A method for activating straw to cooperatively stabilize and remediate cadmium and arsenic compound pollution in farmland in situ

Abstract

The present application relates to the field of in-situ remediation of heavy metal pollution of cultivated land, and discloses a method for in-situ remediation of cadmium and arsenic composite pollution of cultivated land by activated straw and plants in cooperation. After the non-carbonized straw is pretreated by citric acid, L-cysteine is contacted with the first trivalent iron source Fe-A under weak acid conditions to form a pre-complexation activation solution, and is infiltrated into the surface of the straw, the cutting section and the near-surface pores in batches; then the second trivalent iron source Fe-B is added to form an iron-containing hydroxyl anchor interface under near-neutral conditions. After leaching, the cumulative iron loss rate is less than 20%, and the conductivity change rate of the leaching solution is less than 10% for two consecutive times, then the cadmium and arsenic composite pollution of the 5-15 cm rhizosphere pre-laying layer of the cultivated layer soil is applied at 0.5%-1.5% of the dry weight of the soil, and the application is completed 7-20 days before the artemisia planting. The present application can form a sulfur / iron-containing cooperative retention interface on the straw decomposition interface, which is used for reducing the effective state of cadmium and limiting the migration risk of arsenic.

Description

Technical Field

[0001] This invention relates to the field of in-situ remediation technology for heavy metal pollution in arable land, and in particular to a method for in-situ stabilization and remediation of cadmium-arsenic compound pollution in arable land by activating straw and synergistically using plants. Background Technology

[0002] In the remediation of cadmium and arsenic co-polluted farmland, the occurrence forms of cadmium and arsenic in the soil and their responses to pH, redox state, and organic matter release processes are not consistent. When lime, phosphate, iron salts, or ordinary organic materials are applied alone, cadmium availability often decreases while arsenic mobility increases, or arsenic is fixed while cadmium can still be continuously absorbed by plants.

[0003] Returning straw to the field can provide organic matter and improve soil physicochemical properties, but when ordinary straw releases dissolved organic carbon in the early stages of decomposition, it may alter the rhizosphere reducing environment and increase the risk of arsenic migration. Sulfur-containing small molecule ligands or organic acids can affect the speciation of cadmium, but if it enters the rhizosphere directly in a free state, it may also increase metal mobility, making it difficult to simultaneously regulate cadmium and arsenic.

[0004] Therefore, there is a need for a method that can pre-form detectable and reproducible anchor points on the straw decomposition interface and match the interface with the growth sequence of plant roots, so as to limit the increase in the proportion of available arsenic while reducing the risk of cadmium absorption from the edible parts of crops. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a method for activating straw and synergistically using plants to stabilize and remediate cadmium and arsenic compound pollution in arable land, in order to solve the problem that ordinary straw return to the field, single iron salt passivation, iron-modified straw biochar, or free small molecule ligand regulation cannot simultaneously address the reduction of available cadmium and the restriction of arsenic migration.

[0006] To achieve the above objectives, this invention provides a method for the in-situ stabilization and remediation of cadmium-arsenic compound pollution in arable land through the synergistic effect of activated straw and plants, comprising the following steps: S1. Cut the uncarbonized straw into straw segments of 0.5-2.0 cm and sieve out straw powder with a particle size of less than 0.2 cm; pre-treat the straw segments with a 0.08-0.20 mol / L citric acid aqueous solution to adjust the moisture content of the straw to 45%-60% by wet basis mass fraction, treat at 50℃-65℃ for 3-6 h, and adjust the pH of the straw extrusion liquid to 5.2-6.0 to obtain citric acid-conditioned straw. S2. The trivalent iron source is divided into two segments, Fe-A and Fe-B. The initial total iron content provided by Fe-A and Fe-B is 0.35-0.75 mol / kg based on dry straw. Among them, Fe-A accounts for 35%-60% of the total iron molar content of Fe-A and Fe-B, and Fe-B accounts for 40%-65% of the total iron molar content of Fe-A and Fe-B. L-cysteine ​​is pre-complexed with Fe-A under pH 4.8-5.6 conditions, and the molar ratio of L-cysteine ​​to Fe-A is 1.5:1-3.0:1 to obtain a sulfur-containing iron pre-complexed activation solution. S3. Apply the sulfur-iron pre-complexing activation solution to the citric acid-conditioned straw in several batches, so that the sulfur-iron pre-complexing activation solution wets the surface of the straw, the cut surface and the near-surface pores. S4. Add Fe-B to the straw treated by S3 in portions, and control the pH of the straw extrusion liquid to 5.8-6.8 and the straw moisture content to 55%-62% by wet basis mass fraction, so that Fe-B forms an iron-containing hydroxyl interface component on the straw surface, cut surface and near-surface pores. S5. The straw treated in S4 is rinsed. When the ratio of the cumulative amount of iron rinsed to the initial total amount of iron added is less than 20%, and the change rate of the conductivity of the rinsing solution is less than 10% for two consecutive times, the rinsing is stopped, and activated straw is obtained. S6. Apply the activated straw at 0.5% to 1.5% of the soil dry weight into the 5-15cm rhizosphere layer of the cadmium-arsenic compound contaminated soil in cultivated land, and complete the application 7 to 20 days before transplanting Artemisia argyi; do not directly irrigate the rhizosphere with free L-cysteine ​​solution during the plant growth period.

