Removal of Pb(II) from Water by FeSiB Amorphous Materials

Advanced Abstract: Amorphous materials have shown great potential in removing azo dyes in wastewaters. In this study, the performance of FeSiB amorphous materials, including FeSiB amorphous ribbons (FeSiB AR ), and FeSiB amorphous powders prepared by argon gas atomization (FeSiB AP ) and ball-milling (FeSiB BP ), in removing toxic Pb(II) from aqueous solution was compared with the widely used zero valent iron (ZVI) powders (Fe CP ). The results showed that the removal efﬁciency of all the amorphous materials in removing Pb(II) from aqueous solution are much better than Fe CP . Pb(II) was removed from aqueous solution by amorphous materials through the combined effect of absorption, (co)precipitation and reduction. Furthermore, FeSiB AP and FeSiB BP have relatively higher removal efﬁciencies than FeSiB AR due to a high speciﬁc surface area. Although the FeSiB BP has the highest removal efﬁciency up to the ﬁrst 20 min, the removal process then nearly stopped due to aggregation.


Introduction
The rapid development of the modern industry brings more and more threat to our environment. For example, a great deal of lead-containing sewage is produced by smelting, manufacturing, and the oil industry. Excessive lead intake may cause neurological, hematological, and immunological disease. Therefore, these lead ions in the sewage must be removed before it is discharged into the environment. In previous decades, ZVI was widely used to remove various heavy metal ions in water, but it can be easily oxidized, which causes the degradation capability to decay rapidly [1]. In order to improve its degradation capability, various methods have been proposed, including nanotechnology [2,3] and bimetallic technology [4,5], but new limitations such as high production costs, physiological toxicity, and aggregation restrict their application.
Recent reports show that Fe-based amorphous materials in ribbon state or powder state can not only degrade azo dyes from the aqueous solution more efficiently than the wildly used Fe CP but also exhibit relatively stable reusability due to their homogeneous microstructure, thermodynamically metastable nature, and the existence of metalloid elements [6][7][8][9][10]. Furthermore, researchers found that ball-milled amorphous powders can further improve the degradation capacity in degrading azo dyes due to the uneven topography and the stored deformation energy induced by ball milling [11][12][13][14][15]. In view of this, Fe-based amorphous materials in various states may also be able to efficiently remove lead ions in the wastewater.
In this paper, the removal capability and mechanism of lead ion (Pb(II)) from aqueous solution by FeSiB amorphous ribbons were investigated and compared with those of the widely used Fe CP . Furthermore, considering that a more specific surface area always leads to a higher reaction efficiency, the removal capacity and mechanism of FeSiB amorphous powders prepared by argon gas-atomization and ball-milling were also studied. Figure 1 shows the XRD spectrum of the four kinds of removal materials. Sharp crystalline peaks of a single α-Fe phase are observed on the spectrum of Fe CP . The spectra of the other materials present a broad hump in the 2θ range of 40-50 • , indicating their amorphous structure. Furthermore, there are tiny crystalline peaks superimposed on the broad humps, indicating that a small part of the samples may crystallize during the preparation process of the amorphous materials.
pieces of 3 mm × 3 mm × 0.03 mm. Of these, part of the ribbons were u removal material for Pb(II), and the rest were annealed at 653 K (below t tion temperature) for 1 h in an argon atmosphere and then ground by a hi mill under an argon atmosphere for 4 h at 200 r/min, with a ball-to-powde 10:1. Afterwards, the pulverized fine powders were filtered by mesh s meshes per inch and used as removal material for Pb(II).
Pb(II) solutions were prepared by dissolving Pb(NO3)2 salt in ultra-p reaction tests were conducted in 1000 mL beakers placed in a temperat water-bath device and continuously stirred at 200 r/min. All tests were co removal materials of 1 g/L and Pb(II) solutions of 500 mL, with a concen mg/L at pH 5. About 5 mL of solution was extracted at fixed time interva and filtered through a 0.2 μm membrane. Then, they were tested by Ind pled Plasma Optical Emission Spectrometer (ICP-OES, ICAP-7000, Therm tific, Waltham, MA, USA) to determine their concentration. Cycling exp carried out one after another with the same ribbons.
