Method for repairing heavy metal polluted sludge

Abstract

The invention relates to the technical field of soil body repairing, and discloses a method for repairing heavy metal polluted sludge, which comprises the following steps: by taking an iridium tantalum coating titanium electrode as an anode and a conductive electrode as a cathode, adding a complexing agent into heavy metal polluted soft soil, putting one end of the anode and one end of the cathode into the heavy metal polluted soft soil, after the other ends of the two ends are jointly and electrically connected with a power source, the soft soil polluted by the heavy metal is subjected to soil body restoration and soil body reinforcement by adopting a vacuum combined intermittent multi-stage step-by-step pressurization electroosmosis technology; the vacuum combined intermittent multistage step-by-step pressurization electroosmosis technology comprises the following steps: under vacuum, applying voltage to heavy metal polluted soft soil step by step through a power supply, an anode and a cathode, and carrying out electroosmosis by adopting power-on and power-off intermittent treatment. According to the method, vacuum preloading, electroosmosis reinforcement and electrodynamic remediation are combined, heavy metal ions in the heavy metal polluted soft soil are removed, and the heavy metal polluted soft soil is subjected to soil body reinforcement.

Description

Technical Field

[0001] The invention belongs to the technical field of soil restoration, and in particular relates to a method for restoring heavy metal polluted sludge. Background Art

[0002] Inorganic and organic pollutants are the main sources of pollution in contaminated sludge. Heavy metals are the main pollutants among inorganic pollutants. Due to the poor mobility of heavy metal pollutants in soil, they stay in the soil for a long time and are not easily degraded by microorganisms, resulting in the continuous accumulation of heavy metals, which has a serious impact on the surrounding environment and the health of residents. The remediation methods for heavy metal contaminated soil include ex situ remediation and in situ remediation. For soft soil, it has the characteristics of high water content and low permeability, which makes the remediation agents used in various remediation methods unable to enter the dense structure of the contaminated soil, resulting in poor remediation effects.

[0003] In order to solve the above problems, in 1809 Reuss used the potential difference to move the liquid in the soil and reinforce the soil. Therefore, existing research proposes to use electroosmosis to reinforce soft soil silt with high water content and low permeability; in addition, the use of electroosmosis will also cause heavy metal ions in the soil to be enriched in specific areas under the action of the electric field, or discharged from the soil with the electroosmosis tail water, thereby achieving the repair of the contaminated soil. The existing technology also studies electrodynamic remediation technology, which reinforces soft soil by adding flocculants, or improves the desorption capacity of heavy metals by adding chelating agents.

[0004] However, existing research focuses only on electroosmosis reinforcement or electrodynamic remediation, and the two are often not used in combination because of the mutual influence between the two. Electroosmosis reinforcement will be accompanied by a decrease in soil permeability, hindering the discharge of heavy metals in the soil. If the drainage rate during the electroosmosis process is too fast, the heavy metals will not be fully desorbed, and the water in the soil will be discharged under the action of the electric field, thus affecting the removal rate of heavy metals; at this time, the electrodes of electrodynamic remediation and the added flocculants or chelating agents may become new sources of pollution, causing secondary pollution to the soil. Summary of the invention

[0005] In view of the deficiencies of the above-mentioned prior art, the present invention provides a method for repairing heavy metal contaminated sludge. The present invention uses an iridium-tantalum coated titanium electrode as an anode and a conductive electrode as a cathode. After adding a chelating agent to the soft soil contaminated by heavy metals, a vacuum combined with intermittent multi-stage step-by-step pressurization electroosmosis technology is adopted. By combining vacuum preloading, electroosmosis reinforcement and electrodynamic repair, heavy metal ions in the soft soil contaminated by heavy metals are removed, and the soft soil contaminated by heavy metals is reinforced.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for repairing heavy metal polluted sludge, using an iridium-tantalum coated titanium electrode as an anode, the iridium-tantalum coated titanium electrode is purchased, and the reason for using the iridium-tantalum coated titanium electrode as the anode is that it has excellent corrosion resistance, heavy metal removal ability and drainage ability, and using a conductive electrode as a cathode. There are no strict requirements for the selection of the cathode electrode, and any metal electrode that can achieve conductivity can be applied to the present invention, adding a chelating agent to the soft soil polluted by heavy metals, placing one end of the anode and the cathode in the soft soil polluted by heavy metals, and after the other ends are electrically connected to a power supply, a vacuum combined intermittent multi-stage step-by-step pressurized electroosmosis technology is used to repair and reinforce the soft soil polluted by heavy metals;

[0008] The vacuum combined intermittent multi-stage step-by-step pressurized electroosmosis technology is as follows: under vacuum, voltage is applied step by step to the soft soil contaminated by heavy metals through a power supply, an anode and a cathode, and electroosmosis is carried out by intermittent treatment of power on and power off.

[0009] Preferably, the operation of the intermittent multi-stage step-by-step pressurized electroosmosis technology is: repeatedly power on for 3-4 hours and power off for 1-2 hours at a voltage of 12-20V, for a total of 48 hours; continue to repeat power on for 3-4 hours and power off for 1-2 hours at a voltage of 24-40V, for a total of 48 hours; then repeat power on for 6-8 hours and power off for 2-4 hours at a voltage of 36-60V, for a total of 48 hours. The more step-by-step loading times, the higher the heavy metal removal rate.

[0010] Preferably, the vacuum pressure is 20-80 KPa.

[0011] Preferably, several pipes are inserted into the soft soil contaminated by heavy metals, the pipes are close to the middle of the soft soil contaminated by heavy metals and the cathode, the middle is the middle position between the cathode and the anode, and the chelating agent is discharged into the soft soil contaminated by heavy metals through the pipes, so as to facilitate the addition of the chelating agent into the dense structure of the soft soil contaminated by heavy metals.

[0012] Preferably, the complexing agent is selected from carboxymethyl cellulose, chitosan or sodium alginate.

[0013] Preferably, the complexing agent is selected from sodium alginate.

