Method for evaluating environmental risk of heavy metal release from decomposition of plant litter in contaminated land

Abstract

The application discloses a kind of environmental risk assessment methods of heavy metal decomposition and release of plant litter in contaminated land, comprising: obtaining the environmental parameters of contaminated land;Collecting the litter mixed sample of surface plant litter layer, detecting the moisture content of litter mixed sample and the content of various heavy metals;Through litter releasable heavy metal analysis method, detect the releasable content of various heavy metals in litter mixed sample;According to the data obtained and detected above, calculate the annual release flux of litter heavy metals per unit area of contaminated land;According to the environmental parameters of contaminated land and the annual release flux of litter heavy metals per unit area, evaluate the environmental risk of single heavy metal in litter decomposed and released with rainfall, and the comprehensive environmental risk of litter heavy metals decomposed and released with rainfall.The application can quickly, accurately and economically assess the potential risk of heavy metal decomposition and release of plant litter in contaminated land to the surrounding soil and water environment.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of environmental pollution prevention and control, and particularly relates to a method for evaluating environmental risks of heavy metal decomposition and release of plant litter in a contaminated land. BACKGROUND

[0002] Heavy metal contaminated lands usually exist in areas with intensive industrial and agricultural activities. The soil in these areas has high heavy metal availability, and various plants growing on the soil can absorb excessive heavy metals from the soil, resulting in a large accumulation of heavy metals in the branches and leaves of the plants. After the leaves mature with the growth of the plants, they form plant litter and cover the soil surface, which may cause environmental risks due to the heavy metals carried by the plant litter. Under natural conditions, the plant litter in the contaminated land will gradually release the heavy metals contained therein through physical, chemical and biological processes such as rainwater leaching and microbial activity. These heavy metals can migrate to the surrounding soil or surface water through surface runoff, thereby causing secondary pollution to the surrounding environment and endangering the environment and human health. It is of great significance to clarify the environmental risks of heavy metal release from plant litter in contaminated lands for the prevention and control of soil and water pollution.

[0003] At present, there are few methods for evaluating the flux and environmental risks of heavy metal release from plant litter in contaminated lands. The in-situ litter bag decomposition method or the indoor microcosm soil culture method can be used to analyze the amount of heavy metal release from the litter. However, the traditional in-situ litter bag decomposition method is affected by changes in natural environmental factors in the field, and the determination results of the amount of heavy metal release from the litter have strong randomness. The indoor microcosm soil culture method has a long period and high cost, and it is difficult to quickly obtain the amount of heavy metal release from the plant litter in the contaminated land. SUMMARY

[0004] The present application provides a method for evaluating the environmental risks of heavy metal decomposition and release from plant litter in a contaminated land, which can quickly, accurately and economically evaluate the environmental risks of heavy metal decomposition and release from plant litter in a contaminated land.

[0005] To achieve the above technical purposes, the present application adopts the following technical solutions:

[0006] A method for evaluating the environmental risks of heavy metal decomposition and release from plant litter in a contaminated land, comprising:

[0007] Step 1, obtaining environmental parameters of the contaminated land and collecting a mixed sample of plant litter from the surface plant litter layer;

[0008] Step 2, taking a part of the collected mixed sample of plant litter as a first group of samples, and detecting the moisture content and total content of various heavy metals of the mixed sample of plant litter by pretreating the first group of samples;

[0009] Step 3, another part of the collected mixed sample of litter is taken as a second group of samples, and after pretreatment of the second group of samples, the releasable state content of various heavy metals in the mixed sample of litter is detected by the litter releasable state heavy metal analysis method;

[0010] Step 4, according to the data obtained and detected in steps 1-3, the annual release flux per unit area of litter heavy metals in the contaminated land is calculated;

[0011] Step 5, according to the environmental parameters of the contaminated land and the annual release flux per unit area of litter heavy metals, the environmental risk of a single heavy metal in litter decomposed and released with rainfall and the comprehensive environmental risk of litter heavy metals decomposed and released with rainfall are evaluated.

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

[0013] Further, the method for collecting the mixed sample of litter is: the sample collection time is determined according to the plant leaf fall rule and the crop harvesting time, and the sample collection time is autumn or the two seasons of autumn and spring; the collection site is the undecomposed branch and leaf layer with morphological characteristics of original plant residues that can be distinguished by naked eye; the sample types include but are not limited to dry leaves, dry branches and fallen bark; and the sample weight is more than 200g.

