A method for determining chemical oxygen demand based on variable flow rate resistance scanning

Abstract

The present application relates to the field of environmental protection and pollution control technology, in particular to a chemical oxygen demand determination method based on variable flow rate resistance scanning. The method innovatively updates the nanocrystalline stable electrode with strong oxidizing property under the controllable electrochemical mode reaction condition, improves the electrolysis reaction pool system, measures the exchange current, cell voltage, effluent conductivity and other parameters in the reaction process based on the flow rate-kinetics parameter extremum principle by dynamically adjusting the water inflow rate of the electrochemical reaction pool, establishes a standard curve, and realizes the rapid and accurate determination of the chemical oxygen demand in the water sample. Compared with the traditional method, the method has the advantages of simple process, short response time, less environmental pollution and convenient field operation; compared with the existing electrochemical analysis technology, the application range of pollutants is improved, and the detection effect of the method on the difficult-to-degrade organic pollutants rich in industrial wastewater is better.

Description

I. Technical Field

[0001] This invention relates to the field of environmental protection and pollution control technology, and in particular to a method for determining chemical oxygen demand (COD) based on variable flow rate resistance scanning. This innovative method, under controlled electrochemical reaction conditions, utilizes the principle of flow rate-kinetic parameter extremum optimization. By dynamically adjusting the influent flow rate of the electrochemical reaction tank, it measures parameters such as exchange current, effluent resistivity, and pseudo-chemical reaction kinetic constants during the reaction process, establishing a standard curve to achieve rapid and accurate determination of COD in water, especially in industrial wastewater rich in recalcitrant organic pollutants. II. Background Technology

[0002] Organic matter is a key component of surface water chemistry. However, due to increasing natural processes, human activities, and rapid socio-economic development, organic matter is gradually accumulating in surface waters such as rivers, lakes, and reservoirs. This phenomenon will inevitably have a profound impact on the material cycle, energy flow, structural function, and ecosystem services of freshwater ecosystems. Chemical oxygen demand (COD) measures the amount of oxidant required for the oxidation and decomposition of reducing substances in water, and can be converted into the required oxygen concentration (mg / L). The higher the COD value, the more severe the organic pollution of the water body. Therefore, it is an important comprehensive indicator for assessing the organic matter content and pollution level in water bodies, and is also a key evaluation indicator in my country's "Surface Water Environmental Quality Standard (GB3838-2002)". In my country, COD is widely used in various aspects such as wastewater discharge statistics, wastewater treatment plant removal efficiency assessment, and surface water environmental quality evaluation.

[0003] Currently, chemical oxygen demand (COD) is traditionally determined using chemical titration, which can be further divided into the potassium permanganate method and the potassium dichromate method depending on the oxidant used. The potassium permanganate method is generally used to monitor organic matter in surface water and groundwater, but it has a narrow measurement threshold and relatively high error. The potassium dichromate method, due to its high oxidation efficiency and good reproducibility, is widely used in industrial wastewater and domestic sewage with severe organic pollution, and has become the de facto standard measurement method. COD (potassium dichromate method) is defined as the number of oxygen equivalents consumed when the strong oxidant potassium dichromate method (standard redox power supply E0 = 1.36V) oxidizes organic compounds. Current methods involve adding a fixed volume of potassium dichromate of known concentration to the sample solution being analyzed, and then refluxing the resulting solution after approximately 2 hours. After reflux, the initial concentration of organic matter in the sample is calculated by determining the residual potassium dichromate in the sample using titration or colorimetry. This method has a clear theoretical basis. Under strong acid environment and silver sulfate catalysis, the potassium dichromate method has extremely strong oxidizing power and can oxidize most organic matter. However, the oxidation rate of aromatic hydrocarbons, pyridine and aliphatic hydrocarbons is relatively low. Some inorganic reducing substances, such as nitrites, sulfides and ferrous salts, are also included in the total chemical oxygen demand because they have a certain reducing ability.

