Rapid Removal of Hg(II) from Aqueous Solutions Using Thiol- Functionalized Zn-Doped Biomagnetite Particles Feng He,* ,† Wei Wang, † Ji-Won Moon, ‡ Jane Howe, 3 Eric M. Pierce, † and Liyuan Liang † † Environmental Sciences Division, ‡ Biosciences Division, and 3 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: The surfaces of Zn-doped biomagnetite nano- structured particles were functionalized with (3- mercaptopropyl)trimethoxysilane (MPTMS) and used as a high-capacity and collectable adsorbent for the removal of Hg(II) from water. Fourier transform infrared spectroscopy (FTIR) confirmed the attachment of MPTMS on the particle surface. The crystallite size of the Zn-doped biomagnetite was ∼17 nm, and the thickness of the MPTMS coating was ∼5 nm. Scanning transmission electron microscopy and dynamic light scattering analyses revealed that the particles formed aggregates in aqueous solution with an average hydrodynamic size of 826 ± 32 nm. Elemental analyses indicate that the chemical composition of the biomagnetite is Zn 0.46 Fe 2.54 O 4 , and the loading of sulfur is 3.6 mmol/g. The MPTMS-modified biomagnetite has a calculated saturation magnetization of 37.9 emu/g and can be separated from water within a minute using a magnet. Sorption of Hg(II) to the nanostructured particles was much faster than other commercial sorbents, and the Hg(II) sorption isotherm in an industrial wastewater follows the Langmuir model with a maximum capacity of ∼416 mg/g, indicating two -SH groups bonded to one Hg. This new Hg(II) sorbent was stable in a range of solutions, from contaminated water to 0.5 M acid solutions, with low leaching of Fe, Zn, Si, and S (<10%). KEYWORDS: Zn-doped biomagnetite, nanoparticles, (3-mercaptopropyl)trimethoxysilane (MPTMS), superparamagnetic, mercury sorption, stability ■ INTRODUCTION Mercury (Hg), in particular Hg(II) in water, presents a serious environmental concern, because of its transformation to the potent neurotoxin methylmercury, which can be bioaccumu- lated and biomagnified in aquatic food chains. 1-3 To protect ecosystems, the discharge of mercury into aquatic systems often falls below strict regulatory limits that are much lower than the maximum concentration level of Hg (2 μg/L) regulated by the U.S. Environmental Protection Agency (EPA) for drinking water. For example, at the Y-12 National Security Complex (NSC) in Oak Ridge, TN, the interim level of mercury for effluent discharge to the receptor (East Fork Poplar Creek) is 200 ng/L and the future goal is to meet the ambient water quality criteria for mercury in Tennessee (51 ng/L). Currently, the precipitation/coprecipitation (e.g., with sulfide ions) methods that are the most commonly used processes to treat Hg(II) contaminated water are not able to reduce the concentrations of mercury to acceptable limits. 4,5 In response to these challenges, a variety of materials has been developed and tested for removing low levels of Hg(II) from contaminated waters. Traditional adsorbents such as activated carbon, 6,7 low-cost natural biosorbents such as chitosan, 8,9 natural inorganic ion-exchange materials such as zeolites, 10 clay, 11 peat, 12 bentonite, 13 and others 14 suffer from low selectivity, low capacity, and weak binding affinity for mercury. Recent research has focused on development of thiol- functionalized adsorbents to improve the selectivity for Hg(II), including functionalized clays, 15-17 resins, 7,18 organoceram- ics, 19,20 mesoporous silicates, 21-28 and mesoporous carbon. 29 The thiol-functionalized materials are selective and have a strong binding affinity for Hg 2+ , as a consequence of a soft Lewis acid-base interaction. 30 Unfortunately, the efficiency of thiol-based porous sorbents, such as resins, is impeded by diffusion processes, which limit mercury transport to the adsorption sites (i.e., thiol groups). For microscale mesoporous sorbents (e.g., thiol SAMMS), pore diffusion is improved but the separation of the used materials from water remains problematic. Therefore, a dispersible and recoverable sorbent with highly accessible sorption sites is desired for the removal of Hg(II) from aqueous solutions. Superparamagnetic magnetite nanostructured particles are this type of material. With primary particle sizes of <25 nm, magnetite nanostructured particles offer a large surface area, in addition to superparamagnetic properties. They are attracted to a magnetic field but do not retain magnetic properties when the Received: June 7, 2012 Accepted: August 1, 2012 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am301031g | ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX