In-situ dynamic monitoring method of heavy metal complex

Abstract

The present application relates to optical sensing and environmental monitoring technical field, especially to a kind of heavy metal complex in-situ dynamic monitoring method.It includes: preparation chalcogenide glass conical optical fiber sensing probe;Build in-situ dynamic monitoring system, along the light path direction sequentially include mid-infrared light source, coupling focusing unit, conical optical fiber sensing probe and detector;Carry out light path alignment and baseline debugging;Probe is in-situ immersed in the solution to be measured containing Cu-EDTA complex, and the infrared evanescent wave absorption spectrum is collected;Based on the carboxylate coordination characteristic absorption peak of Cu-EDTA complex at 1591cm ‑1 And 1400cm ‑1 , the time sequence change of characteristic absorption peak intensity is tracked, the state of system is judged, in-situ dynamic monitoring and quantitative analysis are realized.The advantage is: without sample marking and pretreatment, the intensity attenuation of coordination bond characteristic absorption peak and the generation of new peak are tracked, the direct discrimination of complexation and decomplexation two-way reaction process is realized.

Description

Technical Field

[0001] This invention relates to the fields of optical sensing and environmental monitoring technology, and in particular to a method for in-situ dynamic monitoring of heavy metal complexes. Background Technology

[0002] Due to the significant risks that heavy metals pose to human health and environmental safety, the efficient monitoring and removal of heavy metals from industrial wastewater has received increasing attention. Besides free metal ions, wastewater also contains a large amount of heavy metals in chelated form, typically covalently bound to amino or carboxyl groups. Ethylenediaminetetraacetic acid (EDTA) is a classic multidentate chelating agent capable of forming a copper EDTA (Cu-EDTA) complex with a coordination number of six with copper ions over a wide pH range. This complex exhibits an extremely high coordination constant (…). log K f Cu-EDTA (18.8%) remains soluble in water, making it a recalcitrant organometallic pollutant that poses a potential threat to eutrophication, soil metal migration, and aquatic bioaccumulation. Therefore, real-time, online monitoring of Cu-EDTA concentration and its chemical transformation processes (especially decomplexation reactions) is crucial for wastewater discharge control, resource recovery, and environmental risk assessment.

[0003] Currently, common methods for detecting Cu-EDTA concentration include ultraviolet spectrophotometry, high-performance liquid chromatography (HPLC), and atomic absorption spectrometry (AAS). While these methods can detect Cu-EDTA, they generally require offline sample acquisition, digestion, or derivatization pretreatment, making in-situ detection impossible and unable to track the dynamic formation and breaking of coordination bonds during the complexation reaction in real time. Compared to traditional methods, infrared absorption spectroscopy-based detection methods can directly perform in-situ detection without any reagents or pretreatment. Fourier transform infrared (FTIR) spectroscopy has been widely used in the detection of organic compounds. In recent years, with the rapid development of mid-infrared fiber optic technology, mid-infrared fiber evanescent wave spectroscopy sensors, as a novel fiber optic sensing technology, have been applied in chemical and food industries, environmental monitoring, and medical applications. This technology is based on the interaction between the evanescent wave generated by mid-infrared light on the surface of an optical fiber and the vibrational modes of the molecules of the analyte. By analyzing the unique spectrum formed by the absorption of light at specific wavelengths by the substance, qualitative and quantitative analysis of the substance can be achieved. Among the many mid-infrared optical fibers, chalcogenide glass fibers are favored in the development of mid-infrared fiber evanescent wave spectral sensors due to their wide infrared transmission range, good processability, deliquescence resistance, and hydrophobicity. However, existing technologies still have significant shortcomings: (1) conventional detection methods cannot achieve in-situ, dynamic monitoring of the complexation / decomplexation reaction process of Cu-EDTA complexes; (2) neither traditional infrared spectroscopy nor existing fiber optic sensing schemes have proposed a dedicated sensor for high-sensitivity in-situ dynamic monitoring of heavy metal complexes (such as Cu-EDTA), and related technologies are still lacking. In particular, existing research on evanescent wave sensing based on chalcogenide glass fibers focuses on the concentration detection of biomarkers (such as lactic acid, urea, and glucose), without involving the tracking and analysis of metal-ligand coordination chemical reactions, nor revealing the coordination vibration spectrum of carboxylate groups in Cu-EDTA molecules at 1591 cm⁻¹. -1 and 1400cm -1 The correspondence between the characteristic absorption peaks at a given location and the progress of the complexation / decomplexation reaction. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a method for in-situ dynamic monitoring of heavy metal complexes.