Structure and morphology of the materials were investigated by X-(XRD, D/MAX-2400, monochromated Cu-Kα radiation) (Rigaku Ltd., equipped with an Ni filter and a graphite crystal monochromator, and by tron microscopy (SEM, QUANTA FEG 450, FEI Ltd., Hillsborough, USA) an Energy Dispersive X-ray Spectrometer (EDS). Specific surface areas of recorded on the Brunauer-Emmett-Teller (BET, ASAP2020, Micromeritics GA, USA) using the nitrogen method. Surface elemental information of was determined by X-ray photoelectron spectroscopy (XPS, AXIS SUPR Ltd, Manchester, UK). Figure 1 shows the XRD spectrum of the four kinds of removal m crystalline peaks of a single α-Fe phase are observed on the spectrum of F tra of the other materials present a broad hump in the 2θ range of 40their amorphous structure. Furthermore, there are tiny crystalline peaks on the broad humps, indicating that a small part of the samples may cry the preparation process of the amorphous materials.    Figure 2a-d. All the particles are well-dispersed with no aggregation. The particle size of Fe CP is about 20-50 µm. Grain boundaries can be obviously seen from the enlarged surface morphology in Figure 2a, indicating that these particles are polycrystalline, composed of crystal grains with a size of about 2-5 µm, whereas the thickness of FeSiB AR is about 30 µm (Figure 2b). SEM images of both the top and bottom sides of the ribbon are shown in the inset of Figure 2b. It can be seen that both sides of the ribbon are generally smooth, even if ripples are shown on the rollcontact surface. Compared with the round and ellipsoid morphology of the 10-20 µm sized FeSiB AP particles (Figure 2c), the morphology of the FeSiB BP appears to be rather irregular ( Figure 2d). Furthermore, compared with the as-quenched ribbon, thickness of the FeSiB BP reduced to between 5 and 15 µm due to severe mechanical deformation during the ball-milling process, as shown in Figure 2e. Additionally, the surfaces of FeSiB AP are smooth, but the surfaces of FeSiB BP are full of protrusions and microcracks, as shown in the high-magnitude image of Figure 2f.  Figure 2 shows SEM images of the materials. The morphologies of Fe CP , FeSiB AR , FeSiB AP , and FeSiB BP are shown in Figure 2a-d. All the particles are well-dispersed with no aggregation. The particle size of Fe CP is about 20-50 μm. Grain boundaries can be obviously seen from the enlarged surface morphology in Figure 2a, indicating that these particles are polycrystalline, composed of crystal grains with a size of about 2-5 μm, whereas the thickness of FeSiB AR is about 30 μm (Figure 2b). SEM images of both the top and bottom sides of the ribbon are shown in the inset of Figure 2b. It can be seen that both sides of the ribbon are generally smooth, even if ripples are shown on the roll-contact surface. Compared with the round and ellipsoid morphology of the 10-20 μm sized FeSiB AP particles (Figure 2c), the morphology of the FeSiB BP appears to be rather irregular (Figure 2d). Furthermore, compared with the as-quenched ribbon, thickness of the FeSiB BP reduced to between 5 and 15 μm due to severe mechanical deformation during the ball-milling process, as shown in Figure 2e. Additionally, the surfaces of FeSiB AP are smooth, but the surfaces of FeSiB BP are full of protrusions and microcracks, as shown in the high-magnitude image of Figure 2f. The removal effects of the four materials on Pb(II) solutions at room temperature were investigated. Figure 3 compares the removal performance, dependent on reaction time, for the four materials. Ct is the concentration of Pb(II) at a reaction time of t, and C0 is the initial concentration of Pb(II). It can be seen obviously that the removal efficiency of all the amorphous materials are much better than that of Fe CP , among which FeSiB BP has the highest removal efficiency up to the first 20 min. Wang et al. [15] also found that the dye degradation capability of Fe-based amorphous ribbons could be enhanced by ball-milling. However, different from Wang's reported results, the removal process seems to stop working and increasing the reaction time cannot effectively reduce the Pb(II) when the Pb(II) concentration is reduced to about 89% at 20 min. Considering the high residual Pb(II) concentration, despite its high removal rate, FeSiB BP is not an opti- The removal effects of the four materials on Pb(II) solutions at room temperature were investigated. Figure 3 compares the removal performance, dependent on reaction time, for the four materials. C t is the concentration of Pb(II) at a reaction time of t, and C 0 is the initial concentration of Pb(II). It can be seen obviously that the removal efficiency of all the amorphous materials are much better than that of Fe CP , among which FeSiB BP has the highest removal efficiency up to the first 20 min. Wang et al. [15] also found that the dye degradation capability of Fe-based amorphous ribbons could be enhanced by ballmilling. However, different from Wang's reported results, the removal process seems to stop working and increasing the reaction time cannot effectively reduce the Pb(II) when the Pb(II) concentration is reduced to about 89% at 20 min. Considering the high residual Pb(II) concentration, despite its high removal rate, FeSiB BP is not an optimal removal material for Pb(II) from aqueous solution. Furthermore, except for FeSiB BP , the removal process by other materials can be well-fitted by the first-order reaction equation (Equation (1)) [16]:

Removal Capacity of the Materials
where k denotes the reaction rate constant, which is estimated to be 0.026, 0.079, and 0.116 min −1 for the removal processes by Fe CP , FeSiB AR , and FeSiB AP by nonlinear regression analysis, respectively. Therefore, the removal rates of FeSiB AR and FeSiB AP are approximately 21 and 87 times as fast as the widely used Fe CP . mal removal material for Pb(II) from aqueous solution. Furthe the removal process by other materials can be well-fitted by equation (Equation (1)) [16]: where k denotes the reaction rate constant, which is estimate 0.116 min −1 for the removal processes by Fe CP , FeSiB AR , and FeS sion analysis, respectively. Therefore, the removal rates of FeS proximately 21 and 87 times as fast as the widely used Fe CP .  Considering that the particle size of FeSiB AP is much smal cific surface areas of Fe CP and FeSiB AP are measured to be 0.339 analysis, respectively. Thus, the surface area normalized rate FeSiB AP are calculated to be 0.07 and 0.353 L/(m 2 ·min) by the e ρa is the surface area concentration of the sample [17]. There per-unit surface area of FeSiB AP is about 99 times that of Fe CP .