[0014] Preferably, the mass ratio of the complexing agent to the solid components in the soft soil contaminated by heavy metals is 1-2:100.

[0015] Preferably, in the intermittent multi-stage step-by-step pressurized electroosmosis technology, the current intensity of the soft soil contaminated by heavy metals is maintained at 1000-2000mA.

[0016] Preferably, the method for repairing heavy metal contaminated sludge specifically comprises the following steps:

[0017] The anode and cathode are fixed in a model box with a drainage port, and soft soil contaminated by heavy metals is added into the model box. Then the model box is sealed, and the drainage port is sealed and connected to the water-gas separation bottle;

[0018] The anode and cathode are electrically connected to a power source, and after the model box is evacuated by a vacuum pump, a voltage is provided to the electrodes by the power source, and a vacuum combined intermittent multi-stage step-by-step pressurized electroosmosis technology is used to repair and reinforce the soft soil contaminated by heavy metals;

[0019] Specifically, Fig.13 As shown, a model box, a vacuum preloading test device, an electrochemical test device and a grouting device are used to repair heavy metal contaminated sludge. The vacuum preloading test device consists of a vacuum pump, a vacuum gauge, a water-gas separation bottle and a drainage board. The vacuum pump adjusts the vacuum pressure, the vacuum gauge reads the vacuum pressure, and detects whether the equipment has leakage. The water-gas separation bottle stores the discharged water, which is convenient for drainage during electroosmosis reinforcement and electrodynamic repair to achieve soil reinforcement; the electrochemical test device is located in the model box and consists of a power supply, an anode plate and a cathode plate. The anode plate is an iridium-tantalum-titanium anode plate, and the cathode plate is a titanium cathode plate. The titanium cathode is a titanium plate with uniformly distributed small holes and is connected to the drainage board; the grouting device consists of a grouting pipe, a diaphragm pump, a pipeline and an air compressor. The pipeline is inserted into the soft soil contaminated by heavy metals. The complexing agent passes through the diaphragm pump under the negative pressure formed by the air compressor and is discharged into the pipeline through the grouting pipe.

[0020] Preferably, the moisture content of the heavy metal-contaminated soft soil is 75-85%.

[0021] Preferably, the conductive electrode is a conductive metal electrode, and the conductive metal is selected from platinum, gold, silver, copper, titanium or nickel.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. Soft soil has the characteristics of high water content and low permeability, and cannot be repaired by conventional repair methods. The present invention adopts vacuum combined intermittent multi-stage step-by-step pressurization electroosmosis technology to repair and reinforce soft soil contaminated by heavy metals, and adopts conductive cathode, iridium-tantalum coated titanium anode, power supply and chelating agent to facilitate electrodynamic repair, and adopts conductive cathode, iridium-tantalum coated titanium anode and power supply to facilitate electroosmosis reinforcement, and then combines vacuum preloading method to improve soil conductivity, overcomes the technical defect that soft soil cannot be repaired, and realizes the combined repair of electroosmosis reinforcement and electrodynamic repair.

[0024] In order to solve the problem that long-term power supply will cause the soil around the electrode to crack too quickly, the electrode contact resistance will increase significantly, the effective potential will be greatly reduced, and the comprehensive energy consumption and cost will be high, the present invention adopts traditional electro-osmosis test to find the intermittent combined step-by-step loading mode with the lowest comprehensive energy consumption, moderate cost, and the best consolidation effect and heavy metal removal rate.

[0025] When metal electrodes are used to treat soft soil contaminated by heavy metals, the metal electrodes produce a large amount of metal ions in the acidic environment of the anode, which not only causes secondary pollution to the soil but also interferes with the desorption effect of the chelating agent and causes a large potential loss problem. The present invention uses an iridium-tantalum coated titanium electrode with excellent corrosion resistance, heavy metal removal ability and drainage ability as the anode to overcome the technical defects caused by the use of metal electrodes.

[0026] Considering that the solution of conventional EDTA elution solution is acidic, it will affect the properties of the soil after elution and change the pH value of the soil, thereby affecting the removal effect of heavy metals and causing secondary pollution to the soil. The present invention uses biopolymer sodium alginate as a chelating agent, which is not only a degradable organic flocculant, but also has an adsorption effect on heavy metals and has excellent reinforcement and decontamination effects on the soil, thereby overcoming the technical defects caused by the use of EDTA elution solution.

[0027] In order to address the problems of a certain degree of cracking of the soil during the electroosmosis process, which causes a decrease in soil conductivity, uneven soil consolidation, clogging of drainage plates, and inability to discharge weakly bound water in the soil, the present invention combines the advantages of the vacuum preloading method and the electroosmosis method to overcome their respective shortcomings, thus giving rise to the vacuum preloading combined with the electroosmosis method and the electrodynamic repair method. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the schematic diagram of the electroosmosis test and soil sampling points;

[0029] Figure 2 Ni of groups T1-T6 under different loading modes + Pollutant concentration distribution cloud map;

[0030] Figure 3 Figure 2 shows the removal rate of heavy metals at different positions of groups T1-T6 under different loading modes;

[0031] Figure 4 It is the graph of cumulative discharge of groups T1-T6 over time under different loading modes;

[0032] Figure 5 The diagrams of shear strength and moisture content changes of groups T1-T6 under different loading modes, where (a) is the anode area; (b) is the middle area; (c) is the cathode area;

[0033] Figure 6 The curves of the current intensity of groups T1-T6 changing with time under different loading modes;

[0034] Figure 7 Ni groups T1-T6 under different electrode materials and complexing agents +Pollutant concentration distribution cloud map;

[0035] Figure 8 Figure 2 shows the heavy metal removal rates at different locations of groups T1-T6 under different electrode materials and complexing agents;

[0036] Fig. 9 This is the graph showing the cumulative water discharge of groups T1-T6 changing with time under different electrode materials and complexing agents;

[0037] Fig.10 The graphs of shear strength and moisture content of groups T1-T6 under different electrode materials and complexing agents, where (a) is the anode area; (b) is the middle area; (c) is the cathode area;

