CN112892244B

CN112892244B - Biochar modified film for DGT device, DGT device and manufacturing method thereof

Info

Publication number: CN112892244B
Application number: CN202110060637.XA
Authority: CN
Inventors: 王翠苹; 朱青; 薛亚楠; 梁亚峰; 祝凌燕
Original assignee: Nankai University
Current assignee: Nankai University
Priority date: 2021-01-15
Filing date: 2021-01-15
Publication date: 2022-02-15
Anticipated expiration: 2041-01-15
Also published as: CN112892244A

Abstract

The application provides a biochar modified film for a DGT (differential global positioning System) device, which is prepared from biochar powder and agar, wherein the biochar powder is uniformly dispersed in the film; also provided is a method for making a biochar-modified film for a DGT device, comprising: mixing and uniformly dispersing the charcoal powder and ultrapure water to obtain a charcoal mixed solution; mixing a proper amount of agar solution with the biochar mixed solution uniformly to obtain biochar-agar mixed solution; and forming a solidified film by the agar-biochar mixed solution, thereby obtaining the biochar modified film. In addition, the DGT device adopting the biochar modified film as the combined film is also provided. The scheme of the application can be applied to prediction of in-situ heavy metal effectiveness of different types of soil, can realize prediction of biological effectiveness of non-metal elements such As As, and has the advantages of capability of predicting effectiveness of various metal/non-metal ions in different forms, high prediction efficiency, low cost and the like.

Description

Biochar modified film for DGT device, DGT device and manufacturing method thereof

Technical Field

The application belongs to the technical field of environmental pollution treatment, and particularly relates to a biochar modified film for a DGT device, the DGT device and a manufacturing method thereof, which are particularly suitable for predicting the biological effectiveness of in-situ heavy metal in soil of a polluted site.

Background

China is a country seriously harmed by heavy metal pollution, and the potential harm of the heavy metal pollution of soil draws wide attention. The bioavailability of heavy metals is a key parameter for measuring the mobility and ecological influence of heavy metal elements, and the current evaluation method for the bioavailable state of the heavy metals in soil mainly comprises a film diffusion gradient method (DGT). The DGT technology mainly utilizes a free diffusion principle (Fick's first law), and obtains information of (biological) effective state content and spatial distribution, ionic state-complex state binding kinetics and solid-liquid exchange kinetics of a target in an environmental medium through research on the gradient diffusion of the target in a diffusion layer and a buffer kinetic process of the target. The DGT device is formed by superposing a fixed layer (namely a fixed membrane) and a diffusion layer (a diffusion membrane and a filter membrane), target ions pass through the diffusion layer in a diffusion mode, are captured by the fixed membrane and form linear gradient distribution in the diffusion layer. The DGT technology has the characteristics of perfect theoretical system, mature technology, small required equipment, simple operation, high result reliability, strong practicability, easy large-area popularization and application and the like, so the DGT technology is one of the most potential in-situ sampling technologies rapidly since the 90 s of the 20 th century.

However, the traditional DGT technology does not consider the migration process of heavy metals between solid-liquid phases in soil, and ignores the concentration of soluble organic matter (DOM) in soil, pH and Cl with different concentrations^-/NO₃ ^-/SO₄ ^2-And Na⁺/Mg²⁺The regional applicability is not strong due to the influence of hydration conditions such as anion and cation strength, and the application of DGT technology in the prediction of the biological effectiveness of heavy metals in a polluted site is still lacking at present. Market expectation researchers develop novel prediction technologies and related equipment for the bioavailability of heavy metals in site-contaminated soil.

Disclosure of Invention

The application aims to provide a biochar modified film for a DGT device, the DGT device and a manufacturing method thereof, and the biochar modified film for the DGT device has the advantages of being capable of predicting the effectiveness of various metal/non-metal ions in different forms, high in prediction efficiency, low in cost and the like.

In order to achieve the purpose, the following technical scheme is adopted in the application:

a biochar modified film for a DGT device is prepared from biochar powder and agar, wherein the biochar powder is uniformly dispersed in the film.

In a preferred embodiment of the biochar-modified film for a DGT device, the biochar powder has a particle size of 50 μm or less (e.g., 45 μm, 40 μm, 35 μm, 30 μm, 25 μm, 20 μm, etc.).

In the biochar modified film for the DGT device, the biochar powder is optionally prepared from rice straws, corn straws, peanut shells, coconut shells, bamboo leaves and the like, and the application does not limit the biochar modified film.

In a preferred embodiment of the biochar-modified film for a DGT device, the mass ratio of the biochar powder to the agar is 0.5-3.5:1 (e.g., 1:1, 1.5:1, 2:1, 2.5:1, 3:1, etc.).

In the above biochar modified film for a DGT device, as a preferred embodiment, the thickness of the biochar modified film is 0.3-0.5mm (e.g., 0.32mm, 0.35mm, 0.40mm, 0.45mm, 0.48mm, etc.).

A method for making a biochar modified film for a DGT device, comprising:

mixing and uniformly dispersing the charcoal powder and ultrapure water to obtain a charcoal mixed solution;

mixing a proper amount of agar solution with the biochar mixed solution uniformly to obtain biochar-agar mixed solution;

and forming a solidified film by the agar-biochar mixed solution, thereby obtaining the biochar modified film.

