TWI409448B - Process for measurements of concentration of dissolved organic nitrogen (don) with low concentration in water - Google Patents

Abstract

Dissolved organic nitrogen (DON) measurements for water samples with a high dissolved inorganic nitrogen (DIN, including nitrite, nitrate and ammonia) to total dissolved nitrogen (TDN) ratio using traditional methods are inaccurate due to the cumulative analytical errors of independently measured nitrogen species (TDN and DIN). In this study, we present a nanofiltration (NF) pretreatment to increase the accuracy and precision of DON measurements by selectively concentrating DON while passing through DIN species in water samples to reduce the DIN/TDN ratio. Compared to the available dialysis pretreatment method, the NF pretreatment method shows a similar improved performance for DON measurement for aqueous samples and can save at least 20 hours of operating time and a large volume of deionized water, which is beneficial for laboratories involved in DON analysis.

Description

Method for determining low concentration of dissolved organic nitrogen in water

The invention relates to a method for determining the concentration of dissolved organic nitrogen in water at low concentration, in particular to an effective method for determining the dissolved organic nitrogen [DON] in water in the field of environmental protection and water treatment.

According to the current situation, the current environmental water body is generally subject to micro-pollution. The common organic matter content in the raw water exceeds the raw water quality standard, the concentration of nitrogen-containing compounds is high, the water body has odor, the chroma is high, and the algae multiply. In view of the widespread presence of dissolved organic matter (DOM) in water, it is easy to undergo halogenation reaction with chlorine during the disinfection process, and forms chlorination by-products such as trihalomethane, haloacetic acid, haloacetonitrile, furanone [MX], etc. The elimination and control of organic matter in polluted raw water has been the focus of continuous attention in this field. The dissolved organic nitrogen [DON] accounts for about 0.5-10% of the dissolved organic matter [DOM], and mainly contains NHs, amino acids, nitriles, purines, pyrimidines, nitro compounds, and the like. Since most of DON's substances are directly or indirectly carcinogenic or mutagenic to humans, the presence of these compounds can have many adverse effects on drinking water treatment, such as the consumption of oxidants and Disinfectants, produce more haloacetic acid and trihalomethane, cause film fouling, etc., so the understanding and control of DON has gradually become an emerging focus in the field of water research, especially the strong carcinogenic chlorination of DON The production and elimination of the product-halogenated nitromethane [HNMs] and dimethylnitrosamine (including NDMA) has also become an important issue of international concern. As a precursor of HNMs and dimethyl nitrosamines (including NDMA), DON has attracted people's attention.

At present, there is no direct method for the determination of DON, which is mainly obtained by calculating the difference between total dissolved nitrogen [TDN] and inorganic nitrogen [DIN] (including nitrate nitrogen, nitrite nitrogen and ammonia nitrogen). Since the DON content in surface water is generally more than an order of magnitude smaller than DIN, the analysis error of DON is often expanded, for example, TN is 1 mg/L ± 0.1, DIN is 0.9 mg/L ± 0.1, and DON is obtained at this time. The value will be 0.1 mg/L ± 0.2 and the error value will reach 200%. In the case where the actual water sample DIN is high (for example, the TDN of Huangpu River is 3-6 mg/L, NO ₃ ^- is 2-4 mg/L, NH ₄ ⁺ -N is 0.1-2 mg/L; DIN/TDN >80%], due to the error of the analytical instrument and the accuracy of the test method (the accuracy of DIN is 0.1mg/L), the test found that the water quality index is determined according to the standard method, and there are still about 20-30% of the water sample due to the DIN concentration value. Greater than the measured TDN value, resulting in a negative value after the DON concentration calculation, which is consistent with foreign research, thus seriously affecting the research work of DON and its nitrogen-containing disinfection by-products [N-DBPs].