[0004] Because the potassium dichromate method requires the use of large amounts of potassium dichromate as an oxidant and silver sulfate as a catalyst, both of which are expensive and highly toxic chemicals; in addition, the potassium dichromate method involves digestion and titration, which are time-consuming and inefficient, taking anywhere from several hours to 1-2 days, severely limiting the timeliness of the detection; furthermore, the chemical titration after digestion largely depends on the operator's experimental skills, resulting in poor repeatability, and inorganic components such as chloride ions, carbonates, and particulate matter in the water can also interfere with the experimental results, leading to false positives.

[0005] A search of journal articles, conference papers, and invention patents using "Chemical Oxygen Demand (COD)" and "Method" as keywords revealed that domestic and international research on COD replacement technologies mainly falls into the following categories: 1) Replacing the original chemical titration method with spectrophotometry, fluorescence spectrophotometry, and flow injection analysis to improve analytical accuracy and sensitivity. These methods, especially spectrophotometry, have been widely used in water quality testing, and some have even been incorporated into national standards. However, these methods still require a lengthy digestion reaction process, which does not significantly improve the timeliness of detection. 2) Seek new digestion and oxidation methods to replace the original chemical reagents such as potassium dichromate, potassium permanganate, and silver sulfate, to shorten digestion time, improve the efficiency of organic matter digestion, and reduce secondary pollution. Examples include microwave and ultrasonic digestion, and photocatalytic oxidation. While these methods improve digestion efficiency, they still require complex equipment such as microwave digesters and photocatalytic reactors, which are not suitable for field deployment. 3) Directly abandon the chemical oxygen demand (COD) indicator and choose alternative indicators, such as using ultraviolet absorbance (UV254) and fluorescence intensity to characterize the amount of organic matter and the degree of pollution in water bodies. These methods combine the accuracy of spectroscopic methods with the timeliness of deep oxidation methods, but their drawback is that they cannot be correlated with existing COD detection systems; the data from both are not interchangeable and can only serve as reference values ​​for traditional COD detection.

[0006] Patent CN103175882A provides a new approach to measuring chemical oxygen demand (COD) using electrochemistry. They prepared a COD electrochemical sensing membrane based on a glassy carbon electrode, forming a three-electrode system. The COD was predicted based on the electrochemical signal response value formed instantaneously when organic substances were added to the electrolyte system. Similarly, patents TW201617607A and KR1020050003770A, and literature (Analytica Chimica Acta 437(2001)95–105; Analytica Chimica Acta 548(2005)199–204; Analytica Chimica Acta 607(2008)176–182; Analytica Chimica Acta 736(2012)55–61) also adopted similar approaches. The aforementioned technical solutions are undoubtedly feasible, but the biggest problem lies in the fact that these methods acquire electrical signals through an electrochemical workstation applied to a dual-chamber diaphragm electrolytic cell. The operating mode of this workstation limits the adjustment range of current and potential. Furthermore, the presence of the diaphragm or proton exchange membrane can cause the pH of the anode and cathode to rise or fall, resulting in non-reproducible chemical oxygen demand (COD) measurements. More importantly, these methods collect the current response value generated instantaneously upon the addition of organic matter to the electrolytic cell system. Whether this signal originates from changes in liquid junction potential, electroadsorption, electro-oxidation-reduction, changes in resistance, or other non-electrochemical changes cannot be confirmed. Even if it can be confirmed that the signal originates from electro-oxidation, are there cascade or chain reactions leading to incomplete reactions? Can the selected electrodes completely decompose or even mineralize the organic matter? These issues could lead to false negatives in electrochemical COD measurements, especially for industrial wastewater containing recalcitrant organic pollutants, where this phenomenon may be more pronounced.

[0007] The main drawbacks of the electrochemical method for measuring chemical oxygen demand are described in detail below.