[0007] Preferably, the uncarbonized straw is selected from rice straw or wheat straw.

[0008] Preferably, the citric acid aqueous solution in S1 is applied to the straw segment at a ratio of 180-260 mL per 100 g of dry straw as the spraying or soaking contact amount. After treatment, the free liquid is drained or squeezed out, and the actual moisture content of the straw segment is adjusted to 45%-60% based on wet basis mass fraction.

[0009] Preferably, Fe-A and Fe-B are each independently selected from ferric chloride hexahydrate, ferric nitrate nonahydrate, or ferric sulfate hydrate; when ferric sulfate hydrate is used, the dosage of Fe-A and Fe-B is calculated based on the total molar amount of iron they provide.

[0010] Preferably, in step S2, the Fe-A iron salt solution is added dropwise to the L-cysteine ​​solution, and the pH of the system is maintained at 4.8-5.6 using a sodium bicarbonate aqueous solution with a mass fraction of 0.1%-1.0%, and the mixture is stirred for 60-120 min to obtain the sulfur-containing iron pre-complexed activation solution.

[0011] Preferably, in step S3, the sulfur-iron pre-complexed activating liquid is sprayed onto the citric acid-conditioned straw in 4 to 5 applications. After each spraying, the straw is turned over and mixed. After spraying, the thickness of the straw spread is controlled to be 8 to 12 cm, and the straw is left to stand at 20°C to 30°C for 10 to 14 hours.

[0012] Preferably, in step S4, the Fe-B iron salt solution is sprayed onto the straw treated in step S3 in 3 to 4 applications. After each application, the pH of the straw extrusion liquid is adjusted to 5.8 to 6.8 using a sodium bicarbonate aqueous solution with a mass fraction of 0.1% to 1.0%. After all the Fe-B has been added, the straw is left to stand for 20 to 28 hours.

[0013] Preferably, deionized water is used for rinsing in S5, and the amount of deionized water is 100mL per 100g of wet straw. After rinsing, the straw is dried in a ventilated environment at 30℃~40℃ until the moisture content is 30%~40% based on wet basis mass fraction.

[0014] Preferably, the cadmium-arsenic contaminated soil in S6 is the topsoil of the 0-20cm tillage layer, with a soil pH of 5.5-7.0, a total cadmium content of 0.5-3.0 mg / kg, and a total arsenic content of 20-80 mg / kg. After applying the activated straw, the soil moisture content is adjusted to 55%-75% of the field capacity.

[0015] Preferably, the mugwort is a transplanted seedling with a height of 10-18cm, and the plant sample is collected from the above-ground harvested part.

[0016] In this invention, the activator refers to a combination system of citric acid humidification system, L-cysteine, and segmented ferric iron sources used for the targeted modification of the straw decomposition interface. Fe-A is the first-stage ferric iron source used to form a sulfur-containing iron pre-complexed activating solution by contacting L-cysteine; Fe-B is the second-stage ferric iron source used to form an iron-containing hydroxyl interface component on the straw surface, cut surface, and near-surface pores. The total molar amount of iron in Fe-A and Fe-B constitutes the initial total amount of iron added by the ferric iron sources.

[0017] In this invention, the pre-anchoring endpoint refers to the state where the ratio of the cumulative amount of iron leached to the initial total amount of iron added is less than 20%, and the change rate of the conductivity of the leaching solution is less than 10% for two consecutive times; this endpoint is used to confirm that the migratable iron salts and free small molecule components in the activated straw have been reduced to the preset range.

[0018] In this invention, the straw surface, cut surface, and near-surface pores refer to the outer surface, natural pore entrance, cut surface, and near-surface pore area that can be contacted and wetted by the sulfur-iron pre-complexing activation liquid after the uncarbonized straw has been cut into sections, sieved to remove fine powder, and moistened with citric acid; the rhizosphere pre-laying layer refers to the 5-15cm soil layer of activated straw pre-arranged in the 0-20cm tillage layer before sowing or transplanting.

[0019] The beneficial effects of this invention are: This invention first pre-treats the uncarbonized straw with citric acid to make the near-surface pores of the straw surface and cut surface wettable, providing reproducible contact conditions for the subsequent entry of the sulfur-iron pre-complexing activation solution into the straw interface.