Removal Mechanism of the Amorphous Ribbons
Reaction active energy (ΔE) is an effective parameter fo mechanism of various reactions. Figure 4 shows the dependen tration of Pb(II) on reaction time at different temperatures for F the calculated reaction rate constants at different temperatures gy ΔE can be derived using the Arrhenius-type equation, ln k = the gas constant, and A is a constant [16]. ΔE is calculated to b Fe CP and FeSiB AR , indicating that the reaction between FeSiB AR easier. According to the perspective of thermodynamics, diffu in solution have relatively lower activation energies (~8-21 k face-controlled chemical reactions have larger activation ener CP Figure 3. The normalized concentration of Pb 2+ ions dependence on the reaction time. Considering that the particle size of FeSiB AP is much smaller than that of Fe CP , specific surface areas of Fe CP and FeSiB AP are measured to be 0.339 and 0.329 m 2 /g by a BET analysis, respectively. Thus, the surface area normalized rate constants k SA of Fe CP and FeSiB AP are calculated to be 0.07 and 0.353 L/(m 2 ·min) by the equation: k SA = k/ρ a , where ρ a is the surface area concentration of the sample [17]. Therefore, the reaction rate of per-unit surface area of FeSiB AP is about 99 times that of Fe CP .

Removal Mechanism of the Amorphous Ribbons
Reaction active energy (∆E) is an effective parameter for exploring the removal mechanism of various reactions. Figure 4 shows the dependence of normalized concentration of Pb(II) on reaction time at different temperatures for Fe CP and FeSiB AR . Based on the calculated reaction rate constants at different temperatures, the reaction active energy ∆E can be derived using the Arrhenius-type equation, ln k = −∆E/RT + lnA, where R is the gas constant, and A is a constant [16]. ∆E is calculated to be 31.2 and 22.4 kJ/mol for Fe CP and FeSiB AR , indicating that the reaction between FeSiB AR and the Pb(II) solution is easier. According to the perspective of thermodynamics, diffusion-controlled reactions in solution have relatively lower activation energies (~8-21 kJ/mol), whereas the surface-controlled chemical reactions have larger activation energies (>29 kJ/mol) [18,19]. Consequently, the reaction between Fe CP and the Pb(II) solution is surface-controlled, whereas interface resistance in the reaction between FeSiB AR and the Pb(II) solution has been greatly reduced. The results are consistent with the report that ZVI removes Pb(II) from solution, mainly by absorption and coprecipitation [20]. In order to investigate the removal mechanism of the amorphous ribbon, XRD p terns of the reacted FeSiB AR are compared with the reacted Fe CP . Figure 5 shows XR patterns of the FeSiB AR and Fe CP after reaction with the aqueous solution for 40 min room temperature. Diffraction peaks of FeO(OH) appear on both the XRD patterns the reacted FeSiB AR and Fe CP . According to the studies of treatment Pb(II)-contaminated solution by ZVI, the FeO(OH) complexes with strong flocculati coprecipitate with Pb(II) during the reaction process (Equation (2)) [19]. Therefore small part of Pb(II) in aqueous solution was also removed by FeSiB AR by absorption a coprecipitating with Pb(II). Furthermore, compared with Fe CP , a large number of diffr tion peaks of Pb are found on XRD patterns of the reacted FeSiB AR and the precipitate FeSiB AR , indicating that reduction (Equation (3)) played an important role in the remov process of Pb(II) by FeSiB AR . In addition, there are some tiny unknown diffraction pea found on the XRD patterns of the reacted FeSiB AR and the precipitate. Combined w XRD patterns of the reacted FeSiB AP and FeSiB BP , which will be discussed later, these ti diffraction peaks may represent SiO2 and PbO.  Figure 5. XRD patterns of the reacted FeSiB AR and Fe CP , and XRD pattern of the precipitate fr the aqueous solution reacted with FeSiB AR .
