Rapid electrokinetic remediation method for cadmium-polluted rice field soil

Abstract

A rapid electrokinetic remediation method for cadmium-polluted rice field soil comprises the steps that a cadmium activating agent is added into the soil, the adding amount is 0.1 mol / kg to 0.3 mol / kg, the water content of the soil is adjusted, low-voltage direct-current electric fields are arranged at the two ends of the polluted soil, the voltage gradient of the direct-current electric fields is 0.5 V / cm to 2 V / cm, the electrode distance is 0.2 m to 0.5 m, and a cadmium adsorption material is arranged near a cathode or between the cathode and an anode. Through the combined design of activation, migration and adsorption, the remediation efficiency can be greatly improved, the remediation time of traditional electrokinetic remediation is shortened, land occupation in the remediation process is reduced, and meanwhile use of electrolyte and subsequent treatment of cadmium-containing waste liquid are reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of environmental protection and relates to a rapid electric repair method for paddy field soil contaminated by cadmium. Background Art

[0002] The treatment and restoration of farmland soil contaminated by heavy metals has always been one of the hot spots and difficulties in soil environmental science research. The current mainstream restoration of farmland contaminated by heavy metals focuses on reducing the effectiveness of heavy metals in the soil and reducing the accumulation of heavy metals in crops. This type of method cannot reduce the total amount of heavy metals in the soil and it is difficult to completely solve the problem of heavy metal pollution in farmland soil. Therefore, soil heavy metal reduction and restoration technology aimed at reducing the total amount of heavy metals in the soil has attracted more and more attention, and it is also one of the urgent needs for the treatment and restoration of farmland heavy metal contaminated soil.

[0003] Electroremediation technology is a heavy metal contaminated soil reduction technology. It is easy to operate and suitable for different soil types and pollutants. It is especially suitable for the remediation of contaminated soil in sites with heavy and deep pollution. In recent years, electroremediation technology has also begun to be applied to the remediation of contaminated farmland soil. However, farmland pollution has the characteristics of a large area (usually tens of acres to thousands of acres) and shallow pollution depth (usually in the crop root layer, 20cm deep). Therefore, traditional electroremediation is difficult to apply to the remediation of contaminated farmland soil. Generally speaking, traditional electroremediation technology usually requires the establishment of electrode wells to install electrodes, and requires a large amount of electrolyte to saturate the soil. In the later stage of treatment, it is also necessary to treat the waste liquid containing pollutants; in addition, the electrode spacing of electroremediation is generally 1 to 3m. This electrode spacing invisibly expands the migration distance of pollutants in the soil, thereby prolonging the treatment time. In addition, in electroremediation, heavy metals generally accumulate in a certain part of the soil and are difficult to move out of the soil body, which is called the "accumulation effect". It is usually necessary to take methods such as extending the treatment time and adjusting the soil pH to reduce the generation of the "agglomeration effect", which makes it difficult to achieve the purpose of rapid remediation. Therefore, there is an urgent need for a new rapid electric remediation technology for heavy metal pollution in farmland. Summary of the invention

[0004] Technical problem to be solved: In order to overcome the deficiencies of the above-mentioned prior art, the present invention provides a rapid electrokinetic remediation method for cadmium-contaminated paddy field soil, which can avoid the occurrence of "accumulation effect" and quickly remove cadmium in the soil, while reducing the use of electrolyte and subsequent treatment of cadmium-containing waste liquid.

[0005] Technical solution: A rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy field soil, the steps are: adding a cadmium activator to the soil, the addition amount is 0.1-0.3 mol / kg soil, adjusting the soil moisture content, setting a low-voltage DC electric field at both ends of the contaminated soil, the voltage gradient of the DC electric field is 0.5-2V / cm, the electrode spacing is 0.2-0.5m, and cadmium adsorption material is set near the cathode or in the middle of the cathode and anode.

[0006] Preferably, the cadmium activator is a small molecule organic acid.

[0007] More preferably, the above-mentioned small molecule organic acid is citric acid, lactic acid or oxalic acid.

[0008] Preferably, the dosage is 0.1 mol / kg soil when the pH is less than 5.5, 0.15 mol / kg soil when the pH is between 5.5 and 6.5, 0.2 mol / kg soil when the pH is between 6.5 and 7.5, and 0.3 mol / kg soil when the pH is greater than 7.5.

[0009] Preferably, the above-mentioned regulated soil moisture content is not less than 100% of field capacity.

