A method for treating chlorinated hydrocarbon-contaminated groundwater using biochar in conjunction with zero-valent iron.

Abstract

This invention belongs to the field of groundwater remediation technology and discloses a method for treating chlorinated hydrocarbon-contaminated groundwater using biochar in synergy with zero-valent iron (ZVI). The method includes the following steps: (1) mixing biomass with iron-containing materials and then pyrolyzing the mixture under oxygen-limited conditions at a temperature of 700-900°C to obtain biochar-ZVI material; (2) injecting the biochar-ZVI material into the chlorinated hydrocarbon-contaminated groundwater to be treated for reaction, with the reaction time lasting at least 7 days, thereby achieving the treatment of chlorinated hydrocarbons in the chlorinated hydrocarbon-contaminated groundwater; wherein the pH value of the chlorinated hydrocarbon-contaminated groundwater does not exceed 11. The biochar-ZVI material prepared using the specific process of pyrolysis at 700-900°C can effectively treat chlorinated hydrocarbons in chlorinated hydrocarbon-contaminated groundwater. The synergistic chemical effect between biochar and ZVI significantly increases the electron capacity of zero-valent iron in the reaction and enhances the reaction activity.

Description

Technical Field

[0001] This invention belongs to the field of groundwater remediation technology, and more specifically, relates to a method for treating groundwater contaminated with chlorinated hydrocarbons using biochar in conjunction with zero-valent iron. Background Technology

[0002] Chlorinated hydrocarbons (such as trichloroethylene, chloroform, and carbon tetrachloride) have a wide range of applications. For example, carbon tetrachloride was once widely used as a dry cleaning agent and fire extinguisher, while trichloroethylene is commonly used as a metal degreasing agent. However, these compounds are also recognized endocrine disruptors, posing potential risks to human health and ecosystems. The widespread use of chlorinated hydrocarbons in industrial production often leads to their entry into groundwater systems as pollutants, and because they are difficult to degrade rapidly through natural processes, their pollution of groundwater systems is long-term. Furthermore, the poor solubility and higher relative density of chlorinated hydrocarbons than water cause them to easily migrate vertically in groundwater, accumulate in aquitards, and then be slowly released into the groundwater system. This slow-release characteristic exacerbates the persistent pollution of chlorinated hydrocarbons in the environment, posing significant challenges and high treatment costs for groundwater remediation.

[0003] Zero-valent iron (ZVFe) has been widely used in the remediation of chlorinated hydrocarbons due to its high electron capacity and strong reducing properties. Compared with other remediation technologies, ZVFe is particularly prominent in groundwater remediation due to its injectability. However, the long-term feasibility of this method is limited by the potential deactivation of the reaction surface due to oxidation or blockage. To address this issue, researchers have developed various improved schemes. For example, Chinese patent application 202410614427.4 describes a method for enhancing the reduction of chlorinated hydrocarbons with ZVFe using clay minerals and sulfidation treatment; Chinese patent application 201910329393.3 uses tannic acid to enhance the reactivity of ZVFe with chlorinated hydrocarbons; and Chinese patent application 202211111093.6 combines ZVFe with biochar (the interaction between biochar and ZVFe is purely physical, with biochar used to improve the dispersibility of ZVFe) and performs phosphorylation treatment to introduce microstructural cracks, effectively preventing surface passivation by utilizing the interaction between Fe and P elements, thereby maintaining effective electron transfer. These existing studies have provided some assistance in improving the efficiency and persistence of zero-valent iron in the remediation of chlorinated hydrocarbon-contaminated environments.

[0004] Zero-valent iron (ZVFe) remediation technology faces some inherent challenges in treating chlorinated hydrocarbon (CHL)-contaminated groundwater. To overcome the decreased reactivity due to surface passivation, technological improvements typically aim to accelerate the ZVFe reaction rate. However, while this accelerated reaction has its advantages, it also introduces a series of problems. It reduces the persistence or slow-release properties of ZVFe during CHL migration in groundwater, requiring repeated injections at multiple sites and significantly increasing project complexity. Furthermore, although various methods have improved the reaction efficiency of ZVFe, this enhanced reactivity also means reduced persistence in groundwater, especially in redox zones, making ZVFe more susceptible to oxidation by coexisting oxidants, thus weakening remediation efficiency. For example, while sulfidation treatment enhances the reducing power of ZVFe, it also makes it more prone to reacting with oxidants in the environment, limiting its application over large areas. Moreover, current technologies show poor performance in treating residual non-aqueous liquids or CHL contamination in complex sites or low-permeability areas, making it difficult to maintain remediation effects and effectively control pollution over long periods.

