Quantitative determination method of mercury in different occurrence forms in organic matter-rich sedimentary rock

Abstract

The application discloses a quantitative testing method for mercury in different occurrence modes in organic matter-rich sedimentary rock, and belongs to the technical field of geochemical analysis. The application divides the sample to be tested into multiple equal parallel sub-samples, performs constant temperature heating at a single temperature point in an external muffle furnace, so that the sample reaches thermodynamic equilibrium, and ensures that mercury forms corresponding to the thermal stability interval are completely released; the residual mercury content of each sub-sample is determined by using a direct mercury analyzer; based on the mass conservation relation of total mercury-residual mercury = released mercury, the mercury release amount of each temperature interval is inversely calculated through the difference value of adjacent temperature gradients, and a standardized thermal desorption curve without kinetic interference, peak distortion and tailing overlap is reconstructed.

Description

Technical Field

[0001] This invention belongs to the field of geochemical analysis and environmental geochemistry technology, specifically relating to a quantitative testing method for mercury in different occurrence forms in organic-rich sedimentary rocks, which is particularly suitable for high organic matter and high sulfur sedimentary rock systems, and further relating to the application of this method in sedimentary geochemistry, paleovolcanic activity tracing, and hydrothermal mineralization exploration. Background Technology

[0002] Mercury (Hg) is a highly volatile element with multiple chemical forms, and its migration, transformation, and deposition in the environment are controlled by its chemical forms. The main sources of mercury in sediments include volcanic activity, terrestrial weathering, and hydrothermal activity. Volcanic eruptions release large amounts of mercury vapor into the atmosphere, which is then transported and deposited into marine and terrestrial sedimentary systems. Therefore, mercury content in sediments is often used as an important indicator of paleovolcanic activity. In recent years, with the deepening of sedimentary geochemistry research, researchers have found that abnormally high levels of mercury in sediments are not necessarily caused entirely by volcanic activity, but may also be influenced by factors such as organic carbon enrichment, sulfide deposition, and adsorption by clay minerals (Shenet et al., 2020). Therefore, it is difficult to accurately identify the source and geological significance of mercury based solely on changes in total mercury concentration in sediments.

[0003] In existing technologies, mercury speciation analysis methods mainly include continuous chemical extraction, X-ray absorption fine structure spectroscopy, and thermal desorption analysis. Among these methods, thermal desorption is an analytical method based on the different volatilization temperature characteristics of different mercury compounds in sediments. By programming the temperature to gradually volatilize the mercury in the sample and recording the relationship between mercury release intensity and temperature, a mercury thermal analysis curve is obtained. The mercury thermal analysis curve is an important technical tool for mercury speciation analysis, provenance tracing, geological environment inversion, and mineral exploration and evaluation. This technique was first proposed by RJ Waltering through experiments on the artificial synthesis of mercury compounds. Its core discovery is that different chemical forms of mercury compounds, under heating conditions, decompose and reduce within a characteristic temperature range, releasing mercury vapor. There are significant differences in the peak thermal release temperatures among different compounds. This discovery laid an early foundation for using temperature characteristics to distinguish mercury speciation. Simply put, based on the differences in the thermal stability of different mercury compounds, obtaining mercury thermal analysis curves through programmed temperature increases is an important means of speciation identification, provenance tracing, and mineral exploration. The generally accepted order of mercury pyrolysis from low to high temperature is: atomic mercury → mercuric chloride → mercuric sulfide → mercuric oxide → mercuric sulfate → mercury encapsulated in mineral lattices. This order reflects the essential differences in the chemical bond strength, thermal stability, and occurrence state of different mercury compounds, and serves as an important reference for determining the speciation of mercury in this field.

[0004] However, there are also recognized technical limitations in this field: in actual geological sample testing, the matrix composition of the samples is complex and varied. Different types of samples, such as rocks, soils, and single minerals, contain components such as organic matter, clay minerals, sulfides, carbonates, and silicates. During heating, a series of complex chemical reactions occur, including oxidation, decomposition, recrystallization, and solid-phase reactions. These reactions directly change the decomposition environment of mercury compounds, leading to significant differences in pyrolysis temperature, peak position shift, peak broadening, and even peak splitting for the same mercury compound in different matrices. Therefore, those skilled in the art generally agree that it is impossible to directly determine the type of mercury compound from a single temperature peak, and relying solely on peak temperature for mercury speciation will result in significant misjudgments.

[0005] In summary, the overall differences in the pyrolysis curves can reflect the form in which mercury exists, providing a theoretical basis for applying this technology to mineral exploration analysis. However, the above content only remains at the level of scientific discovery, summary of natural laws, and theoretical prospect. Current technologies do not provide any complete, implementable, repeatable, or standardized technical solutions, nor have they established a comprehensive methodology system covering the entire process from sample pretreatment, pyrolysis testing, curve calibration, morphological identification, occurrence state determination to mineral exploration indicators. Specifically, current methods for identifying mercury speciation through thermal desorption in sedimentary geological samples still face the following key challenges: 1) These methods primarily rely on dynamic signals during continuous heating, making them susceptible to kinetic processes such as diffusion, re-adsorption, and gas transport. This can lead to tailing or overlap of release peaks, reducing the ability to distinguish between different mercury speciations. 2) The lack of standardized heating procedures and temperature range divisions across different studies results in poor comparability and hinders the formation of a standardized analytical system. 3) Existing methods primarily rely on peak positions for qualitative or semi-quantitative assessments, lacking quantitative calculation methods based on mass conservation, making it difficult to obtain accurate content of mercury in different speciations. 4) There is a lack of specialized optimization methods for complex geological systems such as sedimentary rocks. Under conditions of high organic matter or high sulfur, the overlap of mercury release peaks becomes more pronounced, further reducing identification accuracy. Therefore, while existing thermal desorption techniques possess the potential for mercury speciation identification, a methodological system that effectively avoids kinetic interference, achieves standardized temperature segmentation, and possesses quantitative analysis capabilities is still lacking. It is necessary to develop a sedimentary mercury speciation analysis method with standardized procedures and quantitative identification capabilities to improve the accuracy of mercury geochemical research. Summary of the Invention