[0014] Further, the method for pretreating and detecting the total content of heavy metals in the first group of samples in step 2 is: first, the first group of samples is subjected to air drying treatment; then, it is placed in a 60℃ oven to bake until constant weight, and is ground into powder; then, it is digested by using HNO3-HClO4 system; finally, it is quantitatively detected by using inductively coupled plasma mass spectrometer to obtain the total content of each heavy metal in the sample.

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

[0016] Further, the method for pretreating and detecting the releasable state content of heavy metals in the second group of samples in step 3 is:

[0017] First, the second group of samples is subjected to air drying treatment; then, the air-dried second group of samples is treated to have an area of about 1cm 2The fragments are weighed, deionized water is mixed with the fragments as an extraction solution, the mixed solution is placed in a centrifugal tube for constant temperature oscillation extraction, the extraction conditions are a solid-liquid ratio of 1:40, a temperature of 25°C, a shaker speed of 180 rpm, and an extraction time of 4 h; after the reaction is completed, the centrifugal tube is placed in a low-speed centrifuge for centrifugation at 4800 r / min for 10 min, the residue is separated and filtered, the filtered mixed solution is digested by using an HNO3-H2O2 system, finally, the corresponding heavy metal concentration in the filtered mixed solution is quantitatively analyzed by using an inductively coupled plasma mass spectrometer, and the releasable content of various heavy metals in the sample is calculated.

[0018] Further, the calculation method of the releasable content of various heavy metals in the sample is as follows:

[0019]

[0020] In the formula, M is the releasable content of a certain heavy metal in the litter mixture sample; C L is the corresponding heavy metal concentration in the filtered mixed solution in the litter releasable heavy metal analysis method; L is the volume of the extraction solution; θ A is the air-dried moisture content of the litter mixture sample; and m is the weight of the fragments mixed with the extraction solution.

[0021] Further, the calculation method of step 4 is as follows:

[0022]

[0023] In the formula, R is the release ratio of a certain heavy metal in the litter mixture sample; n is the sample collection times, n = 1 when sampling only in autumn, and n = 2 when sampling in spring and autumn; θ D is the oven-dried moisture content of the litter mixture sample; C T is the total content of the corresponding heavy metal in the litter mixture sample; M k , θ D,k , C T,k are the releasable heavy metal content, oven-dried moisture content, and total heavy metal content of the litter mixture sample collected in the k-th time in a year, respectively; M A is the annual release flux of a certain heavy metal in the litter mixture sample; B is the annual biomass of litter per unit area; and the weights of the first group of samples and the second group of samples are the same.

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

[0025]

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

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

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

[0029]

[0030] In the formula, P is the comprehensive pollution index of heavy metals; P i,max is the maximum value among the pollution indexes of all heavy metals; P i,avg is the average value of the pollution indexes 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; and when P≥3, the comprehensive environmental risk level is extremely high risk.

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

[0033] Beneficial effects

[0034] The present application is based on the analysis method of releasable heavy metals in litter, obtains the decomposition release parameters of heavy metals in plant litter, further combines the environmental parameters of contaminated land and the annual release flux of heavy metals in litter, evaluates the decomposition release risk of heavy metals in litter of contaminated land, and effectively quantitatively evaluates the environmental risk of heavy metals in litter. The method has high accuracy and evaluation efficiency, wide application range, low implementation cost, and is of great significance to promote the development of environmental risk assessment and remediation of heavy metals in litter in high-pollution areas. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 Flow chart of the environmental risk assessment method for the release of heavy metals from plant litter in a contaminated land plot according to the present application.

[0036] Figure 2 Graph of the risk index of a single heavy metal released from litter in a typical contaminated land plot according to the present application.

[0037] Figure 3 Graph of the comprehensive risk index of heavy metals released from litter in a typical contaminated land plot according to the present application. DETAILED DESCRIPTION

[0038] The embodiments of the present application are described in detail below, which are based on the technical solutions of the present application, and give detailed implementation modes and specific operation processes, and further explain and describe the technical solutions of the present application.

