A method for detecting heavy metals in soil

Abstract

The application provides a heavy metal detection method in soil, and relates to the field of heavy metal detection. The heavy metal detection method in soil comprises the following steps: preparing a composite detection electrode, establishing a standard curve, and detecting a sample. The preparation of the composite detection electrode comprises the following steps: preparing cerium-doped nickel-based metal cluster composite graphene, functional modification, and electrode modification. The heavy metal detection method in soil is aimed at hexavalent chromium in soil, does not need to rely on large and precise instruments, has high detection sensitivity, strong anti-interference ability, good stability, is simple to operate, rapid to detect, and can meet the timeliness requirement of rapid screening or emergency monitoring of a large number of samples.

Description

Technical Field

[0001] This invention relates to the field of heavy metal detection, and in particular to a method for detecting heavy metals in soil. Background Technology

[0002] Chromium and its compounds are important industrial raw materials, widely used in electroplating, leather making, and dyeing industries. The discharge of large quantities of chromium-containing wastewater, waste gas, and waste residue causes serious environmental pollution. As a common heavy metal pollutant, chromium exists in the environment in multiple valence states, primarily trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)), which exhibit distinctly different behaviors. Hexavalent chromium (Cr(VI)) is highly toxic, water-soluble, and easily absorbed by plants, primarily existing as CrO4 in the environment. 2- Cr207 2- Chromium exists but is not easily adsorbed by soil particles, has strong mobility, and easily causes groundwater pollution, exerting mutagenic and carcinogenic effects on organisms. Trivalent chromium (Cr(III)) has low toxicity, mainly existing in the environment as insoluble chromium hydroxide, with poor activity and mild toxicity to plants. Studies have shown that hexavalent chromium (Cr(VI)) is approximately 100 times more toxic than trivalent chromium (Cr(III)). Accurate and rapid detection of hexavalent chromium (Cr(VI)) is of great significance for environmental pollution assessment, risk management, and remediation.

[0003] Currently, the detection of Cr(VI) in soil mainly relies on standard laboratory methods. The standard "Determination of Hexavalent Chromium in Soil and Sediments by Alkali Extraction-Flame Atomic Absorption Spectrophotometry" (HJ 1082-2019) is one of the main methods for detecting Cr(VI) in soil. In addition, spectrophotometric methods based on diphenylcarbazide color development (such as GB / T15555.4-1995) have also been widely used in the detection of Cr(VI). Although these methods can achieve accurate quantification under laboratory conditions, they still have the following drawbacks: (1) The detection process is cumbersome, the detection efficiency is low, and it cannot achieve rapid response, making it difficult to meet the timeliness requirements for rapid screening of large batches of samples or emergency monitoring. (2) The anti-interference ability is poor, requiring cumbersome masking, separation, and other auxiliary means. (3) It requires large-scale precision instruments, which limits the spatiotemporal coverage and real-time performance of the monitoring.

[0004] In pursuit of speed and convenience in detection, electrochemical sensors have been extensively studied. However, existing electrochemical sensors for the detection of hexavalent chromium Cr(VI) in soil still have the following problems: (1) insufficient detection sensitivity, making it difficult to meet the detection requirements of soil environmental quality standards; (2) poor detection selectivity, failing to effectively resist interference from high concentrations of ions (especially other electroactive heavy metal ions) coexisting with hexavalent chromium Cr(VI) in soil; and (3) detection stability and reproducibility need to be further improved.

[0005] Based on this, a method for detecting hexavalent chromium Cr(VI) in soil is provided, which is highly sensitive, has strong anti-interference ability, good stability, and is simple to operate and rapid. This method has important technical significance and research value. Summary of the Invention

[0006] To address the technical problems existing in the prior art, this invention provides a method for detecting heavy metals in soil, specifically targeting hexavalent chromium (Cr(VI)) in soil. This method does not rely on large precision instruments, has high detection sensitivity, strong anti-interference ability, and good stability. At the same time, it is simple to operate and fast, and can meet the timeliness requirements for rapid screening of large batches of samples or emergency monitoring.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0008] A method for detecting heavy metals in soil, the method comprising the following steps: preparing a composite detection electrode, establishing a standard curve, and detecting the sample;

[0009] The preparation of the composite detection electrode includes the following steps: preparing cerium-doped nickel-based metal cluster composite graphene, functionalization modification, and electrode modification;

[0010] The method for preparing cerium-doped nickel-based metal cluster composite graphene is as follows: nickel nitrate hexahydrate, cerium nitrate hexahydrate, and urea are added to a graphene aqueous dispersion, dispersed evenly, and then placed in a closed environment for hydrothermal reaction. The solids are separated and collected, and the solids are washed and dried to obtain cerium-doped nickel-based metal cluster composite graphene.

[0011] The functionalization modification method is to modify cerium-doped nickel-based metal cluster composite graphene with glutathione to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene.

[0012] The electrode modification method is as follows: glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene is mixed evenly with a solvent to obtain a film-forming solution; the film-forming solution is dropped onto the surface of a glassy carbon electrode and then dried to obtain a composite detection electrode;

[0013] The method for establishing the standard curve is as follows: a three-electrode system is established using a composite detection electrode as the working electrode. After obtaining the blank background signal by detecting an acetate-sodium acetate buffer solution with pH=3.7 using square wave voltammetry, the reduction peak current is obtained by detecting hexavalent chromium standard working solutions with gradient concentrations using square wave voltammetry. The standard curve is plotted with hexavalent chromium concentration as the abscissa and reduction peak current as the ordinate.

[0014] The sample detection method is as follows: using the three-electrode system, the soil sample extract is detected by square wave voltammetry to obtain the reduction peak current; the concentration of hexavalent chromium in the soil is calculated according to the standard curve.

[0015] Preferably, in the preparation of cerium-doped nickel-based metal cluster composite graphene, the concentration of the graphene aqueous dispersion is 2-2.5 g / L;

[0016] The weight ratio of graphene in the aqueous dispersions of nickel nitrate hexahydrate, cerium nitrate hexahydrate, urea, and graphene is 58.2-64:17.4:360-380:45-50.

