A core-shell type nano zero-valent iron material, a preparation method and application thereof

Abstract

The application provides a core-shell type nano zero-valent iron material, a preparation method and application, relates to the fields of environmental pollution remediation and nanometer material technology.The inner core of the material is phosphorus-doped nano zero-valent iron, the shell layer is a mixed layer of iron sulfide and iron oxide, and the particle has nano cracks extending from the core to the shell layer.The phosphorus doping causes lattice expansion and reduces the electron escape energy barrier, promoting the release of bulk phase electrons;the mixed layer of iron sulfide and iron oxide inhibits the hydrogen evolution side reaction and strengthens the electron transfer by virtue of hydrophobicity;the nano cracks continuously expose the internal active sites as mass transfer channels.Three synergies realize the efficient remediation of chlorinated organic matter contaminated groundwater or soil.

Description

Technical Field

[0001] This invention relates to the fields of environmental pollution remediation and nanomaterials technology, and in particular to a core-shell type nanomaterial with zero valent iron, its preparation method and application. Background Technology

[0002] Groundwater is an important freshwater resource globally, but with industrial development, chlorinated organic compounds (COPs, such as trichloroethylene (TCE) and tetrachloroethylene (PCE)) have become one of the most common, highly toxic, and difficult-to-degrade pollutants in groundwater.

[0003] Nano-zero valent iron (nZVI) is widely used as an "electron reservoir" for in-situ remediation of COPs pollution due to its excellent reduction potential. However, unmodified nZVI faces a difficult trade-off in practical applications, balancing reactivity, selectivity, and stability. (1) Electron transfer is hindered: Most surface modification strategies can only promote surface electron transfer and have little effect on the release of electrons from the zero-valent iron core; (2) Poor selectivity: The released electrons are prone to react with water to produce hydrogen evolution side reaction (HER), resulting in low electron utilization efficiency for degrading target pollutants; (3) Long-term activity decay: A dense oxide passivation layer is easily formed on the surface of nano zero-valent iron, which hinders the mass transfer channel and eventually leads to material deactivation.

[0004] While traditional surface modification or doping can alleviate the above problems to some extent, traditional surface modification is difficult to penetrate deep into the body-centered cubic (BCC) lattice to change its intrinsic electronic structure.

[0005] Therefore, there is an urgent need to develop a nanomaterial of zero-valent iron that can be synergistically optimized from multiple dimensions, including core electron release, interface electron transfer, and mass transfer channels, in order to achieve efficient, selective, and long-lasting reduction and dechlorination of chlorinated organic matter in groundwater. Summary of the Invention

[0006] To overcome the shortcomings of existing nano-zero-valent iron materials in remediating chlorinated organic pollution by simultaneously achieving high reactivity, selectivity, and stability, this invention employs a stepwise synthesis method involving pre-phosphating and post-sulfurization. This method allows phosphorus atoms to enter the crystal lattice during the nucleation stage of nano-zero-valent iron, lowering the electron escape energy barrier and promoting bulk electron release. Subsequently, a mixture of iron sulfide and iron oxide is formed on the particle surface to prevent water molecules from contacting the iron nucleus and to suppress hydrogen evolution side reactions. At the same time, the core-shell stress mismatch induces multiple Kirkendall effects to generate nanocracks, thereby opening up mass transfer channels and achieving a synergistic improvement in the material's reactivity, selectivity, and stability.

[0007] One objective of this invention is to provide a core-shell type nano-zero-valent iron material, wherein the core of the material is phosphorus-doped nano-zero-valent iron, and the shell is a mixture of iron sulfide and iron oxide. The material has nanocracks extending from the core to the shell.

[0008] Preferably, the molar ratio of phosphorus to iron in the core is (3~8):100.

[0009] Preferably, the molar ratio of phosphorus to iron in the core is 5:100.

[0010] Preferably, the molar ratio of sulfur in the shell to phosphorus in the core is (1~4):1.

[0011] Preferably, the molar ratio of sulfur in the shell to phosphorus in the core is 2:1.

[0012] Preferably, the average particle size of the material is 50-100 nm; and / or, The thickness of the shell is 2.6 ~ 4.5 nm.

[0013] The second objective of this invention is to provide a method for preparing core-shell type nano-zero-valent iron materials, comprising the following steps: S1. Under a protective atmosphere, a reducing agent is added to a solution containing an iron source and a phosphorus source to carry out a reduction reaction and form a phosphorus-doped nano-zero-valent iron suspension. S2. Add a sulfur source solution to the phosphorus-doped nano-zero-valent iron suspension to react, so that a mixture of iron sulfide and iron oxide is formed on the surface of the phosphorus-doped nano-zero-valent iron, to obtain a core-shell nano-zero-valent iron material with phosphorus doping and sulfur coating.

[0014] Preferably, the iron source in step S1 includes ferrous chloride and / or ferrous sulfate; and / or, The phosphorus source includes at least one of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate; and / or, The reducing agent includes sodium borohydride and / or potassium borohydride.

[0015] Preferably, the sulfur source in step S2 includes sodium sulfide and / or sodium dithionite.

[0016] Preferably, the molar ratio of phosphorus in the phosphorus source to iron in the iron source in step S1 is (3~8):100.

[0017] Preferably, the molar ratio of phosphorus in the phosphorus source to iron in the iron source in step S1 is 5:100.

[0018] Preferably, the molar ratio of sulfur in the sulfur source to phosphorus in the phosphorus source in step S2 is (1~4):1.

