CN116124816A - Method for detecting adsorption quantity of micro plastic to heavy metal by using low-field NMR relaxation method - Google Patents
Method for detecting adsorption quantity of micro plastic to heavy metal by using low-field NMR relaxation method Download PDFInfo
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Abstract
The invention provides a method for detecting the adsorption quantity of micro plastic to heavy metal by using a low-field NMR relaxation method, which comprises the following steps: establishing a marked line between the concentration of the target heavy metal ions and the transverse relaxation time; extracting a solution sample from a solution to be detected, and detecting the transverse relaxation time of the solution sample; determining the concentration of heavy metal ions of the solution sample according to the marked line; adding micro plastic particles into a solution to be detected, extracting a solution sample, filtering to obtain a filtered solution, and detecting the transverse relaxation time of the filtered solution; determining the concentration of heavy metal ions in the filtered solution according to the marked line; calculating the adsorption quantity of micro plastic with unit mass to heavy metalq e . The method for detecting the adsorption quantity of the micro plastic to the heavy metal by using the low-field NMR relaxation method does not need complicated sample pretreatment, namely sampling and measuring, is simple, convenient and quick, has low cost, wide detection limit and quick detection, and can realize nondestructive detection.
Description
Technical Field
The invention belongs to the technical field of water area environmental pollution detection or low-field nuclear magnetic resonance, relates to a rapid nondestructive detection method of heavy metals adsorbed by microplastic in aqueous solution, and in particular relates to a method for detecting the adsorption quantity of the microplastic to the heavy metals by using a low-field Nuclear Magnetic Resonance (NMR) relaxation method.
The present invention belongs to the technical category of "testing or analyzing a material by means of determining its chemical or physical properties", in particular "testing or analyzing a material by means of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects".
Background
Microplastic contamination has been found throughout the world, particularly in water environments, where trace of microplastic is found in the ocean, lakes, rivers, and arctic glaciers. The micro-plastic can adsorb various pollutants in the environment, especially in water, under the condition of serious heavy metal pollution, the adsorption and desorption of the heavy metal by the micro-plastic under different conditions can influence the diffusion, transportation and circulation of the heavy metal pollution compounded by the micro-plastic in the ecological environment. Typical heavy metals in water environment comprise copper, chromium, iron, manganese, mercury, lead and the like, water can enter a human body after being polluted by the heavy metals, the water is not easy to metabolize and discharge outside the body, various diseases can be caused by accumulation of the heavy metals in the human body, and particularly, most heavy metals have toxicity and carcinogenicity and can threaten the health of the human body. The microplastic has proved to be biotoxic in the field of aquatic organism toxicity research and in animal cell experiments, and constitutes a potential health threat. Research on migration, transportation and circulation of the micro-plastic in water environment after heavy metal adsorption requires research on adsorption and desorption properties of the micro-plastic to heavy metal under different conditions, and meanwhile, more advanced, rapid and convenient technical means and detection means are required to detect the concentration of heavy metal adsorbed by the micro-plastic.
The main methods for detecting heavy metals in water environment are flow analysis detection method, biochemical analysis method, spectroscopy, chromatography and the like. At present, the micro-plastic is monitored to change the metal concentration index in the heavy metal adsorption process by using a spectroscopic technology and a mass spectrometry combined detection technology, including an Atomic Absorption Spectrometry (AAS), an inductively coupled plasma atomic emission spectrometry (ICP-OES), an Atomic Fluorescence Spectrometry (AFS), a visible ultraviolet spectrophotometry (UV-Vis), an inductively coupled plasma mass spectrometry (ICP-MS) and the like.
The traditional biochemical technology is long in sample detection time, complex in operation, large in sample amount, time-consuming and labor-consuming when being used, and low in efficiency. The traditional optical detection needs to carry out complicated pretreatment on the sample, particularly for the micro-plastic adsorption detection, the micro-plastic needs to be filtered by using a filter, the detection limit is low, the samples with different concentrations need to be diluted, the detection time is long, the sample is consumed in most of detection, and the quick, in-situ and nondestructive real-time monitoring cannot be carried out. Meanwhile, since the micro plastic particle size is small, sometimes the micro plastic particles in the sample cannot be completely removed by the filter, and when the spectrum analysis is used, the residual micro plastic particles in the sample and the heavy metals adsorbed by the residual micro plastic particles may influence the test result.
