Catalytic pyrolysis method for analyzing mercury isotope of marine sediment sample

Abstract

The invention relates to the technical field of mercury isotope analysis, in particular to a catalytic pyrolysis method for mercury isotope analysis of a marine sediment sample. According to the method, pyrolysis is carried out under the action of the composite catalyst, efficient in-situ removal of complex matrixes can be realized in cooperation with pyrolysis conditions, and then the recovery rate of mercury and the accuracy of isotope detection results are improved. The method provided by the invention solves the problems of low mercury recovery rate, instability, serious matrix interference and the like of a marine sediment sample with a complex matrix. According to the method, the mercury recovery rate, the matrix removal efficiency and the accuracy and precision of mercury concentration and isotope analysis of the method are comprehensively verified by utilizing various marine sediment standard substances and actual environmental samples, and the result shows that the method has excellent reliability and accuracy in the aspect of mercury concentration and isotope analysis. The catalytic pyrolysis method provided by the invention can provide key technical support for accurately analyzing mercury records in marine sediments.

Description

Technical Field

[0001] This invention relates to the field of mercury isotope analysis technology, and in particular to a catalytic pyrolysis method for mercury isotope analysis of marine sediment samples. Background Technology

[0002] Mercury (Hg), a highly toxic global pollutant, poses a serious threat to ecosystems and human health due to its long-distance atmospheric transport, persistence, and bioaccumulation, drawing widespread international attention. Since the Industrial Revolution, human activities (such as fossil fuel combustion, industrial production, and mining) have released large amounts of mercury into the atmosphere, resulting in global background mercury levels increasing several times compared to pre-industrial levels. Atmospheric mercury is deposited into marine and terrestrial ecosystems, with the ocean considered the final sink. Marine sediments, as long-term reservoirs of mercury, not only provide a true record of historical changes in natural sources (such as volcanic activity) and anthropogenic sources, but also offer valuable information for reconstructing paleoclimate and paleoenvironmental events.

[0003] In recent years, with the development of multi-receiver inductively coupled plasma mass spectrometry (MC-ICP-MS), mercury stable isotope analysis has become a powerful tool for tracing the source of mercury and revealing its migration and transformation processes in the environment. Mercury has seven stable isotopes (… 196 Hg, 198-202 Hg, 204 Hg), its isotopic fractionation follows mass-dependent fractionation (MDF, with δ¹⁸O⁻). 202 Hg represents) and mass-independent fractionation (MIF, expressed in Δ) 199 Hg and Δ 201 (Hg represents two patterns.) Mercury from different sources (such as volcanic emissions, industrial emissions, and natural background) has different δ values. 202 Hg characteristics, while significant MIF is mainly composed of gaseous elemental mercury (Hg) in the atmosphere. 0 Mercury isotopes are produced by photochemical reactions, making them a "fingerprint" for distinguishing different sources and processes. Therefore, high-precision mercury isotope analysis of marine sediments is crucial for understanding the global mercury cycle, assessing the impact of human activities, and reconstructing geological history events.

[0004] Accurate determination of the mercury isotopic composition in marine sediment samples requires the mercury to be completely and non-fractionally converted into a solution or gaseous form detectable by mass spectrometry. Currently, the two-stage tube furnace pyrolysis method is the most widely used sample pretreatment technique. This method converts mercury in the solid phase into gaseous mercury by pyrolyzing the sample at high temperatures (typically 700-950℃), followed by collection with acidic absorbents (such as KMnO4 / H2SO4, HNO3 / HCl, etc.), and finally isotopic determination. However, when applied to complex matrices like marine sediments, this method reveals several problems. Marine sediments are typically rich in organic matter, sulfides, halogens, and various volatile and semi-volatile elements (such as Te, Se, As, Cd, etc.). During pyrolysis, these components generate a large amount of reducing gases and volatile substances, which may react with gaseous mercury in the gas phase or form precipitates in the absorbent, resulting in incomplete mercury collection, i.e., low and fluctuating recovery rates. More seriously, incomplete recovery can easily lead to mercury isotope fractionation, making the measurement results unable to represent the true mercury isotope composition of the sample. Furthermore, the large amount of matrix elements entering the absorption liquid can cause mass spectrometry interference in subsequent mass spectrometry analysis, further affecting the accuracy of the measurement.

