## 4 Conclusions  
Efficient Hg(II) sorbents based on magnetite particles have been prepared using a simple synthetic procedure from low-cost materials; cost per kilogram [64] was urea (USD $0.2), L-cysteine (USD $10), D-Glucose (USD $0.4), ethylene glycol (USD $0.5), and hydrated ferric chloride (USD $0.65). Utilizing cysteine as a functionalization reagent for magnetic sorbents is much more economic and simpler, compared with productions of functionalized magnetic sorbents via multistep material synthesis [12; 13; 15], expensive functionalization reagent, or high-purity chemicals [10; 12; 13; 15; 16]. These sorbents are easy to separate from aqueous media and demonstrate excellent recyclability metrics. Comparative studies were undertaken for uncoated Fe\({}_{3}\)O\({}_{4}\), carbon-coated Fe\({}_{3}\)O\({}_{4}\) (C@Fe\({}_{3}\)O\({}_{4}\)), and cysteine-functionalized carbon-coated Fe\({}_{3}\)O\({}_{4}\) (Cys-C@Fe\({}_{3}\)O\({}_{4}\)) sorbent particles. The 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) sorbent, having optimum L-cysteine loadings, are highly selective for Hg(II) adsorption, are capable of removing 90% of Hg(II) within 10 min, and are effective even at ultralow metal concentrations. The carbon shell enhances the Hg(II) sorption capacity, preserves the integrity of the Fe\({}_{3}\)O\({}_{4}\) core, and acts as a scaffold, allowing appendage of metal chelating L-cysteine to the particle surface. Surface modification of C@Fe\({}_{3}\)O\({}_{4}\) by cysteine boosts the performance and selectivity of the materials, resulting in enhanced levels of Hg(II) removal with fast adsorption rates, at ultralow Hg(II) concentrations. Moreover, factors potentially influencing adsorption efficiency, such as pH and temperature, were comprehensively studied in respect to both kinetics and thermodynamics. The facile preparation, fast and impressive adsorption performance, good selectivity, ease of separation, and recyclability alludes to the high potential of Cys-C@Fe\({}_{3}\)O\({}_{4}\) as a candidate for selective removal of Hg(II) contaminants from water sources and for its possible adoption in mainstream water treatment processes. The following are available online at [[http://www.mdpi.com/2076-3417/10/22/8262/s1](http://www.mdpi.com/2076-3417/10/22/8262/s1)]([http://www.mdpi.com/2076-3417/10/22/8262/s1](http://www.mdpi.com/2076-3417/10/22/8262/s1)), Table S1 %Breakthrough (BT, of %removal efficiency) and BT time interpolated from the %BT vs. treatment times plot in Figure 4, Table S2: %Breakthrough (BT, of %removal efficiency) and BT time interpolated from the %BT vs. treatment times plotted in Figure S2., Scheme S1: Synthesis process for uncoated Fe\({}_{3}\)O\({}_{4}\), C@Fe\({}_{3}\)O\({}_{4}\), and Cys-C@Fe\({}_{3}\)O\({}_{4}\) samples. Figure S1: Nitrogen adsorption-desorption isotherms and pore size distribution plots (inset) for (a) uncoated Fe\({}_{3}\)O\({}_{4}\), (b) C@ Fe\({}_{3}\)O\({}_{4}\), (c) 250 Cys-C@Fe\({}_{3}\)O\({}_{4}\), (d) 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), and (e)1000 Cys-C@Fe\({}_{3}\)O\({}_{4}\). Figure S2: Hg(II) treatment time dependence of % Breakthrough (BT) at initial Hg(II) concentration of 1000 ppb and sorbent loading of 0.01 g L\({}^{-1}\) at different pH values. High reproducibility of the measurements can be indicated by small standard deviations from five identical batch experiments. Figure S3: Linear fitting of (a) [PERSON] and (b) Freundlich isotherms for Hg(II) adsorption onto uncoated Fe\({}_{3}\)O\({}_{4}\), C@ Fe\({}_{3}\)O\({}_{4}\), and 500 Cys-C@ Fe\({}_{3}\)O\({}_{4}\) and thermodynamic data for Hg(II) adsorption on 500 Cys-C@ Fe\({}_{3}\)O\({}_{4}\). Figure S4: Thermodynamic data for Hg(II) adsorption on 500 Cys-C@ Fe\({}_{3}\)O\({}_{4}\). **Author Contributions:** Formal analysis, investigation, and writing--original draft preparation, A.S. Formal analysis, investigation, and writing--original draft preparation, T.B. Formal analysis, investigation, W.P.-O. Conceptualization, methodology, and supervision, T.S., K.U., and K.S.