In response to this problem, some American researchers have proposed to remove and separate DIN before performing total nitrogen analysis to improve the accuracy of DON measurement in water. The main practices include: (1) using a dialysis membrane of a certain molecular weight to allow most of the DIN in the water to pass out while retaining DON, and (2) using a catalytic reduction method to reduce and remove the nitrate. Since the second method may cause DON to adsorb on the catalyst, and the separation effect is greatly affected by the reduction reaction conditions, the practical application is less feasible in the DON analysis; and the first method has been adopted internationally. 100Dalton dialysis membrane to remove DIN, can be effectively applied to DON analysis of water sample with DIN/TDN>0.6, but the device, dialysis tube and dialysis membrane of this method are very expensive and require a large amount of ultrapure water (at least 144 liters), long analysis time (at least 24 hours or more), so it still has great limitations in practical applications.

In view of this, the inventors have long been rich in design and development and practical production experience in the relevant industries, and in particular, provide an effective method for determining the concentration of dissolved organic nitrogen in water to achieve the determination of dissolved organic nitrogen [DON] in water. For the purpose of having a better practical value.

In order to achieve the above object, the solution adopted by the present invention is to apply nano film separation technology to improve the accuracy of the measurement of dissolved organic nitrogen [DON] concentration. Nanofiltration is a pressure-driven membrane separation process between reverse osmosis and ultrafiltration. It has a high removal rate for high-valent ions and organic matter, and a high throughput for monovalent ions, which is in line with The key to solving the DON measurement problem is the main purpose of the method for improving the accuracy of DON measurement.

The invention is a method for accurately determining the concentration of dissolved organic nitrogen, Nanofilm pretreatment is used to increase the accuracy of the measurement of dissolved organic nitrogen in water. In the preparation process before pretreatment, the water sample is filtered with a microfiltration membrane having a pore diameter of 0.22 μm or more and 0.45 μm or less to remove particulate impurities, and the filtered water sample is adjusted to have a pH range of 6.0. - 9.0, a selective nano film with a molecular weight cut off of 150-500 Dalton, using a vertical (dead-end) filtration or cross-flow filtration method to concentrate DON in the water sample 6-15 times; The DON content in the final water sample can be obtained by subtracting the sum of other measured inorganic nitrogen contents (including nitrate nitrogen, nitrite nitrogen and ammonia nitrogen) from the total nitrogen, and dividing by the actual concentration factor; thereby, the DON test is improved. The accuracy and accuracy of the direct water sample method solve the problems of DON often negative values and large standard deviation of the concentration; and the whole measurement process is simple and effective, easy to operate, low cost, and can be widely applied to surface and groundwater monitoring. And monitoring and analysis of large, medium and small water purification plants and sewage plants.

Specifically, the present invention further comprises: pretreatment of the environmental water sample before the nanofiltration, parameter control and operation mode during the nanofiltration pretreatment process, and related water quality analysis (eg, total nitrogen, nitrate nitrogen, nitrous acid) The concentration of salt nitrogen, ammonia nitrogen, and finally DON was calculated. In the preparation before the nanofiltration, the environmental sample is first filtered with a microfiltration membrane of a cellulose acetate membrane or a polyethersulfone having a pore size of 0.22 or 0.45 μm to remove particulate impurities. Compared with other membranes, the membrane function has the characteristics of low porosity, low protein adsorption rate and good macromolecular retention, and does not contain nitrogen. It does not determine the subsequent water samples due to the release of nitrogen from the membrane. DON has an effect]; after taking the filtered water sample about 300-500mL, using a base (such as sodium hydroxide [NaOH]) and an acid (such as hydrochloric acid [HCl]) to adjust the pH range of 6-9, the molecular weight cutoff is The 150-500 Dalton selective nanofilm is subjected to a dead-end or cross-flow filtration to concentrate the DON in the water sample by about 6-15 times. Water quality analysis used total nitrogen analyzer, ion chromatograph and spectrophotometer to determine the content of total nitrogen, nitrate nitrogen, nitrite nitrogen and ammonia nitrogen in the concentrate; the water sample DON content was determined by total nitrogen minus other inorganic nitrogen After the sum of the contents, it is obtained by dividing by the actual concentration factor.