[0006] Purpose of the invention: To provide a quantitative testing method for mercury in different occurrence forms in organic-rich sedimentary rocks, solving the problems of strong interference from traditional thermal desorption kinetics, inaccurate quantification, peak distortion, and low standardization, so as to achieve high-resolution, high-precision, repeatable, and comparable quantitative determination of mercury in different occurrence forms, thereby improving the accuracy of mercury source identification, paleovolcanic tracing, and mineral exploration prediction.

[0007] To achieve the above objectives, this invention divides the sample into multiple equal-volume parallel subsamples and heats them at a single temperature in an external muffle furnace to achieve thermodynamic equilibrium at that temperature, ensuring the complete release of mercury in the corresponding form. Subsequently, a direct mercury analyzer is used to determine the residual mercury content of each subsample. Based on the mass balance relationship of total mercury - residual mercury = released mercury, the mercury release amount in each temperature range is inverted by the difference between adjacent temperature gradients, and finally, an interference-free thermal desorption curve is reconstructed.

[0008] This method transforms the traditional continuous dynamic release process into multiple independent static equilibrium processes, thereby eliminating kinetic interferences such as diffusion hysteresis, re-adsorption, and peak superposition in principle. The measurement results are determined solely by the thermodynamic stability of the mercury's state of occurrence, rather than by the instrument's dynamic response and heating process control, thus significantly improving the accuracy of mercury speciation identification and quantification.

[0009] The present invention specifically includes the following steps:

[0010] S1 Sample Pretreatment:

[0011] Target geological samples were collected, and surface impurities, weathering layers, and contaminants were removed. The samples were then crushed, ground in an agate mortar, passed through a 200-mesh standard sieve, and dried to constant weight at 40℃ to obtain a uniform, moisture-free, and particle-free powder sample. The sample was then precisely divided into multiple parallel subsamples, with each subsample having a mass deviation of ≤±0.1 mg.

[0012] S2 graded constant temperature heating treatment:

[0013] Place parallel samples in clean quartz boats / nickel boats and then into muffle furnaces / tube furnaces. Set a gradient temperature (e.g., 200℃, 300℃, 400℃, 500℃, 600℃, 700℃) for unidirectional heating from low to high temperature. Rapidly raise the temperature to the set point at 30-50℃ / min, then hold it at that temperature for 5-10 min (extend to 10 min for high organic matter / high sulfur samples). This ensures that each sample reaches thermodynamic equilibrium at a fixed temperature, guaranteeing the complete release of mercury speciation within the corresponding thermal stability range and avoiding kinetic disturbances and speciation superposition caused by continuous heating.

[0014] S3 Residual Mercury Content Determination:

[0015] The residual mercury content of each sample after isothermal heating was determined using a direct mercury analyzer (DMA). Based on the mass balance relationship that total mercury - residual mercury = mercury release at that temperature range, the true mercury release at each temperature interval was calculated by inverting the difference between adjacent temperature gradients, and a standardized mercury pyrolysis curve without interference, distortion, or kinetic shift was reconstructed. Specifically: 1) The total mercury content THg of the unheated sample was determined using a direct mercury analyzer (DMA); 2) The residual mercury content RHg(T) of the heated samples at each temperature point was determined under the same conditions; 3) The cumulative mercury release was calculated: ER(T) = THg - RHg(T); 4) The incremental mercury release within the interval was calculated: ΔER(T) n -T n-1 ) = ER(T n ) - ER(T n-1 5) Using the temperature range as the abscissa and ΔER as the ordinate, reconstruct a standardized thermal desorption curve that is free from interference, distortion, and kinetic shift.

[0016] S4 Matrix Interference Correction:

[0017] Blank samples with the same lithology, geological background, and no mineralization anomalies were selected and subjected to staged isothermal testing, residual mercury testing, and inversion calculations simultaneously with the samples to be tested. This process eliminated matrix background signals such as organic matter pyrolysis, sulfide oxidation, and carbonate decomposition, improving the accuracy of the curves. Matrix calibration employed a real-time synchronous testing method to avoid systematic errors caused by instrument status, carrier gas flow rate, and temperature drift.

[0018] S5 Curve Shape Recognition and Shape Judgment:

[0019] Global feature extraction and multi-dimensional morphology identification were performed on the reconstructed standardized mercury pyroelectric curves, including peak number, peak shape, peak width, peak position range, rising slope, falling slope, peak area distribution, and high-temperature tailing characteristics. Mercury compound types were not determined based on any single temperature peak value. Morphology was classified according to temperature range.

[0020] •T1: <200℃: Weakly adsorbed mercury

[0021] •T2: 200-400℃: Organically bound mercury

[0022] •T3: 400-550℃: Mercury in sulfide-bound state

[0023] •T4: ≥600℃: Residual mineral lattice state mercury

[0024] S6 Geological Application and Prospecting Indicator Analysis:

[0025] Based on the identified mercury occurrence state combinations, and according to the preset morphology-mineralization correspondence rules, the mineralization type, mineralization intensity, mineralization depth, and favorable mineralization zones are determined to complete mineral exploration prediction and exploration instructions. Specifically:

[0026] 1. High proportion of mercuric sulfide, main peak located at 350℃-550℃, and significant bimodal morphology → indicating hydrothermal mineralization;

[0027] 2. High proportion of mercury encapsulated in mineral lattices, with continuous release even at 650℃-800℃ → Indicates sedimentary metamorphic / magmatic hydrothermal mineralization;

[0028] 3. Only a single peak at low temperatures, with no significant release at medium and high temperatures → surface adsorption of mercury, with no indication of deep mineralization;

[0029] 4. Abnormal enrichment of mercury sulfide → a strong indicator of paleovolcanic activity.

