Mercury removal efficiency of disulfide- and thiol-functionalized lanthanide coordination polymers

Abstract

To compare efficiency of disulfide and thiol groups in removing mercury from aqueous medium without noteworthy influence from structural differences, a series of new [LnIII(dtba)1.5(H2O)2] (LnIII = EuIII (I), GdIII (II) and TbIII (III), H2dtba = 4,4′-dithiobenzoic acid) were synthesized and characterized. The single crystal structure of I was elucidated and is described. Reaction of II with hydrazine gave IISH containing disulfide and thiol groups. Experimental data confirmed the preserved framework structure and the co-existing of disulfide and thiol groups in IISH. Robustness of II and IISH over a wide range of pH (2–10) was confirmed and their mercury removal performances at different pH were evaluated in terms of removal efficiencies (%R), equilibrium uptake capacities (qe) and distribution constant (Kd). The dependence of these parameters on pH is reported. The best values of %R, qe and Kd could be achieved at pH 10 at which surfaces of the adsorbents were negatively charged; 86%R, 429 mg g−1, and 6.04 × 103 mL g−1 (II), and 98%R, 490 mg g−1 and 5.08 × 104 mL g−1 (IISH). At pH 7, influences of the initial concentration of mercury on performances of the adsorbents as well as the adsorption isotherms and kinetics were examined from which the better performance of IISH has been concluded. The characterization of the adsorptions by the Langmuir model and the pseudo-second-order kinetic as well as their excellent consistency with the experimental data are included. At neutral pH, selectivity to the adsorption of mercury and tolerance to common anions were illustrated. The better affinity between mercury and thiol group and therefore its contribution to the better performance of IISH was then ascertained by a computational study.