CN113740369A - Reduction method for detecting heavy metal ions based on in-situ low-field nuclear magnetic resonance relaxation method - Google Patents
Reduction method for detecting heavy metal ions based on in-situ low-field nuclear magnetic resonance relaxation method Download PDFInfo
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Abstract
The invention discloses a novel method for reducing and detecting heavy metal ions based on an in-situ low-field nuclear magnetic resonance technology, namely, paramagnetic ion concentration and proton transverse relaxation rate (1/T)2) The linear relation between the two is that the T in the heavy metal ion reduction process is monitored in real time by using a CPMG pulse sequence and in-situ illumination2The increase of the concentration of the paramagnetic ions is quickly detected, and the reduction efficiency of the heavy metal ions is further represented. Meanwhile, the method uses an SE-SPI selective layer sequence to monitor the reduction condition of heavy metal ions on different layers of the reaction solution in real time. The method is simple to operate, low in cost and high in accuracy, and does not need to pretreat the original solution or separate products. The method is expected to be popularized in the aspect of removing other heavy metal ions.
Description
Technical Field
The invention belongs to the technical field of low-field nuclear magnetic resonance, relates to detection of heavy metal ion treatment technology, and particularly relates to a method for detecting a chemical reaction of a heavy metal ion in real time by an in-situ low-field nuclear magnetic resonance relaxation method.
Background
Heavy metals (chromium, mercury, lead, etc.) have been used by humans for thousands of years, and the rapid development of industry and population has led to increased exposure of people to heavy metals over the past few years. Of all the toxic heavy metal ions, hexavalent chromium (cr (vi)) is a common surface and ground water contaminant. It has acute toxicity to most organisms, strong carcinogenicity, and high solubility in water. World Health Organization (WHO) regulation, body of waterThe content of Cr (VI) in the alloy is not higher than 0.05 mg/L. Therefore, for the health of organisms worldwide, it is important to remove cr (vi) or reduce the concentration thereof in the water body. Unlike common heavy metals such as lead, cadmium, copper, etc., chromium is mainly present in two forms, trivalent chromium (Cr) (III) which is low in toxicity and hexavalent chromium (Cr) (VI) which is high in toxicity. At present, various methods for treating Cr (VI) -containing wastewater have been developed, wherein the reduction of Cr (VI) to Cr (III) is one of the most effective methods for treating Cr (VI) -containing wastewater because Cr (III) has low toxicity and is easily treated with Cr (OH)3The precipitate is conveniently removed as solid waste. In recent years, photocatalytic technology, which utilizes the conversion of solar-chemical energy, has been recognized as a clean, efficient, low-cost and non-hazardous product method for reducing cr (vi) to cr (iii).
Currently, ultraviolet spectrophotometry or atomic absorption spectrometry is generally used to quantitatively analyze the concentration of heavy metal ions. However, uv spectrophotometry can only detect ions having an optical absorption band in the uv-visible region. Therefore, colorless heavy metal ions cannot be analyzed by ultraviolet spectrophotometry without color development treatment. In addition, the results of the analysis may be subject to other coexisting ions (e.g., Mo)6+、Hg+、Hg2+、V5+、Fe3+) The interference of (2). Atomic absorption spectroscopy has the advantage of element selectivity, but is expensive to operate. Meanwhile, in the process of photocatalytic Cr (VI) reduction, the distance of the light source can influence the efficiency of Cr (VI) reduction, which is shown in that the reduction of Cr (VI) to Cr (III) at each layer of the cross section of the reaction solution at different distances from the light source is different. At present, the traditional ultraviolet spectrophotometry and atomic absorption spectrometry cannot realize the research of Cr (VI) reduction process at different levels. Therefore, the development of a new method for quantifying the concentration of heavy metal ions, which is rapid, simple, low in cost, accurate and free of pretreatment, has important significance for evaluating the reduction performance of heavy metals, and meanwhile, the method can detect the research on the reduction process of heavy metals by using reaction solutions of different layers, and has great value.
