Removal of Pb(II) Ions from Wastewater by Using Polyethyleneimine-Functionalized Fe 3 O 4 Magnetic Nanoparticles

: A class of polyethyleneimine (PEI)-functionalized Fe 3 O 4 magnetic nanoparticles (MNPs) has been facilely produced through a solvothermal process. The synthetic MNPs have been characterized by multiple technologies and then used for Pb(II) ion sorption from the aqueous media in di ﬀ erent conditions. It was found the Pb(II) adsorption behaviors could be well ﬁtted by the pseudo second-order kinetic and Langmuir isotherm models. The maximum Pb(II) adsorption capacity at 25 ◦ C and pH 5.0 was calculated to be 60.98 mg / g. Moreover, e ﬀ ects of temperature, pH, and electrolyte of aqueous phase on the Pb(II) adsorption capacity of MNPs have been carefully examined. The Pb(II) adsorbing capacity was enhanced with temperature or pH rising, but reduced with the addition of various electrolytes. Additionally, the recyclability of synthetic MNPs has been also assessed. PEI-functionalized MNPs could still maintain good adsorption performance after ﬁve cycles of Pb(II) removal. These results indicated that the PEI-functionalized Fe 3 O 4 MNPs could be readily synthesized and served as a desirable and economic adsorbent in Pb(II)-contaminated wastewater treatment.


Introduction
Along with the rapid industrial development, plenty of wastewaters containing heavy metals (e.g., lead, mercury, copper, cadmium, and chromium) are produced, especially in the fields of electroplating, leather-making, textile dyeing and printing, and battery production [1][2][3][4]. Unlike organic pollutants, heavy metals cannot be decomposed. As a result, if the wastewaters are unable to be properly treated, heavy metals will always exist in environment or accumulate in organisms through the food cycle [5], resulting in serious damage to the environmental safety and human health because of their high toxicity and carcinogenicity. Among the heavy metals, Pb(II) pollution is now attracting considerable attention due to its high neurotoxicity and hematotoxicity. The Pb(II) ion is readily assimilated and will be chronically accumulated inside the human body, leading to neurasthenia, anemia, anorexia, stomach ache, diarrhea, and kidney damage [2][3][4]. Thus, it is strongly necessary to design and prepare effective materials or technologies to eliminate Pb(II) from wastewaters in prior to its discharge.
Traditional approaches, including chemical precipitation, ion exchange, membrane filtration, electrolysis etc., were previously reported to effectively remove Pb(II) ion from water phase. However, these methods were usually limited because of their complicated operation process, potential secondary pollution, high operating cost, or difficulty in recycling [6][7][8]. Therefore, adsorption is now still regarded as the most popular techniques for heavy metal removal owing to its simple process, inexpensiveness, and high efficiency. So far, several types of sorbent materials, including clay minerals [9,10], zeolite [11], carbon materials [12][13][14][15], natural and synthetic polymers etc. [16][17][18][19], have been reported to isolate Pb(II) from water phase. Nevertheless, extensive use of these sorbents was very restrictive due to their complex operations (such as filtering and centrifugation) in adsorbent recollection after usage. To overcome this defect, magnetic nanoparticles (MNPs) were widely applied to eliminate pollutant from wastewaters in recent years [20][21][22]. The MNPs were usually surface-modified with the chemical substances containing amidogen, carboxyl, hydroxyl, sulfhydryl, or phosphate groups; after surface modification, the MNPs can be easily dispersed in water phase via simple oscillation or stirring, but also exhibit higher affinity for heavy metals via the generation of complexes or chelates [23,24]. More importantly, the well-designed MNPs are able to be magnetically recycled from wastewater facilely.
Recently, the MNPs were also reported to be loaded onto biochar [2,3], grapheme [23], zeolite [25], litchi peel [5], spent coffee grounds [4], chitosan, alginate beads, etc. [1,26], and the prepared magnetic sorbents showed good performance for Pb(II) adsorption. Nonetheless, the synthetic procedure of above-mentioned magnetic adsorbents involved several steps and was relatively complex, and there remains a need to develop easy-to-synthesize MNPs for Pb(II) sorption. Consequently, in the present work, a class of polyethyleneimine (PEI)-functionalized Fe 3 O 4 MNPs was facilely prepared via a solvothermal process, and directly used for Pb(II) isolation from water environment. In our previous work [27], a series of PEI-functionalized Fe 3 O 4 MNPs were prepared at various solvothermal reaction times and applied for simultaneous separation of emulsified oil, Cr(VI), and Cu(II) from aqueous environment. As part of our ongoing research, the fabricated PEI-functionalized MNPs were further applied to isolate Pb(II) from water phase. The kinetic and thermodynamic behaviors for Pb(II) adsorption were carefully studied; meanwhile, the impacts of temperature, pH, and electrolyte on Pb(II) adsorbing capacity were examined in detail. Besides, the reusability of synthetic PEI-functionalized MNPs was further investigated.