[0005] The present invention aims to provide a method for in-situ dynamic monitoring of heavy metal complexes, comprising the following steps: A method for in-situ dynamic monitoring of heavy metal complexes, characterized by comprising the following steps: S1. A chalcogenide glass tapered fiber sensing probe is fabricated using a fused taper process to form a tapered waist sensing region with evanescent field enhancement effect; the tapered fiber sensing probe is made of germanium-arsenic-selenium-tellurium quaternary chalcogenide glass fiber. S2. Construct an in-situ dynamic monitoring system, which includes a mid-infrared light source, a coupling focusing unit, a tapered fiber optic sensing probe and detector along the optical path; perform optical path alignment and baseline adjustment. S3. Immerse the tapered fiber optic sensing probe in situ in the test solution containing Cu-EDTA heavy metal complex and continuously collect mid-infrared evanescent absorption spectra. S4. Based on the acquired spectra, trace the carboxylate coordination bonds in the Cu-EDTA heavy metal complex at 1591 cm⁻¹. -1 The temporal variation of the intensity of the characteristic absorption peak at 1616 cm⁻¹ and its relationship with the carboxyl group in free EDTA. -1 By observing the wavenumber shift of the characteristic absorption peak, the system can be identified as being in the complexation formation stage, reaction equilibrium stage, or decomposition stage, thus enabling in-situ dynamic monitoring and quantitative analysis of the Cu-EDTA complexation / decomposition reaction process.

[0006] Preferably, in step S4, the discrimination in the decomposition stage also includes tracking 1725cm. -1 The generation and intensity change of the characteristic absorption peak of the protonated carboxyl group, with a peak at 1591 cm⁻¹. -1 The attenuation of the characteristic absorption peak at 1725 cm⁻¹ -1 The enhancement and coexistence of characteristic absorption peaks serve as the basis for determining the occurrence of decomplexation reactions.

[0007] Preferably, in step S4, the criterion for determining the complexation formation stage is: the Cu-EDTA heavy metal complex at 1591 cm⁻¹ -1 The intensity of the characteristic absorption peak at 1616 cm⁻¹ increases with time, and the position of the characteristic peak changes from 1616 cm⁻¹. -1 Towards 1591cm -1 Offset.

[0008] Preferably, in step S4, the quantitative analysis includes establishing a 1591 cm⁻¹ -1 The linear relationship between the absorbance of the characteristic absorption peak and the concentration of Cu-EDTA heavy metal complex was established, enabling accurate quantitative detection within the concentration range of 0–100 mM.

[0009] Preferably, the overall length of the tapered fiber optic sensing probe is 5-8 cm, and the diameter of the tapered waist is 20-50 μm.

[0010] Preferably, the overall length of the tapered fiber optic sensing probe is 5cm and the diameter of the tapered waist is 30μm.

[0011] Preferably, the mid-infrared light source is an integrated light source of a Fourier transform infrared spectrometer, a quantum cascade laser array, or a supercontinuum light source; the detector is a mercury cadmium telluride detector.

[0012] Preferably, the coupling focusing unit includes a planar gold mirror, an off-axis parabolic gold mirror, and a zinc selenide lens arranged in sequence; Planar gold mirrors are used to change the direction of light paths; Off-axis parabolic gold mirrors are used to efficiently focus parallel infrared light and reduce coupling loss; Zinc selenide lenses are used to assist focusing and improve the coupling efficiency between light and optical fibers.

[0013] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. Achieved specific discrimination of the Cu-EDTA complexation / decomplexation bidirectional reaction process. Unlike spectroscopic detection methods that only provide static concentration information, this invention simultaneously tracks the 1591 cm⁻¹... -1 The intensity variation of the characteristic peak at 1616 cm⁻¹ and its relationship with 1616 cm⁻¹ -1 The wavenumber shift at that point, combined with 1725cm -1 Monitoring the formation of new peaks can clearly determine the specific stage of the system in complex formation, reaction equilibrium, or decomposition, providing direct molecular vibrational evidence for the study of chemical reaction mechanisms.