[0019] The third objective of this invention is to provide an application of core-shell nano-zero-valent iron material in the remediation of groundwater or soil contaminated by chlorinated organic compounds, wherein the chlorinated organic compounds include at least one of trichloroethylene, tetrachloroethylene, carbon tetrachloride, or chloroform.

[0020] The beneficial effects of this invention are: The core-shell nano-zero-valent iron material provided by this invention achieves a synergistic effect of three factors: the phosphorus-doped core, the mixed layer of iron sulfide and iron oxide on the outer shell, and the nanocracks induced by stress mismatch in the two layers, thus realizing a simultaneous and significant improvement in reactivity, selectivity, and stability.

[0021] Specifically, phosphorus atoms entering the zero-valent iron lattice cause expansion and lower the electron escape energy barrier, promoting the release of bulk electrons and increasing the trichloroethylene degradation rate to 18.9 times that of unmodified nano-zero-valent iron. The mixture of iron sulfide and iron oxide utilizes its hydrophobicity to prevent water molecules from contacting the iron core, thus suppressing hydrogen evolution side reactions. Simultaneously, its conductivity efficiently guides the electrons released from the core to the target pollutants, increasing electron utilization efficiency to 27.8 times that of unmodified nano-zero-valent iron. Nanocracks act as mass transfer channels, continuously exposing internal active sites and promoting the diffusion of reactant molecules into the particle interior and the effective transfer of electrons to the reaction interface. The synergistic effect of these three factors enables the material to exhibit excellent degradation performance in the remediation of groundwater and soil contaminated with chlorinated organic compounds. Attached Figure Description

[0022] Figure 1 SEM images of different materials prepared in Examples 1-3 and Comparative Examples 1-3; Wherein (a) is the SEM image of nZVI obtained in Comparative Example 1; (b) is the SEM image of P-nZVI obtained in Comparative Example 2; and (c) is the SEM image of S-nZVI obtained in Comparative Example 3. post SEM image of -nZVI; (d) is the SEM image obtained in Example 2. post SEM image of / P-nZVI(1); (e) is the SEM image obtained in Example 1. post SEM image of / P-nZVI(2); (f) is the SEM image obtained in Example 3. post SEM images of / P-nZVI(4); Figure 2 HRTEM images of different materials prepared in Examples 1-3 and Comparative Examples 1-3; (a) is the HRTEM image of nZVI obtained in Comparative Example 1; (b) is the HRTEM image of P-nZVI obtained in Comparative Example 2; and (c) is the HRTEM image of S-nZVI obtained in Comparative Example 3. post HRTEM image of -nZVI; (d) is the S obtained in Example 2. postHRTEM image of / P-nZVI(1); (e) S prepared in Example 1 post HRTEM image of / P-nZVI(2); (f) is the S obtained in Example 3. post HRTEM image of / P-nZVI(4); Figure 3 The XRD patterns of different materials prepared in Examples 1-3 and Comparative Examples 1-3 are shown. Figure 4 This is a comparison diagram of the lattice constants of different materials prepared in Examples 1-3 and Comparative Examples 1-3; Figure 5 The results of BET specific surface area and water contact angle tests for different materials prepared in Examples 1-3 and Comparative Examples 1-3 are shown. Figure 6 The degradation kinetics curves of trichloroethylene by different materials prepared in Examples 1-3 and Comparative Examples 1-3 are shown. Figure 7 The graph shows a comparison of the degradation performance of trichloroethylene by different materials prepared in Examples 1, 4, 5, 6, 7 and Comparative Examples 4, 5. Figure 8 S obtained in Example 1 post / P-nZVI(2) Broad-spectrum degradation efficiency of trichloroethylene, tetrachloroethylene, carbon tetrachloride and chloroform. Detailed Implementation

[0023] The present application will now be described in further detail with reference to embodiments. In the following description, certain specific details are included to provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented without employing one or more of these specific details, but using other methods, components, materials, etc. Unless otherwise required by the present invention, the terms "comprising" and "including" should be interpreted in an open-ended, inclusive sense, meaning "including but not limited to". Throughout this specification, "an embodiment," "an embodiment," "a preferred embodiment," or "some embodiments" means that at least one embodiment includes a specific reference element, structure, or feature related to that embodiment. Therefore, the phrases "in an embodiment," "in an embodiment," "in a preferred embodiment," or "in some embodiments" appearing in different places throughout the specification do not necessarily all refer to the same embodiment. Furthermore, specific elements, structures, or features may be combined in one or more embodiments in any suitable manner.

[0024] According to a first aspect of the present invention, a core-shell nanomaterial for zero-valent iron is provided, wherein the core of the material is phosphorus-doped nano-zero-valent iron; the shell is a mixture of iron sulfide and iron oxide; and the material has nanocracks extending from the core to the shell.

[0025] In this invention, phosphorus-doped nano zero-valent iron (P-nZVI) refers to the introduction of a phosphorus source during the nucleation stage of nano zero-valent iron, so that phosphorus atoms are introduced into the zero-valent iron lattice, forming a doped nanomaterial with zero-valent iron as the main body and phosphorus atoms doped inside the lattice. The core of this invention employs phosphorus doping. By introducing phosphorus atoms into the body-centered cubic lattice during the zero-valent iron nucleation stage, lattice expansion is induced, lowering the electron escape energy barrier and thus opening a channel for the release of bulk electrons to the surface, solving the core bottleneck of electron transfer obstruction in traditional materials. The shell is a mixture of iron sulfide and iron oxide. Its hydrophobicity prevents water molecules from contacting the iron core, thereby suppressing hydrogen evolution side reactions. At the same time, its conductivity efficiently guides the electrons released from the core to the particle surface for use by pollutants, solving the problem of poor reaction selectivity. The nanocracks extending from the core to the shell inside the particle originate from the core-shell stress mismatch between the lattice expansion stress caused by phosphorus doping and the binding force of the mixture of iron sulfide and iron oxide. This stress induces multiple Kirkendall effects, spontaneously forming mass transfer channels that connect the inside and outside, continuously exposing internal active sites. Combined with the antioxidant effect of the sulfide shell, this solves the stability problem of long-term activity decay in materials.

