Environmental risk evaluation method for heavy metal decomposition and release of plant litters in polluted land parcels

Abstract

The invention discloses an environmental risk evaluation method for decomposition and release of plant litter heavy metals in a polluted land parcel. The environmental risk evaluation method comprises the following steps: acquiring environmental parameters of the polluted land parcel; collecting a litter mixed sample of the surface plant litter layer, and detecting the moisture content of the litter mixed sample and the content of various heavy metals; detecting the releasable contents of various heavy metals in the litter mixed sample through a litter releasable heavy metal analysis method; according to the obtained and detected data, calculating the annual release flux of litter heavy metal in unit area of the polluted land parcel; and according to the environmental parameters of the polluted land parcel and the annual release flux of the litter heavy metal in unit area, evaluating the environmental risk of decomposition and release of single heavy metal in the litter along with rainfall and the comprehensive environmental risk of decomposition and release of the litter heavy metal along with rainfall. The method can quickly, accurately and economically evaluate the potential risk of decomposition and release of the plant litter heavy metal in the polluted land parcel to the surrounding soil and water environment.

Description

Technical Field

[0001] The invention belongs to the technical field of environmental pollution prevention and control, and specifically relates to an environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot. Background Art

[0002] Heavy metal contaminated plots are usually found in areas with intensive industrial and agricultural activities. The heavy metal effectiveness of the soil in these areas is high, and the various plants growing on them can absorb excessive heavy metals in the soil, resulting in a large accumulation of heavy metals in the branches and leaves above the ground. As the leaves grow and mature with the plants, they will form plant litter that covers the soil surface, and the heavy metals they carry may pose an environmental risk. Under natural conditions, plant litter in contaminated plots will gradually release the heavy metals they contain in physical, chemical and biological processes such as rainwater leaching and microbial activities. These heavy metals can migrate to nearby soil or surface water bodies through surface runoff and other pathways, resulting in secondary pollution of the surrounding environment, endangering the environment and human health. Clarifying the environmental risks caused by the release of heavy metals from plant litter in contaminated plots is of great significance for the prevention and control of soil and water pollution.

[0003] At present, there are few methods for evaluating the release flux and environmental risk of heavy metals in plant litter from polluted plots. The release of heavy metals in litter can be analyzed by the in-situ litter bag decomposition method or the indoor microcosm soil cultivation method. However, the traditional in-situ litter bag decomposition method is affected by the changes in natural environmental factors in the wild, and the results of the determination of heavy metal release in litter are highly random; while the indoor microcosm soil cultivation method has a long cycle and high cost, and it is difficult to quickly obtain the release of heavy metals in plant litter from polluted plots. Summary of the invention

[0004] The present invention provides an environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot, which can quickly, accurately and economically assess the environmental risk of the decomposition and release of heavy metals in plant litter in a polluted plot.

[0005] In order to achieve the above technical objectives, the present invention adopts the following technical solutions:

[0006] An environmental risk assessment method for the decomposition and release of heavy metals in plant litter in polluted land, comprising:

[0007] Step 1, obtaining environmental parameters of the polluted plot and collecting mixed litter samples of the surface plant litter layer;

[0008] Step 2, taking a portion of the collected litter mixed sample as a first group of samples, pre-treating the first group of samples, and detecting the moisture content and total content of multiple heavy metals in the litter mixed sample;

[0009] Step 3, taking another part of the collected litter mixed sample as a second group of samples, pre-treating the second group of samples, and then detecting the releasable content of various heavy metals in the litter mixed sample by a litter releasable heavy metal analysis method;

[0010] Step 4, calculating the annual release flux of heavy metals per unit area of ​​litter in the polluted plot based on the data obtained and detected in steps 1-3;

[0011] Step 5: Based on the environmental parameters of the contaminated plots and the annual release flux of heavy metals in litter per unit area, evaluate the environmental risk of the decomposition and release of single heavy metals in litter with rainfall, as well as the comprehensive environmental risk of the decomposition and release of heavy metals in litter with rainfall.