[0005] Therefore, there is an urgent need for a technology that can both ensure remediation efficacy and improve the slow-release properties of zero-valent iron (ZVFe). Regarding slow-release properties, Chinese patent application 202210241095.0 describes a method for preparing ZVFe by mixing ZVFe powder, biochar, biocarbon source, emulsifier, stabilizer, and water. This method aims to differentiate the long-term, medium-term, and short-term remediation capabilities of ZVFe. Chinese patent application 202210087096.4 introduces a moldable slow-release agent for in-situ remediation of chlorinated hydrocarbon pollutants and its preparation method. This method involves a combination of emulsifier, reusable oil, micronized iron powder, industrial syrup, thickener, stabilizer, disintegrant, and water. This combination effectively creates an environment for the slow release of ZVFe electrons, enhancing the slow-release properties of ZVFe. However, the use of thickeners and stabilizers may hinder the effective transfer of electrons, reducing remediation efficiency. The purpose of using such materials is to improve remediation effectiveness, reduce construction frequency, and minimize environmental impact. Nevertheless, these materials still face some drawbacks in the remediation of chlorinated hydrocarbon groundwater. For example, the preparation process is complex, requiring the design of specific embedding composite materials tailored to different groundwater environments. For instance, Chinese patent application 202210087096.4 designs specific materials for the living conditions of anaerobic organisms, and these technical challenges may lead to decreased contact efficiency with contaminants in practical applications. Summary of the Invention

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, the present invention aims to provide a method for treating chlorinated hydrocarbon-contaminated groundwater using biochar in synergy with zero-valent iron (ZVI). This method utilizes a specific pyrolysis process at 700-900℃ to prepare biochar-synergistic ZVI materials, enabling effective treatment of chlorinated hydrocarbons in groundwater. The preparation process of the biochar-synergistic ZVI materials in this invention is simple, overcoming the limitations of traditional physical slow-release methods. By leveraging the synergistic chemical interaction between biochar and ZVI, the electron capacity of ZVI in the reaction is significantly increased, enhancing reactivity and effectively solving the problem of ineffective reaction of ferrous iron (Fe2+) produced after the oxidation of ZVI, thereby improving electron storage and utilization efficiency. Furthermore, this method significantly reduces the cost of groundwater remediation and is adaptable to a wide range of scales, effectively addressing various chlorinated hydrocarbon pollution scenarios. This invention not only improves remediation efficiency but also optimizes economics, providing a more effective and sustainable technical solution for the remediation of chlorinated hydrocarbon-contaminated groundwater.

[0007] To achieve the above objectives, according to the present invention, a method for treating groundwater contaminated with chlorinated hydrocarbons using biochar in synergistic zero-valent iron is provided, characterized by comprising the following steps:

[0008] (1) Biomass and iron-containing materials are mixed and then pyrolyzed at a temperature of 700-900℃ under oxygen-limited conditions to obtain biochar-ZVI material;

[0009] (2) The biochar-ZVI material is injected into the groundwater containing chlorinated hydrocarbons to be treated for reaction, and the reaction time lasts for at least 7 days, so as to achieve the treatment of chlorinated hydrocarbons in the groundwater containing chlorinated hydrocarbons; wherein the pH value of the groundwater containing chlorinated hydrocarbons does not exceed 11.

[0010] As a further preferred embodiment of the present invention, step (1) specifically includes the following sub-steps:

[0011] (1-1) Prepare dry biomass with a particle size ≤0.5mm, and pyrolyze the biomass at a temperature of 300℃~460℃ for 30min~2h under limited oxygen conditions to obtain precursor char.

[0012] (1-2) Weigh and mix the precursor carbon and iron-containing materials at a mass ratio of (0.1-0.5):1, then add water for impregnation and stirring, and then let stand; then separate the solid and liquid, dry the obtained solid, and pyrolyze it at a temperature of 700-900℃ for 1-3 hours under oxygen-limited conditions to obtain biochar-ZVI material.

[0013] As a further preferred embodiment of the present invention, in step (1-2), the amount of water added satisfies the water-to-solid mass ratio of (10-25):1.

[0014] The stirring time is more than 2 hours, and the settling time is more than 12 hours.

[0015] As a further preferred embodiment of the present invention, in step (2), the concentration of the biochar-ZVI material added to the groundwater containing chlorinated hydrocarbons to be treated is 0.05-1.0 g / L.

