Comparative study of Pb(II) adsorption onto MIL-101 and Fe-MIL-101 from aqueous solutions

In the present paper, the metal-organic framework-101 (MIL-101) and the iron-doped MIL-101 (Fe-MIL-101) were synthesised using the hydrothermal process. The obtained materials were characterised by means of X-ray diffraction, scanning electron microscopy, and nitrogen adsorption/desorption isotherms. The obtained Fe-MIL-101possessed MIL-101's structure with a large specific areas (2407 m2 g-1for Fe-MIL-101and 3360 m2 g-1for MIL-101). MIL-101 and Fe-MIL-101 were used for Pb(II) adsorption from aqueous solutions. Various factors affecting adsorption, namely contact time, initial concentration, temperature, pH, and adsorbent recycling were investigated. The Arrhenius and Eyring equations were employed to calculate the kinetic parameters, viz. activation energy (Ea), enthalpy (ΔH#), entropy (ΔS#), and free energy (ΔG#) of the sorption process. The thermodynamic parameters, namely changes in standard Gibbs free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) were derived to predict the nature of the process. The equilibrium data of adsorption of Pb(II) onto MIL-101 and Fe-MIL-101 were well fitted to both Langmuir and Freundlich isotherms. The maximum monolayer adsorption capacity of Fe-MIL-101(86.20 mg·g-1) was much higher than that of MIL-101(57.96 mg g-1). It was considered that MIL-101 containing iron provided a much larger adsorption capacity and faster adsorption kinetics than MIL-101. The carboxyl group in the MIL-101 framework played a vital role for the effective Pb(II) removal from aqueous solutions, while the surface functional groups being responsible for the Pb(II) adsorption on Fe-MIL-101 were considered to be hydroxy groups that formed on the iron oxide.