Synthesis of new functionalized magnetic nano adsorbents and adsorption performance for Hg(II) ions

Fe3O4@SiO2-NH-nNG-SPTZ (including Fe3O4@SiO2-NH-2NG-SPTZ, Fe3O4@SiO2-NH-3NG-SPTZ) were prepared by 2-mercapto-5-(4-pyridyl)-1,3,4-thiadiazole and the reaction products of 3, 6-dichloropyridazine and cyanuric chloride with Fe3O4@SiO2-NH2 respectively. Fe3O4@SiO2-S-nNG-SPTZ (including Fe3O4@SiO2-S-2NG-SPTZ, Fe3O4@SiO2-S-3NG-SPTZ) were prepared by 2-mercapto-5-(4-pyridyl)-1,3,4-thiadiazole and the reaction products of 3, 6-dichloropyridazine and cyanuric chloride with Fe3O4@SiO2-SH respectively. Four novel adsorbents were characterized by SEM, TEM, FT-IR and XRD. Hg(II) ions removal efficiency of these adsorbents were more than 95%, in 15–20 min, at pH 7.0–8.0. It is easy to separate these adsorbents that adsorb mercury ions from the solution through magnets. These adsorbents have similar adsorption mechanism and have application value in the treatment of mercury ions in sewage and are worth studying.


Introduction
The wastewater produced in the production process of non-ferrous metal smelting, electroplating, textile, papermaking and other industries usually contains a large number of heavy metal ions, resulting in heavy metal pollution in a variety of water bodies, such as rivers, lakes, groundwater and so on [1,2,3]. Heavy metals are highly toxic and difficult to degrade, which will seriously damage the ecological environment, and can enter the human body through the food chain, accumulate in the body, and finally pose a serious threat to human health [4]. In recent years, the treatment methods of heavy metal polluted water mainly include adsorption method, oxidation-reduction method, membrane separation method, ion exchange method and precipitation method [5,6,7,8,9]. The adsorption method has the advantages of simple operation, low cost and environmental protection. Therefore, finding a suitable adsorbent is the research focus in this field.
In recent years, various types of adsorbents have emerged, including activated carbon, biosorbent, nano materials and so on [10,11,12]. Magnetic nanoparticles (MNPs) have the advantages of large specific surface area, low cost, environmental protection, high adsorption efficiency and easy separation and recovery under the action of external magnetic field. Magnetic nano adsorbents are widely used in cell separation, protein separation, nucleic acid separation, enzyme fixation, protein detection, enzyme detection, bacteria and virus detection, magnetic resonance imaging, drug separation, organic matter adsorption and separation in water treatment, food safety monitoring, etc. There are usually several methods to prepare magnetic nanoparticles: hydrothermal method, micro lotion method, sol-gel method (sol-gel), coprecipitation method and so on. According to literature [13], our research group adopted a simple and efficient method to synthesize magnetic nanoparticles, and grafted functional groups to increase the adsorption capacity.
They have become a research hotspot In dealing with heavy metal pollution [14]. Magnetic ferric oxide nanoparticles are widely used in biomedicine, catalysis, magnetic recording materials [15,16] and other fields due to their unique physical and chemical properties and good biocompatibility. With magnetic separation characteristics and high adsorption efficiency, they also show good performance in the separation and enrichment of heavy metal ions in aqueous environment [17]. However, unmodified MNPs are easy to be oxidized in air, easy to agglomerate in solution and poor dispersion, which reduces their adsorption performance [18]. MNPs are usually modified with substances with specific active functional groups. Common active functional groups mainly include nitrogen-containing groups (pyridyl, thiadiazole, triazole) [19,20] and sulfur-containing groups (sulfhydryl, sulfonic acid) [21,22,23]. On the one hand, the existence of these groups can improve the stability of nanoparticles and prevent agglomeration; On the other hand, functional groups can selectively adsorb specific heavy metal ions through complexation effect, electrostatic attraction or ion exchange, and improve the adsorption efficiency.
In this work, Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ (including Fe 3 O 4 @SiO 2 -NH-2NG-SPTZ, Fe 3 O 4 @SiO 2 -NH-3NG-SPTZ) were prepared by 2-mercapto-5-(4-pyridyl)-1,3,4-thiadiazole and the reaction products of 3, 6dichloropyridazine and cyanuric chloride with Fe 3 O 4 @SiO 2 -NH 2 (MNPs coated by 3-aminopropyltriethoxysilane) respectively. In the same way, Fe 3 O 4 @SiO 2 -S-nNG-SPTZ (including Fe 3 O 4 @SiO 2 -S-2NG-SPTZ, Fe 3 O 4 @SiO 2 -S-3NG-SPTZ) were prepared by 2-mercapto-5-(4-pyridyl)-1,3,4-thiadiazole and the reaction products of 3, 6-dichloropyridazine and cyanuric chloride with Fe 3 O 4 @SiO 2 -SH (MNPs coated by 3mercaptopropyltriethoxysilane) respectively. On the one hand, these adsorbents have the advantages of good dispersion, large specific surface area, strong adsorption capacity, environmental protection and easily separated from solution by an external magnetic field, their synthetic methods are simple and easy to operate. On the other hand, these adsorbents have longer functional groups floating in the water, which not only increases the adsorption sites and adsorption area, but also easily clamps mercury ions like the tentacles of jellyfish. Hg(II) ions are the most frequently reported heavy metal ions with high toxicity, can accumulate in organisms, and even lead to brain and liver damage or even death in severe cases. Our research group has been committed to preparing efficient adsorbents to adsorb mercury ions from wastewater. So, four absorbents were investigated systematically for removal efficiency and conditions of Hg(II) ions.