The selection of the pH value is further determined by the following test method: the water is tested by using the water of the water plant, and the pH value of the water sample after passing through the microfiltration membrane of 0.22 or 0.45 μm is adjusted by using HCl and NaOH, and the method of the invention is applied. The measured values of DON in the same water sample under different pH conditions were analyzed to explore the effect of pH on the selective nanofilm, and the optimum pH condition when the DON value was the largest was obtained.

The suitable concentration factor is further determined by the following test scheme: the water is tested by using different volumes of water in the waterworks, and the pH of the water sample after passing through the 0.22 or 0.45 μm microfiltration membrane is adjusted to 6.5 using HCl and NaOH. -7.5, applying the method proposed by the invention, controlling the volume of the concentrated liquid after pretreatment by nano film is about 50 mL, analyzing the concentration of the water sample DON under different concentration multiples, thereby evaluating the concentration multiple to DON The effect of the measurement is taken to obtain the best concentration factor for the largest DON measurement.

The selection of selective nanofilm type with molecular weight cutoff of 150-500 Dalton is further determined by the following test scheme: using the same volume of water from the waterworks for testing, using HCl, NaOH to adjust the microfiltration through 0.22 or 0.45 μm. The pH value of the post-membrane water sample was 6.5-7.5, and the concentration ratio was controlled to be about 6 times. The influence of different types of nano-films on the measured value of DON was analyzed, and the maximum value of DON was obtained, and the standard after repeated test was obtained. The type of nano film with the smallest deviation.

(1)‧‧‧ water samples

(2)‧‧‧Nano film

(3) ‧‧‧Filter equipment

(31)‧‧‧Filter Cup

(32)‧‧‧Magnetic mixer

(4) ‧ ‧ container

(5) ‧‧‧nitrogen

First: Schematic diagram of the apparatus for pretreating the vertical membrane of the nano film of the present invention

The second figure: the flow chart of the pretreatment sweep filtration method of the nano film of the invention

Third figure: actual operation flow chart of the present invention

Figure 4: Comparison of the effects of different nanofilm types on the determination results of the present invention

Figure 5: Comparison of the effects of different pH values on the measurement results of the present invention

Figure 6: Comparison of the effects of different concentration times on the measurement results of the present invention

Figure 7: Comparison of direct measurement and application of the pretreatment method to the DN measurement results of micro-polluted Huangpu River raw water and clean water

For a more complete and clear disclosure of the technical means, the object of the invention and the effect of the present invention, the following detailed description, and the accompanying drawings,

First, please refer to the first to third figures, which is a schematic diagram of a filtering device and a flow chart of a method for determining the concentration of dissolved organic nitrogen according to the present invention, and the specific steps thereof are as follows:

(S1): For the preparation process before pretreatment, the water sample to be tested (1) is first filtered through a 0.22 or 0.45 μm cellulose acetate or polyestersulfone microfiltration membrane. In order to remove suspended solids and particulate impurities in water [if it is not possible to measure immediately, the water sample to be tested should be stored in a refrigerator at 4 ° C].

(S2): The pH of the water sample to be tested is adjusted to 6.0-9.0 by using sodium hydroxide [NaOH] and hydrogen chloride [HCl].

(S3): Place the selective nano film (2) soaked in ultrapure water [not less than 24 hours] into the filtering device (3) (using a vertically filtered filter cup (31)] The filtrate is placed in a clean container (4) to collect the filtrate; the filtration device (3) should be sealed to prevent evaporation of the sample moisture.

(S4): 200 mL of ultrapure water is poured into the filter cup (31), and the rotation speed of the magnet force mixer (32) is adjusted to be greater than 100 rpm (generally not exceeding 200 rpm, for example, 120 rpm). At the same time, the valve is opened to pass high-purity nitrogen gas (5), the gas pressure is adjusted to 0.5 MPa, and the film pre-compacting is performed for about 0.5-1 h.