[0016] As a further preferred embodiment of the present invention, in step (2), the concentration of chlorinated hydrocarbons in the groundwater containing chlorinated hydrocarbons to be treated is between 20 and 1000 ppm.

[0017] As a further preferred embodiment of the present invention, in step (1), the biomass is wood waste, preferably at least one of bamboo and coconut shell.

[0018] As a further preferred embodiment of the present invention, in step (1), the biomass satisfies the mass ratio of the dry biomass to the iron-containing material as (0.3-2.0):1.

[0019] The iron-containing material is at least one of ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, ferric oxide, zero-valent iron, and magnetite.

[0020] Compared with existing technologies, the present invention utilizes a specific process of pyrolysis at 700-900℃ to prepare biochar-synergistic zero-valent iron (ZVI) materials. Because no passivation is performed and no additional activators are required, it effectively solves the problems of surface passivation and deactivation caused by excessively rapid reaction rates in existing ZVI materials. In existing technologies, sulfidation modification is typically performed to avoid surface passivation of ZVI materials, which significantly reduces the material's migration performance and limits its remediation range for in-situ groundwater remediation. The biochar-ZVI (ZVI) material of this invention not only utilizes the dispersibility of biochar but also enhances the material's efficiency in treating chlorinated hydrocarbons and improves its stability under specific temperature conditions. Unlike the simple combination of biochar and zero-valent iron described in patents 202210241095.0, 202311551167.2, and 201710646025.2, this invention uses biochar-ZVI material where ZVI is uniformly dispersed on the surface of the biochar during preparation, solving the problem of self-aggregation in water and enhancing the stability and reactivity of the material. More importantly, this invention utilizes the divalent iron (Fe(II)) generated during the reduction of chlorinated hydrocarbons by biochar-ZVI. Due to the presence of biochar, compared to Fe(II) existing alone (e.g., Fe(II) existing alone...), the... 2+ Or Fe(OH)2) exhibits a stronger reducing ability. Fe alone... 2+Alternatively, Fe(OH)2 may not be able to effectively reduce chlorinated hydrocarbons. This intermediate biochar-Fe(II) composite material can effectively reduce chlorinated hydrocarbons, significantly enhancing the ability of the oxidation product Fe(II) of ZVI to degrade chlorinated hydrocarbon compounds, thereby improving treatment efficiency and remediation effect.

[0021] The following is a detailed chemical reaction process:

[0022] Biochar-Fe 0 + RCl + H + → Biochar - Fe(II) + RH + Cl - (1)

[0023] 2 Biochar - Fe(II) + RCl + H + → 2 biochar - Fe(III) + RH + Cl - (2)

[0024] This invention utilizes a biochar-ZVI material prepared under specific conditions. In this material, the biochar not only promotes the uniform distribution of ZVI but also reacts with it to form Fe(III) after the reduction of chlorinated hydrocarbons (Equation (2)), achieving the further reduction of Fe(III) to Fe(II). Furthermore, Fe(II) can further react with chlorinated hydrocarbons (Equation (1)), thereby significantly improving the processing efficiency of chlorinated hydrocarbons. In the prior art, biochar prepared at low temperatures (300℃~500℃) is typically used to reduce Fe(III) because the oxygen-containing functional groups such as phenolic hydroxyl groups on low-temperature biochar can provide the necessary electrons. However, the specific surface area of ​​low-temperature biochar is relatively low (usually less than 10 m²). 2 The limited dispersion of ZVI in the initial stage of the reaction restricts its performance because ZVI is prone to aggregation, hindering its effective dispersion and thus affecting its remediation effect on chlorinated hydrocarbons. In contrast, this invention uses biochar-ZVI prepared at a high temperature of 700-900℃, which has a larger specific surface area (~600m²). 2 The biochar not only promotes the effective dispersion of ZVI in the early stages of the reaction, but also reacts with the formed Fe(III) to reduce it to Fe(II), which then participates in the reaction with chlorinated hydrocarbons again. Although the high-temperature biochar of this invention has a low content of oxygen-containing functional groups, its unique structure and high electron content enable it to effectively promote the reduction of Fe(III) to Fe(II) by biochar-ZVI and reduce chlorinated hydrocarbons through multiple pathways, maximizing the utilization of the potential electrons in the biochar, thereby improving the treatment efficiency of chlorinated hydrocarbons and the possibility of environmental remediation.