Experimental equipment
The infrared spectrometer is Nicolet Nexus 470 FT-IR spectrometer (Nicolet, USA). Flame atomic adsorption spectrometric (FAAS) equipment is a Perkin Elmer Zeeman 1100 B spectrometer (Uberlingen, Germany). Recorded on micrographs and morphology of the adsorbents are obtained at 5.0 kV on a supra 40vp field emission scanning electron microscopy (FEI, Shanghai, USA) and a transmission electron microscope (TEM) of H-7500 (Hitachi, Japan), respectively. The pH measurements is a mettler-toledo delta 320 pH meter.

Synthesis of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ
The following methods and approaches, such as the process of synthesizing magnetic nano adsorbent and grafting functional groups through substitution reaction, are often used by our research group. The method is reliable and the experimental effect is good.

Synthesis of Fe 3 O 4 @SiO 2 -NH-nNG
At room temperature and under N 2 , 0.20 g Fe 3 O 4 @SiO 2 -NH 2 was dispersed into 30.00 mL isopropanol and stirred mechanically for 30 min. Then add 0.30 g 3,6-dichloropyridazine to the mixture and continue stirring for 6 h. Separate with external magnetic field, wash with absolute ethanol, and bake in a vacuum oven at 60 C for 6 h to obtain Fe 3 O 4 @-SiO 2 -NH-2NG.

Synthesis of Fe 3 O 4 @SiO 2 -SH
Add 0.20 g MNPs and 30.00 mL anhydrous ethanol into a 100 mL three mouth bottle and ultrasonic for 15 min. After MNPs are evenly dispersed, add 0.80 mL TPTES and 1 mL distilled water into a three mouths bottle. Mechanical stirring for 6 h under the protection of nitrogen at room temperature. The product is separated by external magnetic field, washed with absolute ethanol and dried in a vacuum oven at 60 C for 6 h.

