Modification Of The Mg-Al Hydrotalcite And Its Adsorption Performance On Pb(Ⅱ)

Hydrotalcites (LDHs), a layer material with positive charge, is currently applied in many fields as catalysts, adsorbents and functional auxiliary agents. In the thesis, the modified Mg-Al LDH was prepared using ammonium sulfate, the formation of modified Mg-Al LDH was studied, and its adsorption performance on Pb(Ⅱ) was also carried out.The Mg-Al LDH with Mg/Al molar ratio 3.0 was modified using ammonium sulfate as modifier. By IR, XRD and SEM analyses as well as the point of zero charge (pHPZC) measurement, the formation of modified Mg-Al LDH was investigated. The results showed that the modified Mg-Al LDH had lower Al3+ content in brucite-like sheets, bigger layer spacing, smaller size, irregular crystal shape and lower pHPZC in comparasion with the Mg-Al LDH precursor. The formation processe of modifying Mg-Al LDH was revealed as follows. The H+ generated by (NH4)2SO4 in the reaction system approached the edge of the sheet, and reacted with the OH from the Al(OH) at the edge to produce H2O. Then, the Al3+ was away from the sheet due to the electrostatic repulsion of the strueturally positive charge on the sheet. The OH from the M-OH-M was brought, leading to form new adsorption sites.The optimal conditions of modifying Mg-Al LDH was obtained by single factor and the response surface analyses. Compared with the Mg-Al LDH precursor, the remove rate of the modifying Mg-Al LDH was raised by 18.3%, when 0.25 g Mg-Al LDH precursor was added in 36.8 mL (NH4)2SO4 solution with mass fraction 18.2 wt%at 59.3℃for 4 h.In the initial Pb(Ⅱ) concentration of 70-230 mg·L-1, the pseudo-second-order could well describe the adsorption kinetics of modified Mg-Al LDH to Pb(Ⅱ). The adsorption of modified Mg-Al LDH to Pb(Ⅱ) was chemical adsorption, which was endothermic reaction withΔH 22.94 kJ·mol. From 25 to 85℃, the adsorption isotherms correlated well with the Langmuir isotherm model. The saturated adsorption capacity increased with the increase of temperature, where the saturated adsorption capacity were 201.6 mg·g-1 at 25℃,230.9 mg·g-1 at 45℃,263.1 mg·g-1 at 65℃and 285.7 mg·g-1 at 85℃, respectivity.