efficiency for heavy metal ions absorption and reusability. In this paper, Zn（NO3）2·6H2O and Al（NO3）3·9H2O were selected as zinc and aluminum sources to synthesis Zn/Al-LDHs（including Zn2Al-LDHs and Zn₃Al-LDHs） by co-precipitation method. Al（NO3）3·9H2O, coal gasification slag and Huozhou lignite ash were selected as different aluminum sources to co-precipitate with Mg（NO3）2·6H2O for synthesizing three type of Mg/Al-LDHs（including Mg2Al-LDHs、Q-Mg2Al-LDHs and H-Mg2Al-LDHs） with different aluminum sources. Crystal structure, morphology and thermal stability of LDHs and calcined LDHs（LDOs） were characterized by X-ray diffraction（XRD）, scanning electron microscope（SEM）, Fourier Transform Infrared Spectroscopy（FT-IR）, Brunauer-Emmett-Teller surface area measurement（BET）, and thermogravimetric analyzer（TGA）.XRD analysis shows that the LDHs synthesized have the characteristic diffraction peaks of 003, 006, 009, 110 and 113 ascribed to hydrotalcite, and the intensities of characteristic diffraction peaks from Zn₃Al-LDHs is higher than Zn2Al-LDHs. As for the Mg/Al–LDHs prepared by different sources of aluminum, the order of the diffraction peak intensity is Mg2Al-LDHs > H-Mg2Al-LDHs> Q-Mg2Al-LDHs. SEM images indicate that structured lamellar structure is closely packed in both Zn₃Al-LDHs and Mg2Al-LDHs synthesized, and there also exist obvious layer structure in H-Mg2Al-LDHs and Q-Mg2Al-LDHs, but the layer structure is more incompact and less regularity than that Zn₃Al-LDHs and Mg2Al-LDHs. FT-IR analysesindicate that there are-OH,-C=O and M-O-M functional groupsin LDHs synthesized, inferring that LDHs is composed of OH-, CO32-, M2+ and M3+. TGA shows that LDHs lost their interlayer water and interlayer anions（OH-, CO32- and a small amount of NO3-, etc.） at 100 oC and 200-400 oC, respectively. BET analyses show that the order of the specific surface area is Mg2Al-LDOs>H-Mg2Al-LDHs>Zn₃Al-LDHs>Q-Mg2Al-LDHs. The surface area of Mg2Al-LDOs is 273.24 m2/g,which is the maximum in all LDOs. Zn₃Al-LDOs has the maximum average pore diameter（29.1 nm） and Q-Mg2Al-LDOs possesses the minimum pore diameter（5.15 nm）.Zn₃Al-LDOs, Mg2Al-LDOs, Q-Mg2Al-LDOs and H-Mg2Al-LDOs were used as adsorbent for the removal of Cr（VI） in simulate wastewater, and the effects of p H, temperature（T）, time（t）, absorbent dosage（m）and initial concentration on the Cr（VI） adsorption were investigate through single factor rotates. The results show that the optimum conditions for the adsorption of Cr（VI）on these LDOs are p H=2-4, T=25-35 oC, t=80 min,m=40-60 mg/m L and c0=100 mg/L. The maximum Cr（VI） adsorption capacity of are126.9, 107.25, 95.38 and 85.75 mg/g for Zn₃Al-LDOs, Mg2Al-LDOs, Q-Mg2Al-LDOs and H-Mg2Al-LDOs, respectively. Although adsorption capacity of Q-Mg2Al-LDOs and H-Mg2Al-LDOs is slightly lower than that of Zn₃Al-LDOs and Mg2Al-LDOs, it is higher than many other type of adsorbents. The kinetic and isotherm of Cr（VI） adsorption on Zn₃Al-LDOs, Mg2Al-LDOs, Q-Mg2Al-LDOs and H-Mg2Al-LDOs can be described with the pseudo-second-order kinetic model and Langmuir isotherm, respectively. Thermodynamic parameters suggest that the Cr（VI） adsorption process on these LDOs is an spontaneous, exothermic and entropy increasing process. The LDOs after adsorbing Cr（VI） were also characterized with XRD, SEM and FT-IR analyses. The results indicate that Cr（VI） can enter into the interlayer of LDHs, and LDOs refresh their to its original layered structure, implying LDHs possess an obvious “memory effect” during Cr（VI） absorption process.