Physical auxiliary methods such as ultrasound, ultraviolet (UV) irradiation, magnetization, and ball milling are commonly used to activate zero-valent iron (ZVI), thereby significantly enhancing the reactivity of the system and effectively degrading pollutants. For instance, Pan et al.^[19] found that pre-magnetization could effectively promote the corrosion of zero-valent iron in the ZVI/H₂O₂ system by magnetizing zero-valent iron for 2 minutes under a static uniform magnetic field of 200 mT, thus improving its oxidative removal of antibiotics in water. Similarly, Du et al.^[49] discovered that the ZVI/H₂O₂ system, activated by a weak magnetic field, could basically remove sulfamethoxazole (SMX) from water within 15 minutes.. Pan et al.^[19] also enhanced the contaminant removal efficiency of the ZVI/H₂O₂ system by external UV irradiation. The results showed that within 60 minutes, the removal rate of SMT in water by the standalone ZVI/H₂O₂ system was less than 10%, whereas the removal rate of the UV/ZVI/H₂O₂ system significantly increased to 99.4%. Additionally, Ambika et al.^[47] significantly improved the reactivity of ZVI by ball milling (10 hours) to strip the passivation film on the ZVI surface, thereby enhancing the ability of the ZVI/H₂O₂ system to simultaneously treat Cr(VI) and phenol in wastewater (Figure 4a). Segura et al.^[31] utilized ultrasonic (5 minutes) assisted ZVI/H₂O₂ Fenton-like technology, which enhanced the reactivity of the ZVI/H₂O₂ system and greatly improved the mineralization rate of phenol in sewage. Chen et al.^[37] adopted microwave (MW) synergistic ZVI/H₂O₂ Fenton-like technology to treat high-concentration landfill leachate. The results indicated that at 14 minutes, compared with the ZVI/H₂O₂ system, the COD removal rate of the MW-ZVI/H₂O₂ system significantly increased from 17.90% to 76.38% (Figure 4b). Although external auxiliary means can achieve effective removal and mineralization of pollutants, these methods often rely on excessively low initial pH and are costly, limiting the large-scale application of physically assisted ZVI/H₂O₂ Fenton-like technology.

In recent years, nanomaterials have become a research hotspot in the field of environmental remediation. Among them, nanoscale zero-valent iron possesses outstanding advantages such as high surface area, rapid kinetics, high reactivity, and small particle size, and is used to construct advanced oxidation systems to degrade pollutants in water^［50-53］. For example, Zhang et al.^［54］ successfully synthesized nanoscale zero-valent iron by the borohydride reduction method. The results showed that the degradation rate of Norfloxacin (NOR) by the nZVI/H₂O₂ system was over 95% within 40 minutes, with a mineralization rate of about 50%. Zha et al.^［55］ used nanoscale zero-valent iron as a catalyst to enhance its reactivity with H₂O₂, thus oxidatively degrading 86.5% of Amoxicillin (AMX) in water and removing 71.2% of COD. Yi Yunqiang et al.^［56］ found that under the same reaction conditions, compared with the traditional Fenton method, the degradation rate of Metronidazole (MNZ) and the removal rate of TOC in the nZVI/H₂O₂ system were 1.5 times and 7.1 times those of the Fe²⁺/H₂O₂ system, respectively. Additionally, Pirsaheb et al.^［57］ successfully synthesized nanoscale zero-valent iron composites using Tragacanth gum extracted from Astragalus shrubs, effectively alleviating the bottleneck issue of nanoscale zero-valent iron being prone to agglomeration and deactivation. Within 60 minutes, the system's removal rate of AMX reached 90%. Similarly, Conde-Cid et al.^［58］ successfully synthesized nanoscale zero-valent iron using black tea extract, significantly enhancing the reaction activity. The results showed that the nZVI/H₂O₂ system could remove more than 90% of Sulfadiazine (SDZ) within 1 hour. Machado et al.^［59］ green-synthesized nanoscale zero-valent iron using oak leaf extract. The oak leaf extract enhanced the dispersion of zero-valent iron and the biodegradability of AMX, thereby enabling the nZVI/H₂O₂ system to completely remove AMX in the aqueous phase within 15 minutes (Figure 5a). Ouyang et al.^［60］ successfully green-synthesized nanoscale iron-based composites using tea polyphenols, effectively improving the utilization of iron, thereby efficiently degrading Lincomycin in water and effectively reducing its ecological toxicity (Figure 5b). Although nanoscale zero-valent iron has excellent performance, its effectiveness is affected by some drawbacks. For instance, nZVI has high surface energy, making it highly prone to agglomeration, poor air stability, and easy to cause secondary pollution during preparation. Among these, using green extracts to synthesize nanoscale zero-valent iron can effectively overcome the problem of causing secondary pollution during preparation, showing great application prospects^［57-60］.