Synthesis of Fe 3 O 4 @SiO 2 -S-nNG
Under room temperature and nitrogen protection. 0.20 g Fe 3 O 4 @-SiO 2 -SH was dispersed into 30.00 mL isopropanol and stirred mechanically for 30 min. Then add 0.30 g 3,6-dichloropyridazine and 0.2 mL DIPEA to the mixture and continue stirring for 6 h. Separate with external magnetic field, wash with absolute ethanol, and bake in a vacuum oven at 60 C for 6 h to obtain Fe 3 O 4 @SiO 2 -S-2NG. DIPEA is added to absorb HCl.
In the same way, 0.36 g cyanuric chloride was used instead of 0.30 g of 3,6-dichloropyridazine to prepare Fe 3 O 4 @SiO 2 -S-3NG.

Synthesis of Fe 3 O 4 @SiO 2 -S-nNG-SPTZ
0.50 g Fe 3 O 4 @SiO 2 -S-2NG was dispersed into 50.00 mL ethanol and stirred mechanically for 30 min. Then add 0.50 g HS-PTZ and 0.3 mL of DIPEA into the mixture, heat and reflux for 3 h. Separate with external magnetic field, wash with absolute ethanol, and bake in a vacuum oven at 60 C for 6 h to obtain the product Fe 3 O 4 @SiO 2 -S-2NG-PTTs. DIPEA a is added to absorb HCl. 0.50 g of HS-PTZ is excessive and has been guaranteed Fe 3 O 4 @SiO 2 -S-2NG complete reaction.

Synthesis route and application roadmap of adsorbents
From Scheme 1, we can see the raw materials required for the synthesis of all adsorbents. From Scheme 2, we can see the operation process of the synthetic adsorbent in removing mercury ions in water and the process of recovering the adsorbent in the presence of magnet.

Batch procedure
First, Take 0.10 g respectively Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ or In these experiments, the mixture will be dispersed in the ultrasonic wave at room temperature for 10 min. After the experiment, the adsorbent that adsorbs mercury ions is recovered with a magnet, washed with distilled water to neutral, and the concentration of mercury ions in the filtrate is measured by FASS. Each experiment is repeated three times.
The adsorption capacity and removal efficiency of these experiments were calculated by the following Eqs. (1) and (2): where Q represents the adsorption capacity (mg g À1 ), E is removal efficiency (%), Co and Ce are the initial and equilibrium concentration of Hg(II)ions (mg L À1 ), W is the mass of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ or Fe 3 O 4 @SiO 2 -S-nNG-SPTZ and V is the volume of Hg(II) ions solution (L). Figure 1 is the infrared spectrum of the adsorbent. The absorption peak at 1096 cm À1 should be caused by Si-O-Si stretching vibration, and the absorption peak at 812 cm À1 is its symmetric stretch [25]. The Si-O-Si or O-Si-O bending mode causes an absorption peak at 471 cm À1 . The absorption peak at 590 cm À1 is assigned to the presence of Si-O-Fe [26]. The aromatic ring and presence of Ar-H cause the band at 1650 and 3000 cm À1 , respectively [27]. The bands at 1500 cm À1 is the C-N stretching mode [27]. About 700 cm À1 is C-S band [28,29], No absorption at 2600 cm À1 indicates that all sulfhydryl groups have reacted completely [30]. Table 1 After these adsorbents adsorbed heavy metal ions in aqueous solution, the research group selected one of them to demonstrate its separation effect from the solution under the action of magnet. It can be seen from Figure 2 that when the magnet is close to the uniform suspension, the magnetic nanospheres in the solution are obviously attracted from far to near and gathered on the bottle wall. When the magnet is close to the solution to a certain distance, it will be found that all the magnetic nanospheres in the bottle are attracted to the bottle wall without exception. It shows that the adsorbent has very strong magnetism, be easy to separate from the solution, and has strong operability. Figure 3 shows scanning electron microscope of pure Fe 3 O 4 , Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ respectively. It can be seen from the figure that their shapes are spherical, indicating Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ all have spherical nuclei of Fe 3 O 4 . The surface of pure Fe 3 O 4 ball is relatively rough, while the surface of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @-SiO 2 -S-nNG-SPTZ are smooth, round and has the formation of foam like substances, which is due to the fact that the surface is coated with silica gel grafted with functional groups. Figure 4 shows transmission electron micrograph of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ. It can be clearly seen from the inside structure of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ is divided into two layers. The black ball in the inner layer is Fe 3 O 4 nano ball, and the outer layer is clearly covered with white colloid, which is silica gel. The spheres are different sizes, but they are all attached with silica gel.