Disclosure of Invention
The present invention aims to at least partially overcome the drawbacks of the prior art and to provide a method for detecting the amount of adsorption of a micro plastic to a heavy metal by means of a low field NMR relaxation method.
The invention also aims to provide a method for detecting the adsorption quantity of the micro-plastics to the heavy metals by using a low-field NMR relaxation method, which can efficiently, simply and nondestructively detect the concentration of the heavy metals adsorbed by the micro-plastics.
The invention also aims to provide a method for detecting the adsorption quantity of the micro plastic to the heavy metal by using a low-field NMR relaxation method, and the defects of the traditional spectrum mass spectrometry method for detecting the concentration of the heavy metal are overcome.
In order to achieve one of the above objects or purposes, the technical solution of the present invention is as follows:
a method for detecting the adsorption of a microplastic to a heavy metal using a low field NMR relaxation method, the method comprising:
step S1: establishing a marked line between the concentration of the target heavy metal ions and the transverse relaxation time;
step S2: extracting a solution sample with a preset volume from a solution to be detected containing target heavy metal ions, and detecting the transverse relaxation time of the solution sample;
step S3: determining the concentration of heavy metal ions of the solution sample according to marked lines between the concentration of the target heavy metal ions and the transverse relaxation time;
step S4: adding micro plastic particles with preset mass into a solution to be detected, after keeping for a preset time, extracting a solution sample with preset volume from the solution to be detected containing target heavy metal ions and the micro plastic particles, filtering to obtain a filtered solution, and detecting the transverse relaxation time of the filtered solution;
step S5: determining the concentration of heavy metal ions in the filtered solution according to marked lines between the concentration of the target heavy metal ions and the transverse relaxation time;
step S6: calculating the adsorption quantity of micro plastic with unit mass to heavy metalq e 。
According to a preferred embodiment of the present invention, the step S1 includes:
step S11: preparing standard solution of target heavy metal ions, preparing the standard solution into a plurality of standard solutions with different concentrations by using ultrapure water, and respectively filling the standard solution into different containers;
step S12: measuring the transverse relaxation time of the standard liquid with different concentrations by using a CPMG pulse sequence, and calculating to obtain the low-field nuclear magnetic resonance transverse relaxation rate, wherein the low-field nuclear magnetic resonance transverse relaxation rate is 1/T 2i -1/T 20 Then obtaining the linear relation between the concentration of the target heavy metal ions and the low-field nuclear magnetic resonance transverse relaxation rate; wherein T is 20 T2 value, T of ultrapure water 2i The value T2 of the heavy metal ions in the ith standard solution is T2, and the transverse relaxation time is T2;
step S13: and obtaining a marked line between the concentration of the target heavy metal ions and the transverse relaxation time according to the linear relation.
According to a preferred embodiment of the present invention, the step S12 further includes:
after the radio frequency switch of the low-field nuclear magnetic resonance instrument is started, and before the transverse relaxation time of the standard liquid with different concentrations is measured by using a CPMG pulse sequence, the standard liquid is checked and corrected by using ultrapure water, and initial parameters are adjusted.
According to a preferred embodiment of the present invention, in step S4, after adding micro plastic particles of a predetermined mass to the solution to be measured, the micro plastic particles are uniformly dispersed by using an ultrasonic machine, and then the solution to be measured containing the micro plastic particles is placed in a constant temperature water bath oscillator to make the micro plastic adsorb heavy metals.
According to a preferred embodiment of the present invention, in step S4, "detecting the transverse relaxation time of the filtered solution" comprises: placing the filtered solution into a chromatographic bottle, placing the chromatographic bottle filled with the filtered solution into a test coil of a low-field nuclear magnetic resonance instrument, opening a radio frequency unit of the low-field nuclear magnetic resonance instrument, adjusting parameters, and testing the transverse relaxation time of the filtered solution by using a CPMG pulse sequence.
