Study On Synthesis Of Al₂O₃/bentonite As Adsorbent And Its Adsorption Of Cd（Ⅱ）

In this research, a new composite adsorbent Al2O3/bentonite was optimized synthesized through sol-gel method. The result showed that the synthetic effect of Al2O3/Bentonite was the best when the sol pH of sol was5.0, dose ratio of Al/bentonite was4mmol/g, dose ratio of CTAB/bentonite was20mg/g, calcinated temperature was350℃and calcinated time was3h, respectively. The composite adsorbent was named Ben-A. Compared with bentonite, the Cd(II) removal rate of Ben-A improved by10%and the residual turbidity of Ben-A decreased from282NTU to14NTU, which meaned Ben-A was much more stable in the solvent of water. Surface area analysis showed that the special surface area of Ben-A was63.18m2/g which was higher than11.08m2/g of bentonite. The Scanning Electron Microscope analysis showed that Bentonite had more microporous structure. According to X-Ray Diffraction analysis, the montmorillonite crystal layer spaces became smaller as the result of heat treatment and the bentonite had less impurities after purified. The Fourier Transform-Infared Spectroscopy analysis showed that aluminum ions and bentonite formed chemical bonds in the Ben-A.Using batch adsorption experiment to investigate the impact of main factors on cadmium removal. It was found that cadmium removal rate was about87%when the initial solution pH ranging from5.0to7.0. When the initial solution pH was7.0, the initial cadmium concentration was20mg/L and the dosage of Ben-A was5g/L, the residual concentration of cadmium was below0.1mg/L, which meeted the state wastewater discharge standards. Ben-A could reach91%of the adsorption capacity after reacting for3h and get the adsorption equilibrium after reacting for6h. The particle size of Ben-A had less influence on the removal rate of Cd(Ⅱ), whlie the coexisting ions had a great influence and the order of their interference to the removal rate of Cd(Ⅱ) was:Al3+>Ca2+>K+>Na+, besides the Cd(Ⅱ) removal rate reduced when the concentration of coexisting ions improved.The adsorption of Cd(Ⅱ) by Ben-A satisfied pseudo second order model and Freundlich isotherm equation model. The reaction activation energy Ea=19.45kJ/mol, ΔS=44.18kJ/mol, ΔH=16.34kJ/mol,ΔG<0, AH>0and ΔG<0indicated that the whole adsorption process was spontaneous endothermic reaction. X-Ray Diffraction analysis showed that d(001) of montmorillonite increased from1.41nm to1.54nm after adsorption process, which proved Cd(II) had entered into the montmorillonite layer. According to Scanning Electron Microscope analysis, the crystal particles of Ben-A became larger. The study showed the main removal mechanism was ion exchange, the cadmium was removed from water by exchanging with the cation in Ben-A.Ben-A could be well desorbed by NaCl. After regenerated for4times with0.2mol/L NaCl, the mass loss of Ben-A was less than10%and the removal rate of Cd(II) remained as high as98%, that meaned the prepared adsorbent Ben-A owned well regeneration. On the one hand, it was conducive to the recovery of heavy metal, on the other hand, it was conducive to reuse of adsorbent so as to reduce the treatment costs. So Ben-A was nontoxic and eco-friendly, which had a good prospect in water treatment.