AU2020101750A4

AU2020101750A4 - Method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid

Info

Publication number: AU2020101750A4
Application number: AU2020101750A
Authority: AU
Inventors: Lesheng An; Jingjing CAO; Qian Chen; Baoshi Jin; Leyong Kou; Qiang Wang; Qun Zhang; Xiaoqian Zhang; Kuan ZHAO; Baohua ZHOU
Original assignee: Anqing Normal University
Current assignee: Anqing Normal University
Priority date: 2020-08-10
Filing date: 2020-08-10
Publication date: 2020-09-17
Anticipated expiration: 2028-08-10

Abstract

The present invention discloses a method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid, comprising: collection, air drying and sieving of soil samples; preparation of low molecular weight organic acid standard solutions; extraction of a heavy metal from soil with a low molecular weight organic acid and detection; establishing a model for extracting a heavy metal from soil with a low molecular weight organic acid; and determination of an organic acid concentration threshold at the maximum extraction amount of heavy metal from soil. The present invention can quantitatively characterize the ability of low molecular weight organic acids to extract heavy metals from soil, and the extraction results of different soils are comparable. The present invention has a short experiment period and low cost, which facilitates the promotion. The method of the present invention is not restricted by any time and place conditions, can be performed in a laboratory, and has good controllability. The method of the present invention can determine an organic acid concentration threshold for extracting heavy metal from soil, and is applied to the remediation of heavy metal contaminated soils in farmland with good operability and low cost, and is well worth promoting.

Description

METHOD FOR ESTABLISHING A FITTING MODEL FOR EXTRACTING A HEAVY METAL FROM SOIL WITH A LOW MOLECULAR WEIGHT ORGANIC ACID

Technical field

The present invention relates to the technical field of agricultural biological environment, in particular to a method for establishing a fitting model for extracting a heavy metal from the soil with a low molecular weight organic acid.

Background art

Low molecular weight organic acids refer to carbon-chain aliphatic organic compounds containing at least one carboxyl group (-COOH), mainly containing carbon, hydrogen and oxygen, with a relative molecular mass of less than 200, such as formic acid, acetic acid, malic acid, citric acid, oxalic acid, etc. These organic acids perform synthesis, transport, transformation and secretion at the plant-soil interface, have great effects on plant-soil interface processes such as absorption of plant nutrients, activation of soil nutrients, adsorption and desorption of heavy metals, and soil formation, which is an adaptive response of plants to environmental stress.

3 Heavy metals refer to elements with a relative density greater than 5g/cm .

There are more than 10 kinds of common heavy metals in soil, such as copper (Cu), nickel (Ni), cobalt (Co), lead (Pb), zinc (Zn), cadmium (Cd), chromium (Cr), etc., which can be divided into five forms: exchangeable, carbonate, Fe-Mn oxide, organic and residual forms. The migration and transformation process of heavy metals in soil is affected by many factors, such as the nature and morphological characteristics of heavy metals themselves, the physical and chemical properties of soil and the types of plants; moreover, the discharge of waste liquids from industrial and mining enterprises, the sedimentation of heavy metals in the atmosphere, and the use of pesticides and fertilizers in agriculture may cause hard-to-degrade heavy metals to accumulate in the soil, thus resulting in heavy metal pollution in the soil.

Many studies have shown that low molecular weight organic acids can enhance the capacity to extract heavy metals from the soil and promote the increase of content of exchangeable heavy metals, thereby enhancing plant absorption. This is the basic principle of plant extraction and restoration. However, the relationship between the concentration of organic acids and the content of heavy metals extracted therewith is still unclear. Therefore, the establishment of a model between exogenous organic acids and the extracted soil heavy metals is helpful to understand the threshold of organic acids to extract heavy metals from the soil. On the one hand, it can be applied to the remediation of heavy metal soil pollution and the determination of the minimum concentration of combined organic acids, and it is helpful to deepen the theory and technological innovation of plant extraction and restoration; on the other hand, the model is applied to the study of extracting heavy metals from the soil by organic acids secreted from plant root. Organic acids secreted from roots have a strong effect in the rhizosphere area. Heavy metal contaminated soil can be remediated by cultivating species with higher organic acid contents secreted from roots. The combination of phytoextraction and remediation technology with rhizosphere remediation technology improves theoretical and technical level of remediation of heavy metal contaminated soil, thereby reducing the content of heavy metals in the soil and restoring the soil to arable levels.