In a preferred embodiment of the method for producing a biochar-modified film for a DGT device, the biochar powder has a particle size of 50 μm or less (e.g., 45 μm, 40 μm, 35 μm, 30 μm, 25 μm, 20 μm, etc.).

In the method for manufacturing a biochar-modified film for a DGT device, as a preferred embodiment, the mass ratio of the biochar powder to the ultrapure water in the biochar mixed solution is 1:8-15 (e.g., 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, etc.).

In the above method for manufacturing a biochar-modified thin film for a DGT device, as a preferred embodiment, the method for preparing the agar solution comprises: mixing agar powder and ultrapure water at a mass ratio of 1-3:100 (such as 1.2:100, 1.5:100, 2:100, 2.5:100, 2.8: 100), heating, and keeping at 75-85 deg.C (such as 76 deg.C, 78 deg.C, 80 deg.C, 82 deg.C, 84 deg.C) to make the solution transparent.

In the above method for manufacturing a biochar-modified thin film for a DGT device, as a preferred embodiment, the volume ratio of the biochar mixed solution to the agar solution is 1:2 to 4 (e.g., 1:2.5, 1:3, 1:3.5, etc.); more preferably 1: 2.5-3.

In the above method for producing a biochar-modified film for a DGT device, according to a preferred embodiment, the step of forming the agar-biochar mixture into a coagulated film comprises:

cutting a Polytetrafluoroethylene (PTFE) film with standard thickness into the middle of two glass plates to form a U-shaped compartment with certain thickness (determined according to the thickness required by the biochar modified film), and fixing three edges of the U-shaped compartment;

then the biochar-agar mixed solution is injected into a gap between two glass plates, and bubbles between the glass plates are extruded out during injection; and after the injection is finished, horizontally placing and cooling the glass plates, and solidifying the mixed liquid between the glass plates to obtain the biochar modified film.

A DGT device adopts the biochar modified film as a binding membrane.

The DGT device comprises a DGT core module and a shell for accommodating and fixing the DGT core module, wherein the DGT core module is provided with a plurality of connecting holes; the shell comprises a base and a cover cap, and an exposure window is arranged on the cover cap; the DGT core module comprises a combination membrane, a diffusion membrane and a filter membrane which are arranged in sequence, wherein the filter membrane seals the exposure window.

In the above DGT device, as a preferred embodiment, the diffusion membrane is an agar diffusion membrane; further, the agar diffusion membrane is a coagulation membrane formed by using an agar solution obtained by dissolving agar in ultrapure water, and the mass ratio of the agar to the ultrapure water is 1-3:100 (such as 1.2:100, 1.5:100, 2:100, 2.5:100, 2.8:100, and the like).

In a preferred embodiment of the DGT device, the thickness of the agar diffusion membrane is 0.6-1.0mm (e.g., 0.7mm, 0.8mm, 0.9mm, etc.).

A method of making a DGT device, comprising:

preparing an agar solution;

preparing an agar diffusion membrane;

preparing a biochar modified combined film by adopting the preparation method of the biochar modified film;

assembling to form the DGT device.

In a preferred embodiment of the method for manufacturing a DGT device, the step of preparing the agar solution includes:

mixing appropriate amount of agar powder and ultrapure water at a mass ratio of 1-3:100 (such as 1.2:100, 1.5:100, 2:100, 2.5:100, 2.8:100, etc.), stirring, heating, and maintaining at 75-85 deg.C (such as 76 deg.C, 78 deg.C, 80 deg.C, 82 deg.C, 84 deg.C, etc.) to obtain transparent agar solution.

In a preferred embodiment of the method for manufacturing a DGT device, the step of preparing the agar diffusion membrane comprises:

firstly, cutting a polytetrafluoroethylene film with standard thickness and two organic glass plates to manufacture a U-shaped compartment with certain thickness (determined according to the required thickness of an agar diffusion film), and fixing three edges of the U-shaped compartment;

then, injecting the agar solution into a gap between two glass plates, wherein bubbles between the glass plates need to be extruded out during injection; and after the injection is finished, horizontally placing the glass plate, cooling, and solidifying the solution between the glass plates to obtain the agar diffusion membrane.

In the above method for manufacturing a DGT device, as a preferred embodiment, the agar diffusion membrane and the biochar-modified binding membrane are prepared, then placed in ultrapure water, 0.3wt% sodium nitrate or 0.3wt% sodium chloride solution, and stored at 0-4 ℃ for use, so as to ensure the membrane performance for a long time.

In the manufacturing method of the DGT device, as a preferred embodiment, the assembling to form the DGT device includes: the agar diffusion membrane, the biochar modified film and the filter membrane are sequentially arranged on the base, and then the three membranes are fixed on the base through the cover cap.

Compared with the prior art, the beneficial effects of the application include but are not limited to:

1) the biological carbon modified film for the DGT device and the DGT device have small influence on soil heavy metal biological effectiveness prediction due to physicochemical properties of different types of soil, and can be applied to soil in-situ heavy metal effectiveness prediction of different types of soil;

2) the biochar modified film for the DGT device and the DGT device can be used for predicting the bioavailability of heavy metals in field soil, and when the biochar modified film is used, the biochar has the characteristic of efficiently adsorbing different types of heavy metals, so that not only can the prediction of metal cations such As cadmium be realized, but also the bioavailability prediction of non-metal elements such As As can be realized, and therefore the biochar modified film has the advantages of capability of predicting the effectiveness of various metal/non-metal ions with different forms, high prediction efficiency, low cost and the like;

3) the biochar modified film for the DGT device and the manufacturing method of the DGT device are simple in process, easy to obtain raw materials and low in cost.