Biochar (BC) has gained popularity in various fields due to its advantages such as low cost, high performance, and wide availability of production materials, showing a very promising development prospect^［61-64］. To improve the stability and dispersion of zero-valent iron (ZVI), thereby enhancing the reaction efficiency of the ZVI/H₂O₂ Fenton-like system, researchers have proposed using biochar as a carrier for zero-valent iron. For instance, Mao et al.^［63］ prepared biochar by pyrolyzing Miscanthus and fabricated biochar-supported nanoscale zero-valent iron (BC/nZVI) via a liquid-phase reduction precipitation method, effectively alleviating the severe agglomeration issue of nZVI particles, thus improving the degradation efficiency of the ZVI/H₂O₂ Fenton-like system for ciprofloxacin (CIP) in water, with a CIP removal rate above 70% (Figure 6a). Zhang et al.^［64］ prepared biochar from bamboo wood chips and synthesized biochar-supported nanoscale zero-valent iron (nZVI-BC) using traditional liquid-phase reduction methods. The characterization results of nZVI-BC showed that BC can enhance the adsorption of ornidazole (ONZ) antibiotics and alleviate the agglomeration of nZVI. Compared to the 48.3% ONZ removal rate in the nZVI/H₂O₂ system, the reaction activity of nZVI-BC with H₂O₂ is higher, with the nZVI-BC/H₂O₂ system removing over 63% of ONZ within 12 minutes, and when the nZVI/BC mass ratio is 1:2, the highest ONZ removal rate reaches 80.1% (Figure 6b). Despite the good loading performance and low cost of biochar, it still faces issues such as slow adsorption rates, large usage amounts, and potential secondary pollution caused by migration with water flow. Based on the enhancement of ZVI/H₂O₂ Fenton-like technology through biochar modification, future research could further focus on improving the adsorption performance of biochar and overcoming its potential to cause secondary pollution.

Advanced oxidation technology is one of the effective treatment methods for removing antibiotic pollutants in water because it generates highly oxidative radicals such as hydroxyl radicals (·OH) and sulfate radicals (SO₄^-·), which further attack and degrade pollutants^{［65, 66］}. Therefore, some researchers have improved the generation of highly oxidative radicals in the ZVI/H₂O₂ Fenton-like system by adding external oxidants, significantly enhancing the system's pollutant removal efficiency. For example, Wu et al.^［65］ significantly reduced the pH value of the ZVI/H₂O₂ Fenton-like system and simultaneously acidified the pollutants by externally adding persulfate (PS). Within 30 minutes, the degradation rate of SMT (50 mg/L) reached up to 96%, and another eight typical PPCPs (i.e., bisphenol A, indomethacin, norfloxacin, tetracycline, paracetamol, carbamazepine, phenacetin, and sulfamethoxazole) in the same water sample were also efficiently degraded (degradation rates of 77%~100%). Similarly, Li et al.^［66］ enhanced the corrosion rate of iron in the ZVI/H₂O₂ system and the decomposition rate of H₂O₂ by adding PS, significantly improving the removal efficiency of erythromycin (ERY) in wastewater. Within 90 minutes, the system completely degraded ERY (Figure 7). Although adding external oxidants can significantly enhance the pollutant removal efficiency of the ZVI/H₂O₂ Fenton-like system, it may cause secondary pollution from sulfate radicals, posing a threat to the ecological environment and human health.