(S5): After the film is pre-compacted, the residual ultrapure water is poured out. Use a measuring cylinder to accurately measure a certain volume [300-500 mL] of the water sample filtered through a 0.22 or 0.45 μm microfiltration membrane, pour into a filter cup (31), and adjust the rotation speed of the magnet mixer (32) to be greater than 100 rpm. It is 120 rpm]. At the same time, the valve is opened to pass high-purity nitrogen (5), and the gas pressure is adjusted to 0.5 MPa.

(S6): When the volume of the filtrate to be collected is about 250 mL, the nitrogen gas (5) is stopped. The volume of the filtrate is accurately determined to calculate the volume of the concentrate, after which the concentration factor for the raw water can be determined. Pour the concentrate into a clean glass bottle for storage.

The above steps S2 to S6 are the nano film pretreatment process of the sample; the following steps are carried out:

(S7): Correlation water quality analysis of concentrated liquids, including total nitrogen (using German liquidTOC for total nitrogen [TN] detection), nitrate nitrogen and nitrite nitrogen [NO ₃ ^- and NO ₂ ^- , using American Dian's DANEOX ion The chromatographic measurement], ammonia nitrogen [NH ₄ ⁺ , using a colorimetric method, and measurement using a UNICO ultraviolet spectrophotometer at wavelengths of 420 nm and 540 nm, respectively).

(S8): The measured value is subjected to a difference calculation, that is, DON=TN-NO ₃ ^- -NH ₄ ⁺ -NO ₂ ^- , and the obtained DON concentration is divided by the concentration factor to obtain the DON concentration of the raw water.

In order to make the method of the present invention more clearly understand the difference between the measurement process and the final calculated value, the following examples of practical detection are specifically mentioned as saying Bright:

Implementation case 1

The water sample filtered by a tap water mill with 0.22 or 0.45 μm microfiltration membrane is used as the water sample to be tested. The pH is adjusted to neutral using HCl and NaOH, and different types of nanofiltration membranes are used for DON test. The pretreatment; controlling the concentration of the water sample is about 6 times, comparing the effects of the three types of nanofilm on the measurement results. As shown in the fourth figure, under the condition of no nanofiltration pretreatment, the same water sample directly measures the DON concentration in the water sample by the difference method, and the concentration range of the four repeated measurements is -0.1-+. 0.4mg/L, the standard deviation was 0.21mg/L, and the effective DON concentration value could not be obtained at all. After pretreatment with different nano membranes, the standard deviation of DON concentration was significantly reduced, which effectively improved the measurement results. Reproducibility. Since different types of nano-films have different organic matter retention and adsorption properties, this directly affects the recovery rate of DON, resulting in slightly different DON values for different types of nano-films. After calculation, the nano-film 3 has the highest organic recovery rate of 80-95%, and thus has the highest DON measurement value.

Implementation case 2

After the water of a water plant is filtered through a 0.22 or 0.45 μm microfiltration membrane, the water sample is the water sample to be tested, and the pH of the water sample is between 3 and 10, which affects the concentration of the pretreatment method DON. In the test, the nano film 3 was used, and the concentration ratio of the control water sample was 6 times. As shown in the fifth figure, the DON measurement result and the DIN/TDN ratio change were displayed under different pH conditions. It can be seen from the figure that the concentration of DON is the highest when the pH is between 6.5 and 7.5. The reason is that the interception effect of the inorganic film on the inorganic ions in the water is different due to different pH values, which eventually leads to DIN/TDN. The specific gravity is different; the DIN/TDN content in the water is maintained at about 60% at pH=5.0-9.0, which is basically the same; otherwise, the DIN/TDN will be too high, which will result in the determination of DON. accurate.