[0030] Beneficial effects:

[0031] (1) By transforming the traditional continuous heating thermal desorption process into a discrete processing process of multi-sample graded preheating, and by performing inversion calculation of mercury release in each temperature range based on the principle of mass balance, the dependence on the analysis of thermal release peak shape is fundamentally eliminated, and interference from kinetic factors such as diffusion hysteresis, re-adsorption and peak overlap is effectively avoided. This makes the quantitative results of mercury in different occurrence forms have clear physical meaning, thereby significantly improving the accuracy and interpretability of the analysis results.

[0032] (2) At the same time, the graded preheating-residual mercury inversion method and the DMA segmented temperature program analysis method are organically combined. The former is used for high-precision quantitative analysis, and the latter is used to quickly obtain heat release characteristics. The two correct and complement each other, which not only improves the analysis efficiency, but also enhances the stability and consistency of the results. This breaks through the limitations of a single method in terms of accuracy or efficiency, and forms a universal analytical technology system.

[0033] (3) In view of the problem that mercury has a complex occurrence form and serious overlap of heat release intervals in high organic matter or high sulfur sedimentary rocks, this invention realizes the staged release of mercury in different forms through graded isothermal control, and realizes interval quantification through inversion method, which effectively avoids the misjudgment caused by peak superposition in traditional programmed heating method, thereby significantly improving the identification accuracy of key components such as organic mercury and sulfide mercury.

[0034] (4) The method of the present invention only involves standard experimental operation steps such as conventional sample pretreatment, constant temperature heating and direct mercury analysis. It does not require complex chemical reagent treatment or special equipment modification. The test process is clear and the parameters are controllable. It is easy to form a standard operating procedure (SOP) and is suitable for promotion and application under different laboratory conditions.

[0035] (5) This method mainly relies on conventional equipment such as muffle furnace (or tube furnace) and DMA direct mercury analyzer, and there are no strict restrictions on the instrument model. There is no need to develop a special thermal desorption system or complex combination device, which lowers the technical implementation threshold and facilitates rapid deployment and application on existing experimental platforms. Attached Figure Description

[0036] Figure 1 Curves showing the changes in residual mercury content in carbonaceous mudstone, pyrite, and coal samples after heating at different temperatures. The horizontal axis represents the heating temperature (°C), and the vertical axis represents the residual mercury content (ppb).

[0037] Figure 2 The comparison of standard thermal desorption curves of mercury in different occurrence forms shows the typical thermal release peak shapes and temperature ranges of weakly adsorbed mercury, organically bound mercury, sulfide-bound mercury, and residual mineral lattice mercury.

[0038] Figure 3 The effect of isothermal time and heating rate on the completeness of mercury release.

[0039] Figure 4 The residual mercury content of each sample was determined using a direct mercury analyzer. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Traditional programmed heating methods involve continuous heating on a single sample. The mercury release process is affected by diffusion control, re-adsorption effects, and overlapping release temperatures between different mercury forms, leading to broadening and even superposition of release peaks, making it difficult to effectively distinguish between different mercury forms. This invention, based on the fundamental principles of mercury pyrolysis curves, abandons the approach of judging based on a single temperature peak. Instead, it focuses on the overall curve shape, combining matrix correction to eliminate interference. It determines the mercury occurrence state through comprehensive characteristics such as peak number, peak shape, peak width, peak position interval, slope, and area distribution, ultimately providing mineral exploration indications. Its principle fully conforms to the thermal behavior of geological samples, while simultaneously solving the interference problems, discrimination difficulties, and application challenges that existing technologies cannot address. The core lies in the discretization and reconstruction of the traditional continuous heating thermal desorption curve through a staged preheating—residual mercury measurement—mass conservation inversion method, thereby obtaining the mercury pyrolysis release distribution characteristics unaffected by kinetic effects.

[0042] This invention transforms a "continuous release process" into a "discrete mass difference problem" by dividing a sample into multiple parallel subsamples and heating them separately under different constant temperature conditions, thereby measuring the residual mercury content. Based on the principle of mass conservation, it achieves the inversion calculation of mercury release in each temperature range, and then reconstructs the thermal desorption curve. Furthermore, the temperature range division for different forms of mercury is not empirically determined, but rather a result constrained by both experimental data and geochemical mechanisms. The reconstruction steps are as follows:

[0043] S1. After the sample is dried at low temperature, it is ground into a uniform powder using an agate mortar and pestle, and then sieved through a 200-mesh sieve to improve the uniformity of the sample and thus reduce the test error.

[0044] S2. The sample is divided into multiple parallel subsamples of equal volume and placed separately in a muffle furnace for preheating at different set temperatures. To avoid the re-adsorption and diffusion effects of mercury during the heating process, and to eliminate mutual interference between different mercury release temperature ranges, a heating rate of 30-50℃ / min is set and maintained at a constant temperature for a certain period. The purpose is to ensure that the sample reaches thermodynamic equilibrium under these temperature conditions.

[0045] S3. Determination of residual mercury content: After heating, the residual mercury content in each sample is determined using a direct mercury analyzer (DMA).