[0020] This invention allows L-cysteine ​​to first contact with Fe-A to form a sulfur-containing iron pre-complexed activation solution, which is then applied in stages to citric acid-conditioned straw. This method can preferentially retain sulfur-containing coordination components at the straw decomposition interface, reducing the risk of free L-cysteine ​​directly entering the rhizosphere and causing fluctuations in cadmium and arsenic migration.

[0021] This invention further improves the detectability and reproducibility of the activated straw interface retention state by forming an iron-containing hydroxyl interface component under near-neutral conditions in the later stage using Fe-B, and by using the cumulative amount of leached iron and the rate of change in conductivity of two consecutive leaching solutions as pre-anchoring endpoints.

[0022] This invention involves applying activated straw into a 5-15cm rhizosphere layer 7-20 days before transplanting Artemisia argyi, so that the straw decomposition interface matches the vigorous growth period of the plant roots, which is conducive to forming a temporal connection between the regulation of cadmium exchange state or weak acid extraction state and the restriction of arsenic migration.

[0023] In summary, compared with ordinary straw return to the field, iron-modified straw biochar, direct application of free small molecule ligands, or passivation with iron salt alone, this invention can simultaneously reduce the risk of cadmium absorption in the upper part of Artemisia argyi and control the proportion of available arsenic in the soil during the same rhizosphere remediation process, making it suitable for in-situ remediation of cadmium-arsenic co-contaminated soil in arable land. Detailed Implementation

[0024] The present invention will be further described below with reference to embodiments. These embodiments are used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, all percentages in the embodiments are mass percentages, soil dry weight is calculated as dry matter based on air-dried soil, and straw moisture content is calculated as wet basis mass fraction.

[0025] Example 1: This embodiment provides a method for the in-situ stabilization and remediation of cadmium-arsenic co-contaminated soil by activating the sulfur-iron pre-complex interface with straw and synergistically using Artemisia argyi.

[0026] Take 1000g of air-dried rice straw with a moisture content of 8.6% (by mass). Chop the straw into 0.5-2.0cm long segments using a straw cutter, and sieve to remove straw powder with a particle size smaller than 0.2cm. Dissolve 192.1g of anhydrous citric acid in deionized water and bring the volume to 10L to prepare a 0.10mol / L citric acid solution. Take 2200mL of the citric acid solution as the spray contact volume and add it to the straw segments in four spraying applications. After each spraying, stir at 60r / min for 5min. After spraying, drain off any unabsorbed free liquid and adjust the actual moisture content of the straw to 52% using hot air conditioning. Place the conditioned straw under 60℃ hot air conditioning for 4h, cool it to 25℃, and adjust the pH of the straw extrusion liquid to 5.6 with a 0.5% sodium bicarbonate aqueous solution to obtain citric acid-conditioned straw.

[0027] 60.6 g of L-cysteine ​​was dissolved in 4.0 L of deionized water, and the solution temperature was controlled at 25 °C. Separately, 67.6 g of ferric chloride hexahydrate was dissolved in 1.0 L of deionized water to obtain a Fe-A iron salt solution. The Fe-A iron salt solution was added dropwise to the L-cysteine ​​solution over 60 min, maintaining the pH of the system at 5.1–5.4 using a 0.5% sodium bicarbonate aqueous solution during the addition. After the addition was complete, the mixture was stirred at 200 rpm for 90 min to obtain a sulfur-iron pre-complexed activated solution. The molar ratio of L-cysteine ​​to Fe-A was 2.0:1.

[0028] The sulfur-iron pre-complexing activation solution was sprayed onto the citric acid-conditioned straw in five applications, with each application followed by 8 minutes of stirring. After spraying, the straw was spread to a thickness of 8–12 cm and left to stand at 25°C for 12 hours to allow the sulfur-iron pre-complexing activation solution to penetrate the straw surface, cut surfaces, and near-surface pores. During the standing period, the straw was stirred once every 4 hours at a stirring speed of 40–60 r / min.

[0029] Separately, 81.1 g of ferric chloride hexahydrate was dissolved in 1.5 L of deionized water to obtain a Fe-B iron salt solution. The Fe-B iron salt solution was sprayed three times onto straw that had been soaked in the pre-complexed activation solution. After each spraying, the pH of the straw extrusion liquid was adjusted to 6.1–6.5 with a 0.5% sodium bicarbonate aqueous solution, and the straw moisture content was maintained at 55%–62%. After all the Fe-B iron salt solution was added, the straw was allowed to stand at 30°C for 24 h to allow Fe-B to form an iron-containing hydroxyl interface component on the straw surface, cut surface, and near-surface pores. The initial total iron content provided by Fe-A and Fe-B was 0.55 mol / kg based on dry straw.

