An intensive study on the magnetic effect of mercapto-functionalized nano-magnetic Fe3O4 polymers and their adsorption mechanism for the removal of Hg(II) from aqueous solution

A series of mercapto-functionalized core-shell structured nano-magnetic Fe3O4 polymers (SH-Fe3O4-NMPs) with different amount of magnetic core were synthesized and characterized by XRD, EA, VSM, TG-DTG and XPS. Their applications as adsorbents for Hg(II) removal from aqueous solutions were intensively investigated. The adsorption process of Hg(II) by SH-Fe3O4-NMPs was found highly pH dependent and related to the content of Fe3O4 magnetic core in the adsorbents. The optimized content of magnetic core for SH-Fe3O4-NMPs was found to be at 5.88%. Kinetic studies showed that the adsorption of Hg(II) by SH-Fe3O4-NMPs followed pseudo-second-order model, suggesting a chemisorption process. Activation energy (Ea) for the Hg(II) removal was found to be 22.1 kJ mol(-1), indicating that the diffusion process might be the rate-controlled step. Thermodynamic studies suggested that the adsorption processes of Hg(II) onto the SH-Fe3O4-NMPs were endothermic and entropy favored in nature with the enthalpy changes (Delta H-theta) at 7.26-16.54 kJ mol(-1) and the entropy changes (Delta S-theta) at 27.60-56.78J mol(-1) K. The adsorption processes fit the Langmuir isotherms well with the maximum adsorption capacities of Hg(II) onto the SH-Fe3O4-NMPs at 129.9-256.4 mg g(-1) with the content of Fe3O4 in SH-Fe3O4-NMPs varying from 0% to 22.37%. The selective adsorption of Hg(II) by SH-Fe3O4-NMPs can be achieved when Ca2+, Mg2+. Na+ or Cu2+ ions coexisted. XRD and XPS analyses results of the adsorbents before and after Hg(II) adsorption suggested that Hg(II) ions had been successfully adsorbed onto SH-Fe3O4-NMPs via coordination interactions, and Hg(II) might be partially reduced to Hg2Cl2 through solid state redox reaction via charge transport on the surface of the SH-Fe3O4-NMPs, in which the Fe3O4 magnetic core in the adsorbents might play an important role. (C) 2012 Elsevier B.V. All rights reserved.