[0012] Furthermore, the environmental parameters include: average annual rainfall, underlying surface impermeability, land use type, main vegetation type and annual biomass of litter.

[0013] Furthermore, the method for collecting mixed litter samples is: determine the sample collection time according to the plant leaf fall pattern and crop harvest time, and the sample collection time is autumn or spring and autumn; the collection location is the undecomposed dead branches and leaves layer whose original plant residue morphological characteristics can be distinguished by the naked eye; the sample types include but are not limited to dead leaves, dead branches and fallen bark; the sample weight is more than 200g.

[0014] Furthermore, in step 2, the method for pretreating the first group of samples and detecting the total content of heavy metals is: first air-dry the first group of samples; then bake them in an oven at 60°C to constant weight and grind them into powder; then use a HNO3-HClO4 system to digest them; finally, use an inductively coupled plasma mass spectrometer for quantitative detection to obtain the total content of each heavy metal in the samples.

[0015] Furthermore, the air-dried moisture content and the oven-dried moisture content of the litter mixed sample are calculated by weighing the first group of samples before air-drying, after air-drying, and after oven-drying.

[0016] Furthermore, in step 3, the method for pre-treating the second group of samples and detecting the releasable content of heavy metals is as follows:

[0017] The second group of samples were first air-dried; then the air-dried second group of samples were processed to an area of ​​about 1cm 2fragments, weigh several fragments, mix deionized water as the extraction solution with the fragments; then place the mixed solution in a centrifuge tube for constant temperature oscillation extraction, the extraction conditions are solid-liquid ratio 1:40, temperature 25°C, shaking speed 180rpm, and extraction time 4h; after the reaction is completed, place the centrifuge tube in a low-speed centrifuge at 4800r / min for 10 minutes, separate the residue and filter, and the filtered mixed solution is digested with HNO3-H2O2 system; finally, an inductively coupled plasma mass spectrometer is used to quantitatively analyze the corresponding heavy metal concentration in the filtered mixed solution, and the releasable content of various heavy metals in the sample is calculated.

[0018] Furthermore, the releasable content of various heavy metals in the sample is calculated as follows:

[0019]

[0020] Where, M is the releasable content of a certain heavy metal in the litter mixed sample; C L is the corresponding heavy metal concentration in the mixed solution after filtration in the analysis method of releasable heavy metals in litter; L is the volume of the extraction solution; θ A is the air-dried moisture content of the litter mixture sample; m is the weight of the debris mixed with the extraction solution.

[0021] Furthermore, the calculation method of step 4 is:

[0022]

[0023] Where R is the release ratio of a certain heavy metal in the litter mixed sample; n is the number of sample collections. When sampling is conducted only once in autumn, n=1; when sampling is conducted in spring and autumn, n=2; θ D is the drying moisture content of the litter mixed sample; C T is the total content of the corresponding heavy metals in the litter mixed sample; M k ,θ D,k , C T,k are the releasable heavy metal content, drying moisture content, and total heavy metal content corresponding to the kth collection of the mixed sample of fallen objects in a year; M A is the annual release flux of a certain heavy metal in the litter mixed sample; B is the annual biomass of litter per unit area; the weight of the first group of samples is the same as that of the second group of samples.

[0024] Furthermore, the environmental risk characterization method for evaluating the decomposition and release of a single heavy metal in litter with rainfall is:

[0025]

[0026] In the formula, C w is the concentration of heavy metals released by litter decomposition in rainfall runoff; MA is the annual release flux of heavy metals from litter; W is the average annual rainfall volume per unit area; is the surface runoff coefficient; I is the impermeability of the underlying surface; P i is the pollution index of the ith heavy metal; C w,i is the concentration of the ith heavy metal released by litter decomposition during rainfall; C s,i is the standard concentration of the ith heavy metal;

[0027] When P i <1, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is no pollution; when 1≤P i <2, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is mild pollution; when 2≤P i When P < 3, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is moderate pollution; when P i When ≥3, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is severe pollution.