Disclosure of Invention
The invention aims to overcome the defects of the existing detection method and design a rapid, simple, low-cost, accurate and pretreatment-free quantitative detection method for heavy metal reduction. The invention adopts a low-field nuclear magnetic resonance relaxation method to realize the rapid detection of paramagnetic ions, simultaneously detects the photocatalytic reduction process in real time through an in-situ illumination device, realizes the representation of the reduction performance, and monitors the reduction condition of heavy metal ions on different layers of reaction liquid in real time through the same in-situ illumination device. The method has the advantages of rapidness, real-time performance, low cost and no need of pretreatment, well eliminates the interference of other ions, and realizes the in-situ detection of the reduction process of the reaction solution in different cross sections for the first time.
The invention discloses a method for detecting heavy metal ion reduction based on an in-situ low-field nuclear magnetic resonance relaxation method, which utilizes the linear relation between the concentration of paramagnetic ions and the transverse relaxation rate of protons, monitors T in the heavy metal ion reduction (Cr (VI) reduction) process in real time through in-situ illumination by using a CPMG pulse sequence and2rapidly detecting the increase of the concentration of paramagnetic ions (Cr (III)), and further characterizing the reduction efficiency of heavy metal ions (Cr (VI)); and meanwhile, the reduction condition of heavy metal ions (Cr (VI)) in different layers of the reaction solution is monitored in real time by using an SE-SPI selective layer sequence. The specific process is as follows:
(1) adding the suspension containing the catalyst powder and the heavy metal ion solution into a matched low-field nuclear magnetic tube, and performing ultrasonic homogenization;
(2) assembling in-situ light source equipment, putting the nuclear magnetic tube filled with the sample to be detected into a cavity of a low-field nuclear magnetic resonance spectrometer, and introducing light into the nuclear magnetic tube through an optical fiber;
(3) using Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence to system T2The variation of the value with the illumination time is detected in situ.
(4) Using Spin Echo-Single Point Imaging (SE-SPI) pulse sequence to carry out T treatment on different layers of the system2The variation of the value with the illumination time is detected in situ.
In the invention, the adding amount of the catalyst powder is 0.5-10 mg, the volume of the added heavy metal ion solution is 1-2 mL, and the light source and the nuclear magnetic tube are connected by an optical fiber;
in the invention, the whole in-situ test process does not need product separation;
in the invention, the diameter of the low-field nuclear magnetic tube is 10mm, the height is 100mm, and the added Cr (VI) solution is 1 mg/L-20 mg/L;
in the invention, the optical fiber used is a customized metal-free wrapped optical fiber bundle;
in the invention, the method can rapidly and quantitatively detect the increase of the concentration of paramagnetic ions (Cr (III)).
The design principle of the invention is that heavy metal ions are reduced into paramagnetic ions by photocatalysis, the concentration of the paramagnetic ions has influence on proton relaxation, and T can be measured by low-field nuclear magnetism2Quantifying the effect of paramagnetic ion concentration on proton relaxation by numerical values, establishing T2The relationship with paramagnetic ion concentration, finally by T2To study the process of photocatalytic reduction of heavy metal ions into paramagnetic ions. On the basis, the photocatalytic process can be monitored in real time under the condition of no need of product separation by virtue of in-situ light source equipment. According to the invention, the in-situ illumination device can be built only by connecting the optical fiber with the xenon lamp light source through the condenser lens. In-situ photocatalysis can be realized only by introducing the optical fiber into a low-field nuclear magnetic tube filled with a mixture to be subjected to photocatalytic reaction; t of passing test proton during light irradiation2The photocatalytic process can be monitored in real time; meanwhile, the photocatalysis process of the reaction liquid in different spatial layers can be monitored in real time by utilizing an SE-SPI pulse sequence, and no other technical means can realize a similar detection process at present.
Therefore, the equipment provided by the invention is simple and convenient to build, simple in operation steps and novel in technical means. The result obtained by the method is consistent with the result trend of the traditional ultraviolet spectrophotometry, but the technical means of the method is simpler.
Compared with the prior art, the invention has the beneficial effects that:
(1) the method can quickly detect the change of the concentration of paramagnetic ions in the solution, does not need product separation, and has the detection time of less than 1min which is far faster than other existing detection means.