In addition, it was reported that hydroxide ions (OH -) were generated by the re In order to investigate the removal mechanism of the amorphous ribbon, XRD patterns of the reacted FeSiB AR are compared with the reacted Fe CP . Figure 5 shows XRD patterns of the FeSiB AR and Fe CP after reaction with the aqueous solution for 40 min at room temperature. Diffraction peaks of FeO(OH) appear on both the XRD patterns of the reacted FeSiB AR and Fe CP . According to the studies of treatment on Pb(II)-contaminated solution by ZVI, the FeO(OH) complexes with strong flocculation coprecipitate with Pb(II) during the reaction process (Equation (2)) [19]. Therefore, a small part of Pb(II) in aqueous solution was also removed by FeSiB AR by absorption and coprecipitating with Pb(II). Furthermore, compared with Fe CP , a large number of diffraction peaks of Pb are found on XRD patterns of the reacted FeSiB AR and the precipitate of FeSiB AR , indicating that reduction (Equation (3)) played an important role in the removal process of Pb(II) by FeSiB AR . In addition, there are some tiny unknown diffraction peaks found on the XRD patterns of the reacted FeSiB AR and the precipitate. Combined with XRD patterns of the reacted FeSiB AP and FeSiB BP , which will be discussed later, these tiny diffraction peaks may represent SiO 2 and PbO.
Reduction : In order to investigate the removal mechanism of the amorp terns of the reacted FeSiB AR are compared with the reacted Fe C patterns of the FeSiB AR and Fe CP after reaction with the aqueous room temperature. Diffraction peaks of FeO(OH) appear on bo the reacted FeSiB AR and Fe CP . According to the studi Pb(II)-contaminated solution by ZVI, the FeO(OH) complexes w coprecipitate with Pb(II) during the reaction process (Equation small part of Pb(II) in aqueous solution was also removed by FeS coprecipitating with Pb(II). Furthermore, compared with Fe CP , a l tion peaks of Pb are found on XRD patterns of the reacted FeSiB A FeSiB AR , indicating that reduction (Equation (3)) played an impor process of Pb(II) by FeSiB AR . In addition, there are some tiny unk found on the XRD patterns of the reacted FeSiB AR and the prec XRD patterns of the reacted FeSiB AP and FeSiB BP , which will be di diffraction peaks may represent SiO2 and PbO.  Figure 5. XRD patterns of the reacted FeSiB AR and Fe CP , and XRD patte the aqueous solution reacted with FeSiB AR .
In addition, it was reported that hydroxide ions (OH -) were tion of Fe 0 with H2O (Equations (4) and (5)) at a pH above 4.5 [21]  In addition, it was reported that hydroxide ions (OH − ) were generated by the reaction of Fe 0 with H 2 O (Equations (4) and (5)) at a pH above 4.5 [21], leading to the increase in the pH of the solution. Then, oxides and hydroxides of iron and lead cover the ZVI particles or form precipitates (Equations (6)-(11)) [2,22,23]. However, some of the reactions cannot be observed by the XRD patterns.
In view of the limited resolution of XRD, XPS is used to further investigate the surface chemical composition of FeSiB AR and Fe CP . Figure 6 shows Pb 4f 7/2 spectra of the reacted FeSiB AR and Fe CP . It can be observed that only Pb(II) (138.7 eV) can be found on the surface of the reacted Fe CP . However, both Pb(II) (138.5 eV) and Pb(136.7 eV) [24] are found on the surface of the reacted FeSiB AR . This discovery further confirms the XRD results showing that the removal of Pb(II) by Fe CP is mainly through absorption and (co)precipitation, while the removal mechanism of Pb(II) by FeSiB AR involves absorption, (co)precipitation, and reduction.