[0010] Preferably, the voltage gradient is 1.5 V / cm.

[0011] Preferably, the electrode spacing of the above DC electric field is 0.4 m, and the electrode depth is 0.2 m.

[0012] Preferably, the anode material is a ruthenium-iridium-titanium electrode, and the cathode material is graphite or a stainless steel plate / mesh; the number of the cathode and anode electrodes is arranged in pairs in a 1:1 ratio.

[0013] Preferably, the cadmium adsorption material is activated carbon or ion exchange resin.

[0014] Preferably, the above-mentioned cadmium adsorption material is used in an amount of 10 g / kg soil.

[0015] Beneficial effects: ① The remediation efficiency is high and the time consumption is short, which is significantly better than the traditional electric remediation technology; ② The technology of setting cadmium adsorption materials in the soil avoids the use of large amounts of electrolyte in traditional electric remediation and the subsequent treatment of cadmium-containing wastewater; ③ It effectively avoids the occurrence of the "accumulation effect", shortens the remediation time, and achieves the purpose of rapid soil remediation. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Distribution of total cadmium in soil after switching electric field treatment.

[0017] Figure 2 Distribution of total cadmium in soil after fixed electric field treatment. DETAILED DESCRIPTION

[0018] By adding activators to the soil to activate the cadmium in the soil and adjusting the soil moisture content, a low-voltage DC electric field is set at both ends of the contaminated soil. The cadmium in the soil migrates under the action of the electric field. At the same time, adsorption materials are set at appropriate locations in the soil to adsorb and fix the mobile cadmium, shortening the migration distance of the cadmium and achieving the purpose of rapid soil repair.

[0019] The specific steps taken are:

[0020] ① Land preparation: The soil of the paddy field to be repaired should be prepared, the soil particles should be smaller than 5mm, the land should be kept flat and have water storage capacity;

[0021] ② Add activator: Use small molecule organic acid (citric acid, lactic acid, oxalic acid, etc.) as activator, prepare the activator into solution with water, and spray it evenly on the soil surface. According to the different soil pH, the dosage is 0.1-0.3 mol / kg soil. The higher the soil pH, the more dosage is needed.

[0022] ③ Regulate soil moisture: spray irrigation water into the soil to make the field water holding capacity of the soil reach 100% or keep it flooded;

[0023] ④Buried electrodes: Insert electrodes vertically into the soil, with an electrode spacing of 0.2 to 0.5 m and an electrode depth of 0.2 m. The anode material is a corrosion-resistant electrode material, such as ruthenium iridium titanium electrode, and the cathode material is conductive, such as graphite, stainless steel plate / net, etc. The cathode and anode electrodes are arranged in pairs in a 1:1 ratio;

[0024] ⑤Installation of adsorbent material: There are two installation positions for adsorbent material. One is to install the adsorbent material between the cathode and the soil with a fixed direction electric field. The other is to set the adsorbent material in the middle of the anode and the cathode with an exchange electric field that regularly exchanges the positive and negative electrodes. The types of adsorbent materials include activated carbon, ion exchange resin, etc., and the dosage is 10g / kg soil.

[0025] ⑥ Set up a DC electric field: Use wires to connect multiple anodes and cathodes in series, and connect them to the positive and negative electrodes of a DC power supply. Determine the voltage based on the distance between the electrodes, and the voltage gradient is 0.5 to 2.0 V / cm. Convert AC power into DC power through a DC regulated power supply for repair use.

[0026] ⑦ Operation: Continuously supply power, and the restoration can generally be completed in 7 days; after the restoration is completed, the adsorption material and electrodes are removed, and the land can be reclaimed.

[0027] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0028] Embodiment 1:

[0029] Cadmium-contaminated paddy soil from Jiujiang, Jiangxi Province was collected as test soil. The soil had a pH of 4.20, a conductivity of 0.46 mS / cm, and a total cadmium content of 2.51 mg / kg, which was 1.67 times the 1.50 mg / kg control value standard (pH ≤ 5.5) in the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard (Trial) GB15618-2018". The experimental treatment design in Table 1 was adopted, with a voltage gradient of 1 V / cm, a treatment time of 7 days, and an electrode spacing of 0.2 m. T1 was the control treatment, and no strengthening measures were adopted; T2 treatment set the adsorption material activated carbon in the middle of the soil column, and switched the electric field at 3.5 days. T3 treatment added citric acid as an activator at a ratio of 0.1 mol / kg soil on the basis of T2. T4 replaced the activated carbon with ion exchange resin as the adsorption material on the basis of T3. After the experiment, the soil was divided into 5 equal parts from the anode to the cathode, marked as S1 to S5. After air-drying, the total cadmium content in the soil was determined, and the total cadmium removal rate in the soil was calculated.