One effective method to address the bottleneck issues of the Fenton reaction is to regulate the Fe³⁺/Fe²⁺ cycle by altering the coordination environment of iron, thereby promoting the generation of strongly oxidative radicals such as ·OH^{[53, 67]}. Currently, the most extensively studied approach is introducing complexing agents with multiple coordination sites to construct a complexing agent-Fenton-like system. Moreover, some complexing agents possess reducing capabilities themselves; introducing organic or inorganic reducing agents with reductive functions into Fenton or Fenton-like systems can promote the Fe³⁺/Fe²⁺ cycle and the generation of ·OH within the system, thus enhancing its pollution removal efficiency^{[68, 69]}. For instance, Ouyang et al.^[68] used tea polyphenols, which have both complexing and reducing abilities, as promoters to lower the pH of the reaction system and effectively promoted the Fe³⁺/Fe²⁺ cycle in the ZVI/H₂O₂ system, significantly improving the removal efficiency of lincomycin (Lincomycin, LCM). The pollutant could be degraded by more than 97% around 90 minutes, and compared with the ZVI/H₂O₂ system alone, its degradation rate constant increased about 26 times, while greatly reducing secondary pollution. Similarly, Ouyang et al.^[70] found that using oxalic acid, hydroxylamine hydrochloride, and ascorbic acid as reductive promoters could also reduce the system pH, promote iron corrosion, and effectively enhance the degradation efficiency of the ZVI/H₂O₂ system for LCM, completely degrading the pollutant within 90 minutes. Hwang et al.^[71] utilized ascorbic acid as a complexing agent to improve the reaction efficiency of the ZVI/H₂O₂ system, and results showed that the system could almost completely remove endosulfan from the aqueous phase within 24 hours. Additionally, the research group led by Fang Zhanqiang^[69] enhanced the ZVI/H₂O₂ Fenton-like system by externally adding tea extract with antioxidant properties. Results indicated that extracts from different types of tea all promoted the removal of NOR by the ZVI/H₂O₂ Fenton-like system, among which green tea extract (Green tea extract, GT) had the best effect on promoting the ZVI/H₂O₂ Fenton-like system. GT could effectively promote the Fe³⁺/Fe²⁺ cycle, increasing the NOR removal rate from 56.69% to 97.68%, while the reaction rate was improved by 5.3 times. The research group also used guava leaf extract and eucalyptus leaf extract as green promoters for the ZVI/H₂O₂ Fenton-like system, which significantly improved the reaction efficiency and overcame the bottleneck problems of low reagent utilization and narrow applicable pH range in the ZVI/H₂O₂ Fenton-like system (Fig. 8a, b))^{[72, 73]}. Cao et al.^[74] employed Moringa seed extract as a bioflocculant, effectively dispersing zero-valent iron in the system and improving the mineralization rate of pollutants in leachate (72.6%), which is 1.21 times higher compared to the ZVI/H₂O₂ system without Moringa seed extract (Fig. 8c). Zou Yachen et al.^[75] used gelatin extracted from animals as a coagulant aid, which could almost completely remove copper ions, nickel ions, and lead ions from wastewater within 60 minutes, and its sludge production was only 20% of the traditional Fenton system (Fig. 8d). Furthermore, Ling et al.^[76] introduced chloride ions into water by adding sodium chloride, accelerating the corrosion of ZVI by H₂O₂ and the formation of ROS, thereby improving the degradation efficiency of organic pollutants. Oral et al.^[77] used pyrite as a promoter, suppressing ZVI particle aggregation, significantly lowering solution pH, and enhancing iron dissolution, thereby markedly improving the removal of diclofenac by the ZVI/H₂O₂ system.