In view of the limited resolution of XRD, XPS is used t face chemical composition of FeSiB AR and Fe CP . Figure 6 sho acted FeSiB AR and Fe CP . It can be observed that only Pb(II) (1 surface of the reacted Fe CP . However, both Pb(II) (138.5 eV found on the surface of the reacted FeSiB AR . This discover results showing that the removal of Pb(II) by Fe CP is mai (co)precipitation, while the removal mechanism of Pb(II) by (co)precipitation, and reduction. Furthermore, the XPS spectra of FeSiB AR before and af Figure 7. The surface of the as-received FeSiB AR was surroun of iron, silicon oxide, and boron oxide. However, the XPS pe Ar + sputtered at 0.4 nm/s for 20 s, suggesting that the thickn the surface of the FeSiB AR produced in the atmosphere i compared with the nominal composition of Fe78Si9B13 amor tios of Fe:Si:B on the surface of the as-received FeSiB AR is 47 Furthermore, the XPS spectra of FeSiB AR before and after reaction are compared in Figure 7. The surface of the as-received FeSiB AR was surrounded by oxide and hydroxide of iron, silicon oxide, and boron oxide. However, the XPS peak of O 1s disappeared after Ar + sputtered at 0.4 nm/s for 20 s, suggesting that the thickness of the oxidation layer on the surface of the FeSiB AR produced in the atmosphere is about 8 nm. Furthermore, compared with the nominal composition of Fe 78 Si 9 B 13 amorphous ribbon, the atomic ratios of Fe:Si:B on the surface of the as-received FeSiB AR is 47:37:16, indicating that Si and B atoms are enriched on the surface due to a lower binding energy and the larger diffusion coefficient of Si and B elements [25]. The enrichment of Si and B element may leave an incompact  [26][27][28], indicating the reactions of Equations (4)- (11) involved in the Pb(II) removal process by FeSiB AR . Therefore, the combined effect of absorption, (co)precipitation, and reduction lead to the high removal efficiency of FeSiB AR , which is similar to the removal mechanism of Pb(II) by nano ZVI [29].
Pb(II) removal process by FeSiB AR . Therefore, the combined effect of abso (co)precipitation, and reduction lead to the high removal efficiency of FeSiB AR , w similar to the removal mechanism of Pb(II) by nano ZVI [29].
SEM images of the precipitate and reacted FeSiB AR were also observed. Fi shows microscopic morphology of the precipitate. The main features observed large number of particles with a size of about 5 μm surrounded by a thin layer o tion product. It can be seen from the enlarged image in Figure 8b that the partic surrounded by a villous product layer. EDS elemental analyses in Figure 8d sho the particles are mainly Pb crystallines. This result is in accordance with the XRD yses that show that the precipitates are mainly Pb. The irregular multi-prism mor gy of crystal Pb can be clearly seen from the backscattered electron image of the pa in Figure 8c due to the large difference in atomic number between Pb and the oth ments.  SEM images of the precipitate and reacted FeSiB AR were also observed. Figure 8 shows microscopic morphology of the precipitate. The main features observed are a large number of particles with a size of about 5 µm surrounded by a thin layer of reaction product. It can be seen from the enlarged image in Figure 8b that the particles are surrounded by a villous product layer. EDS elemental analyses in Figure 8d show that the particles are mainly Pb crystallines. This result is in accordance with the XRD analyses that show that the precipitates are mainly Pb. The irregular multi-prism morphology of crystal Pb can be clearly seen from the backscattered electron image of the particles in Figure 8c due to the large difference in atomic number between Pb and the other elements.   Figure 9b,c, respectively. Cracks have emerged on the surface of the residual product layer, as shown in Figure 9b. Following this, pieces of product layer tear along the cracks due to agitation and expose a fresh amorphous matrix. Then, a new product layer generates, as shown in Figure 9c. It can be seen that lamellar PbO, irregular multi-prism Pb, and worm-like iron oxide or silicon oxide have covered the fresh amorphous matrix. Furthermore, the high-magnitude image of region C in Figure 9d shows that the new product layer is a loose porous structure. The easily detached and loose porous product layer not only helps the elements exchange and accelerates the reaction process but also contributes to the reuse and durability performance of the FeSi-B AR .    Figure 9b,c, respectively. Cracks have emerged on the surface of the residual product layer, as shown in Figure 9b. Following this, pieces of product layer tear along the cracks due to agitation and expose a fresh amorphous matrix. Then, a new product layer generates, as shown in Figure 9c. It can be seen that lamellar PbO, irregular multi-prism Pb, and worm-like iron oxide or silicon oxide have covered the fresh amorphous matrix. Furthermore, the high-magnitude image of region C in Figure 9d shows that the new product layer is a loose porous structure. The easily detached and loose porous product layer not only helps the elements exchange and accelerates the reaction process but also contributes to the reuse and durability performance of the FeSiB AR .   Figure 9b,c, respectively. Cracks have emerged on the surface of the residual product layer, as shown in Figure 9b. Following this, pieces of product layer tear along the cracks due to agitation and expose a fresh amorphous matrix. Then, a new product layer generates, as shown in Figure 9c. It can be seen that lamellar PbO, irregular multi-prism Pb, and worm-like iron oxide or silicon oxide have covered the fresh amorphous matrix. Furthermore, the high-magnitude image of region C in Figure 9d shows that the new product layer is a loose porous structure. The easily detached and loose porous product layer not only helps the elements exchange and accelerates the reaction process but also contributes to the reuse and durability performance of the FeSi-B AR . ZVI is the typical material for water purification, but fast corrosion always leads to rapid decay of its efficiency [28]. To verify the reusability of FeSiB AR , repeated removal experiments were carried out for six 90 min cycles. Figure 10 shows the removal efficiency of FeSiB AR for six cycles. Although the removal efficiency gradually decreases after every cycle, removal efficiency up to 54% is still present until the sixth cycle, indicating that FeSiB AR can be reused conveniently for a few times without any treatment.