[0005] To overcome these challenges, researchers have tried various improvement methods, such as optimizing the heating program and using different absorbent formulations, but the effects have been limited. The problem of low mercury recovery remains particularly difficult to solve for samples with high organic matter or high sulfur content. Therefore, developing a novel pretreatment method that can effectively remove matrix interference and achieve quantitative mercury recovery has become a critical technical bottleneck that urgently needs to be overcome in the field of mercury isotope research on marine sediments. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a catalytic pyrolysis method for mercury isotope analysis of marine sediment samples. The catalytic pyrolysis method of this invention can effectively remove matrix interference, achieve high mercury recovery, and does not produce isotope fractionation, resulting in accurate and reliable results, making it particularly suitable for complex matrices such as marine sediments.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a catalytic pyrolysis method for mercury isotope analysis of marine sediment samples, comprising the following steps: contacting a composite catalyst with a marine sediment sample and then heating the sample from room temperature to the pyrolysis temperature to perform catalytic pyrolysis; The heating rate is ≤19℃ / min; The temperature of the catalytic pyrolysis is 800~1100℃, and the holding time is ≥40min; The composite catalyst comprises 60-80% MnO2, 10-30% Co3O4 and 10% CaO by mass percentage.

[0008] Preferably, the composite catalyst comprises 70% MnO2, 20% Co3O4 and 10% CaO by mass percentage.

[0009] Preferably, the heating rate is 15~19℃ / min.

[0010] Preferably, the holding time for the catalytic pyrolysis is 40-60 min.

[0011] Preferably, the mass ratio of the marine sediment sample to the composite catalyst is 1:(5~20).

[0012] Preferably, the step of bringing the composite catalyst into contact with the marine sediment sample includes: spreading the marine sediment sample flat and then covering its surface with the composite catalyst.

[0013] Preferably, the catalytic pyrolysis is carried out in a two-stage tubular furnace system.

[0014] Preferably, before heating, the two-stage tube furnace system is purged with argon gas.

[0015] Preferably, the composite catalyst is obtained by mixing MnO2, Co3O4 and CaO.

[0016] Preferably, the mixing includes grinding and mixing.

[0017] This invention provides a catalytic pyrolysis method for mercury isotope analysis of marine sediment samples, comprising the following steps: contacting a composite catalyst with the marine sediment sample and then heating from room temperature to the pyrolysis temperature for catalytic pyrolysis; the heating rate is ≤19℃ / min; the catalytic pyrolysis temperature is 800~1100℃, and the holding time is ≥40min; the composite catalyst comprises 60-80% MnO2, 10-30% Co3O4, and 10% CaO by mass percentage. This invention, by conducting pyrolysis in the presence of a composite catalyst, and by adjusting the heating rate, pyrolysis temperature, and pyrolysis time, enables efficient in-situ removal from complex matrices, thereby improving the mercury recovery rate and the accuracy of isotope detection results.

[0018] The mechanism of action of the composite catalyst is as follows: MnO2 is a strong oxidant that, at high temperatures, can undergo redox reactions with reducing substances (such as H2S and SO2) or volatile metals / metalloids (such as Te and As) produced by pyrolysis, converting them into oxides or salts with higher thermal stability and less volatility. For example, MnO2 can react with Te to generate non-volatile MnTeO3, thereby fixing Te in the solid residue. The addition of Co3O4 accelerates the oxidative fixation process of interfering substances by MnO2, improving the reaction efficiency. In addition to acting as a physical dispersant, CaO can also absorb acidic gases (such as SO2 and HCl), further purifying the carrier gas. The synergistic effect of multiple components achieves highly efficient in-situ removal from complex matrices.

[0019] The catalytic pyrolysis method provided by this invention solves the problems of low and unstable mercury recovery and severe matrix interference faced by marine sediment samples with complex matrices. This invention comprehensively validates the method's mercury recovery rate, matrix removal efficiency, and the accuracy and precision of mercury concentration and isotope analysis using various marine sediment standards and actual environmental samples. The results show that this method has excellent reliability and accuracy in mercury concentration and isotope analysis. The catalytic pyrolysis method provided by this invention will provide key technical support for accurately interpreting the mercury record in marine sediments, and promote the development of related earth sciences and environmental sciences. Attached Figure Description