According to a preferred embodiment of the present invention, in said step S4, after turning on the rf unit of the low-field nmr, the filtered solution is subjected to an inspection correction using ultrapure water before the transverse relaxation time of the filtered solution is tested using a CPMG pulse sequence.
According to a preferred embodiment of the present invention, the adsorption amountq e To balance the adsorption quantity，The calculation formula of the equilibrium adsorption quantity is as follows:
wherein ,C i the concentration of heavy metal ions in the solution sample is mg/L;C e the concentration of heavy metal ions in the filtered solution is mg/L;Mthe mass of the micro plastic is g;Vthe unit is L, which is the volume of the solution to be measured.
According to a preferred embodiment of the present invention, the step S1 further includes: and analyzing the sensitivity of nuclear magnetic signals of heavy metals with different concentrations in low-field nuclear magnetic resonance to obtain the concentration detection sensitivity under different heavy metal concentration ranges.
According to a preferred embodiment of the invention, the heavy metal ion is a paramagnetic metal ion.
According to a preferred embodiment of the invention, the microplastic particles are selected from polystyrene materials having a particle size of 1.5 μm.
Compared with the prior art, the invention has the following beneficial effects:
(1) The method for detecting the adsorption quantity of the micro plastic to the heavy metal by using the low-field NMR relaxation method does not need complicated sample pretreatment, namely sampling and measuring, is simple, convenient and quick, and has low cost.
(2) The method for detecting the adsorption quantity of the micro plastic to the heavy metal by using the low-field NMR relaxation method has wide detection limit, can cover ppb-ppm level sample concentration detection, and compared with ICP-MS test, the method has the advantages that the sample is not required to be diluted, the detection is faster, the CPMG pulse sequence is used for testing the T2 value, the detection time is lower than 1 minute, and the detection time is far lower than other detection means.
(3) The volume of the required sample is small, the test can be completed by 1-2ml, the sample can not be lost, and nondestructive detection can be realized.
(4) The method for detecting the adsorption quantity of the micro-plastics to the heavy metals by using the low-field NMR relaxation method is particularly suitable for detecting the adsorption quantity of the micro-plastics to the heavy metals, and has few interference items.
Drawings
FIG. 1 is a flow chart of a method for detecting the amount of adsorbed heavy metals of microplastic using low field NMR relaxation method according to an embodiment of the invention;
FIG. 2 shows the relationship between heavy metal ion concentration and transverse relaxation time;
FIG. 3 shows a linear relationship between heavy metal ion concentration and transverse relaxation rate;
FIG. 4 shows the transverse relaxation times of samples at different time points;
FIG. 5 shows the concentration of heavy metal ions as a function of time; and
FIG. 6 is a graph showing the comparison of Cr (III) concentration during adsorption of microplastic with the method of detecting the adsorption amount of the microplastic to the heavy metal using the low-field NMR relaxation method of the present invention, and the detection using ICP-MS.
Detailed Description
Exemplary embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein the same or similar reference numerals denote the same or similar elements. Furthermore, in the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. It may be evident, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in the drawings in order to simplify the drawings.
Low-field nuclear magnetic resonance (Nuclear Magnetic Resonance, NMR) is a non-destructive, accurate and quantitative detection method for detecting the distribution and content of hydrogen protons in a substance. In the aspect of pollutant detection, the method can realize quantitative analysis of the concentration of the hydrocarbon-containing paramagnetic heavy metal ions and can also monitor the real-time dynamic change of the heavy metal ions in some mediums. The invention is based on a low-field nuclear magnetic resonance relaxation method, uses CPMG (Carr-Purcell-Meiboom-Gill) pulse sequences and utilizes paramagnetic heavy metal ion concentration and hydrogen proton low-field nuclear magnetic resonance transverse relaxation rate [1/T ] 2i -1/T 20 ]（T 20 T2 value, T of ultrapure water 2i T2 values corresponding to heavy metal ions with different concentrations, T2 is transverse relaxation time), and a marked line is established to form a corresponding relation between T2 and the concentration of the heavy metal ions, so that the concentration of the heavy metal ions can be obtained by using T2, and the adsorption quantity of the micro-plastic to the heavy metal is calculated according to the concentration of the heavy metal in the solution.