Metals 2022, 12, x FOR PEER REVIEW ZVI is the typical material for water purification, but fast corrosion always le rapid decay of its efficiency [28]. To verify the reusability of FeSiB AR , repeated re experiments were carried out for six 90 min cycles. Figure 10 shows the remov ciency of FeSiB AR for six cycles. Although the removal efficiency gradually decrea ter every cycle, removal efficiency up to 54% is still present until the sixth cycle, i ing that FeSiB AR can be reused conveniently for a few times without any treatment The summarized removal mechanism of FeSiB AR is illustrated in Figure 11. F can be rapidly covered by oxidations of Fe, Si, and B in the atmosphere. However oxygenated solution environment, Fe 2+ and B 3+ preferentially dissolved into the so leaving a porous and incompact layer of Si oxide on the surface of FeSiB AR . This n provides an incompact transport channel for element exchange, but also mak product layer easy to be peeled off from the ribbon and keeps the ribbon having tively stable reusability. Furthermore, the Fe 0 beneath the oxide layer acts as an e donor through the anodic reaction, Fe→Fe 2+ + 2e − , and releases a steady stream ions into the solution. At the same time, Pb 2+ is adsorbed on the oxidation layer o B AR and reduced by Fe 0 . In addition, the reaction of Fe 0 with H2O generates hyd ions (OH − ) and increases the pH of the solution. Then, oxides and hydroxides and lead may cover the FeSiB AR or form precipitates. In this process, the hydrox iron can form surface complexes with Pb(II) and act as an adsorbent of Pb(II), wh precipitate of lead oxide can separate Pb(II) from the solution. The summarized removal mechanism of FeSiB AR is illustrated in Figure 11. FeSiB AR can be rapidly covered by oxidations of Fe, Si, and B in the atmosphere. However, in an oxygenated solution environment, Fe 2+ and B 3+ preferentially dissolved into the solution, leaving a porous and incompact layer of Si oxide on the surface of FeSiB AR . This not only provides an incompact transport channel for element exchange, but also makes the product layer easy to be peeled off from the ribbon and keeps the ribbon having a relatively stable reusability. Furthermore, the Fe 0 beneath the oxide layer acts as an electron donor through the anodic reaction, Fe→Fe 2+ + 2e − , and releases a steady stream of Fe 2+ ions into the solution. At the same time, Pb 2+ is adsorbed on the oxidation layer of FeSiB AR and reduced by Fe 0 . In addition, the reaction of Fe 0 with H 2 O generates hydroxide ions (OH − ) and increases the pH of the solution. Then, oxides and hydroxides of iron and lead may cover the FeSiB AR or form precipitates. In this process, the hydroxides of iron can form surface complexes with Pb(II) and act as an adsorbent of Pb(II), while the precipitate of lead oxide can separate Pb(II) from the solution.
Metals 2022, 12, x FOR PEER REVIEW 9 of 15 ZVI is the typical material for water purification, but fast corrosion always leads to rapid decay of its efficiency [28]. To verify the reusability of FeSiB AR , repeated removal experiments were carried out for six 90 min cycles. Figure 10 shows the removal efficiency of FeSiB AR for six cycles. Although the removal efficiency gradually decreases after every cycle, removal efficiency up to 54% is still present until the sixth cycle, indicating that FeSiB AR can be reused conveniently for a few times without any treatment. The summarized removal mechanism of FeSiB AR is illustrated in Figure 11. FeSiB AR can be rapidly covered by oxidations of Fe, Si, and B in the atmosphere. However, in an oxygenated solution environment, Fe 2+ and B 3+ preferentially dissolved into the solution, leaving a porous and incompact layer of Si oxide on the surface of FeSiB AR . This not only provides an incompact transport channel for element exchange, but also makes the product layer easy to be peeled off from the ribbon and keeps the ribbon having a relatively stable reusability. Furthermore, the Fe 0 beneath the oxide layer acts as an electron donor through the anodic reaction, Fe→Fe 2+ + 2e − , and releases a steady stream of Fe 2+ ions into the solution. At the same time, Pb 2+ is adsorbed on the oxidation layer of FeSi-B AR and reduced by Fe 0 . In addition, the reaction of Fe 0 with H2O generates hydroxide ions (OH − ) and increases the pH of the solution. Then, oxides and hydroxides of iron and lead may cover the FeSiB AR or form precipitates. In this process, the hydroxides of iron can form surface complexes with Pb(II) and act as an adsorbent of Pb(II), while the precipitate of lead oxide can separate Pb(II) from the solution. In conclusion, the removal of Pb(II) by Fe CP is a surface-controlled process and is mainly controlled by absorption and coprecipitation, but the results suggest that, because of the special amorphous structure, FeSiB AR not only removes Pb(II) in