Drawings

FIG. 1 is a schematic structural diagram of a biochar-modified DGT (B-DGT) device fabricated in example 1 of the present application;

in FIG. 2, a shows different soil samples and their corresponding pH values, B is a comparison schematic diagram of effective state Cd extracted from soil by a traditional method, and C is a comparison schematic diagram of effective state Cd extracted from soil by a traditional C-DGT device and a novel B-DGT device;

a in FIG. 3 shows DOM concentrations in different soils, and B is a comparison graph of the effectiveness of Cd predicted by soils with different DOM concentrations on a B-DGT device and a C-DGT device;

FIG. 4 is a graph showing the comparison of the anion composition concentration in different soils, and B is a graph showing the prediction of Cd effectiveness against B-DGT and C-DGT for soils with different anion composition concentrations;

FIG. 5A shows a comparison graph of the concentrations of cation constituents in different soils, B is a comparison graph of prediction of Cd effectiveness for B-DGT and C-DGT for soils of different cation constituent concentrations;

FIG. 6 is a schematic diagram showing the pH values of the soil collected at different areas 4 of a metal contaminated site, which is used in the prediction and verification experiment of the biological effectiveness of the B-DGT on the heavy metals in the site soil prepared in example 1;

a and b in fig. 7 show diffusion coefficients of Cd and As in the agar gel diffusion membrane, respectively;

FIG. 8 is a schematic diagram of correlation analysis of the effective state of Cd extracted by the B-DGT device and the C-DGT device and the content of the effective state of Cd in soil;

FIG. 9 is a schematic diagram of the correlation analysis between the As available state extracted by the B-DGT device and the C-DGT device and the content of the As available state in the soil;

FIG. 10 is a graph showing the effect of the combination membrane prepared by mixing different biochar mixed solutions with agar solution in example 2.

Detailed Description

Compared with the traditional DGT technology, the biochar modified film for the DGT device, the DGT device and the manufacturing method thereof adopt a novel biochar modified film as a combined film. The biological carbon has rich microporous structure, large specific surface area and strong adsorption capacity, and can adsorb heavy metals in various forms; in addition, the raw materials for preparing the biochar are cheap and easy to obtain, safe, environment-friendly and pollution-free, and have wide application prospects in the aspects of soil improvement, carbon sequestration, emission reduction, pollution treatment and the like. The production and obtaining process of the biochar not only realizes the resource utilization of the biomass, but also can protect the ecological environment and the sustainable agricultural development. The inventor develops a novel DGT technology with biochar as a binding phase, predicts the heavy metal bioavailability rule influence angle from the soil water chemistry condition to the biochar modified DGT technology, constructs a test method suitable for the bioavailability of typical heavy metal pollutants in soils with different properties, applies verification in the actual contaminated site soil, provides technical support for accurately evaluating the risk of the heavy metal pollutants, and provides basis for scientifically and reasonably formulating the repair target of the site contaminated soil. See the examples below for details.

The following examples are presented to facilitate a better understanding of the present application and are not intended to limit the present application.

The experimental procedures in the following examples are conventional unless otherwise specified.

Other test materials used in the following examples were purchased from a conventional biochemical reagent store unless otherwise specified. The rice straw charcoal powder used in the embodiment is self-made by the inventor, and the method comprises the following steps: taking rice straws as a raw material, crushing the rice straws simply, then crushing the rice straws by using a high-speed crusher, sieving the crushed rice straws by using a 100-mesh sieve, and drying the crushed rice straws for 2 hours in a forced air drying oven at the temperature of 80 ℃ to remove water; putting a proper amount of straw powder into a vacuum tube furnace, heating to 100 ℃ at a heating rate of 5 ℃/min, preheating for 1h, then heating to 700 ℃ at a heating rate of 5 ℃/min, and carrying out constant-temperature anaerobic ignition for 4 h; pulverizing the prepared biochar, sieving with a 320-mesh sieve, drying at 80 ℃ for 6h, and storing in a dryer in a dark place for later use.

Example 1

1. Biochar modified film diffusion gradient device (B-DGT) and manufacturing method thereof

The structure of the B-DGT used in the present embodiment is shown in fig. 1, and includes a DGT core module and a housing for accommodating and fixing the DGT core module; the shell comprises a base 1 and a cap 2, and an exposure window 21 is arranged on the cap 2; the DGT core module comprises a binding membrane 3, a diffusion membrane 4 and a filter membrane 5 which are sequentially arranged, wherein the exposure window 21 is sealed by the filter membrane 5; the combination membrane 3 is a biochar modified thin film prepared in the embodiment, the base 1 and the cap 2 are made of ABS plastic, the diffusion membrane 4 is an agar diffusion membrane prepared in the embodiment, and the filter membrane 5 is made of PES.