(2) The test method is quick and simple, only specific pulses are needed to be applied to the sample, the experiment parameters are adjusted, and the system to be tested is not damaged.
(3) The original solution in the system does not need to be subjected to color development treatment, and ions which do not develop color can also be used for detection.
(4) The in-situ light source device has the advantages of ingenious design, simple operation and low cost, can detect the reduction process of photocatalysis heavy metal ions in real time, and simultaneously evaluates the reduction performance.
(5) The layer selection experiment can observe the reduction state of the photocatalytic heavy metal ions on different spatial layer surfaces of the sample in real time, so that the reduction condition of the heavy metal ions on each layer surface with different distances from the light source in the real solid-liquid reaction environment can be further understood.
Drawings
FIG. 1 is a CMPG decay curve chart showing different Cr (III) concentrations of single components in example 1 of the present invention;
FIG. 2 shows T of a one-component Cr (III) system in example 1 of the present invention2A graph of variation with Cr (III) concentration;
FIG. 3 shows 1/T in example 1 of the present invention2A plot of the change in cr (iii) ion concentration;
FIG. 4 is a CMPG decay curve chart of different Cr (VI) -Cr (III) ion concentrations of mixed components in example 1 of the present invention;
FIG. 5 shows T of the mixed Cr (VI) -Cr (III) system in example 1 of the present invention2A graph of the variation with the Cr (VI) -Cr (III) concentration;
FIG. 6 shows 1/T in example 1 of the present invention2A plot of the concentration of cr (vi) -cr (iii) ions as a function of time;
FIG. 7 is a schematic diagram of the overall appearance of in-situ low-field nuclear magnetic resonance monitoring photocatalytic Cr (VI) reduction in an embodiment of the present invention;
FIG. 8 is the visible light photocatalytic Cr (VI) reduction activity of the photocatalyst measured by UV spectrophotometry in example 2 of the present invention;
FIG. 9 is a low-field nuclear magnetic T of the composite photocatalyst in example 2 of the present invention2A graph of changes with illumination time;
FIG. 10 isIn the embodiment 3 of the invention, (a) a layer selection schematic diagram and (b) T with different concentrations on different layers2A value;
FIG. 11 is a schematic diagram of (a) layer selection and (b) T layers of a composite photocatalyst in example 3 of the present invention2Graph of variation with illumination time.
Detailed Description
The preferred embodiments of the present invention will be described in detail with reference to the following examples, but it should be understood that those skilled in the art can reasonably change, modify and combine the examples to obtain new embodiments without departing from the scope defined by the claims, and that the new embodiments obtained by changing, modifying and combining the examples are also included in the protection scope of the present invention.
The following describes the embodiments of the present invention in further detail with reference to the drawings and examples.
The specific technical laws for realizing the purpose of the invention are as follows: a method for quantitatively detecting the reduction of heavy metal ions in chemical reaction based on in-situ low-field nuclear magnetic resonance relaxation technology features that the paramagnetic ion concentration and transverse proton relaxation rate (1/T)2) The linear relation between the two is that the T in the heavy metal ion reduction process is monitored in real time by using a CPMG pulse sequence and in-situ illumination2The increase of the concentration of the paramagnetic ions is rapidly detected, and the reduction efficiency of the heavy metal ions is further represented. Meanwhile, the method uses an SE-SPI selective layer sequence to monitor the reduction condition of heavy metal ions on different layers of the reaction solution in real time. The method comprises the following specific steps:
(1) adding the suspension containing the catalyst powder and the heavy metal ion solution into a matched low-field nuclear magnetic tube, and performing ultrasonic homogenization;
(2) assembling in-situ light source equipment, putting the nuclear magnetic tube filled with the sample to be detected into a cavity of a low-field nuclear magnetic resonance spectrometer, and introducing light into the nuclear magnetic tube through an optical fiber;
(3) using Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence to system T2The value is carried out along with the change of illumination timeDetecting in situ;
(4) using Spin Echo-Single Point Imaging (SE-SPI) pulse sequence to carry out T treatment on different layers of the system2The variation of the value with the illumination time is detected in situ.