[0020] Figure 1 The mercury recovery rate of (a) standard GSS4a mixed with sample NH1 at different mass ratios after catalytic pyrolysis is compared with the mercury recovery rate of (b) different samples (NH2, NH3, HZW, GBW07335, GBW07336) under conditions with and without catalyst; the error bars represent one standard deviation (1 SD). Figure 2 The effect of varying the holding time on the catalytic pyrolysis results of the NH1 sample was investigated, with a fixed heating time of 50 min. (a) Changes in mercury recovery rate; (b) δ 202 Changes in Hg composition; (c) Δ 199 Changes in Hg composition; error bars represent 1 SD; Figure 3 To maintain a fixed holding time of 40 min, the effect of varying the heating time on the catalytic pyrolysis results of the NH1 sample was investigated; (a) changes in mercury recovery rate; (b) δ 202 Changes in Hg composition; (c) Δ 199 Changes in Hg composition; error bars represent 1 SD; Figure 4The effect of different catalyst ratios (MnO2 content) on the catalytic pyrolysis recovery and isotopic composition of NH1 samples is shown. The purple area represents the "true value" range of the isotopic composition of NH1 samples (mean ± 2SD); the error bars represent 1SD. Figure 5 The mercury isotope composition (δ¹⁸O) was measured after the standard substance GSS4a was mixed with the sample NH1 in different proportions. 202 Hgvs 1 / Δ 199 Hg) linear fit plot; solid line is the linear fit line, dark orange band is the 95% confidence interval; Figure 6 A heatmap showing the correlation between mercury recovery rate from catalytic pyrolysis and the content of certain matrix elements in the absorbent. Figure 7 The concentration changes of refractory elements in the pyrolysis absorbent before and after the addition of the catalyst; Figure 8 The concentration changes of moderately volatile elements in the pyrolysis absorbent before and after the addition of the catalyst; Figure 9 The concentration changes of highly volatile elements in the pyrolysis absorbent before and after the addition of the catalyst; Figure 10 Mercury isotope composition (δ¹²) of different samples with and without catalyst. 202 Hg vs Δ 199 For comparison with Hg, the error bar represents 1 SD. Detailed Implementation

[0021] This invention provides a catalytic pyrolysis method for mercury isotope analysis of marine sediment samples, comprising the following steps: contacting a composite catalyst with a marine sediment sample and then heating the sample from room temperature to the pyrolysis temperature to perform catalytic pyrolysis; The heating rate is ≤19℃ / min; The temperature of the catalytic pyrolysis is 800~1100℃, and the holding time is ≥40min; The composite catalyst comprises 60-80% MnO2, 10-30% Co3O4 and 10% CaO by mass percentage.

[0022] Unless otherwise specified, all raw materials used in this invention are commercially available products well known in the art.

[0023] The composite catalyst will be explained below.

[0024] In this invention, the composite catalyst comprises 60-80% MnO2, 10-30% Co3O4 and 10% CaO by mass percentage. In specific embodiments, the content of MnO2 in the composite catalyst can be 60%, 63%, 65%, 68%, 70%, 72%, 75%, 78% or 80%; the content of Co3O4 can be 10%, 13%, 15%, 18%, 20%, 25%, 28% or 30%.

[0025] In this invention, the role of MnO2 in the composite catalyst is as a strong oxidant, which reacts with reducing substances (such as H2S, SO2) or volatile metals / metalloids (such as Te, As) generated by pyrolysis at high temperatures to convert them into oxides or salts with higher thermal stability and less volatility. The addition of Co3O4 accelerates the oxidation and fixation process of interfering substances by MnO2 and improves the reaction efficiency. The role of CaO is to prevent the catalyst particles from physically agglomerating at high temperatures. In addition, it can absorb acidic gases to further purify the carrier gas.

[0026] In this invention, the composite catalyst is preferably obtained by mixing MnO2, Co3O4 and CaO.

[0027] Before mixing, the MnO2, Co3O4, and CaO are preferably dried. In embodiments of the present invention, the drying temperature is preferably 105°C, and the drying time is preferably 4 hours.

[0028] This invention does not have special requirements for the mixing process; it is sufficient to ensure that all components are mixed evenly. In embodiments of this invention, the mixing is a grinding process.

[0029] After obtaining the composite catalyst, the present invention contacts the composite catalyst with a marine sediment sample, and then heats it from room temperature to the pyrolysis temperature to carry out catalytic pyrolysis.