In summary, chelating agents can form relatively stable complexes with Fe³⁺ or Fe²⁺, thereby maintaining the solubility of Fe³⁺ and Fe²⁺ and enhancing the Fenton-like reaction activity. Furthermore, when the chelating agent added to the ZVI/H₂O₂ Fenton-like system has reducing capability, an appropriate amount of reducing chelating agent can effectively promote iron cycling and thus improve the Fenton-like reaction efficiency. When the introduced promoter contains acidic groups and exhibits acidity, it often reduces the pH value of the system, promotes iron corrosion, and effectively enhances the decontamination efficiency of the system. Additionally, coagulant aids can also effectively disperse zero-valent iron in the system, increase iron utilization, and thus enhance reaction efficiency. It is worth noting that using reducing green promoters (such as plant extracts) to enhance ZVI/H₂O₂ Fenton-like technology, such as tea polyphenols and tea extract, can often alleviate the agglomeration of iron, reduce the pH of the system, effectively promote iron cycling, and greatly reduce secondary pollution, showing great application potential. However, the promoters used in current research often have the problems of high cost or difficult preparation; therefore, future research can focus on finding low-cost, environmentally friendly, and easily prepared green promoters.

The surface of zero-valent iron (ZVI) is prone to passivation in the environment, which greatly increases the consumption of ZVI. Meanwhile, passivation significantly reduces the corrosion rate and reactivity of ZVI, making it difficult to continuously release Fe²⁺, reducing the removal efficiency of pollutants in the reaction system and leading to the formation of some iron sludge, thus limiting the large-scale application of ZVI/H₂O₂ Fenton-like technology ^[42,78-83]. Therefore, some researchers have adopted acid-washing modification of ZVI to remove the passivation layer on its surface, thereby improving its reactivity and pollution removal efficiency. Wang Qi et al. ^[84] applied acid-washing to remove the passivation film on the surface of ZVI powder, significantly enhancing the reactivity of ZVI. In the acid-washed ZVI/H₂O₂ system, aniline in water was completely degraded within 10 minutes, and the removal rates of chromium and antimony reached up to 99% at 90 minutes, achieving simultaneous removal of three pollutants in wastewater (Figure 9a). Liang et al. ^[35] found that acid-washed ZVI promoted the precipitation of Fe²⁺ and the generation of strong oxidizing radicals in the system, greatly improving the reactivity. Under the reaction condition of 40°C, the water content of the sludge treated by the acid-washed ZVI Fenton-like system decreased to 54.81 wt%, which was 18.47% less than that of the ZVI/H₂O₂ system, significantly improving the sludge dewatering performance (Figure 9b). Huang Ting et al. ^[48] showed that the rate of wastewater treatment by the acid-modified ZVI/H₂O₂ system was significantly improved, with a COD removal rate of 20% at 2 hours, effectively increasing the B/C ratio and improving the biodegradability of wastewater. Therefore, acid modification can significantly enhance the reactivity and pollution removal performance of ZVI, reduce the amount of H₂O₂ used, and decrease the production of iron sludge. However, there are non-negligible problems with the acid modification method, such as the consumption of large amounts of acid reagents, the loss of ZVI, and the secondary pollution easily caused by waste acid and iron salts after modification.

