## 1 Introduction  
Mercury contamination of ground and surface water, and bioaccumulation of this heavy metal in aquatic and marine animals, presents serious concerns to public health and food safety worldwide [1; 2; 3; 4; 5]. Damage to the central nervous system, liver, kidneys, and heart in humans can be attributed to mercury exposure, the extent of which depends on the mercury form, dosage, and age and underlying health of the individual [6]. Major sources of environmental mercury contamination arise from waste effluents, such as those from mining, the chloralkali process, and petroleum industries [7]. Due to its acute toxicity, the World Health Organization (WHO) and European Union (EU) define permissible limits of total mercury in drinking water as being not greater than 1.0 ppb [8]. Based on literatures, though with its high toxicity, the number of papers reporting Hg(II) removal from water [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23] have been found much less than those focused on other heavy metal pollutant such as ions of Cu, Pb, Zn, Cr, or Cd. Scopus-sorbents comparative details of the performance of several sorbents for Hg(II) removal are given, however, they are limited [12; 13; 24]. Besides, reports of effective sorbents for the removal of Hg(II) in water at low initial concentrations (\(\leq\)100 ppb) are scarce [10; 12; 13; 14; 15; 16]. Adsorption of Hg(II) has been found to be less effective at ultralow initial concentrations (\(\leq\)20 ppb) [10] and is hampered due to the very small difference in Hg(II) concentration in solution relative to that at the sorption sites, resulting in very slow adsorption rates [25; 26]. Therefore, treatment of Hg(II) contaminated water at such ultralow concentrations is very challenging, especially with the need to ensure quality levels comply with the WHO and EU permissible limits for drinking water (1.0 ppb) [8; 26]. Commercially available and laboratory-scale synthetic activated carbon materials have been extensively reported as sorbents for Hg(II) ions, however, with low adsorption capacities (less than 20 mg/g) [24]. Recently, magnetic sorbents, i.e., zero-valent iron, magnetite (Fe\({}_{3}\)O\({}_{4}\)), and maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)), easily recovered by application of an external magnetic field, have attracted great interest as effective sorbents for the removal of metal ions from water [17; 18; 19; 20; 21; 22; 27]. To enhance the stability of such sorbents, the magnetic core should be coated by a protective shell to avoid corrosion, as is possible at low pH or under other extreme conditions. Shell materials can be organic or inorganic, with stabilities dependent on the chemical nature of the material. For example, carbon shell coatings proved more stable than those of functionalized (chitosan, polyamidoamine, and mesoporous silica) materials at low/high pH and at high temperature/pressure [28; 29; 30; 31; 32]. Previously, [PERSON] et al. [14] utilized a magnetic powdered activated carbon material at high sorbent dosage (1.0 g/L) in the adsorption of Hg(II) from aqueous solutions containing 100 ppb of the toxic metal. The Hg(II) removal efficiency was only 84% after 180 min, additionally having no evidence of the sorbent recyclability. Effective sorbents for water remediation based on carbon-coated Fe\({}_{3}\)O\({}_{4}\) materials have been fabricated recently, through one-step solvothermal treatment of an aqueous Fe(III) solution or aqueous Fe(III) and Fe(II) mixture in the presence of urea and glucose [29; 33; 34]. The high adsorption capacity of metal ions on these materials was due to the presence of oxygen-containing functional groups on the carbon surface, which electrostatically interact with metal ions. Surface oxygen functionalities can be further modified chemically, resulting in greater adsorption site density and, more importantly, the potential for selective metal ion adsorption.