[0030] In this invention, the contact between the composite catalyst and the marine sediment sample preferably includes: spreading the marine sediment sample flat and then covering its surface with the composite catalyst. In an embodiment of this invention, specifically, the marine sediment sample is spread flat on the bottom of a quartz boat that has been pre-calcined at 800°C, then the composite catalyst is covered on the surface of the marine sediment sample, and the ends of the quartz boat are sealed with quartz wool. The purpose of pre-calcining the quartz boat is to remove mercury from the quartz boat itself. In this invention, the mass ratio of the marine sediment sample to the composite catalyst is preferably 1:(5~20), and in specific embodiments it can be 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:12, 1:14, 1:16, 1:18, or 1:20.

[0031] In this invention, the catalytic pyrolysis is preferably carried out in a two-stage tubular furnace system. Specifically, the present invention places a quartz boat containing a composite catalyst and a marine sediment sample in the isothermal zone of the first-stage furnace (decomposition furnace) for decomposition. The temperature of the second-stage furnace (holding furnace) is always maintained at 950°C to ensure that all gaseous mercury and its compounds can pass through smoothly, further removing organic matter in the gas and preventing condensation in the pipeline.

[0032] Before heating, the present invention preferably purges the two-stage tube furnace system with argon gas. The present invention removes air from the pipelines through argon purging.

[0033] In this invention, the heating rate is ≤19℃ / min, preferably 15~19℃ / min, and in specific embodiments it can be 19, 18.5, 18, 17.5, 17, 16 or 15℃ / min. In this invention, the heating rate is preferably linear.

[0034] In this invention, the temperature of the catalytic pyrolysis is 800~1100℃, and in specific embodiments it can be 800℃, 850℃, 900℃, 950℃, 1000℃, 1050℃ or 1100℃, with a holding time ≥40min, preferably 40-60min. In embodiments of this invention, the holding time of the catalytic pyrolysis can be 40min, 45min, 50min, 55min or 60min.

[0035] After catalytic pyrolysis is completed, the present invention preferably continues to purge with argon gas to ensure that all mercury vapor is enriched.

[0036] The catalytic pyrolysis method for mercury isotope analysis of marine sediment samples provided by the present invention will be described in detail below with reference to the embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0037] The samples and reagents used in the following examples and comparative examples are as follows: This invention used four actual marine sediment samples: NH1, NH2, and NH3 were collected from the South China Sea, and HZW was collected from Hangzhou Bay, China. Simultaneously, four national standard reference materials were used for method verification and quality control: GSS4a and GSS5a are soil standard reference materials, and GBW07335 and GBW07336 are marine sediment standard reference materials.

[0038] The catalyst was prepared by mixing analytically pure manganese dioxide (MnO2), cobalt tetroxide (Co3O4), and calcium oxide (CaO) powders (Sinopharm Chemical Reagent Co., Ltd.) in a specific mass ratio. All experimental water was Milli-Q ultrapure water (18.2 MΩ·cm). All acids used in the experiments (HCl, HNO3) were of analytical grade and purified by sub-boiling distillation before use. The standard solutions NIST SRM 3133 (Hg), NIST SRM 3177 (Hg), and NIST SRM 997 (Tl) used for mercury isotope assays were purchased from the National Institute of Standards and Technology (NIST).

[0039] Analytical methods 1. Total mercury concentration analysis The total mercury concentration (THg) in solids of the samples and standards was determined using a direct mercury analyzer (DMA-80, Milestone), a method that does not require sample digestion. The mercury concentration in the absorbent was determined using cold vapor atomic fluorescence spectrometry (CV-AFS, Brooks Rand Model III). The method's recovery was calculated by comparing the total mercury content in the absorbent after catalytic pyrolysis with the total mercury content in the sample directly measured by the DMA-80.

[0040] The total mercury (THg) concentrations of the samples used in this invention are shown in Table 1. The THg concentrations of standard materials GSS4a and GSS5a, measured using a DMA-80 direct mercury analyzer, were 72 ± 2 ng / g (n = 6, 1SD) and 743 ± 37 ng / g (n = 2, 1SD), respectively, highly consistent with the values ​​certified in the national standard materials certificates. The THg concentrations of marine sediment standard materials GBW07335 and GBW07336 were 19 ± 1 ng / g (n = 5, 1SD) and 35 ± 1 ng / g (n = 5, 1SD), respectively. The THg concentrations of the four actual environmental samples were: NH1 (223 ± 8 ng / g, n = 6), NH2 (56 ± 2 ng / g, n = 6), NH3 (38 ± 1 ng / g, n = 6), and HZW (114 ± 7 ng / g, n = 6). These data will serve as the baseline for calculating the catalytic pyrolysis recovery rate.