In the embodiment of the invention, the method for detecting the adsorption capacity of the micro plastic to the heavy metal by using the low-field NMR relaxation method is related to the method for detecting the adsorption capacity of the micro plastic to the heavy metal by using the low-field NMR relaxation method, wherein the method for detecting the adsorption capacity of the micro plastic to the heavy metal is the basis of the method for detecting the adsorption capacity of the micro plastic to the heavy metal, the method for detecting the adsorption capacity of the micro plastic to the heavy metal is the expansion of the method for detecting the adsorption capacity of the micro plastic to the heavy metal, the sample solution containing the micro plastic is sampled once and calculated once, and if the adsorption capacity of the micro plastic to the heavy metal is to be detected, multiple sampling detection (under different adsorption time) is needed in the adsorption process, and an adsorption kinetic curve is obtained by observing the adsorption capacity at different time, so as to achieve the aim of detecting/analyzing/researching the adsorption capacity of the micro plastic to the heavy metal, for researching the adsorption capacity of the micro plastic to the heavy metal, the migration, transportation and circulation problems in water environment after the micro plastic adsorbs the heavy metal are required to be obtained.
The method for detecting the adsorption quantity of the micro plastic to the heavy metal by using the low-field NMR relaxation method mainly comprises the following steps:
step S1: establishing a marked line between the concentration of the target heavy metal ions and the transverse relaxation time;
the nuclear magnetic resonance signal is the current signal generated by the hydrogen protons in the pore medium cutting the coil as they relax. According to the principle of nuclear magnetic resonance, the nuclear magnetic resonance signal is proportional to the hydrogen proton content in the sample to be measured. After the rf field is applied, the process of recovering the nuclei from the non-equilibrium high-energy state to the equilibrium low-energy state is called relaxation, and the time elapsed is the relaxation time. The nuclei of the excited state transfer energy to the nuclei of the equilibrium state to return themselves to the equilibrium state, and the time that elapses when the energy decays to 1/e of the maximum value is defined as T2, also called the transverse relaxation time, where e is a natural constant.
Firstly, preparing standard solution of target heavy metal ions, preparing the standard solution into a plurality of standard solutions with different concentrations by using ultrapure water, and respectively filling the standard solution into different containers;
secondly, measuring the transverse relaxation time of the standard solutions with different concentrations by using a CPMG pulse sequence, and calculating to obtain the low-field nuclear magnetic resonance transverse relaxation rate, wherein the low-field nuclear magnetic resonance transverse relaxation rate is 1/T 2i -1/T 20 Then obtaining the linear relation between the concentration of the target heavy metal ions and the low-field nuclear magnetic resonance transverse relaxation rate; wherein T is 20 T2 value, T of ultrapure water 2i The value T2 of the heavy metal ions in the ith standard solution is T2, and the transverse relaxation time is T2;
then, a reticle between the target heavy metal ion concentration and the transverse relaxation time is obtained according to the linear relation.
The low-field nuclear magnetic resonance is suitable for analysis of paramagnetic metal ion concentration, and thus, the target heavy metal ion is set as a paramagnetic metal ion. In the above steps, after the radio frequency switch of the low-field nuclear magnetic resonance apparatus is started, and before the transverse relaxation time of the standard liquid with different concentrations is measured by using the CPMG pulse sequence, the ultra-pure water is used for checking and correcting, and the initial parameters are adjusted.
Step S2: extracting a solution sample with a preset volume from a solution to be detected containing target heavy metal ions, and detecting the transverse relaxation time of the solution sample; the predetermined volume of the solution sample is a minute volume, for example, 1-2ml, and the transverse relaxation time of the solution sample is detected in the same manner as in step S1.