[0030] After settling, the straw was rapidly rinsed with 100 mL of deionized water per 100 g of wet straw, and the rinsing solution was collected. Rinsing was stopped when the ratio of the cumulative rinsed iron to the initial total iron added was less than 20%, and the change rate of the conductivity of the rinsing solution was less than 10% for two consecutive cycles. The rinsed straw was then dried in a ventilated environment at 35°C until the moisture content reached 30%–40%, yielding activated straw.

[0031] Collect topsoil contaminated with cadmium and arsenic, air-dry it, and pass it through a 5mm sieve. The soil pH was 6.3, the total cadmium content was 1.2 mg / kg, and the total arsenic content was 45 mg / kg. Add the activated straw at 1.0% of the soil dry weight and apply it to the 5-15cm rhizosphere pre-laying layer within the 0-20cm topsoil layer. Apply the straw 10 days before transplanting Artemisia argyi. After application, adjust the soil moisture content to 65% of field capacity and pre-condition for 10 days. After pre-conditioning, transplant Artemisia argyi seedlings. The seedlings should be 12-15cm tall, with one seedling per pot. During the plant growth period, maintain the soil moisture content at 55%-75% of field capacity. Do not apply additional phosphate passivating agents with high phosphorus content, and do not directly apply free L-cysteine ​​to the rhizosphere.

[0032] Example 2: Take 1000g of air-dried rice straw, cut it into 0.5-1.5cm segments, and sieve out powder with a particle size smaller than 0.2cm. Spray 2000mL of 0.08mol / L citric acid aqueous solution into the straw segments in four portions to adjust the straw moisture content to 46%. Keep it at 55℃ for 3 hours, and adjust the pH of the straw extrusion liquid to 5.3.

[0033] Dissolve 45.4 g of L-cysteine ​​in 3.5 L of deionized water; dissolve 67.6 g of ferric chloride hexahydrate in 1.0 L of deionized water to prepare the Fe-A iron salt solution, making the molar ratio of L-cysteine ​​to Fe-A 1.5:1. Add the Fe-A iron salt solution dropwise to the L-cysteine ​​solution, maintaining the pH at 5.0–5.3, and stir for 80 min to obtain a sulfur-iron pre-complexed activation solution. Spray this activation solution onto citric acid-conditioned straw in four applications and let it stand at 25 °C for 10 h.

[0034] Separately, 125.6 g of ferric chloride hexahydrate was dissolved in 2.0 L of deionized water to prepare the Fe-B iron salt solution, ensuring that Fe-A and Fe-B accounted for 35% and 65% of the total iron molar amount, respectively. The initial total iron addition was 0.715 mol / kg based on dry straw. The Fe-B iron salt solution was sprayed onto the straw in three applications, and the pH of the straw extrusion liquid was controlled at 6.2–6.6. The straw was then allowed to stand at 30°C for 20 hours. After leaching to reach the pre-anchoring endpoint, activated straw was obtained. The activated straw was then applied to a 5–15 cm rhizosphere pre-laying layer at 0.5% of the dry weight of the soil to be remediated, and this application was completed 7 days before the transplanting of Artemisia argyi.

[0035] Example 3: Take 1000g of air-dried rice straw, cut it into 1.0-2.0cm segments, and sieve out powder with a particle size smaller than 0.2cm. Spray 2400mL of 0.20mol / L citric acid aqueous solution into the straw segments in 5 divided doses to adjust the straw moisture content to 58%. Keep it at 65℃ for 6 hours, and adjust the pH of the straw extrusion liquid to 5.8.

[0036] Dissolve 75.7 g of L-cysteine ​​in 4.5 L of deionized water; dissolve 67.6 g of ferric chloride hexahydrate in 1.0 L of deionized water to prepare the Fe-A iron salt solution, making the molar ratio of L-cysteine ​​to Fe-A 2.5:1. Add the Fe-A iron salt solution dropwise to the L-cysteine ​​solution, maintaining the pH at 5.2–5.5, and stir for 100 min to obtain a sulfur-iron pre-complexed activation solution. Spray this activation solution onto citric acid-conditioned straw in 5 applications and let it stand at 25 °C for 14 h.

[0037] Separately, 101.4 g of ferric chloride hexahydrate was dissolved in 1.8 L of deionized water to prepare the Fe-B iron salt solution, ensuring that Fe-A and Fe-B accounted for 40% and 60% of the total iron molar amount, respectively. The initial total iron addition was 0.625 mol / kg based on dry straw. The Fe-B iron salt solution was sprayed onto the straw in four applications, and the pH of the straw extrusion liquid was controlled at 6.3–6.8. The straw was then allowed to stand at 30°C for 28 hours. After leaching to reach the pre-anchoring endpoint, activated straw was obtained. The activated straw was then applied as a 5–15 cm rhizosphere pre-laying layer at 1.5% of the dry weight of the soil to be remediated, and this application was completed 20 days before the transplanting of Artemisia argyi.