[0008] The principle of electrochemical detection of chemical oxygen demand is not complicated. According to the reaction mechanism proposed by Yu et al. (Electrochemistry Communications 9 (2007) 2280), firstly, H2O is discharged at the anode, generating physically adsorbed active oxygen such as hydroxyl radicals on the electrode surface:

[0009]

[0010] BDD[] represents the unoccupied surface sites of the reverse reaction assuming a constant discharge reaction and neglecting the H2O discharge; k1 is the electrochemical rate constant of the H2O discharge; BDD(OH) corresponds to the adsorbed hydroxyl radical. Hydroxyl radicals are non-selective, very strong oxidants capable of reacting with organic pollutants to produce dehydrogenated or hydroxylated derivatives until complete mineralization is achieved.

[0011]

[0012] Where R is the reactant, and k2 is the electrochemical rate constant for the combustion of organic matter. In case I, k1 >> k2, and the available hydroxyl radicals are sufficient to ignite the reactant. Therefore, the following equation can be derived:

[0013] I=nFAk2Z c C R (3)

[0014] Where n is the number of participating electrons, F is the Faraday constant, A is the area of ​​the BDD electrode, and Z... c It is the stoichiometric factor for the complete combustion of organic matter, c R It is the surface concentration of the reactants. Since one oxygen molecule is equivalent to four transferred electrons, and according to the definition of chemical oxygen demand, nc R It can be replaced by COD / 8000 (32000mg O2 mol) -1 O2 / 4mol e mol -1 O2). Therefore, equation (3) can be rewritten as:

[0015] I=FAk2Z c COD / 8000 (4)

[0016] In case II, k1 << k2cR, the overall reaction is controlled by the mechanism of equation (1), and due to insufficient physical adsorption of hydroxyl radicals, the current I total Becomes saturated, I total It can be simplified to the form given by the following formula:

[0017] I total =3FAk1Γ0 (5)

[0018] Where Γ0 is the density of surface sites available for adsorbing hydroxyl radicals. As described above, it is known that as long as the physically adsorbed hydroxyl radicals are sufficient to react in equation (2), the current of the working electrode will change proportionally with the concentration of reactants or chemical oxygen demand.

[0019] However, this technical approach has the following serious problems:

[0020] 1) Reaction formula (3) assumes two constants that do not change with species, and the stoichiometric factor Z for the complete combustion of organic matter. c and the surface concentration c of the reactants R However, in reality, due to differences in the chemical structure stability, surface electrostatic properties, dipole moment size, and hydrophilicity / hydrophobicity of organic matter, these two values ​​may vary considerably under different conditions.

[0021] 2) The definition of k1 during the reaction process is not clear. In reality, hydroxyl radicals are not only present on the electrode surface, but may also partially migrate into the electrolyte at a certain distance from the electrode. Moreover, due to the extremely strong reactivity of hydroxyl radicals, they may undergo quenching reactions with superoxide anions, hydrogen peroxide, peroxide radicals, some anions, and hydroxyl radicals, resulting in a residual hydroxyl radical concentration far lower than the theoretical value. Furthermore, the k1 value is closely related to the electrolyte environment and the type of organic matter, and cannot be applied to all detection environments.

[0022] 3) The definition of k2 in the reaction process is also unclear. If only the first elementary reaction between organic matter and hydroxyl radical is considered, the reaction formulas (2) to (4) are correct. However, the measurement of chemical oxygen demand requires the oxygen equivalent required to completely mineralize organic matter into inorganic substances such as carbon dioxide and water. From the second elementary reaction onwards, the main factor may no longer be hydroxyl radical, but rather a chain reaction induced by various reactive oxygen species or organic radicals. At this time, k2 may not be a universal constant, let alone a mineralization constant.

[0023] 4) Due to the uncertainty of k1 and k2, reaction formula (5) also has the above problems. More importantly, if only elementary reactions are considered, due to the great differences in the reaction kinetics between different organic compounds and hydroxyl radicals, the electrochemical signal reaction may not be the chemical oxygen demand, but the molar number of the organic compound itself. This is also the reason why the (pseudo) chemical oxygen demand concentration and the standard solution concentration have a good correlation in the above literature reports.