In the step (1), the catalyst is 2Ag/g-C3N4、5Ag/g-C3N4、Ag32NCs/g-C3N4One or more of the following; preferably, it is 5Ag/g-C3N4、Ag32NCs/g-C3N4。
In the step (1), the adding amount of the catalyst powder is 0.5-10 mg; preferably, it is 5 mg.
In the step (1), the heavy metal ion solution is one or more of Cr (VI) solution and the like; preferably, it is a cr (vi) solution.
In the step (1), the volume of the solution added with the heavy metal ions is 1 mL-2 mL, and the concentration of the solution added with the heavy metal ions is 1 mg/L-20 mg/L.
In the step (1), the diameter of the low-field nuclear magnetic tube is preferably 10mm, and the height of the low-field nuclear magnetic tube is preferably 100 mm.
In the step (1), the ultrasonic condition is that the ultrasonic power is 40W, the temperature is 20 ℃ and the time is 5 min.
The whole in-situ test process of the invention does not need product separation.
In the step (2), the in-situ light source device is assembled by the following steps: and connecting the designed optical fiber bundle consisting of the bare fibers without metal wrapping with a light source through a condenser.
The light source is one or more of a common xenon lamp light source, a mercury lamp light source, a metal halide lamp light source and the like; preferably, the present invention uses a general 300W xenon lamp as a light source. It is noted that the present invention has no substantial requirements for the light source and the illumination intensity.
The invention designs an optical fiber bundle consisting of bare fibers without metal wrapping, which mainly aims to avoid the interference of an optical fiber metal wrapping layer on nuclear magnetic signals, ensure the stability of illumination and avoid the condition of no light introduction caused by the damage of a single optical fiber.
The light source and the nuclear magnetic tube are connected by the optical fiber.
In the step (2), the optical fiber is a customized metal-free wrapped optical fiber bundle.
The method of the invention can rapidly carry out quantitative detection on the increase of the concentration of the paramagnetic ions.
"paramagnetic ion" specifically refers to an ion having a strong intrinsic magnetic moment, such as cr (iii), cu (ii), etc., as is well known in the art.
In the invention, a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence is used for the system T2The determination of the value is a well known routine operation.
In the invention, a Spin Echo-Single Point Imaging (SE-SPI) pulse sequence is used for carrying out T treatment on different layers of a system2The determination of the value is a well known routine operation.
The height of the light source from the liquid level in all tests of the invention must be kept consistent, and the height can be adjusted according to requirements. The distance between the light sources is 3 cm.
In one embodiment, the method of the present invention specifically comprises the steps of:
step 1: installation of equipment
And connecting the designed optical fiber bundle consisting of the bare fibers without metal wrapping with a light source through a condenser.
Step 2: sample loading
The test sample was added to a low-field nuclear magnetic tube having a diameter of 10mm and a height of 100 mm. And then putting the low-field nuclear magnetic tube into a low-field nuclear magnetic resonance instrument.
And step 3: the optical fiber is led into the nuclear magnetic tube
The other end of the optical fiber is led into a nuclear magnetic tube placed in low-field nuclear magnetism, and the optical fiber is a bare optical fiber bundle. The rest of the wrapped optical fiber (non-bare optical fiber) and the light source device are placed outside the low-field nuclear magnetic instrument.
In the invention, the single optical fiber and the optical fiber bundle which are led into the low-field nuclear magnetic tube are all exposed; the single optical fiber outside the nuclear magnetic tube is bare, and the optical fiber bundle formed by the single optical fiber is wholly wrapped by the protective layer.
And 4, step 4: pre-illumination signal acquisition
And before the light is turned on, the data of the sample put into the low-field nuclear magnetic instrument is preliminarily collected every 5 min.
And 5: signal acquisition in illumination
And (3) turning on a xenon lamp light source, and carrying out relaxation data acquisition on the sample placed in the low-field nuclear magnetic instrument every 5min or 15 min.
Step 6: analysis of results
Carrying out inversion processing on the data obtained in the step 4 and the step 5 to obtain T2The value is obtained. The data inversion process is well known in the art. The results were analyzed by mapping software and the final results were compared to standard uv spectrophotometry.
