A porous composite material modified electrode and a preparation method and application thereof

Abstract

The application discloses a kind of porous composite material modified electrode and its preparation method and application, belong to chemical analysis detection technical field.The porous composite material modified electrode includes conductive substrate and modification layer fixed on the surface of conductive substrate, and modification layer is the biochar-polyurethane composite material with porous structure, the surface of modification layer contains C-O, N-H and-OH functional group, and the BET multipoint specific surface area of modification layer is 8.77m² / g-36.22m² / g, and BJH median pore size is 2.06nm-5.95nm.The application is by the biochar-polyurethane composite material with high specific surface area (8.77m² / g-36.22m² / g) and abundant surface functional group is modified on the surface of conductive substrate, increases one step open circuit enrichment operation before electrochemical scanning, makes the Cr 6+ High-efficiency pre-concentration is realized in the modification layer, and the problems of Cr 6+ The electrochemical detection has the problem of insufficient response under low concentration condition.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis and detection technology, specifically to a porous composite material modified electrode, its preparation method, and its application. Background Technology

[0002] Hexavalent chromium (Cr) 6+ The biotoxicity and environmental persistence of Cr are widely recognized, and strict environmental regulation of it relies on accurate and sensitive detection technologies. Current national standards specify the limits for Cr... 6+ Laboratory analytical methods, such as diphenylcarbazide spectrophotometry, are technically mature, but the operation involves multiple steps such as color development and extraction, which is time-consuming, and requires complex pretreatment for colored or turbid water samples. Inductively coupled plasma mass spectrometry (ICP-MS) is relatively sensitive, but its detection equipment is expensive and its maintenance costs are high, which cannot meet the growing market demand for on-site and low-cost rapid detection.

[0003] Electrochemical sensing technology is a sensor technology that combines electrochemical processes with sensitive elements for gas measurement, solution analysis, and the measurement of other physicochemical parameters. Electrochemical sensing technology is one of the ideal ways to solve the above-mentioned problems.

[0004] Currently, electrodes (such as glassy carbon electrodes and gold electrodes) are used directly to detect Cr in water. 6+ When performing voltammetry measurements, due to the presence of Cr in natural water bodies 6+ Most ions are present at trace levels, with very few reaching the electrode surface and participating in electron transfer reactions per unit time. This results in an extremely weak Faraday current signal, easily drowned out by charging current and instrument noise. Although some studies have modified the electrode surface with highly conductive carbon nanomaterials (such as graphene and carbon nanotubes) to increase the active area, these materials are susceptible to Cr... 6+ Lacking specific chemical affinity, the effect of simple physical sensitization is limited and cannot fundamentally solve the problem of insufficient response under low concentration conditions. Summary of the Invention

[0005] This invention provides a porous composite material modified electrode, its preparation method, and its application, aiming to solve the problems of Cr in the prior art. 6+ Electrochemical detection suffers from insufficient response under low concentration conditions.

[0006] The technical solution provided by this invention is as follows: The first aspect of the present invention provides a porous composite material modified electrode, comprising a conductive substrate and a modification layer fixed to the surface of the conductive substrate. The modification layer is a biochar-polyurethane composite material with a porous structure. The surface of the modification layer contains CO, NH and -OH functional groups. The BET multi-point specific surface area of ​​the modification layer is 8.77 m² / g-36.22 m² / g, and the BJH median pore size is 2.06 nm-5.95 nm.

[0007] Optionally, the modification layer is composed of biochar particles and polyurethane prepolymer.

[0008] Optionally, the biochar particles are at least one of coconut shell activated carbon and rice husk biochar, and the particle size of the biochar particles is 0.15 mm to 0.3 mm.

[0009] A second aspect of the present invention provides a method for preparing a porous composite material modified electrode, comprising: The biochar-polyurethane composite material is cut into thin sheets with a thickness of 1mm-3mm and fixed to the surface of the conductive substrate using conductive adhesive or physical clamping; or The biochar-polyurethane composite material is ground into powder, the powder is mixed with a conductive binder to form a slurry, the slurry is drop-coated or printed onto the surface of the conductive substrate and then dried and cured.

[0010] Optionally, the preparation method of the biochar-polyurethane composite material is as follows: Coconut shell activated carbon or rice husk biochar is crushed into activated carbon particles with a particle size of 50-100 mesh. The activated carbon particles are dried in sequence and deionized water is added to make a suspension slurry. Polyurethane prepolymer is added to the suspension slurry for foaming. After standing and setting, the biochar-polyurethane composite material is obtained. The mass ratio of the coconut shell activated carbon or rice husk biochar to the polyurethane prepolymer is 1:(1-2).

[0011] A third aspect of the present invention provides an application of the above-mentioned porous composite material modified electrode in the rapid on-site detection of hexavalent chromium in environmental water or industrial wastewater.

[0012] Optionally, when using the porous composite material-modified electrode for rapid on-site detection of hexavalent chromium, the following steps are included: The porous composite material modified electrode was immersed in the water sample to be tested, and open-circuit enrichment was performed under stirring conditions to increase the Cr content in the water sample. 6+ The adsorption and concentration are concentrated into the modification layer on the surface of the porous composite material modified electrode; The porous composite material-modified electrode was transferred to an electrolyte solution, and a cathode scan was performed using differential pulse voltammetry to record Cr. 6+ The current value of the reduction peak; According to the pre-established Cr 6+ The concentration-reduction peak current standard curve, with the Cr 6+ The current value of the reduction peak is converted to the Cr content in the water sample being tested. 6+ The concentration.

[0013] Optionally, the open-path enrichment time is 5 min to 30 min; The pH value of the water sample to be tested was 4.0-6.0; The electrolyte solution is a 0.1 mol / L to 0.5 mol / L hydrochloric acid or sulfuric acid solution.

[0014] Optionally, the scanning parameters of the differential pulse voltammetry are: initial potential 0.2 V, termination potential -0.6 V, potential increment 2mV-5mV, pulse amplitude 25mV-50mV, and pulse width 40ms-60ms. The Cr 6+ The potential of the reduction peak is -0.25V to 0.35V.

