Adsorption Effects of Phosphate and Chromium from Aqueous Solution by Adsorption Using LDH and Mg/Fe/Al Modified Montmorillonites
|School||Henan Agricultural University|
|Keywords||modified montmorillonite Layered double hydroxides isothermal adsorption kinetics thermodynamics|
The Mg-Al Layered double hydroxides was synthesized by the coprecipitation method. Characteristics of phosphate adsorption on Mg-Al layer double hydroxide (Mg-Al LDH) and Na-montmorillonite (Na-Mt) and Mg/Fe/Al modifiedmontmorillonite (Mg-Al-Mt and Fe-Mt and Fe-Al-Mt) were studied by isotherm adsorption and kinetic experiments at different pH values.The results were drawn as follows:1. The percentage of phosphate adsorption on Na-Mt and Fe-Mt and Fe-Al-Mt decreased,and that on Mg-Al LDH increased with solution pH increase. The adsorptin effects of phosphate adsorption on Mg-Al LDH、Fe-Mt and Fe-Al-Mt were about 95%, higher 40% and 80% than that of Mg-Al-Mt and Na-Mt, respectively. The percentage of chromium adsorption on Na-Mt were about 2~8%,and were not obvious;The percentage of chromium adsorption on Fe-Mt and Fe-Al-Mt decreased with solution pH increase. The impact of solution pH for Mg-Al and Mg-Al-Mt were not obvious.2. The order of maximum adsorption capacity (Q_m) from Langmuir equation was Fe-Al-Mt > Fe- Mt > Mg-Al LDH > Mg-Al-Mt> Na-Mt; K_F vaules of relatively adsorption capacity form Freundlich equation was the order of Mg-Al>Fe-Mt >Fe-Al-Mt >Mg-Al-Mt>Na-Mt.The Langmuir and Freundlich equations were used to describe chromium equilibrium sorption on LDH and modified montmorillonite. The values of Q_m for Mg-Al and Mg-Al-Mt are larger than Fe-Mt and Fe-Al-Mt, and that value for Na-Mt is smallest of all. The values of Q_m for Mg-Al were the order of 1Mg-1Al> 4Mg-1Al> 2Mg-1Al > 1Mg-2Al.3. The thermodynamics of the adsorption process indicates endothermic with phosphate. The positive values of△H decreased with the percentage of phosphate adsorption increased. The negative values of△G decreased with the temperature increased indicate the spontaneous nature of phosphate adsorption. The positive values of△S indicated a increased randomness during the adsorption of phosphate onto LDH and modified montmorillonite.4. The pseudo-second order kinetics provide the best correlation of the experimental data of various modified Mt and Mg-Al-LDH, and the determination coefficients(R~2)were 0.999. There also was correlation coefficient R~2(0.89~0.94)on various minerals fitted by Elovich equation. Parabolic diffusion equation was most suitable to descrirbe the phosphate adsorption on Na-Mt, and because phosphate adsorption is a diffusion process by inlaid in mineral layers and don’t involve chemical adsorption processes. The chromium adsorption kinetic date were fitted with pseudo-first, pseudo-second kinetic models, Elovich epuation and Parabolic diffusion equation. Parabolic diffusion equation was most suitable to descrirbe the chromium adsorption on Na-Mt,The pseudo-second order kinetics provide the best correlation of the experimental data of LDH and modified montmorillonite. The adsorptin effects of chromium adsorption on 2Mg-1Al and 1Mg-1Al were about 95%, higher than that of 4Mg-1Al and 1Mg-2Al, 70% and 81%,respectively.5. The concomitant anions have different extent adverse effect on adsorbent, but the differences were less than 10%, so the effect of concomitant anions for phosphate adsorption on LDH and modified montmorillonite were not abvious.