[0014] 2. High-sensitivity quantitative detection was achieved. By leveraging the enhancement effect of tapered optical fibers on the evanescent field, this invention established a 1591 cm⁻¹ [fiber optic cable]. -1 The absorbance of the characteristic peak showed a good linear relationship with the Cu-EDTA concentration (0~100mM), and the detection limit was as low as 0.65mM.

[0015] 3. Provides a label-free direct monitoring method: Based on mid-infrared absorption spectroscopy, this invention directly detects the vibrational characteristics of molecular bonds without the need for complex sample pretreatment or reagent addition, simplifying the operation process and enabling direct identification of reactants and products.

[0016] 4. The wavenumber shift phenomenon of Cu-EDTA coordination bonds is applied inversely to the real-time discrimination of reaction progress. This is achieved by observing the characteristic peak from 1616 cm⁻¹. -1 Towards 1591cm -1 The pattern of displacement, and the 1591cm in the discomplexation reaction. -1 Peak and 1725cm -1 The relationship between the ebb and flow of peaks is studied, and these spectral patterns are used to construct quantitative criteria for judging the direction and process of reactions.

[0017] 5. The entire process of triggering and evolving the decomplexation reaction was tracked. This was achieved through real-time monitoring of the acid-induced decomplexation process at 1591 cm⁻¹. -1 Peak attenuation kinetics and 1725 cm⁻¹ -1The present invention provides key kinetic parameters such as reaction rate and equilibrium time for peak generation kinetics, providing real-time data support for the optimized control of decomplexation timing in water treatment processes. Attached Figure Description

[0018] Figure 1 This is a flowchart of an in-situ dynamic monitoring method for heavy metal complexes provided in an embodiment of the present invention.

[0019] Figure 2 This is a schematic diagram of the structure of the tapered fiber optic sensing probe provided in an embodiment of the present invention.

[0020] Figure 3 This is a schematic diagram of the optical path of the in-situ dynamic monitoring system provided in an embodiment of the present invention.

[0021] Figure 4 This is a comparison between the Cu-EDTA infrared spectrum measured by this invention and the traditional attenuated total reflectance infrared spectrum (ATR spectrum).

[0022] Figure 5 This is an infrared spectrum variation diagram of Cu-EDTA solutions of different concentrations provided according to an embodiment of the present invention.

[0023] Figure 6 This is a schematic diagram illustrating the linear relationship between the absorption peak intensity and the Cu-EDTA concentration according to an embodiment of the present invention.

[0024] Figure 7 The EDTA and Cu provided according to embodiments of the present invention 2+ Complexation to form Cu-EDTA solution at 1700-1300 cm⁻¹ -1 The graph shows the dynamic changes in the infrared spectrum within the specified range. Here, +0, +40, +80, +120, +160, +200, +240, and +280 μL correspond to the amounts of CuCl2 solution of the same concentration added to the EDTA solution.

[0025] Figure 8 Cu provided according to embodiments of the present invention Dynamic changes in in-situ infrared spectra during the EDTA decomplexation reaction.

[0026] Figure 9 This is a schematic diagram showing the change in the intensity of the acid hydrolysis complexation characteristic peak over time, according to an embodiment of the present invention.

[0027] Figure label: 1. Fourier transform infrared spectrometer; 2. Flat gold mirror; 3. Off-axis parabolic gold mirror; 4. Zinc selenide lens; 5. Mid-infrared fiber optic sensor; 51. Conical waist; 52. Conical region; 6. Mercury cadmium telluride detector. Detailed Implementation

[0028] In the following description, embodiments of the invention will be described with reference to the accompanying drawings. In the description below, the same modules are denoted by the same reference numerals. Where the same reference numerals are used, their names and functions are also the same. Therefore, their detailed description will not be repeated.