[0026] In summary, phosphorus doping promotes electron release, the mixture of iron sulfide and iron oxide inhibits side reactions and enhances electron transfer, and radial cracks facilitate mass transfer and expose active sites. The synergistic effect of these three factors enables core-shell nano-zero-valent iron materials to exhibit excellent degradation performance in the remediation of groundwater and soil contaminated with chlorinated organics.

[0027] In a preferred embodiment of the present invention, the molar ratio of phosphorus to iron in the core is (3~8):100. For example, it is 3:100, 4:100, 5:100, 6:100, 7:100 or 8:100.

[0028] If the molar ratio of phosphorus to iron in the core is less than 3:100, the phosphorus doping level is too low, and the concentration of phosphorus atoms in the zero-valent iron lattice is insufficient to induce effective lattice expansion. The Fe-Fe bond length change is not significant, the electron escape energy barrier of the iron core cannot be sufficiently reduced, and the improvement in bulk electron release capability is limited. Simultaneously, insufficient lattice expansion stress will lead to insufficient core-shell stress mismatch between the subsequent mixture of iron sulfide and iron oxide layers, making it difficult to induce a strong multiple Kirkendall effect. The density and extension of nanocracks are insufficient, and the construction effect of mass transfer channels is greatly reduced.

[0029] If the molar ratio of phosphorus to iron in the core is higher than 8:100, the phosphorus doping level is too high. Excessive phosphorus atoms entering the crystal lattice will cause severe lattice distortion and may even generate iron phosphide (FeP, Fe2P, etc.) impurities instead of maintaining the phosphorus doping state in the crystal lattice. The formation of impurities will reduce the effective content of zero-valent iron and weaken the material's reductive dechlorination activity. In addition, excessively high phosphorus doping levels will also lead to increased particle agglomeration and uneven particle size distribution during nucleation, affecting the overall specific surface area and dispersibility of the material.

[0030] The present invention preferably controls the molar ratio of phosphorus to iron within the range of (3~8):100, which ensures that sufficient phosphorus atoms enter the crystal lattice to cause moderate expansion and fully reduce the work function to promote the release of bulk electrons, while avoiding excessive lattice distortion and impurity phase generation caused by excessive doping, thus achieving the optimization of lattice engineering.

[0031] In a preferred embodiment of the present invention, the molar ratio of phosphorus to iron in the core is 5:100.

[0032] When the molar ratio of phosphorus to iron is 5:100, the phosphorus-induced lattice expansion stress and the subsequent sulfidation-formed shell achieve optimal stress mismatch, thus inducing the most complete multiple Kirkendall effect and nanocracks. Experiments show that when the molar ratio of phosphorus to iron is 5:100, the material exhibits the best overall performance: the lattice constant increases from 2.855 Å to 2.867 Å, ​​the TCE dechlorination reaction rate is 18.9 times that of unmodified nZVI, and the electron utilization efficiency reaches 36.9%, all superior to other ratios.

[0033] In a preferred embodiment of the present invention, the molar ratio of sulfur in the shell to phosphorus in the core is (1~4):1; for example, 1:1, 2:1, 3:1 or 4:1.

[0034] The amount of sulfur added is measured based on the amount of phosphorus doping, rather than directly on the molar amount of iron, because the amount of phosphorus doping determines the degree of lattice expansion, i.e. the magnitude of the expansion stress of the core; while the amount of sulfur added determines the thickness, density, and binding force of the shell. Only by matching and controlling the amount of sulfur relative to the amount of phosphorus can the optimal matching relationship between the core expansion stress and the shell binding force be obtained, thereby inducing a full multiple Kirkendall effect.

[0035] If the molar ratio of sulfur in the shell to phosphorus in the core is less than 1:1, the sulfur doping level is too low. The resulting mixture of iron sulfide and iron oxide on the surface of the P-nZVI particles is too thin or discontinuous, failing to form a complete hydrophobic barrier. Water molecules can still contact the iron core surface, the hydrogen evolution side reaction cannot be effectively suppressed, and electron utilization efficiency is difficult to improve. Simultaneously, insufficient shell porosity results in a still strong binding force on the core, and the core-shell stress mismatch is insufficient to induce a full Kirkendall effect.

[0036] If the molar ratio of sulfur in the shell to phosphorus in the core is greater than 4:1, the sulfur doping is too high. Excessive sulfur ions continuously react on the particle surface, resulting in an excessively thick layer of mixed iron sulfide and iron oxide. Although a complete hydrophobic layer can be formed, it will hinder the outward transfer of zero-valent iron electrons from the core, reducing the material's reduction activity. In addition, excessive sulfur may also cause the iron core to be over-etched, destroying the integrity of the core-shell structure, and even generating a large amount of free iron sulfide impurities, weakening the overall performance of the material.

[0037] The present invention preferably controls the molar ratio of sulfur in the shell to phosphorus in the core within the range of (1~4):1. This ensures that a continuous and complete mixture of iron sulfide and iron oxide is formed on the particle surface to effectively suppress hydrogen evolution side reactions and generate a moderate stress mismatch, while avoiding an excessively thick shell that hinders electron transfer or excessive etching of the iron core, thus achieving the optimization of surface modification.