[0038] Fig.11 Curves of the current intensity of groups T1-T6 changing with time under different electrode materials and complexing agents;

[0039] Fig.12 Anode electrode corrosion diagram and average energy consumption coefficient diagram of groups T1-T6 under different electrode materials and complexing agent conditions, where A is the anode electrode corrosion diagram and B is the average energy consumption coefficient diagram;

[0040] Fig.13 It is the schematic diagram of vacuum combined electrochemical test and soil sampling points;

[0041] Fig.14 Ni of group T1-T6 under vacuum combined electrochemical loading conditions 2+ Pollutant concentration distribution cloud map;

[0042] Fig.15 Figure 2 shows the removal rate of heavy metals at different positions of groups T1-T6 under vacuum combined electrochemical loading conditions;

[0043] Fig.16 This is a curve of the cumulative water discharge of groups T1-T6 changing with time under vacuum combined electrochemical loading conditions;

[0044] Fig.17 The diagram shows the changes in shear strength and moisture content of the T1-T6 group under vacuum combined electrochemical loading conditions, where (a) is the anode area; (b) is the middle area; (c) is the cathode area;

[0045] Fig.18 This is a curve of the current intensity of groups T1-T6 changing with time under vacuum combined electrochemical loading conditions;

[0046] Fig.19 It is the average energy consumption coefficient diagram of T1-T6 groups under vacuum combined electrochemical loading conditions;

[0047] Fig. 20Ni after T1-T6 group tests under vacuum combined electrochemical loading conditions 2+ Concentration mean values ​​plot. DETAILED DESCRIPTION

[0048] The technical solutions in the present invention will be clearly and completely described below in combination with the specific implementation of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0049] Example 1

[0050] A method for repairing heavy metal contaminated sludge comprises the following steps:

[0051] Pour the soft soil contaminated by heavy metals into the model box, the soil sample depth is 160mm, the mass is 10kg, the water content is 80%, and the amount of nickel in the soil sample is 800mg / kg;

[0052] According to the mass ratio of sodium alginate to the solid components in the soft soil contaminated by heavy metals of 1:100, sodium alginate was weighed, and then 5L of pure water was used to prepare a sodium alginate solution; an iridium-tantalum coated titanium electrode was used as the anode and a titanium sheet was used as the cathode. Fig.13 Assemble according to the connection method shown;

[0053] After assembly, sodium alginate solution was grouted at a volume of 1000 mL / d, and under a vacuum pressure of 80 kPa, voltages of 20 V, 40 V and 60 V were applied in sequence during the electroosmosis process, and each was powered on for 48 h. When the voltage was 20 V, the power was turned on for 4 h and then off for 2 h, when the voltage was 40 V, the power was turned on for 4 h and then off for 2 h, and when the voltage was 60 V, the power was turned on for 8 h and then off for 4 h, and the intermittent operation was repeated.

[0054] Example 2

[0055] A method for repairing heavy metal contaminated sludge comprises the following steps:

[0056] Pour the soft soil contaminated by heavy metals into the model box, the soil sample depth is 160mm, the mass is 10kg, the water content is 80%, and the amount of nickel in the soil sample is 800mg / kg;

[0057] According to the mass ratio of sodium alginate to the solid components in the soft soil contaminated by heavy metals of 2:100, sodium alginate was weighed, and then 5L of pure water was used to prepare a sodium alginate solution; an iridium-tantalum coated titanium electrode was used as an anode and a conductive electrode was used as a cathode. Fig.13 Assemble according to the connection method shown;

[0058] After assembly, sodium alginate solution was grouted at a rate of 1000 mL / d, and under a vacuum pressure of 60 kPa, voltages of 15 V, 35 V and 55 V were applied in sequence during the electroosmosis process, and each was powered on for 48 h. When the voltage was 15 V, the power was turned on for 3 h and then off for 1 h. When the voltage was 35 V, the power was turned on for 3 h and then off for 1 h. When the voltage was 55 V, the power was turned on for 7 h and then off for 3 h, and the intermittent operation was repeated.

[0059] Example 3

[0060] Pour the soft soil contaminated by heavy metals into the model box, the soil sample depth is 160mm, the mass is 10kg, the water content is 80%, and the amount of nickel in the soil sample is 800mg / kg;

[0061] According to the mass ratio of sodium alginate to the solid components in the soft soil contaminated by heavy metals of 1.5:100, sodium alginate was weighed, and then 5L of pure water was used to prepare a sodium alginate solution; an iridium-tantalum coated titanium electrode was used as an anode and a conductive electrode was used as a cathode. Fig.13 Assemble according to the connection method shown;

[0062] After assembly, sodium alginate solution was grouted at a rate of 1000 mL / d, and under a vacuum pressure of 20 kPa, voltages of 12 V, 24 V and 36 V were applied in sequence during the electroosmosis process, and each was powered on for 48 h. When the voltage was 12 V, the power was turned on for 4 h and then off for 2 h, when the voltage was 24 V, the power was turned on for 4 h and then off for 2 h, and when the voltage was 36 V, the power was turned on for 8 h and then off for 4 h, and the intermittent operation was repeated.

[0063] The vacuum combined intermittent multi-stage step-by-step pressurized electroosmosis technology proposed in the present invention is obtained through experimental comparison. The following is a specific experimental study, and the research method and results are as follows:

[0064] 1. Detection method:

[0065] (1) Detection of heavy metal pollutant content: After drying, grinding and sieving the soil to be tested, weigh 0.5 g and put it into a polytetrafluoroethylene crucible. After adding ultrapure water to moisten it, add hydrochloric acid, nitric acid, hydrofluoric acid and perchloric acid in sequence. Heat until the sample decomposition product turns white or light yellow. Finally, rinse and dissolve the residue with nitric acid solution and ultrapure water. After cooling, make the volume up to 50 mL. Use inductively coupled plasma emission spectrometry to detect the concentration of heavy metal pollutants at different locations in the soil.