Summary of the invention

The technical problem to be solved by the present invention is to overcome the defects of the above background art, and the present invention provides a method for establishing a fitting model for extracting a heavy metal from the soil with a low molecular weight organic acid.

The present invention is realized by the following technical solution: a method for establishing a fitting model for extracting heavy metals from soil with a low molecular weight organic acid, comprising:

Step 1: collection, air-drying and sieving of soil samples: collecting surface soils from the field and placing in a marked cloth bag, air-drying naturally in an indoor ventilated area, picking out the roots and debris therein, and then crushing with a wooden stick, sieving through a soil sieve, grinding with a mortar to make them all pass through an aperture sieve, uniformly mixing, sampling by quarter method and storing;

Step 2: preparation of low molecular weight organic acid standard solutions: preparing single low molecular weight organic acid standard solutions with an A.R. grade organic acid, and marking them after the preparation for later use;

Step 3: extraction of heavy metal from the soil with low molecular weight organic acid and detection: adding the prepared single low molecular weight organic acid solutions to weighed and quantitated air-dried soil samples respectively, mixing well, shaking, standing for a period of time, taking respective supernatants, and analyzing the content of the heavy metal in the respective supernatants by an atomic absorption spectrophotometer;

Step 4: establishing a model for extracting a heavy metal from soil with a low molecular weight organic acid: establishing a model between the concentration of organic acid and the content of heavy metal extracted therewith according to the concentrations of organic acid and the heavy metal extracted therewith, the model equation is as follows:

M=Ma+ M. x+M N b+N()

b +'3 in equation (1), N represents the concentration of organic acid (mmol/L); M represents the content of soil heavy metal extracted with organic acid (mg/Kg soil sample); MO represents the content of heavy metal extracted with an organic acid concentration of 0 mmol, i.e. the background value of heavy metal content; Mmax represents the extraction amount when the heavy metal content is saturated by the organic acid concentration, i.e. the maximum extraction amount; b is a constant;

Step 5: determining the organic acid concentration threshold at the maximum extraction amount of heavy metal from soil: obtaining the following equation (2) by incorporating M=Mmax into the equation (1),

M = MO +M=XN b+N (2);

and obtaining an absolute value of the organic acid concentration threshold Nmax used to extract heavy metal from soil as shown in equation (3):

N (MU - MO) b MO (3).

As one of the preferred modes of the present invention, the surface soils in the step 1 are 0-20 cm soils.

As one of the preferred modes of the present invention, the soil sieve in step 1 is of a 100-mesh.

As one of the preferred modes of the present invention, the aperture sieve in step 1 is of a 200-mesh.

As one of the preferred modes of the present invention, the mortar in step 1 is an agate mortar.

As one of the preferred modes of the present invention, the organic acid can be either formic acid or malic acid.

As one of the preferred modes of the present invention, the concentration of the low molecular weight organic acid standard solution is 0-100 mmol/L.

As one of the preferred modes of the present invention, the standing after mixing well and shaking in step 3 is carried out for 2 hours.

Compared with the prior art, the present invention has the following advantages: (1) the present invention can quantitatively characterize the capacity of low molecular weight organic acids to extract heavy metals from the soil, and the extraction results of different soils are comparable; (2) the present invention has a short experimental period and low cost, which facilitates the promotion;(3) the method of the present invention is not restricted by any time and place conditions, and can be performed in a laboratory with good controllability;(4) the method of the present invention can determine the organic acid concentration threshold for extracting heavy metals from the soil, and is applied to the remediation of heavy metal contaminated soil in farmland with good operability and low cost, and is well worth promoting.

Embodiments

The following is a detailed description of the examples of the present invention.

These examples are implemented on the premise of the technical solution of the present invention. Detailed embodiments and specific operation procedures are given, but the protection scope of the present invention is not limited to the following examples.