The B-DGT used in the embodiment is self-made in a laboratory, and the specific method is as follows:

1.1 agar solution and agar diffusion Membrane preparation steps:

mixing proper amount of agar powder and ultrapure water in the mass ratio of 1.5:100, heating with a magnetic heating stirrer and keeping the temperature at about 80 ℃ all the time to make the solution transparent.

A U-shaped PTFE compartment with the thickness of 0.8mm is manufactured by cutting a polytetrafluoroethylene film with the thickness of 0.4mm and two organic glass plates, and three sides of the U-shaped PTFE compartment are fixed by a fixing device (a long tail clamp). At this time, the transparent agar solution was introduced into the gap between the two glass plates, and care was taken to squeeze out the air bubbles between the glass plates when the solution was injected so as not to affect the quality of the agar diffusion membrane. After the injection, the glass plates were placed as horizontally as possible and cooled at room temperature for 1h, at which time the solution between the glass plates solidified to form an agar diffusion membrane. The fixing device was removed and the two glass plates were separated, circular thin pieces having a diameter of 2.51cm were cut out of the agar diffusion membrane with a designed die, taken out, put in a NaCl solution having a mass concentration of 0.3%, and stored in a refrigerator at 4 ℃ for use.

1.2 preparation steps of the biochar modified film (combined film):

the rice straw charcoal powder is ground and sieved by a 320-mesh sieve to ensure that the rice straw charcoal powder has smaller particle size distribution (the particle size is less than 50 mu m), so that the subsequent homogenization and uniform mixing of the charcoal mixed solution are facilitated.

Mixing the sieved charcoal powder and ultrapure water according to the mass ratio of 1:10, uniformly stirring to obtain a charcoal mixed solution, mixing the agar solution prepared by the method (step 1.1) with the charcoal mixed solution, and uniformly mixing the charcoal mixed solution and the agar solution according to the volume ratio of 1.5:4 to obtain the charcoal-agar mixed solution. The strip single sheet is placed between two organic glass plates to form a U-shaped PTFE compartment with the thickness of 0.4mm, and three sides of the U-shaped PTFE compartment are respectively fixed by a fixing device (a long tail clamp). At this time, the biochar-agar mixture was introduced into the gap between the two glass plates, and the solution was injected while paying the same attention to squeeze out the air bubbles between the glass plates. After the injection is finished, the glass plates are placed horizontally as much as possible, and are cooled for 1 hour at room temperature, and at the moment, the mixed liquid between the glass plates is solidified to form the biochar modified film. Taking down the fixing device and separating the two glass plates, cutting out a circular slice with the diameter of 2.51cm from the biochar modified film by using a designed die, taking out and putting in NaCl solution with the mass concentration of 0.3%, and storing in a refrigerator at 4 ℃ for later use.

1.3 biochar modified DGT device (B-DGT) preparation:

the biochar modified film (as a binding membrane), the agar diffusion membrane and the PES filter membrane prepared in the above steps are assembled into an ABS plastic supporting device in a corresponding order, as shown in FIG. 1. The assembled B-DGT (Biochar-DGT) devices are respectively put into plastic package bags, and simultaneously, a proper amount of ultrapure water is added into the plastic package bags to keep humidity, and then the plastic package bags are stored in a refrigerator at 4 ℃.

The C-DGT used in the embodiment is self-made in a laboratory, and the specific method is the same as the preparation method of the B-DGT, and the difference is that: in C-DGT, the diffusion membrane is prepared from polyacrylamide and agar solution, and the binding membrane is prepared from Chelex 100 and agar solution.

2. Effect test

2.1 soil pretreatment, soil pH, soluble organic matter (DOM) and anion and cation determination.

Taking 0-20 cm of soil without pollution on the surface layer from 11 provinces, naturally drying, screening out impurities by using a 2mm sieve to homogenize, and storing in a cool and dry place for later use.

The pH range of the 11 kinds of soil is 6.08-9.35 (see figure 2), and the DOM concentration range of the soil is 29.01-483.00 mg/kg^-1(see FIG. 3(a)), the content of Cd in the soil is 0.10-0.29 mg/kg measured by BCR method^-1The measured concentrations of partial anions and cations in the soil are shown in the following tables 1 and 2:

TABLE 1 partial anion content in soil (g.kg)^-1)

Note: in the table, "/" indicates no detection.

TABLE 2 content of partial cations in soil (g.kg)^-1)

Note: in the table, "/" indicates no detection.

2.2 Effect of different types of soil pH on B-DGT prediction of heavy Metal bioavailability

The section 2.3 and 2.4 are experimental laboratory contamination simulation contaminated soil. 400g of the 11 kinds of soil are weighed and put into a plastic flowerpot, and ultrapure water is added into the flowerpot to keep the water content of the soil at about 60-70%. Adding a Cd standard substance into soil according to the set concentration (the concentration of cadmium in the soil is 10mg/kg), taking the soil without the Cd standard substance as a blank control group, and placing the blank control group and the soil together under the condition of room temperature for aging culture for three months. After the soil is aged, respectively using BCR method and CaCl₂Extracting heavy metal effective state components in soil by using a method, an HCl method, a DTPA method and a DGT technology, referring to b and c in figure 2; the pH of the blank was measured, see a in fig. 2.