[0015] Optionally, the linear range of the standard curve is 2 μg / L-200 μg / L, and the detection limit is 0.5 μg / L-2.0 μg / L; Biochar-polyurethane composite material for Cr 6+ The maximum monolayer adsorption capacity is 3.61 mg / g-4.06 mg / g.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention modifies the working electrode surface with a biochar-polyurethane composite material possessing high specific surface area (8.77 m² / g - 36.22 m² / g) and abundant surface functional groups, adding an open-circuit enrichment step before electrochemical scanning to reduce Cr in the water sample. 6+ Highly efficient pre-concentration is achieved within the modified layer. This biochar-polyurethane composite material exhibits high Cr content. 6+ The adsorption behavior conforms to a pseudo-second-order kinetic model (with a correlation coefficient R² of 0.994-0.998), indicating that chemisorption is the rate-determining step, with a fast adsorption rate, and effective enrichment can be completed within 10-20 minutes. The isothermal adsorption behavior conforms to the Langmuir monolayer adsorption model (with a correlation coefficient R² of 0.966-0.977). The maximum monolayer adsorption capacity of coconut shell activated carbon-based materials reaches 4.06 mg / g, and that of rice husk biochar-based materials reaches 3.61 mg / g. This high adsorption capacity means that even in water samples containing Cr... 6+Even at concentrations only in the μg / L range, a concentration gradient much higher than that of the bulk solution can be established within the modified layer, resulting in a strong cumulative reduction current during subsequent electrochemical scanning. Experimental results show that the detection limit using the porous composite material modified electrode provided by this invention reaches 0.8 μg / L, with sensitivity improved by tens of times compared to the unmodified bare electrode. Attached Figure Description

[0017] Figure 1 This is a scanning electron microscope (SEM) image of the biochar-polyurethane composite material in an embodiment of the present invention. Figure 2 This is a flowchart illustrating the rapid on-site detection of hexavalent chromium using a porous composite material-modified electrode, as described in an embodiment of the present invention. Figure 3 The biochar-polyurethane composite material prepared in the embodiments of the present invention is effective against Cr. 6+ Adsorption kinetics curves; Figure 4 The biochar-polyurethane composite material prepared in the embodiments of the present invention is effective against Cr. 6+ Isothermal adsorption curves; Figure 5 The biochar-polyurethane composite material prepared in the embodiments of the present invention is effective against Cr. 6+ The fitting diagram of the quasi-second-order dynamic model, where (a) represents material A and (b) represents material B; Figure 6 The biochar-polyurethane composite material prepared in the embodiments of the present invention is effective against Cr. 6+ The fitting plots of the Langmuir isotherm adsorption model, where (a) represents material A and (b) represents material B; Figure 7 The biochar-polyurethane composite material prepared in this embodiment of the invention adsorbs Cr. 6+ The following are X-ray photoelectron spectroscopy (XPS) images, where (a) represents material A and (b) represents material B. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of the present invention, and not all embodiments.

[0019] Therefore, the following detailed description of the embodiments of the present invention provided in conjunction with the accompanying drawings is intended merely to illustrate selected embodiments of the invention and is not intended to limit the scope of protection claimed by the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0020] This invention provides a porous composite material modified electrode, comprising a conductive substrate and a modification layer fixed to the surface of the conductive substrate. The modification layer is a biochar-polyurethane composite material with a three-dimensional interconnected porous structure. The surface of the modification layer contains CO, NH and -OH functional groups. The BET multi-point specific surface area of ​​the modification layer is 8.77m² / g-36.22m² / g, and the BJH median pore size is 2.06nm-5.95nm.

[0021] In traditional electrochemical detection, trace amounts are limited by diffusion and mass transfer, resulting in a very small number of ions reaching the surface of the conductive substrate (i.e., the working electrode) per unit time. This leads to a weak Faraday current signal, which is difficult to distinguish from background noise. This invention modifies the surface of a conductive substrate with a biochar-polyurethane composite material possessing a high specific surface area (8.77 m² / g - 36.22 m² / g) and abundant surface functional groups, forming a porous composite modified electrode. This electrode is used to detect Cr in water samples. 6+ During detection, this is equivalent to adding an open-circuit enrichment step before the electrochemical scan, allowing the Cr in the water sample to be detected. 6+ Efficient pre-concentration is achieved within the modified layer. The CO, NH, and -OH functional groups distributed on the surface of the biochar-polyurethane composite material affect Cr. 6+ Anions (HCrO4- / CrO4-) 2- It exhibits a specific complexing tendency. The median pore size of the BJH in the modified layer is 2.06 nm-5.95 nm, which enables the modified layer to form a well-ordered ion transport network, allowing trace amounts of Cr in the water to be transported. 6+ Rapid diffusion to the active sites of the porous composite material modified electrode significantly accelerates the ion mass transfer rate, solving the problem of trace Cr in the original bare electrode. 6+ The problem is that the number of electrodes reaching the working electrode surface is small and the current signal is weak.

[0022] The biochar-polyurethane composite material with a three-dimensional interconnected porous structure used in this invention possesses excellent mechanical strength and structural stability. The modified layer is firmly bonded to the electrode substrate and is not easily detached or pulverized during multiple enrichment-scanning-cleaning cycles. After the measurement is completed, the porous composite modified electrode is cyclically scanned several times in a blank electrolyte solution within the potential range of 0.2V-0.6V, which electrochemically removes residual chromium species from the modified layer, thus regenerating the electrode. Experiments show that even after more than 20 consecutive uses, the relative standard deviation of the peak current response value can still be controlled within 5%. This characteristic significantly reduces the consumable cost per test, making this method economically feasible in large-scale sample screening scenarios.

[0023] The biochar-polyurethane composite material used in this invention possesses both high adsorption capacity and electron supply capability, enabling the construction of a continuous reaction domain on the working electrode surface, encompassing adsorption-concentration-reduction-signal output. This allows for the conversion of difficult-to-detect low-concentration chemical information into a clear and measurable current signal. The scanning electron microscope (SEM) morphology image of the biochar-polyurethane composite material provided by this invention is shown below. Figure 1 As shown.

[0024] Preferably, the modification layer is composed of biochar particles and polyurethane prepolymer.

[0025] Specifically, the modified layer of this invention is composed of biochar particles and polyurethane prepolymer, which work synergistically to solve the existing Cr problem. 6+ The technical limitations of electrochemical detection electrodes, specifically their effects, are as follows: Biochar particles, as the core functional component of porous composite modified electrodes, play two roles: firstly, they possess excellent conductivity and a natural porous structure, providing fundamental electrochemical active sites for the modification layer, achieving physical sensitization, and effectively enhancing the electrode's electronic conductivity; secondly, biochar particles themselves are naturally rich in functional groups such as CO, -OH, and NH, which can interact with Cr through coordination, electrostatic adsorption, and other processes. 6+ This enables specific recognition, compensating for the limitations of existing pure carbon nanomaterials on Cr. 6+ It lacks specific chemical affinity and has limited physical sensitization effect; at the same time, biochar particles are readily available and have a stable porous structure, providing a basic framework support for the formation of the porous structure of the modification layer.