The experimental principle of the present invention is as follows. A low molecular weight organic acid is the most active form of carbon cycle at the plant-soil interface. It contains at least one carboxyl group with a molecular weight below 200, can regulate the nutrient cycle at the plant-soil interface and respond to adversity stress, and adjust the absorption-desorption of heavy metals, etc. The concentration and existing form of heavy metals in the soil are affected by many factors. Low molecular weight organic acids can regulate the existing form of heavy metals in the soil; they can react with heavy metals bound to carbonates, heavy metals bound to Fe-Mn oxides etc. in the soil to increase the content of free-form heavy metals in the soil. Many studies have shown that low molecular weight organic acids can increase the concentration of free-form heavy metals in the soil. However, the relationship between the concentration of specific organic acids and the content of heavy metals extracted therewith is still unclear. Therefore, it is very necessary to establish a relationship between the concentration of organic acids and the content of heavy metals extracted therewith to obtain an organic acid concentration threshold quantitatively. The application of this technology can improve the remediability of heavy metal contaminated soil and restore the soil to arable level.

Based on the above experimental principle, the present invention provides a method for establishing a fitting model for extracting a heavy metal from the soil with a low molecular weight organic acid, the method comprising:

Step 1: collection, air-drying and sieving of yield soil samples

collecting surface soils (0-20cm) randomly from the field and placing in a marked cloth bag, air-drying naturally in an indoor ventilated area, picking out the roots and debris therein, and then crushing with a wooden stick, sieving through a soil sieve of a 100-mesh, further grinding with an agate mortar to make them all pass through an aperture sieve of a 200-mesh, uniformly mixing, sampling by quarter method, and storing;

Step 2: preparation of low molecular weight organic acid solution

preparing standard solutions of a single low molecular weight organic acid having a concentration of 0-100 mmol/L (0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively, 8 concentration gradients in total) with an A.R. (analytical reagent) grade organic acid, and marking the prepared solutions for later use;

Step 3: extraction of a heavy metal from soil with a low molecular weight organic acid and detection

weighing 2.0 g soil samples and placing them in 8 triangular flasks respectively, adding 50ml of prepared single low molecular weight organic acid (formic acid, malic acid) standard solutions (having a concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L) to the 8 triangular flasks respectively, mixing well, shaking, and standing for 2 h, taking the respective supernatants respectively, and analyzing the contents of heavy metal in the respective supernatants by atomic absorption spectrophotometer;

Step 4: establishing a model for extracting a heavy metal from soil with a low molecular weight organic acid

establishing a model between the concentration of organic acid and the content of heavy metal extracted therewith according to the concentrations of organic acid and the contents of the heavy metal extracted therewith, the model equation is as follows:

M=M+ M. xN b+N (IV)

in equation (1), N represents the concentration of organic acid (mmol/L); M represents the content of soil heavy metal extracted with organic acid (mg/Kg soil sample); MO represents the content of heavy metal extracted with an organic acid concentration of 0 mmol, i.e. the background value of heavy metal content; Mmax represents the extraction amount when the heavy metal content is saturated by the organic acid concentration, i.e. the maximum extraction amount; b is a constant;

Step 5: determining the organic acid concentration threshold at the maximum extraction amount of heavy metal from the soil

the above Mmax represents the extraction amount when the content of heavy metal is saturated by the concentration of organic acid, obtaining the following equation (2) by incorporating M=Mmax into the equation (1),

M = M + Mx b+N (2);

and obtaining the absolute value of the organic acid concentration threshold Nmax used to extract heavy metal from the soil as shown in equation (3):

INO = (MI - M)b

The present invention is further illustrated by the following 8 specific examples:

Example 1

Surface soils (0-20cm) were randomly collected from the field and placed in a marked cloth bag. Soils in 10 areas in total were collected and marked as S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, respectively, air-dried naturally in indoor ventilated area, and the roots and debris in the soils were picked out. The soils were crushed with a wooden stick, passed through a soil sieve with a 100-mesh, and then were ground with an agate mortar to make them all pass through an aperture sieve of a 200-mesh. After uniformly mixed, samples are taken by quarter method and stored;

Preparation of formic acid standard solutions: single formic acid standard solutions having a concentration of 0-100 mmol/L (0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively, 8 concentration gradients in total in total) were prepared with an is A.R. (analytical reagent) grade formic acid, and marked after the preparation for later use;

Extraction of heavy metal from the soil with formic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks respectively, 50ml of the prepared single formic acid standard solutions (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively) were added to the triangular flasks respectively, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal copper (Cu) in the supernatants was analyzed by atomic absorption spectrophotometer;

x N M=M+Mi b+N

Establishment of a model for extracting heavy metal Cu from the soil with formic acid: establishing a model between the concentration of formic acid and the content of Cu extracted therewith by means of equations according to the concentrations of formic acid and the contents of Cu extracted therewith, see table 1;