[0024] Therefore, the current technical approach is relatively successful in detecting easily degradable organic pollutants, but it is prone to false negatives or non-reproducibility when detecting recalcitrant organic pollutants, requiring further adjustments and improvements. III. Summary of the Invention

[0025] This invention addresses the problems existing in the prior art. The objective of this invention is to provide an innovative method for determining chemical oxygen demand (COD) based on variable flow rate resistance scanning. Under controlled electrochemical reaction conditions, this method, based on the principle of flow rate-kinetic parameter extremum optimization, dynamically adjusts the influent flow rate of the electrochemical reaction tank and measures parameters such as exchange current, tank voltage, and effluent resistivity during the reaction process to establish a standard curve. This enables rapid and accurate determination of COD in water, especially in industrial wastewater rich in recalcitrant organic pollutants.

[0026] The technical improvement ideas of this invention are as follows:

[0027] 1) Improvement of electrocatalytic electrodes. Replacing traditional ruthenium-iridium, lead oxide, and BDD anodes with nanocrystalline stable electrodes improves the efficiency of electrocatalytic oxidation and the electrode's selectivity for hydroxyl radicals;

[0028] 2) The serpentine flow channel reaction chamber is replaced with the frame reaction chamber, and the continuous flow reaction is replaced with the batch reaction, which improves the controllability of the electrochemical reaction. At the same time, due to the continuous flow reaction mode, even if hydroxyl radicals still undergo various quenching reactions on the electrode surface, their steady-state concentration can remain basically consistent during the reaction process, which can ensure the relative stability of k1.

[0029] 3) The change in resistivity of the electrochemically effluent is used instead of the change in current response to measure the chemical oxygen demand concentration, thus avoiding the interference of adsorption effects, polarization effects, capacitance effects and short-lived highly active free radicals inside the electrolytic cell and on the electrode surface.

[0030] 4) Most importantly, the effective residence time required for complete mineralization of organic matter in water samples is determined by using variable flow rate, so as to obtain the kinetic constant k2 for complete mineralization of organic matter.

[0031] The following section presents the chemical theoretical design and mathematical derivation of the proposed improvement scheme of this invention:

[0032] When the influent flow rate is at its minimum, it can be assumed that an electrochemical oxidation reaction with an infinitely long residence time is occurring in the electrolytic cell. With an infinitely abundant supply of hydroxyl radicals and sufficiently thorough oxidation, it can be assumed that any organic matter can be completely mineralized, and the effluent concentration is zero regardless of the influent concentration. When the influent flow rate is at its maximum, it can be assumed that no electrochemical reaction occurs in the electrolytic cell, and the influent organic matter concentration is equal to the effluent concentration. At this point, if the effluent resistivity is plotted against the influent flow rate, an inverse S-shaped curve will be obtained. Tangents to the curve at both ends, with slopes less than 0.05, can be considered close to a steady state. Tangents to the inflection point in the center of the S-shaped curve will yield the slope of the effluent resistivity relative to the influent flow rate, i.e.:

[0033]

[0034] At this point, the inflection point in the S-curve is not fixed, but varies with the ease of electrocatalytic degradation of organic matter. When the degradation reaction is difficult to occur, the inflection point shifts towards the minimum flow rate; when the degradation reaction is easy to occur, the inflection point shifts towards the maximum flow rate. The physical meaning of the inflection point slope is the reciprocal of the volume of solution passing through for the same charge transfer number, i.e.:

[0035]

[0036] Therefore, the inflection point slope at this point is not affected by the ease of organic matter degradation and is positively correlated with the theoretical chemical oxygen demand (ThOD). Since the physical meaning of the slope value partially reflects the changing trend of the electrochemical reaction, it is defined as a "pseudo-chemical reaction kinetic constant" in this invention. By establishing a linear relationship between ThOD and the "pseudo-chemical reaction kinetic constant," the shortcomings of traditional electrochemical detection techniques can be overcome. The value measured using this technique can truly reflect the concentration of ThOD in water, or in other words, it can approximate the theoretical oxygen demand (ThOD) as closely as possible.