Example 1: rapid detection of paramagnetic Cr (III) ions at different concentrations
Step 1: preparation of different concentrations of Cr (III) ion
One component Cr (III) System: 1mg/mL of Cr (III) standard solution is diluted to 25mL of Cr (III) solution with different concentrations: 1mg/mL, 5mg/mL, 10mg/mL, 15mg/mL, and 20 mg/mL.
Mixed component cr (vi) -cr (iii) system: preparing a Cr (III) standard solution with the concentration of 1mg/ml and a Cr (VI) solution with the concentration of 100mg/L into Cr (VI) -Cr (III) mixed solutions with different concentrations: 20-0mg/mL, 15-5mg/mL, 10-10mg/mL, 5-15mg/mL, and 0-20 mg/mL.
Step 2: low field nuclear magnetic resonance test
Adding 1.5ml of Cr (III) solution with different concentrations in the step 1 into a low-field nuclear magnetic tube, and testing T by using a CPMG sequence2The value is obtained.
The model and specific parameters of the low-field nuclear magnetic instrument in the step 2 are as follows: the low-field nuclear magnetic analyzer (NMI20-015V-I) has the magnetic field intensity of 0.5 +/-0.08T, the proton resonance frequency of 21.3MHz and the diameter of a probe coil of 15 mm. The sampling parameters are as follows: the oversampling waiting Time (TW) is 8000ms, the echo Time (TE) is 1ms, the Number of Echoes (NECH) is 15000, and the number of accumulated samples (NS) is 4.
FIG. 1 shows the CMPG decay of a single component Cr (III) ion solution at a concentration of from 1mg/L to 20mg/LThe graph is subtracted. As can be seen from FIG. 1, the signal-to-noise ratio (SNR) of these signals is relatively high under the field strength test of about 0.05T, which is favorable for testing T through fitting2. FIG. 2 shows T of a one-component Cr (III) solution2The value change profile. As can be seen from FIG. 2, as the paramagnetic Cr (III) ion concentration in the system increases, the T of the system increases2The values show a decreasing trend. This is due to the unpaired electrons of the paramagnetic ion in solution and in the solvent1The H nucleus undergoes dipolar interaction, resulting in a rapid relaxation of the hydrogen protons. FIG. 3 is 1/T2Analytical plot of change with cr (iii) ion concentration. As can be seen in FIG. 3, 1/T2Shows a good linear relation with Cr (III) ion concentration, determines the coefficient (R)2) At 0.998, the fit equation is shown in fig. 3.
In order to reflect the change of Cr (VI) and Cr (III) in the photocatalytic system more truly, the invention researches the mixed component Cr (VI) -Cr (III) system according to the same method as the single component Cr (VI). FIG. 4 is a CMPG decay curve diagram of the concentration gradient of Cr (VI) and Cr (III) from 20-0mg/L to 0-20mg/L in the mixed component Cr (VI) -Cr (III) system. As can be seen from FIG. 4, the SNR of the signal in the mixed component Cr (VI) -Cr (III) system is consistent with that of the single component Cr (III), which shows that the change of the system does not affect the test, and the system T can be obtained by fitting2The value of (c). FIG. 5 shows T of a mixed component Cr (VI) -Cr (III) solution2The value change profile. As can be seen from FIG. 5, as the concentration of Cr (VI) -Cr (III) ions in the Cr (VI) -Cr (III) system decreases and the concentration of paramagnetic Cr (III) ions increases, the T of the system2The values also show a decreasing trend. FIG. 6 shows 1/T of the mixed components2Analytical graph of change with Cr (VI) -Cr (III) ion concentration. As can be seen from FIG. 6, 1/T2Shows a good linear relation with the ion concentration of the mixed component, R2The fit equation is shown in fig. 6, 0.991. The close fit of the equations found by the comparative fitting equations for the single and mixed components confirms that Cr (VI) does not contribute to T for Cr (III)2The test has an impact. Thus can use T2The paramagnetic Cr (III) ion concentration in the mixed components can be rapidly judged.