[0066] The sampling points for heavy metal pollutant content detection are as follows: Figure 1 As shown, three test data are collected at each sampling point and the average value is taken. For the removal rate of heavy metals, in the soil near the anode, middle and cathode, the calculation formula is:

[0067] Where: Ci is the initial heavy metal concentration in the soil; Cf is the heavy metal concentration in the anode, middle or cathode soil after the test.

[0068] (2) Moisture content test: Multiple soil samples were collected at each sampling point and moisture content tests were performed.

[0069] (3) Shear strength test: Use cross-plate shear test to test the strength of soil at different locations.

[0070] (4) Energy consumption calculation: Use the unit drainage energy consumption C as an indicator for calculation.

[0071] 2. Experimental study on grouting electrochemical reinforcement and repair of nickel-contaminated sludge under different loading modes:

[0072] The present invention selects dredged silt from Dongtou, Wenzhou, and refers to the untreated silt soil as soft soil. The results show that particles with a particle size of less than 75 μm account for more than 76%, and particles with a particle size of less than 5 μm account for 20%. Its physical properties are measured according to the "Standard for Geotechnical Test Methods" (GB / T 50123-2019), the moisture content is measured by the drying method, and the organic matter content is measured by the combustion method (CJ / T249-2007, 2007). The results are shown in Table 1:

[0073] Table 1 Basic properties of soft soil

[0074]

[0075] Dissolve NiCl2·6H2O in water to obtain a solution, add the solution into soft soil, prepare the soft soil with 800 mg / kg heavy metal pollution, and prepare the soil with a water content of 80% according to the test requirements.

[0076] 2.1 Experimental plan

[0077] Based on the electroosmosis test, iron was used as the electrode material, and a chelating agent of 0.05 mol / L EDTA was added by grouting. Pure water was used as the control group. The soft soil contaminated by heavy metals was loaded into the model box, and then potential probes were arranged, connected to the circuit, and voltage was applied to explore the effects of electroosmotic repair and reinforcement of contaminated sludge under different loading modes, analyze the heavy metal desorption mechanism, and preliminarily determine the optimal loading mode.

[0078] Table 2 Basic conditions for different loading mode tests

[0079]

[0080] In the table, EO (Electro-Osmosis) means constant voltage electro-osmosis test, SEO (Step-by-step loading voltage Electro-Osmosis) means step-by-step loading voltage electro-osmosis test, and I-SEO (Intermittent combined cascade loading voltage electroosmosis) means intermittent combined step-by-step pressure electro-osmosis.

[0081] 2.2 Heavy metal content detection and interference desorption analysis

[0082] (1) Heavy metal nickel concentration in soil: Figure 2 The following is a distribution diagram of pollutants in different areas of each group. The vertical axis represents the color corresponding to different concentrations, and the horizontal axis represents the soil at different distances from the anode. As the color tone changes from dark green to dark red, the concentration of heavy metal nickel increases. When EDTA is used as a complexing agent, heavy metal nickel ions are enriched in the middle area. When pure water is used as a complexing agent, heavy metals are enriched in the cathode area. Among them, the T3 group has the lowest concentration in the anode area, which is 293.84 mg / kg, the T2 group has the highest concentration in the middle, which is 1236.8 mg / kg, and the T4 group has the lowest concentration in the cathode area, which is 213.76 mg / kg. When EDTA is added to the soil, the desorbed nickel ions will react with EDTA. NiEDTA 2- The stability constant of the complex is several orders of magnitude higher than that of other substances. The specific chemical reaction equation is shown in 2-2:

[0083] Ni 2+ +Na2EDTA 2- =NiEDTA 2- +2Na + (2-2)

[0084] (2) Removal rate of heavy metal nickel in soil: Figure 3 As shown, the experimental results of groups T1-T6 showed obvious differences. In group T1, nickel ions were mainly enriched in the cathode area, and heavy metal nickel ions in groups T2-T6 were mainly enriched in the middle area. In addition, the heavy metal removal rates of group T1 in each area were significantly different compared with other groups, while the removal rates between groups T2-T6 were relatively close.

[0085] The removal rates of the anode and cathode areas of the T2 group were both above 55%, and the enrichment ratio of heavy metal nickel ions in the middle soil was about 55%, while the metal removal rates of the anode and middle areas of the T1 group were 37.78% and 30.06%, respectively, and it was enriched at the cathode by about 37%, which was at a lower level than the removal rates of the groups with EDTA added. The reason is that the use of pure water as a chelating agent lacks the desorption capacity of the chelating agent, and the release of heavy metal nickel ions depends on the acidic environment formed in the anode area. In this environment, nickel ions can be desorbed and flow to the cathode with the electrolyte under the action of the electric field force. After reaching the cathode, the nickel ions react with OH due to the alkaline cathode area. - The ions react to form a precipitate and accumulate in the cathode area. At the same time, some nickel ions are discharged into the cathode pool along with the electrolyte.

[0086] For the soil in the middle area, the added EDTA breaks the original adsorption equilibrium state inside the soil. The heavy metal nickel ions adsorbed in the soil are complexed and form complexes under the action of the chelating agent. Since the complex is weakly negatively charged, a part of the complex will flow toward the anode area, but at the same time, it will flow toward the cathode under the action of the electric field force. The migration rate is greater than the speed of the complex moving toward the anode caused by the weak negative charge, which eventually leads to the accumulation of heavy metals in the middle position; for the soil in the cathode area, EDTA can exert its desorption ability. Since the electric field force on the cathode area is smaller, which is less than the speed of the complex moving toward the anode caused by the weak negative charge, the generated weakly negatively charged complex begins to gradually move toward the anode.

