The rapid cooling technology is to transfer the heat of the melt to the cooling medium quickly through the contact between the high temperature melt and the low temperature and high thermal conductivity medium, so as to achieve the purpose of cooling the alloy melt very rapidly. the commonly used methods are single roll strip casting, copper mold casting, and gas atomization.

The method of preparing MGs by mechanical alloying is to make the material deform, break and cold weld repeatedly through the collision and friction between the alloyed material and the hard steel ball in the high energy ball mill for a long time, so as to improve the atomic mobility and dislocation density at the interface of the material until the crystal structure is completely lost (Fig. 5). The ball-milled powders have rough surface, irregular shape, larger specific surface area and higher catalytic efficiency than atomized powders [26,27]. In addition, the residual stress in the ball-milled powders will lead to the increase of free energy, thus further improving the catalytic activity [28]. In order to obtain amorphous powders with small particle size and rough surface, the researchers combined gas atomization method with mechanical alloying method. For example, Surreddi et al. [29] first prepared Al₈₄Gd₆Ni₇Co₃ amorphous powders by gas atomization method, and then the powders were milled for different time. It is found that the original crystal phase is amorphized. At the same time, the regular spherical structure disappears and presents an irregular rough surface. With the extension of ball milling time, the initial crystallization temperature of amorphous increases while the crystallization temperature of intermetallic phase decreases, and the hardness of the powders increase obviously. The production of MGs by mechanical alloying has the advantages of simple technological process, low equipment requirements, and high yield. The disadvantage is that the production cycle is long, the loss of equipment is serious, and the output is small. Therefore, it is not suitable for industrial production and is mainly used for laboratory research.

The inhibition of nucleation is mainly the inhibition of heterogeneous nucleation. In the alloy design and equipment selection, attention should be paid to improving the purity of the material, doping the secondary elements which are quite different from the atomic size of the main elements, and the crystal structure of the selected cooling medium is quite different from that of the crystal nucleus [30]. The commonly used methods are tube drop method, molten glass purification method and suspension melting method [31]. These methods are to reduce the contact between the melt and the container in the cooling process, avoid heterogeneous nucleation and improve the amorphous forming ability of the material.

The above methods are all segmented operation, and the material size is small, so it is not suitable for the production of large parts and materials. For this reason, the zone melting technology (Fig. 6) is developed to produce rod-like MGs. The rod-like alloy material is placed in the groove of water-cooled copper die, and the material is melted by high-energy heat source. At the same time, the relative motion between the copper mold and the heat source causes the rapid movement of the solid-liquid interface. The huge temperature gradient between the melting zone and the solid zone rapidly cools the melting zone to form an amorphous structure [32]. This process has been widely used in industrial production. The size of rod-shaped MGs prepared by this process can reach 200 × 10 × 10 mm.

The chemical reduction method is to use the reducing agent containing metal-like metal (KBH₄ or NaH₂PO₂) to drop into the metal salt solution, so that the metal ion is reduced to metal, and then the metal particles are repeatedly leached and dried to obtain nano-sized amorphous particles. The preparation parameters such as pH value of solution, metal elements, ratio of reactants and temperature have important effects on the microstructure and catalytic performance of MGs. Compared with other processes mentioned above, the preparation conditions of the chemical reduction method are mild, the morphology of the nanoparticles can be controlled and the composition of the alloy is not limited by the eutectic composition, and the atomic ratio can be changed arbitrarily [33]. This method has been widely used in the production of nanometer amorphous catalysts.