[0026] Polyurethane prepolymer, as a binder and structure-regulating component in porous composite modified electrodes, serves two main purposes: First, it acts as a binder, firmly attaching loose biochar particles to the conductive substrate surface, effectively preventing the modified layer from detaching or peeling off during testing, thus significantly improving the repeatability and lifespan of the porous composite modified electrode. Second, during the curing process, it precisely regulates the pore structure of the biochar-polyurethane composite material, stabilizing the median BJH pore size of the modified layer within the optimal range of 2.06 nm to 5.95 nm, synergistically constructing an ordered mesoporous transport network with the biochar particles, and optimizing the Cr content. 6+ The enrichment and mass transfer efficiency; third, to improve the dispersion of biochar particles on the conductive substrate surface, avoid biochar particle agglomeration, make the modified layer film uniform, dense and stable, reduce defects on the surface of porous composite material modified electrode, and reduce background noise during the detection process.

[0027] In summary, the composite of biochar particles and polyurethane prepolymer exhibits a synergistic effect. Biochar particles provide conductivity, specific functional groups, and a basic porous framework, while the polyurethane prepolymer ensures bonding and immobilization, precise pore size control, film stability, and functional group replenishment. The combination of these two components allows the modified layer to stably possess a defined BET specific surface area and BJH median pore size. This solves the problems of weak signals and poor anti-interference capabilities of bare electrodes, while overcoming the shortcomings of pure carbon-modified electrodes, such as insufficient specificity and poor low-concentration response. Ultimately, this achieves effective control of trace Cr in water. 6+ It provides accurate and sensitive detection while also ensuring electrode stability and practicality, meeting the market demand for on-site and low-cost rapid detection.

[0028] Preferably, the biochar particles are at least one of coconut shell activated carbon and rice husk biochar, and the particle size of the biochar particles is 0.15mm-0.3mm.

[0029] Specifically, in this embodiment, the biochar particles are preferably at least one of coconut shell activated carbon or rice husk biochar, and the particle size is limited to 0.15-0.3 mm. The reason is that both coconut shell activated carbon and rice husk biochar possess natural porous structures and excellent conductivity, and the raw materials are inexpensive and readily available, meeting the requirements for low-cost detection. Coconut shell activated carbon, in particular, has regular pores and a large specific surface area, providing more active sites and enhancing the activity of Cr. 6+ Adsorption and electron transfer; rice husk biochar is rich in silicon and polar functional groups, which can enhance the adsorption of Cr. 6+ The specific coordination and electrostatic adsorption of these two components allow them to be used individually for different scenarios, and their combined use provides complementary advantages. Compared to other biochars, they have fewer impurities and stronger stability, which can reduce interference with detection signals. Limiting the particle size to 0.15mm-0.3mm is key to achieving synergy with polyurethane prepolymer and the median pore size of BJH: excessively large particle sizes easily agglomerate, leading to uneven film formation and pore blockage in the modified layer, hindering Cr... 6 Mass transfer is impaired, and adhesion to the polyurethane prepolymer is reduced, making it prone to detachment; if the particle size is too small, it is difficult to be fixed by the polyurethane prepolymer, and the pores of the modified layer are too dense, hindering Cr 6 The particle size allows the biochar to be uniformly dispersed in the polyurethane prepolymer, and the polyurethane prepolymer is used to precisely control the optimal pore size of 2.06nm-5.95nm, maximizing the exposure of active functional groups, improving enrichment and electron transfer efficiency, and ensuring electrode stability and detection sensitivity.

[0030] This invention provides a method for preparing the above-mentioned porous composite material modified electrode, comprising the following steps: Biochar-polyurethane composite materials are cut into thin sheets with a thickness of 1mm-3mm and fixed to the surface of a conductive substrate using conductive adhesive or physical clamping methods; or The biochar-polyurethane composite material is ground into powder, the powder is mixed with a conductive binder to form a slurry, the slurry is drop-coated or printed onto the surface of a conductive substrate and then dried and cured.

[0031] Specifically, the method provided by this invention does not depend on a specific type or material of working electrode. The biochar-polyurethane composite material can be cut into sheets and then bonded together to adapt to conventional disc electrodes such as glassy carbon electrodes and gold electrodes; it can also be ground into powder and mixed with a conductive binder, and then modified onto the surface of a conductive substrate by drop coating or printing to adapt to commercial disposable electrode sheets.

[0032] Preferably, the preparation method of the above-mentioned biochar-polyurethane composite material is as follows: Coconut shell activated carbon or rice husk biochar is crushed into activated carbon particles with a particle size of 50-100 mesh. The activated carbon particles are dried in sequence and deionized water is added to make a suspension slurry. Polyurethane prepolymer is added to the suspension slurry for foaming. After standing and setting, biochar-polyurethane composite material is obtained. The mass ratio of coconut shell activated carbon or rice husk biochar to polyurethane prepolymer is 1:(1-2).

[0033] Specifically, in this embodiment, biochar is pulverized into 50-100 mesh (corresponding to 0.15mm-0.3mm), which is the basis for synergistic effects with the polyurethane prepolymer and for controlling pore size. Drying removes moisture and impurities from the activated carbon particles, avoiding interference with the foaming of the polyurethane prepolymer and ensuring that the pore size remains stable at 2.06nm-5.95nm; deionized water ensures uniform dispersion of the biochar, laying the foundation for thorough mixing and uniform foaming. Foaming and settling are key to constructing a porous structure and are directly related to pore size requirements. The foaming of the polyurethane prepolymer forms micropores, and the activated carbon particles act as a framework to prevent pore collapse and guide the formation of regular channels; settling stabilizes the pore size within the optimal range, providing a stable pore size for Cr 6+ Enrichment and mass transfer are guaranteed.

[0034] The mass ratio of coconut shell activated carbon or rice husk biochar to polyurethane prepolymer should be 1:(1-2). If the ratio is too high, it indicates insufficient polyurethane prepolymer, resulting in poor adhesion and uncontrolled pore size; if the ratio is too low, it indicates excessive polyurethane prepolymer, leading to dense pores that block mass transfer channels and reduce detection sensitivity. This ratio achieves a balance, ensuring accurate pore size and stable adhesion, and fully leveraging the advantages of both.