[0017] The hydrothermal reaction temperature is 110-120℃, and the hydrothermal reaction time is 10-12h.

[0018] Furthermore, the functionalization modification method involves adding cerium-doped nickel-based metal cluster composite graphene into a glutathione aqueous solution, stirring at room temperature in a nitrogen atmosphere, separating and collecting the solids, and washing and drying the solids to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene.

[0019] Preferably, in the functional modification, the concentration of the glutathione aqueous solution is 1-1.2 mmol / L;

[0020] The mass-to-volume ratio of cerium-doped nickel-based metal cluster composite graphene to glutathione aqueous solution was 10 mg: 11-13 mL;

[0021] Stirring at room temperature for 2-3 hours.

[0022] Preferably, in the electrode modification, the mass-to-volume ratio of glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene to solvent is 2.8-3.2 mg: 1 mL;

[0023] The solvent consists of a Nafion perfluorosulfonic acid ion exchange resin solution and an ethanol solution; the volume ratio of the Nafion perfluorosulfonic acid ion exchange resin solution to the ethanol solution is 1:8.9-9.1.

[0024] The mass concentration of the Nafion perfluorosulfonic acid ion exchange resin solution is 4-5 wt%.

[0025] The volume concentration of the ethanol solution is 40-45%.

[0026] Furthermore, in the establishment of the standard curve, the three-electrode system uses a composite detection electrode as the working electrode, a platinum wire as the counter electrode, and Ag / AgCl / 2.5-3M KCl as the reference electrode.

[0027] The measurement parameters for detecting the gradient concentration of hexavalent chromium standard working solution by square wave voltammetry are as follows: initial potential +0.35V to +0.45V, termination potential -0.45V to -0.35V, potential increment 3-5mV, square wave amplitude 20-30mV, frequency 10-20Hz, and equilibrium time 20-30s.

[0028] The reduction peak current obtained by detecting hexavalent chromium standard working solutions of gradient concentrations using square wave voltammetry is the reduction peak current that appears in the range of +0.1V to +0.2V.

[0029] Furthermore, the sample detection method involves diluting the soil sample extract until the hexavalent chromium concentration falls within the linear range of the standard curve, recording the dilution factor D, and obtaining the sample solution to be tested. Using the three-electrode system, the sample solution to be tested is detected by square wave voltammetry to obtain the reduction peak current of the sample solution to be tested, and the concentration C of hexavalent chromium in the sample solution to be tested is obtained using the standard curve.

[0030] Furthermore, the detection method also includes: sample pretreatment;

[0031] The sample pretreatment method is as follows: a soil sample of mass m is placed in a potassium phosphate buffer solution with pH=7.2, dispersed evenly, and then magnesium chloride solution is added. After extraction by shaking at room temperature at 200-300 rpm, the supernatant is separated and collected, and the solids are filtered out to obtain a sample extract of volume V.

[0032] Preferably, in the sample pretreatment, the concentration of the magnesium chloride solution is 0.4-0.5 mol / L;

[0033] The mass-to-volume ratio of soil sample, potassium phosphate buffer, and magnesium chloride solution was 1 g: 10-12 mL: 0.9-1 mL.

[0034] Preferably, in the sample detection, the measurement parameters of the sample liquid to be tested by square wave voltammetry are: initial potential +0.35V to +0.45V, termination potential -0.45V to -0.35V, potential increment 3-5mV, square wave amplitude 20-30mV, frequency 10-20Hz, and equilibrium time 20-30s.

[0035] The concentration of hexavalent chromium in soil is calculated using the following formula: (C×V×D) / (m×1000); where C is the concentration of hexavalent chromium in the sample solution to be tested, μg / L; V is the volume of the sample extract, L; D is the dilution factor of the soil sample extract; and m is the mass of the soil sample, kg.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) In the method for detecting heavy metals in soil of the present invention, in the preparation of the composite detection electrode, a hydrothermal method is used to prepare a composite material with highly conductive graphene as the framework and cerium-doped nickel-based metal clusters grown in situ on the surface; under the premise of nickel-based redox pair, Ce is introduced by cerium doping. 3+ / Ce 4+The redox pair, through synergistic catalysis, accelerates the reduction reaction of chromium ions, significantly amplifying the electrochemical signal and improving detection sensitivity. Simultaneously, glutathione functionalization improves the dispersibility of the composite material in the film-forming solution, enhancing the formation of a uniform and dense active layer on the electrode surface; it also protects the Ni-Ce active centers, improving electrode stability and storage resistance. A three-electrode system was established using the aforementioned composite detection electrode as the working electrode. Square wave voltammetry (SWV) was employed for detection. After establishing a standard curve and following sample detection procedures, the concentration of Cr(VI) in the soil was determined. The aforementioned techniques work synergistically to target hexavalent chromium (Cr(VI)) in soil without relying on large precision instruments, exhibiting high detection sensitivity, strong anti-interference ability, and good stability. Furthermore, the operation is simple and rapid, meeting the timeliness requirements for rapid screening of large batches of samples or emergency monitoring.

[0038] (2) The method for detecting heavy metals in soil of the present invention has high sensitivity, strong anti-interference ability and good stability for detecting hexavalent chromium Cr(VI) in soil, with an intra-batch RSD (precision) of 3.8-4.5%; and the relative error rate between the detection results and the detection results of standard HJ1082-2019 "Determination of hexavalent chromium in soil and sediment by alkaline extraction-flame atomic absorption spectrophotometry" is 3.5-4.3%; at the same time, the spiked recovery rate is 94.9-97.5%, and the accuracy is consistent at different concentration levels; the detection limit for hexavalent chromium Cr(VI) in soil is 0.009 mg / kg.

[0039] (3) In the method for detecting heavy metals in soil of the present invention, the RSD (precision) of the prepared composite detection electrode is 4.2%, the preparation method of the composite detection electrode is stable and controllable, and the response consistency of the prepared composite detection electrode is good; the current retention rate of the composite detection electrode after 30 consecutive measurements can still reach 95.3%, and the continuous measurement stability is good; the current retention rate of the composite detection electrode after 14 days of storage can still reach 90.2%, and the long-term stability is good; in addition, the composite detection electrode can also effectively avoid the interference of common ions in soil on the detection results, and has good anti-interference ability. Detailed Implementation

[0040] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific embodiments are now described. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0041] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, "first," "second," etc., are used to distinguish similar objects and are not used to describe a particular order or sequence. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0042] This invention provides a method for detecting heavy metals in soil, comprising the following steps: preparing a composite detection electrode, establishing a standard curve, sample pretreatment, and sample detection.