Step S3: determining the concentration of heavy metal ions of the solution sample according to marked lines between the concentration of the target heavy metal ions and the transverse relaxation time;
step S4: adding micro plastic particles with preset mass into a solution to be detected, after keeping for a preset time, extracting a solution sample with preset volume from the solution to be detected containing target heavy metal ions and the micro plastic particles, filtering to obtain a filtered solution, and detecting the transverse relaxation time of the filtered solution;
in the step, after micro plastic particles with preset mass are added into the solution to be measured, the micro plastic particles are uniformly dispersed by utilizing an ultrasonic machine, and then the solution to be measured containing the micro plastic particles is placed into a constant temperature water bath oscillator to enable the micro plastic to adsorb heavy metals. And, the "detecting the transverse relaxation time of the filtered solution" includes: placing the filtered solution into a chromatographic bottle, placing the chromatographic bottle filled with the filtered solution into a test coil of a low-field nuclear magnetic resonance instrument, opening a radio frequency unit of the low-field nuclear magnetic resonance instrument, adjusting parameters, and testing the transverse relaxation time of the filtered solution by using a CPMG pulse sequence. Likewise, the predetermined volume of the solution sample is a minute volume, for example, 1-2ml.
The CPMG pulse sequence is adopted, namely, after 90-degree pulse application is finished, a series of 180-degree complex phase pulses are added, so that a T2 value of a sample after the influence of magnetic field non-uniformity is eliminated is obtained.
In step S4, advantageously, after switching on the radio frequency unit of the low field nmr, the filtered solution is subjected to an inspection correction using ultrapure water before testing the transverse relaxation time using a CPMG pulse sequence.
Step S5: determining the concentration of heavy metal ions in the filtered solution according to marked lines between the concentration of the target heavy metal ions and the transverse relaxation time;
step S6: calculating unit massAdsorption amount of micro plastic to heavy metalq e 。
After the transverse relaxation time T2 of the filtered solution is obtained, the concentration of the heavy metal in the filtered solution can be calculated by combining the marked line between the target heavy metal ion concentration and the T2.
In the step S6, the adsorption amountq e To balance the adsorption quantity，The calculation formula of the equilibrium adsorption quantity is as follows:
wherein ,C i the concentration of heavy metal ions in the solution sample is mg/L;C e the concentration of heavy metal ions in the filtered solution is mg/L;Mthe mass of the micro plastic is g;Vthe unit is L, which is the volume of the solution to be measured.
According to a preferred embodiment of the present invention, the step S1 further includes: and analyzing the sensitivity of nuclear magnetic signals of heavy metals with different concentrations in low-field nuclear magnetic resonance to obtain the concentration detection sensitivity under different heavy metal concentration ranges.
Based on the method for detecting the adsorption capacity of the micro plastic to the heavy metal, when the step S4 is executed at a plurality of different preset times (different time points) for the solution to be detected, namely, sampling is carried out after the different preset times, and filtering and detecting the transverse relaxation time are carried out for the plurality of samples, the method for detecting the adsorption capacity of the micro plastic to the heavy metal is formed, and in the method, the following steps can be introduced: the data were analyzed and model fitted. Calculating the adsorption amount of the micro plastic with unit mass to the heavy metal at different time points according to the measured heavy metal ion concentration data of the samples at different time pointsq e Drawing an adsorption dynamics curve; and then, fitting the adsorption dynamics curve obtained by the experiment with the quasi-primary dynamics model and the quasi-secondary dynamics model, and analyzing the characteristics of adsorption behaviors and adsorption generation mechanism.
The following describes in detail the steps for detecting the amount of adsorption of the micro plastic to the heavy metal by taking Cu (II) as an example:
firstly, cu (II) solutions with different concentrations are prepared, and a CPMG pulse sequence is used to establish marked lines of Cu (II) concentration and T2 value.