Preparation s of Fe 3 O 4 MNPs
PEI-functionalized Fe 3 O 4 MNPs were synthesized through a typical solvothermal method. First, PEI (3.0 g), NaAc (6.0 g), and FeCl 3 ·6H 2 O (2.0 g) were completely dissolved in EG (65 mL), and its solution was then introduced into a high pressure reactor (100 mL). The reaction lasted for 3 h at 200 • C, except as otherwise indicated. Thereafter, the resulting product was naturally cooled and was then rinsed by using ethanol and water for several times, respectively. The synthetic PEI-functionalized MNPs had good water dispersibility. As a contrast, pute Fe 3 O 4 MNPs were synthesized without any addition of PEI via a similar process.

Characterization
Particle size and morphology of synthetic MNPs were examined by scanning electron microscope (SEM, SIRION-100, FEI company, Eindhoven, The Netherlands), and their particle size distribution was determined via statistical analysis of at least 200 nanoparticles in SEM image. Chemical structure of samples was analyzed by using Fourier-transform infrared spectra (FTIR, Nicolet 6700, Thermo Scientific, Boston, MA, USA). The Brunauer-Emmett-Teller (BET) (Quantachrome Instruments system, Boynton Beach, FL, USA) was applied for specific surface area measurement. The Pb(II) ion concentration was measured by using UV-Vis spectrophotometry (UV-2600, Shimadzu, Kyoto, Japan) with xylenol orange as the complexing agent.

Batch Adsorption Experiments
The batch adsorption test was performed at 25 • C and pH 5.0, where the Pb(II) concentration was controlled at 50 mg/L, unless otherwise mentioned. The nanoparticle dosage and its adsorption time were 1000 mg/L and 120 min, respectively. For the adsorption kinetic experiment, the adsorbing quantity at time t (q t ) was determined within 120 min. Adsorption isotherms were obtained at 25 • C via changing initial Pb(II) concentration between 10 and 150 mg/L. Equilibrium adsorbing capacity (q e ) was calculated by the equation (q e = (C 0 − C e )V/M), where C 0 (mg/L) represents the initial Pb(II) concentration, C e (mg/L) represents the residual Pb(II) concentration in solution when the Pb(II) adsorption reached equilibrium, V (L) represents the solution volume, and M (g) represents the adsorbent mass. The pH impact on Pb(II) adsorption was evaluated in the range of 2.0 to 5.0, while the temperature effect was assessed at 25, 35, and 45 • C, respectively. The influence of electrolyte on Pb(II) sorption was investigated with addition of CaCl 2 , KCl or NaCl. The spent MNPs were regenerated by washing with aqueous HCl solution (1.0 mol/L), followed by rinsing with deionized water. The recycling experiment was repeated for five times to evaluate its reusability of PEI-functionalized MNPs.