[0030] Table 1. Processing design of switching electric field

[0031] deal with Activator Adsorption material Adsorbent material location Electric field method Voltage gradient T1 none none none One-way electric field 1V / cm T2 none Activated carbon Middle position, S3 3.5 days to switch the electric field direction 1V / cm T3 Citric Acid Activated carbon Middle position, S3 3.5 days to switch the electric field direction 1V / cm T4 Citric Acid Cation exchange resin Middle position, S3 3.5 days to switch the electric field direction 1V / cm

[0032] Figure 2 Table 2 shows the distribution of soil total cadmium content and removal rate after 7 days of electrokinetic treatment. For T1, soil cadmium migrated to the cathode, showing a trend of gradually increasing from the anode to the cathode. The total cadmium in the soil of sections S1 and S2 near the anode decreased significantly, and the removal rate could reach 79.7%, but the soil cadmium accumulated significantly in S4-S5, indicating that the cadmium migrated from the anode accumulated in the soil near the cathode (i.e., the "accumulation effect"), resulting in a total cadmium removal rate of only 5.02%. In the T2 treatment, activated carbon adsorption material was set in the middle of the soil column, and the electric field was switched at the same time. The soil cadmium migrated under the action of the electric field and was adsorbed and fixed when passing through the activated carbon, thus separating it from the soil. According to the research results, the soil cadmium content in sections S2-S4 was lower than that in S1 and S5, indicating that the activated carbon material set adsorbed the cadmium in the soil, and the total cadmium removal rate was 23.3%. T3 added citric acid for activation on the basis of T2, further improving the migration of cadmium in the electric field, thereby improving the removal rate of total cadmium in the soil to 31.6%. T4 replaced the adsorption material from activated carbon to cation exchange resin, which greatly improved the removal rate of total cadmium in the soil to 61.8%.

[0033] Table 2. Removal rate of total soil cadmium in switching electric field treatment

[0034] deal with S1 S2 S3 S4 S5 Total removal rate T1 79.7％ 78.9％ 55.4％ -63.7％ -125％ 5.02％ T2 13.5％ 45.8％ 47.0％ 27.5％ -17.5％ 23.3％ T3 53.8％ 25.5％ 14.9％ -7.97％ 71.7％ 31.6％ T4 36.7％ 41.0％ 66.7％ 84.9％ 79.7％ 61.8％

[0035] Note: Negative values ​​indicate that soil cadmium levels are higher than the original values.

[0036] Embodiment 2:

[0037] The cadmium-contaminated paddy soil in Jiujiang, Jiangxi Province was used as the test soil, and the soil properties were the same as those in Example 1. The experimental treatment design of Table 3 was adopted, with an electric field in a fixed direction, a treatment time of 7 days, and an electrode spacing of 0.2m. Among them, T1 was a control treatment, and no strengthening measures were adopted; T5 treatment was to set an adsorption material cation exchange resin between the cathode and the soil to improve the adsorption and fixation capacity of cadmium migrated to the cathode area; T6 treatment was to add citric acid for pre-saturation on the basis of T5 treatment, and the dosage of citric acid was 0.1mol / kg soil, the purpose was to activate soil cadmium and strengthen its migration in the electric field; T7 was to increase the voltage to 1.5V / cm on the basis of T6, which can further accelerate the migration rate of soil cadmium, thereby achieving the purpose of removing cadmium in the soil in a shorter time. After the experiment, the soil was divided into 5 equal parts from the anode to the cathode, marked as S1~S5, and the total cadmium content of the soil was measured after air drying, and the total cadmium removal rate of the soil was calculated.