[0037] To achieve the above objectives, the present invention provides a method for determining chemical oxygen demand based on variable flow rate resistance scanning, the specific implementation of which includes the following steps:

[0038] Step 1: Weigh 4.251g of potassium hydrogen phthalate dried at 105℃ for 2h, dissolve it in ultrapure water, and dilute it to 1000mL. Mix well to obtain a standard solution of chemical oxygen demand. The theoretical chemical oxygen demand of this standard solution is 5000mg / L. Dilute it to different multiples for later use.

[0039] Step 2: Collect water samples, filter them through a 0.45μm glass fiber membrane, and add 98% concentrated sulfuric acid to adjust the pH to less than 3. The concentration of Cl in the water sample is then adjusted accordingly. – Depending on the ion concentration, add silver nitrate until no more white precipitate forms. Take the supernatant and filter it again through a 0.45 μm glass fiber membrane for analysis.

[0040] Step 3: Use the peristaltic pump (2) to inject the standard solution prepared in step 1 or the water sample collected in step 2 into the inlet of the electrochemical reaction cell (4), filling the (7) serpentine flow channel in the nanocrystalline stable anode (6) to form a continuous flow;

[0041] Step 4: Control the peristaltic pump output of (2) gradually increasing or decreasing variable flow rate with a pre-programmed control until the reaction ends, and continuously collect the value of the flow rate change with reaction time;

[0042] Step 5: Simultaneously with step 4, start the (10) DC pulse power supply and output a tank voltage of 3.0 to 20.0V with a duty cycle of 1:5 to 1:20, and collect the value of the change of the exchange current with the reaction time on the (10) DC pulse power supply;

[0043] Step 6: Simultaneously with step 4, start the (13) online resistivity detector and collect the value of the change of solution resistivity with reaction time on the (16) electrical signal acquisition platform of the (15) computer;

[0044] Step 7: Based on the values ​​of flow rate versus reaction time obtained in Step 4, exchange current versus reaction time obtained in Step 5, and resistivity versus reaction time obtained in Step 6, plot the correlation curves of reaction time versus flow rate, reaction time versus resistivity, reaction time versus exchange current, flow rate versus exchange current, flow rate versus resistivity, and exchange current versus resistivity, respectively.

[0045] Step 8: Establish the correlation curve between flow rate and pseudo-chemical kinetic constant;

[0046] Step 9: Draw tangents at the lowest and highest extremes of the flow rate-pseudochemical kinetic constant correlation curve to determine the effective range of flow rate; draw tangents at the inflection points of the flow rate-pseudochemical kinetic constant curve to determine the sampling points for the chemical oxygen demand electrical signal data.

[0047] Step 10: Establish a standard curve for the correlation between chemical oxygen demand (COD) and pseudo-chemical kinetic constant using standard solutions with different dilution ratios. Substitute the pseudo-chemical kinetic constant obtained from the collected water samples into the standard curve to obtain the true value of COD.

[0048] For the above technical solution, further defined, the variable flow rate resistance scanning reaction system described in step 3 consists of the following components: (1) inlet, (2) peristaltic pump, (3) peristaltic pump tube, (4) electrochemical reaction cell inlet, (5) stainless steel perforated plate, (6) nanocrystalline stable anode, (7) serpentine flow channel, (8) tantalum cathode, (9) stainless steel pressure plate, (10) DC pulse power supply, (11) electrode connection line, (12) anode liquid outlet, (13) online resistivity detector, (14) waste liquid outlet, (15) computer, and (16) electrical signal acquisition platform.

[0049] The advantages of this invention are: compared with traditional methods, this method has a simple process, short response time, less environmental pollution, and is convenient for field operations; compared with existing electrochemical analysis techniques, it improves the applicability to pollutants, especially showing better detection effects on recalcitrant organic pollutants abundant in industrial wastewater. IV. Description of the attached drawings

[0050] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the description of the specific embodiments are briefly explained below. Figure 1 The following is a schematic diagram of the variable current resistance scanning reaction system described in this invention. The reference numerals are as follows: (1) Inlet, (2) Peristaltic pump, (3) Peristaltic pump tube, (4) Electrochemical reaction cell inlet, (5) Stainless steel perforated plate, (6) Nanocrystalline stable anode, (7) Serpentine flow channel, (8) Tantalum cathode, (9) Stainless steel pressure plate, (10) DC pulse power supply, (11) Electrode connection line, (12) Anode liquid outlet, (13) Online resistivity detector, (14) Waste liquid outlet, (15) Computer, (16) Electrical signal acquisition platform.