solution by In conclusion, the removal of Pb(II) by Fe CP is a surface-controlled process and is mainly controlled by absorption and coprecipitation, but the results suggest that, because of the special amorphous structure, FeSiB AR not only removes Pb(II) in solution by absorption, (co)precipitation, and reduction, but also produces a loose porous product layer. The combination of these effects leads to the high removal efficiency of FeSiB AR . Figure 12 shows XRD patterns of the FeSiB AP and FeSiB BP after a reaction with the aqueous solution for 40 min at room temperature. The particles are too small to be distinguished from the precipitate, so the samples used for the XRD tests, included the reacted particles and the precipitate. XRD patterns of the FeSiB AP and FeSiB BP show nearly the same diffraction peaks that Pb, PbO, FeO(OH), and SiO 2 are superimposing on the broad hump of amorphous particles. The reaction product is consistent with the reaction product of FeSiB AR . 022, 12, x FOR PEER REVIEW absorption, (co)precipitation, and reduction, but also produces a layer. The combination of these effects leads to the high removal e Figure 12 shows XRD patterns of the FeSiB AP and FeSiB BP af aqueous solution for 40 min at room temperature. The particles a tinguished from the precipitate, so the samples used for the XRD acted particles and the precipitate. XRD patterns of the FeSiB AP an the same diffraction peaks that Pb, PbO, FeO(OH), and SiO2 are broad hump of amorphous particles. The reaction product is consi product of FeSiB AR .  Figure 13 shows SEM images of the FeSiB AP after reaction wit for 20 min at room temperature. The reacted FeSiB AP maintains morphology and is covered with heterogeneous phases. It can be that the phases appear to have the same morphologies as the pr acted FeSiB AR . Minor amounts of irregular multi-prism Pb are also the reacted FeSiB AP , as shown in high resolution in Figure 13c. Ho Pb particles are much smaller than those produced by the reaction the Pb(II) solution, which may be related to the short reaction ti persed Pb crystalline due to the large specific surface area of the F can be seen from region B that pieces of product layer have peele uct layer composed of worm-like oxides has generated, as shown Figure 13d. The same phenomenon also appears on the surface Therefore, it can be concluded that FeSiB AR and FeSiB AP have ne mechanism. Compared with FeSiB AR , it is the high specific surfac higher removal efficiency of FeSiB AP .  Figure 13 shows SEM images of the FeSiB AP after reaction with the aqueous solution for 20 min at room temperature. The reacted FeSiB AP maintains its round or ellipsoid morphology and is covered with heterogeneous phases. It can be seen from Figure 13b that the phases appear to have the same morphologies as the product layers of the reacted FeSiB AR . Minor amounts of irregular multi-prism Pb are also present on surface of the reacted FeSiB AP , as shown in high resolution in Figure 13c. However, the sizes of the Pb particles are much smaller than those produced by the reaction between FeSiB AR and the Pb(II) solution, which may be related to the short reaction time and relatively dispersed Pb crystalline due to the large specific surface area of the FeSiB AP . Additionally, it can be seen from region B that pieces of product layer have peeled off and a new product layer composed of worm-like oxides has generated, as shown in high resolution in Figure 13d. The same phenomenon also appears on the surface of the reacted FeSiB AR . Therefore, it can be concluded that FeSiB AR and FeSiB AP have nearly the same reaction mechanism. Compared with FeSiB AR , it is the high specific surface area that leads to the higher removal efficiency of FeSiB AP . It is interesting that the specific surface area of FeSiB BP is measured to be 0.0649 m 2 /g, corresponding to one fifth of the specific surface area of FeSiB AP , but FeSiB BP has the highest removal efficiency after the first 20 min. This phenomenon must be related to the special microstructure induced by the ball-milling process. According to Wang [11], huge residual stress and plastic deformation energy are stored in the FeSiB BP due to intense plastic deformation during the ball-milling process, which may facilitate the reaction activity and contribute to the low reaction active energy. Figure 14 shows SEM images of the FeSiB BP after reaction for 20 min at room temperature. Compared with FeSiB AP , the reacted FeSiB BP and the reaction products gather together, as shown in Figure 14a, which may be responsible for the slowing of the removal process after reacting for 20 min.

Removal Mechanism of the Amorphous Powders
It can be seen from the enlarged image of region B in Figure 14c that the aggregation is composed of reaction product particles (marked as P), flakes, and the FeSiB BP residue (marked as R).