[0038] Example 4: Take 1000g of air-dried rice straw, cut it into 0.5-2.0cm segments, and sieve out powder with a particle size smaller than 0.2cm. Spray 2200mL of 0.12mol / L citric acid aqueous solution into the straw segments in four portions to adjust the straw moisture content to 53%. Keep it at 60℃ for 4 hours, and adjust the pH of the straw extrusion liquid to 5.6.

[0039] Dissolve 90.9 g of L-cysteine ​​in 5.0 L of deionized water; dissolve 67.6 g of ferric chloride hexahydrate in 1.0 L of deionized water to prepare the Fe-A iron salt solution, making the molar ratio of L-cysteine ​​to Fe-A 3.0:1. Add the Fe-A iron salt solution dropwise to the L-cysteine ​​solution, maintaining the pH at 5.1–5.4, and stir for 90 min to obtain a sulfur-iron pre-complexed activation solution. Spray this activation solution five times onto wheat straw pretreated with citric acid, and let it stand at 25 °C for 12 h.

[0040] Separately, 45.1 g of ferric chloride hexahydrate was dissolved in 1.5 L of deionized water to prepare the Fe-B iron salt solution, ensuring that Fe-A and Fe-B accounted for 60% and 40% of the total iron molar amount, respectively. The initial total iron addition was 0.417 mol / kg based on dry straw. The Fe-B iron salt solution was sprayed onto the straw in three applications, and the pH of the straw extrusion liquid was controlled at 6.0–6.4. The straw was then allowed to stand at 30°C for 24 hours. After leaching to reach the pre-anchoring endpoint, activated straw derived from wheat straw was obtained. The activated straw was then applied as a 5–15 cm rhizosphere pre-laying layer at 1.0% of the dry weight of the soil to be remediated, and the application was completed 14 days before the transplanting of Artemisia argyi.

[0041] Comparative example: The blank control (CK) was not treated with straw, iron salts, or L-cysteine, but was treated under the same potting conditions, Artemisia argyi transplanting time, moisture content management, and testing methods as in Example 1. It was used to reflect the basic risk level of soil contaminated with cadmium and arsenic.

[0042] The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 does not involve citric acid conditioning, L-cysteine / Fe-A pre-complexation, and Fe-B post-terminal interface formation. Instead, air-dried rice straw is cut into 0.5-2.0 cm pieces and directly applied to a 5-15 cm rhizosphere pre-laying layer at 1.0% of the soil dry weight. The other soil sources, plant materials, application layers, transplanting time, and testing conditions are the same as in Example 1.

[0043] The difference between Comparative Example 2 and Example 1 is that Comparative Example 2 does not involve pre-complexation and post-interface formation on the surface of straw, the cut surface, and near-surface pores. Instead, 10 days before transplanting, the same amount of straw, L-cysteine, Fe-A iron salt, and Fe-B iron salt are directly mixed into a 5-15cm rhizosphere pre-laying layer. The sources of other raw materials, total dosage, soil, plant materials, and test conditions are the same as in Example 1.

[0044] The difference between Comparative Example 3 and Example 1 is that L-cysteine ​​is not added to Comparative Example 3, the original volume of L-cysteine ​​solution is made up with deionized water, the iron molar amount corresponding to Fe-A is incorporated into the Fe-B section, so that the total iron molar amount is consistent with that of Example 1, and the remaining straw treatment, citric acid humidification, application amount, layer position, advance amount and test conditions are the same as those of Example 1.

[0045] The difference between Comparative Example 4 and Example 1 is that Comparative Example 4 does not include the Fe-B post-addition step, but instead incorporates the total iron molar amounts corresponding to Fe-A and Fe-B into the sulfur-containing iron contact liquid in the front stage and adds it all at once, so that the initial total iron amount, L-cysteine ​​dosage, citric acid for straw conditioning, application amount, layer position, and advance amount are all the same as in Example 1.

[0046] The difference between Comparative Example 5 and Example 1 is that Comparative Example 5 uses the same activated straw as Example 1, but it is applied to the rhizosphere layer of 5-15 cm on the day of transplanting of Artemisia argyi instead of 10 days before transplanting. The other raw materials, equipment, preparation steps and test conditions are the same as those in Example 1.

[0047] Comparative Example 6 was used to simulate the latest existing technology for iron-modified straw biochar. Rice straw was pyrolyzed under limited oxygen to produce straw biochar, which was then subjected to iron salt co-precipitation treatment with a ferric chloride hexahydrate solution with the same initial total iron content as in Example 1. After washing until the supernatant was nearly neutral, it was dried and applied to the soil layer of 0-10 cm at 1.0% of the soil dry weight. Other soil and Artemisia argyi test conditions were the same as in Example 1.