The experimental method comprises the following steps: weighing 10g of a blank control group, air-drying the soil sample which is sieved by a 16-mesh sieve, placing the soil sample into a 50mL centrifuge tube, and adding 25mL of 0.01M CaCl according to the soil-water ratio (2:5)₂The solution was measured for soil pH with a pH meter. 60.00g of soil which is contaminated and aged by sieving with a 100-mesh sieve is weighed by a quartering method and put into a plastic beaker, and ultrapure water is added into each beaker to ensure that the soil in each beaker is in a mud-water mixed state (about 100 percent of water holding capacity), and then the beaker is sealed and placed for 24 hours in a dark place. And (3) taking about 20g of soil sample from the beaker, placing the soil sample into a clean culture dish, uniformly coating a proper amount of mud on windows of B-DGT and C-DGT, fully coating the windows, reversely buckling the DGT, and slightly screwing the DGT into the soil sample in the culture dish. And covering the culture dish, putting the culture dish into a plastic packaging bag, horizontally placing the culture dish, adding a proper amount of ultrapure water into the bag, and placing the plastic packaging bag for 24 hours with a small opening. Opening the DGT device shell, taking out the combined membrane, putting the combined membrane into a centrifuge tube, and using 1mL of super pure HNO₃(1mol·L^-1) And (5) eluting for 24h, and determining the content of the heavy metal in the binding membrane, namely the heavy metal effective state predicted by DGT. Each process set 3 sets of repetitions.

The pH of 11 kinds of soil is shown in figure 2, which shows that the soil in Yunnan (CK1), Heilongjiang (CK2) and Guangxi (CK3) is weak acidic, the soil in Chongqing (CK4) and Hebei (CK5) is weak alkaline, and the soil in Tianjin (CK6), inner Mongolia (CK7), Gansu (CK8), Zhejiang (CK9), Tibet (CK10) and Ningxia (CK11) is alkaline, wherein the pH values of the Tibet (CK10) and Ningxia (CK11) are the highest, and are respectively 9.32 and 9.35.

Adopts the traditional BCR method, HCl method and CaCl₂The method, the DTPA method and the traditional C-DGT and the novel modified B-DGT extract or predict the concentration of the effective Cd in the soil, and the method, the DTPA method and the traditional C-DGT and the novel modified B-DGT are shown as (B) and (C) in figure 2. In fig. 2(b), the dispersion degree of the content of available Cd extracted by the three chemical extractants compared with the content of available Cd in soil (measured by BCR method) is: HCl process<DTPA process<CaCl₂The method is carried out. Correlation analysis can be carried out to obtain that correlation coefficients of the effective state Cd content in the soil and the effective state Cd content are respectively-0.410, -0.324 and-0.051, which indicates that the effective state Cd content extracted by the three chemical extraction methods is negatively correlated with the effective state Cd content in the soil, but the correlation is weaker, namely the accuracy of the three extraction methods is as follows: HCl process>DTPA process>CaCl₂The method is carried out. Referring to FIG. 2(C), under the same soil conditions, the capacity of B-DGT to extract available Cd in soil is generally higher than that of C-DGT. For different soils with similar content of Cd in an effective state, the content of Cd in the effective state extracted by B-DGT in acid soil is obviously higher than that in alkaline soil, but the content of Cd in the effective state extracted by B-DGT in Heilongjiang soil is far less than that in Yunnan soil under similar conditions, which may be caused by higher content of organic matters in the Heilongjiang soil. C-DGT also shows similar change trend, and has better correlation with B-DGT (r is 0.775, p)<0.01). Therefore, B-DGT can be used for predicting the effectiveness of heavy metals in soil with different pH values instead of C-DGT.

2.3 influence of different types of soil DOM concentrations on extraction of heavy metals

Soil soluble organic matter (DOM) is an important component of a solid phase part of soil, and can form a complex with heavy metal elements, so that the bioavailability of the heavy metals in the soil is changed. The B-DGT device prepared by the embodiment is less influenced by soil DOM.

The experimental method comprises the following steps: weighing 2.0g of blank control group, air-drying, sieving with 100 mesh sieve, placing in a 50mL centrifuge tube, and adding 10mL of ultrapure water according to the water-soil ratio of 1:5Centrifuging, and filtering the supernatant with a 0.45-micron water system filter membrane to obtain a soil DOM solution; placing B-DGT and C-DGT in the contaminated aged soil sample for 24h according to the method, opening DGT, taking out binding membrane, and adding 1mL of 1 mol. L^-1And (5) eluting for 24h by nitric acid, and determining the content of the heavy metal in the DGT binding membrane, namely the heavy metal effective state predicted by DGT. Each process set 3 sets of repetitions.

The DOM concentration of 11 soils and the Cd content of soil available were first determined and summarized in fig. 3 (a). The DOM concentration in 11 soils was: heilongjiang (CK2) > Yunnan (CK1) > Hebei (CK5) > Chongqing (CK4), Tianjin (CK6) > Zhejiang (CK9), Gansu (CK8) > inner Mongolia (CK7) is not less than Tibet (CK10) and Ningxia (CK11) is not less than Guangxi (CK 3). The higher the DOM content in the soil is, the faster the migration rate of Cd in the soil is, and meanwhile, the combination of DOM and Cd ions and the characteristic that DOM can be adsorbed by charcoal enable the adsorption capacity of the charcoal binding membrane of the B-DGT device to be larger, so that the content of the heavy metal biological effective state measured by B-DGT is more accurate than that of C-DGT. However, the B-DGT and the C-DGT have no correlation with the DOM concentration in the soil, namely the C-DGT and the B-DGT are not influenced by the DOM concentration in the soil when predicting the effectiveness of the heavy metal in the soil, and can be applied to prediction of the bioavailability of the heavy metal in the soil with different specific DOM characteristics (see (B) in figure 3).