[0025] In the application of ZVI for treating chlorinated hydrocarbons, a key challenge is the increase in pH value in localized areas of the water body during the reaction (refer to chemical reaction formulas (3) and (4)). This alkaline condition makes the continued reaction of ZVI more difficult. In particular, when the oxidized Fe(II) is further converted to Fe(III), Fe(III) has strong adsorption properties and easily forms a coating on the ZVI surface, thus hindering further chemical reactions of ZVI. Specifically, as the pH increases, Fe(II) generated by the oxidation of ZVI is extremely sensitive to oxides and will be rapidly converted to Fe(III). Its strong adsorption properties will significantly hinder the reactivity of ZVI (refer to chemical reaction formulas (3) to (6)). In this invention, biochar prepared under specific high-temperature conditions is introduced. This biochar has a rich aromatic structure and a high specific surface area, which can effectively promote the reduction of Fe(III) to Fe(II). This process can still be carried out effectively in an alkaline environment, which can expand the remediation pH range for treating chlorinated hydrocarbon-contaminated groundwater and increase the remediation area of ​​groundwater. This enables the invention to adapt to more complex groundwater environmental conditions, enhances the applicability and effectiveness of remediation projects, and further improves the universal applicability and practical application value of environmental remediation technology.

[0026] Fe 0 + RCl + H2O→Fe 2+ + RH + Cl - + OH - (3)

[0027] 2Fe 0 + O2 + 2H2O → 2Fe 2+ + 4OH - (4)

[0028] Fe 2+ + Oxic → Fe 3+ (5)

[0029] Fe 3+ + 3OH - → Fe(OH)3 (6)

[0030] In this invention, the combination of biochar and iron significantly enhances the reducing power of biochar. Specifically, the originally stable carbon structure, under the influence of iron, can unleash its potential reducing power, thereby promoting electron release and optimizing the electron supply mechanism. Typically, biochar prepared, especially at 700 to 900°C, maintains a stable physicochemical structure before and after the reaction, making it difficult to oxidize. However, the biochar-iron composite material prepared in this invention exhibits a synergistic effect exceeding the sum of the effects of the individual components, further improving the treatment efficiency of chlorinated hydrocarbons.

[0031] In the preparation of biochar-ZVI, this invention involves pyrolyzing a mixture of biomass and iron-containing materials under oxygen-limited conditions at 700-900°C to obtain biochar-ZVI material. Preferably, the biomass raw material is first pyrolyzed at 300°C to 460°C to obtain precursor char, which further increases the specific surface area of ​​the carbon material, allowing it to load more iron when in contact with the iron-containing material, thus enriching the final biochar-ZVI material with more iron. Simultaneously, with the introduction of the precursor char preparation step, the processing temperature of the precursor char should not exceed 460°C to avoid releasing excessive reducing gases during the precursor char preparation process. This could prevent sufficient reducing gases from being generated during the subsequent oxygen-limited pyrolysis of the precursor char with the iron-containing material at 700-900°C, thus failing to reduce the iron-containing material to ZVI and reducing remediation efficiency (alternatively, biomass can be directly mixed with the iron-containing material and pyrolyzed directly at 700-900°C under oxygen-limited conditions). The biochar-ZVI composite material prepared by this invention maintains high activity and stability, providing an efficient and economical solution for the remediation of groundwater contaminated with chlorinated hydrocarbons.

[0032] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The biochar-ZVI material used in this invention has significant advantages in remediating groundwater contaminated with chlorinated hydrocarbons. The biochar-ZVI material prepared by this invention using a specific process of pyrolysis at 700-900℃ not only enhances the reducing power of biochar but also allows both Fe(II) and Fe(III) (the latter reduced to Fe(II)) to effectively export electrons, thereby improving the reduction efficiency of chlorinated hydrocarbons. This method effectively transforms the shortcomings of existing technologies into advantages, ensuring that the biochar-ZVI material can continuously and effectively treat chlorinated hydrocarbon pollution.

[0034] 2. The preparation process of the biochar-ZVI material in this invention is simple and requires no additional reducing agent or binder. Iron-based materials are widely available; the gas generated during pyrolysis promotes the reaction between the iron-based materials and biomass, forming structurally stable biochar-ZVI with good reducibility. Preferably, this invention involves first pyrolyzing the biomass raw material at a temperature of 300℃ to 460℃ to obtain precursor char, then mixing it with iron-containing materials and performing oxygen-limited pyrolysis at 700-900℃. This two-step preparation process ensures that more iron-containing materials are loaded onto the precursor char.