Characterizations
In our previous study [27], the influence of reaction time on the morphology of PEI-functionalized Fe 3 O 4 MNPs was carefully investigated; when the solvothermal reaction lasted for 3 h, the submicrometer Fe 3 O 4 sphere was fully formed, which was also confirmed by X-ray diffraction and FTIR results. The Fe 3 O 4 microsphere prepared at 3 h showed superparamagnetic behavior, and its magnetization saturation was~62 emu/g [27]. The morphology and size distribution of spherical MNPs are shown in Figure 1a-d. The size of PEI-functionalized Fe 3 O 4 MNPs ranged from 100 to 350 nm, and its average size was estimated to be~209 nm; in comparison, the size of pure Fe 3 O 4 MNPs ranged from 100 to 450 nm, and its average size increased to 256 nm. According to the nitrogen adsorption-desorption experiments, the surface area of PEI-functionalized MNPs was calculated to be~45 m 2 /g. Moreover, the PEI-functionalized MNPs could be well dispersed in water, whereas the pure Fe 3 O 4 MNPs sunk together in water phase. These results suggested that PEI could not only stabilize the crystalline grain in reaction system and decrease the size of synthetic Fe 3 O 4 MNPs, but also significantly improve the water dispersibility of MNPs. Figure 1e shows the FTIR spectra of pure and PEI-functionalized Fe 3 O 4 MNPs, as well as the FTIR spectra of PEI-functionalized MNPs after Pb(II) adsorption. The vibration peak at~1638 cm −1 was ascribed to the COO − group [27], suggesting the existence of carboxylate in Fe 3 O 4 microsphere. The carboxylate groups were mainly originated from sodium acetate and coordinated with iron ion. The vibration peaks at approximately 2920 and 2857 cm −1 were assigned to the methyl and methylene groups. Meanwhile, as compared with pure Fe 3 O 4 MNPs, a characteristic vibration peak at 1595 cm −1 was observed for PEI-functionalized Fe 3 O 4 MNPs, which corresponded with the bending vibration of -NH 2 group [28], suggesting the successful coating of PEI on Fe 3 O 4 MNPs. After Pb(II) adsorption, characteristic band of -NH 2 group disappeared at 1595 cm −1 but moved to 1521 cm −1 instead, suggesting the Pb(II) binding with -NH 2 groups through chelating interaction mechanisms. Meanwhile, the peak of COO − group became stronger and shifted slightly to 1628 cm −1 , which was ascribed to the adsorption of carboxylate groups from aqueous solution along with Pb(II) capture for charge balance [28]. This is because Pb(Ac) 2 was used as the source of Pb(II) in this study, and hence the carboxylate groups was also existed in the aqueous solution.
the resulting PEI-functionalized Fe3O4 MNPs were applied to adsorb Pb(II) from aqueous environment. Figure 2b compares the adsorption capacity between the Fe3O4 MNPs with and without PEI functionalization. It was found that the pure Fe3O4 MNPs did not show any adsorption ability toward Pb(II) ion. This result further suggested that the chelating interaction between Pb(II) and amine group of PEI played a very important role for Pb(II) capture, whereas the carboxylate groups on Fe3O4 MNPs could not adsorb Pb(II) via charge attraction. Figure 2b also shows the Pb(II) adsorption kinetics on PEI-functionalized MNPs. Fast adsorption was observed at early stage and its adsorption equilibrium was reached merely after 20 min. Herein, the adsorption kinetics was analyzed by using pseudo first-order (Equation (1)) and pseudo second-order (Equation (2)) kinetic models, which are presented as follows, where k1 (1/min) represents the pseudo first-order rate constant, and k2 (g/(mg•min)) represents the pseudo second-order rate constant. The simulated curves and relevant kinetic parameters are shown in Figure 2c,d and Table 1. Pb(II) adsorption kinetics could be perfectly simulated by the pseudo second-order model according to the obtained correlation coefficient (R 2 = 0.999). Meanwhile, the calculated qe (34.014 mg/g) was approximate to the experimentally determined value (33.651 mg/g). These results indicated that Pb(II) sorption on PEI-functionalized Fe3O4 MNPs

Adsorption Kinetics
First, the adsorption capacity of MNPs prepared at various solvothermal reaction times was examined. It is evident in Figure 2a that its adsorbing capacity was reduced gradually with extending solvothermal reaction time from 3 to 12 h, due to the decreasing amount of PEI coated on the MNPs [27]. Therefore, in this study, the solvothermal reaction time was mainly kept at 3 h and the resulting PEI-functionalized Fe 3 O 4 MNPs were applied to adsorb Pb(II) from aqueous environment. Figure 2b compares the adsorption capacity between the Fe 3 O 4 MNPs with and without PEI functionalization. It was found that the pure Fe 3 O 4 MNPs did not show any adsorption ability toward Pb(II) ion. This result further suggested that the chelating interaction between Pb(II) and amine group of PEI played a very important role for Pb(II) capture, whereas the carboxylate groups on Fe 3 O 4 MNPs could not adsorb Pb(II) via charge attraction. Figure 2b also shows the Pb(II) adsorption kinetics on PEI-functionalized MNPs. Fast adsorption was observed at early stage and its adsorption equilibrium was reached merely after 20 min. Herein, the adsorption kinetics was analyzed by using pseudo first-order (Equation (1)) and pseudo second-order (Equation (2)) kinetic models, which are presented as follows, where k 1 (1/min) represents the pseudo first-order rate constant, and k 2 (g/(mg·min)) represents the pseudo second-order rate constant. The simulated curves and relevant kinetic parameters are shown in Figure 2c,d and Table 1. Pb(II) adsorption kinetics could be perfectly simulated by the pseudo second-order model according to the obtained correlation coefficient (R 2 = 0.999). Meanwhile, the calculated q e (34.014 mg/g) was approximate to the experimentally determined value (33.651 mg/g). These results indicated that Pb(II) sorption on PEI-functionalized Fe 3 O 4 MNPs was mainly dominated by chemisorption, consistent with the metal-amine complexation adsorption mechanism [5].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 9 was mainly dominated by chemisorption, consistent with the metal-amine complexation adsorption mechanism [5].   Figure 3a shows the Pb(II) adsorption isotherm at 25 °C. The adsorbing capacity was continually enhanced with increasing initial Pb(II) concentration over the studied range. In this study, Langmuir and Freundlich isothermal models were applied to simulate the adsorbing isotherms; theoretically, the former is used to describe a monolayer adsorption process, whereas the latter is applied to depict a multilayer adsorption process [5]. The Langmuir and Freundlich models are given in Equations (3)