[0038] In a preferred embodiment of the present invention, the molar ratio of sulfur in the shell to phosphorus in the core is 2:1.

[0039] When the molar ratio of sulfur in the shell to phosphorus in the core is 2:1, the density and thickness of the sulfurized shell are moderate, forming an optimal stress mismatch with the phosphorus-doped expanded core, inducing sufficient multiple Kirkendall effects, while achieving an optimal balance between hydrophobicity and conductivity. Experiments show that the material's overall performance is optimal when the S / P molar ratio is 2:1.

[0040] In a preferred embodiment of the present invention, the average particle size of the material is 50-100 nm; and / or, The thickness of the shell is 2.6 ~ 4.5 nm.

[0041] If the particle size is less than 50 nm, the surface energy of the particles is too high, making them prone to aggregation and difficult to magnetically separate. Simultaneously, the stress mismatch effect between the sulfide shell and the phosphorus-doped core is not significant, making it difficult to form fully developed nanocracks. If the particle size is greater than 100 nm, the specific surface area decreases significantly, the number of surface active sites decreases, and the degradation reaction rate declines. Therefore, a particle size range of 50–100 nm can ensure sufficient specific surface area and abundant surface active sites, while also ensuring sufficient stress mismatch between the core and shell to induce crack formation, while simultaneously considering material dispersibility and magnetic separation and recovery performance.

[0042] According to a second aspect of the present invention, a method for preparing a core-shell type nanomaterial of zero valent iron is provided, comprising the following steps: S1. Under a protective atmosphere, a reducing agent is added to a solution containing an iron source and a phosphorus source to carry out a reduction reaction and form a phosphorus-doped nano-zero-valent iron suspension. S2. Add a sulfur source solution to the phosphorus-doped nano-zero-valent iron suspension to react, so that a mixture of iron sulfide and iron oxide is formed on the surface of the phosphorus-doped nano-zero-valent iron, and a core-shell nano-zero-valent iron material with phosphorus doping and sulfur coating is obtained.

[0043] In this invention, step S1 involves reducing both the phosphorus and iron sources during the nano-zero-valent iron nucleation stage, allowing phosphorus atoms to enter the crystal lattice during the zero-valent iron crystallization process, thus achieving body-centered cubic lattice doping. After phosphorus doping, the lattice expands, the Fe-Fe bond length increases, and the electron escape barrier of the iron core is lowered, making it easier for bulk electrons to be released.

[0044] Secondly, relying solely on lattice doping to promote electron release is insufficient if the electrons are ultimately consumed by water molecules, thus failing to achieve efficient degradation of the target pollutants. In step S2 of this invention, a sulfur source is introduced after the formation of P-nZVI particles, causing sulfur ions to grow in situ on the particle surface, forming a mixture of iron sulfide and iron oxide. This shell is hydrophobic, effectively preventing water molecules from contacting the iron core, thereby suppressing the hydrogen evolution side reaction. Simultaneously, the iron sulfide is conductive, efficiently guiding the electrons released from the core to the particle surface for use by the target pollutants, thus improving electron utilization efficiency.

[0045] Finally, the core expansion stress induced by phosphorus doping and the low binding force of the mixed iron sulfide and iron oxide shell create a core-shell stress mismatch. This stress mismatch induces multiple Kirkendall effects within the particle, generating numerous nanocracks extending from the core to the shell. These cracks not only increase the specific surface area and shorten the mass transfer distance but also continuously expose the internal active sites. Combined with the antioxidant effect of the external mixed iron sulfide and iron oxide layer, this gives the material long-term stability.

[0046] In summary, step S1 addresses the electron release problem, while step S2 addresses the electron utilization and mass transfer problem; these two steps are performed separately rather than simultaneously. If phosphorus and sulfur are introduced simultaneously in one step, phosphorus will have difficulty preferentially entering the crystal lattice to play a doping role, sulfur will be unable to form a surface shell in a directional manner, and core-shell stress mismatch will not be generated to induce multiple Kirkendall effects, thus the synergistic effect of the three will not be achieved.

[0047] In a preferred embodiment of the present invention, the iron source includes ferrous chloride and / or ferrous sulfate; the iron source is, for example, ferrous chloride, ferrous sulfate, ferrous chloride, and ferrous sulfate; and / or, The phosphorus source includes at least one of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate; for example, the phosphorus source is any one of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate; or, the phosphorus source is a mixture of sodium dihydrogen phosphate and potassium dihydrogen phosphate, a mixture of sodium dihydrogen phosphate and disodium hydrogen phosphate, a mixture of sodium dihydrogen phosphate and dipotassium hydrogen phosphate, a mixture of potassium dihydrogen phosphate and disodium hydrogen phosphate, or a mixture of disodium hydrogen phosphate and potassium dihydrogen phosphate. Any one of the following mixtures of dipotassium dihydrogen phosphate; or, the phosphorus source is any one of the following mixtures of sodium dihydrogen phosphate, potassium dihydrogen phosphate, and disodium hydrogen phosphate; or, the phosphorus source is any one of the following mixtures of sodium dihydrogen phosphate, potassium dihydrogen phosphate, and disodium hydrogen phosphate; or, the phosphorus source is any one of the following mixtures of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, and disodium hydrogen phosphate; and / or, The reducing agent includes sodium borohydride and / or potassium borohydride; the reducing agent is, for example, sodium borohydride, potassium borohydride, and sodium borohydride.

[0048] Ferrous iron in the iron source can be directly reduced to zero-valent iron by sodium borohydride and / or potassium borohydride during liquid-phase reduction, making the reaction pathway simple and efficient. Chloride ions and sulfate ions are common companion ions that do not affect the reduction reaction process and are easily removed by subsequent washing, making them inexpensive and readily available.