M xN ME = MO + mx b+N

(M - M) MO

Determination of formic acid concentration threshold at the maximum extraction amount of Cu from the soil: the above Mmax represents the extraction amount when Cu content was saturated by the concentration of formic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of formic acid concentration threshold used to extract the maximum amount of Cu from the soil was obtained, as shown in table 1:

Table 1: Fitting equation for extracting Cu with formic acid and formic acid concentration threshold at the maximum extraction amount of Cu

formic acid determinant concentration soil sample fitting equation coefficient P threshold R2

2. 0573 x N SI M =0.3674+ 7 0.9818 0.0024 367.6935 79.9400 V S2 M 0.5914+ O0'715Y 0.9860 0.0017 6935.315 398.9791+ N

S3 M 0.1339 - 0.6571x N 0.9315 0.0179 112.1703 28.7072 i N S4 M =0.1646+ 0.5988 Y N 0.9734 00043 53.47466 20.2716+ N SS M= 0.2521 L.8483x N 0,9908 0,0092 1522.142 240.4035+ N

S6 M =0.2046+ 0.3421x N 0.8746 0.0157 21.93367 32.6373+ N S7 M =0.1240 10.4732 x N 0.9595 0.0082 28.8504 10.2447 +AN

S8 U.0.1639+ 0.3226xN 0.9242 0.0209 23.88043 24.6629+ N S9 M= .1117+ 063 18xV 0.9626 0.0374 37.619 5.6517-+N Sio A 0.1278 + 0.5002x.N 0.9778 0.0033 10.333 2.1028+ N

Note: M represents the content of Cu extracted with formic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 2:

Surface soils (0-20cm) were randomly collected from the field and placed in a marked cloth bag. Soils in 10 areas in total were collected and marked as S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, respectively, air-dried naturally in indoor ventilated area, and the roots and debris in the soils were picked out. The soils were crushed with a wooden stick, passed through a soil sieve with a 100-mesh, and then were ground with an agate mortar to make them all pass through an aperture sieve of a 200-mesh. After uniformly mixed, samples are is taken by quarter method and stored;

Preparation of formic acid standard solutions: single formic acid standard solutions having a concentration of 0-100 mmol/L (0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively, 8 concentration gradients in total) were prepared with an A.R. (analytical reagent) grade formic acid, and marked after the preparation for later use;

Extraction of heavy metal from the soil with formic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, were added to the 8 triangular flasks 50ml of prepared single formic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively), mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal zinc (Zn) in the supernatants was analyzed by atomic absorption spectrophotometer;

M=M M xN 0 b+N Establishment of a model for extracting heavy metal Zn from the soil with formic acid: establishing a model between the concentration of formic acid and the content of Zn extracted therewith by means of equations according to the concentrations of formic acid and the contents of Zn extracted therewith, see table 2;

M =MMmax xN Mr= = MO + M ,

b+N

(M 1 -M) MO Determination of formic acid concentration threshold at the maximum extraction amount of Zn from the soil: the above Mmax represents the extraction amount when Zn content was saturated by the concentration of formic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of formic acid concentration threshold used to extract the maximum amount of Zn from soil was obtained, as shown in table 2:

Table 2: Fitting equation for extracting Zn with formic acid and formic acid concentration threshold at the maximum extraction amount of Zn

formic acid concentration determinant 2 soil sample fitting equation coefiint P threshold coefficient R

(mmo/L)

Si M = 0.3739+ 10.3336xN 0.9421 0.0034 797.069 29.9230 + N S2 M 0.6052+ 6.6277x N 0.9574 0.0004 138.5394 13.9218+ N S3 M 0.6567 + 6.0396 N 0.9753 0.0006 266.0638 32.4591 A S4 M 1,3477+ 9.0833x N 0.9583 0.0095 411.7977 71.7436+ N S5 M =0.4416+5.5662xN 0.9590 0.0017 185.9826 16.0266+ N xN M= 0.5243+ 29.9051 S6 132.5689+ N 0.9956 0.0044 7428.915