An analytical grade copper sulfate crystal having a mass of 0.1g was weighed using a balance into a flask having a total volume of 500ml, 100ml of ultrapure water was weighed using a measuring cylinder, and CuSO having a concentration of 1000ppm was prepared by adding the mixture into the flask 4 A solution. 1ml, 2ml, 4ml, 6ml and 8ml of prepared solutions were respectively removed by a pipette, added to 5 glass bottles having a volume of 10ml, and 9ml, 8ml, 6ml, 4ml and 2ml of ultrapure water were respectively added by the pipette, and diluted 10, 5, 2.5, 1.6 and 1.25 times by the pipette, thereby obtaining standard solutions having concentrations of 100, 200, 400, 600 and 800 ppm.
The radio frequency switch of the low-field nuclear magnetic resonance instrument is turned on, the main frequency and the sampling frequency are adjusted by using the oil sample, the initial parameters are adjusted, and ultrapure water test correction (the T2 value of ultrapure water is approximately equal to 2800) is carried out when the device is used for the first time. Standard solutions of Cu (ii) concentrations of 100, 200, 400, 600, 800ppm were tested for T2 values using a CPMG pulse sequence, respectively, as shown in fig. 2. Calculated to obtain the low-field nuclear magnetic resonance transverse relaxation rate (1/T) 2i -1/T 20, wherein ,T20 T2 value, T of ultrapure water 2i For the T2 value of the heavy metal ion of the i-th standard solution, T2 is transverse relaxation time), a linear relation between the concentration of the heavy metal ion and the transverse relaxation rate is established, as shown in figure 3, the fitting data is good, and R 2 0.9996. Then, a reticle between the target heavy metal ion concentration and the transverse relaxation time is obtained according to the linear relation.
Then, an adsorption experiment was performed to obtain a sample.
The experimental samples were prepared in glass vials with a volume of 30ml using ultrapure water. 24ml of ultrapure water and 6ml of a standard solution of 1000ppm of copper sulfate were added to a glass bottle using a pipette, to obtain 30ml of a Cu (II) metal solution having a concentration of 200 ppm. 0.9g of PS (polystyrene) microplastic powder having a particle diameter of 1.5 μm was weighed out using a balance and added to the prepared solution. The microplastic in the solution system was uniformly dispersed using an ultrasonic machine, placed in a thermostatic water bath shaker, and adsorbed at 25℃and 50 rpm. At five times of 0.1, 2, 4.67, 6.67 and 48 hours, 2ml of the sample is respectively extracted by a disposable syringe, and the sample is respectively injected into chromatographic bottles with the volume of 2ml through needle filters with the aperture of 0.22 mu m, refrigerated and the like.
The sample solution was then tested for T2 value.
Placing the chromatographic flask containing the sample into a test coil of a low-field nuclear magnetic resonance apparatus, the diameter of the test coil is 25mm, opening a radio frequency unit of the low-field nuclear magnetic resonance apparatus, adjusting parameters, testing the T2 values of the samples at different moments by using a CPMG pulse sequence before the adjusting step, and adding the sample of ultrapure water once as correction (the T2 value of ultrapure water is approximately equal to 2800) every 10 samples tested in the test process.
Then, the heavy metal concentration of the sample solution was calculated.
And (3) obtaining T2 values of samples at different moments, carrying out algebraic calculation on a linear equation of the Cu (II) concentration and the T2 value corresponding to the concentration marked lines corresponding to the heavy metal ions, and obtaining the heavy metal concentrations of the samples at different moments.
The concentration of the copper sulfate solution (corresponding to the solution sample in step S3) is known, and the concentration of the obtained sample solution (corresponding to the filtered solution in step S5) is calculated, and the amount of adsorption of the micro plastic to the metal per unit mass at different times can be calculated according to the foregoing formula.
In the invention, the model of the low-field nuclear magnetic resonance apparatus used in the experiment is MesoMR23-060H-I (New Mich. Suzhou), and the model of the dual-function constant-temperature water bath oscillator used in the experiment is SHA-B (Bona technology, hangzhou Tech, tech.). The CPMG pulse sequence used in the preceding steps has specific implementation parameters of a main frequency (MHz) of 21, a 90 degree pulse width (μs) of 6.00, a sampling frequency (kHZ) of 100, a waiting time (TW, ms) of 10000, an echo time (TE, ms) of 1.5, an echo Number (NECH) of 18000, and an accumulation Number (NS) of 4.