[0087] The heavy metal nickel ions of the T3 and T4 groups were mainly enriched in the middle area, just like the T2 group, and the heavy metal removal rates were improved compared with the T1 group. Among them, the removal rate of the T3 group was the highest in the anode area, reaching 65.27%, and the removal rate in the cathode area was above 65%, and the corresponding enrichment rate in the middle area was 47.21%; the anode heavy metal removal rate of the T4 group was slightly lower than that of the T3 group by 1%, the enrichment rate in the middle area was 45.25%, and the removal rate in the cathode area increased by about 1%; the heavy metal nickel of the T5 and T6 groups was mainly concentrated in the middle area, and the average removal rates of the heavy metal nickel in the anode area and cathode area of ​​the two groups were 61.73% and 64.53% respectively, while the average enrichment rate in the middle area was 44.72%. The biggest difference in heavy metal removal rate between the two groups of tests is in the cathode area. The heavy metal content of the soil near the cathode of the T5 group, which uses step-by-step loading combined with intermittent power-on (power on for 4 hours and power off for 2 hours), decreased by 61.17%, and the heavy metal content of the soil in the cathode area of ​​the T6 group decreased by 67.89%. There is a relatively obvious difference between the two. In summary, the heavy metal nickel removal rate of the T5 and T6 groups using intermittent combined step-by-step loading is slightly lower than that of the T3 and T4 groups using only step-by-step loading, which is related to the shorter effective power-on time in the test method using intermittent combined step-by-step loading. The present invention improves the removal rate of heavy metal nickel by changing different loading methods.

[0088] 2.3 Discharge, moisture content and shear strength

[0089] (1) Displacement

[0090] Figure 4 The curve of the cumulative water displacement of each group over time is as follows. The water displacement collected from group T1 to group T6 is 1140mL, 1205mL, 1290mL, 1515mL, 1580mL and 1550mL, respectively.

[0091] It was found that the drainage of the T3 and T4 groups exceeded that of the T2 group. The reason was that the initial voltage of the T2 group was too high, which caused the soil to crack prematurely, reduced the contact area with the electrode, and increased the interface resistance. The T3 and T4 groups adopted the step-by-step pressurization method to effectively slow down the current decay and improve the drainage capacity in the middle and late stages. In particular, the T4 group had the highest final drainage, which was 17.4% higher than the T3 group, indicating that the more the number of step-by-step voltage loading levels, the more the drainage capacity of the soil can be improved.

[0092] The drainage volume of group T5 and group T6 was also greater than that of group T2. The final drainage volume of group T5 was greater than that of group T6, indicating that under the same on-off ratio, if the power-off time was too long, the drainage process would stagnate in the middle and late stages. At the same time, the water near the cathode would partially flow back due to capillary phenomena and reverse hydraulic gradient. Therefore, too long an intermittent power-on cycle would reduce the drainage efficiency, which was not conducive to the smooth progress of the electro-osmosis test.

[0093] In summary, the drainage capacity of the intermittent combined step-by-step loading voltage test is the strongest, followed by the step-by-step loading voltage loading mode. The more the loading voltage levels, the better the drainage effect of the electroosmosis method.

[0094] (2) Moisture content and shear strength

[0095] Figure 5 The moisture content and shear strength distributions of each group at different positions from the anode. As the distance from the anode continues to shorten, the moisture content gradually decreases and the shear strength increases accordingly. This phenomenon is related to the aggregation of pore water to the cathode under the action of the electric field in the electro-osmosis test.

[0096] The average values ​​of moisture content and cross-plate shear strength at all measuring points of groups T1-T6 were taken, and the average moisture content was 52.90%, 47.61%, 46.66%, 43.09%, 41.37%, and 42.49%, respectively. The average cross-plate shear strength was 34.33%, 39.67%, 39.67%, 43.33%, 45.33%, and 44.67, respectively. The average moisture content of groups T2-T6 was lower than that of group T1, and the average cross-plate shear strength was higher than that of group T1, which was related to the EDTA of groups T2-T6. The moisture content of group T1 after electroosmosis was much higher than that of the other five groups. Since its heavy metal concentration was also high, the average shear strength of group T1 was not significantly different from that of the other groups, indicating that the high heavy metal concentration had a certain effect on the shear strength. Similarly, the average moisture content of group T2 was slightly higher than that of group T3, but its average shear strength was the same as that of group T3, which was also related to its higher heavy metal concentration.

[0097] 2.4 Current intensity

[0098] like Figure 6 As shown in the figure, the initial current intensity of group T1 is much higher than that of groups T3 to T6, which is because the initial loading voltage of group T1 is 36V, which is higher than 12V of groups T3 to T6. In addition, groups T2 and groups T3 to T6 also show similar phenomena. Comparing groups T1 and T2, the initial current intensity of group T2 is higher than that of group T1, because group T2 added EDTA chelating agent to the soil through PVC pipe at the beginning of the experiment, which increased the content of ions in the soil and improved the conductivity inside the soil, while group T1 used pure water as a control test, and the ion concentration in the soil did not increase significantly, so the initial current intensity of group T2 was higher than that of group T1.

[0099] Comparing the time when the current of each group began to decay, it was found that the time when the current of group T1 and group T2 began to decay was earlier than that of group T3-T6. The main reason was that the initial loading voltage of group T1 and group T2 was too high, which led to rapid drainage in the early stage, causing soil contraction, making the anode and soil separate quickly, increasing the interface resistance, and having a greater impact on the current, resulting in the current decay time of group T1 and group T2 earlier than that of other groups. Group T1 began to decay 4 hours after power-on, earlier than group T2, which was related to the continuous addition of EDTA to increase the conductivity of the soil and reduce the current resistance between soil bodies.