[0038] Table 3. Treatment design for fixed electric field

[0039] deal with Activator Adsorption material Adsorbent material location Voltage gradient Electric field direction T1 none none none 1V / cm Fixed electric field T5 none Cation exchange resin Near the cathode, S6 1V / cm Fixed electric field T6 Citric Acid Cation exchange resin Near the cathode, S6 1V / cm Fixed electric field T7 Citric Acid Cation exchange resin Near the cathode, S6 1.5V / cm Fixed electric field

[0040] Figure 2 Table 4 shows the distribution of total cadmium content and removal rate in the soil after 7 days of electrokinetic treatment. In the T1 treatment, no strengthening measures were taken, and cadmium accumulated in the S4-S5 section of the soil, with a total cadmium removal rate of only 5.02%. In the T5 treatment, a cation exchange resin was set near the cathode as an adsorption material for cadmium ions. On the one hand, it can slow down the alkalinization of the soil near the cathode, and on the other hand, it can also timely adsorb cadmium migrated from the soil and reduce the accumulation of cadmium in the soil. The accumulation of cadmium near S4 and S5 in this treatment is significantly lower than that in T1, and the total cadmium removal rate is increased to 22.5%. In the T6 treatment, citric acid was added to pre-saturate the soil, which can activate the cadmium in the soil and facilitate the migration of cadmium. From Figure 2 From the results, the total cadmium content in each section of the soil has decreased significantly, and is significantly lower than T1 and T5, with a removal rate ranging from 63.7% to 84.9%, and a total removal rate of 77.1%. The T7 treatment further increased the voltage gradient. Generally speaking, the higher the voltage gradient, the faster the migration of ions in the soil. From the results, the removal rate of total cadmium in each section of the soil under T7 treatment reached 89.2% to 92.0%, and the average removal rate reached 90.8%; the total cadmium content in the soil after treatment was 0.20 to 0.27 mg / kg, which is lower than the screening value standard of 0.30 mg / kg in the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard (Trial) GB15618-2018", indicating that the cadmium-contaminated soil has been completely repaired.

[0041] Table 4. Removal rate of total soil cadmium in fixed electric field treatment

[0042] deal with S1 S2 S3 S4 S5 Total removal rate T1 79.7％ 78.9％ 55.4％ -63.7％ -125％ 5.02％ T5 87.5％ 78.8％ 12.6％ -2.44％ -64.2％ 22.5％ T6 84.9％ 79.7％ 80.9％ 76.5％ 63.7％ 77.1％ T7 90.0％ 92.0％ 92.0％ 90.8％ 89.2％ 90.8％

[0043] Note: Negative values ​​indicate that soil cadmium levels are higher than the original values.

[0044] Through the analysis of the above examples, it can be known that the removal rate of total cadmium in soil by single electric field is 5.02%, the removal rate of total cadmium in soil by single activator treatment is 8.30%, and the removal rate of total cadmium in soil by cation exchange resin as adsorption material is 17.5%. When the electric field, activator and cation exchange resin are combined, the removal rate of total cadmium in soil reaches 77.1%, which is 2.5 times the sum of the removal rates of 30.8% when the three are treated alone. This shows that the combination of the three has an unexpected synergistic effect, which is not obvious to technicians in this field. The reason for this synergistic effect is that the use of activator (citric acid) greatly increases the mobility of soil cadmium, which moves to the cathode under the action of the electric field; because the adsorption material (cation exchange resin) is set in the cathode area, on the one hand, it has an effect on the OH generated by the cathode. - It has a certain buffering capacity to prevent the precipitation of cadmium that moves to the cathode. On the other hand, it can timely adsorb the cadmium that moves from the soil and reduce the accumulation of cadmium in the soil near the cathode. The decrease in the concentration of cadmium in the soil near the cathode is in turn further conducive to the migration of cadmium to the cathode, thereby improving the overall removal rate of cadmium. In addition, under this combination condition, increasing the voltage gradient (1.5V / cm) can further increase the soil cadmium removal rate to 90.8%. However, if only the voltage gradient is increased without combining the activator and adsorption material, the soil cadmium removal rate will not be significantly improved. This is because although increasing the voltage gradient can speed up the migration rate of cadmium to the cathode, the increase in the voltage gradient will cause the cathode to produce more OH - If OH cannot be balanced in time - This will lead to the accumulation of cadmium near the cathode, making it difficult to significantly increase the removal rate of cadmium. This shows from another perspective that the combination of the three has a significant synergistic effect.