[0043] The preparation of the composite detection electrode includes the following steps: preparing cerium-doped nickel-based metal cluster composite graphene, functionalization modification, and electrode modification.

[0044] The method for preparing cerium-doped nickel-based metal cluster composite graphene is as follows: nickel nitrate hexahydrate, cerium nitrate hexahydrate, and urea are added to a graphene dispersion (deionized water as solvent). After stirring at room temperature for 20-30 minutes, the mixture is transferred to a hydrothermal reactor with a polytetrafluoroethylene liner. The hydrothermal reactor is sealed, and the temperature is increased to 110-120°C at a heating rate of 0.8-1.2°C / min. The mixture is then kept at this temperature for 10-12 hours for hydrothermal reaction. After naturally cooling to room temperature, the hydrothermal reactor is opened, and the material in the hydrothermal reactor is transferred to a centrifuge tube. The solids are collected by centrifugation at 8000-10000 rpm. The solids are washed with deionized water and anhydrous ethanol, dried, and ground evenly to obtain cerium-doped nickel-based metal cluster composite graphene.

[0045] In the preparation of cerium-doped nickel-based metal cluster composite graphene, the concentration of the graphene dispersion is 2-2.5 g / L;

[0046] The weight ratio of graphene in the dispersions of nickel nitrate hexahydrate, cerium nitrate hexahydrate, urea, and graphene is 58.2-64:17.4:360-380:45-50.

[0047] In this embodiment of the invention, a hydrothermal method is used to prepare a composite material with highly conductive graphene as the framework and cerium-doped nickel-based metal clusters grown in situ on the surface through in-situ composite formation. Under the premise of nickel-based redox pairs, cerium doping introduces Ce. 3+ / Ce 4+ The redox pair accelerates the reduction reaction of chromium ions through synergistic catalysis, significantly amplifying the electrochemical signal and improving detection sensitivity.

[0048] The functionalization modification method is as follows: cerium-doped nickel-based metal cluster composite graphene is added to a glutathione aqueous solution with a concentration of 1-1.2 mmol / L, ultrasonically dispersed for 20-30 min, transferred to a reaction vessel, and the air in the reaction vessel is completely replaced with nitrogen. Under nitrogen protection, the mixture is stirred at room temperature for 2-3 h. The material in the reaction vessel is then transferred to a centrifuge tube, centrifuged at 8000-10000 rpm to separate and collect the solids. The solids are washed with deionized water and dried to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene.

[0049] In the aforementioned functionalization modification, the mass-to-volume ratio of cerium-doped nickel-based metal cluster composite graphene to glutathione aqueous solution is 10 mg: 11-13 mL.

[0050] In this embodiment of the invention, cerium-doped nickel-based metal cluster composite graphene is contacted with glutathione. The active functional groups of glutathione coordinate with the metal ions on the surface of the cerium-doped nickel-based metal cluster composite graphene to form an organic molecular self-assembled layer. On the one hand, the functionalization modification of glutathione can improve the dispersibility of the composite material in the subsequent film-forming solution, and enhance the formation of a uniform and dense active layer on the electrode surface. On the other hand, glutathione can protect the Ni-Ce active centers, improving the stability and storage resistance of the electrode.

[0051] The electrode modification method is as follows: a Nafion perfluorosulfonic acid ion exchange resin solution (4-5 wt%) and an ethanol solution (40-45% vol%) are mixed evenly at a volume ratio of 1:8.9-9.1 to obtain a mixed solvent; glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene is added to the mixed solvent and ultrasonically dispersed for 20-30 min to obtain a film-forming solution; 5-6 μL of the film-forming solution is dropped onto the surface of a glassy carbon electrode, and after evaporation and drying in air, a composite detection electrode is obtained.

[0052] In the electrode modification, the mass-to-volume ratio of glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene to the mixed solvent is 2.8-3.2 mg:1 mL.

[0053] In this embodiment of the invention, a uniform thin film layer is formed on the electrode surface by combining a perfluorosulfonic acid ion exchange resin Nafion solution with an ethanol solution, and the glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene encapsulated therein is fixed on the electrode surface, thereby further improving the stability and storage resistance of the electrode.

[0054] The establishment of the standard curve includes the following steps: preparing standard working solution, detection calibration, and data processing.

[0055] The method for preparing the standard working solution is as follows: using a Cr(VI) standard stock solution with a concentration of 1000 mg / L, and using an acetate-sodium acetate buffer solution (pH=3.7) as a diluent, the Cr(VI) standard stock solution is gradually diluted to the following concentrations of Cr(VI) standard working solutions: 0 μg / L, 0.5 μg / L, 1 μg / L, 5 μg / L, 10 μg / L, 20 μg / L, 50 μg / L, 100 μg / L, and 200 μg / L, to obtain Cr(VI) standard working solutions with gradient concentrations.

[0056] The calibration method is as follows: A three-electrode system is established using the aforementioned composite detection electrode as the working electrode, a platinum wire as the counter electrode, and Ag / AgCl (2.5-3M KCl) as the reference electrode. This three-electrode system is inserted into an electrolytic cell containing 8-12 mL of acetate-sodium acetate buffer solution (pH=3.7). After purging with nitrogen to remove oxygen, the system is cleaned and reset at a constant potential of -0.25V (vs. Ag / AgCl) to -0.15V (vs. Ag / AgCl) for 60-90 seconds. The cleaned and reset three-electrode system is then removed and inserted into another electrolytic cell containing 8-12 mL of acetate-sodium acetate buffer solution (pH=3.7). The measurement parameters for square wave voltammetry (SWV) are as follows: initial potential +0.35V (vs. Ag / AgCl) to +0.45V (vs. Ag / AgCl), termination potential -0.45V (vs. Ag / AgCl) to -0.35V (vs. Ag / AgCl), potential increment 3-5mV, square wave amplitude 20-30mV, frequency 10-20Hz, and equilibrium time 20-30s. Measurements were performed on an acetate-sodium acetate buffer solution (pH=3.7) to obtain the electrolyte blank background signal.