[0046] Quality control assurance during testing:

[0047] Operational quality control: The instrument background was blank. The mercury analyzer was used to test the room temperature to 700 degrees Celsius. An empty nickel boat and a blank background were used to ensure that the half-peak intensity of the mercury integral curve of the instrument was below 0.0009. The stability of the mercury standard sample was tested. Mess-4 marine sediment samples were used as the standard (the reference value of the standard is 88±10 ppb) to verify the stability of the instrument and the effectiveness of the procedure.

[0048] Sample testing: Weigh equal amounts of sample (e.g., 50-100 mg) and place them into the nickel boat in sequence. Perform the test using the interpolation method of "standard-sample-standard".

[0049] Quality control indicators: Monitor whether the total mercury recovery rate and the distribution of each mercury form in the control standard are within the expected range (e.g., total mercury recovery rate between 95% and 105%, and the content of each mercury form within ±2 SD of the average value). If the result of a quality control standard drifts, stop the test immediately, check the instrument status (e.g., whether the gold amalgam trap needs regeneration, whether the light source is stable, etc.), and continue the test after it returns to normal.

[0050] S4. Thermal Desorption Curve Reconstruction: The mercury release amount in each temperature range is calculated by the difference between adjacent stages, thereby reconstructing the thermal desorption curve. The residual mercury content of each sample after isothermal heating is determined using a direct mercury analyzer (DMA). Based on the mass conservation relationship that total mercury – residual mercury = mercury release amount in that temperature range, the true mercury release amount in each temperature range is calculated by inverting the difference between adjacent temperature gradients, reconstructing a standardized mercury pyrolysis curve without interference, distortion, or kinetic shift. Specifically: 1) Take the unheated original sample and determine the total mercury content THg using a direct mercury analyzer (DMA); 2) Determine the residual mercury content RHg(T) of the samples after heating at each temperature point under the same conditions; 3) Calculate the cumulative mercury release: ER(T) = THg - RHg(T); 4) Calculate the incremental mercury release in the interval: ΔER(T) n -T n-1 ) = ER(T n ) - ER(T n-1 5) Using the temperature range as the abscissa and ΔER as the ordinate, reconstruct a standardized thermal desorption curve that is free from interference, distortion, and kinetic shift.

[0051] Data processing: The integral signal (i.e., the amount of mercury released in that temperature range) is obtained from the instrument software. Based on the instrument's built-in standard curve, the signal is converted into mercury concentration. Quantitative results for various forms of mercury are directly obtained: weakly adsorbed mercury, organically bound mercury, sulfide-bound mercury, and residual mineral mercury.

[0052] Verify mass balance: The sum of the contents of the four mercury forms should be consistent with the total mercury content measured individually, and the difference should be within an acceptable range (<5%).

[0053] Example 1: To verify the effects of isothermal time and heating rate on the complete release of mercury in different forms during the staged preheating treatment of this invention, ensure the accuracy of mercury release measurement, and further optimize experimental parameters and improve method reliability, this experiment was conducted. Coal samples (mainly organically bound mercury) and pyrite samples (mainly sulfide-bound mercury) were selected as test objects. The experiment focused on variable experiments for the isothermal time and heating rate at temperature programs T2 and T3 to determine the optimal experimental parameters and provide a standardized basis for subsequent sample testing.

[0054] 1. Experimental Materials and Conditions

[0055] (1) Experimental samples: High organic carbon coal samples (organic carbon content 80%) and high sulfide pyrite samples (total sulfur content 32%) were selected. They were pretreated by the S1 step mentioned above (low temperature drying, grinding with an agate mortar, and sieving with a 200-mesh sieve) to ensure the uniformity of the samples and reduce test errors.

[0056] (2) Variable setting: Set two variable parameters, namely the isothermal time (3 minutes, 5 minutes, 10 minutes) and the heating rate (30℃ / minute, 50℃ / minute). Each experimental condition is tested in parallel 5 times, and the average value is taken as the final result to reduce random error.

[0057] (3) Determination of reference values: Using the “graded preheating-residual mercury inversion method” described above in this invention, the reference content of organically bound mercury in coal samples was determined in advance to be 1600 ppb, and the reference content of sulfide-bound mercury in pyrite samples was 460 ppb. These were used as reference standards for calculating the recovery rate of mercury under various experimental conditions.

[0058] (4) Testing and Calculation Methods: Samples under different conditions were preheated in stages, and the residual mercury content was determined. The amount of speciation mercury released under each condition was calculated, and the recovery rate was calculated by "specified mercury release amount / reference content × 100%" to assess the completeness of mercury release.

[0059] (5) Temperature range: 200-400℃ for coal samples and 400-550℃ for pyrite.

[0060] Data results as follows Figure 3 ,according to Figure 3 The experimental data and tables show that in both coal samples (mainly organically bound mercury) and pyrite samples (mainly sulfide-bound mercury), the isothermal time and heating rate have a significant impact on the completeness of mercury release.

[0061] Based on the experimental results of this embodiment, the core principle for determining the optimal parameters of the graded preheating treatment of this invention is: to adopt "adaptive heating rate + sufficient isothermal time" for different matrix samples. The specific optimization scheme is as follows:

[0062] (1) Isothermal time: It is recommended to set the isothermal time at the end of the target temperature range to 5-15 minutes. For samples with high organic matter and complex matrix (such as coal and carbonaceous mudstone), 10-15 minutes of isothermal time is preferred. For samples with simple matrix (such as pure pyrite), 5-10 minutes of isothermal time can be used to ensure that the target form of mercury is completely released and thermodynamic equilibrium is reached.

[0063] The experimental results show that, with the extension of the isothermal time, both the release and recovery rates of speciation mercury in the two types of samples exhibit a significant upward trend.

[0064] ① Coal sample: When the temperature is maintained for 3 minutes, the recovery rate is only 81%-91%, indicating that the organically bound mercury is not completely released; when the temperature is maintained for 5 minutes, the recovery rate increases to 91%-98%, and the mercury release is close to the benchmark value; when the temperature is maintained for 10 minutes, the recovery rate reaches 95%-100%, and the organically bound mercury is basically completely released.