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0030] This invention provides a method for in-situ dynamic monitoring of heavy metal complexes, the flowchart of which is shown below. Figure 1 Specifically, it includes the following steps: S1. Fabrication of a tapered fiber optic sensing probe: A germanium-arsenic-selenium-tellurium (Ge-As-Se-Te) quaternary chalcogenide glass fiber was selected. This fiber has a length of 4000~800 cm. -1 It has a wide transmission window within the range, with an optical fiber cladding diameter of 250μm and a core diameter of 200μm. The middle section of the optical fiber is processed into a tapered shape using a fused taper process, with an overall fiber length of 5~8cm, a tapered waist diameter of 20~50μm, and a tapered region length of 1~3cm. The two ends of the tapered optical fiber are polished to make the end faces smooth and flat without scratches, thereby reducing light scattering caused by the end faces and forming an optical fiber sensing probe that can efficiently couple mid-infrared light. Preferably, the overall length of the tapered optical fiber is 5cm, and the diameter of the tapered waist is 30μm; the drawing process of the tapered optical fiber is as follows: S11. Clamp both ends of the chalcogenide glass optical fiber onto two moving platforms controlled by linear motors, and adjust the optical fiber to keep it straight. S12. At the center of the preset cone area, a heating source is used to locally heat the fiber segment; when the fiber material in the heated area reaches a uniformly softened state, two moving platforms driven by linear motors, under the synchronous control of the motion controller, begin to move at a constant speed in opposite directions along the fiber axis at a preset speed program. S13. Under continuous heating and stretching, the softened region gradually becomes thinner and longer, and is drawn into a tapered optical fiber with a waist diameter of 30μm; after heating is stopped, the optical fiber is naturally cooled and solidified in the air. The structure of the tapered fiber optic sensing probe is as follows: Figure 2As shown, the tapered fiber optic sensing probe has a symmetrical double-cone structure. The middle section of the fiber is formed into a tapered region 52 by fusion tapering, and the central region of the tapered region 52 is the tapered waist 51. The tapered waist 51 is the core sensing region for evanescent wave enhancement.

[0031] S2. Build and debug the in-situ dynamic monitoring system for heavy metal complexes: This step builds a stable optical detection platform for the subsequent in-situ dynamic monitoring of heavy metal complexes. The system includes a mid-infrared light source, a coupling focusing unit (arranged in sequence with a plane gold mirror, an off-axis parabolic gold mirror and a zinc selenide lens), a conical fiber optic sensing probe, and a detector along the optical path propagation direction. Among them, the mid-infrared light source is an integrated light source of a Fourier transform infrared spectrometer, a quantum cascade laser array, or a supercontinuum light source. All of the above light sources can provide mid-infrared light covering the characteristic absorption band of Cu-EDTA. The coupling and focusing unit includes a planar gold mirror, an off-axis parabolic gold mirror, and a zinc selenide lens arranged in sequence. The planar gold mirror is used to change the direction of the light path and achieve a compact optical layout. The off-axis parabolic gold mirror is used to efficiently focus parallel infrared light and reduce coupling loss. The zinc selenide lens is used to provide high transmittance in the mid-infrared band and assists in focusing to improve the coupling efficiency between light and optical fiber. The tapered fiber optic sensing probe serves as both the light transmission medium and the sensing core, transmitting infrared light and generating evanescent waves in the waist region of the cone to achieve evanescent wave absorption detection. The detector is a mercury cadmium telluride detector, which is used to receive infrared light carrying sample information with high sensitivity and convert the optical signal into an electrical signal, providing a basis for subsequent spectral analysis.

[0032] Combination Figure 3 The optical path structure shown includes the following sub-steps: S21. Layout and alignment of core optical components: Using a Fourier transform infrared spectrometer 1 as the mid-infrared light source, a plane gold mirror 2, an off-axis parabolic gold mirror 3, and a zinc selenide lens 4 are arranged sequentially on its outgoing optical path. The angle of the plane gold mirror 2 is adjusted so that the infrared light is reflected and redirected at 45° to enter the off-axis parabolic gold mirror 3. The position and orientation of the off-axis parabolic gold mirror 3 are adjusted to efficiently focus the parallel infrared light to form a convergent beam. The zinc selenide lens 4 further focuses and collimates the beam so that its spot size matches the fiber input end, providing conditions for subsequent efficient coupling. S22. Installation and Coupling of the Tapered Fiber Optic Sensing Probe: The tapered chalcogenide glass fiber optic sensing probe prepared in S1 is installed in the sample cell: both ends of the fiber are fixed with fiber clamps to ensure that the axis coincides with the incident and outgoing light paths; the fiber input end is aligned with the focusing point of the zinc selenide lens 4, and the fiber output end is aligned with the photosensitive surface of the mercury cadmium telluride detector 6; by finely adjusting the relative position of the fiber, the zinc selenide lens 4, and the mercury cadmium telluride detector 6, the output signal intensity of the detector is observed until the coupling efficiency reaches the preset threshold, forming a stable optical path; S23. Evanescent Field Stability and Baseline Adjustment: Inject a blank aqueous solution (free of heavy metal complexes) into the sample cell and perform a system baseline test: Start the Fourier transform infrared spectrometer 1 and continuously acquire the infrared spectrum of the blank solution; adjust the light source power and detector gain to ensure a stable and interference-free evanescent field is formed in the fiber cone waist region, and the spectral baseline is stable with no obvious noise; complete the system performance calibration and confirm that the optical path is in the test band (850-4000 cm⁻¹). -1 It has sufficient sensitivity and signal-to-noise ratio to meet the accuracy requirements of subsequent dynamic monitoring; S24. Confirmation of in-situ monitoring conditions: After the system is built, confirm that: the fiber optic sensing probe can be completely immersed in the liquid to be tested without any air bubbles adhering to it; the optical path coupling efficiency is stable and the signal intensity fluctuation is less than the set range; the system does not require labeling, adding reagents or complex pretreatment of the sample to be tested, and can directly perform in-situ and continuous monitoring of the liquid.