It should be noted that, the nuclear magnetic system is sensitive to the change of the environmental temperature, and in order to reduce the test error caused by the temperature, the temperature control work of the laboratory should be performed.
The following results were obtained according to the method of detecting the adsorption performance of the microplastic to heavy metal by the low-field NMR relaxation method in the above examples:
t2 values of samples at the time points of 0.1 and 48 hours are obtained, and as shown in FIG. 4, the change of the T2 value with time in a solution system during the process of adsorbing heavy metals by the microplastic can be observed.
The T2 value is converted into the concentration of heavy metal ions, so that the change condition of the concentration of the heavy metal in the solution system when the adsorption process is carried out is obtained, and as shown in figure 5, the concentration reduction of the heavy metal ions before and after the adsorption can be seen.
In addition, the step S1 of the method can obtain the sensitivity of nuclear magnetic signals of heavy metals with different concentrations in a nuclear magnetic system, can obtain a heavy metal concentration range with high concentration detection precision, and provides a basis for concentration setting in the process of designing an adsorption experiment. It can be seen from FIG. 2 that the concentration of Cu (II) varies over a certain interval at a concentration of less than 600ppm, with a larger range of T2 value variation, i.e. with higher sensitivity using nuclear magnetic resonance detection at this concentration range.
The invention also compares the concentration of Cr (III) in the process of detecting the micro-plastic adsorption by using a low-field nuclear magnetic resonance apparatus with the concentration detected by using ICP-MS to obtain the graph shown in figure 6, and finds that the trend of the data detected by adopting the method is similar to that of the ICP-MS detection, and the detection initial value is closer to the configured known concentration, so that the accuracy is higher.
Compared with the prior art, the invention has the following beneficial effects:
(1) The method for detecting the adsorption quantity of the micro plastic to the heavy metal by using the low-field NMR relaxation method does not need complicated sample pretreatment, namely sampling and measuring, is simple, convenient and quick, and has low cost.
(2) The method for detecting the adsorption quantity of the micro plastic to the heavy metal by using the low-field NMR relaxation method has wide detection limit, can cover ppb-ppm level sample concentration detection, and compared with ICP-MS test, the method has the advantages that the sample is not required to be diluted, the detection is faster, the CPMG pulse sequence is used for testing the T2 value, the detection time is lower than 1 minute, and the detection time is far lower than other detection means.
(3) The volume of the required sample is small, the test can be completed by 1-2ml, the sample can not be lost, and nondestructive detection can be realized.
(4) The method for detecting the adsorption quantity of the micro-plastics to the heavy metals by using the low-field NMR relaxation method is particularly suitable for detecting the adsorption quantity of the micro-plastics to the heavy metals, and has few interference items.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention. The scope of applicability of the present invention is defined by the appended claims and equivalents thereof.
Claims (10)
1. A method for detecting the adsorption of a microplastic to a heavy metal by a low field NMR relaxation method, the method comprising:
step S1: establishing a marked line between the concentration of the target heavy metal ions and the transverse relaxation time;
step S2: extracting a solution sample with a preset volume from a solution to be detected containing target heavy metal ions, and detecting the transverse relaxation time of the solution sample;
step S3: determining the concentration of heavy metal ions of the solution sample according to marked lines between the concentration of the target heavy metal ions and the transverse relaxation time;
step S4: adding micro plastic particles with preset mass into a solution to be detected, after keeping for a preset time, extracting a solution sample with preset volume from the solution to be detected containing target heavy metal ions and the micro plastic particles, filtering to obtain a filtered solution, and detecting the transverse relaxation time of the filtered solution;
step S5: determining the concentration of heavy metal ions in the filtered solution according to marked lines between the concentration of the target heavy metal ions and the transverse relaxation time;
step S6: calculating the adsorption quantity of micro plastic with unit mass to heavy metalq e 。
2. The method for detecting the adsorption amount of the micro plastic to the metal by the low field NMR relaxation method according to claim 1, wherein the step S1 comprises:
step S11: preparing standard solution of target heavy metal ions, preparing the standard solution into a plurality of standard solutions with different concentrations by using ultrapure water, and respectively filling the standard solution into different containers;
step S12: measuring the transverse relaxation time of the standard liquid with different concentrations by using a CPMG pulse sequence, and calculating to obtain the low-field nuclear magnetic resonance transverse relaxation rate, wherein the low-field nuclear magnetic resonance transverse relaxation rate is 1/T 2i -1/T 20 Then obtaining the linear relation between the concentration of the target heavy metal ions and the low-field nuclear magnetic resonance transverse relaxation rate; wherein T is 20 T2 value, T of ultrapure water 2i The value T2 of the heavy metal ions in the ith standard solution is T2, and the transverse relaxation time is T2;
step S13: and obtaining a marked line between the concentration of the target heavy metal ions and the transverse relaxation time according to the linear relation.