[0065] ② Pyrite sample: When the temperature is constant for 3 minutes, the recovery rate is only 67%-80%, and the release of sulfide-bound mercury is incomplete; when the temperature is constant for 5 minutes, the recovery rate increases to 80%-93%; when the temperature is constant for 10 minutes, the recovery rate reaches 89%-100%, and the release of sulfide-bound mercury is complete.

[0066] The core reason is that the isothermal stage is designed to ensure that all target forms of mercury within the target temperature range are completely decomposed and released, ensuring that the sample reaches thermodynamic equilibrium under these conditions, thereby eliminating quantitative errors caused by incomplete release. Simultaneously, the isothermal time needs to be optimized based on the complexity of the sample matrix: for example, for samples with high organic matter and high porosity (such as coal and carbonaceous mudstone), the organic matter pyrolysis process is complex, and the binding force between mercury and organic matter is strong, requiring a longer isothermal time to ensure complete release of organically bound mercury; while for pyrite samples with relatively simple matrices, the time can be adjusted appropriately according to the actual situation, but it is recommended not to be less than 5 minutes to ensure test accuracy.

[0067] (2) Heating rate: For samples with high organic matter and high sulfur (coal, carbonaceous mudstone), a heating rate of 30℃ / min is preferred; for samples with relatively simple matrix (such as pyrite, common mudstone), a heating rate of 30-50℃ / min can be selected according to the actual situation to avoid incomplete mercury release and peak overlap.

[0068] Different types of sedimentary rock samples exhibit significant differences in mineral composition, organic matter content, and mercury occurrence states, resulting in varying mercury thermal release behaviors. Using a uniform heating rate can easily lead to incomplete mercury release and overlapping peaks. Therefore, this invention, based on a unified temperature range for mercury occurrence, introduces a matrix adaptation adjustment mechanism to optimize the heating rate according to the sample matrix characteristics, thereby improving the method's applicability and testing accuracy.

[0069] The experimental results show that, under the same isothermal time, the recovery rate of both types of samples is higher at a heating rate of 30℃ / min than at a heating rate of 50℃ / min, especially for coal samples (high organic matter, high sulfur samples), the difference is more obvious: the recovery rate of coal samples reaches 97%-100% at a heating rate of 30℃ / min after 10 minutes of isothermal heating, while the recovery rate is 95%-99% at a heating rate of 50℃ / min; the recovery rate of pyrite samples is also about 1-4 percentage points higher at a heating rate of 30℃ / min than at 50℃ / min under the same isothermal conditions.

[0070] The core reason is that in high-organic-matter and high-sulfur samples (such as coal and carbonaceous mudstone), the organic matter undergoes intense pyrolysis during heating. If the heating rate is too fast (e.g., 50℃ / min), it will lead to a lag in the pyrolysis of organic matter, resulting in delayed or incomplete mercury release. It may also cause the release peaks of different forms of mercury to overlap, affecting the accuracy of subsequent integration and quantification. Reducing the heating rate to 30℃ / min can effectively alleviate the problem of lag in organic matter pyrolysis, ensuring that the release peaks of different forms of mercury are spaced out in time, clearly distinguishing the mercury release signals in each temperature range. This not only ensures the completeness of mercury release but also provides a clear and reliable signal basis for subsequent reconstruction of standardized pyrolysis curves and quantitative analysis of mercury forms.

[0071] Based on the experimental results of this embodiment, the core principle for determining the optimal parameters of the graded preheating treatment of this invention is: to adopt "adaptive heating rate + sufficient isothermal time" for different matrix samples. The specific optimization scheme is as follows:

[0072] (1) Isothermal time: It is recommended to set the isothermal time at the end of the target temperature range to 5-15 minutes. For samples with high organic matter and complex matrix (such as coal and carbonaceous mudstone), 10-15 minutes of isothermal time is preferred. For samples with simple matrix (such as pure pyrite), 5-10 minutes of isothermal time can be used to ensure that the target form of mercury is completely released and thermodynamic equilibrium is reached.

[0073] (2) Heating rate: For samples with high organic matter and high sulfur (coal, carbonaceous mudstone), a heating rate of 30℃ / min should be used first in the T2 and T3 heating programs; for samples with relatively simple matrix (such as pyrite, common mudstone), a heating rate of 30-50℃ / min can be selected according to the actual situation to avoid incomplete mercury release and peak overlap.

[0074] The entire process strictly follows the principle of unidirectional heating from low temperature to high temperature. The reason is that, on the one hand, different forms of mercury have increasing thermal stability. At low temperature, weakly bound mercury is released preferentially. If the process directly enters the high temperature stage, it will cause the release of low-temperature mercury and high-temperature mercury mixed together, thus losing the ability to distinguish them. On the other hand, organic matter and sulfides in sedimentary rocks may undergo thermally induced transformation. If the temperature sequence is disordered, it will change the original occurrence state and affect the test results.

[0075] Example 2: Experimental Measurement and Curve Reconstruction of Graded Isothermal-Mass Conservation Inversion of Typical Geological Samples

[0076] This embodiment uses three typical geological samples: carbonaceous mudstone (total organic carbon content: 6.5%, total sulfur content: 10%), high-sulfur pyrite (total sulfur content: 32%), and high-organic coal (organic carbon content: 80%). Based on the graded isothermal heating-residual mercury mass conservation inversion method of this invention, the quantitative analysis of speciation of mercury and the reconstruction of standardized thermal desorption curves are completed according to a unified standard calculation process.