[0100] By analyzing the attenuation amplitude of each group, it was found that the attenuation amplitude of groups T2-T4 was smaller than that of group T1, and the attenuation of group T4 was the smallest. The attenuation amplitude of groups T3 and T4 was smaller than that of group T2 because the initial loading voltage of group T2 was too high, which caused the soil to crack, reduced the contact area with the electrode and increased the interface resistance. Groups T3 and T4 adopted a step-by-step loading voltage mode, which effectively suppressed the current attenuation. By comparing groups T3 and T4, it was found that the attenuation amplitude of group T4 was large, indicating that the more loading levels, the smaller the current attenuation amplitude. In addition, intermittent combined step-by-step loading voltage can slow down the chemical reaction at the electrode and reduce anode corrosion. At the same time, the gas that hinders the contact between the electrode and the soil can be discharged during the power-off period. Therefore, after the soil is powered off and then powered on, the current intensity will increase. Excessive current intensity will cause the soil to consolidate too quickly, causing the soil to crack rapidly, resulting in an increase in soil resistance and a sharp drop in current intensity. Ultimately, the consolidation and decontamination effects of the soil are not ideal. Therefore, the current intensity needs to be stable in a certain range for a long time to obtain good soil treatment effect. The test results show that maintaining the current intensity at 1000mA-2000mA is the best choice.

[0101] 3. Experimental study on the effect of different electrode materials and complexing agents on the grouting electrochemical reinforcement and repair of nickel-contaminated sludge:

[0102] 3.1 Selection of electrode materials

[0103] When selecting electrode materials, their stability in strong acid or strong alkali environments, as well as the conductivity and electrochemical activity of the electrode itself should be considered. The present invention uses a graphite electrode with strong stability and low price as one of the control groups, but the electrochemical oxidation of the graphite electrode is relatively weak. Therefore, the present invention also studies the effect of titanium electrodes on electroosmosis. However, when the titanium electrode participates in electroosmosis as an anode, it releases active oxygen and forms TiO2, resulting in a decrease in the conductivity of the electrode. Therefore, a specially treated titanium electrode is selected, that is, iridium dioxide and tantalum pentoxide coatings are loaded on the electrode surface to form an iridium-tantalum coated titanium electrode (Ti|IrO2-Ta2O5) to optimize its conductivity.

[0104] 3.2 Selection of complexing agent

[0105] Complexing agent is one of the main factors affecting the electrodynamic repair effect. The present invention selects chitosan, sodium alginate, and carboxymethyl cellulose biopolymers as complexing agents, wherein chitosan as a flocculant and coagulant can purify heavy metal ions in water, and sodium alginate has low toxicity and good biocompatibility, and can improve the adsorption capacity of heavy metals. Existing studies have found that sodium alginate can not only remove heavy metal ions in water, but also be used as a desorption agent for heavy metals in contaminated soil.

[0106] 3.3 Experimental plan

[0107] Table 3 Basic conditions for different electrode materials and complexing agents tests

[0108]

[0109]

[0110] 3.3 Heavy metal content detection and interference desorption analysis

[0111] (1) Heavy metal nickel concentration in soil

[0112] like Figure 7 As shown, the heavy metal nickel ions of groups T1 to T3 using EDTA were enriched in the middle area. The concentration of group T3 was the lowest in the anode and cathode areas, which were 245.84 mg / kg and 233.28 mg / kg respectively, and the concentration of group T1 in the middle area was the highest, 1145.2 mg / kg. However, there was no large-scale enrichment in groups T4 to T6 using biopolymers as chelating agents. The heavy metal concentrations in each area were about 400 mg / kg. The concentrations in each area of ​​group T6 were the lowest, which were 246.88 mg / kg, 285.68 mg / kg and 304.88 mg / kg in the anode, middle and cathode areas respectively, all showing a trend of low anode concentration and high cathode concentration. The comprehensive removal rate was greatly improved compared with the first three groups.

[0113] (2) Removal rate of heavy metal nickel in soil

[0114] like Figure 8As shown, compared with the iron electrode, the graphite electrode and the iridium-tantalum-coated titanium electrode have improved in heavy metal removal. Among them, the enrichment degree in the middle area of ​​the T2 group (graphite electrode) and the T3 group (iridium-tantalum-coated titanium electrode) decreased by 2.5% and 3% respectively compared with the 43.15% of the T1 group (iron electrode), while the heavy metal removal rate in the cathode area increased by 1% and 3.7% respectively compared with the 67.17% of the iron electrode. Compared with the removal rates in the middle area and the cathode area, which are not much different, the T2 and T3 groups in the anode area have a greater improvement than the T1 group. The heavy metal removal rates of the T2 and T3 groups are 66.84% and 69.27%, respectively, which are 10.7% and 13.1% higher than those of the T1 group. This is because the iron ion concentration in the anode area of ​​group T1 is too high, which causes the EDTA that moves to the anode to combine with the iron ions to form a complex before participating in the desorption of nickel ions, thereby consuming part of the EDTA. Since groups T2 and T3 use inert electrodes, no additional metal ions will be produced, thus not affecting the desorption of EDTA in the anode area of ​​the soil, thereby further improving the removal rate.

[0115] Groups T3 to T6 use different chelating agents to desorb heavy metal nickel ions. Among them, group T3 uses EDTA. Since nickel ions will form a stable complex with EDTA, and the EDTA-heavy metal complex is weakly negatively charged, directional migration and enrichment will occur, so that nickel ions are mainly enriched in the middle area. Groups T4 and T5 use cellulose and chitosan as chelating agents, respectively. They will also form nickel metal gels similar to complexes with desorbed nickel ions, and be discharged from the soil through electromigration under the action of the electric field. Group T6 uses sodium alginate, which undergoes ion exchange with nickel ions on the surface of the soil to form an unstable alginate gel. Due to the high electroactivity of sodium alginate, it can release Ni through a strong electrolysis reaction under a strong electric field. 2+ , and discharged from the soil along with the liquid.