The ability of dyes to show different colors is based on the conjugated systems such as azo bond and nitrogen-oxygen bond in dye molecules. When visible light is irradiated on the dye, the conjugated system can selectively absorb specific wavelength energy. The light in other bands is reflected and presents different colors [37]. The decolorization reaction of MGs with organic dyes is to destroy the chromophoric groups such as C=O, -N=N-, -NO₂, and -N=O, which can be used as chromogenic groups. The results show that the chromaticity removal rate of MGs for commonly used dye-contaminated wastewater is close to 100%. For example, Jia et al. [38] developed a multi-component Fe₈₃Si₂B₁₁P₃C₁^AR catalyst using arc melting and melt spinning technology to treat mixed aqueous solutions containing rhodamine B, methylene blue, methyl orange, and complex organic structures. As shown in Fig. 7, the solution is gradually decolorized to colorless within 30 min. The decolorization rate of the above four different organic wastewater is 100%. By comparing the degradation rates of the three dyes, it is found that the degradation rate of methylene blue with amino group by Fe₈₃Si₂B₁₁P₃C₁^AR is the fastest, methyl orange with azo bond as chromophore is the second, and rhodamine B with phenylamino group and carbonyl group as chromophore is the slowest. This shows that the different functional groups in the dyes have a direct effect on the decolorization reaction. Xie et al. [39] compared the decolorization performance of methyl orange wastewater by Fe₇₆B₁₂Si₉Y₃^AR and crystalline zero-valent iron (ZVI). After 1 h, the wastewater treated by Fe₇₆B₁₂Si₉Y₃^AR lost color completely, while the wastewater treated by ZVI still had high chromaticity. In order to understand the degradation process, the mass spectrometric analysis of the intermediate products is shown in Fig. 8. Methyl orange is first decomposed into N-dimethylaniline, N-dimethyl-p-phenylenediamine, and sulfonamide, followed by further degradation into small molecular organic compounds such as benzenesulfonic acid, and partially decomposed into carbon dioxide and water. The authors believe that the excellent properties of Fe₇₆B₁₂Si₉Y₃^AR are mainly due to the heterogeneous structure composed of local iron-rich and poor iron clusters, rather than the large specific surface area and strong residual stress of the powders. Table 1 shows the decolorization rate of some MGs to dye wastewater.

The chromaticity of dyes can reflect the content of dyes in solution to a certain extent, but the decrease of chromaticity does not prove that dyes are completely decomposed, only that the chromophoric groups in dye molecules are destroyed [47]. In order to more accurately reflect the performance of MGs in the degradation of dyes, the researchers proposed to use organic carbon content (TOC) to measure the degree of mineralization of polymer dyes into small molecules such as CO₂, H₂O, and inorganic salts [48]. Jia et al. [49] used Fe₇₈Si₉B₁₃^AR to treat brilliant red 3B-A solution, most of the dyes can be degraded into inorganic salts such as H₂O, CO₂, NO₃⁻ , and SO₄²⁻ within 5 min, and the final removal rate of TOC can reach 80%. However, due to the existence of Cu and Nb, Fe_73.5Si_13.5B₉Cu₁Nb₃^AR is difficult to form pores on the surface of the strip, so the reaction activity is reduced, the reaction time is extended to 30 min, and the removal rate of TOC is reduced to 65%. Table 2 lists several MGs removal rates of TOC in dye wastewater.

Ions are inevitably released into the water during the reaction between MGs and pollutants, which not only causes secondary pollution to the environment, but also reduces the recycling utilization of the catalyst. There are two main sources of leaching ions: (1) After the molecular structure of the dye is destroyed, the metal ions contained in it break away from the bulk structure and enter into the solution. (2) The redox reaction occurs between MGs and dyes, and part of MGs loses electrons and is leached in solution. It is difficult to reduce the amount of ions released from dye molecules in the process of degradation, so it is necessary to reasonably design the structure and composition of MGs to reduce the leaching of catalysts. On the one hand, choose metals and metal-like elements that have no pollution to the environment, such as Fe, Si, B and so on. On the other hand, corrosion resistant elements such as Ni, Nb are added to it. In order to reduce the leaching rate of MGs, Liang et al. [51] doped 3% Nb into the alloy, the initial dosage of Fe_73.5Si_13.5B₉Cu₁Nb₃^AR was 2 g/L, and the leaching amounts of Fe and Si after reaction with malachite green are 28 mg/L and 0.05 mg/L, respectively. Wang et al. [53] obtained that the leaching amount of Fe after the degradation of methylene blue by Fe₇₈Si₉B₁₃^AR is only 26 mg/L, and the SiO₂ attached to the surface of the strip after the reaction played a role in reducing the leaching of the catalyst to a certain extent. Some scholars believe that the proper leaching of non-toxic ions can improve the electrical conductivity of the solution and promote the electron transfer on the interface of MGs, thus improving the degradation rate of pollutants [52].