2.4 Effect of the concentration of different types of anions and cations in the soil on the prediction of the effective B-DGT of heavy metals

The experimental method comprises the following steps: weighing 5.0g of blank control group, air drying, sieving with 100 mesh sieve, placing in 50mL centrifuge tube, adding ultrapure water to constant volume to 50mL, centrifuging, collecting supernatant, filtering with 0.22 μm water system filter membrane, and filtering with Thermo Hyper SepTM C₁₈Organic matter components are removed by the SPE small column, and the measuring sensitivity of an instrument is improved. Measuring F of the extracted soil solution by ion chromatography^-、Cl^-、NO₂ ^-、SO₄ ^2-、NO₃ ^-And K⁺、Na⁺Plasma concentration, measurement of Mg in solution by ICP-MS²⁺And (4) concentration. Then placing B-DGT and C-DGT in the contaminated aged soil sample for 24h according to the method, opening the DGT and taking out the binding membrane,eluting with 1mL of 1 mol. L-1 nitric acid for 24h, and determining the content of the heavy metal in the binding membrane, namely the heavy metal effective state predicted by DGT. Each process set 3 sets of repetitions.

The B-DGT prepared by the method is hardly influenced by the type and concentration of anions in soil. The present inventors examined the total amount and composition analysis of anions in 11 kinds of soils and summarized them in Table 1 and FIG. 4(a), and the results showed that F in soil^-And NO₂ ^-Has a small content of F in 11 kinds of soil^-The concentration is less than 0.1 g/kg^-1，NO₂ ^-The concentration is less than 0.2 g/kg^-1，SO₄ ^2-And NO₃ ^-The main component of the soil anions, the sum of the two in 11 kinds of soil accounts for more than 50% of all the anions. Notably, NO in Yunnan (CK1) and Heilongjiang (CK2) soils₃ ^-In a much greater amount than the other anionic components; the proportions of main anions in the soil of Hebei (CK5) and Gansu (CK8) are Cl^->SO₄ ^2->NO₃ ^-And Cl is found in Gansu soil sample^-The concentration is as high as 67.78 g/kg^-1And SO₄ ^2-The concentration is as high as 37.89 g/kg^-1. Therefore, the total amount of anions in Gansu soil is much higher than that in other soil, the total amount of anions in Hebei soil is inferior, and the total amount of anions in other 9 kinds of soil is lower than 2.6 g.kg^-1。

Performing correlation analysis on the concentration of the available Cd in the soil extracted by the C-DGT and the B-DGT and the change of the concentration of anions in the soil (see figure 4(B)), performing paired sample T test (95% confidence interval) by using SPSS statistical software, and displaying that the correlation coefficient r between the concentration of the available Cd in the soil extracted by the C-DGT and the concentration of the anions in the soil is 0.514, and p is greater than 0.05; for B-DGT, the correlation coefficient r between the concentration of the effective state Cd in the extracted soil and the concentration of anions in the soil is 0.009, p is greater than 0.05, and the correlation coefficient r indicates that the effective state Cd in the extracted soil and the concentration of the anions in the soil have no correlation between the effective state Cd and the concentration of the anions in the extracted soil, namely C-DGT is influenced by the concentration of the anions in the soil to a certain extent when predicting the effectiveness of heavy metals in the soil, and B-DGT is hardly influenced by the concentration of the anions in the soil, so that the stability of prediction results of the B-DGT in the soil with different anion concentrations is ensured.

In addition, the total amount and composition of cations in 11 soils were analyzed as shown in FIG. 5. Obviously, the total amount of cations in Gansu soil is much higher than that in other soils (almost all K)⁺) The total amount of cations in Hebei soil is less than that in Hebei soil (mainly Na)⁺) The total amount of cations in the other 9 kinds of soil is less than 1.3 g.kg^-1And the content of cations in the soil in Guangxi province is minimum and is lower than 0.1 g/kg^-1. Performing correlation analysis on the concentration of the effective Cd in the soil extracted by the C-DGT and the B-DGT and the change of the concentration of the cations in the soil, performing pair sample T test in a 95% confidence interval by using SPSS statistical software, and displaying that the correlation coefficients of the concentration of the effective Cd in the soil extracted by the C-DGT and the B-DGT and the concentration of the cations in the soil are respectively 0.223 to 0.215, p is 0.223 to 0.223>0.05, which shows that the two have no correlation with the concentration of the cations in the soil, namely that the C-DGT and the B-DGT are not influenced by the concentration of the cations in the soil when predicting the effectiveness of the heavy metals in the soil. Therefore, the B-DGT prepared by the embodiment can be suitable for predicting the bioavailability of heavy metals in soils with different cation concentrations.