[0057] Then, the aforementioned three-electrode system was removed and inserted into Cr(VI) standard working solutions of various concentrations. Under the same measurement parameters, SWV measurements were performed on the Cr(VI) standard working solutions of various concentrations. The reduction peak current lp (μA) of each concentration of Cr(VI) standard working solution in the range of +0.1V to +0.2V was recorded. After measuring the previous concentration of Cr(VI) standard working solution, the aforementioned constant potential cleaning and reset, and blank background signal measurement operations were repeated, and then SWV measurements were performed on the next concentration of Cr(VI) standard working solution. The constant potential cleaning and reset was to remove measurement residues and oxides from the electrode surface and to fully expose and reset the electrode active potential to its initial state.

[0058] In this embodiment of the invention, by measuring the signal response of Cr(VI) standard working solutions of various concentrations under specific measurement parameter conditions, a dataset of "concentration-current signal" correspondence is established; and by optimizing the setting of measurement parameters, the signal-to-noise ratio is improved, and the detection sensitivity and accuracy are enhanced.

[0059] The data processing method involves plotting a standard curve with the Cr(VI) concentration C (μg / L) of the Cr(VI) standard working solution as the abscissa and the reduction peak current lp (μA) as the ordinate. A linear regression equation of Cr(VI) standard working solution concentration - reduction peak current lp is then fitted, resulting in lp = KC + b, and a linear correlation coefficient R is required. 2 ≥0.995.

[0060] In this embodiment of the invention, the discrete "concentration-current signal" data points obtained in the detection calibration are transformed into a linear function that can be directly used for calculation through mathematical modeling.

[0061] The sample pretreatment method is as follows: After the collected soil is naturally air-dried, solid foreign objects such as stones and plant tissues are removed, the soil is ground evenly and then sieved to obtain a soil sample, and the mass m (kg) of the soil sample is recorded; the soil sample is placed in potassium phosphate buffer (pH=7.2), dispersed evenly, and then magnesium chloride solution with a concentration of 0.4-0.5 mol / L is added. After mixing evenly, the sample is extracted by shaking at room temperature at 200-300 rpm for 50-60 min, and then centrifuged at 6000-7000 rpm for 10-15 min. The supernatant is collected, and after filtering to remove solids, the obtained filtrate is the sample extract, and the total effective volume V (L) of the sample extract is recorded.

[0062] In the sample pretreatment, the potassium phosphate buffer is a K2HPO4 / KH2PO4 buffer.

[0063] The mass-to-volume ratio of soil sample, potassium phosphate buffer, and magnesium chloride solution was 1 g: 10-12 mL: 0.9-1 mL.

[0064] In this embodiment of the invention, a potassium phosphate buffer solution with pH=7.2 was used to extract soil samples to inhibit the reduction of Cr(VI) during the extraction process, thus ensuring extraction efficiency and the stability of Cr(VI). At the same time, magnesium chloride flocculation precipitation was used to reduce background interference in subsequent electrochemical detection.

[0065] The sample detection method is as follows: the sample extract is diluted with an acetate-sodium acetate buffer solution (pH=3.7) to make the Cr(VI) concentration fall within the linear range of the standard curve, and the sample solution to be tested is obtained, and the dilution factor D is recorded; after adjusting the pH of the sample solution to be tested to 3.7, it is introduced into an electrolytic cell, and the SWV of the sample solution to be tested is measured using the aforementioned three-electrode system under the aforementioned measurement parameter conditions to obtain the reduction peak current lp (μA) of the sample solution to be tested; the Cr(VI) concentration C (μg / L) in the sample solution to be tested is obtained using the standard curve, and combined with the mass m (kg) of the soil sample and the total effective volume V (L) of the sample extract recorded in the aforementioned sample pretreatment step, the Cr(VI) concentration in the soil is calculated according to the following formula: Cr(VI) (mg / kg) = [(C×V×D) / (m×1000)].

[0066] In this embodiment of the invention, the same measurement parameters as those used in the detection calibration are used to perform electrochemical detection on the diluted sample extract. After obtaining the Cr(VI) concentration using the standard curve, the Cr(VI) concentration in the soil sample is calculated.

[0067] The aforementioned technical methods work together synergistically to achieve high sensitivity, strong anti-interference ability, and good stability in detecting hexavalent chromium (Cr(VI)) in soil, while also being easy to operate and rapid in detection.

[0068] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described below in conjunction with some specific embodiments.

[0069] Example 1

[0070] This embodiment provides the preparation of a composite detection electrode used in the detection of heavy metals in soil. The specific steps are as follows:

[0071] (1) Preparation of cerium-doped nickel-based metal cluster composite graphene

[0072] Nickel nitrate hexahydrate, cerium nitrate hexahydrate, and urea were added to a graphene dispersion (deionized water as solvent). After stirring at room temperature for 25 minutes, the mixture was transferred to a hydrothermal reactor with a polytetrafluoroethylene liner. The reactor was sealed, and the temperature was increased to 110°C at a rate of 1°C / min. The mixture was then kept at this temperature for 12 hours and allowed to cool naturally to room temperature. The reactor was then opened, and the contents were transferred to a centrifuge tube. The solids were collected by centrifugation at 10,000 rpm. The solids were washed with deionized water and anhydrous ethanol, dried, and ground until homogeneous to obtain cerium-doped nickel-based metal cluster composite graphene.

[0073] The concentration of the graphene dispersion was 2.3 g / L.

[0074] The weight ratio of graphene in the dispersion of nickel nitrate hexahydrate, cerium nitrate hexahydrate, urea and graphene is 58.2:17.4:360:45.

[0075] (2) Functional modification

[0076] Cerium-doped nickel-based metal cluster composite graphene was added to a 1 mmol / L glutathione aqueous solution and ultrasonically dispersed for 20 min. Then, it was transferred to a reaction vessel, and the air in the reaction vessel was completely replaced with nitrogen. Under nitrogen protection, the mixture was stirred at room temperature for 2.5 h. The material in the reaction vessel was then transferred to a centrifuge tube and centrifuged at 10,000 rpm to collect the solids. The solids were washed with deionized water and dried to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene.