The redox reaction is easy to occur, which requires the reductant to have a negative electrode potential and the oxidant to have a positive electrode potential. The standard electrode potential of Fe is −0.44 V, which has good reducibility in the removal of organic matter and oxidizing pollutants. As shown in Eqs. (4)–(11), When Fe comes into contact with pollutants, it is easy to lose electrons and convert them into Fe²⁺ or Fe³⁺ ions. The electrons generated are captured by organic compounds or heavy metal salts to reduce and degrade the pollutants. In addition, Fe reacts with H₂O or H⁺ ions to form an active hydrogen atom [H], which has a strong reducibility, resulting in the breaking of the chemical bond (-N=N-) in the molecules of organic pollutants [57]. Therefore, as a traditional environmental functional material, ZVI have been widely used in the treatment of groundwater or industrial wastewater.(4) Fe0→Fe2++2e−

(5) Fe2+→Fe3++ e−

(6) 2H2O(l) + 2e−→H2(g) + 2OH−(aq)

(7) H2O(l)→H+(aq) + OH−(aq)

(8) H+(aq) + e−→[H]

(9) [H] + [H]→H2(g)

(10) R−N = N−R′ + 4H + (aq) + 4e−→R−NH2+ R′−NH2

(11) R−N = N−R′ + 4[H]→R−NH2+ R′−NH2

Fe-MGs is a special kind of ZVI, in order to compare the difference between Fe-MGs and ZVI in the treatment of pollutants, some researchers have carried out the kinetic analysis of the reaction process. Tang et al. [58] used amorphous ribbons Fe₈₆B₁₄^AR, crystalline ribbons Fe₈₆B₁₄^CR, and ZVI to degrade Direct Blue 6 under the same external conditions. The apparent reaction rate constant k_obs was obtained by fitting the quasi-first-order kinetic equation (Eq. (12)). It is found that the k_obs of Fe₈₆B₁₄^AR in the degradation reaction is 1.8 and 89 times higher than that of Fe₈₆B₁₄^CR and ZVI, respectively, indicating that the catalytic rate of amorphous materials is higher than that of crystalline materials. In addition, the formation of oxide layer can be controlled by fine-tuning the content of B, which can further improve the catalytic efficiency of Fe-B amorphous ribbons. On this basis, Tang et al. [59] further explored the degradation process of orange II by ternary amorphous ribbons Fe₇₉Si₁₃B₈^AR and ZVI. Using Arrhenius formula (Eq. (13)), the apparent activation energy ΔE of Fe₇₉Si₁₃B₈^AR is 17.98 kJ/mol, and that of ZVI is 31.98 kJ/mol. Therefore, Fe₇₉Si₁₃B₈^AR is more likely to be excited to change from a conventional state to an active state capable of chemical reaction. Fig. 12 compares the ΔE of Fe-MGs and ZVI in the degradation of typical azo dyes acid orange (a) and direct blue 6 (b). For the same dye, the ΔE of all Fe-MGs is lower than that of ZVI, indicating that Fe-MGs in metastable state have higher reaction activity than crystalline ZVI, which are more suitable to be used as environmental functional materials in the field of water treatment. In order to show that Fe-MGs generally have higher k_obs and lower ΔE in catalytic reaction, the kinetic parameters of catalytic degradation of various dyes by different Fe-MGs are summarized in Table 3.(12) ln(Ct /C0) = -kobs⋅t