[0033] S3. Configure the solution system to be tested, immerse the sensor probe in situ, and continuously acquire mid-infrared evanescent wave spectra to obtain continuous spectral data, as detailed below: S31. Preparation of the test solution system: Based on the monitoring target, prepare an aqueous solution containing heavy metal-ligand complexes (such as Cu-EDTA), or an aqueous solution containing free heavy metal ions (Cu... 2+ The reaction stock solution of EDTA and its ligand is prepared; the pH of the reaction system is controlled between 3 and 11 to simulate actual wastewater or experimental environment. S32. In-situ immersion of the sensor probe: Inject the prepared test solution into the sample cell, ensuring the liquid level completely covers the tapered fiber optic sensor probe, with no air bubbles adhering to the fiber surface; for dynamic reaction monitoring, acid, alkali, or other reagents can be added to the system through the sample inlet above the sample cell to induce complexation or decomplexation reactions; after the addition is complete, wait for the temperature, pH value, and flow state of the solution system to stabilize before starting the spectral acquisition program to avoid external disturbances affecting the monitoring results; S33. Set spectral acquisition parameters: Start the mid-infrared light source (corresponding spectral analysis device, preferably a Fourier transform infrared spectrometer), and set the spectral acquisition range to 800~4000 cm⁻¹. -1 The resolution is 4cm.-1 The number of scans is 32 to 128, and the acquisition interval is 1 to 60 seconds, in order to balance temporal resolution and signal-to-noise ratio; preferably, the number of scans is set to 64, and the acquisition interval is set to 10 seconds. S34. Start real-time spectral acquisition and data processing: continuously acquire mid-infrared evanescent absorption spectra; mid-infrared light is transmitted in the optical fiber by total internal reflection, and an evanescent field is excited on the surface of the cone waist 51. The evanescent wave interacts with the heavy metal-ligand (Cu-EDTA) complex molecules in the solution. The complex molecules absorb infrared light of specific wavelengths, forming a spectral signal containing the characteristic absorption peaks of the coordination bonds; the acquired spectral data is used for subsequent dynamic analysis after baseline correction, background subtraction and noise filtering.

[0034] S4. Spectral feature analysis, reaction process identification, and quantitative analysis; specifically including: S41. Precise identification of characteristic peaks: Based on the acquired continuous in-situ infrared spectrum, the Cu-EDTA coordination bond is located at 1591 cm⁻¹. -1 and 1400cm -1 The specific absorption peak at the location eliminates background interference from water and substrate; S42. Dynamic Trend Analysis: Real-time Tracking of 1591cm -1 and 1400cm -1 The temporal variation and wavenumber shift of characteristic peak intensity can be used to determine whether the system is in the complexation formation stage, reaction equilibrium stage, or decomposition stage; by tracking the 1616~1591 cm⁻¹... -1 Wavenumber shift was used to determine the complexation process; by tracking 1591cm -1 Peak attenuation at 1725cm -1 The waxing and waning of peaks indicate the decomposition process; S43. Quantitative conversion and kinetic law analysis: Based on the pre-established calibration relationship between characteristic peak intensity and Cu-EDTA heavy metal complex concentration, the real-time content of complex in the system is converted in real time to achieve dynamic quantitative detection; the characteristic peak time series data is fitted to obtain key kinetic information such as reaction rate and equilibrium time.

[0035] S5. Summary of monitoring results: Organize the spectral data, kinetic parameters and concentration calculation results, and combine them with the standard curve to clarify the dynamic laws of complexation and decomplexation reactions.