S7 M =0.9650+ 5'7456x N 0.9123 0.0023 39.70028 8.0138+N SS M=-.3343-+ 7MbxN 0.9615 0.0015 101.3385 21.7760+ N 59 A =L.1580+- 3.730 x N 0.8083 0.0367 26.78519 1 .4244+ N S10 M =-0.5184+ 2.9579 x N 0.9731 0.0007 14.2492 2.1249+ N

Note: M represents the content of Zn extracted with formic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 3:

Extraction of heavy metal from the soil with formic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, 50ml of prepared single formic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively) were added to the 8 triangular flasks, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal lead (Pb) in the supernatants was analyzed by atomic absorption spectrophotometer;

M= 0+ Mra xN b+N Establishment of a model for extracting heavy metal Pb from the soil with formic acid: establishing a model between the concentration of formic acid and the content of Pb extracted therewith by means of equations according to the concentrations of formic acid and the contents of Pb extracted therewith, see table 3;

Mi= MO + "m xN b+N

(M - )•b N"I M

Determination of formic acid concentration threshold at the maximum extraction amount of Pb from the soil: the above Mmax represents the extraction amount when Pb content was saturated by the concentration of formic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of formic acid concentration threshold used to extract the maximum amount of Pb from soil was obtained, as shown in table 3:

Table 3: Fitting equation for extracting Pb with formic acid and formic acid concentration threshold at the maximum extraction amount of Pb formic acid determinant concentration soil sample fitting equation coefficient P threshold R2 INMIA(mmol/L)

SI M = 4.7692+ 0.8911 0.0039 16.54067 22.4120 +N S2 M = 4.224+ 7.6452 x N 0.9384 0.0009 6305831 10.1884+N S3 M =5.5118+ 6.0955x N 0,9235 0.0016 0.738071 6.9695+ N

S4 M =6.4585+ 8.6362 N 0.9706 0.01 6,800048 20.1672 + N S5 M -5.81U3+ 7.2568xN 0.7998 0.0179 5.489144 22.2212+ N S6 M =6.5616+ 8.6141 x N 0390 0,0009 61665 19.7781+N N M=6.6105+ 12,6623x LS 7 121,2140+N 07714 0.0250 110.9693

8 Al 6.3592 + .-_ 09477 0-0006 2.45635 6, S467+N S9 M =4.6851+ 7.9148 x N 0.8432 0.0097 9.669936 140275+N S10 M =6.2992+.4 09707 0.0001 1302633 45 0854 + N

Note: M represents the content of Pb extracted with formic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 4:

Extraction of heavy metal from the soil with formic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, 50ml of prepared single formic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, is 100 mmol/L, respectively) were added to the 8 triangular flasks, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal manganese (Mn) in the supernatants was analyzed by atomic absorption spectrophotometer;

M=M 0 ± Mn=xN M xN b+ N Establishment of a model for extracting heavy metal Mn from the soil with formic acid: establishing a model between the concentration of formic acid and the content of Mn extracted therewith by means of equations according to the concentrations of formic acid and the contents of Mn extracted therewith, see table 4;

M = M +MxN b+N

(M1 - M - MO

Determination of formic acid concentration threshold at the maximum extraction amount of Mn from the soil: the above Mmax represents the extraction amount when Mn content was saturated by the concentration of formic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of formic acid concentration threshold used to extract the maximum amount of Mn from soil was obtained, as shown in table 4:

Table 4: Fitting equation for extracting Mn with formic acid and formic acid concentration threshold at the maximum extraction amount of Mn

formic acid determinant concentration soil sample fitting equation coefficient P threshold R2 N. (mmol/L)

S1 M -1.5949+ 120.61 26 xN 0.9714 <0.0001 3825.98 49.9319+ N

S2 At= 0 9 0 84 100.9743 x N 0.9773 <0.0001 3751.127 34.0528+ N S3 M -1.3781 123,2778x N D.9940 <0.0001 6490.21 71.508 4 N S4 M = 1.8369 229.4936 XA 0.9925 <0.0001 22176.6 176.0954 + N

S5 A= -1.7494+ 125.5557 x N 09796 Ofw0 383276 52.6690+ N S6 M -5. 38 8 4 , 510.1632 x N 0.9945 <0.0001 14669.8 153,3250+ N S7 M = 2.4550+ 73.356 x M 0.9836 <0.0001 1168.743 40.4671+ N