The invention has been shown above that the method of the invention has the advantages of rapidness, real-time performance, low cost and no need of pretreatment, and can well eliminate the interference of other ions.
Referring to fig. 7, an overall appearance schematic diagram of in-situ low-field nuclear magnetic resonance monitoring photocatalytic cr (vi) reduction is shown, the in-situ low-field nuclear magnetic resonance measurement method for the photocatalytic cr (vi) reduction reaction designed by the present invention includes the following steps: firstly, adding a suspension containing catalyst powder and a heavy metal ion solution into a low-field nuclear magnetic tube, then assembling light source equipment, putting the nuclear magnetic tube into a cavity of a low-field nuclear magnetic resonance spectrometer, introducing light into the nuclear magnetic tube through an optical fiber, and carrying out in-situ detection on the nuclear magnetic tube. The device can realize real-time monitoring of the reduction process of Cr in the heterogeneous phase reduction reaction process of photocatalytic Cr (VI) under the reaction condition.
Example 2: in-situ low-field nuclear magnetic resonance real-time detection of Ag/g-C3N4Photocatalytic Cr (VI) reduction of
Step one, preparation of catalyst
g-C3N4The preparation of (1): weighing 10g of urea, adding the urea into an alumina crucible, covering the crucible, putting the alumina crucible into a muffle furnace, heating to 520 ℃ at the speed of 5 ℃/min, keeping the temperature for 2h, and naturally cooling to room temperature to obtain g-C3N4And (3) sampling.
Ag/g-C3N4The preparation of (1): taking 0.2g of g-C prepared in the step3N4Adding the mixture into 50mL of deionized water, carrying out ultrasonic dispersion for 1h, then adding a certain amount of 1mg/mL silver nitrate solution, carrying out magnetic stirring for 0.5h, then adding a certain amount of freshly prepared sodium borohydride solution (the mass ratio of the Ag to be reduced to the sodium borohydride is 1:6), putting the mixture into an ice water bath, carrying out stirring reaction for 1h, carrying out suction filtration, washing the mixture for multiple times by using deionized water, and carrying out vacuum drying at 60 ℃ for 12 h. Photocatalytic samples with Ag loadings of 1 wt%, 2 wt%, 5 wt% and 10 wt% were prepared using the above procedure and labeled as 1Ag/g-C3N4、2Ag/g-C3N4、5Ag/g-C3N4And 10Ag/g-C3N4。
Step two, ultraviolet spectrophotometry performance characterization test
Uniformly dispersing 20mg of catalyst in 50ml of 20mg/L K at room temperature2Cr2O7And adding 0.165mL of citric acid with the concentration of 100mg/mL into the solution as a cavity sacrificial agent, and magnetically stirring the mixed solution for 0.5h in the dark to ensure that the catalyst and the pollutants reach adsorption-desorption balance. The dark-treated solution was placed on a filter with a cut-off filter (. lamda.)>420nm) under a 300W xenon lamp, visible light was simulated for photocatalytic experiments. During the irradiation with visible light, 1mL of the reaction solution was taken from the reaction cell at given time intervals, filtered through a 0.45 μm polytetrafluoroethylene filter, and the concentration of Cr (VI) was measured by an ultraviolet-visible spectrophotometer at a maximum absorption wavelength of 540nm by using a modified Diphenylaminourea (DPC) color development method.
Step three, in-situ low-field nuclear magnetic resonance test
5mg of catalyst and 1.5ml of 20mg/L Cr (VI) solution are added into a nuclear magnetic tube, the nuclear magnetic tube is placed into a low-field nuclear magnetic cavity, one end of an optical fiber is connected to a xenon lamp light source, and the other end of the optical fiber is introduced into low-field nuclear magnetic to prepare for testing and collecting signals.
Under the condition of keeping out of the sun, performing T on the sample in the nuclear magnetic tube at regular intervals2And (6) testing. Then turning on the xenon lamp light source, and in the process of illumination, carrying out T on the sample in the nuclear magnetic tube at regular intervals2And (6) testing.