[0036] Experimental Verification and Monitoring Effect Analysis: Typical experiments were conducted to verify the quantitative detection capability and in-situ dynamic monitoring performance of the method of this invention. The results are as follows: Figures 3-8 As shown.

[0037] (1) Quantitative detection of Cu-EDTA concentration: The sample to be tested is an aqueous solution containing Cu-EDTA complex. During the test, the infrared spectrum of the tapered fiber optic sensor probe in ultrapure water is first collected as the background baseline. Then, Cu-EDTA standard solutions of different concentration gradients are injected into the sample cell in sequence to complete the mid-infrared spectral detection.

[0038] Figure 4 This figure shows a comparison between the Cu-EDTA infrared spectrum measured by this invention and the traditional attenuated total reflectance infrared spectrum (ATR spectrum). As can be seen from the figure, the spectrum measured by this invention has a lower threshold at 1591 cm⁻¹. -1 and 1400cm -1 A significant absorption peak appears nearby, perfectly corresponding to the characteristic peak position of the ATR spectrum, and the absorption intensity is significantly higher than that of the ATR method, indicating that the evanescent wave enhancement effect of this invention effectively improves the detection sensitivity and makes the characteristic signal easier to identify. Specifically, 1591 cm⁻¹ -1 This is the peak of the asymmetric stretching vibration of the carboxylate group, at 1400 cm⁻¹. -1 The peaks are due to the symmetric stretching vibration of the carboxylate group, and both are typical characteristic absorption peaks of Cu-EDTA complexes.

[0039] Figure 5 The image shows the infrared spectra of Cu-EDTA solutions at different concentrations. As the Cu-EDTA concentration increases from 1 mM to 100 mM, the infrared spectrum at 1591 cm⁻¹... -1 With 1400cm -1 The characteristic absorption peak intensities at the specified locations all showed a regular increase, and the peak shapes were clear and without obvious interference, indicating that there is a good positive correlation between the signal intensity and the concentration of the target analyte in this method.

[0040] Further extraction of the intensity data of the two characteristic peaks mentioned above established a linear relationship between the absorption peak intensity and the Cu-EDTA concentration, as shown in the following figure. Figure 6 As shown in the figure, within the concentration range of 0–100 mM, the absorption intensity of both characteristic peaks exhibits a good linear correlation with the concentration, with the 1591 cm⁻¹ peak being the most significant. -1 The linear response of the peak is more significant. Based on this linear relationship, this method can achieve accurate quantitative detection of Cu-EDTA complexes in water, with a detection limit as low as 0.65 mM, demonstrating that this invention has excellent quantitative analysis capabilities.

[0041] (2) In-situ monitoring of complexation reaction process: Taking the combination process of EDTA and copper ions as the research object, CuCl2 solution was gradually added to the EDTA solution to simulate the actual process of heavy metal complexation in water, and the mid-infrared spectrum was continuously collected in real time using this monitoring system.

[0042] The specific experimental conditions were as follows: CuCl2 solution was gradually added to 240 μL of EDTA stock solution, and the spectrum was collected in situ throughout the process. The dynamic changes in the spectrum are as follows: Figure 7 As shown in the figure, with the continuous addition of copper ions, the characteristic absorption peak of the carboxylate group in the EDTA molecule decreased from 1616 cm⁻¹. -1 It gradually shifted towards lower wavenumbers and eventually stabilized at 1591 cm⁻¹. -1 This peak shift directly reflects the Cu 2+ The gradual formation of coordination bonds between EDTA molecules: The stretching vibration frequency of the carboxylate group in free EDTA is relatively high. When it coordinates with copper ions to form a Cu-EDTA complex, the electron cloud density changes, the vibrational frequency decreases, and the characteristic peak shifts to a lower wavenumber. The key to the above experimental phenomenon lies in the asymmetric stretching vibration frequency (1616 cm⁻¹) of the carboxylate group in the free EDTA molecule. -1 The vibrational frequency of the coordinated carboxylate group in the Cu-EDTA complex is higher than that of the Cu-EDTA complex (1591 cm⁻¹). -1 The fundamental reason for this difference lies in the fact that when the carboxylate ion reacts with Cu²⁺... + After the formation of a coordinate bond, the lone pair of electrons on the carboxyl oxygen atom is partially transferred to Cu. 2+ The absence of empty orbitals leads to a decrease in the bond order and an increase in the bond length of the C=O bond, thereby lowering its stretching vibration frequency. Therefore, the characteristic peak changes from 1616 cm⁻¹. -1 Towards 1591cm -1 The gradual shift of the peak position essentially reflects the dynamic process of the gradual formation of Cu-EDTA coordination bonds, and the rate and magnitude of the peak position shift have a quantitative correlation with the progress of the coordination reaction. Based on this understanding of the mechanism, this invention is the first to use the peak position shift as a real-time criterion for judging the progress of the complexation reaction.