Implementation case 3

After the water from a tap water factory is filtered through a 0.22 or 0.45 μm microfiltration membrane, the water sample is the water sample to be tested. The test uses a method of adjusting the volume of the filtered water sample to maintain the volume of the fixed concentrate [50 cm ³ ]. The ratio of the total volume of the water sample to be tested to the area of the nanofiltration membrane is 2.5, ⁵ , 7.5, 12.5 and 25 cm ³ /cm ² , and the corresponding concentration multiples are ² , 4, 6, 10 and 20, and the analysis and comparison of the DON of the concentrated liquid, the results are shown in the sixth figure, as can be seen from the figure, the ratio of the total volume of the water sample to be tested and the area of the nanofiltration membrane is controlled at 7.5-12.5 cm. ³ /cm ² , corresponding to the concentration factor of 6-10 times, the obtained concentrate has the lowest DIN ratio and the highest DON content. When the concentration factor is too high, it will cause obvious concentration polarization, so the amount of DON penetrating nano film to be measured is obviously increased, and the measured DON concentration is significantly reduced.

Implementation case 4

Measured and analyzed the changes of DON concentration in four different sampling dates and treatment units in a water purification plant in Shanghai, and compared the direct measurement and the DON concentration measured by pretreatment with nano membrane filtration. ,result Shown in the seventh picture. From the seventh figure, it can be found that the DON in the water to be tested which has not been pretreated by nano film filtration cannot be measured. Because of the high DIN/TDN interference, the error of TN and DIN analysis is large, so that DON is calculated in the pre-ozone and precipitation process. The average concentration becomes negative, and the standard deviation of the measurement results are high. It is impossible to understand the change of DON in the process of water purification by the analysis method. The analysis results after pretreatment with nanofilm were good, all DONs were positive, and the standard deviation of repeated tests was within 10%. Through this method, it can be understood that the change of DON in each unit of drinking water treatment process is the same as that of dissolved organic carbon [DOC], and is consistent with the characteristics of each treatment unit.

The analytical method of the invention can effectively reduce the proportion of inorganic nitrogen (containing nitrate, nitrite and ammonia nitrogen) in the total nitrogen while carrying out the concentration of DON at a low concentration, thereby greatly improving the accuracy and accuracy of the DON test. Degree, overcomes the problem that DON often has negative values and large standard deviation of deviation in direct water sample determination method; the detection method is simple, rapid and effective, and has easy operation and low cost, and can be widely used for surface and groundwater monitoring, large DON monitoring and related research and analysis of medium and small water purification plants and sewage treatment plants.

The above is only a part of the embodiments of the present invention, and is not intended to limit the present invention. It is intended to be included in the scope of the present invention.

From the above, the composition and use of the components show that the present invention has many advantages compared with the prior art, and is described as follows:

1. The method for determining the concentration of dissolved organic nitrogen in water at low concentration according to the present invention The method can effectively reduce the concentration of inorganic nitrogen [nitrate nitrogen, nitrite nitrogen and ammonia nitrogen] in the concentration of total dissolved organic nitrogen in the concentrated water sample, and increase the content of dissolved organic nitrogen, so as to greatly reduce the utilization difference. The concentration deviation that may be caused by the calculation of the organic nitrogen content, thereby ensuring the accuracy and accuracy of the measured concentration.

2. Compared with the internationally proposed dialysis membrane pretreatment method, the pretreatment time can be saved by at least 5 times, and the ultrapure water usage can be saved by at least 720 times, and the invention has simple operation, convenient control and economic feasibility. Extremely high.

In summary, the embodiments of the present invention can achieve the expected use efficiency, and the specific structure disclosed therein has not been seen in similar products, nor has it been disclosed before the application, and has completely complied with the provisions of the Patent Law. And the request, the application for the invention of a patent in accordance with the law, please forgive the review, and grant the patent, it is really sensible.