[0077] The mass-to-volume ratio of cerium-doped nickel-based metal cluster composite graphene to glutathione aqueous solution is 10 mg: 12 mL.

[0078] (3) Electrode modification

[0079] A mixed solvent was prepared by mixing Nafion perfluorosulfonic acid ion exchange resin solution (5 wt%) and ethanol solution (45 vol%) at a volume ratio of 1:9. Glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene was added to the mixed solvent and ultrasonically dispersed for 20 min to obtain a film-forming solution. 6 μL of the film-forming solution was dropped onto the surface of a glassy carbon electrode and evaporated and dried in air to obtain a composite detection electrode.

[0080] The mass-to-volume ratio of glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene to the mixed solvent was 3 mg: 1 mL.

[0081] Example 2

[0082] This embodiment provides the preparation of a composite detection electrode used in the detection of heavy metals in soil. The specific steps are as follows:

[0083] (1) Preparation of cerium-doped nickel-based metal cluster composite graphene

[0084] Nickel nitrate hexahydrate, cerium nitrate hexahydrate, and urea were added to a graphene dispersion (deionized water as solvent). After stirring at room temperature for 20 minutes, the mixture was transferred to a hydrothermal reactor with a polytetrafluoroethylene liner. The reactor was sealed, and the temperature was increased to 120°C at a rate of 1.2°C / min. The mixture was then kept at this temperature for 10 hours and allowed to cool naturally to room temperature. The reactor was then opened, and the contents were transferred to a centrifuge tube. The solids were collected by centrifugation at 10,000 rpm. The solids were washed with deionized water and anhydrous ethanol, dried, and ground until homogeneous to obtain cerium-doped nickel-based metal cluster composite graphene.

[0085] The concentration of the graphene dispersion was 2.3 g / L.

[0086] The weight ratio of graphene in the dispersion of nickel nitrate hexahydrate, cerium nitrate hexahydrate, urea and graphene is 58.2:17.4:360:45.

[0087] (2) Functional modification

[0088] Cerium-doped nickel-based metal cluster composite graphene was added to a 1 mmol / L glutathione aqueous solution and ultrasonically dispersed for 30 min. Then, it was transferred to a reaction vessel, and the air in the reaction vessel was completely replaced with nitrogen. Under nitrogen protection, the mixture was stirred at room temperature for 3 h. The material in the reaction vessel was then transferred to a centrifuge tube, and the solids were collected by centrifugation at 10,000 rpm. The solids were washed with deionized water and dried to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene.

[0089] The mass-to-volume ratio of cerium-doped nickel-based metal cluster composite graphene to glutathione aqueous solution is 10 mg: 11 mL.

[0090] (3) Electrode modification

[0091] A mixed solvent was prepared by mixing Nafion perfluorosulfonic acid ion exchange resin solution (5 wt%) and ethanol solution (45 vol%) at a volume ratio of 1:9. Glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene was added to the mixed solvent and ultrasonically dispersed for 30 min to obtain a film-forming solution. 6 μL of the film-forming solution was dropped onto the surface of a glassy carbon electrode and dried by evaporation in air to obtain a composite detection electrode.

[0092] The mass-to-volume ratio of glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene to the mixed solvent was 2.8 mg: 1 mL.

[0093] Example 3

[0094] This embodiment provides a method for detecting heavy metals in soil, using the composite detection electrode prepared in Example 1. The specific steps are as follows:

[0095] 1. Establish a standard curve

[0096] (1) Prepare standard working solution

[0097] Using a Cr(VI) standard stock solution with a concentration of 1000 mg / L, and with acetate-sodium acetate buffer solution (pH=3.7) as the diluent, the Cr(VI) standard stock solution was serially diluted to the following concentrations of Cr(VI) standard working solutions: 0 μg / L, 0.5 μg / L, 1 μg / L, 5 μg / L, 10 μg / L, 20 μg / L, 50 μg / L, 100 μg / L, and 200 μg / L, to obtain Cr(VI) standard working solutions with gradient concentrations.

[0098] (2) Testing and calibration

[0099] Using the aforementioned composite detection electrode as the working electrode, platinum wire as the counter electrode, and Ag / AgCl (3M KCl) as the reference electrode, a three-electrode system was established. This three-electrode system was inserted into an electrolytic cell containing 10 mL of acetate-sodium acetate buffer solution (pH=3.7). After purging with nitrogen to remove oxygen, the system was cleaned and reset at a constant potential of -0.2V (vs. Ag / AgCl) for 60 s. The cleaned and reset three-electrode system was then removed and inserted into another electrolytic cell containing 8-12 mL of acetate-sodium acetate buffer solution (pH=3.7). The square wave voltammetry (SWV) measurement parameters were set as follows: initial potential +0.4V (vs. Ag / AgCl), termination potential -0.4V (vs. Ag / AgCl), potential increment 4 mV, square wave amplitude 25 mV, frequency 15 Hz, and equilibrium time 30 s. The acetate-sodium acetate buffer solution (pH=3.7) was measured to obtain the blank background signal of the electrolyte.

[0100] Then, using the aforementioned three-electrode system, the electrodes were inserted into Cr(VI) standard working solutions of various concentrations. Under the same measurement parameters, SWV measurements were performed on each concentration of Cr(VI) standard working solution. The reduction peak current lp (μA) of each concentration of Cr(VI) standard working solution in the range of +0.1V to +0.2V was recorded. Furthermore, after measuring the previous concentration of Cr(VI) standard working solution, the aforementioned constant potential cleaning and reset, and blank background signal measurement operations were repeated before performing SWV measurements on the next concentration of Cr(VI) standard working solution. The constant potential cleaning and reset was performed to remove measurement residues and oxides from the electrode surface and to fully expose and reset the electrode active potential to its initial state. To verify reproducibility, each concentration of Cr(VI) standard working solution was measured in parallel three times.