[0048] The difference between Comparative Example 7 and Example 1 is that Comparative Example 7 does not use citric acid aqueous solution for moisture conditioning pretreatment, but uses deionized water to adjust the straw moisture content to 52%. The remaining L-cysteine / Fe-A pre-complexation, Fe-B post-addition, rinsing, application amount, layer position and advance amount are the same as in Example 1.

[0049] The difference between Comparative Example 8 and Example 1 is that Comparative Example 8 does not perform pre-anchoring leaching after completing the Fe-B post-treatment, but directly dries the treated straw and applies it to the 5-15cm rhizosphere pre-laying layer at 1.0% of the soil dry weight. The other raw material sources, preparation conditions, application time and testing conditions are the same as those in Example 1.

[0050] Test method: Soil sampling and sample preparation: Three parallel basins were set up for each treatment, with each basin containing 5.0 kg of air-dried soil contaminated with cadmium and arsenic that had passed through a 5 mm sieve. Soil sampling, sample preparation, preservation, and transfer were performed in accordance with HJ 166-2026; day 0 after application refers to the sampling time point within 24 hours after the application and mixing of activated straw or control material. Continuous test results are recorded as the mean ± standard deviation of the three parallel basins.

[0051] Soil pH, Eh, and dissolved organic carbon tests: Rhizosphere soil samples were collected at 0, 30, and 60 days after application. pH was determined using the potentiometric method according to HJ 962-2018, with a soil-to-water ratio of 2.5:1. For Eh, platinum electrodes and reference electrodes were inserted into the rhizosphere layer of each pot, and stable readings were recorded. For dissolved organic carbon, fresh soil was shaken at a soil-to-water ratio of 5:1 for 1 hour, centrifuged, filtered through a 0.45 μm filter membrane, and measured using the combustion oxidation-non-dispersive infrared absorption method.

[0052] Test of cadmium extraction state in weak acid in soil: Take air-dried soil samples that have passed through a 2 mm sieve, extract them with 0.11 mol / L acetic acid solution at a liquid-to-soil ratio of 40 mL: 1 g and shake for 16 h. After centrifugation and filtration, determine the cadmium concentration by ICP-MS. The results are expressed as mg / kg dry soil.

[0053] Soil arsenic extractable by sodium bicarbonate: Air-dried soil samples that have passed through a 2mm sieve were extracted with 0.5mol / L sodium bicarbonate solution at a liquid-to-soil ratio of 25mL:1g and shaken for 16h. The pH of the extract was controlled at 8.5±0.2. After filtration, the arsenic concentration was determined by ICP-MS. The results are expressed as mg / kg dry soil.

[0054] Tests on iron loss rate and conductivity change rate of leachate: After the activated straw was prepared, it was continuously leached with deionized water at a liquid-to-solid ratio of 10 mL: 1 g. The total iron concentration and conductivity of the leachate were measured for each leaching. Iron loss rate = cumulative iron loss in the leachate / initial total iron added × 100%; conductivity change rate between two consecutive tests = |ECn - ECn-1| / ECn-1 × 100%. When the iron loss rate was less than 20% and the conductivity change rate between two consecutive tests was less than 10%, it was recorded as reaching the pre-anchoring endpoint. The iron-containing leachate was collected and disposed of according to the laboratory metal-containing waste liquid specifications.

[0055] Total sulfur retention rate test of straw: Take straw samples before and after activation, dry and crush them, and then use an elemental analyzer to determine the total sulfur content. The retention rate is calculated as (total sulfur content of straw after activation / total sulfur content after theoretical addition) × 100%.

[0056] Tests on cadmium and arsenic accumulation in the aboveground parts of the plant: The aboveground parts of Artemisia argyi were collected 60 days after transplanting, washed, blanched at 105℃ for 30 min, and then dried at 65℃ to constant weight. The dry weight of the aboveground parts was recorded, and the samples were pulverized and sieved. After microwave digestion, the cadmium and arsenic contents were determined by ICP-MS, and the results were expressed as mg / kg dry weight. The accumulation of Cd or As in the aboveground parts was converted to μg / pot according to the elemental content and the corresponding dry weight.

[0057] Quality control: For each batch of samples, method blanks, parallel samples, and soil / plant standard materials are set up. The recovery rates of Cd and As spikes are controlled at 80% to 120%, and the relative deviation of parallel samples does not exceed 20%. The correlation coefficient of the instrument calibration curve is not less than 0.999.