88 A = 2.0635+ 66.9-57 x N 0.9669 0.0002 859.7375 27.3387 +N S9 W =0.7552 114,0929x N 0.9828 <0.0001 5705.59 38.079+ N SIO A =0.0343 +-1243477x N 0.9935 <c-0.0001 526634,7 145.3067+ N

Note: M represents the content of Mn extracted with formic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 5:

Preparation of malic acid standard solutions: single malic acid standard solutions having a concentration of 0-100 mmol/L (0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively, 8 concentration gradients in total) were prepared with an A.R. (analytical reagent) grade malic acid, and marked after the preparation for later use;

Extraction of heavy metal from the soil with malic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, 50ml of prepared single malic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively) were added to the 8 triangular flasks, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal copper (Cu) in the supernatants was analyzed by atomic absorption spectrophotometer;

M=M MxN 0 b+N

Establishment of a model for extracting heavy metal Cu from the soil with malic acid: establishing a model between the concentration of malic acid and the content of Cu extracted therewith by means of equations according to the concentrations of malic acid and the contents of Cu extracted therewith, see table 5;

M=MO M xN b+N

(M -M) MO

Determination of malic acid concentration threshold at the maximum extraction amount of Cu from the soil: the above Mmax represents the extraction amount when Cu content was saturated by the concentration of malic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of malic acid concentration threshold used to extract the maximum amount of Cu from the soil was obtained, as shown in table 1:

Table 5: Fitting equation for extracting Cu with malic acid and malic acid concentration threshold at the maximum extraction amount of Cu

malic acid determinant concentration soil sample fitting equation coefficient P threshold R2 INI (nmol/L)

SI M = .2286+ 164908xAN 0.9930 <0.0001 6575190711 9127.3626+ N S2 Al =0.3243+ 9.8783 x 0.9660 0.0002 2459.470046 ) 83,4840+-N S3 M =-0.2263+ 2,769x N 0.9650 0.0350 398.5146879 30.7459+N S4 M=0.0421+ 7.1845xN 0.9941 0.0059 30756,4977 181,2%4+N S5 MA= -. 0024 + 1.0433x N 0.9947 0.0004 5801.979308 S3.3162+ N S6 M= -0.8333+ 2.4944-xN 0.9960 0.0040 26.10685077 6,5375+ N S7 M 0.1944+ 1.9529xN 0.9818 0.0025 1444.707254 159.7106+ N s$ M =0.1253+ 0.9436 0,0008 248.1044577 23,4995+ N 89 M =0.1401 - 0.7676x N 0.9946 <0.0001 21.27498216 4.7500+ N SIO Al- 0.2865+-. 9737x 0.9679 0.0010 18.4562971 7.6946+;N

Note: M represents the content of Cu extracted with malic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 6:

Extraction of heavy metal from the soil with malic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, 50ml of prepared single malic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively) were added to the 8 triangular flasks, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal zinc (Zn) in the supernatants was analyzed by atomic absorption spectrophotometer;

M=M M.x N 0 b+N Establishment of a model for extracting heavy metal Zn from the soil with malic acid: establishing a model between the concentration of malic acid and the content of Zn extracted therewith by means of equations according to the concentrations of malic acid and the contents of Zn extracted therewith, see table 6;

MmaM =MO =M + mna xN ,

b+N

(M -M 0 Nj|= m •eb, )

M0

Determination of malic acid concentration threshold at the maximum extraction amount of Zn from the soil: the above Mmax represents the extraction amount when Zn content was saturated by the concentration of malic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of malic acid concentration threshold used to extract the maximum amount of Zn from soil was obtained, as shown in table 6:

Table 6: Fitting equation for extracting Zn with malic acid and malic acid concentration threshold at the maximum extraction amount of Zn

malic acid determinant concentration soil sample fitting equation coefficient P threshold R2 I (mmnoUL)

6.1633.xNV Si M = -0.2973+ N 0.9104 0.0080 135.7269273 6.2458+N S2 M = 0.7489+ 8,5948 x N 09918 <0.0001 202.2846721 19.3083+,N S3 M = 0.5374+ 5.4869 N 0.9561 0,0019 87.99868385 9.5546+ N S4 M = -0.8397+ 13.0686 x-N 0.9974 0.0001 245.4135022 14,8166_+N