[0045] Embodiment 3:

[0046] The cadmium-contaminated paddy soil in Xiangtan, Hunan was selected as the test soil. The optimal technical parameters obtained by T7 were used to carry out the electrokinetic remediation test treatment, and the treatment was compared with the T7 treatment of cadmium-contaminated paddy soil in Jiujiang, Jiangxi as the test soil to evaluate the adaptability of this technology on different cadmium-contaminated soils. The soil properties of Xiangtan soil are as follows: pH is 6.90, conductivity is 0.18mS / cm, and total cadmium content is 12.5mg / kg, which is 4.16 times the 3.0mg / kg control value standard (pH≤5.5) in the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard (Trial) GB15618-2018". The experimental treatment design in Table 5 was adopted, with a fixed direction electric field and a treatment time of 7 days. T7 was the treatment using Jiujiang soil; T8 and T9 were treated with Xiangtan soil. The treatment conditions of T8 were the same as those of T7. In T9 treatment, considering the high pH of Xiangtan soil, the dosage of citric acid was increased to 0.15mol / kg soil. After the experiment, the soil was divided into 5 equal parts from the anode to the cathode, marked as S1 to S5. After air-drying, the total cadmium content in the soil was determined, and the removal rate of the total cadmium in the soil was calculated.

[0047] Table 5. Technical adaptability studies on different soils

[0048]

[0049] Table 6 shows the distribution of total cadmium content and removal rate in soil after 7 days of electrokinetic treatment. Since there are large differences in soil properties between Xiangtan soil and Jiujiang soil, especially in soil pH, the former soil pH is 6.95, which is much higher than the latter's 4.17. Therefore, under the same sampling treatment measures, especially the same citric acid dosage conditions, the total cadmium removal rate of Xiangtan soil in S4 and S5 sections is lower than that of Jiujiang soil. This is mainly because the citric acid dosage of 0.1 mol / kg soil is not enough to balance the soil pH at S4 and S5 and activate the soil cadmium, resulting in the accumulation of cadmium migrated from the anode in S4 and S5 sections. In the T9 treatment, the dosage of citric acid was further increased to 0.15 mol / kg soil, and the removal rate of total cadmium in the soil was improved, especially the total cadmium removal rate of the S4 section reached more than 90%, but the soil cadmium in the S5 section still accumulated. Therefore, the dosage of citric acid should be further increased to improve the total cadmium removal rate of the S5 section. Based on the analysis and calculation of the relationship between soil pH and total cadmium removal rate in the above test, the empirical value of citric acid dosage when achieving 90% soil total cadmium removal rate is given, see Table 7.

[0050] Table 6. Total cadmium content and removal rate of soil after different soil treatments

[0051]

[0052] Note: Negative values ​​indicate that soil cadmium content is higher than the starting value.

[0053] Table 7. Empirical values ​​of soil citric acid dosage

[0054] Soil pH <5.5 5.5～6.5 6.5～7.5 >7.5 Citric acid dosage (mol / kg soil) 0.1 0.15 0.2 0.3

[0055] The contents explained in the above embodiments should be understood as these embodiments are only used to more clearly illustrate the present invention, rather than to limit the scope of the present invention. Those skilled in the art can also obtain other embodiments based on these embodiments without creative work, and these embodiments all belong to the protection scope of the present invention.

Claims

1. A rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil, characterized in that: The steps are: adding a cadmium activator into the soil in an amount of 0.1-0.3 mol / kg soil, adjusting the soil moisture content, setting a low-voltage direct current electric field at both ends of the contaminated soil, the voltage gradient of the direct current electric field is 0.5-2 V / cm, the electrode spacing is 0.2-0.5 m, and setting a cadmium adsorption material near the cathode or between the cathode and the anode.

2. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The cadmium activator is a small molecule organic acid.

3. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 3, characterized in that: The small molecule organic acid is citric acid, lactic acid or oxalic acid.

4. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: When the pH is less than 5.5, the dosage is 0.1 mol / kg soil; when the pH is between 5.5 and 6.5, the dosage is 0.15 mol / kg soil; when the pH is between 6.5 and 7.5, the dosage is 0.2 mol / kg soil; when the pH is greater than 7.5, the dosage is 0.3 mol / kg soil.

5. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The soil moisture content is adjusted to not less than 100% of field capacity.

6. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The voltage gradient is 1.5 V / cm.

7. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The electrode spacing of the DC electric field is 0.4 m, and the electrode depth is 0.2 m.

8. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The anode material is a ruthenium-iridium-titanium electrode, and the cathode material is graphite and a stainless steel plate / net; the number of the cathode and anode electrodes is arranged in pairs in a ratio of 1:

1.

9. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The cadmium adsorption material is activated carbon or ion exchange resin.

10. The rapid in-situ electrokinetic remediation method for cadmium-contaminated paddy soil according to claim 1, characterized in that: The cadmium adsorption material is used in an amount of 10 g / kg soil.

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