[0028] Furthermore, the method for characterizing the comprehensive environmental risk of heavy metals released from litter during decomposition with rainfall is as follows:

[0029]

[0030] Where, P is the comprehensive pollution index of heavy metals; P i,max It is the maximum value among all heavy metal pollution indices; P i,avg is the average value of the pollution index of all heavy metals;

[0031] When P<0.7, the comprehensive environmental risk level is safe; when 0.7≤P<1, the comprehensive environmental risk level is warning level; when 1≤P<2, the comprehensive environmental risk level is moderate risk; when 2≤P<3, the comprehensive environmental risk level is high risk; when P≥3, the comprehensive environmental risk level is extremely high risk.

[0032] Furthermore, the plurality of heavy metals include but are not limited to Cd, Pb, Cu, Zn and As.

[0033] Beneficial Effects

[0034] The present invention is based on the analysis method of releasable heavy metals in litter, obtains the parameters of heavy metal decomposition and release in plant litter, and further combines the environmental parameters of polluted plots and the annual release flux of heavy metals in litter to evaluate the decomposition and release risk of heavy metals in litter in polluted plots, and effectively quantifies and evaluates the environmental risk of heavy metals in litter. The method has high accuracy and evaluation efficiency, a wide range of applications, and low implementation costs, and is of great significance for promoting the development of environmental risk assessment and restoration and management of heavy metals in litter in highly polluted areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The present invention is a flow chart of the environmental risk assessment method for heavy metal release in plant litter of contaminated land.

[0036] Figure 2 This is a risk index diagram of a single heavy metal released from litter in a typical polluted plot in an embodiment of the present invention.

[0037] Figure 3 This is a comprehensive risk index diagram of heavy metals released from litter in a typical polluted plot in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following is a detailed description of an embodiment of the present invention. This embodiment is based on the technical solution of the present invention, and provides a detailed implementation method and a specific operation process to further explain the technical solution of the present invention.

[0039] An environmental risk assessment method for the decomposition and release of heavy metals from plant litter in contaminated land around an abandoned zinc smelter in Hunan Province is used as an example. Figure 1 As shown, the following steps are included:

[0040] Step 1: Select representative polluted plots based on the results of the soil heavy metal content sampling survey; obtain environmental parameters such as the average annual rainfall, underlying surface impermeability, land use type, main vegetation type and annual biomass of litter at the corresponding points through on-site investigation and data query; the research points include woodlands and abandoned lands whose main vegetation types are deciduous broad-leaved forests, shrubs and grasses, and paddy fields where the main crop type is rice.

[0041] In addition, according to the pattern of plant leaf fall and crop harvest time, autumn sampling (September) was selected to collect plant litter samples with obvious morphological characteristics of original plant residues.

[0042] Step 2: taking a portion of the collected litter mixed samples as the first group of samples, pre-treating the first group of samples, and detecting the moisture content and the total content of various heavy metals in the litter mixed samples.

[0043] First, the first group of samples were air-dried; then they were placed in a 60°C oven and baked to constant weight and ground into powder; then they were digested using a HNO3-HClO4 system; finally, the heavy metal content of the digestion solution was analyzed and determined using an inductively coupled plasma mass spectrometer (ICP-MS) to obtain the total content of each heavy metal in the sample (dry weight basis).

[0044] The air-dried moisture content and oven-dried moisture content of the litter mixed sample are calculated by weighing the first group of samples before air-drying, after air-drying, and after oven-drying.