[0101] (3) Data processing

[0102] A standard curve was plotted with the Cr(VI) concentration C (μg / L) of the Cr(VI) standard working solution as the x-axis and the reduction peak current lp (μA) as the y-axis. The linear regression equation for the Cr(VI) standard working solution concentration versus the reduction peak current lp was obtained by fitting the curve: lp = 0.0802C + 0.0127, and the linear correlation coefficient R0. 2 =0.9987.

[0103] 2. Sample pretreatment

[0104] Three soil samples were collected from different regions. After the collected soil was naturally air-dried, solid foreign objects such as stones and plant tissues were removed. The soil was then ground evenly and sieved to obtain soil samples No. 1-3. 0.002 kg (m) of each soil sample was weighed and placed in 20 mL of K2HPO4 / KH2PO4 buffer (pH=7.2). After being dispersed evenly, 2 mL of 0.5 mol / L magnesium chloride solution was added. After mixing evenly, the mixture was extracted by shaking at 200 rpm at room temperature for 60 min and centrifuged at 7000 rpm for 10 min. The supernatant was collected and filtered to remove solids to obtain the extracts of samples No. 1-3. The total effective volume V (L) of the extracts was recorded for each sample.

[0105] 3. Sample testing

[0106] The extracts of each sample were diluted with an acetate-sodium acetate buffer solution (pH=3.7) to bring the Cr(VI) concentration into the linear range of the standard curve, and the dilution factor D was recorded. After adjusting the pH of the sample solution to 3.7, it was introduced into an electrolytic cell. Using the aforementioned three-electrode system and under the aforementioned measurement parameters, the SWV of the sample solution was measured to obtain the reduction peak current lp (μA). The Cr(VI) concentration C (μg / L) in the sample solution was obtained using the standard curve. Combined with the mass m (kg) of the soil sample and the total effective volume V (L) of the sample extract recorded in the aforementioned sample pretreatment steps, the Cr(VI) concentration in the soil was calculated using the following formula: Cr(VI) (mg / kg) = [(C×V×D) / (m×1000)]. Each sample extract was measured in triplicate.

[0107] The detection concentration C of Cr(VI) in the extract of sample No. 1 was 15.2 μg / L, the dilution factor D was 2, and the calculated concentration of Cr(VI) in the soil was 0.334 mg / kg, with an intra-batch RSD (precision) of 4.1%. The content of hexavalent chromium in soil sample No. 1 corresponding to the extract of sample No. 1 was determined using the standard HJ1082-2019 "Determination of hexavalent chromium in soil and sediments by alkaline solution extraction-flame atomic absorption spectrophotometry". Based on the standard test results, the relative error rate was calculated to be 3.5%.

[0108] The detection concentration C of Cr(VI) in the extract of sample No. 2 was 8.5 μg / L, and the dilution factor D was 1. The calculated concentration of Cr(VI) in the soil was 0.094 mg / kg, and the intra-batch RSD (precision) was 4.5%. The content of hexavalent chromium in soil sample No. 2 corresponding to the extract of sample No. 2 was determined by standard HJ1082-2019 "Determination of hexavalent chromium in soil and sediments by alkaline extraction-flame atomic absorption spectrophotometry". Based on the standard test results, the relative error rate was calculated to be 4.0%.

[0109] The detection concentration C of Cr(VI) in the extract of sample No. 3 was 25.4 μg / L, and the dilution factor D was 3. The calculated concentration of Cr(VI) in the soil was 0.838 mg / kg, and the intra-batch RSD (precision) was 3.8%. The content of hexavalent chromium in soil sample No. 3 corresponding to the extract of sample No. 3 was determined by standard HJ1082-2019 "Determination of hexavalent chromium in soil and sediments by alkaline solution extraction-flame atomic absorption spectrophotometry". Based on the standard test results, the relative error rate was calculated to be 4.3%.

[0110] 4. Detection of spiked recovery rate

[0111] Take three equal volumes of sample extract from sample No. 2. Add Cr(VI) standard working solution with a concentration of 20 μg / L to the first sample extract, controlling the spiked concentration at 0.1 mg / kg. Using the aforementioned three-electrode system and under the aforementioned measurement parameters, detect the spiked sample extract. Based on the detection results of the spiked sample extract, the recovery rate is calculated to be 94.9%. Add Cr(VI) standard working solution with a concentration of 100 μg / L to the second sample extract, controlling the spiked concentration at 0.2 mg / kg. The aforementioned three-electrode system, under the aforementioned measurement parameters, was used to detect the spiked sample extract. Based on the detection results of the spiked sample extract, the recovery rate was calculated to be 97.0%. A Cr(VI) standard working solution with a concentration of 200 μg / L was added to the third sample extract, controlling the spike concentration to 0.5 mg / kg. Using the aforementioned three-electrode system, under the aforementioned measurement parameters, the spiked sample extract was detected. Based on the detection results of the spiked sample extract, the recovery rate was calculated to be 97.5%.

[0112] 5. Detection limit

[0113] Using the aforementioned three-electrode system and under the aforementioned measurement parameters, the noise level of the acetic acid-sodium acetate buffer solution (pH=3.7) was measured to obtain the electrolyte noise level. The aforementioned Cr(VI) standard working solution with a concentration of 1 μg / L was gradually diluted, and the detection limit was calculated with a signal-to-noise ratio (S / N) of 3. The method detection limit for Cr(VI) in soil was found to be 0.009 mg / kg.

[0114] Furthermore, to evaluate the response consistency of the composite detection electrode, a reproducibility test was conducted on the composite detection electrode of Example 1. Specifically, the preparation method of the composite detection electrode of Example 1 was repeated 5 times to prepare 5 composite detection electrodes.

[0115] The aforementioned Cr(VI) standard working solution with a concentration of 20 μg / L was used as the test solution. Using the aforementioned three-electrode system and under the aforementioned measurement parameters, each newly prepared composite detection electrode was used to detect the test solution, and the reduction peak current lp of the test solution was recorded. Each composite detection electrode was measured in parallel three times, and the average value was taken as the response value of the electrode.

[0116] The intra-batch RSD (precision) of the aforementioned five composite detection electrodes was calculated to be 4.2%, indicating that the preparation method of the composite detection electrodes is stable and controllable, and the prepared composite detection electrodes have good response consistency.