[0035] This invention also provides an application of the above-mentioned porous composite material modified electrode in the rapid on-site detection of hexavalent chromium in environmental water or industrial wastewater. No large instruments or organic reagents are required; only a portable electrochemical workstation and a porous composite material modified electrode are needed. A single detection time does not exceed 30 minutes, with a spiked recovery rate of 95%-99% and a relative standard deviation of less than 5%.

[0036] like Figure 2 As shown, the following steps are included when using a porous composite material modified electrode for rapid on-site detection of hexavalent chromium: S1. Immerse the porous composite material modified electrode into the water sample to be tested, and perform open-circuit enrichment under stirring conditions to increase the Cr content in the water sample. 6+ Adsorption and concentration are concentrated into the modification layer on the surface of the porous composite material modified electrode.

[0037] S2. The porous composite material modified electrode was transferred to an electrolyte solution, and a cathode scan was performed using differential pulse voltammetry to record the Cr content. 6+ The current value of the reduction peak.

[0038] S3, based on the pre-established Cr 6+ Concentration-reduction peak current standard curve, Cr 6+ The current value of the reduction peak is converted to the Cr content in the water sample being tested. 6+ The concentration.

[0039] Specifically, S1 is the open-path enrichment step, the purpose of which is to address the Cr content in the water sample. 6+ The issue of trace amounts of Cr. Stirring conditions can accelerate the process. 6+ Diffusion onto the surface of the porous composite material-modified electrode allows the porous structure of the modified layer (BJH median pore size 2.06 nm-5.95 nm) and surface functional groups (CO, NH, -OH) to specifically adsorb and concentrate Cr. 6+ To achieve trace Cr 6+ Rapid enrichment provides a recognizable signal basis for subsequent detection, avoiding the need for low-concentration Cr. 6+ The signal is too weak to be detected.

[0040] S2 involves transferring the porous composite material-modified electrode into an electrolyte solution and employing differential pulse voltammetry for cathode scanning, which is crucial for ensuring detection accuracy. Transferring to the electrolyte solution avoids interference from impurities in the water sample, ensuring stable electron transfer during the scanning process; differential pulse voltammetry offers strong anti-interference capabilities and high sensitivity, enabling precise capture of Cr. 6+ By restoring the peak current, the influence of charging current and instrument noise is eliminated, ensuring the accuracy of signal detection.

[0041] The S3 uses a standard curve to convert concentration, adapting to the needs of rapid on-site detection. A pre-established concentration-reduction peak current standard curve allows for the conversion of current values ​​to Cr... 6+ The rapid concentration conversion requires no complex calculations and is easy to operate, meeting the core needs of low-cost and rapid on-site testing while ensuring the reliability of test results.

[0042] Traditional Cr 6+Traditional detection methods typically separate enrichment and detection operations. Taking solid-phase extraction-spectrophotometry (SPE-Spectrophotometry) as an example, multiple steps are required sequentially, including column enrichment, eluent collection, addition of a colorimetric reagent, and colorimetric determination, taking over an hour in total. Furthermore, the use of eluent can introduce secondary contamination and operational errors. This invention integrates enrichment and signal conversion functions into a single porous composite material modified electrode interface. The detection process requires only three steps: immersion of the porous composite material modified electrode in the water sample for enrichment (5-30 min) → transfer to an electrolyte solution → execution of differential pulse voltammetry (approximately 2 min). The entire process requires no organic solvents, no separate elution operation, and no sample digestion. The only required equipment is a portable electrochemical workstation and a porous composite material modified electrode. The operation can be performed by non-professionals after simple training. This feature gives this method a significant advantage in grassroots environmental monitoring stations, industrial wastewater discharge outlets, and emergency on-site screening for sudden pollution incidents.

[0043] Preferably, in the above detection method, the open-circuit enrichment time is 5 min-30 min, the pH value of the water sample to be tested is 4.0-6.0, and the electrolyte solution is a 0.1 mol / L-0.5 mol / L hydrochloric acid or sulfuric acid solution.

[0044] The scanning parameters for differential pulse voltammetry are: initial potential 0.2 V, termination potential -0.6 V, potential increment 2mV-5mV, pulse amplitude 25mV-50mV, and pulse width 40ms-60ms.

[0045] Cr 6+ The potential of the reduction peak is -0.25V to 0.35V.

[0046] The linear range of the standard curve was 2 μg / L–200 μg / L, and the detection limit was 0.5 μg / L–2.0 μg / L; the biochar-polyurethane composite material for Cr 6+ The maximum monolayer adsorption capacity is 3.61 mg / g-4.06 mg / g.

[0047] Specifically, limiting the open-circuit enrichment time to 5-30 minutes strikes a balance between efficiency and enrichment effect: too short a time will result in trace amounts of Cr... 6+ The signal is weak because the Cr cannot be fully adsorbed onto the modified layer; the detection time is too long, increasing the detection time and not meeting the requirements for rapid on-site detection. This range can achieve Cr 6+ Balancing efficient enrichment with rapid detection.

[0048] The pH value of the water sample to be tested is 4.0-6.0, which can ensure the Cr content. 6+ Adsorption and detection stability: This acidic range can maintain Cr 6+ The ionic form is matched to the adsorption activity of the functional groups on the surface of the modified layer, avoiding excessive pH from causing Cr.6+ Hydrolysis and low-corrosion electrodes ensure smooth adsorption and subsequent detection.

[0049] The electrolyte solution should be 0.1 mol / L to 0.5 mol / L hydrochloric acid or sulfuric acid. The key is to ensure stable electron transfer: this concentration range provides sufficient ions to reduce solution resistance, while hydrochloric acid and sulfuric acid do not contain interfering ions, thus avoiding interference with Cr. 6+ Restore the peak signal to ensure the accuracy of scanning detection.

[0050] The precise capture of the reduction peak is achieved by limiting the scanning parameters of the differential pulse voltammetry: an initial potential of 0.2V and a termination potential of -0.6V, which can completely cover the Cr peak. 6+ The reduction peak potential (-0.25V to 0.35V), with a potential increment of 2mV to 5mV, a pulse amplitude of 25mV to 50mV, and a pulse width of 40ms to 60ms, can balance scanning sensitivity and speed, effectively eliminate noise interference, and accurately capture the reduction peak current.