[0035] 3. The biochar-ZVI material in this invention exhibits weak heterogeneous reactivity with oxygen, which means low storage costs during preparation and low loss costs during transport in groundwater. Especially in groundwater environments with complex redox variations, this weak reactivity allows the material to better adapt to environmental conditions, significantly expanding the remediation scope of chlorinated hydrocarbon-contaminated groundwater and enhancing the practicality and efficiency of environmental remediation projects. Attached Figure Description

[0036] Figure 1 The graph shows a comparison of the carbon tetrachloride treatment effects obtained by adding biochar, Fe(II), ZVI, Fe3O4, and biochar-ZVI to simulated chlorinated hydrocarbon-contaminated groundwater in Example 2.

[0037] Figure 2 This is a comparison chart showing the carbon tetrachloride treatment effects obtained by adding biochar-ZVI to simulated chlorinated hydrocarbon-contaminated groundwater at different pH values ​​in Examples 3 and 4.

[0038] Figure 3 The image shows the XPS plot of the Fe valence state distribution on the solid after the biochar reacts with Fe(III) in Example 9.

[0039] Figure 4 The graph shows a comparison of the carbon tetrachloride treatment effects obtained by treating simulated chlorinated hydrocarbon-contaminated groundwater with biochar-ZVI prepared at different temperatures in Examples 5 and 6.

[0040] Figure 5 The image shows the carbon tetrachloride treatment effect obtained by the synergistic treatment of simulated chlorinated hydrocarbon-contaminated groundwater by biochar and Fe(II) in Example 7.

[0041] Figure 6 The images show the XRD characterization of biochar-ZVI materials prepared at different temperatures in Example 1.

[0042] Figure 7 The image shows the carbon tetrachloride treatment effect obtained by treating simulated chlorinated hydrocarbon-contaminated groundwater after ZVI and biochar-ZVI materials were oxidized in water for 1 day in Example 10. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0044] Example 1: Preparation of biochar-ZVI material

[0045] The biochar-ZVI in this invention is prepared by combining biomass with iron-containing materials.

[0046] Specifically, in this embodiment:

[0047] First, bamboo from Longyan, Fujian Province, was selected as biomass. The raw material (i.e., bamboo) was crushed to a particle size of ≤0.5mm (the target particle size can be obtained by sieving). After washing, it was dried at 80℃ for 24 hours. Then, 100g of biomass (i.e., the dry basis mass of biomass is 100g) was weighed and subjected to oxygen-limited pyrolysis in a muffle furnace (in this embodiment, oxygen limitation is achieved by placing the biomass in a crucible, covering it, and pyrolyzing it under air conditions; of course, a protective atmosphere can also be used to directly carry out pyrolysis under a protective atmosphere). The heating rate was 10℃·min-1, and the final temperature was 460℃ and held for 1 hour. The resulting solid material was named precursor char (each 1g of precursor char corresponds to 3.3g of dry basis biomass).

[0048] Subsequently, the precursor char and FeSO4·7H2O were mixed at a mass ratio of 1:5 and impregnated at a water-to-solid ratio of 20:1 (mass ratio). The mixture was stirred for 2 hours and then allowed to stand overnight. The mixture was then subjected to high-temperature heat treatment in a high-temperature furnace. Specifically, the sample was placed in a muffle furnace under oxygen-limited conditions (in this embodiment, the mixture was placed in a crucible, covered, and subjected to high-temperature heat treatment in air; alternatively, a protective atmosphere could be used for direct high-temperature heat treatment). The temperature was increased to 700°C at a rate of 10°C·min⁻¹ and maintained at this temperature for 2 hours. After cooling to room temperature, the biochar-ZVI material was obtained, designated as the "Biochar (700°C)-ZVI" sample. Before use, it should be stored in a sealed desiccator to ensure its dryness and storage stability.

[0049] After mixing the precursor carbon with FeSO4·7H2O in the same dosage as described above, the mixture was subjected to high-temperature heat treatment at 500℃ and 900℃ respectively. The resulting samples were designated as "Biochar (500℃)-ZVI" and "Biochar (900℃)-ZVI" samples, respectively, thus obtaining biochar-ZVI materials prepared at different temperatures.