(2)‧‧‧Nano film

(3) ‧‧‧Filter equipment

(31)‧‧‧Filter Cup

(32)‧‧‧Magnetic mixer

(4) ‧ ‧ container

(5) ‧‧‧nitrogen

Claims (10)

A method for determining the concentration of dissolved organic nitrogen in water is carried out by filtering the water sample to be tested through a microfiltration membrane to remove suspended solids and particulate impurities, and then adjusting with sodium hydroxide [NaOH] and hydrogen chloride [HCl]. The pH value of the water sample to be tested is such that the pH of the water sample to be tested is between 6.0 and 9.0, and the water sample is continuously poured into a vertical (dead-end) filtering device equipped with a nano film. The transmembrane pressure is adjusted between 0.3 and 0.6 MPa, and the water sample can be filtered and concentrated, and the ratio of the total volume of the concentrated water sample to the area of the nanofiltration membrane is controlled at 5-10 cm ³ /cm. ² , the water sample concentration factor is 6-15 times, in order to improve the accuracy of the analysis of dissolved organic nitrogen [DON] in the water sample. A method for measuring a concentration of a low-concentration dissolved organic nitrogen in water according to the first aspect of the invention, wherein the water sample is filtered to remove particulate impurities by using a microfiltration membrane having a pore diameter of 0.22 μm or more and 0.45 μm or less. The method for determining the concentration of a low concentration of dissolved organic nitrogen in water according to the second aspect of the patent application, wherein the microfiltration membrane is made of a cellulose acetate or a polyethersulfone. Determination of low concentration solubility in water as described in claim 1 A method for concentrating organic nitrogen, wherein a nano film having a molecular weight cut off of 100-500 Dalton is used to filter the water sample, and the nano film is immersed in ultrapure water for more than 24 hours before use to remove Preservation solution. The method for determining the concentration of low-concentration dissolved organic nitrogen in water according to the fourth aspect of the patent application, wherein when the vertical (dead-end) filtration is used, in order to prevent concentration polarization during the filtration process, the rotor is stirred. Set the speed to 100-200 rpm to avoid the water flow being too intense and the nano film to contact the air. A method for determining the concentration of dissolved organic nitrogen in water is carried out by filtering the water sample to be tested through a microfiltration membrane to remove suspended solids and particulate impurities, and then adjusting with sodium hydroxide [NaOH] and hydrogen chloride [HCl]. The pH value of the water sample to be tested is such that the pH of the water sample to be tested is between 6.0 and 9.0, and the water sample is continuously poured into a cross-flow filtration device equipped with a nano film. By adjusting the transmembrane pressure between 0.3 and 0.6 MPa, the water sample can be filtered and concentrated, and the ratio of the total volume of the concentrated water sample to the area of the nanofiltration membrane is controlled at 5-10 cm ³ / Cm ² , the water sample concentration factor is 6-15 times, in order to improve the accuracy of the analysis of dissolved organic nitrogen [DON] in the water sample. Determination of low concentration solubility in water as described in claim 6 A method of organic nitrogen concentration, wherein the water sample is filtered to remove particulate impurities by using a microfiltration membrane having a pore diameter of 0.22 μm or more and 0.45 μm or less. The method for determining the concentration of a low concentration of dissolved organic nitrogen in water according to the seventh aspect of the patent application, wherein the microfiltration membrane is made of a cellulose acetate or a polyethersulfone. A method for determining a concentration of a low concentration of dissolved organic nitrogen in water as described in claim 6 wherein a nanofilm having a molecular weight cut off of 100 to 500 Dalton is used to filter the water sample, and the nanometer is filtered. The film should be immersed in ultrapure water for more than 24 hours before use to remove the preservation solution. A method for determining a low concentration of dissolved organic nitrogen in water as described in claim 1 or 6, wherein the concentrated water sample is subjected to analysis of total nitrogen, nitrate nitrogen, nitrite nitrogen and ammonia nitrogen, and water sample The concentration of dissolved organic nitrogen is equal to the value of total nitrogen in the water sample after concentration minus nitrate nitrogen, nitrite nitrogen and ammonia nitrogen, which is obtained by dividing the actual concentration factor.