[0051] Cr 6+ The reduction peak potential is limited to -0.25V to 0.35V, enabling specific identification: this potential range is for Cr. 6+ The characteristic potential of reduction can avoid interference from the reduction signals of other ions in the water sample, ensuring that the current signal is consistent with Cr. 6+ Precise correspondence of concentration.

[0052] The standard curve has a linear range of 2 μg / L–200 μg / L and a detection limit of 0.5 μg / L–2.0 μg / L, suitable for trace detection needs: the linear range covers Cr in natural water bodies. 6+ Common trace detection ranges; low detection limits can meet the requirements for low concentrations of Cr. 6+ The testing requirements align with the needs of on-site trace monitoring; the maximum monolayer adsorption capacity of 3.61 mg / g-4.06 mg / g ensures the modified layer's ability to detect trace amounts of Cr. 6+ The full adsorption of the substance provides support for the low detection limit.

[0053] The technical solution of the present invention will be further described in detail below through specific embodiments. Experimental methods not specified in the embodiments are generally performed under conventional conditions or conditions recommended by the manufacturer.

[0054] Example 1 This embodiment provides a biochar-polyurethane composite material, which is made by combining coconut shell activated carbon and polyurethane prepolymer through a foaming process.

[0055] The preparation of biochar-polyurethane composite materials includes the following steps: (1) Wash commercially available coconut shell activated carbon with deionized water to remove surface dust, air dry naturally, and then crush it in a crusher. Use a standard sieve to collect the particle components with a particle size range of 0.15mm-0.3mm (corresponding to 50 mesh-100 mesh), dry them in an oven at 105℃ to constant weight, and obtain coconut shell activated carbon particles, which are then sealed and stored for later use.

[0056] (2) Weigh 30g of the above-mentioned coconut shell activated carbon particles, add 30g of deionized water, and stir continuously in a container until a uniform suspension without obvious stratification is formed. While stirring, quickly add 48g of polyurethane prepolymer (i.e., 16g of prepolymer for every 10g of coconut shell activated carbon), and continue stirring at high speed for 40 seconds before stopping. At this time, the volume of the mixture expands significantly. Let it stand for 2-5 minutes to complete the solidification and shaping, and then place it in a ventilated place to air dry naturally to obtain the biochar-polyurethane composite material, denoted as material A.

[0057] According to nitrogen adsorption-desorption tests, the BET multi-point specific surface area of ​​material A is 36.22 m² / g, the BJH median pore size is 5.95 nm, and the cumulative pore volume is 0.029 cm³ / g, which belongs to typical mesoporous materials.

[0058] Example 2 This embodiment provides a biochar-polyurethane composite material, which is made by combining rice husk biochar and polyurethane prepolymer through a foaming process.

[0059] The preparation of biochar-polyurethane composite materials includes the following steps: (1) Wash the rice husk biochar with deionized water to remove dust, air dry it naturally, crush it, sieve out the particle components with a particle size of 0.15mm-0.3mm, dry it for later use, and obtain rice husk biochar particles.

[0060] (2) Weigh 30g of the above rice husk biochar particles, add 35g of deionized water, and stir until a uniform suspension is formed. Add 45g of polyurethane prepolymer (i.e., 15g of prepolymer for every 10g of rice husk biochar) while stirring, stop stirring at high speed for 40 seconds, let stand for 2min-5min to complete the curing and shaping, and air dry to obtain the biochar-polyurethane composite material, denoted as material B.

[0061] According to nitrogen adsorption-desorption tests, the BET multi-point specific surface area of ​​material B is 8.77 m² / g, the BJH median pore size is 2.06 nm, the cumulative pore volume is 0.036 cm³ / g, and the isotherm type conforms to type IV in the IUPAC classification.

[0062] To verify the functional basis of the biochar-polyurethane composite material used in the detection method, its adsorption properties and surface chemical state were characterized.

[0063] Take 0.3g each of material A and material B (grinded to a particle size no greater than 1mm), and add 100mL of Cr solution with an initial concentration of 20mg / L to each. 6+ The solution (prepared with potassium dichromate, pH adjusted to 5.0 with hydrochloric acid and sodium hydroxide) was used for adsorption by shaking at a rate of 150 r / min under constant temperature conditions of 25℃. Samples were taken at preset time points, filtered through a 0.45 μm filter membrane, and the residual Cr in the solution was determined by atomic absorption spectrophotometry. 6+ Concentration, used to calculate the amount of adsorption per unit mass of material.

[0064] The adsorption kinetics results are shown in Table 1. Within the first 12 hours of adsorption, materials A and B achieved 82.3% and 75.2% of their respective equilibrium adsorption capacities, exhibiting rapid adsorption kinetics. A pseudo-second-order kinetic model was used to fit the experimental data; the equation is as follows: (1) In equation (1), Q t Let Q be the amount of adsorption at time t. e To balance the adsorption amount, K S The value is a quasi-second-order rate constant. The fitting results are shown in Table 2.

[0065] Table 1. Effects of two materials on Cr at different times 6+ Adsorption capacity (mg / g)

[0066] Table 2 Fitting parameters of the pseudo-second-order dynamic model

[0067] The above results indicate that the adsorption process of Cr6+ by both materials is controlled by chemisorption, which can achieve significant adsorption and enrichment effects in a short time.

[0068] In the isothermal adsorption experiment, the Cr content was controlled. 6+ Initial concentrations were set at 5 mg / L, 10 mg / L, 15 mg / L, 20 mg / L, 25 mg / L, 30 mg / L, 40 mg / L, and 50 mg / L. Residual concentrations were measured after 24 hours of adsorption to reach equilibrium. The equilibrium data were fitted using the Langmuir model, and the equation is as follows: (2) (2) In the formula, C e To achieve equilibrium concentration, Q e To balance the adsorption amount, Q m K represents the maximum monolayer adsorption capacity, and K1 is the Langmuir adsorption constant. The fitting results are shown in Table 3.

[0069] Table 3 Fitting parameters of the Langmuir isothermal adsorption model

[0070] The biochar-polyurethane composite material prepared in this invention has a positive effect on Cr 6+ The adsorption behavior conforms to a pseudo-second-order kinetic model, with a fitting correlation coefficient R² of 0.994–0.998; the isothermal adsorption behavior conforms to the Langmuir monolayer adsorption model, with a fitting correlation coefficient R² of 0.966–0.977. The biochar-polyurethane composite material exhibits good adsorption properties for Cr... 6+ The maximum monolayer adsorption capacity is 3.61 mg / g-4.06 mg / g.

