| In this paper, a hierarchical Mg-Al layered double hydroxide(LDH) framework and a Ni-Al LDH were fabricated via the in-situ crystallization synthesis technique on Zeolite sphere and Ni foam, CO(NH2)2 and NH4NO3 were chosen as activation agent of A1 on the 4A type Zeolite surface and Ni on the Ni-foam as well as precipitant.MgAlCO3- LDH@Zeolite, LDO@Zeolite and NiAlNO3-LDH@NF were synthesized by controlling the reaction conditions such as aging temperature, aging time, concentration of Mg2+ or Al3+ and the mol ratio of raw materials and activation agent, S/L, calcintion temperature and time. The samples were characterized by XRD and SEM. Experimental results showed that as aging temperature 120 ℃, aging time 12 h,Mg(NO3)2·6H2O/CO(NH2)2 1:3, Mg2+ 2.0 and 2.5 mol/L, S/L 0.06 g/m L for LDH@Zeolite, and Al(NO3)3·9H2O/ NH4NO3 2:3, 100 ℃, 48 h, Al3+ 10 mmol/L, 4.5g/L for LDH@NF, the two as-prepared LDHs were constructed from nanosheets with well-defined shapes and sizes, and displays flower like LDH microspheres composed of numerous LDH nanoplatelets, which was confirmed by XRD and SEM. The layered structure were destroyed by calcination at 500 ℃, but the specific surface area and pore volume had signifieantly inereased.The adsorption of heavy metal ion Cr(Ⅵ) on LDO@Zeolite and LDH@NF from aqueous solutions was investigated. The performances of LDO@Zeolite for adsorpt ion and desorption of Cr(Ⅵ) at different tempemtures were also studied. The influence of in-situ synthesis and operation cinditions of concentration of Mg2+ or Al3+, calcination temperature, pH value, temperature, adsorption time and Cr(Ⅵ) concentration had been investigated. The results indicated that the optimum calcination conditions was 500℃Roasting 4h for LDO@Zeolite. The LDH-Cr@Zeolite possessed a layered structure of hydrotalcites with high crystallinity as the LDH@Zeolite. The brucite-like sheets were still held in LDH-Cr@Zeolite. The adsorbility of LDO@Zeolite to Cr(Ⅵ) was substantially 50 % higher than that of LDH@Zeolite because of the structure“memory effect” of hydrotalcites. The best adsorption condition was as follows: pH 4.0,adsorption dosage 0.25 g/mL, temperature 500 ℃, Cr(Ⅵ) concentration 50 mg/L and adsorption time 90 min, theη % was 96 % and adsorption capacity was 833.3 μg/g.The results showed that for LDH@NF the optimum condition was as follows: pH5.0,adsorption dosage 3.4 g/L, temperature 25 ℃, Cr(Ⅵ) concentration 50 mg/L and adsorption time 240 min, the η % was 80 % and adsorption capacity was 10.05 mg/g.It was found that Adsorption rates and isotherms of the LDH adsorbents for Cr(Ⅵ)were investigated in batch experiments. Desorption behaviors and regeneration of Cr(Ⅵ)-adsorbed LDHs at the solid liquid interface were alsostudied. The adsorption kinetics of the two kinds of hierarchical LDH frameworks for Cr(Ⅵ) was appropriately described by a pseudo-second-order model, LDO@Zeolite(R2=0.998,k2=0.0026);LDH@NF(R2=0.999,k2=0.0237). Sorption isotherms of both Cr(Ⅵ) was also studied,which can be fitted by the Langmuir model more satisfactorily than the Freundlich (LDO@Zeolite:R2=0.992,q0=833.33 μg/g,b=0.002; LDH@NF:R2=0.998,q0=9.56 mg/g,b=0.205). The LDH@NF exhibited excellent sorption capacity for Cr(Ⅵ) and good regenera-tion performance. Furthermore, the sorption-regeneration performances of LDH framework was 50~60 % of original adsorption. |
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