[0041] Table 1 Mercury content of the samples used in this invention

[0042] 2. Mercury isotope analysis The mercury isotope composition in the absorption liquid was analyzed at the Institute of Geochemistry, Chinese Academy of Sciences, using a multi-detector inductively coupled plasma mass spectrometer (MC-ICP-MS, Neptune Plus, Thermo Fisher Scientific). Prior to analysis, Hg(II) in the absorption liquid was reduced to Hg using SnCl2 (3%, w / v). 0 The mercury isotope composition was mixed with Tl (thallium) aerosol and generated as an internal standard (NIST 997, internal standard) via a desolvation nebulizer (Aridus II), and then introduced into the mass spectrometer via inert gas stripping. Instrumental mass bias was corrected using the sample-standard cross-validation (SSB) method with NIST SRM 3133 standard solution. Following the convention recommended by Blum and Bergquist (Blum & Bergquist, 2007), the mercury isotope composition was expressed as δ and Δ for mass fractionation (MDF) and non-mass fractionation (MIF), respectively: δ xxx Hg (‰) = [( xxx Hg / 198 Hg) sample / ( xxx Hg / 198 Hg) NIST 3133 -1] × 1000, Δ xxx Hg (‰) = δ xxx Hg - δ xxx Hg × β xxx .in xxx Hg is the mass number of each Hg isotope, for example, 199, 200, 201, 202, and 204; β xxx It is a mass-related proportionality factor estimated through theoretical kinetics. 199 Hg, 200 Hg, 201 Hg and 204 The β values ​​for Hg were 0.2520, 0.5024, 0.7520, and 1.4930, respectively. The external precision (2SD) obtained from long-term analysis of NIST SRM 3133 standard solutions for δ... 202 Hg is better than ±0.10‰, for Δ 199 Hg was within ±0.05‰. The NIST 3177 standard solution was measured every 10 samples. The overall mean and uncertainty of NIST 3177 were δ. 202 Hg: -0.52 ± 0.12‰; Δ 199Hg: 0.01 ± 0.04‰ (n = 10, 2SD), consistent with previous reports (Yin, 2022; Yu, 2020; Zerkle, 2020). Uncertainty was analyzed using the maximum standard deviation (2SD) of NIST 3177.

[0043] 3. Matrix elemental analysis To evaluate the catalyst’s removal efficiency for matrix elements, the concentrations of various potential interfering elements in the absorbent before and after pyrolysis were analyzed using inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7900), including refractory elements (V, Cr, Ni, Mo, U, Ba), moderately volatile elements (Mn, Cu, Sb, As), and highly volatile elements (Zn, Tl, Pb, Cd, Te).

[0044] Examples 1-4 and Comparative Examples 1-6 MnO2, Co3O4 and CaO powders, which were dried in an oven at 105℃ for 4 hours, were thoroughly ground and mixed in an agate mortar at a mass ratio of 7:2:1 until the color was uniform.

[0045] In order to improve experimental efficiency while ensuring recovery rate, this invention uses NH1 samples with high organic matter content as the target for catalytic pyrolysis.

[0046] Mercury enrichment was performed in a two-stage tubular furnace system. Approximately 100-300 mg of sediment sample NH1 was accurately weighed and spread evenly on the bottom of a quartz boat pre-ignited at 800°C. About 2 g of composite catalyst was uniformly covered on the sample surface. The ends of the quartz boat were then sealed with quartz wool (pre-cleaned at 450°C for 2 h) to prevent particle release during combustion. The quartz boat containing the sample was placed in the isothermal zone of the first-stage furnace (decomposition furnace). The absorption apparatus consisted of a 25 mL borosilicate glass bubble bottle and a custom-made elbow-shaped sintered glass tube, each containing 5 mL of 40% (v / v) anti-aqua regia (HNO3 / HCl = 3 / 1, v / v) as Hg. 0 The absorbent liquid.

[0047] The pyrolysis procedure was set as follows: First, the system was purged with high-purity argon gas (flow rate 50 mL / min) for 15 minutes to remove air from the pipeline. Then, the temperature rise program of the first-stage furnace (decomposition furnace) was started, linearly increasing from room temperature (20-35℃) to 950℃ within a specified time (see Tables 2 and 3), and held at 950℃ for a period of time (see Tables 2 and 3). The temperature of the second-stage furnace (holding furnace) was maintained at 950℃ to ensure that all gaseous mercury and its compounds could pass through smoothly and prevent condensation in the pipeline. After pyrolysis, argon gas was continued to flow for 15 minutes to ensure that all mercury vapor was carried into the absorbent. After each set of experiments, the decomposition furnace was allowed to cool to room temperature before the next set of experiments was conducted.