[0071] Material A and Material B were respectively immersed in 20 mg / L Cr. 6+ After treatment in solution for 24 hours, the material was removed, rinsed, dried, and then subjected to X-ray photoelectron spectroscopy (XPS) analysis. The results showed that Cr was detected on the surface of both materials at a binding energy of approximately 577 eV. 3+ The 2p3 / 2 characteristic peak was detected with Cr at approximately 579 eV-580 eV. 6+ The 2p3 / 2 characteristic peak confirms that the material adsorbs Cr. 6+ Simultaneously, it can utilize the electron supply effect of surface functional groups to partially convert Cr... 6+ Reduced to Cr 3+ .

[0072] Figure 3 This is an adsorption kinetic curve of Cr6+ on the biochar-polyurethane composite material prepared in this invention. The horizontal axis represents the adsorption time t (h), and the vertical axis represents the adsorption amount Q per unit mass. t (mg / g); CACLS represents coconut shell activated carbon-based composite material A, and RHBLS represents rice husk biochar-based composite material B. The curves show the effect of the two materials on Cr 6+ All of them exhibit rapid adsorption characteristics, achieving over 80% equilibrium adsorption within the first 12 hours, meeting the requirements for rapid on-site enrichment.

[0073] Figure 4 The biochar-polyurethane composite material prepared for this invention has a positive effect on Cr 6+ The isothermal adsorption curve, with Cr on the horizontal axis. 6+ equilibrium concentration C e (mg / L), with the ordinate representing the equilibrium adsorption capacity Q. e (mg / g). Figure 4 This indicates that with Cr 6+ As the concentration increases, the adsorption capacity gradually increases and tends to saturate, exhibiting typical monolayer adsorption behavior; the adsorption capacity of coconut shell activated carbon-based materials is higher than that of rice husk-based materials.

[0074] Figure 5 The biochar-polyurethane composite material prepared for this invention has a positive effect on Cr 6+ The fitting plots of the pseudo-second-order kinetic model are shown, where (a) represents material A and (b) represents material B. The horizontal axis represents the adsorption time t (h), and the vertical axis represents the adsorption amount Q. t (mg / g). Figure 5 The results indicate that the pseudo-second-order kinetic model has a significantly better fit than the pseudo-first-order model, suggesting that chemisorption is the rate-determining step in the adsorption process.

[0075] Figure 6 The biochar-polyurethane composite material prepared for this invention has a positive effect on Cr 6+ The Langmuir isotherm adsorption model fitting plots are shown, where (a) represents material A and (b) represents material B. The horizontal axis represents Cr. 6+ equilibrium concentration C e (mg / L), with the ordinate representing the equilibrium adsorption capacity Q. e (mg / g). Figure 6 This indicates that the Langmuir model has a better fit, proving that the material is more sensitive to Cr. 6+ This represents the monolayer adsorption behavior of uniformly active sites.

[0076] Figure 7 The biochar-polyurethane composite material prepared in this invention adsorbs Cr 6+ The following are X-ray photoelectron spectroscopy (XPS) images, where (a) represents material A and (b) represents material B, and the horizontal axis represents the binding energy (eV). Figure 7 Cr appears at approximately 577 eV. 3+ Characteristic peaks appear for Cr at 579 eV–580 eV. 6+ Characteristic peaks confirm that the material adsorbs Cr 6+ At the same time, it can be partially reduced to Cr 3 + It possesses intrinsic reducing activity.

[0077] Example 3 Material A obtained in Example 1 was cut into circular pieces with a diameter of approximately 3 mm and a thickness of approximately 2 mm using a sharp blade. A glassy carbon electrode with a diameter of 3 mm was polished on chamois leather to a mirror finish using 1.0 μm, 0.3 μm, and 0.05 μm α-alumina polishing powders, followed by ultrasonic cleaning and drying with high-purity nitrogen. A thin layer of conductive carbon adhesive was uniformly coated onto the surface of the glassy carbon electrode, and the cut material A circular pieces were then adhered to it. The electrode was allowed to cure naturally at room temperature for 2 hours, thus obtaining a porous composite material modified electrode modified with material A.

[0078] Cr prepared with potassium dichromate 6+The standard stock solution (100 mg / L) was serially diluted to obtain standard working solutions with concentrations of 0 μg / L, 2 μg / L, 5 μg / L, 10 μg / L, 20 μg / L, 50 μg / L, 100 μg / L, and 200 μg / L. 0.01 mol / L KCl was added to each solution as a supporting electrolyte, and the pH was adjusted to 5.0 with dilute hydrochloric acid.

[0079] The porous composite material modified electrode prepared in Example 3 was used as the working electrode, the Ag / AgCl (saturated KCl) electrode as the reference electrode, and the platinum wire as the counter electrode. The porous composite material modified electrode was sequentially immersed in the aforementioned standard solutions of various concentrations, and enriched in an open circuit for 15 min under magnetic stirring at 200 r / min. After enrichment, the porous composite material modified electrode was removed, its surface was gently rinsed with a small amount of deionized water, and then transferred to an electrochemical cell containing a 0.1 mol / L HCl electrolyte solution.

[0080] Differential pulse voltammetry was used for cathode scanning with the following parameters: initial potential 0.2V, termination potential -0.6V, potential increment 4mV, pulse amplitude 50mV, and pulse width 50ms. The reduction peak current value at approximately -0.3V (vs. Ag / AgCl) was recorded. After each measurement, the porous composite modified electrode was placed in a blank electrolyte and cyclically scanned five times in the range of 0.2V–0.6V to remove residual chromium species until the peak current disappeared.

[0081] Test results show that in Cr 6+ When the standard solution concentrations were 2 μg / L, 5 μg / L, 10 μg / L, 20 μg / L, 50 μg / L, 100 μg / L, and 200 μg / L, the corresponding reduction peak current values ​​were 0.41 μA, 0.78 μA, 1.52 μA, 2.87 μA, 6.63 μA, 12.96 μA, and 25.31 μA, respectively. The peak current value I (μA) was used to correlate with Cr... 6+ Linear regression analysis of concentration C (μg / L) yielded the standard curve equation: I = 0.126C + 0.251, with a correlation coefficient R² = 0.996. The detection limit was calculated to be 0.8 μg / L (S / N = 3) by continuously measuring the concentration corresponding to three times the standard deviation of the peak current values ​​obtained from 11 consecutive determinations using a blank solution.

