## 3 Results and Discussion
### Sorbent Recyclability  
As shown in the Figure 6 inset, 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) sorbent can be well dispersed in aqueous media and can be harvested easily by magnetic separation. Desorption of Hg(II) from the sorbent surface was examined to provide a measure of sorbent stability and recyclability, both being important parameters for industrial applications. For 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), experiments were conducted using an eluent solution (10 mL) composed of 0.1 M HCl and 5% (w/v) thiourea. Prior to measurements, the used sorbent was washed three times with deionized water. After adding the eluent solution to the used sorbent, desorption was allowed to proceed by shaking the suspension for 1 h at a constant temperature. It is possible that dissolution of magnetite particles could occur during desorption under the experimental conditions. Atomic absorption spectroscopy of the eluent solution after the first desorption cycle indicated an Fe concentration of 0.34 ppm for 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\), and 0.51 ppm and 5.94 ppm for C@Fe\({}_{3}\)O\({}_{4}\) and uncoated Fe\({}_{3}\)O\({}_{4}\) nanoparticles, respectively, highlighting the importance of the surface coating on maintaining core integrity. Results of desorption and adsorption for five cycles using the 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) sorbent are shown in Figure 6. These reveal that the sorbent maintains high desorption efficiency (90%) and Hg uptake (90 mg.g\({}^{-1}\)) performance over the first three cycles. However, adsorption efficiencies significantly decreased for the fourth and fifth cycles, while the desorption efficiency only slightly decreased. This behavior could result from saturation of the binding sites and/or degradation of bound cysteine moieties at low pH [62; 63]. Table 4 highlights examples of sorbents applied to removal of ultralow concentrations (<20 ppb of Hg(II)), in comparison to 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) material. From these, 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) shows comparable performance at much lower sorbent loadings, although longer contact times were employed in  
Figure 6: Hg(II) uptake (solid line) and percentage Hg(II) desorbed (dashed line) over five adsorption/desorption cycles using 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) sorbent. The inset shows the separation of the material from the aqueous media using an external magnetic field. this work. Thio-silica-coated magnetite required very low sorbent loadings for Hg(II) removal, but its preparation is more complex, requiring more steps than that needed for Cys-C@Fe\({}_{3}\)O\({}_{4}\)[15]. Acrylamide-hydroxyl-MOFs [10] and MOF-74-Zn [16] show lower removal efficiencies using higher sorbent loadings and are nonmagnetic, thus cannot be separated for reuse by an application of an external magnet. Furthermore, the adsorption capacity and satisfactory level of sorbent performance in removal of ultralow concentrations of Hg(II) based on the WHO and EU drinking water standards is reported in Table 5, in which 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) material shows superior comparative performance at low sorbent loadings. Comparative PC values of 500 Cys-C@Fe\({}_{3}\)O\({}_{4}\) material, at varying initial Hg(II) concentrations (5-1000 ppb), were evaluated using Equation (2). They are reported in Table 5. The obtained PC values of the developed magnetic material at the studied Hg(II) concentration range were found being much higher than other sorbents. In particular, the significantly high PC values of the Cys-C@Fe\({}_{3}\)O\({}_{4}\) material at ultralow Hg(II) initial concentrations (\(\leq\)20 ppb) should be attributed to the high chelating ability of Hg(II) ions and the -NH\({}_{2}\) and -SH functional groups present on the surface of magnetic material.