[0051] Figure 2 The reaction time-flow rate correlation curve for wastewater sample #4 in Example 2 of this invention;

[0052] Figure 3 The reaction time-resistivity correlation curve of wastewater sample #4 in Example 2 of this invention;

[0053] Figure 4 The reaction time-exchange current correlation curve for wastewater sample #4 in Example 2 of this invention;

[0054] Figure 5 The flow rate-exchange current correlation curve for wastewater sample #4 in Example 2 of this invention;

[0055] Figure 6 The flow rate-resistivity correlation curve of wastewater sample #4 in Example 2 of this invention;

[0056] Figure 7 This is the exchange current-resistivity correlation curve of wastewater sample #4 in Example 2 of the present invention;

[0057] Figure 8 This is the correlation curve of flow rate and pseudo-chemical kinetic constant for wastewater sample #4 in Example 2 of the present invention;

[0058] Figure 9 The standard curve of chemical oxygen demand concentration-reaction kinetic constant established in Example 2 of this invention; V. Detailed Implementation Methods

[0059] The present invention will now be described in detail with reference to the accompanying drawings and embodiments:

[0060] Example 1:

[0061] A method for determining chemical oxygen demand (COD) based on variable flow rate resistance scanning includes the following steps: First, 4.251 g of potassium hydrogen phthalate dried at 105℃ for 2 h is dissolved in ultrapure water and diluted to 1000 mL. The solution is mixed to obtain a COD standard solution with a theoretical COD value of 5000 mg / L. This standard solution is then diluted to different concentrations for later use. Water samples are collected, filtered through a 0.45 μm glass fiber membrane, and then 98% concentrated sulfuric acid is added to adjust the pH to less than 3. The determination of COD is based on the concentration of Cl in the water sample. – Depending on the ion concentration, silver nitrate is added until no more white precipitate is formed. The supernatant is then filtered again through a 0.45 μm glass fiber membrane for analysis. The standard solution prepared in step 1 or the water sample collected in step 2 is injected into the inlet of the electrochemical reaction cell (4) using a peristaltic pump (2), filling the serpentine channel (7) in the nanocrystalline stable anode (6) to form a continuous flow. The peristaltic pump (2) is controlled by a pre-programmed variable flow rate that gradually increases or decreases until the reaction ends, and the flow rate is continuously collected as the reaction time changes. At the same time, the DC pulse power supply (10) is started, and the cell voltage of 3.0 to 20.0 V is output with a duty cycle of 1:5 to 1:20. The exchange current is collected as the reaction time changes on the DC pulse power supply (10). At the same time, the online resistivity detector (13) is started, and the signal is collected on the (16) electrical signal acquisition platform of the (15) computer. Collect the values ​​of resistivity as a function of reaction time; based on the obtained values ​​of flow rate as a function of reaction time, exchange current as a function of reaction time, and resistivity as a function of reaction time, plot the correlation curves of reaction time-flow rate, reaction time-resistivity, reaction time-exchange current, flow rate-exchange current, flow rate-resistivity, and exchange current-resistivity; establish the correlation curve of flow rate-pseudochemical kinetic constant; draw tangents at the lowest and highest extreme values ​​of the correlation curve of flow rate-pseudochemical kinetic constant to determine the effective range of flow rate; draw tangents at the inflection points of the curve of flow rate-pseudochemical kinetic constant to determine the sampling points of chemical oxygen demand (COD) electrical signal data; establish a standard curve of the correlation between COD and pseudochemical kinetic constant using standard solutions with different dilution ratios; substitute the pseudochemical kinetic constant values ​​obtained from the collected water samples into the standard curve to obtain the true value of COD.