[0082] Example 4 Material B obtained in Example 2 was cut and pasted onto the surface of a glassy carbon electrode in the same manner as in Example 3 to obtain a porous composite material modified electrode modified with material B.

[0083] The porous composite material modified electrode prepared in Example 4 was used, and the operating steps of Example 3 were repeated. The standard curve equation was obtained as: I = 0.098C + 0.187, with a correlation coefficient R² = 0.993. The detection limit was 1.5 μg / L. The sensitivity of the porous composite material modified electrode is approximately 75%-80% of that of material A, but it has the advantage of lower cost, making it suitable for routine monitoring scenarios where sensitivity requirements are relatively lenient.

[0084] Example 5 Material A obtained in Example 1 was ground to a particle size of less than 0.1 mm. 5 mg of the powder was weighed and dispersed in 1 mL of 0.5 wt% Nafion ethanol solution, and ultrasonically dispersed for 30 min to form a uniform suspension. 10 μL of the above suspension was uniformly drop-coated onto the surface of the pretreated glassy carbon electrode (loading of approximately 0.71 mg / cm²), and dried under an infrared lamp to obtain a porous composite material modified electrode.

[0085] Example 6 Prepare a Cr solution with a concentration of 50 μg / L 6+ Several portions of the solution were prepared, and their pH values ​​were adjusted to 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 respectively using dilute HNO3 or NaOH. The porous composite material electrode prepared in Example 3 was used to modify the electrode. The enrichment time was fixed at 15 min, and the remaining steps were the same as in Example 3. The reduction peak current values ​​under each pH condition were recorded. Each condition was measured three times, and the average value was taken. The results are shown in Table 4.

[0086] Table 4. Reduction peak current values ​​(μA) under different enrichment pH conditions

[0087] The results show that the peak current reaches its maximum at pH 5.0. When the pH is below 4.0, the oxygen- and nitrogen-containing functional groups on the material surface are highly protonated, affecting the Cr content in the form of oxygen-containing anions. 6+ Electrostatic attraction weakens; when pH is above 6.0, Cr 6+ The oxidizing power is relatively reduced. Therefore, the optimal enrichment pH is determined to be 5.0.

[0088] Example 7 Prepare a Cr solution with a concentration of 50 μg / L 6+ Several portions of solution (pH 5.0) were prepared. The porous composite material modified electrode prepared in Example 3 was used, and the enrichment time was controlled at 2 min, 5 min, 10 min, 15 min, 20 min, 30 min and 45 min respectively, with other conditions the same as in Example 3. The reduction peak current at each time point was measured, and the results are shown in Table 5.

[0089] Table 5. Reduction peak current values ​​(μA) at different enrichment times

[0090] The results showed that the peak current increased significantly with time in the initial stage of enrichment, but the growth rate slowed down significantly after 20 minutes and gradually plateaued. Considering both detection sensitivity and analysis efficiency, 15-20 minutes was selected as the recommended enrichment time.

[0091] Example 8 In the peak current acquisition step, the effects of HCl electrolyte solution concentrations of 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.5 mol / L, and 1.0 mol / L on Cr were investigated. 6+ The influence of reduction peak shape and peak current. Experiments show that when the HCl concentration is in the range of 0.1 mol / L to 0.5 mol / L, the reduction peak shape is sharp and symmetrical, and the background current is stable. At too low a concentration (0.05 mol / L), the solution conductivity is insufficient, and the peak potential shifts slightly; at too high a concentration (1.0 mol / L), the hydrogen evolution background current increases, interfering with the measurement. Therefore, the preferred HCl concentration is 0.1 mol / L to 0.5 mol / L.

[0092] Example 9 Preparation of Cr 6+ A matrix solution with a concentration of 20 μg / L and a pH of 5.0 was prepared. Different concentrations of potential coexisting ions were added to the solution, and measurements were performed according to the procedure in Example 3. A relative error within ±5% was considered to indicate no significant interference. The results are shown in Table 6. Table 6. Coexisting ion pairs at 20 μg / L Cr 6+ Interference effects of measurement

[0093] The results showed that, in the presence of coexisting ions at all concentration multiples, Cr 6+ The relative errors of the measurements were all less than 5%, indicating that this method has good anti-interference ability. Higher concentrations of Cu... 2+ or Zn 2+ Although it may be adsorbed in small amounts, its reduction peak potential is similar to that of Cr in this method. 6+ The detection potential is separated, and no peak height superposition interference is generated.

[0094] Example 10 Water samples were collected from a drainage ditch in an electroplating industrial park (sample 1) and from a scenic urban river (sample 2). After filtration through a 0.45 μm mixed cellulose membrane to remove suspended particulate matter, the pH was measured and adjusted to 5.0. The two samples were then subjected to Cr content analysis using the method described in Example 3. 6+The content was determined and verified by comparison with the national standard method—diphenylcarbazide spectrophotometry (GB / T 7467-1987).

[0095] To further evaluate the accuracy of the method, 10.0 μg / L and 50.0 μg / L of Cr were added to the samples, respectively. 6+ The standard solution was analyzed using the same procedure, and the spiked recovery rate was calculated. Each sample and spiked level was analyzed in triplicate, and the results are shown in Table 7.

[0096] Table 7 Cr in actual water samples 6+ Measurement results and spiked recovery experimental data (n=3)

[0097] Note: Cr in sample 1 was determined by spectrophotometry. 6+ The content was 23.9 μg / L, which is consistent with the results of this method.

[0098] The above results show that the present invention can maintain high accuracy under complex matrix conditions in actual water bodies, with a spiked recovery rate between 95% and 99% and a relative standard deviation of less than 5%, demonstrating good practicality and reliability.

[0099] Example 11 The porous composite material modified electrode prepared in Example 3 was used in a Cr solution with a concentration of 50 μg / L. 6+ The standard solution underwent 20 consecutive cycles of enrichment-determination-washing. After each determination, the porous composite modified electrode was placed in blank 0.1 mol / L HCl and electrochemically cleaned by cyclic scanning 5 times within a potential range of 0.2 V–0.6 V. The reduction peak current of each determination was recorded, and the results are shown in Table 8.

[0100] Table 8. Peak current variation of porous composite material modified electrode after 20 consecutive uses.