S5 M =1.5361+ 5.2707 N 09990 <0.0001 62.17023459 25.5716+ N S6 M = -0.8 7 19 30334xN 0.9894 0.0001 1514.844908 41.39731 N S7 M = 0 .9 4 17 + 4.4326x N 0.8297 0.0120 25.90353821 6.9877 N 58 M =0.8532+ 12.2889x N 0.9853 0.0002 199.7507978 14.9031+N 59 H =1.0777+ -5.2906xN 0.8293 0.0120 80.69636579 20.6429+ N S10 M= 1.3322+ 21.0449xN 0.9779 0.0005 2875.812439 194.3497+N

Note: M represents the content of Zn extracted with malic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 7:

Extraction of heavy metal from the soil with malic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, 50ml of prepared single malic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively) were added to the 8 triangular flasks, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal lead (Pb) in the supernatants was analyzed by atomic absorption spectrophotometer;

M=M MxN b+ N Establishment of a model for extracting heavy metal Pb from the soil with malic acid: establishing a model between the concentration of malic acid and the content of Pb extracted therewith by means of equations according to the concentrations of malic acid and the contents of Pb extracted therewith, see table 7;

Mm xN MI= = MO + - ,

b+N

(M -M) MO

Determination of malic acid concentration threshold at the maximum extraction amount of Pb from the soil: the above Mmax represents the extraction amount when Pb content was saturated by the concentration of malic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of malic acid concentration threshold used to extract the maximum amount of Pb from soil was obtained, as shown in table 7:

Table 7: Fitting equation for extracting Pb with malic acid and malic acid concentration threshold at the maximum extraction amount of Pb

malic acid determinant concentration soil sample fitting equation coefficient P threshold R2 |NI (mnmoL)

SI M=4.6003+ -5.4122xN 0.9478 0.0006 365.531883 -167.9457+ N S2 M = 4.6903+ -1.2714 x N 0.9929 <0.0001 142.4490782 112.0702+ N S3 Al- 3.5493- 3.0297 x N 0.9449 0.007 4.472164246 30.5486+ N S4 M =1.7893+2268x N 0.9410 0.0008 2.230282094 0.89931 N S5 M =5.0134+ 2.7803xN 0.8349 0.0111 10.82906955 24.3117+ N 86 Al =6.1874+ 5.0275 x N 0.8935 0.0037 6.423615957 34.2663+ N

S7 M -4.1885+ 73 0.9074 0.0026 7.89761337 214.7136+ N S8 M = 5.3301+ -7.3217,xr 0.9259 0.0015 605.5624169 -255.1185+,N S9 M= 3.3758 3. 136 8 x.N 0.9484 0.0006 0.604027579 8.5317+N S10 M=3.4454+ 3.4041xN 0.8848 0.0045 0.133869586 11,1679+N

Note: M represents the content of Pb extracted with malic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

Example 8:

Extraction of heavy metal from the soil with malic acid and detection: 2.0 g soil samples were weighed and placed in 8 triangular flasks, respectively, 50ml of prepared single malic acid standard solution (having an concentration of 0 mmol/L, 1 mmol/L, 2 mmol/L, 5 mmol/L, 10 mmol/L, 20 mmol/L, 50 mmol/L, 100 mmol/L, respectively) were added to the 8 triangular flasks, mixed well, and shaked for 2 h. Then the respective supernatants were taken respectively, and the content of heavy metal manganese (Mn) in the supernatants was analyzed by atomic absorption spectrophotometer; b+N Establishment of a model for extracting heavy metal Mn from the soil with malic acid: establishing a model between the concentration of malic acid and the content of Mn extracted therewith by means of equations according to the concentrations of malic acid and the contents of Mn extracted therewith, see table 8;

M=M~zMM =MO+ M xN , b+N

(M -M) Nna |= "M •~b, a MO

Determination of malic acid concentration threshold at the maximum extraction amount of Mn from the soil: the above Mmax represents the extraction amount when Mn content was saturated by the concentration of malic acid, by incorporating M=Mmax into the equation (1), the equation can be obtained. The absolute value of malic acid concentration threshold used to extract the maximum amount of Mn from soil was obtained, as shown in table 8:

Table 8: Fitting equation for extracting Mn with malic acid and malic acid concentration threshold at the maximum extraction amount of Mn

determinant malic acid soil sample fitting equation coefficient P concentration R2 threshold