[0058] Unless otherwise specified, Tables 1 to 3 below are the mean ± standard deviation of three parallel basins; "Not Applicable" in the table indicates that the treatment did not have the corresponding pre-anchoring or sulfur retention steps.

[0059] The test results of the eluent iron loss rate, the rate of change of conductivity in two consecutive tests, and the total sulfur retention rate of straw are shown in Table 1.

[0060] Table 1. Pre-anchoring endpoint and interface retention index of activated straw

[0061] The test results of soil available content and physicochemical indicators are shown in Table 2.

[0062] Table 2. Test results of soil available quantities and physicochemical properties

[0063] The results of cadmium and arsenic tests on the aboveground parts of the plants are shown in Table 3.

[0064] Table 3. Test results of cadmium, arsenic and their accumulation in the aboveground parts of plants.

[0065] As shown in Table 1, the iron loss rate of the leachate in Examples 1-4 was 9.6%-15.7%, and the conductivity change rate was 5.1%-8.2% for two consecutive tests, both reaching the pre-anchoring endpoint. Meanwhile, the total sulfur retention rate of straw in Examples 1-4 was 72.4%-80.9%. In contrast, Comparative Example 2, which used on-site physical mixing of straw, L-cysteine, and iron salts, saw its iron loss rate increase to 33.4%, its conductivity change rate increase to 18.5% for two consecutive tests, and its total sulfur retention rate decrease to 48.7%. Comparative Example 4, while maintaining the same initial total iron content, eliminated the Fe-B interface formation step, yet the iron loss rate and conductivity change rate still increased to 24.8% and 13.2%, respectively. Comparative Example 6, with its iron-modified straw biochar treatment, failed to form a sulfur-iron pre-complexed retention interface. Comparative Examples 7 and 8 indicate that both citric acid conditioning and pre-anchoring leaching affect the interface retention state.

[0066] As shown in Tables 2 and 3, Examples 1-4 all exhibited low levels of Cd weakly acid extractable and As extracted by sodium bicarbonate 30-60 days after application. Specifically, Example 1 showed a Cd weakly acid extractable of 0.52 mg / kg and an As extracted by sodium bicarbonate of 1.46 mg / kg at 60 days. The Cd and As content in the aboveground parts of the plants was 0.54 mg / kg and 0.42 mg / kg, respectively, with Cd and As accumulations of 17.5 μg / pot and 13.6 μg / pot, respectively. Compared to the blank control (CK), the conventional straw return comparison example 1, and the iron-modified straw biochar comparison example 6, the Example groups simultaneously reduced the Cd and As content and accumulation in the aboveground parts of the plants. This indicates that the effect of this invention is not simply to increase plant extractability, but rather is more accurately described as a synergistic effect of in-situ stabilization and remediation of plants and absorption inhibition.

[0067] Compared to Comparative Example 2, the Example group showed lower iron loss rate and higher total sulfur retention rate, and lower Cd / As bioavailability and plant uptake at 60 days, indicating that on-site physical mixing cannot replace pre-complexation and straw interface confinement retention. Compared to Comparative Example 3, Example 1 showed a decrease in both weakly acid-extractable Cd and aboveground Cd at 60 days, indicating that the L-cysteine-mediated Fe-A pre-complexation interface contributes to the regulation of bioavailability of cadmium. Compared to Comparative Example 4, Examples 1 and 3 showed a decrease in sodium bicarbonate-extractable As, DOC, and aboveground As, indicating that the formation of iron-containing hydroxyl interface components in the Fe-B post-complexation stage helps limit the risk of DOC-induced arsenic migration during straw decomposition.

[0068] Compared with Comparative Example 5, the soil available state indicators in the Example group were more stable at 30 and 60 days, indicating that applying activated straw into a 5-15 cm rhizosphere pre-laying layer 7-20 days before Artemisia argyi transplanting is beneficial for matching the straw decomposition interface with the vigorous root growth period. Compared with Comparative Example 7, citric acid humidification pretreatment improved the wetting effect of the sulfur-iron pre-complexed activation solution on the straw surface, cut surface, and near-surface pores; compared with Comparative Example 8, pre-anchoring leaching reduced early fluctuations after migratable iron salts and free small molecule components entered the rhizosphere.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention. Those skilled in the art can make appropriate adjustments to the parameters, straw source, type of trivalent iron source, application time and application layer in the embodiments without departing from the concept of the present invention.