[0101] The results show that the peak current decay of the porous composite material modified electrode is less than 7% in the first 10 uses, and it can still maintain more than 88% of the initial response after 20 uses, demonstrating good reusability.

[0102] Comparative Example 1 Using an unmodified bare glassy carbon electrode, under the same experimental conditions, 200 μg / L Cr was directly subjected to... 6+The standard solution was subjected to differential pulse voltammetry scanning, following the same procedure as in Example 3 (the enrichment step is omitted). The results showed that the bare glassy carbon electrode exhibited only extremely weak reduction current disturbances above the noise level, making it impossible to identify a clear reduction peak, and the signal-to-noise ratio was far below 3. This comparative example fully demonstrates the importance of the enrichment function of the modified layer for achieving trace Cr... 6+ The crucial role of detection.

[0103] Comparative Example 2 Commercially available coconut shell activated carbon powder without foaming and composite treatment was ground and then used to prepare a powder-modified electrode according to the drop-coating method of Example 5. Under the same conditions, 50 μg / L Cr... 6+ The solution was tested. The results showed that the reduction peak current value of the powder-modified electrode was only about 35% of that of the porous composite material-modified electrode in Example 3, and the relative standard deviation of five consecutive measurements was as high as 12.5% ​​(while that of the porous composite material-modified electrode in Example 3 was 4.3%). The reason for this is that the powder layer, which is simply physically stacked, lacks three-dimensional interconnected pores, and the effective reaction area is much smaller than that of the foamed structure material of this invention. In addition, the bonding force between the powder and the substrate is weak, and it is easy to fall off and be lost during stirring enrichment and potential scanning.

[0104] Comparative Example 3 As disclosed in Example 6, the detection signal significantly decreased when the enrichment pH deviated from 5.0. In actual sample analysis, without pH adjustment (e.g., the original pH of sample 1 was 7.2), the recovery rate of direct determination was only about 60%. This comparative example illustrates that adjusting the pH of the test solution to the range of 4.0-6.0 recommended by this invention is a necessary condition to ensure the accuracy of the method.

[0105] In summary, this invention provides a porous composite material modified electrode, its preparation method, and its application. By organically combining functional materials with three-dimensional porous structures, high adsorption capacity, and intrinsic reducing activity with electrochemical voltammetry, it achieves the control of trace amounts of Cr. 6+ This method offers high sensitivity, high selectivity, and rapid detection. It is simple to operate, has strong anti-interference capabilities, and low analytical costs, making it a promising candidate for rapid on-site monitoring of hexavalent chromium in environmental water bodies and industrial wastewater.

[0106] The above description is merely a preferred embodiment of the present invention and does not limit the scope of protection of the present invention. For those skilled in the art, several improvements and equivalent substitutions can be made to the technical solutions of the present invention without departing from the principles and spirit of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A porous composite material modified electrode, characterized in that, The invention includes a conductive substrate and a modification layer fixed to the surface of the conductive substrate. The modification layer is a biochar-polyurethane composite material with a porous structure. The surface of the modification layer contains CO, NH and -OH functional groups. The BET multi-point specific surface area of ​​the modification layer is 8.77 m² / g-36.22 m² / g, and the BJH median pore size is 2.06 nm-5.95 nm.

2. The porous composite material modified electrode according to claim 1, characterized in that, The modified layer is composed of biochar particles and polyurethane prepolymer.

3. The porous composite material modified electrode according to claim 2, characterized in that, The biochar particles are at least one of coconut shell activated carbon and rice husk biochar, and the particle size of the biochar particles is 0.15mm-0.3mm.

4. A method for preparing a porous composite material modified electrode, used to prepare the porous composite material modified electrode as described in any one of claims 1-3, characterized in that, include: The biochar-polyurethane composite material is cut into thin sheets with a thickness of 1mm-3mm and fixed to the surface of the conductive substrate by conductive adhesive or physical clamping. or The biochar-polyurethane composite material is ground into powder, the powder is mixed with a conductive binder to form a slurry, the slurry is drop-coated or printed onto the surface of the conductive substrate and then dried and cured.

5. The preparation method according to claim 4, characterized in that, The preparation method of the biochar-polyurethane composite material is as follows: Coconut shell activated carbon or rice husk biochar is crushed into activated carbon particles with a particle size of 50-100 mesh. The activated carbon particles are dried in sequence and deionized water is added to make a suspension slurry. Polyurethane prepolymer is added to the suspension slurry for foaming. After standing and setting, the biochar-polyurethane composite material is obtained. The mass ratio of the coconut shell activated carbon or rice husk biochar to the polyurethane prepolymer is 1:(1-2).

6. The application of a porous composite material modified electrode as described in any one of claims 1-3 in the rapid on-site detection of hexavalent chromium in environmental water or industrial wastewater.

7. The application according to claim 6, characterized in that, When using the porous composite material-modified electrode for rapid on-site detection of hexavalent chromium, the following steps are included: The porous composite material modified electrode was immersed in the water sample to be tested, and open-circuit enrichment was performed under stirring conditions to increase the Cr content in the water sample. 6+ The adsorption and concentration are concentrated into the modification layer on the surface of the porous composite material modified electrode; The porous composite material-modified electrode was transferred to an electrolyte solution, and a cathode scan was performed using differential pulse voltammetry to record Cr. 6+ The current value of the reduction peak; According to the pre-established Cr 6+ The concentration-reduction peak current standard curve, with the Cr 6+ The current value of the reduction peak is converted to the Cr content in the water sample being tested. 6+ The concentration.

8. The application according to claim 7, characterized in that, The open-path enrichment time is 5 min-30 min; The pH value of the water sample to be tested was 4.0-6.0; The electrolyte solution is a 0.1 mol / L to 0.5 mol / L hydrochloric acid or sulfuric acid solution.

9. The application according to claim 8, characterized in that, The scanning parameters of the differential pulse voltammetry method are: initial potential 0.2 V, termination potential -0.6 V, potential increment 2mV-5mV, pulse amplitude 25mV-50mV, and pulse width 40ms-60ms. The Cr 6+ The potential of the reduction peak is -0.25V to 0.35V.

10. The application according to claim 9, characterized in that, The linear range of the standard curve is 2 μg / L-200 μg / L, and the detection limit is 0.5 μg / L-2.0 μg / L. Biochar-polyurethane composite material for Cr 6+ The maximum monolayer adsorption capacity is 3.61 mg / g-4.06 mg / g.

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