[0049] Sodium dihydrogen phosphate (NaH2PO4), potassium dihydrogen phosphate (KH2PO4), and disodium hydrogen phosphate (Na2HPO4) or dipotassium hydrogen phosphate (K2HPO4) can all dissociate into PO43 in solution. - or HPO4² - / H2PO4 - Under alkaline reducing conditions, it reacts with Fe²⁺. + They coexist and can be uniformly dispersed in the reaction system in anionic form. When zero-valent iron nucleates, phosphorus atoms can enter the body-centered cubic lattice along with the iron crystallization process, achieving in-situ lattice doping. Furthermore, the selected phosphorus sources are all common analytical-grade reagents with good solubility, suitable for large-scale applications.

[0050] Boronhydrides have extremely strong reducing power and can reduce Fe²⁺. + Rapid reduction to zero-valent iron meets the conditions for room temperature liquid-phase synthesis; the reduction byproduct metaborate is highly water-soluble and easy to wash away, and sodium / potassium borohydride can slowly release hydrogen bubbles in aqueous solution, which plays a role in stirring and dispersing the reaction system and helps to obtain nanoparticles with uniform particle size.

[0051] In a preferred embodiment of the present invention, the sulfur source in step S2 includes sodium sulfide and / or sodium dithionite.

[0052] Sodium sulfide dissociates directly into S² when dissolved in water. - At room temperature, it reacts with iron species (Fe²⁺) on the surface of P-nZVI particles. + / Fe³ + The reaction rapidly produces iron sulfide (FeS). x The reaction conditions are mild and the shell formation efficiency is high; moreover, sodium sulfide is a common analytical grade reagent, which is inexpensive, readily available, and suitable for large-scale production.

[0053] Sodium dithionite has reducing properties and can slowly decompose in water to produce S²⁺. - or S2O4² - In the reaction system, it can both provide a sulfur source to generate a mixture of iron sulfide and iron oxide, and maintain a reducing atmosphere to prevent the nano-zero-valent iron from being oxidized during sulfidation, thus helping to maintain the high activity of the core zero-valent iron. Meanwhile, the reaction byproducts of sodium dithionite are highly water-soluble and easily removed by washing.

[0054] Both sodium sulfide and sodium dithionite can provide active sulfur species (S²) in aqueous solution at room temperature. - ), reacts with the P-nZVI surface to generate FeS x Thin-shelled; both are soluble salts that can be uniformly added to the reaction system in step S2 in solution form, ensuring that the sulfidation reaction occurs uniformly on the particle surface, forming a continuous, complete and controllable thickness mixture layer of iron sulfide and iron oxide.

[0055] In a preferred embodiment of the present invention, the molar ratio of phosphorus in the phosphorus source to iron in the iron source in step S1 is (3~8):100. For example, it is 3:100, 4:100, 5:100, 6:100, 7:100 or 8:100.

[0056] In a preferred embodiment of the present invention, the molar ratio of phosphorus in the phosphorus source to iron in the iron source in step S1 is 5:100.

[0057] In a preferred embodiment of the present invention, the molar ratio of sulfur in the sulfur source to phosphorus in the phosphorus source in step S2 is (1~4):1. For example, it is 1:1, 2:1, 3:1 or 4:1.

[0058] In a preferred embodiment of the present invention, the molar ratio of sulfur in the sulfur source to phosphorus in the phosphorus source in step S2 is (1~4):1.

[0059] In a preferred embodiment of the present invention, the preparation method of core-shell type nano-zero-valent iron material specifically includes the following steps: S1. Dissolve the iron source and the phosphorus source in water so that the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (3~8):100, to obtain a mixed solution containing the iron source and the phosphorus source; prepare a reducing agent solution. The mixed solution is placed in a reaction vessel and a protective atmosphere is introduced to remove oxygen for 20-60 minutes; the protective atmosphere includes nitrogen and / or argon; the reducing agent solution is added dropwise at a rate of 1-5 mL / min under stirring, preferably at a rate of 2-3 mL / min; a black suspension is produced by the reaction, and stirring is continued for 20-60 minutes, preferably at a stirring rate of 400-800 rpm, so that phosphorus element enters the zero-valent iron lattice during the nucleation process of nano-zero-valent iron, and a phosphorus-doped nano-zero-valent iron suspension is obtained; The iron source includes ferrous chloride and / or ferrous sulfate; the phosphorus source includes at least one of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate; the reducing agent includes sodium borohydride and / or potassium borohydride. S2. Prepare a sulfur source solution; the sulfur source includes sodium sulfide and / or sodium dithionite; The sulfur source solution is added dropwise to the phosphorus-doped nano-zero-valent iron suspension at a rate of 1~5 mL / min, so that the molar ratio of sulfur element in the sulfur source to phosphorus element in the phosphorus source in step S1 is (1~4):1. The reaction is continued to be stirred for 0.5~2 hours, so that sulfur ions react on the surface of phosphorus-doped nano-zero-valent iron to form a mixture of iron sulfide and iron oxide. The reaction product was magnetically separated, washed 3-5 times with oxygen-free deionized water, then washed 2-3 times with anhydrous ethanol, and then freeze-dried under vacuum for 12-24 hours to obtain a core-shell nano-zero-valent iron material with phosphorus doping and sulfur coating.

[0060] According to a third aspect of the present invention, there is an application of a core-shell nano-zero-valent iron material in the remediation of groundwater or soil contaminated with chlorinated organics, said chlorinated organics including at least one of trichloroethylene, tetrachloroethylene, carbon tetrachloride or chloroform.