[0045] Step 3, another part of the collected litter mixed sample is taken as the second group of samples, and the second group of samples is pretreated, and then the releasable content of various heavy metals in the litter mixed sample is detected by the litter releasable heavy metal analysis method; the second group of samples are first air-dried; and the air-dried second group of samples are processed into an area of ​​about 1cm 2 Fragments; weigh 2.00g of the second group of fragment samples, and then place the fragment samples in a 100mL centrifuge tube, add 80ml of deionized water for constant temperature oscillation extraction; the extraction conditions are solid-liquid ratio 1:40, temperature 25℃, shaker speed 180rpm, and extraction time 4h; after the reaction is completed, place the centrifuge tube in a low-speed centrifuge at 4800r / min for 10 minutes, pass through a 0.45μm filter membrane, transfer the filtrate to a polytetrafluoroethylene beaker, and use the HNO3-H2O2 system for digestion; use an inductively coupled plasma mass spectrometer for quantitative analysis to calculate the releasable content of various heavy metals in the sample. The calculation formula is:

[0046]

[0047] Where, M is the releasable content of a certain heavy metal in litter (mg / kg); C L is the corresponding heavy metal concentration in the mixed solution in the analysis method of releasable heavy metals in litter (mg / L); L is the volume of the added extraction solution (L); θ A is the air-dried moisture content of litter (dimensionless); m is the weight of debris mixed with the extraction solution (kg).

[0048] When constructing the mixed solution, the weight of the added fragment sample is weighed to be more than 2 g, and the added deionized water is mixed at a ratio of 1:40 to obtain a more stable effect.

[0049] Step 4, calculate the annual release flux of heavy metals per unit area of ​​litter based on the environmental parameters of the contaminated plot obtained in step 1, the air-dried moisture content, oven-dried moisture content and total content of various heavy metals in the litter obtained in step 2, and the releasable content of each heavy metal obtained in step 3.

[0050] The calculation formulas for the litter heavy metal release ratio and the litter heavy metal annual release flux are as follows:

[0051]

[0052] Where R is the ratio of heavy metal release from litter (dimensionless); n is the number of samplings. In this case, sampling was only done once in autumn, so n = 1; θD is the moisture content of litter after drying (dimensionless); C T is the total content of various heavy metals in the first group of samples (dry weight, mg / kg); M A is the annual heavy metal release flux of litter (mg / ha / yr); B is the annual biomass of litter per unit area (kg / ha / yr). By searching the literature, the average biomass of litter in the case site forest and abandoned land was 4298kg / ha / yr, and that in paddy field was 6540kg / ha / yr.

[0053] Step 5: According to the environmental parameters of the polluted plots and the annual release flux of heavy metals in litter, the environmental risk of the decomposition and release of a single heavy metal in the litter with rainfall is evaluated, and the concentration of heavy metals released by the decomposition of litter with rainfall runoff (the heavy metals involved in this embodiment are Cd, Pb, Cu, Zn and As) is calculated, and compared with the Class III limits of Cd (<0.005 mg / L), Pb (<0.05 mg / L), Cu (<1.0 mg / L), Zn (<1.0 mg / L), and As (<0.05 mg / L) content specified in the "Surface Water Environmental Quality Standard" (GB 3838-2002), and the environmental risk and risk pollution level of a single heavy metal in the plant litter of the polluted plots around the abandoned zinc smelter with the decomposition and release of rainfall are evaluated:

[0054]

[0055] In the formula, C w is the concentration of heavy metals released by litter decomposition with rainfall (mg / L); M A is the annual release flux of heavy metals from litter (mg / ha / yr); W is the average annual rainfall volume per unit area (L / ha / yr); is the surface runoff coefficient (dimensionless); I is the underlying surface impermeability (%); the underlying surface impermeability I takes empirical values ​​according to different land use types, among which forest land and abandoned land are 35, and paddy field is 75. i is the risk index of the ith heavy metal (dimensionless); C w,i is the concentration of the ith heavy metal released by litter decomposition with rainfall (mg / L); C s,i is the standard concentration of the ith heavy metal (mg / L).