[0050] Furthermore, XRD characterization was performed on the "Biochar (500℃)-ZVI" sample, the "Biochar (700℃)-ZVI" sample, and the "Biochar (900℃)-ZVI" sample, and the results are as follows. Figure 6As shown, both the "Biochar (700℃)-ZVI" and "Biochar (900℃)-ZVI" samples have obvious characteristic peaks of zero-valent iron (ZVI); however, no characteristic peaks of zero-valent iron (ZVI) were observed in the "Biochar (500℃)-ZVI" sample, indicating that the iron element was mainly supported in the form of Fe3O4.

[0051] Example 2

[0052] To simulate groundwater contamination by chlorinated hydrocarbons, 500 mL of water containing carbon tetrachloride was placed in a brown glass bottle, and the pH was adjusted to 6.5 with a carbon tetrachloride concentration of 50 ppm. The reaction bottle was then placed in a glove box to simulate an anaerobic environment in the groundwater.

[0053] Then, five control experiments were conducted, specifically:

[0054] a. Add 0.5g of biochar-ZVI material (i.e., the "biochar (700℃)-ZVI" sample prepared in Example 1) to 500mL of simulated chlorinated hydrocarbon-contaminated groundwater; the Fe mass ratio was determined to be approximately 25% by acid dissolution. For ease of comparison, the Fe element addition amount in subsequent groups was also 0.125g.

[0055] b. Add Fe to 500 mL of simulated chlorinated hydrocarbon-contaminated groundwater. 2+ (Specifically, FeSO4·7H2O compound was added) as a control; the corresponding amount of FeSO4·7H2O added was 0.621g;

[0056] c. Add Fe3O4 to 500 mL of simulated chlorinated hydrocarbon-contaminated groundwater as a control; the corresponding Fe3O4 dosage is 0.173 g.

[0057] d. Add ZVI (i.e., zero-valent iron, from purchased micron-sized iron powder) to 500 mL of simulated chlorinated hydrocarbon-contaminated groundwater as a control; the corresponding ZVI dosage was 0.125 g.

[0058] e. Biochar was added to 500 mL of simulated chlorinated hydrocarbon-contaminated groundwater as a control. The biochar was obtained by pyrolyzing the precursor char prepared in Example 1 at 700°C under a nitrogen atmosphere for 2 hours (this biochar did not contain any iron-containing materials; its specific surface area was measured to be 600 m²). 2 / g), the amount of biochar added is 0.5g;

[0059] Next, the reactors containing the five sets of samples were placed on a magnetic stirrer and stirred continuously at a speed of 600 r / min. Samples were taken at time points of 8, 24, 48, 72, 120, 168 and 336 h to analyze and determine the concentration of carbon tetrachloride.

[0060] like Figure 1 As shown, biochar (corresponding to group e), Fe 2+ (Corresponding to group b) had almost no reduction effect on carbon tetrachloride; the removal rate of carbon tetrachloride using ZVI alone (corresponding to group d) was 45.55% at 168h and 48.57% at 336h; the removal rate of carbon tetrachloride using Fe3O4 alone (corresponding to group c) was 33.61% at 168h and 34.68% at 336h; the removal rate of carbon tetrachloride using the biochar-ZVI system (corresponding to group a) was 95.99% at 168h and 96.9% at 336h, showing the best removal effect.

[0061] Example 3

[0062] This embodiment is largely the same as Embodiment 2, except that the pH of the simulated chlorinated hydrocarbon-contaminated groundwater in Embodiment 2 is first adjusted to 3.0, and the material used is biochar-ZVI material (i.e., the "biochar (700℃)-ZVI" sample prepared in Embodiment 1).

[0063] like Figure 2 As shown, when the pH of the polluted water is 3.0, the removal rate of carbon tetrachloride is 83.62% after 168 hours and 92.1% after 336 hours, which also shows a good removal effect.

[0064] Example 4

[0065] This embodiment is largely the same as Embodiment 2, except that the pH of the simulated chlorinated hydrocarbon-contaminated groundwater in Embodiment 2 is first adjusted to 10.0, and the material used is biochar-ZVI material (i.e., the "biochar (700℃)-ZVI" sample prepared in Embodiment 1).

[0066] like Figure 2 As shown, when the pH of the polluted water is 10.0, the removal rate of carbon tetrachloride is 81.54% after 168 hours and 89.9% after 336 hours, which also shows a good removal effect.

[0067] Example 5

[0068] This embodiment is largely the same as Example 2, except that the biochar-ZVI material prepared at 500℃ in Example 1 is used (i.e., “Biochar (500℃)-ZVI” sample).