[0061] Example 1 S postPreparation of P-nZVI(2) materials S1. Prepare a 0.1 M FeCl2 solution as the iron source; prepare a KH2PO4 solution as the phosphorus source, making the P / Fe molar ratio 5%; prepare an excess of NaBH4 solution as the reducing agent; mix the iron and phosphorus source solutions in a three-necked flask and purge with high-purity nitrogen for 30 minutes to remove oxygen. Under magnetic stirring (600 rpm), slowly and uniformly add the NaBH4 solution, producing a black suspension. Continue stirring for 30 minutes to allow phosphorus atoms to fully participate in iron nucleation, obtaining a phosphorus-doped nano-zero-valent iron (P-nZVI) suspension.

[0062] S2. Prepare a Na2S solution as a sulfur source, with an S / P molar ratio of 2:1; add the Na2S solution dropwise to the above P-nZVI suspension, and continue stirring for 1 hour to allow sulfur ions to react on the particle surface to form a thin shell of iron sulfide. Post-processing: The reaction products were separated using a magnet, washed three times each with oxygen-free deionized water and anhydrous ethanol, and then dried in a vacuum freeze dryer for 24 hours to obtain a core-shell nanomaterial of zero-valent iron with phosphorus doping and sulfur coating, denoted as S. post / P-nZVI(2).

[0063] Example 2 S post Preparation of P-nZVI(1) material The difference from Example 1 is that the Na₂S solution was prepared with an S / P molar ratio of 1:1, while all other parameters and operating procedures were the same as in Example 1. This yields a material with an S / P molar ratio of 1:1, denoted as S. post / P-nZVI(1).

[0064] Example 3 S post Preparation of P-nZVI(4) materials The difference from Example 1 is that the Na₂S solution was prepared with an S / P molar ratio of 4:1, while all other parameters and operating procedures were the same as in Example 1. The resulting material with an S / P molar ratio of 4:1 is denoted as S. post / P-nZVI(4).

[0065] Example 4 S post Preparation of / P-nZVI(P3) material The difference from Example 1 is as follows: S1. Prepare a 0.1 M FeSO4 solution as an iron source; prepare a NaH2PO4 solution as a phosphorus source, so that the P / Fe molar ratio is 3%; prepare an excess of KBH4 solution as a reducing agent; prepare a Na2S2O4 solution as a sulfur source, so that the S / P molar ratio is 2:1. Iron and phosphorus source solutions were mixed in a three-necked flask and deoxygenated by purging with high-purity argon gas for 30 minutes. KBH4 solution was slowly and uniformly added dropwise under magnetic stirring (500 rpm), producing a black suspension. Stirring was continued for 40 minutes to obtain phosphorus-doped nano-zero-valent iron (P-nZVI).

[0066] S2. Add Na2S2O4 solution dropwise to the above P-nZVI suspension and continue stirring for 1.5 hours to allow sulfur ions to react on the particle surface to form a mixture of iron sulfide and iron oxide. The post-processing operation is the same as in Example 1, resulting in a material with a P / Fe molar ratio of 3% using the FeSO4 / NaH2PO4 / KBH4 / Na2S2O4 system, denoted as S. post / P-nZVI(P3).

[0067] Example 5 S post Preparation of / P-nZVI(P8) material The difference from Example 1 is as follows: S1. Prepare a 0.1 M FeCl2 solution as an iron source; prepare a K2HPO4 solution as a phosphorus source, so that the P / Fe molar ratio is 8%; prepare an excess NaBH4 solution as a reducing agent; prepare a Na2S solution as a sulfur source, so that the S / P molar ratio is 2:1. Iron and phosphorus source solutions were mixed in a three-necked flask and deoxygenated by purging with high-purity nitrogen for 40 minutes. NaBH4 solution was slowly and uniformly added dropwise under magnetic stirring (700 rpm), and a black suspension was produced. Stirring was continued for 20 minutes to obtain phosphorus-doped nano-zero-valent iron (P-nZVI).

[0068] Step S2 is performed in the same manner as in Example 1, resulting in a material with a P / Fe molar ratio of 8% using the FeCl2 / K2HPO4 system, denoted as S. post / P-nZVI(P8).

[0069] Example 6 S post Preparation of P-nZVI(2)-FeSO4 materials The difference from Example 1 is that FeSO4 is used instead of FeCl2 as the iron source, the P / Fe molar ratio remains 5%, and the S / P molar ratio remains 2:1; all other parameters and operating procedures are the same as in Example 1. The resulting material using FeSO4 as the iron source is denoted as S. post / P-nZVI(2)-FeSO4.

[0070] Example 7 S post Preparation of P-nZVI(2)-Na2S2O4 material The difference from Example 1 is that Na2S2O4 is used instead of Na2S as the sulfur source, while the S / P molar ratio remains 2:1; all other parameters and operating procedures are the same as in Example 1. Thus, a material using Na2S2O4 as the sulfur source is obtained, denoted as S. post / P-nZVI(2)-Na2S2O4.

[0071] Comparative Example 1 Preparation of unmodified nano-zero valent iron (nZVI) Prepare a 0.1 M FeCl2 solution as the iron source; prepare an excess NaBH4 solution as the reducing agent; The iron source solution was placed in a three-necked flask and high-purity nitrogen was purged for 30 minutes to remove oxygen. NaBH4 solution was then slowly and uniformly added dropwise under magnetic stirring (600 rpm). A black suspension was produced, and stirring continued for another 30 minutes. The reaction products were separated using a magnet, washed three times each with oxygen-free deionized water and anhydrous ethanol, and then dried in a vacuum freeze dryer to obtain unmodified nano-zero-valent iron material, denoted as nZVI.