[0116] The removal rate of the T3 group in the anode and cathode regions was the highest, at 69.27% ​​and 70.84%, but due to its enrichment in the middle region, the enrichment rate in the middle region was 40.21%. The T4 (cellulose) group had the lowest anode heavy metal removal rate of 45.66%, and the removal rates in the middle and cathode regions were 43.25% and 39.28%, respectively; the removal rates of the T5 (chitosan) group in each region were improved, with the removal rates from the anode to the cathode being 56.12%, 49.15% and 47.17%, respectively; and the T6 (sodium alginate) group had the best removal rate, with the removal rates in each region reaching 69.14%, 64.29% and 61.89%, respectively. Therefore, although the T3 group had the highest removal rate in the anode and cathode regions, due to its enrichment in the middle region, the comprehensive removal rate was not high, only 33.3%. And because there was no enrichment in the T4-T6 groups, the comprehensive heavy metal removal rate was improved compared with the T3 group. The comprehensive removal rate of the T6 group was the highest, reaching 65.11%. In summary, the use of inert electrode materials not only reduces the possibility of secondary pollution to the soil, but also reduces the impact of iron ions on the chelating agent, and slightly improves the removal rate of heavy metal pollution. The selection of chelating agents will have a significant impact on the removal effect of heavy metals. Among them, the comprehensive removal rate of sodium alginate as the chelating agent is the best, and sodium alginate does not cause secondary pollution to the soil.

[0117] 3.4 Discharge, moisture content and shear strength

[0118] (1) Displacement: Fig. 9 As shown in the figure, with the extension of the power-on time, the overall drainage volume of each group showed a trend of continuous increase. In the early stage of electroosmosis, due to the use of different electrode materials and complexing agents, differences appeared between the drainage volumes. As the experiment continued, this difference became more and more obvious. After the experiment, the drainage volumes collected from groups T1 to T6 were counted as: 1585mL, 1690mL, 1825mL, 1740mL, 1790mL, and 1990mL, respectively.

[0119] First, by comparing the control tests of T groups 1-T3, it was found that the final drainage of T3 group was higher than that of T1 group and T2 group, which was related to the different electrode materials selected during the test. Among them, the soil drainage of T1 group using iron electrode and T2 group using graphite electrode was relatively close, with the final drainage of 144h being 1585mL and 1690mL respectively, while the drainage of T3 group using iridium tantalum electrode was the largest, which was 1825mL. Overall, the drainage of T1 group was higher than that of T2 group in 0-48h, but as the test continued, the final drainage was lower than that of T2 group. It may be because the conductivity of iron electrode is stronger than that of graphite electrode, so it can make the free water and weakly bound water in the soil be discharged faster in the early stage of electroosmosis, while the graphite electrode is an inert electrode. Compared with the iron electrode, it does not participate in the electrode reaction itself, so it will not corrode as the test progresses, preventing the effective conduct of the test. In addition, the initial potential gradient is small, and the corrosion of the iron electrode is not obvious. However, as the test progresses, the high potential will aggravate the corrosion of the iron electrode, so the final drainage volume of the T1 group is lower than that of the T2 group. The T3 group uses iridium-tantalum coated titanium electrodes, which are coated with a layer of metal oxides with electrocatalytic activity on the surface, which improves the conductivity of the titanium electrode. The coating can further prevent the titanium electrode from contacting with the active oxygen released by the anode to form TiO2, avoiding passivation and affecting its conductivity. Therefore, the T3 group has the strongest drainage capacity.

[0120] Secondly, by comparing the final drainage of the T3-T6 group test, it was found that the final drainage of the T3-T5 group was almost the same, and the final drainage of the T4 and T5 groups decreased by 85mL and 35mL respectively compared with the T3 group, while the drainage capacity of T6 was significantly improved, increasing by 165mL compared with the T3 group. The main reason may be that sodium alginate, as a strong polyelectrolyte, can be electrolyzed under the action of an electric field and act as an electron transfer medium to improve the efficiency of electron transfer. In addition, the strong complexing ability of sodium alginate enables it to effectively desorb heavy metal ions in the soil and migrate under the action of an electric field, thereby enhancing the electroosmotic drainage capacity. This also further confirms the good effect of sodium alginate in heavy metal removal.

[0121] The above results show that the drainage capacity of the soil can be further improved by changing the electrode material and the chelating agent. Among them, the T6 group using iridium-tantalum coated titanium electrode and sodium alginate as the chelating agent has the best drainage effect. This is because the iridium-tantalum coated titanium electrode has good conductivity and corrosion resistance, which makes the potential loss smaller. In addition, sodium alginate is a strong polyelectrolyte and a strong chelating agent, which can improve the electron transfer efficiency and desorb heavy metals in soil particles, thereby accelerating the transmission speed of heavy metal ions in the electric field, increasing the current intensity during the test, and further improving the drainage effect of the test.

[0122] (2) Moisture content and shear strength

[0123] like Fig.10 As shown, as the distance from the anode continues to shorten, the moisture content gradually decreases and the shear strength increases accordingly. The average moisture content and shear strength at all measuring points of group T1-T6 are taken, and the average moisture content is 41.37%, 40.61%, 39.95%, 40.56%, 39.74%, and 38.56%, respectively. The average cross-plate shear strength is 45.33, 46.33, 47.67, 51.67, 47.33, and 50.33. By comparing the average moisture content and shear strength of group T1-T6, it can be seen that the average moisture content of group T6 is the lowest, which is consistent with the final drainage phenomenon at the end of each test, but the average cross-plate shear strength of group T4 is the highest, followed by group T6. This may be related to the continuous addition of sodium carboxymethyl cellulose in the T4 group during the experiment, which has a flocculating effect on the soil, causing the fine soil particles in the soil to condense into a large particle, improving the integrity of the soil and increasing the strength of the soil.

[0124] In addition, a higher concentration of heavy metal nickel can reduce the repulsive force between soil particles, allowing fine particles to gradually form agglomerates, and can also increase shear strength. Since the comprehensive removal rate of the T4 group is lower than that of the T6 group, although its average water content is higher than that of the T6 group, its average shear strength is the largest, 1.34 kPa higher than that of the T6 group. In addition, since the addition of the chelating agent may reduce the pore size and connectivity between soil particles and reduce the promotion of electrochemical drainage, the T4 group has a lower drainage volume but the largest shear strength. Similarly, the water content of the T3 group is slightly higher than that of the T5 group, but its average shear strength is higher than that of the T5 group, which is also related to its higher heavy metal concentration than that of the T5 group.