[0077] Unified standard calculation process:

[0078] 1) Take the unheated original sample and determine the total mercury content (THg) using a direct mercury analyzer (DMA);

[0079] 2) The residual mercury content RHg(T) of the subsamples after heating at each temperature point was determined under the same conditions;

[0080] 3) Calculate the cumulative mercury release: ER(T) = THg - RHg(T);

[0081] 4) Calculate the incremental mercury release over the interval: ΔER(T) n -T n-1 ) = ER(T n ) - ER(T n-1 );

[0082] 5) Using the temperature range as the abscissa and ΔER as the ordinate, reconstruct a standardized thermal desorption curve that is free from interference, distortion, and kinetic shift.

[0083] A total of 45 parallel subsamples were prepared for this test, including 15 carbonaceous mudstone samples, 15 pyrite samples, and 15 coal samples. These samples underwent graded isothermal heating treatment at room temperature, 50℃, 100℃, 150℃, 200℃, 250℃, 300℃, 350℃, 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, and 700℃, respectively. The residual mercury content of each subsample was then determined using a direct mercury analyzer. The results can be found in [reference missing]. Figure 4

[0084] See Figure 1 It can be seen that the carbonaceous mudstone sample showed almost no significant mercury loss below 200℃, indicating that only weakly bound or physically adsorbed mercury existed in this temperature range, with low thermal stability but limited content. Subsequently, in the 200–400℃ range, the residual mercury content showed a slow decreasing trend, indicating that this stage mainly corresponds to the gradual decomposition and release of organically bound mercury with moderate thermal stability. This process corresponds to the organic matter pyrolysis temperature range (approximately 250–350℃). Figure 1The results show a high degree of consistency with the experimental data. Furthermore, within the 400–500℃ range, the mercury content significantly decreases, indicating a concentrated release of large amounts of mercury during this stage. This characteristic is consistent with experimental and theoretical understanding of the thermal decomposition of mercury sulfide (HgS) within the approximately 450–550℃ range, suggesting that this range primarily corresponds to sulfide-bound mercury. When the temperature rises above 600℃, the mercury content tends to stabilize, with only a small amount of slow release, indicating that the remaining mercury is mainly contained within the crystal lattice or mineral structure, representing a high-temperature stable residual state. Therefore, based on the aforementioned experimental curve morphology characteristics and the known thermochemical decomposition temperature constraints, this invention classifies the occurrence forms of mercury in the deposition as follows: 1) T1: <200℃: weakly adsorbed mercury; 2) T2: 200–400℃: organically bound mercury; 3) T3: 400–550℃: sulfide-bound mercury; 4) T4: <600℃: residual mineral mercury.

[0085] In the pyrite sample, the mercury content remained stable (approximately 490 ppb) throughout the temperature range from room temperature to 400℃, without a significant decrease. This indicates that almost no mercury was released within this temperature range, and the contents of weakly adsorbed mercury, organically bound mercury, and mercury with moderate or lower thermal stability accounted for a very low proportion. Subsequently, in the 400–500℃ range, the mercury content showed a significant and concentrated decrease, rapidly dropping from approximately 490 ppb at 400℃ to approximately 0 ppb at 500℃. This characteristic is completely consistent with experimental and theoretical understanding of the thermal decomposition of mercury sulfide (HgS) in the approximately 450–550℃ range, indicating that this range is the concentrated release stage of sulfide-bound mercury in pyrite and is the only dominant form of mercury in pyrite. When the temperature rises above 500℃, the mercury content becomes completely stable, with only trace amounts remaining. This indicates that the remaining mercury exists in the pyrite lattice in a high-temperature stable residual form, with an extremely low content.

[0086] Therefore, based on the pyrolysis curve characteristics and thermochemical constraints of pyrite samples, the occurrence forms of mercury in pyrite are classified as follows: <200℃: weakly adsorbed / volatile mercury: no obvious release, extremely low content; 200–400℃: organically bound mercury: no obvious release, extremely low content; 400–550℃: sulfide-bound mercury: concentrated release at medium and high temperatures, the absolutely dominant mercury occurrence form in pyrite; ≥600℃: residual mineral mercury: stable at high temperatures, extremely low content.

[0087] In the coal samples, the mercury content showed a continuous and significant decreasing trend within the temperature range of room temperature to 200℃, rapidly decreasing from approximately 1660 ppb at room temperature to approximately 1430 ppb at 200℃. This indicates the presence of a large amount of mercury with extremely low thermal stability within this temperature range, primarily in volatile and weakly adsorbed states. This portion of mercury has weak binding force and is easily released under ambient to low-temperature conditions. Subsequently, in the 200–400℃ range, the residual mercury content exhibited a precipitous and rapid decrease, plummeting from approximately 1430 ppb at 200℃ to approximately 70 ppb at 400℃. This process highly coincides with the organic matter pyrolysis temperature range (approximately 250–350℃, Figure 1), indicating that this stage represents the concentrated decomposition and release of organic matter-bound mercury with moderate thermal stability, which is the main form of mercury found in coal. Furthermore, in the 400–500℃ range, the mercury content had decreased to an extremely low level (<100 ppb). The mercury content in the coal was only slightly and slowly decreasing, with no obvious concentrated release characteristics, indicating that the content of sulfide-bound mercury in the coal was extremely low. When the temperature rose above 500℃, the mercury content became completely stable, with only trace amounts remaining. This indicates that the remaining mercury existed in the mineral lattice in a high-temperature stable residual form, with an extremely low content.