Adsorption Isotherm
Herein, qm (mg/g) represents the maximum adsorbing amount and b (L/mg) refers to the Langmuir isotherm constant, whereas k and n refer to the Freundlich isotherm constants. The simulated curves and parameters are presented in Figure 3b,c and    Figure 3a shows the Pb(II) adsorption isotherm at 25 • C. The adsorbing capacity was continually enhanced with increasing initial Pb(II) concentration over the studied range. In this study, Langmuir and Freundlich isothermal models were applied to simulate the adsorbing isotherms; theoretically, the former is used to describe a monolayer adsorption process, whereas the latter is applied to depict a multilayer adsorption process [5]. The Langmuir and Freundlich models are given in Equations (3) and (4), respectively.

Adsorption Isotherm
Herein, q m (mg/g) represents the maximum adsorbing amount and b (L/mg) refers to the Langmuir isotherm constant, whereas k and n refer to the Freundlich isotherm constants. The simulated curves and parameters are presented in Figure 3b,c and  [29]. Although frequently reported magnetic adsorbents, such as magnetic biochar [2,3], magnetic grapheme [23], and magnetic chitosan and alginate beads [1,26], exhibited much higher adsorption capacity, their extensive applications were restricted to some degree because of the relatively complicated synthetic procedures. As a result, the PEI-functionalized Fe 3 O 4 MNPs remained a class of prospective nanomaterials for Pb(II) sorption from water environment.
magnetic chitosan and alginate beads [1,26], exhibited much higher adsorption capacity, their extensive applications were restricted to some degree because of the relatively complicated synthetic procedures. As a result, the PEI-functionalized Fe3O4 MNPs remained a class of prospective nanomaterials for Pb(II) sorption from water environment.   Figure 4 shows the influences of temperature, pH value, and electrolyte on its adsorption performance. Pb(II) adsorbing capacity was enhanced with temperature rising from 25 to 45 °C (Figure 4a), indicating an endothermic adsorption process. Meanwhile, Pb(II) adsorption was also promoted with pH increasing from 2.0 to 5.0 (Figure 4b), which was in agreement with the adsorption mechanism of metal-amine complexation. At lower pH level, the amine groups was protonated, and therefore the charge repulsion between MNPs and Pb(II) ions became more intensive, leading to the restraint of Pb(II) sorption. Besides, Pb(II) adsorption was restrained with addition of various electrolytes (Figure 4c), presumably due to their competitive sorption of cations. Divalent cation exhibited stronger complexing ability with amine groups, and therefore Ca 2+ imposed a more remarkable inhibiting effect on Pb(II) sorption.