Saturated adsorption capacity
Under the optimum conditions, the adsorption capacity of the adsorbent was investigated. It is obvious from Figure 5 with other type of adsorbent, the adsorption capacity of Hg(II) ions has been greatly improved [31]. The generally good adsorption capacity of these adsorbents may be related to the grafting of long functional groups, which float in the aqueous solution like the tentacles of jellyfish and can effectively clamp down on mercury ions. It can be seen that the maximum adsorption capacity of Fe 3 O 4 @SiO 2 -NH-3NG-SPTZ and Fe 3 O 4 @-SiO 2 -S-3NG-SPTZ for Hg(II) are higher than Fe 3 O 4 @SiO 2 -NH-2NG-SPTZ and Fe 3 O 4 @SiO 2 -S-2NG-SPTZ, which may be Fe 3 O 4 @-SiO 2 -NH-3NG-SPTZ and Fe 3 O 4 @SiO 2 -S-3NG-SPTZ have more HS-PTZ, which increased the adsorption site, which was consistent with the results obtained by EDS spectrum. The maximum adsorption capacity of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ for Hg(II) are generally slightly higher, which may be related to their more functional groups.

Effect of pH
As can be seen from Figure 6, the adsorption of Hg(II) ions by   precipitation loss of Hg(II) ions is too large, which seriously affects the removal efficiency. Figure 7 shows the effect of contacting time on the removal efficiency of Hg(II) ions. As can be seen from the figure, at the beginning, the removal efficiency increases rapidly with time. There are mainly the following reasons, first of all, Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @-SiO 2 -S-nNG-SPTZ are magnetic silica gel spheres, which have many grafted functional groups and long carbon chains. They are like ribbons floating on the surface of silica gel spheres, fully exposing the adsorption sites, which is conducive to the rapid contact between Hg(II) ions and the adsorption sites. Secondly, under shaking conditions, Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ have good dispersion effect and can be very evenly dispersed in water, which is also conducive to the rapid contact between Hg(II) ions and adsorption sites. Finally, the active functional groups grafted by Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @-SiO 2 -S-nNG-SPTZ are excellent heavy metal ion coordination groups, such as pyridyl and thiadiazole groups, which accelerate the coordination with Hg(II) ions and improve the removal efficiency. As the adsorption sites slowly decrease and the adsorbed Hg(II) ions block the coordination sites, the removal efficiency decreases until the adsorption equilibrium is reached. No matter which of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ, the removal efficiency can reach more than 95% in 15-20 min, indicating the excellent adsorption efficiency of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ.

Selection of eluents
The The results show that 0.30 mol/l HCl has the best desorption effect on Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ adsorbed with Hg(II) ions at room temperature, and the desorption efficiency of these absorbents are more than 95%. Therefore, 0.30 mol/l HCl is selected as the eluent in this experiment.

Reusability
After the research group conducted five adsorption and elution experiments, we found the best reuse times of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ. As shown in (see Figure 8), after repeated use for 3 times, the removal efficiency of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ are generally more than 90%, indicating that Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ have certain reusability and stability. And the reusability of Fe 3 O 4 @SiO 2 -NH-3NG-SPTZ is much less, which may be related to the long grafted functional groups and the easy fracture of functional groups in the elution process.