[0062] Example 2:

[0063] Thirteen wastewater samples were collected from a wastewater treatment plant in a chemical industrial park in Ningxia and tested using both the national standard method and the method described in Example 1. Correlation curves were plotted for reaction time-flow rate, reaction time-resistivity, reaction time-exchange current, flow rate-exchange current, flow rate-resistivity, and exchange current-resistivity. A correlation curve for flow rate-pseudochemical kinetic constant was established. Tangents were drawn at the lowest and highest extremes of the flow rate-pseudochemical kinetic constant correlation curve to determine the effective range of flow rate. Tangents were drawn at the inflection points of the flow rate-pseudochemical kinetic constant curve to determine the sampling points for the chemical oxygen demand (COD) electrical signal data. A standard curve for the correlation between COD and the pseudochemical kinetic constant was established using standard solutions with different dilution ratios. The pseudochemical kinetic constant obtained from the collected water samples was substituted into the standard curve to obtain the true value of COD (see Example 1). Figure 2 –9). The results are shown in the table below (all units are mg / L):

[0064]

[0065] As can be seen from the table above, when the wastewater from the wastewater treatment plant in the chemical industrial park is tested using the method described in this invention, the maximum positive deviation is 3.7% and the maximum negative deviation is -3.1%, both of which are less than 5%, showing good consistency with the national standard method.

[0066] Example 3:

[0067] Seven wastewater samples were collected from a livestock farm in Hebei Province and tested using both the national standard method and the method described in Example 1. The results of the national standard method comparison are shown in the table below (all units are mg / L):

[0068] National Standards Law 5377.0 5973.9 587.3 319.6 1378.0 5266.5 779.3 This invention 5469.8 5883.6 587.6 317.1 1348.7 5468.8 742.8 relative deviation 1.7％ -1.5％ 0.1％ -0.8％ -2.1％ 3.8％ -4.7％

[0069] As can be seen from the table above, when the method described in this invention is used to test aquaculture wastewater, the maximum positive deviation is 3.8% and the maximum negative deviation is -4.7%, both of which are less than 5%, showing good consistency with the national standard method.

[0070] Example 4:

[0071] Ten industrial wastewater samples were collected from a coal chemical industrial park in Anhui Province and tested using both the national standard method and the method described in Example 1. The results are shown in the table below (all units are mg / L):

[0072] National Standards Law 457.3 494.4 459.5 161.1 304.5 148.7 197.8 116.0 227.9 449.4 This invention 469.7 516.0 454.3 166.4 299.3 155.6 205.8 119.4 228.3 430.3 relative deviation 2.7％ 4.4％ -1.1％ 3.3％ -1.7％ 4.7％ 4.1％ 2.9％ 0.2％ -4.2％

[0073] As can be seen from the table above, when the method described in this invention is used to test coal chemical wastewater in chemical industrial parks, the maximum positive deviation is 4.7% and the maximum negative deviation is -4.2%, both of which are less than 5%, showing good consistency with the national standard method.

[0074] Example 5:

[0075] Wastewater from the secondary sedimentation tank of a municipal wastewater treatment plant in Beijing was tested multiple times using both the national standard method and the method described in Example 1 to verify the reproducibility of different methods. The results are shown in the table below (all units are mg / L):

[0076] National Standards Law 22.2 21.5 23.5 23.4 21.9 22.9 22.5 21.9 0.73 This invention 22.3 22.5 22.9 22.9 22.8 22.1 22.8 22.5 0.29

[0077] As can be seen from the table above, the dispersion (standard deviation) of the biochemical effluent from the secondary sedimentation tank, after eight consecutive rounds of testing using the method described in this invention, is 0.29, which is much lower than the 0.73 of the national standard method, indicating good reproducibility.

[0078] Example 6:

[0079] Agricultural wastewater collected from an agricultural plantation in Changping District, Beijing, was tested multiple times using both the national standard method and the method described in Example 1 to verify the reproducibility of different methods. The results are shown in the table below (all units are mg / L):

[0080] National Standards Law 119.2 124.6 115.1 115.4 119.2 121.0 114.3 121.1 3.57 This invention 117.8 120.7 121.1 120.3 117.1 118.6 114.5 115.3 2.48

[0081] As can be seen from the table above, the dispersion (standard deviation) of continuous detection of agricultural and domestic sewage using the method described in this invention is 2.48, which is lower than the 3.57 of the national standard method, indicating good reproducibility.