[0069] like Figure 4As shown, when the pH of the simulated groundwater contaminated with chlorinated hydrocarbons is 6.5, the removal rate of carbon tetrachloride is 66.19% after 168 hours and 66.51% after 336 hours.

[0070] Example 6

[0071] This embodiment is largely the same as Example 2, except that the biochar-ZVI material prepared at 900℃ in Example 1 is used (i.e., “Biochar (900℃)-ZVI” sample).

[0072] like Figure 4 As shown, when the pH of the simulated groundwater contaminated with chlorinated hydrocarbons is 6.5, the removal rate of carbon tetrachloride is 97.99% after 168 hours and 98.9% after 336 hours.

[0073] Example 7

[0074] This embodiment is largely the same as Embodiment 2 under the same conditions. This embodiment explores the reduction efficiency of Fe(II), biochar, and biochar-Fe(II) for carbon tetrachloride, and conducts three sets of control experiments. Specifically:

[0075] i. Fe(II) group: still using Fe 2+ (That is, it is still done by adding FeSO4·7H2O compound), and the specific dosage and operation are the same as group b in Example 2;

[0076] ii. Biochar group: The biochar used was obtained by pyrolyzing the precursor char prepared in Example 1 at 700°C under a nitrogen atmosphere for 2 hours. The specific amount and operation were the same as in group e in Example 2.

[0077] iii. Biochar-Fe(II) system: Simultaneous use of Fe from group i 2+ The biochar in group ii was used with a dosage of 0.621 g of FeSO4·7H2O and a usage of 0.5 g of biochar. Specifically, the biochar and Fe(II) were reacted fully in 250 mL of deionized water for 1 hour, and then 250 mL of simulated chlorinated hydrocarbon (100 ppm) contaminated groundwater was added for further reaction.

[0078] The reaction results of groups i and ii are the same as those of groups b and e in Example 2, that is, as... Figure 1 As shown, neither Fe(II) alone nor biochar alone had a significant removal effect on carbon tetrachloride; the reaction results of group iii are as follows. Figure 5As shown, the removal efficiencies of biochar-Fe(II) for carbon tetrachloride were 43.37% and 47.95% at 168 h and 336 h, respectively. The results indicate that biochar and Fe(II) have a synergistic effect in the removal of carbon tetrachloride.

[0079] Example 8

[0080] This embodiment explores the electron capacity in biochar, Fe(II), and biochar-Fe(II) systems. The amount of electrons available in different systems was determined using an electrochemical workstation at 0.61 V via a dielectric electrochemical method.

[0081] The samples in each group are as follows:

[0082] ①. Biochar group: The biochar used was obtained by pyrolyzing the precursor char prepared in Example 1 at 700°C under a nitrogen atmosphere for 2 hours; specifically, an aqueous solution with a concentration of 4 g / L was prepared.

[0083] ②. Fe(II) group: still using Fe 2+ (That is, still in the form of adding FeSO4·7H2O compound); specifically, preparing Fe 2+ An aqueous solution with a concentration of 2 mM;

[0084] ③. Biochar-Fe(II) system: using biochar from group ① and Fe from group ② simultaneously. 2+ The biochar was obtained by reacting it with Fe(II) solution for 2 hours, wherein the concentration of biochar in the mixed solution was 4 g / L, and the concentration of Fe... 2+ The concentration in the mixed solution is 2 mM;

[0085] The results showed that the electron supply of biochar was 1.77 mmol / L, that of Fe(II) was 1.82 mmol / L, and the total electron supply in the biochar-Fe(II) system was 5.51 mol / L. This indicates that when biochar and Fe(II) coexist, the electron supply increases by approximately 53.5% compared to the simple sum of the electron supplies in either system (i.e., (5.51 - 1.77 - 1.82) / (1.77 + 1.82) × 100% = 53.5%), demonstrating a synergistic effect in electron supply. In other words, biochar and Fe(II) exhibit a synergistic effect in terms of electron supply.

[0086] Table 1. Comparison of electron donation from biochar, Fe(II), and biochar-Fe(II).

[0087]

[0088] Example 9

[0089] This example investigates the Fe(II) content after the reaction of biochar with Fe(III). The biochar used was obtained by pyrolyzing the precursor char prepared in Example 1 at 700°C under a nitrogen atmosphere for 2 hours. The Fe(III) used was FeCl3·6H2O. Specifically, 0.5 g of biochar and 0.6 g of FeCl3·6H2O were mixed and stirred evenly in 500 mL of deionized water. After 8 hours, solid-liquid separation was performed. The resulting solid was washed with deionized water and then characterized by XPS to analyze the change in Fe valence state.