[0117] Furthermore, to evaluate the continuous measurement stability of the composite detection electrode, the same composite detection electrode was prepared using Example 1. Using the aforementioned three-electrode system, and under the aforementioned measurement parameter conditions, SWV scans were performed continuously 30 times on a Cr(VI) standard working solution with a concentration of 20 μg / L. Based on the reduction peak current lp obtained in the first measurement, the current retention rate of the composite detection electrode in the 30th measurement was calculated as: (reduction peak current lp obtained in the 30th measurement / reduction peak current lp obtained in the first measurement) × 100%.

[0118] The experiment showed that the current retention rate of the composite detection electrode in Example 1 still reached 95.3% in the 30th measurement, indicating that the composite detection electrode has good continuous measurement stability.

[0119] Furthermore, to evaluate the long-term stability of the composite detection electrode, the same composite detection electrode prepared in Example 1 was placed in a dry environment at 4°C and stored statically. On the 1st, 3rd, 7th and 14th days of storage, the composite detection electrode was taken out and, using the aforementioned three-electrode system, under the aforementioned measurement parameters, SWV scanning was performed on a 20 μg / L Cr(VI) standard working solution. The reduction peak current lp obtained was recorded. After the detection was completed, the composite detection electrode was transferred to an acetate-sodium acetate buffer solution (pH=3.7), and after constant potential cleaning and reset at -0.2V (vs. Ag / AgCl) for 60s, it was washed with ethanol and deionized water, dried with nitrogen, and then stored statically for a longer period.

[0120] Using the reduction peak current lp measured on day 1 as a reference, the current retention rate of the composite detection electrode in the measurement on day 14 is calculated as: (reduction peak current lp measured on day 14 / reduction peak current lp measured on day 1) × 100%.

[0121] The test showed that the composite detection electrode of Example 1 maintained a current retention rate of 90.2% after being stored at 4°C for 14 days, indicating that the composite detection electrode has good long-term stability.

[0122] Furthermore, to evaluate the anti-ionic interference performance of the composite detection electrode, a 20 μg / L Cr(VI) standard working solution was used as the test substrate. Interfering ions (Na+) were added to the test substrate. + K + Ca 2+ Mg 2+ Cu 2+ Pb 2+ Cd 2+ Fe 3+ Al 3+ Cl - NO3 - SO4 2- The concentration of interfering ions was controlled to be 10, 20, 50, 100, 250, 500, and 1000 times that of Cr(VI) in the Cr(VI) standard working solution. After adding different concentration ratios of interfering ions, the aforementioned three-electrode system was used to perform SWV scanning under the aforementioned measurement parameters, and the measured reduction peak current lp was recorded.

[0123] Using the reduction peak current lp of the Cr(VI) standard working solution as a reference, the signal change rate after the addition of interfering ions is calculated as: [(reduction peak current lp after the addition of interfering ions - reference reduction peak current lp) / reference reduction peak current lp] × 100%; the highest interfering ion concentration ratio that causes the signal change rate within ±5% is recorded as the maximum allowable concentration ratio of that ion.

[0124] According to the experiment, Na + K + Cl - NO3 - SO4 2- The maximum permissible concentration ratio for Cr(VI) detection exceeds 1000 times; Ca 2+ Mg 2+ The maximum permissible concentration ratio for Cr(VI) detection is 500 times; Fe 3+ Al 3+ The maximum permissible concentration ratio for Cr(VI) detection is 100 times, and the composite detection electrode exhibits good anti-interference capability against these ions. Meanwhile, Cu...2+ Pb 2+ Cd 2+ The maximum permissible concentration ratio for Cr(VI) detection is 50 times. At concentrations exceeding 50 times, although independent reduction peaks appear, they do not overlap with the Cr(VI) peak. In summary, it can be seen that the composite detection electrode has good resistance to ion interference.

[0125] Comparative Example 1

[0126] Comparative Example 1 uses the preparation method of the composite detection electrode of Example 1 and the detection method of Example 3. The differences are: (1) In the preparation of the composite detection electrode, the addition of cerium nitrate hexahydrate is omitted; (2) In the preparation of the composite detection electrode, the functionalization modification step is omitted, and the product prepared in the previous step is directly used for electrode modification.

[0127] Using the composite detection electrode from Comparative Example 1 to establish the standard curve, the linear regression equation for the concentration of Cr(VI) standard working solution and the reduction peak current lp was obtained: lp = 0.0437C + 0.0151, with a linear correlation coefficient R. 2 =0.9945, with a linear range of 0.5-50 μg / L; the method detection limit for Cr(VI) in soil was 0.033 mg / kg, significantly higher than 0.009 mg / kg in Example 3.

[0128] The composite detection electrode of Comparative Example 1 was used to detect the extracts of samples 1-3 in Example 3. The relative error rate of the detection results compared with the standard HJ1082-2019 "Determination of Hexavalent Chromium in Soil and Sediments by Alkali Extraction-Flame Atomic Absorption Spectrophotometry" was 9.2-10.1%, which was significantly higher than the 3.5-4.3% of Example 3. The detection spike recovery rate was 87.6-104.5%, and its fluctuation range was significantly wider than that of 94.9-97.5% of Example 3.

[0129] It can be seen that after omitting the cerium doping and glutathione functionalization modification of the composite detection electrode in Comparative Example 1, it cannot synergistically catalyze with the nickel-based redox pair, resulting in a weakened amplification effect of the electrical signal, a reduced slope of the standard curve, and a narrower linear range of the standard curve. At the same time, its spiked recovery rate and detection limit are significantly deteriorated, the detection sensitivity is reduced, the detection accuracy and stability are reduced, and the consistency with the detection results of standard HJ1082-2019 deteriorates.

[0130] Furthermore, using the composite detection electrode of Comparative Example 1, after 30 consecutive SWV scans of a 20 μg / L Cr(VI) standard working solution, the current retention rate was 83.7% (95.3% in Example 3). After being stored in a dry environment at 4°C for 14 days, the composite detection electrode was used to perform SWV scans on a 20 μg / L Cr(VI) standard working solution, and the current retention rate was 71.9% (90.2% in Example 3). It can be seen that the lack of glutathione functionalization significantly degrades the stability and storage resistance of the electrode.