[0048] The recovery rates of mercury in each example and comparative example were determined according to the aforementioned analytical methods. The effects of changing the holding time on the catalytic pyrolysis results of the NH1 sample are shown in Table 2 and... Figure 2 The effect of changing the heating time on the catalytic pyrolysis results of the NH1 sample is shown in Table 3 and . Figure 3 a.

[0049] Table 2. Effect of changing the holding time on the catalytic pyrolysis results of NH1 samples

[0050] like Figure 2 As shown in Figure a and Table 2, the holding time has a significant impact on the recovery rate. When the holding time is less than 30 min, the recovery rate is low and unstable; between 30 min and 40 min, the recovery rate shows a jump; when the holding time reaches 40 min, the recovery rate can reach 100.9 ± 1.9% (n = 6, 1SD), and remains stable thereafter (50 and 60 min).

[0051] Table 3. Effect of changing the heating time on the catalytic pyrolysis results of NH1 samples

[0052] like Figure 3 As shown in Figure a and Table 3, the recovery rate gradually increases with the extension of heating time (i.e., the decrease of heating rate). When the heating time reaches 50 min, the recovery rate can stably meet the requirements of isotope testing (>95%), and tends to stabilize at 60 min.

[0053] Taking into account both efficiency and recovery rate, the optimal pyrolysis scheme determined in this invention is: heating time of 50 min and holding time of 40 min. All subsequent experiments adopted this optimized condition.

[0054] Examples 5-7 and Comparative Examples 7-8 Catalyst ratio optimization The only difference from Example 1 is the change in catalyst ratio, as shown in Table 4.

[0055] This invention tested the effects of different ratios (MnO2 content ranging from 50% to 90%) on the pyrolysis recovery and isotopic composition of NH1 samples. The results are shown below. Figure 4 And Table 4. Figure 4 In the diagram, the purple area represents the "true" range (mean ± 2 SD) of the isotopic composition of the NH1 sample. Table 4. Effect of catalyst ratio on pyrolysis recovery and isotopic composition of NH1 samples

[0056] Depend on Figure 4 As shown in Table 4, when the proportion of MnO2 reaches 60%, the mercury recovery rate can be stably higher than 90%, meeting the basic requirements for isotope analysis. From the perspective of isotope composition, this invention uses the average value (δ¹⁸O₂) of the isotopic composition of the NH₁ sample obtained from multiple high-recovery (close to 100%) experiments. 202 Hg = -0.54 ± 0.16‰, Δ 199 Hg = 0.21 ± 0.04‰, n = 5, 2SD) was considered its "true value". Experimental results showed that when the proportion of MnO2 was between 60% and 80%, the measured isotopic composition all fell within the 2SD error range of this "true value", indicating that the results were accurate and reliable. However, when the proportion of MnO2 increased to 90%, although the recovery rate remained high, the measured δ... 202 The Hg value is significantly negative, indicating significant isotopic fractionation. This may be due to the excess of the strong oxidant MnO2 altering the release mechanism or valence state of mercury, leading to isotopic fractionation. Therefore, this invention determines the optimal catalyst ratio range as: 10% CaO, 60-80% MnO2, and 10-30% Co3O4.

[0057] Validation of the reliability and effectiveness of the method 1. Validation of mercury content recovery rate in samples To verify the reliability of the method, the present invention first subjected the standard substance GSS4a to catalytic pyrolysis (pyrolysis conditions as in Example 1), obtaining a recovery rate of 104.7 ± 7.4% (n=6, 1SD). To further eliminate the interference (matrix effect) that may be caused by complex matrices, the present invention mixed GSS4a with high organic matter marine sediment samples NH1 at different mass ratios (1:1, 2:1, 3:1) and then subjected them to pyrolysis, achieving an average recovery rate of 102.0 ± 4.6% (n=8, 1SD) (see Example 1). Figure 1 (a) demonstrates that the method has good resistance to matrix interference.