The EDS elemental analysis of the reaction product particles in Figure 15 shows that the particles are mainly Pb crystalline, according with the reaction precipitates of FeSi-B AR . Additionally, there are more Pb particles found on the surface layer of FeSiB BP than on surface of FeSiB AP , as shown in Figure 14b. It can be deduced from so large a number of reaction products of Pb crystalline that the reduction is enhanced and plays a main role in the removal process of Pb(II) by FeSiB BP , which may be induced by the high deformation energy stored in the FeSiB BP . It is interesting that the specific surface area of FeSiB BP is measured to be 0.0649 m 2 /g, corresponding to one fifth of the specific surface area of FeSiB AP , but FeSiB BP has the highest removal efficiency after the first 20 min. This phenomenon must be related to the special microstructure induced by the ball-milling process. According to Wang [11], huge residual stress and plastic deformation energy are stored in the FeSiB BP due to intense plastic deformation during the ball-milling process, which may facilitate the reaction activity and contribute to the low reaction active energy. Figure 14 shows SEM images of the FeSiB BP after reaction for 20 min at room temperature. Compared with FeSiB AP , the reacted FeSiB BP and the reaction products gather together, as shown in Figure 14a, which may be responsible for the slowing of the removal process after reacting for 20 min.   It can be seen from the enlarged image of region B in Figure 14c that the aggregation is composed of reaction product particles (marked as P), flakes, and the FeSiB BP residue (marked as R).
The EDS elemental analysis of the reaction product particles in Figure 15 shows that the particles are mainly Pb crystalline, according with the reaction precipitates of FeSiB AR . Additionally, there are more Pb particles found on the surface layer of FeSiB BP than on surface of FeSiB AP , as shown in Figure 14b. It can be deduced from so large a number of reaction products of Pb crystalline that the reduction is enhanced and plays a main role in the removal process of Pb(II) by FeSiB BP , which may be induced by the high deformation energy stored in the FeSiB BP .  Furthermore, microcracks formed in the ball-milling process on the original FeSi also affect the reaction process. These microcracks may grow and propagate due to crevice corrosion effect and the huge residual stress in the agitated Pb(II) solution, esp cially when the FeSiB BP is very thin. The thinner the FeSiB BP , the easier it is for the crac Figure 15. SEM image of the reaction products for FeSiB BP and the corresponding EDS elemental mapping images. The insets highlight the details of the particles.
Furthermore, microcracks formed in the ball-milling process on the original FeSiB BP also affect the reaction process. These microcracks may grow and propagate due to the crevice corrosion effect and the huge residual stress in the agitated Pb(II) solution, especially when the FeSiB BP is very thin. The thinner the FeSiB BP , the easier it is for the cracks to penetrate the FeSiB BP and break it into small pieces. Figure 14d shows surface details of the flake. It is obvious that the flake would have broken into many small pieces if it had been further agitated. The flakes in Figure 14c have a similar morphology. The increased specific surface areas induced by the breaking of the FeSiB BP may also contribute to the reaction process.
Therefore, even though the original specific surface areas of FeSiB BP are low, the combined effect of high deformation energy stored in the FeSiB BP and increased specific surface areas make them the most reactive material. However, the aggregation of the reaction product and the FeSiB BP residue leads to the rapid decay of its efficiency after reaction for 20 min.

Conclusions
In summary, the removal efficiency of all the amorphous materials, including FeSiB AR , FeSiB AP , and FeSiB BP , in removing Pb(II) from aqueous solution are much better than the widely used Fe CP . Furthermore, it is interesting to find that FeSiB BP has the highest removal efficiency after the first 20 min, but the removal process stops at 20 min, when the Pb(II) concentration is reduced to about 89%.
The removal mechanism of FeSiB AR was discussed, and the results show that, different from the surface-controlled chemical reaction of Fe CP , Pb(II) was removed from aqueous solution by FeSiB AR through the combined effect of absorption, (co)precipitation, and reduction. In addition, the product layer on the surface of FeSiB AR is easily detached and is a loose porous structure, which not only accelerates the reaction process but also makes FeSiB AR maintain a relatively stable reusability.
FeSiB AP and FeSiB BP have nearly the same reaction mechanism as FeSiB AR , but they have a relatively higher removal efficiency than FeSiB AR , due to high specific surface areas. Although FeSiB BP has the highest removal efficiency up to the first 20 min, aggregation of reaction product and the FeSiB BP residue makes the reaction process nearly halt after the first 20 min of reaction.
This study suggests that all the amorphous materials have great application potential in removal of Pb(II) from wastewater. Despite this, the long-term use of FeSiB BP for Pb(II) removal may need further treatment due to the rapid decay of its efficiency.