Reusability of MNPs
Facile regeneration and fine reusability of MNPs are extremely significant for their practical usage in the view of economic cost. Previous report showed that the thiol-coated Fe3O4 MNPs showed superior adsorbing performance toward Pb(II) ions in aqueous media [21]. Nevertheless, the MNPs were not easy to regenerate due to the extremely high affinity between thiol and heavy metals; for example, the amino/thiol bifunctionalized MNPs should be eluted with HNO3 and ethylene diamine tetraacetic acid (EDTA) sequentially to desorb the Pb(II) ions [30]. In this study, the adsorption capacity of PEI-functionalized MNPs decreased significantly with pH declining as mentioned above. In other words, the Pb(II) ions would be desorbed from MNPs at low pH level, and therefore the spent MNPs could be easily regenerated via rinsing with aqueous hydrochloric acid solution. Accordingly, repetitive sorption and regeneration tests were performed five cycles to examine the recycling performance of PEI-functionalized Fe3O4 MNPs. Results showed that the synthesized MNPs still maintained high adsorbing capacity after five cycles (Figure 4d), indicating the fine recyclability.   Figure 4 shows the influences of temperature, pH value, and electrolyte on its adsorption performance. Pb(II) adsorbing capacity was enhanced with temperature rising from 25 to 45 • C (Figure 4a), indicating an endothermic adsorption process. Meanwhile, Pb(II) adsorption was also promoted with pH increasing from 2.0 to 5.0 (Figure 4b), which was in agreement with the adsorption mechanism of metal-amine complexation. At lower pH level, the amine groups was protonated, and therefore the charge repulsion between MNPs and Pb(II) ions became more intensive, leading to the restraint of Pb(II) sorption. Besides, Pb(II) adsorption was restrained with addition of various electrolytes (Figure 4c), presumably due to their competitive sorption of cations. Divalent cation exhibited stronger complexing ability with amine groups, and therefore Ca 2+ imposed a more remarkable inhibiting effect on Pb(II) sorption.

Reusability of MNPs
Facile regeneration and fine reusability of MNPs are extremely significant for their practical usage in the view of economic cost. Previous report showed that the thiol-coated Fe 3 O 4 MNPs showed superior adsorbing performance toward Pb(II) ions in aqueous media [21]. Nevertheless, the MNPs were not easy to regenerate due to the extremely high affinity between thiol and heavy metals; for example, the amino/thiol bifunctionalized MNPs should be eluted with HNO 3 and ethylene diamine tetraacetic acid (EDTA) sequentially to desorb the Pb(II) ions [30]. In this study, the adsorption capacity of PEI-functionalized MNPs decreased significantly with pH declining as mentioned above. In other words, the Pb(II) ions would be desorbed from MNPs at low pH level, and therefore the spent MNPs could be easily regenerated via rinsing with aqueous hydrochloric acid solution. Accordingly, repetitive sorption and regeneration tests were performed five cycles to examine the recycling performance of PEI-functionalized Fe 3 O 4 MNPs. Results showed that the synthesized MNPs still maintained high adsorbing capacity after five cycles (Figure 4d), indicating the fine recyclability.

Conclusions
In this work, a class of PEI-functionalized Fe3O4 MNPs has been facilely fabricated via solvothermal approach for effective separation of Pb(II) from water phase. PEI functionalization of MNPs was vital for Pb(II) capture, and its sorption mechanism was metal-amine complexation. The Pb(II) adsorption process could be well depicted with the pseudo second-order kinetic and Langmuir isotherm model. The maximum adsorbing capacity at 25 °C and pH 5.0 reached 60.98 mg/g. Moreover, the adsorbing capacity increased with pH or temperature rising, but decreased with addition of electrolyte due to competitive sorption of cations. Besides, the PEI-functionalized Fe3O4 MNPs could still maintained high adsorption capacity after recycling five times, indicating the superior reusability. In summary, the PEI-functionalized Fe3O4 MNPs can be readily synthesized, and they served as a simple but effective tool for Pb(II) removal from the aqueous environment. Funding: This work was funded by the National Natural Science Foundation of China (#21878064 and #41977017).

Conclusions
In this work, a class of PEI-functionalized Fe 3 O 4 MNPs has been facilely fabricated via solvothermal approach for effective separation of Pb(II) from water phase. PEI functionalization of MNPs was vital for Pb(II) capture, and its sorption mechanism was metal-amine complexation. The Pb(II) adsorption process could be well depicted with the pseudo second-order kinetic and Langmuir isotherm model. The maximum adsorbing capacity at 25 • C and pH 5.0 reached 60.98 mg/g. Moreover, the adsorbing capacity increased with pH or temperature rising, but decreased with addition of electrolyte due to competitive sorption of cations. Besides, the PEI-functionalized Fe 3 O 4 MNPs could still maintained high adsorption capacity after recycling five times, indicating the superior reusability. In summary, the PEI-functionalized Fe 3 O 4 MNPs can be readily synthesized, and they served as a simple but effective tool for Pb(II) removal from the aqueous environment.