(13) lnkT= ΔE/RT+ lnA

The above results explain why MGs have higher catalytic activity than crystalline materials from the point of view of kinetics, but it is still at the apparent level. Through the redox potential of MGs in dye wastewater, some researchers explained the deep reason why the k_obs of MGs is higher and ΔE is lower. Weng et al. [64] measured that the redox potentials of Fe₈₅B₁₅^AP and Fe₇₆Si₉B₁₀P₅^AP in methyl orange solution are −0.47 V and −0.45 V, respectively, which are lower than the redox potential of ZVI −0.437 V. Therefore, Fe₈₅B₁₅^AP and Fe₇₆Si₉B₁₀P₅^AP have stronger reductivity. Zhang et al. [55] measured the potentiodynamic polarization curves of pure iron strips and Fe-Si-B amorphous strips in CuSO₄ solution. The electrode potential of Fe-Si-B amorphous ribbons is lower than that of pure iron ribbons due to the presence of non-metallic elements Si and B in the amorphous ribbons. This result also explains the inherent reason why the k_obs of the reaction of Fe-Si-B amorphous ribbons with Cu²⁺ ions is higher than that of ZVI. In order to determine whether the atomic disorder stacking structure in MGs is one of the reasons for the decrease of redox potential, Tang [60] measured the open circuit potential of Fe₇₈Si₉B₁₃^AR, Fe₇₈Si₉B₁₃^CR, and ZVI in methyl orange wastewater under the same external conditions as −0.735 V, −0.471 V, and −0.439 V, respectively. Comparing the open circuit potential of Fe₇₈Si₉B₁₃^AR and Fe₇₈Si₉B₁₃^CR, it is found that the open circuit voltage of the alloy with the same nominal composition is different due to the different arrangement of internal atoms. Therefore, it can be determined that the disorder stacking structure of atoms in MGs is one of the reasons for the decrease of redox potential.

When Fe-MGs degrades pollutants through direct reduction mechanism, the Fe in Fe-MGs loses the outermost electrons as a reducing agent. From the analysis of the above experimental results, it is found that Fe-MGs have higher reducibility than the same component crystalline alloy and ZVI, that is, the outermost electrons of Fe-MGs are easier to break away from the binding force of the nucleus and transfer to the contaminants. In order to compare the electron loss ability of amorphous and crystalline materials, Zhang et al. [66] analyzed the valence band densities of states of Fe₇₈Si₈B₁₄^AR and ZVI by X-ray photoelectron spectroscopy as shown in Fig. 13. There is an obvious difference in the energy band structure between the two materials. The valence band top of Fe₇₈Si₈B₁₄^AR is 0.16 eV lower than that of ZVI, and the empty band is widened accordingly, which makes it easier for electrons in 4 s orbitals to overcome the energy barrier and be activated into the empty band. Therefore, in the process of reductive degradation of dyes, Fe₇₈Si₈B₁₄^AR can provide more excited electrons to participate in the catalytic reaction. The simple disordered stacking of atoms can make the electrons get rid of the constraints of the lattice, which shows the superiority of MGs structure.

Advanced oxidation is a new purification technology for oxidative degradation of organic pollutants, which mainly includes ozonation, photocatalysis, and fenton method [67,68]. Iron-based materials are mainly oxidized and degraded by fenton method. The principle is that Fe²⁺ ions reacts with fenton agent H₂O₂ to form strong oxidizing ·OH (formula 14), and its redox potential can reach 2.7 V. The generated ·OH can quickly oxidize and decompose organic pollutants into CO₂, H₂O, and inorganic small molecular substances. During this process, when Fe²⁺ ions is converted to Fe³⁺ ions, it cannot continue to generate ·OH. Moreover, Fe³⁺ ions can also consume H₂O₂ to produce HO₂· with negative redox potential (Eq. (15)), which seriously inhibits the oxidative decomposition of organic pollutants. At this time, if there is ZVI in the solution, the generated Fe³⁺ ions can be reduced to Fe²⁺ ions again (Eq. (16)) to participate in the formation of OH. The traditional ZVI is easy to be passivated due to the adhesion of the reaction products on its surface, which reduces the long-term reaction activity of ZVI. Using Fe-MGs instead of ZVI can effectively alleviate the passivation phenomenon on the surface of the material and reduce the amount of catalyst.(14) Fe2++ H2O2→Fe3++ OH−+⋅OH