[0056] When P i <1, the risk level is no pollution; when 1≤P i <2, the risk level is light pollution; when 2≤P i <3, the risk level is moderate pollution; when P i When ≥3, the risk level is severe pollution; standard concentration C s,iRefer to "Surface Water Environmental Quality Standard" (GB 3838-2002).

[0057] Figure 2 The environmental risk results of single heavy metals in litter decomposed and released with rainfall showed that the Cd risk indexes of litter in forest land, abandoned land and paddy field were 1.18, 1.52 and 1.84, respectively. The Cd risk index of litter in the three typical polluted plots was greater than 1, which means that there is a risk of Cd release in litter. The risk indices of Pb and Zn in litter in abandoned land were 1.08 and 1.24, respectively, indicating that litter in abandoned land will also cause Pb and Zn in surrounding surface water bodies to exceed the standard.

[0058] After calculating the risk index of each heavy metal, the comprehensive risk index of heavy metals is calculated according to the Nemerow pollution index method:

[0059]

[0060] Where, P is the comprehensive risk index of heavy metals; P i,max It is the maximum value among all heavy metal risk indices; P i,avg is the average value of the risk index of all heavy metals;

[0061] When P<0.7, the risk level is safe; when 0.7≤P<1, the risk level is warning level; when 1≤P<2, the risk level is moderate risk; when 2≤P<3, the risk level is high risk; when P≥3, the risk level is extremely high risk.

[0062] Figure 3 The comprehensive risk index of medium and heavy metals shows that the comprehensive risk index of heavy metals released from litter in three typical polluted plots is as follows: paddy field (1.34) > abandoned land (1.21) > woodland (0.88), among which the risk level of paddy field and abandoned land is moderate risk, and the risk level of woodland is warning level, indicating that the risk of heavy metal release from litter in paddy field and abandoned land is relatively high, and it is necessary to take risk prevention and control measures such as litter management.

[0063] The above embodiments are preferred embodiments of the present application. Ordinary technicians in this field can also make various changes or improvements on this basis. Without departing from the overall concept of the present application, these changes or improvements should fall within the scope of protection required by the present application.

Claims

1. A method for environmental risk assessment of heavy metal decomposition and release from plant litter in polluted land, characterized in that: include: Step 1, obtaining environmental parameters of the polluted plot and collecting mixed litter samples of the surface plant litter layer; Step 2, taking a portion of the collected litter mixed sample as a first group of samples, and detecting the moisture content and the total content of various heavy metals in the litter mixed sample by pre-treating the first group of samples; Step 3, taking another part of the collected litter mixed sample as a second group of samples, pre-treating the second group of samples, and then detecting the releasable content of various heavy metals in the litter mixed sample by a litter releasable heavy metal analysis method; Step 4, calculating the annual release flux of heavy metals per unit area of ​​litter in the polluted plot according to the data obtained and detected in steps 1-3; Step 5: Based on the environmental parameters of the contaminated plots and the annual release flux of heavy metals in litter per unit area, evaluate the environmental risk of the decomposition and release of single heavy metals in litter with rainfall, as well as the comprehensive environmental risk of the decomposition and release of heavy metals in litter with rainfall.

2. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1 is characterized in that: The environmental parameters include: average annual rainfall, underlying surface impermeability, land use type, main vegetation type and annual biomass of litter.

3. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1 is characterized in that: The method for collecting mixed litter samples is: determine the sample collection time according to the plant leaf fall pattern and crop harvest time, and the sample collection time is autumn or spring and autumn; the collection location is the undecomposed dead branches and leaves layer where the morphological characteristics of the original plant residues can be distinguished by the naked eye; the sample types include but are not limited to dead leaves, dead branches and fallen bark; the sample weight is more than 200g.

4. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1 is characterized in that: Step 2: The method for pretreating the first group of samples and detecting the total content of heavy metals is as follows: first, air-dry the first group of samples; then, bake them in a 60°C oven until constant weight is reached, and grind them into powder; then, digest them using a HNO3-HClO4 system; and finally, perform quantitative detection using an inductively coupled plasma mass spectrometer to obtain the total content of each heavy metal in the samples.

5. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 4 is characterized in that: The air-dried moisture content and oven-dried moisture content of the litter mixed sample are calculated by weighing the first group of samples before air-drying, after air-drying, and after oven-drying.

6. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1 is characterized in that: Step 3: The method for pre-treating the second group of samples and detecting the releasable content of heavy metals is as follows: The second group of samples were first air-dried; then the air-dried second group of samples were processed to an area of ​​about 1cm 2 Weigh a number of fragments, and mix deionized water as an extraction solution with the fragments; The mixed solution was then placed in a centrifuge tube for constant temperature oscillation extraction. The extraction conditions were a solid-liquid ratio of 1:40, a temperature of 25°C, a shaking speed of 180 rpm, and an extraction time of 4 hours. After the reaction was completed, the centrifuge tube was placed in a low-speed centrifuge and centrifuged at 4800 r / min for 10 minutes. The separated residue was filtered, and the filtered mixed solution was digested with a HNO3-H2O2 system. Finally, an inductively coupled plasma mass spectrometer was used to quantitatively analyze the corresponding heavy metal concentrations in the filtered mixed solution, and the releasable content of various heavy metals in the sample was calculated.

7. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 6, characterized in that: The calculation method for the releasable content of various heavy metals in the sample is: Where, M is the releasable content of a certain heavy metal in the litter mixed sample; C L is the corresponding heavy metal concentration in the mixed solution after filtration in the analysis method of releasable heavy metals in litter; L is the volume of the extraction solution; θ A is the air-dried moisture content of the litter mixture sample; m is the weight of the debris mixed with the extraction solution.

8. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1 is characterized in that: The calculation method for step 4 is: Where R is the release ratio of a certain heavy metal in the litter mixed sample; n is the number of sample collections. When sampling is conducted only once in autumn, n = 1; when sampling is conducted in spring and autumn, n = 2; θ D is the drying moisture content of the litter mixed sample; C T is the total content of the corresponding heavy metals in the litter mixed sample; M k ,θ D,k , C T,k are the releasable heavy metal content, drying moisture content, and total heavy metal content corresponding to the kth collection of the mixed sample of fallen objects in a year; M A is the annual release flux of a certain heavy metal in the litter mixed sample; B is the annual biomass of litter per unit area; the weight of the first group of samples is the same as that of the second group of samples.

9. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1, characterized in that: The environmental risk characterization method for evaluating the release of a single heavy metal in litter during decomposition with rainfall is: In the formula, C w is the concentration of heavy metals released by litter decomposition in rainfall runoff; M A is the annual release flux of heavy metals from litter; W is the average annual rainfall volume per unit area; is the surface runoff coefficient; I is the impermeability of the underlying surface; P i is the pollution index of the ith heavy metal; C w,i is the concentration of the ith heavy metal released by litter decomposition during rainfall; C s,i is the standard concentration of the ith heavy metal; When P i <1, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is no pollution; when 1≤P i <2, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is mild pollution; when 2≤P i When P < 3, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is moderate pollution; when P i When ≥3, the environmental risk level of the i-th heavy metal in the litter decomposed and released with rainfall is severe pollution.

10. The environmental risk assessment method for the decomposition and release of heavy metals in plant litter in a polluted plot according to claim 1, characterized in that: The method to characterize the comprehensive environmental risk of heavy metals released from litter during rainfall is as follows: Where, P is the comprehensive pollution index of heavy metals; P i,max It is the maximum value among all heavy metal pollution indices; P i,avg is the average value of all heavy metal pollution indexes; When P<0.7, the comprehensive environmental risk level is safe; when 0.7≤P<1, the comprehensive environmental risk level is warning level; when 1≤P<2, the comprehensive environmental risk level is moderate risk; when 2≤P<3, the comprehensive environmental risk level is high risk; when P≥3, the comprehensive environmental risk level is extremely high risk.

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