Isothermal model of adsorbing Hg(II) ions
The adsorption processes of Hg(II) ions by Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ were fitted by Langmuir and Freundlich adsorption isothermal models, respectively. The adsorption isotherm Eqs. (3) and (4) are as follows: where Q max (mg/g) is the theoretical adsorption capacity, Q eq is the equilibrium adsorption capacity (mg/g), C eq (mg/L) is the equilibrium   concentration of Hg(II) ions, b (L/mg) is the adsorption equilibrium constant of Langmuir model, K F is the binding energy constant. n is the Freundlich exponent related to adsorption intensity. From the fitting results in Table 2, the adsorption equilibrium of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ for Hg(II) can be well described by Freundlich model. n ¼ 3.231, 4.326, 2.651 and 2.157, which are all between 0 and 10. The adsorption is easy. It belongs to the adsorption process of multi-layer ion exchange and chemical adsorption, which may be related to Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ are related to the multilayer structure formed by magnetic core, wrapped silica gel and grafted functional groups.

Kinetic model of adsorption of Hg(II) ions
The adsorption kinetic model is used to fit the adsorption process of Hg(II) ions by Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ. At present, pseudo-first-order kinetic equation and pseudo-secondorder adsorption rate equation are widely used [32].
The linear form of the pseudo-first-order kinetic Eq. (5) is as follows: where Q t (mg/g) is the adsorption capacity at any time, Q eq (mg/g) is the equilibrium adsorption capacity, and k 1 (min À1 ) is the pseudo-first-order rate constant. The linear form of the pseudo-second-order adsorption rate Eq. (6) is as follows: where Q t (mg/g) is the adsorption capacity at any time, Q eq (mg/g) is the equilibrium adsorption capacity, and k 2 g/(mg⋅min) is the pseudosecond-order rate constant. The adsorption kinetics of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @-SiO 2 -S-nNG-SPTZ are described by pseudo-first-order kinetic equation and pseudo-second-order kinetic adsorption equation.
As can be seen from Table 3, Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ correlation coefficients R 2 (0.9956, 0.9879, 0.9910, 0.9798) of the pseudo-second-order kinetic model for the adsorption of Hg(II) are greater than those R 2 (0.8256, 0.6651, 0.7210, 0.8823) of the pseudo-second-order kinetic model, the theoretical maximum adsorption calculated by the pseudo-second-order kinetic model are in good agreement with the actual adsorption, indicating that the adsorption process of the adsorbent conforms to the pseudo-secondorder kinetic equation, the adsorption process is mainly chemical    adsorption, The adsorption rate is determined by the number of free adsorption points [33].

Conclusion
In summary, four kinds of novel adsorbents were successfully formed. Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ were confirmed and characterized by FT-IR, SEM, TEM and XRD. Batch experiments were performed to evaluate adsorption performance of Hg(II) ions. Under the normal temperature and neutral condition, just 15-20 min, the removal efficiency of any adsorbent is more than 95% and could be reused for 3 times. The maximum adsorption capacities of Fe 3 O 4 @-SiO 2 -NH-2NG-SPTZ, Fe 3 O 4 @SiO 2 -NH-3NG-SPTZ, Fe 3 O 4 @SiO 2 -S-2NG-SPTZ, Fe 3 O 4 @SiO 2 -S-3NG-SPTZ for Hg(II) were 67.45, 82.39, 70.21 and 88.47 mg/g respectively. The adsorption equilibrium of Fe 3 O 4 @SiO 2 -NH-nNG-SPTZ and Fe 3 O 4 @SiO 2 -S-nNG-SPTZ for Hg(II) can be well described by Freundlich model, fit well with the pseudo-second-order kinetic equation [32]. After the experiments were finished, these adsorbents can be easily recovered from the solution through magnets. From the experimental results, these adsorbents will play a role in the treatment of mercury ions in sewage.

Declarations
Author contribution statement Dun Chen: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Funding statement
Amatjan Sawut was supported by General project of natural science of Xinjiang Autonomous Region (No. 2022D01C22).

Data availability statement
Data included in article/supplementary material/referenced in article.

Declaration of interests statement
The authors declare no conflict of interest.

Additional information
No additional information is available for this paper.