2.5B-DGT (denaturing gradient-denaturing high-performance liquid chromatography) for predicting and verifying biological effectiveness of field soil heavy metal

In addition to the laboratory contamination simulation contaminated soil verification test above, a site actual heavy metal contaminated soil verification test was also performed. Compared with laboratory contamination simulation contaminated soil, the field soil contains high heavy metal concentration, the risk is large, the types of pollutants are multiple, composite pollution is easy to form, and prediction of the effective state of the heavy metal is more difficult. The biological effectiveness of heavy metals Cd and As in the soil of an actual field is determined by adopting the B-DGT and the C-DGT prepared by the embodiment, the heavy metals in the soil are determined by adopting a BCR method, and the content of the heavy metals in a weak acid extraction state is used As effective state components of the heavy metals in the soil.

The experimental method comprises the following steps: the method comprises the steps of collecting the soil (4 positions in total) in different areas of a heavy metal polluted site, placing the soil in a shady and dry place for air drying, removing impurities such as stones, and the like, grinding, sieving with a 16-mesh sieve and a 100-mesh sieve respectively, and measuring the physicochemical properties of the soil.60.00g of soil which is sieved by a 100-mesh sieve is weighed by a quartering method and put into a plastic beaker, a proper amount of ultrapure water is added into the beaker to ensure that the soil is in a soft state (about 40 percent of water holding capacity), the beaker is sealed and placed in a dark place for 48 hours, then the ultrapure water is added into the beaker to ensure that the soil in the beaker is in a mud-water mixed state (about 100 percent of water holding capacity), and then the beaker is sealed and placed in a dark place for 24 hours. And (3) taking about 20g of soil sample from the beaker, placing the soil sample into a clean culture dish, uniformly coating a proper amount of mud on windows of B-DGT and C-DGT, fully coating the windows, reversely turning the DGT device, and slightly screwing the DGT device into the soil sample in the culture dish. And covering the culture dish, putting the culture dish into a plastic packaging bag, horizontally placing the culture dish, adding a proper amount of ultrapure water into the bag, and placing the plastic packaging bag for 24 hours with a small opening. Opening the DGT device shell, taking out the combined membrane, putting the combined membrane into a centrifuge tube, and using 1mL of super pure HNO₃(1mol·L^-1) And (5) eluting for 24h, and determining the content of the heavy metal in the binding membrane, namely the predicted effective state of the heavy metal. Each process set 3 sets of repetitions.

The pH of the soil in different areas 4 of the heavy metal-polluted site collected by the inventor is shown in FIG. 6, wherein the soil at the No. 1 point is acidic (pH is 5.47), and the soil at the No. 2, 3 and 4 points are alkaline (pH is 8.25-8.52).

The properties of the field soil, such as texture, Cation Exchange Capacity (CEC), Organic Matter (OM), and soluble organic carbon (DOC), were measured by dividing the soil into two categories (acidic and alkaline) according to the acidity and alkalinity thereof, as shown in table 3.

TABLE 3 field soil part physicochemical Properties

The diffusion coefficients of Cd (T ═ 21 ℃) and As (T ═ 17 ℃) in the agar diffusion membrane were measured by the diffusion cell method, and the changes with time of the content of heavy metals in the receiving chamber are shown in fig. 7.

According to the linear fitting result, the contents of Cd and As in the receiving chamber have good linear relation with the receiving time (y)₁＝0.0940·x,R²＝0.9407；y₂＝0.3965·x,R²0.9998). The diffusion coefficient D can be calculated by the following formula:

where k is the slope of the increase of the target substance in the receiving chamber with time, Δ g represents the thickness of the diffusion layer, C_STo supply the concentration of the target in the chamber (which needs to be kept constant during the sampling time), A_SIs the exposed area of the diffusion layer (determined by the exposed pore size of the diffusion cell).

The diffusion coefficient D of Cd can be obtained₁＝3.19E-6cm²(T21 ℃ C.), and the diffusion coefficient D of As₂＝1.35E-5cm²/s(T＝17℃)。

And then determining the content of Cd and As in the effective states extracted by the B-DGT and the C-DGT and the content of the heavy metal in the effective states in the soil of the actual field represented by the BCR method, and analyzing the correlation among the three.

FIG. 8(a) shows that the effective state content of Cd extracted by B-DGT and C-DGT has the same trend with the change of the effective state content of Cd in the actual field soil, and is in a significant positive correlation. The correlation analysis results of the three (see figure 8(B)) show that the correlation between the available state content of Cd in the field soil measured by the BCR method and the available state content of Cd extracted by B-DGT is slightly higher than that of C-DGT (r)₁＝0.916，r₂＝0.902，p<0.01) and also exhibits a better correlation (r) between the two DGT extraction techniques₃＝0.965，p<0.01)。

FIG. 9(a) shows that the As available state content extracted by B-DGT and C-DGT has similar trend with the change of the As available state content in the soil of the actual field, and shows a significant positive correlation. The correlation analysis results of the three (see figure 9(B)) show that the content of the As effective state in the field soil measured by the BCR method is slightly lower than the content of the Cd effective state extracted by the C-DGT (r) As the content of the As effective state extracted by the B-DGT₁＝0.769，r₂＝0.826，p<0.01) and the correlation between the two DGT extraction techniques is not significant (r)₃＝0.396，p>0.05). Therefore, the DGT modified by the biochar prepared by the embodiment, namely the novel B-DGT can better predict heavy metals in different types of soil and can also predict the biological effectiveness of Cd and non-metal ions As in field soil.