[0043] (3) In-situ monitoring of the decomplexation reaction process: Based on the research on the complexation reaction, further dynamic monitoring experiments on decomplexation were conducted. By adding a strong acid reagent to the Cu-EDTA equilibrium solution to adjust the pH of the system to 1, the dissociation reaction of the complex was actively triggered, and the spectral evolution was recorded in real time. The dynamic changes of the spectrum over time are as follows: Figure 8 As shown. With the decomplexation reaction proceeding, the 1591 cm⁻¹, representing the Cu-EDTA coordination structure... -1 The intensity of the characteristic absorption peak continuously decreases, while the 1725 cm⁻¹ peak, representing the cationized carboxyl group of the reaction product, also decreases. -1 Characteristic absorption peaks gradually form and intensify. This spectral change directly corresponds to the dissociation process of Cu-EDTA under acidic conditions: after the coordination bonds break, the free EDTA molecules are protonated to form protonated carboxyl groups, whose characteristic absorption peak appears at 1725 cm⁻¹. -1 Near the point of reactivity, the peak intensity gradually increases as the reaction proceeds.

[0044] Simultaneous extraction of 1591cm -1 With 1725cm -1 The data on the intensity changes of the two characteristic peaks over time are used to plot the dynamic change curves as follows: Figure 9 As shown. The curve clearly illustrates the kinetic process of the complexation reaction: 1591 cm⁻¹ -1 Peak intensity decreases rapidly over time and then tends to stabilize at 1725 cm⁻¹. -1 The peak intensity gradually increases over time and eventually reaches equilibrium; the trends of both correspond perfectly to the progress of the decomplexation reaction. In the decomplexation experiment, 1591 cm⁻¹ -1 Peak attenuation and 1725 cm⁻¹ -1 The peak enhancement exhibits a strict mirror image relationship, which directly confirms the dissociation mechanism of the Cu-EDTA complex under acidic conditions at the molecular vibrational level: H + Competing for coordination with the oxygen atom of the carboxylate group, substituting Cu 2+ The coordination bond with EDTA makes Cu 2+ Released from the complex, the EDTA molecule is simultaneously protonated, forming a product containing a protonated carboxyl group (-COOH). 1725 cm -1 The peak is the characteristic absorption of the C=O stretching vibration in the protonated carboxyl group. This invention achieves this by simultaneously tracking the characteristic peak of the reactants (1591 cm⁻¹). -1 ) and product characteristic peak (1725 cm⁻¹) -1 The ebb and flow of these peaks allows for bidirectional verification of the direction and progress of the decomplexation reaction. This "reactant-product paired peak tracking" strategy provides a general method reference for monitoring other metal-ligand complex systems.

[0045] The key technical points of this invention are: (1) By actively regulating and enhancing the penetration depth and intensity of the evanescent field through tapering, the sensor's detection sensitivity for Cu-EDTA molecules in solution is improved. This is the basis for achieving subsequent high-sensitivity detection and dynamic monitoring. (2) Using the above-mentioned tapered fiber optic sensor, for Cu-EDTA complexes, its specific molecular vibration modes (asymmetric stretching vibration and symmetric stretching vibration of carboxylate group) at 1591 cm⁻¹ were established and verified. -1 and 1400cm -1 The linear quantitative relationship between the intensity of the characteristic absorption peak at 1591 cm⁻¹ and the solution concentration. (3) The sensor is combined with a real-time spectral acquisition system to continuously track the 1591 cm⁻¹ spectral density. -1 Peak and 1725 cm -1 The wavenumber shift and intensity variation of the peaks enabled the analysis of the relationship between EDTA and Cu. 2+ Cu-EDTA is formed through complexation, and Cu-EDTA decomplexes under strong acid conditions to release Cu. 2+Label-free in-situ monitoring of two complete reaction kinetic processes. (4) Unlike static concentration measurement using conventional infrared spectroscopy or fiber optic sensing, this invention specifically applies tapered fiber evanescent wave spectroscopy to the real-time tracking of the Cu-EDTA complexation / decomplexation reaction process, by identifying and tracking the characteristic absorption peak (1591 cm⁻¹) directly related to the metal coordination bond. -1 The intensity decay and the new peak (1725 cm) -1 The generation of ) enables direct determination of the reaction direction and process, providing a novel in-situ analytical method for studying the environmental chemical behavior of heavy metal complexes.