[0088] Therefore, based on the pyrolysis curve characteristics and thermochemical constraints of coal samples, the occurrence forms of mercury in coal are classified as follows: <200℃: weakly adsorbed / volatile mercury: released rapidly at low temperatures, with weak binding force, and is a secondary occurrence form in coal; 200–400℃: organic matter bound mercury: released intensively at medium temperatures, synchronous with the pyrolysis of organic matter, and is the absolutely dominant occurrence form of mercury in coal; 400–550℃: sulfide bound mercury: released in small amounts, and is a secondary occurrence form in coal; ≥600℃: residual mineral mercury: stable at high temperatures, with extremely low content.

[0089] Summary of test results for three typical samples

[0090] 1) Pyrite (high sulfur): Mercury release is concentrated in the sulfide-bound state range of 400~500℃, accounting for >95%;

[0091] 2) Coal sample (high organic matter): Mercury release was concentrated in the organically bound state range of 200~400℃, accounting for >84%;

[0092] 3) Carbonaceous mudstone: The organic and sulfide-bound states are clearly separated by bimodal peaks, which is consistent with the characteristics of a composite matrix.

[0093] Based on the pyrolysis curve morphology characteristics of the above three types of samples, the known thermochemical decomposition temperature constraints, and the cross-validation between standard samples and actual sedimentary rock samples, this invention classifies the occurrence forms of mercury in sedimentary rocks into four categories, corresponding to four characteristic temperature ranges:

[0094] <200℃: Weakly adsorbed mercury (volatile and weakly adsorbed mercury): Mercury that is rapidly released at low temperatures has weak binding force and low thermal stability, and is easily migrated and released in room temperature to low temperature environments;

[0095] 200–400℃: Organically bound mercury: Mercury that is released gradually or in a concentrated manner in the medium temperature range, synchronous with the pyrolysis process of organic matter, with moderate thermal stability, and mainly found in the organic matter components of sedimentary rocks;

[0096] 400–550℃: Sulfide-bound mercury: Mercury that is concentrated in the medium-high temperature range, which is consistent with the thermal decomposition temperature range of mercury sulfide (HgS). It has high thermal stability and is mainly found in sulfide minerals such as pyrite.

[0097] ≥600℃: Residual mineral mercury (residual mercury): Mercury that is stable at high temperatures and is released slowly in trace amounts. It exists in mineral lattices or stable mineral structures and has extremely high thermal stability, making it difficult to migrate and release.

[0098] After baseline correction, normalization, and smoothing reconstruction of the original pyroelectric data, a standardized mercury pyroelectric curve is obtained (the vertical axis represents the relative content of residual mercury / release rate, and the horizontal axis represents temperature). This curve is not a simple set of discrete temperature points, but a continuous thermodynamic response spectrum reflecting the thermal stability of the mercury-bearing phase. Its overall morphological characteristics are a comprehensive reflection of the mercury occurrence type, binding strength, and combination of occurrence phases. To address the technical shortcomings of traditional mercury pyroelectric analysis, which relies solely on a single temperature peak to determine the type and morphological classification of mercury compounds, this step performs global feature extraction and multi-dimensional morphological identification on the reconstructed standardized mercury pyroelectric curve. The core features include: extracting seven types of global features: peak number, peak shape, peak width, peak position interval, rising slope, falling slope, peak area distribution, and high-temperature tailing characteristics. Strictly avoiding the determination of mercury compound type based on any single temperature peak, this step avoids peak position shift errors caused by sample matrix and multiple morphological superposition. The validation was conducted using three types of samples: coal (high organic carbon standard sample), pyrite (high sulfide standard sample), and mudstone (actual sedimentary rock sample). The coal sample exhibited an absolute dominant peak in the 200–400℃ range, validating the reasonableness of the range for organically bound mercury. The pyrite sample exhibited a unique dominant peak in the 400–550℃ range, validating the reasonableness of the range for sulfide-bound mercury. The mudstone sample displayed a continuous bimodal structure of "organic secondary peak + sulfide main peak," perfectly matching its "organic carbon + sulfide" compositional characteristics, demonstrating the universality and accuracy of this morphological identification method.

[0099] Traditional methods often use a single peak temperature to determine the type of mercury compound. However, in actual samples, mercury often coexists in a mixed phase, and the peak position is easily shifted by factors such as matrix, heating rate, and mineral encapsulation, leading to identification errors. Therefore, this invention does not rely on a single peak value for qualitative identification, but rather on the coupling of continuous curve morphology changes with temperature ranges: the standardized curve is divided into several continuous temperature ranges according to the thermal stability gradient and characteristic release segment. Each range corresponds to a type of mercury occurrence with similar thermochemical behavior. The range boundaries are jointly constrained by the thermal decomposition temperature of the standard sample (mudstone, coal, pyrite), the curve inflection point, the abrupt change point of the release rate, and the peak concentration range.

[0100] Global feature extraction and interval division of the reconstructed standardized curve are performed as follows:

[0101] <200℃ range: The curve shows a gentle decline, weak peaks or no obvious peaks, and extremely small rising / falling slopes, corresponding to the mercury phase with the lowest thermal stability and weakest binding force;

[0102] In the 200–400℃ range: the curve shows a distinct release peak with a moderate peak width, a steep rising edge, and a gradually decreasing falling edge, which highly overlaps with the organic matter pyrolysis temperature range (250–350℃);

[0103] In the 400–550℃ range: the curve shows a strong release peak, a sharp peak shape, a large slope of the rising and falling edges, and a high peak area ratio, which is consistent with the thermal decomposition temperature range of mercuric sulfide (HgS) (450–550℃);

[0104] Above 600℃: the curve tends to flatten out, only a small amount is released slowly, and there is a high-temperature tail, corresponding to a residual phase with lattice confinement and extremely high thermal stability.

[0105] Based on this, a one-to-one correspondence is established between standardized curve characteristics, temperature range, and mercury occurrence morphology, achieving a precise mapping from curve morphology to occurrence state.