Claims

1. A method for activating straw and synergistically using plants to in-situ stabilize and remediate cadmium-arsenic compound pollution in arable land, characterized in that, Includes the following steps: S1. Cut the uncarbonized straw into straw segments of 0.5-2.0 cm and sieve out straw powder with a particle size of less than 0.2 cm; pre-treat the straw segments with a 0.08-0.20 mol / L citric acid aqueous solution to adjust the moisture content of the straw to 45%-60% by wet basis mass fraction, treat at 50℃-65℃ for 3-6 h, and adjust the pH of the straw extrusion liquid to 5.2-6.0 to obtain citric acid-conditioned straw. S2. The trivalent iron source is divided into two segments, Fe-A and Fe-B. The initial total iron content provided by Fe-A and Fe-B is 0.35-0.75 mol / kg based on dry straw. Among them, Fe-A accounts for 35%-60% of the total iron molar content of Fe-A and Fe-B, and Fe-B accounts for 40%-65% of the total iron molar content of Fe-A and Fe-B. L-cysteine ​​is pre-complexed with Fe-A under pH 4.8-5.6 conditions, and the molar ratio of L-cysteine ​​to Fe-A is 1.5:1-3.0:1 to obtain a sulfur-containing iron pre-complexed activation solution. S3. Apply the sulfur-iron pre-complexing activation solution to the citric acid-conditioned straw in several batches, so that the sulfur-iron pre-complexing activation solution wets the surface of the straw, the cut surface and the near-surface pores. S4. Add Fe-B to the straw treated by S3 in portions, and control the pH of the straw extrusion liquid to 5.8-6.8 and the straw moisture content to 55%-62% by wet basis mass fraction, so that Fe-B forms an iron-containing hydroxyl interface component on the straw surface, cut surface and near-surface pores. S5. The straw treated in S4 is rinsed. When the ratio of the cumulative amount of iron rinsed to the initial total amount of iron added is less than 20%, and the change rate of the conductivity of the rinsing solution is less than 10% for two consecutive times, the rinsing is stopped, and activated straw is obtained. S6. Apply the activated straw at 0.5% to 1.5% of the soil dry weight into the 5-15cm rhizosphere layer of the cadmium-arsenic compound contaminated soil in cultivated land, and complete the application 7 to 20 days before transplanting Artemisia argyi; do not directly irrigate the rhizosphere with free L-cysteine ​​solution during the plant growth period.

2. The method according to claim 1, characterized in that, The uncarbonized straw is selected from rice straw or wheat straw.

3. The method according to claim 1, characterized in that, The citric acid aqueous solution described in S1 is applied to the straw segment at a ratio of 180-260 mL per 100 g of dry straw as a spraying or soaking contact amount. After treatment, the free liquid is removed by leaching or squeezing, and the actual moisture content of the straw segment is adjusted to 45%-60% based on wet basis mass fraction.

4. The method according to claim 1, characterized in that, The Fe-A and Fe-B are each independently selected from ferric chloride hexahydrate, ferric nitrate nonahydrate, or ferric sulfate hydrate; when ferric sulfate hydrate is used, the dosage of Fe-A and Fe-B is calculated based on the total molar amount of iron they provide.

5. The method according to claim 1, characterized in that, In step S2, the Fe-A iron salt solution is added dropwise to the L-cysteine ​​solution, and the pH of the system is maintained at 4.8-5.6 using a sodium bicarbonate aqueous solution with a mass fraction of 0.1%-1.0%. The mixture is stirred for 60-120 min to obtain the sulfur-containing iron pre-complexed activated solution.

6. The method according to claim 1, characterized in that, In step S3, the sulfur-iron pre-complexed activating liquid is sprayed onto the citric acid-conditioned straw in 4 to 5 applications. After each spraying, the straw is turned over and mixed. After spraying, the thickness of the straw spread is controlled to be 8 to 12 cm, and it is left to stand at 20 to 30°C for 10 to 14 hours.

7. The method according to claim 1, characterized in that, In S4, the Fe-B iron salt solution is sprayed onto the straw treated in S3 in 3 to 4 applications. After each spraying, the pH of the straw extrusion liquid is adjusted to 5.8 to 6.8 with a sodium bicarbonate aqueous solution of 0.1% to 1.0% by mass. After all Fe-B is added, the mixture is left to stand for 20 to 28 hours.

8. The method according to claim 1, characterized in that, In S5, deionized water is used for rinsing. The amount of deionized water is 100mL per 100g of wet straw. After rinsing, the straw is dried in a ventilated environment at 30℃~40℃ until the moisture content is 30%~40% based on wet basis mass fraction.

9. The method according to claim 1, characterized in that, The cadmium-arsenic contaminated soil described in S6 refers to the topsoil layer of 0-20cm, with a soil pH of 5.5-7.0, a total cadmium content of 0.5-3.0 mg / kg, and a total arsenic content of 20-80 mg / kg. After applying the activated straw, the soil moisture content is adjusted to 55%-75% of field capacity.

10. The method according to claim 1, characterized in that, The mugwort was a transplanted seedling, with a plant height of 10-18cm. The plant samples were collected from the above-ground harvested parts.

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