[0046] In summary, the technical solution of this invention constructs a mid-infrared fiber evanescent wave spectroscopy sensing system (MIR-FEWS sensing system) and dynamic monitoring method by using tapered chalcogenide glass optical fibers to enhance the evanescent field. This not only achieves highly sensitive quantitative detection of Cu-EDTA, but more importantly, it enables in-situ, dynamic, and label-free monitoring of the entire process of its complexation and decomplexation chemical reactions.

[0047] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0048] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for in-situ dynamic monitoring of heavy metal complexes, characterized in that: Includes the following steps: S1. A chalcogenide glass tapered fiber sensing probe is fabricated using a fused taper process to form a tapered waist sensing region with evanescent field enhancement effect; the tapered fiber sensing probe is made of germanium-arsenic-selenium-tellurium quaternary chalcogenide glass fiber. S2. Construct an in-situ dynamic monitoring system, which includes, in sequence along the optical path, a mid-infrared light source, a coupling focusing unit, a tapered fiber optic sensing probe, and a detector; Perform optical path alignment and baseline adjustment; S3. Immerse the tapered fiber optic sensing probe in situ in the test solution containing Cu-EDTA heavy metal complex and continuously collect mid-infrared evanescent absorption spectra. S4. Based on the acquired spectra, trace the carboxylate coordination bonds in the Cu-EDTA heavy metal complex at 1591 cm⁻¹. -1 The temporal variation of the intensity of the characteristic absorption peak at 1616 cm⁻¹ and its relationship with the carboxyl group in free EDTA. -1 By observing the wavenumber shift of the characteristic absorption peak, the system can be identified as being in the complexation formation stage, reaction equilibrium stage, or decomposition stage, thus enabling in-situ dynamic monitoring and quantitative analysis of the Cu-EDTA complexation / decomposition reaction process.

2. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 1, characterized in that: In step S4, the discrimination in the decomposition stage also includes tracing 1725cm. -1 The generation and intensity change of the characteristic absorption peak of the protonated carboxyl group, with a peak at 1591 cm⁻¹. -1 The attenuation of the characteristic absorption peak at 1725 cm⁻¹ -1 The enhancement and coexistence of characteristic absorption peaks serve as the basis for determining the occurrence of decomplexation reactions.

3. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 1, characterized in that: In step S4, the criterion for determining the complexation formation stage is: Cu-EDTA heavy metal complexes at 1591 cm⁻¹ -1 The intensity of the characteristic absorption peak at 1616 cm⁻¹ increases with time, and the position of the characteristic peak changes from 1616 cm⁻¹. -1 Towards 1591cm -1 Offset.

4. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 1, characterized in that: In step S4, the quantitative analysis includes establishing a 1591cm... -1 The linear relationship between the absorbance of the characteristic absorption peak and the concentration of Cu-EDTA heavy metal complex was established, enabling accurate quantitative detection within the concentration range of 0–100 mM.

5. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 1, characterized in that: The overall length of the tapered fiber optic sensing probe is 5-8 cm, and the diameter of the waist is 20-50 μm.

6. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 5, characterized in that: The overall length of the tapered fiber optic sensing probe is 5cm, and the diameter of the tapered waist is 30μm.

7. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 1, characterized in that: The mid-infrared light source is an integrated light source of a Fourier transform infrared spectrometer, a quantum cascade laser array, or a supercontinuum light source; the detector is a mercury cadmium telluride detector.

8. The method for in-situ dynamic monitoring of heavy metal complexes according to claim 7, characterized in that: The coupling focusing unit includes a planar gold mirror, an off-axis parabolic gold mirror, and a zinc selenide lens arranged in sequence. Planar gold mirrors are used to change the direction of light paths; Off-axis parabolic gold mirrors are used to efficiently focus parallel infrared light and reduce coupling loss; Zinc selenide lenses are used to assist focusing and improve the coupling efficiency between light and optical fibers.

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