[0039] The environmental risk assessment of the release of heavy metals from plant litter in a contaminated land plot around an abandoned zinc smelting plant in Hunan Province is taken as an example. A method for assessing the environmental risk of the release of heavy metals from plant litter in a contaminated land plot is shown in FIG. 1, which includes the following steps: Figure 1

[0040] Step 1: According to the sampling and investigation results of the heavy metal content in the soil, a representative contaminated land plot is selected; through field investigation and data query, the average annual rainfall, impermeable rate of underlying surface, land use type, main vegetation type and litter annual biomass of the corresponding point are obtained; the research point includes forest land and abandoned land with main vegetation type of deciduous broad-leaved forest and shrub-grass, and paddy field agricultural land with main crop type of rice.

[0041] In addition, according to the plant leaf falling rule and crop harvesting time, autumn sampling (September) is selected, and plant litter samples with obvious plant residual body morphological characteristics are collected.

[0042] Step 2: A part of the collected litter mixed sample is taken as the first group of samples, the moisture content and the total content of various heavy metals of the litter mixed sample are detected by pretreating the first group of samples.

[0043] First, the first group of samples is subjected to air drying treatment; then it is placed in a 60℃ oven to bake to constant weight, and is ground into powder; then it is digested by using HNO3-HClO4 system; finally, the heavy metal content of the digestion solution is analyzed and determined by using inductively coupled plasma mass spectrometer (ICP-MS), and the total content (dry weight) of each heavy metal in the sample is obtained.

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

[0045] Step 3: Another part of the collected mixed sample of litter is taken as the second group of samples, the second group of samples is pretreated, and then the releasable content of various heavy metals in the mixed sample of litter is detected by the releasable state heavy metal analysis method of litter; first, the second group of samples is air-dried; then the air-dried second group of samples is treated into fragments with an area of about 1cm 2 ; 2.00g of the second group of fragment samples is weighed, and then the fragment samples are placed in a 100mL centrifugal tube, 80ml of deionized water is added for constant temperature oscillation extraction; the extraction conditions are as follows: solid-liquid ratio 1:40, temperature 25 DEG C, shaking bed speed 180rpm, extraction time 4h; after the reaction is completed, the centrifugal tube is placed in a low-speed centrifuge at 4800r / min for 10 minutes, 0.45mu m filter membrane is used, the filtrate is transferred to a polytetrafluoroethylene beaker, and HNO3-H2O2 system is used for digestion; inductively coupled plasma mass spectrometry is used for quantitative analysis, and the releasable content of various heavy metals in the sample is calculated. The calculation formula is:

[0046]

[0047] In the formula, M is the releasable content of a certain heavy metal in litter (mg / kg); C L is the concentration of the corresponding heavy metal in the mixed solution in the releasable state heavy metal analysis method of litter (mg / L); L is the volume of the added extraction solution (L); theta A is the air-drying moisture content of litter (dimensionless); and m is the weight of the fragments mixed with the extraction solution (kg).

[0048] In the present application, the weight of the added fragment sample is more than 2g, and the effect of mixing the added deionized water at a ratio of 1:40 is more stable.

[0049] Step 4: According to the environmental parameters of the contaminated land plot obtained in step 1, the air-drying moisture content, the oven-drying moisture content and the total content of various heavy metals of litter obtained in step 2, and the releasable content of each heavy metal obtained in step 3, the annual release flux of heavy metals per unit area of litter is calculated.

[0050] The calculation formula of the release proportion of litter heavy metals and the annual release flux of litter heavy metals is as follows:

[0051]

[0052] In the formula, R is the release proportion of litter heavy metals (dimensionless); n is the sampling times, in this case, only one sampling in autumn, so n = 1; thetaD is the litter moisture content (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), and the average biomass of litter in forest and abandoned land is 4298 kg / ha / yr and that in paddy field is 6540 kg / ha / yr, which are obtained by searching literatures.

[0053] Step 5, according to the environmental parameters of the contaminated land and the annual release flux of litter heavy metals, the environmental risk of the decomposition and release of a single heavy metal in litter with rainfall is evaluated, and the concentration of heavy metals released from litter with rainfall runoff is calculated (the heavy metals involved in this embodiment are Cd, Pb, Cu, Zn and As). The concentration is compared with the III class limit value 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) specified in the Environmental Quality Standards for Surface Water (GB 3838-2002), so as to evaluate the environmental risk and risk pollution level of the decomposition and release of a single heavy metal in plant litter of the contaminated land around the abandoned zinc smelting plant with rainfall.