[0131] Unless otherwise stated, all percentages used in this invention are mass percentages.

[0132] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for detecting heavy metals in soil, characterized in that, The detection method includes the following steps: preparing a composite detection electrode, establishing a standard curve, and detecting the sample. The preparation of the composite detection electrode includes the following steps: preparing cerium-doped nickel-based metal cluster composite graphene, functionalization modification, and electrode modification; The method for preparing cerium-doped nickel-based metal cluster composite graphene is as follows: nickel nitrate hexahydrate, cerium nitrate hexahydrate, and urea are added to a graphene aqueous dispersion, dispersed evenly, and then placed in a closed environment for hydrothermal reaction. The solids are separated and collected, and the solids are washed and dried to obtain cerium-doped nickel-based metal cluster composite graphene. The functionalization modification method is to modify cerium-doped nickel-based metal cluster composite graphene with glutathione to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene. The electrode modification method is as follows: glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene is mixed evenly with a solvent to obtain a film-forming solution; the film-forming solution is dropped onto the surface of a glassy carbon electrode and then dried to obtain a composite detection electrode; The method for establishing the standard curve is as follows: a three-electrode system is established using a composite detection electrode as the working electrode; the hexavalent chromium standard working solution with gradient concentrations is detected by square wave voltammetry to obtain the reduction peak current; and a standard curve is plotted with hexavalent chromium concentration as the abscissa and reduction peak current as the ordinate. The sample detection method is as follows: using the three-electrode system, the soil sample extract is detected by square wave voltammetry to obtain the reduction peak current; the concentration of hexavalent chromium in the soil is calculated according to the standard curve.

2. The method for detecting heavy metals in soil according to claim 1, characterized in that, In the preparation of cerium-doped nickel-based metal cluster composite graphene, the concentration of the graphene aqueous dispersion is 2-2.5 g / L; The weight ratio of graphene in the aqueous dispersions of nickel nitrate hexahydrate, cerium nitrate hexahydrate, urea, and graphene is 58.2-64:17.4:360-380:45-50. The hydrothermal reaction temperature is 110-120℃, and the hydrothermal reaction time is 10-12h.

3. The method for detecting heavy metals in soil according to claim 1, characterized in that, The functionalization modification method involves adding cerium-doped nickel-based metal cluster composite graphene into a glutathione aqueous solution, stirring at room temperature in a nitrogen atmosphere, separating and collecting the solid, and washing and drying the solid to obtain glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene.

4. The method for detecting heavy metals in soil according to claim 3, characterized in that, In the aforementioned functionalization modification, the concentration of the glutathione aqueous solution is 1-1.2 mmol / L; The mass-to-volume ratio of cerium-doped nickel-based metal cluster composite graphene to glutathione aqueous solution was 10 mg: 11-13 mL; Stirring at room temperature for 2-3 hours.

5. The method for detecting heavy metals in soil according to claim 1, characterized in that, In the electrode modification, the mass-to-volume ratio of glutathione-functionalized cerium-doped nickel-based metal cluster composite graphene to solvent is 2.8-3.2 mg: 1 mL; The solvent consists of a Nafion perfluorosulfonic acid ion exchange resin solution and an ethanol solution; the volume ratio of the Nafion perfluorosulfonic acid ion exchange resin solution to the ethanol solution is 1:8.9-9.

1. The mass concentration of the Nafion perfluorosulfonic acid ion exchange resin solution is 4-5 wt%. The volume concentration of the ethanol solution is 40-45%.

6. The method for detecting heavy metals in soil according to claim 1, characterized in that, In the establishment of the standard curve, a composite detection electrode is used as the working electrode in the three-electrode system. The measurement parameters for detecting the gradient concentration of hexavalent chromium standard working solution by square wave voltammetry are as follows: initial potential +0.35V to +0.45V, termination potential -0.45V to -0.35V, potential increment 3-5mV, square wave amplitude 20-30mV, frequency 10-20Hz, and equilibrium time 20-30s. The reduction peak current obtained by detecting hexavalent chromium standard working solutions of gradient concentrations using square wave voltammetry is the reduction peak current that appears in the range of +0.1V to +0.2V.

7. The method for detecting heavy metals in soil according to claim 1, characterized in that, The sample detection method is as follows: dilute the soil sample extract until the hexavalent chromium concentration falls within the linear range of the standard curve, record the dilution factor D, and obtain the sample solution to be tested; use the three-electrode system to detect the sample solution to be tested by square wave voltammetry to obtain the reduction peak current of the sample solution to be tested, and use the standard curve to obtain the concentration C of hexavalent chromium in the sample solution to be tested.

8. The method for detecting heavy metals in soil according to claim 7, characterized in that, The detection method further includes: sample pretreatment; The sample pretreatment method is as follows: the soil sample is placed in a potassium phosphate buffer solution with pH=7.2, dispersed evenly, and then magnesium chloride solution is added. After extraction by shaking at room temperature at 200-300 rpm, the supernatant is separated and collected, and the solids are filtered out to obtain the sample extract.

9. The method for detecting heavy metals in soil according to claim 8, characterized in that, In the sample pretreatment, the concentration of the magnesium chloride solution was 0.4-0.5 mol / L; The mass-to-volume ratio of soil sample, potassium phosphate buffer, and magnesium chloride solution was 1 g: 10-12 mL: 0.9-1 mL.

10. The method for detecting heavy metals in soil according to claim 8, characterized in that, In the sample detection, the measurement parameters of the sample liquid to be tested by square wave voltammetry are: initial potential +0.35V to +0.45V, termination potential -0.45V to -0.35V, potential increment 3-5mV, square wave amplitude 20-30mV, frequency 10-20Hz, and equilibrium time 20-30s. The concentration of hexavalent chromium in soil is calculated using the following formula: (C×V×D) / (m×1000); where C is the concentration of hexavalent chromium in the sample solution to be tested, μg / L; V is the volume of the sample extract, L; D is the dilution factor of the soil sample extract; and m is the mass of the soil sample, kg.