[0090] like Figure 3 As shown, the results indicate that 74.53% of the iron in the solid obtained from the reaction is Fe(II), indicating that the reaction of biochar with Fe(III) yields biochar-Fe(II).

[0091] Example 10

[0092] This embodiment explores the effect of the reaction of oxygen with ZVI and biochar-ZVI on the removal of chlorinated hydrocarbons.

[0093] In the ZVI system, 0.125 g of ZVI was added to 250 mL of deionized water and stirred for one day (under air conditions). The liquid was then transferred to a glove box to maintain an oxygen-free atmosphere, and 250 mL of 100 ppm carbon tetrachloride was added. The concentrations of ZVI and carbon tetrachloride, as well as the pH, were consistent with group d in Example 2.

[0094] For the biochar-ZVI system, 0.5 g of biochar-ZVI was added to 250 mL of deionized water and stirred for 1 day (under air conditions). The liquid was then transferred to a glove box to maintain an oxygen-free atmosphere, and 250 mL of 100 ppm carbon tetrachloride was added. The concentrations of biochar-ZVI and carbon tetrachloride, as well as the pH, were consistent with group a in Example 2.

[0095] like Figure 7 As shown, after ZVI was stirred in air, the removal efficiency of carbon tetrachloride decreased significantly, with removal efficiencies of 16.19% and 18.51% at 168 h and 336 h, respectively; while the removal efficiencies of carbon tetrachloride in the biochar-ZVI system were 80.89% and 86.91% at 168 h and 336 h, respectively. This indicates that biochar-ZVI is less affected by oxygen.

[0096] The above embodiments are merely examples. For instance, besides bamboo, other woody waste can be used for biomass (bamboo is, of course, more effective). Also, the iron-containing material can be other solid iron-containing materials, such as common iron salts, iron oxide powders, or zero-valent iron powder, as long as they are easily dispersed in water and can impregnate the biochar (powder particle size can be, for example, at the micrometer level or even finer). Furthermore, in addition to covering the container to isolate oxygen, the oxygen-limited conditions can also be created by using a protective atmosphere (such as nitrogen or argon). The method of this invention also has a considerable removal effect on trichloroethylene, chloroform, and other chlorinated hydrocarbons.

[0097] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for treating chlorinated hydrocarbon-contaminated groundwater using biochar in synergistic zero-valent iron, characterized in that, Includes the following steps: (1) Prepare dry biomass with a particle size ≤0.5 mm, and pyrolyze the biomass at a temperature of 300℃~460℃ for 30 min~2 h under oxygen-limited conditions to obtain precursor char; then, weigh and mix the precursor char with iron-containing materials at a mass ratio of (0.1-0.5):1, then add water for impregnation and stirring, and then let stand; then separate the solid and liquid, dry the obtained solid, and pyrolyze it at a temperature of 700-900℃ for 1-3 hours under oxygen-limited conditions to obtain biochar-ZVI material; (2) The biochar-ZVI material is injected into the groundwater containing chlorinated hydrocarbons to be treated for reaction, and the reaction time lasts for at least 7 days, so as to achieve the treatment of chlorinated hydrocarbons in the groundwater containing chlorinated hydrocarbons; wherein the pH value of the groundwater containing chlorinated hydrocarbons does not exceed 11.

2. The method as described in claim 1, characterized in that, In step (1), the amount of water added satisfies the water-to-solid mass ratio of (10-25):1; The stirring time is more than 2 hours, and the settling time is more than 12 hours.

3. The method as described in claim 1, characterized in that, In step (2), the concentration of the biochar-ZVI material added to the groundwater containing chlorinated hydrocarbons to be treated is 0.05-1.0 g / L.

4. The method as described in claim 1, characterized in that, In step (2), the concentration of chlorinated hydrocarbons in the groundwater containing chlorinated hydrocarbons to be treated is between 20 and 1000 ppm.

5. The method as described in claim 1, characterized in that, In step (1), the biomass is wood waste.

6. The method as described in claim 5, characterized in that, In step (1), the biomass is at least one of bamboo and coconut shell.

7. The method as described in claim 1, characterized in that, In step (1), the biomass satisfies the mass ratio of biomass dry basis to the iron-containing material as (0.3-2.0):1; The iron-containing material is at least one of ferrous sulfate, ferric sulfate, ferrous chloride, ferric chloride, ferric oxide, zero-valent iron, and magnetite.

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