[0054]

[0055] In the formula, C w is the concentration of heavy metals released from litter with rainfall (mg / L) ; M A is the annual release flux of litter heavy metals (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 impermeability of underlying surface ( % ), and the empirical values of the impermeability of underlying surface I are taken according to different land use types, wherein the values of forest and abandoned land are 35, and the value of paddy field is 75. P i is the risk index of the i-th heavy metal (dimensionless) ; C w,i is the concentration of the i-th heavy metal released from litter with rainfall (mg / L) ; C s,i is the standard concentration of the i-th heavy metal (mg / L).

[0056] When P i < 1, the risk level is non-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 ≥ 3, the risk level is heavy pollution; the standard concentration C s,iRefer to the Environmental Quality Standards for Surface Water (GB 3838-2002).

[0057] Figure 2 The environmental risk results of the release of single heavy metals in the litter decomposition by rainfall show that the Cd risk indexes of the litter in the forest land, the abandoned land and the paddy field are 1.18, 1.52 and 1.84 respectively, and the Cd risk indexes of the litter in the three typical contaminated lands are all greater than 1, which means that the litter has the release risk of Cd; the Pb and Zn risk indexes of the litter in the abandoned land are 1.08 and 1.24 respectively, which shows that the litter in the abandoned land can also make the surrounding surface water exceed the standard of Pb and Zn.

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

[0059]

[0060] In the formula, P is the comprehensive risk index of heavy metals; P i,max is the maximum value in the risk indexes of all heavy metals; P i,avg is the average value in the risk indexes of all heavy metals;

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

[0062] Figure 3 The comprehensive risk indexes of heavy metals show that the comprehensive risk indexes of the heavy metals released by the litter in the three typical contaminated lands from high to low are: the paddy field (1.34) > the abandoned land (1.21) > the forest land (0.88), wherein the risk levels of the paddy field and the abandoned land are moderate risk, and the risk level of the forest land is alert, which shows that the litter heavy metal release risk of the paddy field and the abandoned land is higher, and it is necessary to take litter management and other risk prevention and control measures.

[0063] The above examples are the preferred embodiments of the present application, and those skilled in the art can also make various transformations or improvements on the basis of the above examples, and these transformations or improvements should all belong to the scope of protection claimed by the present application without departing from the general concept of the present application.

Claims

1. A method for evaluating environmental risk of heavy metal release from decomposition of plant litter in a contaminated site, characterized by, The application relates to a method for evaluating the environmental risk of heavy metals in plant litter. The method comprises the following steps: step 1, obtaining environmental parameters of a contaminated land plot and collecting a plant litter mixture sample from a litter layer on the ground surface; step 2, taking a part of the collected plant litter mixture sample as a first group of samples, detecting the water content and the total content of various heavy metals in the plant litter mixture sample by pretreating the first group of samples; step 3, taking another part of the collected plant litter mixture sample as a second group of samples, detecting the releasable content of various heavy metals in the plant litter mixture sample by pretreating the second group of samples and then by a plant litter releasable heavy metal analysis method; step 4, calculating the annual release flux of heavy metals per unit area of the plant litter in the contaminated land plot according to the data obtained and detected in steps 1-3; the calculation mode is as follows: wherein R is the release proportion of a certain heavy metal in the plant litter mixture sample; n is the sample collection times, n=1 when sampling is performed only once in autumn, and n=2 when sampling is performed in spring and autumn; and step 5, evaluating the environmental risk of the decomposition and release of a single heavy metal in the plant litter and the comprehensive environmental risk of the decomposition and release of heavy metals in the plant litter according to the environmental parameters of the contaminated land plot and the annual release flux of heavy metals per unit area of the plant litter. The environmental parameters include the average annual rainfall, the impermeable rate of the underlying surface, the land use type, the main vegetation type and the annual biomass of the plant litter. The method for collecting the plant litter mixture sample is as follows: the sample collection time is determined according to the plant leaf falling rule and the crop harvesting time, the sample collection time is autumn or spring and autumn; the sample collection site is the undecomposed branch and leaf layer with the morphological characteristics of the original plant residual body which can be distinguished by naked eyes; the sample types include dry leaves, dry branches and fallen bark; and the sample weight is more than 200g. The method for pretreating the first group of samples and detecting the total content of heavy metals in step 2 is as follows: the first group of samples are first subjected to air drying treatment; then the first group of samples are placed in a 60 DEG C oven and baked until the weight is constant, and then the first group of samples are ground into powder; then the HNO3-HClO4 system is used for digestion; finally, the inductively coupled plasma mass spectrometer is used for quantitative detection, so that the total content of each heavy metal in the sample is obtained. The air-drying water content and the oven-drying water content of the plant litter mixture sample are calculated by weighing the first group of samples before air drying, after air drying and after oven drying. θ D is the oven-dry moisture content of the mixed litter sample; C T is the total content of the corresponding heavy metal in the mixed litter sample; M k , θ D,k , C T,k respectively are the releasable heavy metal content, oven-dry moisture content and total heavy metal content corresponding to the kth collection of the mixed litter sample in a year; M A is the annual release flux of a certain heavy metal in the mixed litter sample; B is the annual biomass of the litter per unit area; the weights of the first group of samples and the second group of samples are the same; The method for pretreating the second group of samples and detecting the releasable content of heavy metals in step 3 is as follows:

2. The method for assessing environmental risk of heavy metal release from plant litter decomposition in a contaminated site according to claim 1, wherein Then the mixed solution is placed in a centrifugal tube for constant-temperature oscillation extraction, the extraction conditions are as follows: the solid-liquid ratio is 1:40, the temperature is 25 DEG C, the shaking bed speed is 180 rpm, and the extraction time is 4h; after the reaction is completed, the centrifugal tube is placed in a low-speed centrifuge for centrifugation at 4800r / min for 10 minutes, the residue is separated and filtered, the filtered mixed solution is subjected to HNO3-H2O2 system digestion; finally, the inductively coupled plasma mass spectrometer is used for quantitative analysis of the corresponding heavy metal concentration in the filtered mixed solution, so that the releasable content of various heavy metals in the sample is calculated.

3. The method according to claim 1, wherein the method is characterized by, The calculation mode of the releasable content of various heavy metals in the sample is as follows:

4. The method according to claim 1, wherein the method is characterized by, The method for characterizing the environmental risk of the decomposition and release of a single heavy metal in the plant litter is as follows:

5. The method for assessing environmental risk of heavy metal release from plant litter decomposition in a contaminated site according to claim 4, wherein The method for characterizing the comprehensive environmental risk of the decomposition and release of heavy metals in the plant litter is as follows:

6. The method according to claim 1, wherein the method is characterized by, ​ The second set of samples were first air dried; the air dried second set of samples were then processed into pieces having an area of about 1 cm 2 The pieces were weighed and mixed with deionized water as the extraction solution. ​ 7. The method according to claim 6, wherein the method is characterized by, ​ In the formula, M is the releasable state content of a certain heavy metal in the mixed sample of litter; C L is the concentration of the corresponding heavy metal in the mixed solution after filtration in the analysis method of releasable state heavy metals in litter; L is the volume of the extraction solution; θ A is the air-drying moisture content of the mixed sample of litter; and m is the weight of the fragments mixed with the extraction solution.

8. The method according to claim 1, wherein the method is characterized by, ​ wherein C w is the concentration of heavy metals released from the decomposition of the litter with the rainfall runoff; 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 i-th heavy metal; C w,i is the concentration of the i-th heavy metal released from the decomposition of the litter with the rainfall; C s,i is the standard concentration of the i-th heavy metal; When P i <1, the environmental risk level of the i th heavy metal in the litter released with rainfall decomposition is non-pollution; when 1≤P i <2, the environmental risk level of the i th heavy metal in the litter released with rainfall decomposition is slight pollution; when 2≤P i <3, the environmental risk level of the i th heavy metal in the litter released with rainfall decomposition is moderate pollution; when P i ≥3, the environmental risk level of the i th heavy metal in the litter released with rainfall decomposition is severe pollution.

9. The method according to claim 1, wherein the method is characterized by, ​ In the formula, P is a heavy metal comprehensive pollution index; P i,max is the maximum value among the pollution indexes of all heavy metals; P i,av is the average value of the pollution indexes of all heavy metals; When P < 0.7, the comprehensive environmental risk level is safe; when 0.7 ≤ P < 1, the comprehensive environmental risk level is alert; when 1 ≤ P < 2, the comprehensive environmental risk level is moderate risk; when 2 ≤ P < 3, the comprehensive environmental risk level is high risk; and when P ≥ 3, the comprehensive environmental risk level is extremely high risk.

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