By strengthening the ZVI/H₂O₂ Fenton-like technology through metal doping, electron transfer reactions between different metals can promote the precipitation of Fe²⁺, enhance the catalytic activity and utilization of zero-valent iron as well as the generation of ·OH, thereby improving the reaction efficiency of the Fenton-like technology^[85]. Jiang et al.^[86] significantly improved the reactivity of ZVI by loading Cu metal particles on micrometer-sized zero-valent iron particles, greatly enhancing the removal efficiency of TC contaminants in aqueous solutions by the Fenton-like system, with a TC removal rate as high as 98% within 3 minutes. Chen et al.^[87] recovered manganese from the cathode materials of spent lithium-ion batteries for hydrothermal synthesis of MnO₂/ZVI composites, which significantly enhanced the catalytic activity of zero-valent iron, with the degradation rate of SDZ in the system reaching about 98.6% at 60 minutes (Figure 10a). Xia et al.^[88] prepared Fe-Cu bimetallic materials via a simple electrodeposition process as Fenton-like catalysts. The results showed that when Cu accounted for 25% of the material, the Cu heteroatoms enhanced the ability of adjacent Fe atoms to adsorb H₂O₂, thereby promoting the generation of more ·OH in the system, while independent Cu clusters also inhibited surface passivation of the catalyst, allowing the Fe-Cu/H₂O₂ system to remove over 99% of phenol within 30 minutes, and the catalyst could be reused sustainably up to 10 times (Figure 10b). Bao et al.^[89] introduced the lanthanide metal cerium (Ce) into the iron-based material (Fe-MIL-101), and the introduction of Ce accelerated electron transfer in the system, significantly improving the catalytic performance of the iron-based material; the degradation efficiency of NOR (10 mg/L) increased from 50.1% to 94.8% within 60 minutes. Although metal doping can significantly enhance catalytic activity, issues such as difficult preparation and high cost limit its large-scale application. By simply introducing other metal ions or synergistically removing wastewater containing metal ions, these problems can be overcome to a certain extent. For example, Yang Bo et al.^[90] promoted the corrosion of zero-valent iron and reduced sludge production by introducing copper ions into the ZVI/H₂O₂ system, and the synergistic effect of copper ions further promoted the generation of ·OH, significantly enhancing the reaction performance; the system could basically completely degrade methylene blue under near-neutral conditions.

In addition to the aforementioned enhancement methods, researchers have also strengthened ZVI/H₂O₂ Fenton-like technology by preparing metal-organic frameworks and synthesizing graphene composite iron-based materials. Xie et al.^[91] successfully prepared zero-valent iron metal-organic frameworks (Metal-organic frameworks, MOFs) embedded in a carbon-based structure, and obtained derivatives (Fe-MOF-based iron/carbon hybrid material, FMC) through direct pyrolysis of MOFs. The carbon structure promotes electron transfer and reaction activity within the system, thereby efficiently degrading and mineralizing AMX in water, with removal and mineralization rates of 100% and 60.41%, respectively, after 1 hour (Figure 11a). Li Yuhui et al.^[92] used waste activated carbon as the carbon source and prepared red mud-based zero-valent iron material (ZVI/RM) through a two-step method of reduction roasting-magnetic separation, improving the utilization rate of zero-valent iron. The system can completely degrade Rhodamine B and SDZ pollutants in water within 10 minutes. Additionally, Masud et al.^[93] synthesized graphene nano iron-based material (rGO-nZVI) using reduced graphene oxide (Reduced graphene oxide, rGO), significantly enhancing the reaction activity of the system and the adsorption performance of the material, achieving almost complete removal of 12 different PPCPs mixed in water within 10 minutes (Figure 11b).