[0072] Comparative Example 2 Preparation of phosphorus-modified nano-zero-valent iron (P-nZVI) alone The difference from Example 1 is that the post-sulfurization in step S2 is not performed; that is, post-processing is performed directly after step S1. All other parameters and operating steps are the same as in Example 1. This yields a material with only phosphorus doping and no sulfur shell, denoted as P-nZVI.

[0073] Comparative Example 3 Sulfur-modified nano-zero-valent iron (S) post Preparation of -nZVI) The difference from Example 1 is that no phosphorus source is added or pre-phosphating is performed. Instead, the iron source solution is directly reduced to obtain an nZVI suspension, followed by post-sulfurization in step S2. All other steps are the same as in Example 1. This yields a material modified only by sulfur and without phosphorus doping, denoted as S. post -nZVI.

[0074] Comparative Example 4 Preparation of one-step sulfur-phosphorus co-modified nano-zero-valent iron (S / P-nZVI) The difference from Example 1 is that the stepwise synthesis is not performed. Instead, the iron, phosphorus, and sulfur sources are mixed, and then the reducing agent is added all at once to carry out the reduction reaction. This allows sulfur and phosphorus to participate in the nucleation process of nano-zero valent iron simultaneously, without any order of reaction. All other parameters (P / Fe=5%, S / P=2:1) ​​and post-processing operations are the same as in Example 1. The resulting one-step co-doped material is denoted as S / P-nZVI.

[0075] Comparative Example 5 Post-phosphating and pre-sulfidation of nano-zero valent iron (P) post Preparation of / S-nZVI) The difference from Example 1 is that the order of introducing sulfur and phosphorus is reversed; only iron and sulfur sources are prepared, without adding a phosphorus source, and pre-sulfurization (S / Fe = 10%) is performed first. After the reduction reaction yields a sulfur-doped nano-zero-valent iron suspension, KH₂PO₄ solution (P / Fe = 5%) is added dropwise in step S2 for post-phosphating. All other operation steps are the same as in Example 1. The resulting material, with the sulfidation followed by phosphating reversed, is denoted as P. post / S-nZVI.

[0076] The materials obtained in Examples 1-3 and Comparative Examples 1-3 were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM).

[0077] SEM images such as Figure 1 As shown, nZVI and S post -nZVI particles are all spherical and retain their chain-like structure. After vulcanization, a FeS shell layer forms on the surface of nZVI, which slightly increases the particle size but effectively suppresses agglomeration, thereby improving particle dispersibility. In contrast, P-nZVI still retains some chain-like structure, presenting as irregular spherical particles with surface defects and cracks. Its spherical outline is slightly deformed, the edges are irregular, and the surface roughness is significantly increased. post / P-nZVI still retains some chain-like and aggregated structures, the particle surface is relatively rough, and some particles show signs of breakage, which is due to radial cracks extending from the outer shell to the iron core.

[0078] HRTEM images such as Figure 2 As shown, all nZVI particles exhibit a typical core-shell structure. The average diameter of the nZVI nanoparticles is 50–100 nm, and the oxide shell thickness is approximately 10–15 nm. Typically, the synthesis of nZVI involves two stages: nucleation and growth. In this study, introducing P into the nucleation process resulted in larger P-nZVI particles with a pronounced radial crack structure. This may be because the P species inhibits Fe³⁺… + The reduction of Fe 0 The supersaturation caused the synthesis process to shift from uniform nucleation to particle-based growth. Furthermore, the circumferential stress difference between the iron core and the outer shell led to the formation of nanoscale cracks. post The outer shell of -nZVI was thin, but no radial cracks were observed. Therefore, the subsequent sulfidation process mainly suppressed Fe 0Core oxidation does not participate in the nucleation process of nZVI. Phosphorylation and post-sulfidation etching significantly reduce the shell thickness of the original nZVI. As the molar ratio of sulfur in the shell to phosphorus in the core increases from 1:1 to 4:1, S... post The shell thickness of the / P-nZVI particles gradually increased from 2.6 nm to 4.5 nm, indicating that the shell thickness can be adjusted by controlling the degree of sulfidation. The densest nanocracks were observed in the nanoparticles with a sulfur to phosphorus molar ratio of 2:1, suggesting that moderate post-sulfidation is beneficial for crack formation.

[0079] S post / P-nZVI(2) was analyzed by crystallography, such as Figure 3 The XRD pattern shown indicates a significant leftward shift in the diffraction peaks of the Fe(110) crystal plane for zero-valent iron; as... Figure 4 As shown, the calculated lattice constant increased from 2.855 Å for pure nZVI to 2.867 Å, ​​confirming that P successfully entered the zero-valent iron lattice, causing lattice expansion.

[0080] like Figure 5 As shown, S post The specific surface area (BET) of / P-nZVI(2) reaches 23.9 m² / g, S post The water contact angle of / P-nZVI(2) increased from 7.94° of the original nZVI to 35.25°, indicating a significant improvement in its hydrophobicity.

[0081] The materials prepared in the examples and comparative examples were tested for their degradation performance and selectivity towards trichloroethylene (TCE): 1. Apply the material to the degradation of TCE wastewater.

[0082] Experimental conditions: 15 mL of HEPES buffer (50 mM, pH=7), 76 μM TCE, and 2 g / L of the test material were added to a 20 mL headspace vial. The reaction was carried out with shaking at 25℃ and 200 rpm. The formula for calculating the residual concentration ratio of pollutants is as follows: C t / C0(%) = Concentration at time t / Initial concentration × 100%; Test results are as follows Figure 6 and Figure 7 As shown in the diagram.