[0106] All three types of samples strictly meet the unified 5-step standard calculation process and mass conservation relationship, verifying that the present invention has high accuracy, high universality and high repeatability in different geological samples, and can achieve accurate quantification of mercury in different occurrence forms.

[0107] Based on the mercury occurrence state combinations obtained from the above discrimination, and combined with the preset morphology-mineralization correspondence rules, a comprehensive judgment is made on the mineralization type, mineralization intensity, mineralization depth, and favorable mineralization sections to achieve mineral exploration prediction and exploration indication. The specific discrimination rules are as follows: 1) High proportion of mercury sulfide, main peak located at 350℃~550℃, and significant bimodal morphology indicate hydrothermal mineralization; 2) High proportion of mercury encapsulated in mineral lattices, with continuous release at 650℃~800℃, indicates sedimentary metamorphic / magmatic hydrothermal mineralization; 3) Only showing a low-temperature single peak, with no obvious release at medium and high temperatures, is judged as surface adsorbed mercury, which does not have the significance of indicating deep mineralization; 4) Abnormal enrichment characteristics of mercury sulfide indicate deep material input signals brought by ancient volcanic activity.

[0108] All three types of samples strictly meet the unified 5-step standard calculation process and material balance relationship, verifying that the present invention has high accuracy, high universality and high repeatability in different geological samples, and can achieve accurate quantification of mercury in different occurrence forms.

[0109] The problem with traditional methods is that the sample temperature changes constantly during continuous heating. Mercury needs time to be released from the sample interior to the surface and then carried away by the carrier gas. If the heating is too rapid, the external temperature may rise before the internal mercury has fully emerged, causing mercury that should have been released in the low-temperature region to be delayed until the high-temperature region (tailing), or to overlap with the signal from the high-temperature region (overlap). Furthermore, the dynamic signal (peak area) measured by traditional methods is related to the release rate, not directly proportional to the total mercury amount. Quantification requires assuming a kinetic model to deconvolve and integrate the peak shape, a complex process with significant errors. This invention maintains a constant temperature for a sufficient time until no more mercury is released, reaching thermodynamic equilibrium. Simultaneously, this invention performs "isothermal-equilibrium" at each temperature point, allowing matrix interference reactions (such as organic matter pyrolysis) to tend to complete at these temperatures. Therefore, when calculating mercury release within a temperature range, the background signal from the blank matrix sample can be simultaneously subtracted, treating matrix interference as static background rather than dynamic signal interference, thus effectively separating the pure mercury release signal.

[0110] In summary, this invention provides a quantitative testing method for mercury in different occurrence forms in organic-rich sedimentary rocks based on hierarchical isothermal and mass balance inversion. By transforming the traditional continuous heating thermal desorption process into multi-sample hierarchical isothermal discretization, and inverting the mercury release amount in each temperature range based on mass balance relationships, this method effectively eliminates kinetic interference and achieves high-precision quantification of standardized thermal desorption curves and different occurrence forms of mercury. This method has a clear operation procedure, controllable experimental parameters, and universal equipment requirements, making it suitable for sedimentary rock samples with complex matrixes such as high organic matter and high sulfur content. It has broad application prospects in sedimentary geochemistry, paleovolcanic activity tracing, and hydrothermal mineralization exploration.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any person skilled in the art can make some modifications or alterations to the disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention, without departing from the scope of the present invention, shall still fall within the scope of the present invention.

Claims

1. A quantitative method for detecting mercury in different occurrence forms in organic-rich sedimentary rocks, characterized in that, Includes the following steps: S1 Sample pretreatment: The sedimentary rock sample was crushed, ground, passed through a 200-mesh sieve, dried at a low temperature of 40℃, and then precisely divided into multiple parallel subsamples. S2 graded isothermal heating: Parallel subsamples are placed in a muffle furnace and heated at a single temperature point according to a unidirectional gradient from low temperature to high temperature until the samples reach thermodynamic equilibrium. S3 Residual Mercury Test and Curve Reconstruction: 1) Take an unheated sample and determine the total mercury content (THg) using a direct mercury analyzer; 2) The residual mercury content RHg(T) of the subsamples after heating at each temperature point was determined under the same conditions; 3) Calculate the cumulative mercury release: ER(T) = THg - RHg(T); 4) Calculate the incremental mercury release over the interval: ΔER(T) n -T n-1 ) = ER(T n ) - ER(T n-1 ); 5) Reconstruct standardized thermal desorption curves without interference, distortion, or kinetic shift by plotting the temperature range on the x-axis and ΔER on the y-axis. S4 Matrix interference correction: Simultaneous processing was performed using blank samples of the same lithology and without mineralization anomalies to subtract matrix background signals; S5 Morphology Identification: Based on the reconstructed thermal desorption curves, mercury is classified into four characteristic temperature ranges and their corresponding mercury occurrence forms in order of increasing thermal stability: weakly adsorbed mercury below 200℃, organically bound mercury between 200-400℃, sulfide-bound mercury between 400-550℃, and residual mineral lattice mercury above 600℃.

2. The method according to claim 1, characterized in that, In step S2, the heating rate is 30–50℃ / min, and the holding time is 5–15min.

3. The method according to claim 2, characterized in that, In step S2, the sample is a high organic matter / high sulfur sample, and the isothermal time is 10-15 min.

4. The method according to claim 1, characterized in that, The quantitative results satisfy the law of conservation of mass: the relative deviation between the total mercury released and the total mercury content in each temperature range is <5%.

5. The method according to claim 1, characterized in that, The sample was divided into multiple parallel subsamples, preheated at different set temperatures, and combined with the determination of residual mercury content to realize the inversion calculation of mercury release in each temperature range and the reconstruction of standardized mercury pyrolysis curves.

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