[0082] The specific embodiments described above are only used to illustrate the spirit of the present invention. The scope of protection of the present invention is not limited thereto. For those skilled in the art, other embodiments can be easily made by means of changes, substitutions or modifications based on the technical content disclosed in this specification. All such other embodiments should be covered within the scope of protection of the present invention.

Claims

1. A method for determining chemical oxygen demand based on variable flow rate resistance scanning, characterized in that, The method includes the following steps: Step 1: Weigh 4.251 g of potassium hydrogen phthalate dried at 105°C for 2 h, dissolve it in ultrapure water, and dilute it to 1000 mL. Mix well to obtain a standard solution of chemical oxygen demand. The theoretical chemical oxygen demand of this standard solution is 5000 mg / L. Dilute it to different multiples for later use. Step 2: Collect water samples, filter them through a 0.45 μm glass fiber membrane, and add 98% concentrated sulfuric acid to adjust the pH to less than 3. The concentration of Cl in the water sample is then adjusted accordingly. – Depending on the ion concentration, add silver nitrate until no more white precipitate forms. Take the supernatant and filter it again through a 0.45 μm glass fiber membrane for analysis. Step 3: The standard solution prepared in Step 1 or the water sample collected in Step 2 is injected into the inlet (4) of the electrochemical reaction cell using a peristaltic pump (2), filling the serpentine flow channel (7) in the nanocrystalline stable anode (6) to form a continuous flow; the variable flow rate resistance scanning reaction system described in Step 3 consists of the following components: inlet (1), peristaltic pump (2), peristaltic pump tube (3), electrochemical reaction cell inlet (4), stainless steel perforated plate (5), nanocrystalline stable anode (6), serpentine flow channel (7), tantalum cathode (8), stainless steel pressure plate (9), DC pulse power supply (10), electrode connection line (11), anolyte outlet (12), online resistivity detector (13), waste liquid outlet (14), computer (15), and electrical signal acquisition platform (16); Step 4: Control the peristaltic pump (2) to output a variable flow rate that gradually increases or decreases until the reaction ends, and continuously collect the value of the flow rate change with the reaction time. Step 5: Simultaneously with step 4, start the DC pulse power supply (10) and output a slot voltage of 3.0~20.0 V with a duty cycle of 1:5~1:20, and collect the value of the exchange current changing with the reaction time on the DC pulse power supply (10); Step 6: Simultaneously with step 4, start the online resistivity detector (13) and collect the value of the change of solution resistivity with reaction time on the electrical signal acquisition platform (16) of the computer (15); Step 7: Based on the values ​​of flow rate versus reaction time obtained in Step 4, exchange current versus reaction time obtained in Step 5, and resistivity versus reaction time obtained in Step 6, plot the correlation curves of reaction time versus flow rate, reaction time versus resistivity, reaction time versus exchange current, flow rate versus exchange current, flow rate versus resistivity, and exchange current versus resistivity, respectively. Step 8: Establish the correlation curve between flow rate and pseudo-chemical kinetic constant; Step 9: Draw tangents at the lowest and highest extremes of the flow rate-pseudochemical kinetic constant correlation curve. When the slope at both ends is less than 0.05, it can be considered close to steady state, thus determining the effective range of flow rate. Draw tangents at the inflection points of the flow rate-pseudochemical kinetic constant correlation curve to determine the sampling points of the chemical oxygen demand (COD) electrical signal data. Determine the pseudochemical kinetic constant value at the COD electrical signal data acquisition points. Step 10: Establish a standard curve for the correlation between chemical oxygen demand (COD) and pseudo-chemical kinetic constant using standard solutions with different dilution ratios. Substitute the pseudo-chemical kinetic constant values ​​obtained from the collected water samples into the standard curve to obtain the true value of COD.

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