[0083] Figure 6 The graphs show the degradation kinetics of trichloroethylene by different materials prepared in Examples 1-3 and Comparative Examples 1-3. Within 6 hours of reaction, Example 1 (S...) postThe TCE concentration of / P-nZVI(2)) decreased most sharply, with Ct / C0 dropping to approximately 10.7%; while the TCE concentration of the single modified materials (P-nZVI, S) decreased most sharply. post The -nZVI curve shows a gradual decrease, with unmodified nZVI almost in a passivated state. In Example 2, the S / P=1:1 ratio showed the second highest activity, with a lower sulfur doping content and a slightly thinner sulfurized shell. In Example 3, the S / P=4:1 ratio resulted in decreased activity: the surface sulfur doping content was higher, and the shell was thicker. Although hydrophobicity was enhanced, the excessively thick FeS... x The layer increases the resistance to electron transfer to the outside, thereby reducing the reducing activity.

[0084] The co-modified materials in Examples 1-3 were significantly superior to the single-modified materials in Comparative Examples 2 and 3, verifying the triple synergistic mechanism of phosphorus-promoted release, sulfur-conducting electrons, and crack-promoted mass transfer.

[0085] Figure 7 The graph shows a comparison of the degradation performance of trichloroethylene by different materials prepared in Examples 1, 4, 5, 6, 7 and Comparative Examples 4 and 5. Example 1 showed the best degradation performance. The degradation curves of Comparative Examples 4 and 5 were significantly slower. The activity of Example 4 was significantly lower than that of Example 1. The activity of Example 5 was between that of P3 and Example 1. The activities of Examples 6 and 7 were close to those of Example 1 but slightly lower.

[0086] In Comparative Example 4, the simultaneous introduction of sulfur and phosphorus significantly reduced activity. Phosphorus could not preferentially enter the zero-valent iron lattice to achieve doping, and sulfur could not specifically form a uniform shell on the particle surface. The two competed with each other during the nucleation stage, resulting in neither sufficient lattice expansion nor radial cracks induced by core-shell stress mismatch.

[0087] Comparative Example 5 showed the lowest activity. When sulfur was introduced first, it preferentially occupied the nucleation sites on the particle surface. Subsequently, phosphorus could not enter the zero-valent iron lattice in large quantities to cause expansion, and it could not form a core-shell stress mismatch from the inside out, so radial cracks could not be generated.

[0088] Figure 8 S obtained in Example 1 post The broad-spectrum degradation efficiency of / P-nZVI(2) for trichloroethylene, tetrachloroethylene, carbon tetrachloride, and chloroform is shown in the figure. The material exhibits extremely high removal rates for chloroform (CHCl3) and carbon tetrachloride (CCl4). It also degrades tetrachloroethylene (PCE) and trichloroethylene (TCE) efficiently, verifying the material's ability to dechlorinate and hydrogenate chlorinated hydrocarbons. The degradation rate of all pollutants remained above 70% within 12 h, fully demonstrating that the material has good broad-spectrum degradation ability for typical chlorinated hydrocarbons with different degrees of chlorination and molecular configurations.

[0089] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A core-shell type nanomaterial with zero valent iron, characterized in that, The core of the material is phosphorus-doped nano-zero-valent iron, and the shell is a mixture of iron sulfide and iron oxide. The material has nanocracks extending from the core to the shell.

2. The core-shell type nano-zero-valent iron material as described in claim 1, characterized in that, The molar ratio of phosphorus to iron in the core is (3~8):

100.

3. The core-shell type nano-zero-valent iron material as described in claim 2, characterized in that, The molar ratio of phosphorus to iron in the core is 5:

100.

4. The core-shell type nano-zero-valent iron material as described in claim 1, characterized in that, The molar ratio of sulfur in the shell to phosphorus in the core is (1~4):

1.

5. The core-shell type nano-zero-valent iron material as described in claim 4, characterized in that, The molar ratio of sulfur in the shell to phosphorus in the core is 2:

1.

6. The core-shell type nano-zero-valent iron material as described in claim 1, characterized in that, The material has an average particle size of 50~100 nm; and / or, The thickness of the shell is 2.6 ~ 4.5 nm.

7. A method for preparing a core-shell type nano-zero-valent iron material as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Under a protective atmosphere, a reducing agent is added to a solution containing an iron source and a phosphorus source to carry out a reduction reaction and form a phosphorus-doped nano-zero-valent iron suspension. S2. Add a sulfur source solution to the phosphorus-doped nano-zero-valent iron suspension to react, so that a mixture of iron sulfide and iron oxide is formed on the surface of the phosphorus-doped nano-zero-valent iron, to obtain a core-shell nano-zero-valent iron material with phosphorus doping and sulfur coating.

8. The preparation method according to claim 7, characterized in that, The iron source mentioned in step S1 includes ferrous chloride and / or ferrous sulfate; and / or, The phosphorus source includes at least one of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, or dipotassium hydrogen phosphate; and / or, The reducing agent includes sodium borohydride and / or potassium borohydride.

9. The preparation method according to claim 8, characterized in that, The sulfur source mentioned in step S2 includes sodium sulfide and / or sodium dithionite.

10. The application of a core-shell type nano-zero-valent iron material as described in any one of claims 1 to 6 in the remediation of groundwater or soil contaminated with chlorinated organic compounds, characterized in that, The chlorinated organic compound includes at least one of trichloroethylene, tetrachloroethylene, carbon tetrachloride, or chloroform.

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