| Summary: | This work reports the preparation and utility of cysteine-functionalized carbon-coated Fe<sub>3</sub>O<sub>4</sub> materials (Cys-C@Fe<sub>3</sub>O<sub>4</sub>) as efficient sorbents for remediation of Hg(II)-contaminated water. Efficient removal (90%) of Hg(II) from 1000 ppb aqueous solutions is possible, at very low Cys-C@Fe<sub>3</sub>O<sub>4</sub> sorbent loadings (0.01 g sorbent per liter of Hg(II) solution). At low metal concentrations (5–100 ppb Hg(II)), where adsorption is typically slow, Hg(II) removal efficiencies of 94–99.4% were achievable, resulting in final Hg(II) levels of <1.0 ppb. From adsorption isotherms, the Hg(II) adsorption capacity for Cys-C@Fe<sub>3</sub>O<sub>4</sub> is 94.33 mg g<sup>−1</sup>, around three times that of carbon-coated Fe<sub>3</sub>O<sub>4</sub> material. The highest partition coefficient (PC) of 2312.5 mgg<sup>−1</sup>µM<sup>−1</sup> was achieved at the initial Hg (II) concentration of 100 ppb, while significantly high PC values of 300 mgg<sup>−1</sup>µM<sup>−1</sup> and above were also obtained in the ultralow concentration range (≤20 ppb). Cys-C@Fe<sub>3</sub>O<sub>4</sub> exhibits excellent selectivity for Hg(II) when tested in the presence of Pb(II), Ni(II), and Cu(II) ions, is easily separable from aqueous media by application of an external magnet, and can be regenerated for three subsequent uses without compromising Hg(II) uptake. Derived from commercially available raw materials, it is highly possible to achieve large-scale production of the functional sorbent for practical applications.
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