The model and specific parameters of the low-field nuclear magnetic instrument in the step 3 are as follows: the low-field nuclear magnetic analyzer (NMI20-015V-I) has the magnetic field intensity of 0.5 +/-0.08T, the proton resonance frequency of 21.3MHz and the diameter of a probe coil of 15 mm. Selecting a CPMG sequence, wherein the sampling parameters are as follows: the oversampling waiting Time (TW) is 8000ms, the echo Time (TE) is 1ms, the Number of Echoes (NECH) is 15000, and the number of accumulated samples (NS) is 4.
FIG. 9 shows Ag/g-C3N4Paramagnetic Cr (III) T obtained by reducing Cr (VI)2Graph of the change in value. As shown in FIG. 9, T is measured under dark conditions (i.e., during the time period of-5 to 0)2The value decreased very slowly during the initial 5 min. Under the condition of illumination, due to the rapid increase of the concentration of paramagnetic Cr (III) ions which are the products of the photocatalytic reduction of Cr (VI),T2the value also decreases rapidly. And Ag/g-C along with the increase of Ag loading capacity3N4T of composite material2The reduction degree is gradually increased, and the reduction degree is ranked as 10Ag/g-C within 10min of illumination time3N4≈5Ag/g-C3N4＞2Ag/g-C3N4＞1Ag/g-C3N4The performance trend is substantially consistent with the results of the ultraviolet spectrophotometry of fig. 8. Thus, passing through T2It is feasible to evaluate the photocatalytic cr (vi) reduction activity of the catalyst with the trend of the values over the time of the light.
Example 3: in-situ low-field nuclear magnetic resonance real-time detection of photocatalytic Cr (VI) reduction process of different layers
Step 1: preparation of the catalyst
Preparation of Ag by simple impregnation method32NCs/g-C3N4And (3) sampling. First 0.04g of g-C3N4Dispersed in 40ml ethanol solution and ultrasonically diffused for 1 h. Then adding Ag into the mixed solution respectively32(MPG)19NCs stock solution was stirred at room temperature for 2 h. Finally, the required nano composite catalyst sample can be obtained through centrifugation, multiple times of alcohol washing and vacuum drying.
Step 2: low field nuclear magnetic stratification experiment
Adding 5mg catalyst and 1.5ml,20mg/L Cr (VI) solution into a nuclear magnetic tube, placing the nuclear magnetic tube into a low-field nuclear magnetic cavity, connecting one end of an optical fiber to a xenon lamp light source, introducing the other end into low-field nuclear magnetic, testing T of different layers at certain intervals2And monitoring the conditions of the photocatalytic Cr (VI) reduction at different levels in real time through an in-situ device.
The model and specific parameters of the low-field nuclear magnetic instrument in the step 2 are as follows: the low-field nuclear magnetic analyzer (NMI20-015V-I) has the magnetic field intensity of 0.5 +/-0.08T, the proton resonance frequency of 21.3MHz and the diameter of a probe coil of 15 mm. Selecting an SE-SPI sequence, wherein sampling parameters are as follows: the oversampling latency (TW) is 8000ms, the echo Time (TE) is 0.8ms, the Number of Echoes (NECH) is 15000, the number of accumulated samples (NS) is 2, and the number of layers (NTI) is 6.
FIG. 10 shows different concentrationsCr (III) at 5mg/L, 10mg/L and 15mg/L, respectively, on 1-6 layers T2The value of (c). As shown in FIG. 10, when the concentration of Cr (III) ions is 5mg/L, each layer T2The values are all 610 ms. When the concentration of Cr (III) ions is 10mg/L, each layer T2The value is 340 ms. When the concentration of Cr (III) ions is 15mg/L, all layers T2Are all 265 ms. For T of different layers with the same concentration2The values remain substantially consistent.
FIG. 11 shows Ag32NCs/g-C3N4Different levels of Cr (VI) reduction Process T2And illumination time. T of all layers before illumination2The values are the same. All levels T are extended with the illumination time2The values all show a decreasing trend due to the reduction of cr (vi) in the system to paramagnetic cr (iii). However, because the distances from different layers to the light source are different and the Cr (VI) reduction conditions of different layers are different, different layers T have different illumination time2The degree of reduction of the value also varies. It can be found that at the 6 th layer T closest to the light source2The layer 1, T, having a smaller degree of significance, and the farthest distance from the light source2The reduction trend was the slowest. The layer selection experiment can show that the photocatalytic Cr (VI) reduction conditions of different layers are different, and the Cr (VI) reduction efficiency is higher as the light source is closer.