Although individual modification techniques have shown good enhancement effects, they often present issues such as high energy consumption and cost. Therefore, researchers have proposed combining two or more modification techniques to overcome these problems. Pan et al^[19] built upon pre-magnetized zero-valent iron (pre-Fe⁰) with additional UV irradiation. The results showed that pre-magnetized ZVI could achieve a faster corrosion rate under UV irradiation. Applying the UV/pre-Fe⁰/H₂O₂ process could completely remove antibiotics such as SMT, oxytetracycline (OTC), and TC within 30 minutes, while the mineralization rate of SMT was as high as 92.1% (Fig. 12a). Tian et al^[94] enhanced the conventional ZVI-EF process by adding a certain amount of MoS₂, applying ultrasonic treatment for 20 seconds to disperse it, and then introducing air to excite the reaction in direct current mode under constant current conditions. The results indicated that this process could completely degrade SMT within 10 minutes, whereas the degradation rate of the traditional ZVI-EF process was only 19.4% (Fig. 12b). Huang Danwei et al^[95] prepared micron-scale sulfidated zero-valent iron (S-ZVI) through mechanical ball milling. SEM and XRD characterization results showed that FeS replaced the passivation film (iron oxide) on the surface of ZVI, accelerating electron transfer from ZVI to H₂O₂, thereby increasing the release rate of Fe²⁺. The constructed S-ZVI/H₂O₂ system could rapidly and thoroughly degrade all selected pollutants; tetracycline could be completely degraded in 20 seconds, and the apparent rate constant of phenol degradation was 51 times that of the ZVI/H₂O₂ system (Fig. 12c). Similarly, Feng et al^[96] prepared sulfidated zero-valent iron (S-ZVI) through mechanical ball milling to enhance its reactivity and efficiently activate H₂O₂ to oxidize various organics over a wide pH range. Mondal et al^[36] utilized UV irradiation in synergy with nZVI/H₂O₂ technology to remove CIP from water under near-neutral pH conditions. The results indicated that CIP could be completely removed within 25 minutes, with a relatively high mineralization rate (Fig. 12d). Conde-Cid et al^[58] green-synthesized nano-zero-valent iron using black tea extract and further employed photocatalytic-assisted nZVI/H₂O₂ systems, which could completely remove SDZ from water within 5 minutes (Fig. 12e). Wan et al^[97] synthesized Ce⁰/Fe⁰-reduced graphene oxide (Ce⁰/Fe⁰-RGO) nanocomposites using chemical reduction methods. RGO alleviated the agglomeration of Ce⁰ and Fe⁰, improving the catalyst's stability. The removal rate of SMT reached 99% at 30 minutes, with a mineralization rate as high as 73%.

In summary, different enhancement measures can effectively improve the reaction efficiency of ZVI/H₂O₂ Fenton-like technology for degrading organic pollutants through various pathways. Therefore, the main effects of the aforementioned enhancement measures on improving ZVI/H₂O₂ Fenton-like technology were summarized, and the results are shown in Figure 13. By analyzing Figure 13, it can be seen that the effects of different enhancement measures all include increasing the reactivity of zero-valent iron, while the modification pathways of zero-valent iron by different enhancement measures vary. Physical enhancement measures such as ball milling, magnetization, and ultrasonication improve its reactivity by promoting the corrosion of zero-valent iron; nano-scale zero-valent iron has advantages such as high surface area and high reactivity due to its ultrafine particle size; biochar loading can reduce the agglomeration of zero-valent iron and also has an adsorption effect on pollutants; adding oxidants improves its reactivity by promoting the corrosion of zero-valent iron and lowering the system's pH value; adding non-oxidizing promoters, based on the effects of added oxidants, often can improve the stability and dispersion of zero-valent iron and effectively promote iron cycling; acid washing enhances the corrosion rate of zero-valent iron by stripping its passivation film; metal doping enhancement measures not only improve the corrosion efficiency of zero-valent iron and accelerate electron transfer but also have a synergistic catalytic effect on the system by the doped metal itself; the synergistic action of multiple enhancement measures can combine the advantages of each measure to efficiently enhance the system's reactivity, but its operation is often more complex. In addition, these enhancement measures also have non-negligible problems, such as ecological risks associated with the preparation process and existence of nano-scale zero-valent iron itself; enhancement measures like adding oxidants, acid washing, metal doping, and biochar loading may cause secondary pollution; physical auxiliary measures such as ball milling, UV irradiation, and magnetization are too costly. In contrast, using green extracts (plant extracts) with chelating and reducing capabilities as promoters to enhance ZVI/H₂O₂ Fenton-like technology can reduce the system's pH value to some extent, effectively improve the coordination environment of iron and promote iron cycling effectively, while greatly reducing the generation of secondary pollution, having the advantages of low cost, high efficiency, and environmental friendliness, and possessing great application potential.