SI M= 2.72(X- 2.04 xN 0.9834 <0.00 1 28626472 38.9718+,%

S2 Al 410319+ 244.9X65yx N 0.9841 <0.0001 2,6397.3225 37.2873- N S3 M= -0,4331 + 34523 0.9854 <0.0001 39263.72118 49.3271 ,,

S4 M- 15.3273 - 408.3766 x 0.9768 <0.0001 1022.863577 37.0021 A S5 l= -2.8498+ 322.9455xN 0.9794 <0.0001 3873.212017 33.8798+ N S6 Al 19A887 + 44 2 .4147 KN 0.9699 0.0002 529.4894235 22.3403 - N 57 275,0166xN S7 Al 10.7299 2 6 0.9901 <0.0001 1351300885 54.8621+ N Sa M -4.671 1+179.025 5 x' N 0.9912 <0.000 I 1350114683 36.1707 + N S9 M =7.71 4 2 7 0.9832 <0.0001 1650193706 42.9732+ N

S10 Mf =L9285+ 2 0.9902 <0.000I 4888 613331 385275- N

Note: M represents the content of Mn extracted with malic acid (mg/Kg soil sample), and N represents the concentration of organic acid (mmol/L).

The technical effects of the eight examples were as follows:

M x N b+N The results of the above eight examples are as follows: it is quite feasible to characterize the extraction of a heavy metal from soil with a low molecular weight organic acid using the model equation. The determinant coefficient R2 of the fitting equation is between 0.8293-0.9990, and is significant (P<0.05). The fitting equation for extracting manganese from soil with formic acid in Example 4 and the fitting equation for extracting manganese from soil with malic acid in Example 8 have very strong significance (P<0.005), which is indicates that the fitting model equations are quite feasible. Malic acid

(HOOCCH2CHOHCOOH) contains two carboxyl groups, it is dissociated in two steps with dissociation constants of Kal= 4.x1O4 and Ka2=8.9x10- 6

, respectively; formic acid (HCOOH) has only one carboxyl group, and its dissociation constant is Ka=1.8xlO4, so the ability of extracting heavy metals from soil is malic acid>formic acid, which is consistent with other studies; moreover, the order of the extraction of heavy metals by the two organic acids is Mn>Pb>Cu>Zn, which is related to the total amount and existing forms of the four elements in soil, which is consistent with the actual situation. At the same time, the calculated organic acid concentration threshold at the maximum extraction amount of heavy metals has a certain reference value, which provides theoretical support for the immobilization of heavy metals and phytoremediation in soil, and has important significance for the remediation of heavy metal contaminated soil.

The above are only preferred embodiments of the invention and are not intended to limit the present invention. Any modification, equivalent replacement and improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid, comprising:

Step 3: extraction of a heavy metal from soil with a low molecular weight organic acid and detection: adding the prepared single low molecular weight organic acid solutions to weighed and quantitated air-dried soil samples respectively, mixing well, shaking, standing for a period of time, taking respective supernatants, and analyzing the content of heavy metal in the respective supernatants by an atomic absorption spectrophotometer;

M M + b+XN xN b+N (I)

wherein in equation (1), N represents the concentration of organic acid (mmol/L); M represents the content of heavy metal from soil extracted with organic acid (mg/Kg soil sample); MO represents the content of heavy metal extracted with an organic acid concentration of 0 mmol, i.e. the background value of heavy metal content; Mmax represents the extraction amount when the heavy metal content is saturated by the organic acid concentration, i.e. the maximum extraction amount; b is a constant;

Step 5: determining an organic acid concentration threshold at the maximum extraction amount of heavy metal from soil: obtaining the following equation (2) by incorporating M=Mmax into the equation (1),

Mir M xN M = MO + M=k b+N (2);

N..0, I (M1 -MO).b

MOU (3).

2. The method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid according to claim 1, wherein the surface soils in step 1 are 0-20 cm soils; the soil sieve in step 1 is of a

100-mesh; and the aperture sieve in step 1 is of a 200-mesh.

3. The method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid according to claim 1, wherein the mortar in step 1 is an agate mortar.

4. The method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid according to claim 1, wherein the organic acid is any of formic acid and malic acid; and the concentration of the low molecular weight organic acid standard solutions is 0-100mmol/L.

5. The method for establishing a fitting model for extracting a heavy metal from soil with a low molecular weight organic acid according to claim 1, wherein the standing after mixing well and shaking in step 3 is carried out for 2 hours.