(15) Fe3++ H2O2→Fe2++ HO2⋅+ H+

(16) Fe + 2Fe3+→3Fe2+

As the effect of fenton reaction on the degradation of pollutants is greatly affected by the external environment, some researchers explore the best process parameters under different experimental conditions. Wang et al. [69] studied the effects of temperature, pH, and H₂O₂ concentration on the degradation of rhodamine B. The results show that the complete degradation of rhodamine B can be achieved in about 10 min by adding 0.5 g Fe₇₈Si₉B₁₃^AR catalyst and 1.6 mol/L H₂O₂ to the solution of rhodamine B at the temperature of 295 K and pH = 3 and the concentration of 20 mg/L. The degradation process accords with the description of pseudo-first-order kinetic model. The strips still have good structural stability after being reused for three times, and there are no obvious performance loss. The most direct factor affecting the efficiency of oxidative degradation is the amount of ·OH. Properly increasing the amount of catalyst and H₂O₂ is helpful to the rapid formation of ·OH. Jia et al. [49] used Fe₇₈Si₉B₁₃^AR and Fe_73.5Si_13.5B₉Cu₁Nb₃^AR as catalysts to degrade brilliant red by fenton. As shown in Fig. 14, with the increase of amorphous ribbons and H₂O₂ content, the rate of catalytic degradation increases and the decolorization rate increases. According to Eq. (17), it can be understood that more effective Fe²⁺ ions can be extracted from the ribbons with higher fenton dosage. However, when the concentration of Fe²⁺ ions in the solution is too high, it will react with ·OH (Eq. (18)), and the consumption of the resulting ·OH leads to the decrease of degradation efficiency [70].(17) Fe + H2O2= Fe2++ 2OH−

(18) Fe2+ + ⋅OH = Fe3++ OH−

The fenton system can be carried out spontaneously without external energy. If the external energy can be input in the reaction process, the catalytic performance of the system can be effectively improved. For example, increasing the temperature of the system can significantly increase the reaction rate, because at high temperature, the reactant can easily cross the energy barrier to an active state capable of chemical reaction. At the same time, high temperature is beneficial to the diffusion of matter and accelerate the reaction [69]. Light radiation is also an effective way of energy input. When the fenton system is irradiated with high intensity light, the rate of photochemical reaction of Fe³⁺ ions to Fe²⁺ ions is accelerated through Eq. (19). This is because the unstable electrons of 4s² orbitals in amorphous Fe atoms are easily activated, resulting in rapid electron transfer [71]. In order to improve the light energy efficiency of MGs, Yang et al. [72] prepared composites MGs/TiO₂ by doping TiO₂ into Fe₇₈Si₉B₁₃^AP. Under visible light irradiation, the decolorization rate of methylene blue degraded by TiO₂ is 3 times higher than that of crystalline alloy (CA)/TiO₂ (Fig. 15a). The synergistic strengthening mechanism of MGs/TiO₂ is shown in Fig. 15b. The action mechanism of MGs is similar to that of ZVI, but it has enhanced performance. The catalyst is excited by photons, and the electrons transition from the valence band (VB) of TiO₂ to the conduction band (CB), resulting in electron-hole pairs. The electrons produced by TiO₂ prevent the formation of oxide layer on the surface of metallic glass by reducing Fe³⁺ ions to Fe²⁺ ions.(19) Fe(OH)2++ hv = ⋅OH+Fe2+