Example 2

This example is the same as example 1, section 1.2, except that the ratio of the biochar mixture to the agar solution is different. In this embodiment, the effect of preparing a binding membrane by mixing a biochar mixed solution and an agar solution in different ratios is shown in fig. 10, where a in fig. 10 shows that the binding membrane prepared according to the volume ratio of the biochar mixed solution to the agar solution of 1:1 can cause local aggregation of biochar, which affects the effect of the binding membrane; in fig. 10, c shows a binding membrane prepared by 1:5 volume ratio of biochar mixture to agar solution, which has poor mechanical strength and is not favorable for membrane extraction and retention, although biochar has good dispersibility; and b in fig. 10 shows that the binding membrane prepared according to the volume ratio of the biochar mixed solution to the agar solution of 1:2.5 has the advantages of both the biochar mixed solution and the agar solution, so that the mechanical strength of the membrane is ensured, and the biochar has better dispersibility in the membrane.

Finally, it should also be noted that, in the present application, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

While the application has been disclosed by the description of specific embodiments thereof, it should be understood that various modifications, adaptations, and equivalents may occur to one skilled in the art and are within the spirit and scope of the appended claims. Such modifications, improvements and equivalents are intended to be included within the scope of the claims.

Claims

1. A biochar modified film for a DGT device is characterized by being prepared from biochar powder and agar, wherein the biochar powder is uniformly dispersed in the film, the particle size of the biochar powder is less than or equal to 50 micrometers, and the mass ratio of the biochar powder to the agar is 1.5-3.5: 1;

the manufacturing method of the biochar modified film comprises the following steps:

2. The biochar-modified film for a DGT device of claim 1, wherein the thickness of the biochar-modified film is 0.3-0.5 mm.

3. A method for manufacturing a biochar modified film for a DGT device is characterized by comprising the following steps:

mixing and uniformly dispersing the charcoal powder and ultrapure water in a mass ratio of 1:8-15 to obtain a charcoal mixed solution, wherein the grain size of the charcoal powder is less than or equal to 50 μm;

mixing a proper amount of agar solution with the biochar mixed solution uniformly to obtain biochar-agar mixed solution; the preparation method of the agar solution comprises the following steps: mixing and stirring a proper amount of agar powder and ultrapure water according to a mass ratio of 1-3:100, heating and keeping the temperature at 75-85 ℃ all the time to enable the solution to be in a transparent state; the volume ratio of the biochar mixed solution to the agar solution is 1: 2-4; in the biochar-agar mixed solution, the mass ratio of the biochar powder to the agar is 1.5-3.5: 1;

4. The method of claim 3, wherein the forming the agar-biochar mixture into a solidified film comprises:

cutting a polytetrafluoroethylene film with standard thickness between two glass plates to form a U-shaped compartment with certain thickness, and fixing three edges of the U-shaped compartment;

5. A DGT device, characterized in that a biochar-modified membrane as claimed in claim 1 or 2 is used as a binding membrane.

6. The DGT device of claim 5, comprising a DGT core module and a housing for receiving and fixing the DGT core module; the shell comprises a base and a cover cap, and an exposure window is arranged on the cover cap; the DGT core module comprises a combination membrane, a diffusion membrane and a filter membrane which are arranged in sequence, wherein the filter membrane seals the exposure window.

7. The DGT device of claim 6, wherein the diffusion membrane is an agar diffusion membrane.

8. The DGT apparatus of claim 7, wherein the agar diffusion membrane is a coagulated membrane formed using an agar solution obtained by dissolving agar in ultrapure water, and the mass ratio of agar to ultrapure water is 1-3: 100.

9. DGT device according to claim 7 or 8, characterised in that the agar diffusion membrane has a thickness of 0.6-1.0 mm.

10. A method of making a DGT device, comprising:

step one, preparing an agar solution;

step two, preparing an agar diffusion membrane;

step three, a step of preparing a biochar modified binding membrane, which is prepared by the method for preparing the biochar modified film according to any one of claims 3 to 4;

and step four, assembling to form the DGT device.

11. The method of claim 10, wherein the step of preparing the agar solution in the first step comprises:

mixing and stirring a proper amount of agar powder and ultrapure water according to a mass ratio of 1-3:100, heating and keeping the temperature at 75-85 ℃ all the time to obtain a transparent agar solution.

12. The method of making a DGT device according to claim 10 or 11, wherein the step of preparing the agar diffusion membrane comprises:

firstly, cutting a polytetrafluoroethylene film with standard thickness and two organic glass plates to manufacture a U-shaped compartment with certain thickness, and fixing three edges of the U-shaped compartment;

then, injecting the agar solution obtained in the step one into a gap between two glass plates, wherein bubbles between the glass plates need to be extruded during injection; and after the injection is finished, horizontally placing the glass plate, cooling, and solidifying the solution between the glass plates to obtain the agar diffusion membrane.

13. The method of claim 10 or 11, wherein the agar diffusion membrane and the biochar-modified conjugate membrane are prepared and then placed in ultrapure water, 0.3wt% sodium nitrate solution or 0.3wt% sodium chloride solution and stored at 0-4 ℃ for further use.