The invention relates to a novel method for reducing and detecting heavy metal ions based on an in-situ low-field nuclear magnetic resonance technology, namely, paramagnetic ion concentration and proton transverse relaxation rate (1/T)2) The linear relation between the two is that the T in the heavy metal ion reduction process is monitored in real time by using a CPMG pulse sequence and in-situ illumination2The increase of the concentration of the paramagnetic ions is quickly detected, and the reduction efficiency of the heavy metal ions is further represented. Meanwhile, the method uses an SE-SPI selective layer sequence to monitor the reduction condition of heavy metal ions on different layers of the reaction solution in real time. The method is simple to operate, low in cost and high in accuracy, and does not need to pretreat the original solution or separate products. The method is expected to be popularized in the aspect of removing other heavy metal ions.
The above description is only a preferred embodiment of the present invention, and those skilled in the art can make modifications or equivalent substitutions within the spirit of the present invention, such as the reaction involving paramagnetic metal ions in chemical reactions, and variations made according to the spirit of the present invention should fall within the scope of the present invention.
Claims (10)
1. The method for detecting the reduction of the heavy metal ions based on the in-situ low-field nuclear magnetic resonance relaxation method is characterized in that the linear relation between the concentration of paramagnetic ions and the transverse relaxation rate of protons is utilized, a CPMG pulse sequence is used, and the T in the reduction process of the heavy metal ions is monitored in real time through in-situ illumination2The change of the paramagnetic ion concentration is detected quickly, and the reduction efficiency of the heavy metal ions is further represented; and meanwhile, the reduction condition of the heavy metal ions on different layers of the reaction solution is monitored in real time by using an SE-SPI (sequence of sequence initiation) selective layer.
2. The method of claim 1, wherein the heavy metal ion solution is a cr (vi) solution.
3. The method of claim 1, wherein the specific process is as follows:
(1) adding the suspension containing the catalyst powder and the heavy metal ion solution into a matched low-field nuclear magnetic tube, and performing ultrasonic homogenization;
(2) assembling in-situ light source equipment, putting the nuclear magnetic tube filled with the sample to be detected into a cavity of a low-field nuclear magnetic resonance spectrometer, and introducing light into the nuclear magnetic tube through an optical fiber;
(3) using Carr-Purcell-Meiboom-Gill pulse sequence to system T2The value is detected in situ along with the change condition of the illumination time;
(4) using Spin Echo-Single Point Imaging pulse sequence to carry out T Imaging on different levels of the system2The variation of the value with the illumination time is detected in situ.
4. The method of claim 3, wherein the catalyst is 2Ag/g-C3N4、5Ag/g-C3N4、Ag32NCs/g-C3N4One or more of (a).
5. The method of claim 3, wherein the ultrasound is performed at an ultrasound power of 40W, at a temperature of 20 ℃ and for a time of 5 min.
6. The method of claim 3, wherein the catalyst powder is added in an amount of 0.5mg to 10mg, the volume of the solution of heavy metal ions is 1mL to 2mL, and the light source and the nuclear magnetic tube are connected by an optical fiber.
7. The method of claim 1, wherein the entire in situ test procedure does not require product isolation.
8. The method of claim 3, wherein the low-field nuclear magnetic tube has a diameter of 10mm and a height of 100mm, and the heavy metal ion solution is added in an amount of 1mg/L to 20 mg/L.
9. The method of claim 3, wherein the optical fibers used are custom metal free wrapped fiber bundles.
10. The method according to claim 3, wherein the preliminary data collection is performed on the sample placed in the low-field nuclear magnetic instrument every 5min before the light is turned on; and after the light source is turned on, relaxation data acquisition is carried out on the sample placed in the low-field nuclear magnetic instrument every 5min or 15 min.
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