[0058] To demonstrate the effectiveness of this method, comparative experiments were conducted on three other samples (NH2, NH3, HZW) with and without a catalyst (results are shown in...). Figure 1 (b) Under conventional pyrolysis conditions without a catalyst, the recoveries of NH2, NH3, and HZW were 57.2 ± 19.2% (n=3), 64.4 ± 12.1% (n=3), and 78.1 ± 2.3% (n=2), respectively. It can be seen that, especially for NH2 and NH3 samples, the recoveries were not only low but also had extremely large relative standard deviations, indicating serious problems with conventional methods for these samples. In contrast, after using the optimized catalytic pyrolysis method of this invention, the recoveries of these three samples increased to 99.8 ± 1.7% (n=3), 100.0 ± 4.6% (n=2), and 96.9 ± 7.1% (n=3), respectively, achieving not only quantitative recovery but also significantly improved stability (precision).

[0059] 2. Mercury isotope analysis verification of the sample Accurate determination of the mercury isotopic composition in marine sediments requires a high and stable mercury recovery rate. In experiments involving adjusting and optimizing the pyrolysis furnace heating program, this invention simultaneously tracked changes in the sample's isotopic composition (see...). Figure 2 b and c and Figure 3 (b and c). Experimental results show that when the recovery rate reaches a stable state under the conditions of holding time ≥ 40 min and heating time ≥ 50 min, the measured δ 202 Hg and Δ 199 The Hg value tends to stabilize and exhibits good consistency. For example, under the conditions of a heating time of 50 min and a holding time of 40 min, the isotopic composition of NH1 is δ. 202 Hg = -0.54 ± 0.08‰, Δ 199 Hg = 0.21 ± 0.01‰ (n=3, 1SD), which is completely consistent with the results obtained with longer holding times (50 and 60 min) within the error range. A noteworthy phenomenon is that even under conditions of low recovery (e.g., a heating time of 30 min and a recovery rate of 73.1%), the measured isotopic composition did not differ significantly from the results under high recovery conditions. This may suggest that for samples like NH1, the isotopic composition of different forms of mercury is relatively homogeneous. However, to ensure the accuracy and universality of isotopic analysis results, from a methodological perspective, achieving quantitative recovery is an indispensable and reliable strategy for eliminating bias and adapting to multiple sample types.

[0060] To further verify the accuracy of isotope analysis, this invention conducted a two-terminal mixing experiment. This invention uses the standard substance GSS4a (δ¹⁸O₁⁻) with significantly different mercury isotope compositions. 202Hg = -1.67 ± 0.12‰, Δ 199 Hg ( -0.32 ± 0.03‰) was mixed with NH1 samples at ratios of 1:1, 2:1, and 3:1 based on the total mercury content. For example... Figure 5 As shown, the measured mercury isotope composition of the mixed sample is highly consistent with the theoretical mixing line constructed based on GSS4a and NH1 as two endmembers (R 2 =0.99287), and all experimental data points are within the 95% confidence interval. Furthermore, the experimental measurements agree well with the theoretical values ​​calculated by the isotope binary mixing model (e.g., the measured δ¹⁸O of the 3:1 mixture sample). 202 The Hg value was -1.38‰ (the theoretical value is -1.39‰). This result fully demonstrates the accuracy of this method in determining the mercury isotopic composition of the sample, and no obvious isotopic fractionation effect or matrix interference was observed.

[0061] 3. Verification of matrix removal efficiency The core advantage of this invention lies in its highly efficient matrix removal capability. This invention analyzed the concentrations of various potential interfering elements in the pyrolysis absorbent under catalytic and non-catalyst conditions (the experiment with a catalyst is described in Example 1) using ICP-MS. For example... Figure 6 As shown in the heatmap, the concentration of most interfering elements is significantly negatively correlated with mercury recovery rate; that is, as mercury recovery rate increases (using a catalyst), the content of these elements decreases sharply. Figure 6 In the diagram, red indicates a positive correlation, blue indicates a negative correlation, and the larger the area of ​​the circle, the stronger the correlation.

[0062] Figure 7 , Figure 8 , Figure 9 The changes in the content of refractory elements (V, Cr, Ni, Mo, U, Ba), moderately volatile elements (Mn, Cu, As, Sb), and highly volatile elements (Zn, Tl, Pb, Cd, Te) before and after the addition of the catalyst were shown. The results indicate that the addition of the catalyst reduced the concentration of almost all interfering elements in the absorbent by more than an order of magnitude. The removal effect was particularly significant for elements considered major interfering agents, such as Te, As, Sb, and Cd. For example, the Te content in the absorbent decreased by approximately an order of magnitude. This demonstrates that the catalyst can effectively immobilize these volatile or semi-volatile elements in the solid residue at high temperatures, preventing them from entering the gas phase and being captured by the absorbent.

