Mechanism study about the adsorption of Pb(II) and Cd(II) with iron-trimesic metal-organic frameworks

This study examined the effects of two kinds of iron-trimesic metal-organic frameworks (MOFs) MIL-100(Fe) and Fe-BTC] on adsorption of Pb(II) and Cd(II). For both materials, adsorption of Pb and Cd nearly approached equilibriums in the initial 2 min with high pseudo-second-order kinetics rate constants (0.895 g.mg(-1).min(-1) for Pb and 1.416 g.mg(-1).min(-1) for Cd). Both materials showed better adsorption performance with the increase of solution pH (from 2 to 7). Adsorption processes were endothermic, entropy-increased and spontaneous. The higher enthalpy change (Delta H-0) and entropy change (Delta S-0) values for Cd adsorption reflected the more difficult adsorption of Cd compared with Pb, which is related with more negative hydration enthalpy (Delta H-hyd(Theta)) and larger ionic radius of Cd. Multilayer adsorption phenomena appeared with no maximum adsorptive capacity observed. Both Langmuir-Freundlich and Freundlich-BET models could well describe isotherms. Multilayer adsorption may be attributed to the formation of inner-sphere complexation and outer-sphere complexation. BockrisDevanathan-Muller (BDM) electrical double layer theory was used for illustrating adsorption mechanism. The fixed-bed adsorption experiments with Fe-BTC showed outstanding elimination performance in initial operating period ( > 99.8% for Pb and > 99.6% for Cd), with effective filtration volume of 980 mL for Pb and 300 mL for Cd. Recyclability test showed that Fe-BTC could be well regenerated by ethylenediamine tetraacetic acid disodium salt (EDTA-2Na). This study showed that MIL-100(Fe) and Fe-BTC are potential for practical heavy metal removal applications.