3. The method for detecting the adsorption amount of the micro plastic to the metal by the low field NMR relaxation method according to claim 2, wherein the step S12 further comprises:
after the radio frequency switch of the low-field nuclear magnetic resonance instrument is started, and before the transverse relaxation time of the standard liquid with different concentrations is measured by using a CPMG pulse sequence, the standard liquid is checked and corrected by using ultrapure water, and initial parameters are adjusted.
4. The method for detecting the adsorption amount of the micro plastic to the heavy metal by the low-field NMR relaxation method according to claim 2, wherein:
in step S4, after adding micro plastic particles with preset mass into the solution to be measured, uniformly dispersing the micro plastic particles by using an ultrasonic machine, and then placing the solution to be measured containing the micro plastic particles into a constant temperature water bath oscillator to enable the micro plastic to adsorb heavy metals.
5. The method for detecting the adsorption amount of the micro plastic to the heavy metal by the low field NMR relaxation method according to claim 4, wherein:
in step S4, "detecting the transverse relaxation time of the filtered solution" includes: placing the filtered solution into a chromatographic bottle, placing the chromatographic bottle filled with the filtered solution into a test coil of a low-field nuclear magnetic resonance instrument, opening a radio frequency unit of the low-field nuclear magnetic resonance instrument, adjusting parameters, and testing the transverse relaxation time of the filtered solution by using a CPMG pulse sequence.
6. The method for detecting the adsorption amount of the micro plastic to the heavy metal by the low field NMR relaxation method according to claim 5, wherein:
in said step S4, after switching on the radio frequency unit of the low field nuclear magnetic resonance apparatus, before testing the transverse relaxation time of the filtered solution using the CPMG pulse sequence, an inspection correction is performed using ultrapure water.
7. The method for detecting an adsorption amount of a micro plastic to a metal by a low field NMR relaxation method according to claim 5, wherein the adsorption amountq e To balance the adsorption quantity，The calculation formula of the equilibrium adsorption quantity is as follows:
wherein ,C i the concentration of heavy metal ions in the solution sample is mg/L;C e the concentration of heavy metal ions in the filtered solution is mg/L;Mthe mass of the micro plastic is g;Vthe unit is L, which is the volume of the solution to be measured.
8. The method for detecting the adsorption amount of the micro plastic to the heavy metal by the low field NMR relaxation method according to claim 4, wherein:
the step S1 further includes: and analyzing the sensitivity of nuclear magnetic signals of heavy metals with different concentrations in low-field nuclear magnetic resonance to obtain the concentration detection sensitivity under different heavy metal concentration ranges.
9. The method for detecting the adsorption amount of micro plastic to metal by using a low field NMR relaxation method according to any one of claims 1 to 8, wherein:
the heavy metal ions are paramagnetic metal ions.
10. The method for detecting the adsorption amount of micro plastic to metal by using a low field NMR relaxation method according to any one of claims 1 to 8, wherein:
the microplastic particles are selected from polystyrene materials with the particle size of 1.5 mu m.
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