[0063] Method application examples This invention applies the established catalytic pyrolysis method to the analysis of actual samples and standard substances. For example... Figure 1 (b) Figure 10As shown, without the addition of a catalyst, the recoveries of GBW07335 and GBW07336 are low, and their measured δ values ​​are low. 202 The relatively negative Hg value is likely due to kinetic fractionation caused by the preferential volatilization of lighter mercury isotopes during incomplete release. However, the recovery rate significantly improved after the addition of a catalyst (see Example 1 for experiments with a catalyst), resulting in a more reliable isotopic composition. This invention reports the mercury isotopic compositions of these two marine sediment standard materials for the first time: GBW07335 is δ... 202 Hg = -1.24 ± 0.08‰, Δ 199 Hg = -0.08 ± 0.03‰ (n=2, 1SD); GBW07336 is δ 202 Hg = -1.38 ± 0.10‰, Δ 199 Hg = -0.11 ± 0.02‰ (n=2, 1SD). These data can provide important references for future related research.

[0064] Furthermore, this invention analyzed two sediment samples from different marine environments. The mercury isotope composition of the deep sediments (NH2) in the South China Sea is δ¹⁸. 202 Hg = -1.39 ± 0.18‰, Δ 199 Hg = -0.17 ± 0.05‰ (n=3,1SD), while the composition of Hangzhou Bay sediments (HZW) is δ 202 Hg = -3.52 ± 0.11‰, Δ 199 Hg = -0.11 ±0.05‰ (n=3, 1SD). Both δ 202 The significant differences in Hg values ​​reflect the marked differences in their mercury sources. Hangzhou Bay is influenced by a large influx of rivers carrying terrigenous detritus, resulting in its extremely negative δg values. 202 Mercury levels are likely a typical characteristic of terrestrial inputs. Mercury in deep sediments of the South China Sea, however, may more accurately reflect background inputs from the open ocean. This application demonstrates the significant potential of this method in accurately distinguishing mercury from different sources and in conducting environmental tracing studies.

[0065] In summary, the catalytic pyrolysis method established in this study is simple, efficient, and reliable, effectively overcoming the bottlenecks of traditional methods and providing a powerful technical means for the accurate analysis of mercury isotopes in complex environmental samples. The widespread application of this method will greatly promote research in areas such as paleoenvironmental reconstruction using marine sediments and other environmental archives, global mercury cycling, and pollution source tracing.

[0066] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for mercury isotope analysis of marine sediment samples by catalytic pyrolysis, characterized in that, The method comprises the following steps: catalytic pyrolysis of the marine sediment sample after the composite catalyst is contacted with the marine sediment sample and the temperature is raised from room temperature to pyrolysis temperature; the rate of the temperature raising is ≤19℃ / min; the temperature of the catalytic pyrolysis is 800-1100℃, and the holding time is ≥40min; the composite catalyst comprises 60-80% of MnO2, 10-30% of Co3O4 and 10% of CaO in terms of mass percentage.

2. The catalytic pyrolysis process of claim 1, wherein, the composite catalyst comprises 70% of MnO2, 20% of Co3O4 and 10% of CaO in terms of mass percentage.

3. The catalytic pyrolysis process of claim 1, wherein, the rate of the temperature raising is 15-19℃ / min.

4. The catalytic pyrolysis process of claim 1, wherein, the holding time of the catalytic pyrolysis is 40-60min.

5. The catalytic pyrolysis process of claim 1, wherein, the mass ratio of the marine sediment sample to the composite catalyst is 1: (5-20).

6. The catalytic pyrolysis process of claim 1, wherein, the contacting of the composite catalyst with the marine sediment sample comprises: the marine sediment sample is laid flat and the composite catalyst is covered on the surface of the marine sediment sample.

7. The catalytic pyrolysis process of claim 1, wherein, the catalytic pyrolysis is carried out in a double-section tube furnace system.

8. The catalytic pyrolysis process of claim 7, wherein, before the temperature raising, the double-section tube furnace system is purged with argon.

9. The catalytic pyrolysis process according to claim 1 or 2, c h a r a c t e r i z e d in that the composite catalyst is obtained by mixing MnO2, Co3O4 and CaO.

10. The catalytic pyrolysis process of claim 9, wherein, the mixing comprises grinding and mixing.

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