[0125] 3.5 Current intensity

[0126] like Fig.11 As shown in the figure, the currents of the groups using different electrode materials and different complexing agents have large variations, but the overall variation trend is the same, showing a phenomenon of first rising and then falling every 48 hours, and the initial current intensity of each group is weak. On the one hand, it is because the initial potential gradient is small, which affects the current intensity; on the other hand, in the early stage of electroosmosis repair, the migration of water-soluble ions in the soil leads to weak current intensity. The continuous addition of complexes and the desorption of heavy metal ions increase the ion concentration in the soil, causing the current intensity to increase continuously.

[0127] First, the initial current intensity of each group was analyzed and it was found that there were differences in the initial current intensity of the T1-T3 group. The initial current intensity of the T3 group was much higher than that of the T2 group, and similar to that of the T1 group. This was because the conductivity of iron and iridium tantalum titanium was better than that of the graphite electrode. In addition, the comparison of the T3-T6 group showed that the initial current intensity of T6 was the highest. The main reason may be that sodium alginate, as a strong polyelectrolyte, can be electrolyzed under the action of an electric field and act as an electron transfer medium to improve the electron transfer efficiency. In addition, the strong complexing ability of sodium alginate enables it to effectively desorb heavy metal ions in the soil and increase the conductivity of the soil. This is consistent with the heavy metal removal rate.

[0128] Secondly, by comparing Group T1 and Group T3, it was found that although the initial current intensity of Group T2 was smaller than that of Group T1, as the experiment progressed, its current intensity gradually approached that of Group T1, and was eventually higher than that of Group T1. The current of Group T3 was higher than that of Group T1 and Group T2 throughout the entire process. This is because as the experiment progresses, the iron electrode will oxidize, resulting in a loss of potential, while the iridium-tantalum-coated titanium electrode, like the graphite electrode, will not cause electrode corrosion as the experiment progresses. This is also reflected in the changes in the water discharge of each group.

[0129] Finally, by comparing the T3-T6 groups with different complexing agents, it can be seen that the T6 group has the highest current intensity, followed by the T5 group, with the highest current values ​​of 2795mA and 2731mA respectively. Except for the current value of 1% sodium alginate being lower than that of 1% chitosan in the middle period (54h-70h), the current value of 1% sodium alginate is higher than that of 1% chitosan at other times, and sodium alginate has a higher current in the late stage of electroosmosis. The above results show that sodium alginate and chitosan, as strong polyelectrolytes, can be electrolyzed under the action of electric field and act as electron transfer media to increase the current and conductivity during electroosmosis; on the other hand, it is because of the strong complexing ability of sodium alginate and chitosan that they can effectively desorb heavy metal ions in the soil, increase the conductivity of the soil, and improve the transmission capacity of heavy metal ions in the electric field.

[0130] 3.6 Electrode corrosion and energy consumption per unit of water discharge

[0131] The corrosion degree of the electrode S (%) and the unit energy consumption P (W·mL) required to discharge a unit volume of water -1 ) is determined by the following relationship:

[0132]

[0133]

[0134] Where: M0 and M f is the anode mass before and after the test (g); P is the energy consumption required to discharge a unit volume of water (W·mL -1 ); Q is the final displacement of the electroosmotic test (mL); Ut is the voltage at a certain moment during the electroosmosis process (V); I t is the current at a certain moment during the electroosmosis test (A); t is the electroosmosis treatment time (h).

[0135] like Fig.12 As shown in Figure A, the anode corrosion rates of groups T1 to T6 are 23.69%, 0.2%, 0.31%, 0.12%, 0.24%, and 0.28%, respectively. The electrode corrosion rate diagrams of each group also show that the corrosion of the iron electrode is very serious, while almost no corrosion occurs when the inert electrode is used. This shows that the iron electrode has undergone severe oxidation and electrolysis during the electroosmosis process, which also explains the large drop in the current intensity of the iron electrode in the later period and the presence of excess iron ions in the anode of group T1.

[0136] like Fig.12 As shown in Figure B, by comparing the T1 group and the T3 group, it was found that the energy consumption coefficient of the T3 group was the largest, which was 1.424W / mL, and the energy consumption of the T2 group was the smallest, which was 1.308W / mL. This was related to the current intensity during the test of each group. The same was true for the T3 group and the T6 group. Since the current intensity of 1% sodium alginate was higher than that of other groups from the beginning to the end, its average energy consumption coefficient was the highest, reaching 1.519W / mL, but in general, the average energy consumption coefficients of each group were not much different, all between 1.3-1.52W / mL.

[0137] In summary, comparing the T1 group and the T3 group, although the T3 group has the highest average energy consumption coefficient, it is superior to the other two groups in terms of heavy metal removal rate and shear strength, and it will not cause secondary pollution to the soil. Therefore, the iridium-tantalum coated titanium electrode with better treatment effect is selected; similarly, comparing the T3 group and the T6 group, it is found that although the T6 group has similar problems, it has greatly improved in the reinforcement and remediation of contaminated sludge.

[0138] 4. Experimental study on grouting vacuum combined with electrochemical reinforcement and repair of nickel-contaminated sludge

[0139] 4.1 Experimental plan

[0140] Table 4 Vacuum combined electrochemical test plan for different loading modes

[0141]

[0142]

[0143] Among them, T5 is the solution of Example 1 of the present invention.

[0144] 4.2 Heavy metal content detection and interference desorption analysis

[0145] (1) Heavy metal nickel concentration in soil: Fig.14 As shown, the heavy metal removal rates of groups T2-T5 were significantly improved compared with group T1 which only used electrochemical remediation.

[0146] (2) Removal rate of heavy metal nickel in soil: Fig.15 As shown, the results of groups T2 to T5 are roughly the same, with a significant improvement over group T1. Group T4 has the highest removal rate of heavy metal nickel, and the removal rate of group T5 is slightly lower than that of group T4.

[0147] Groups T2 to T5 used different vacuum combined electrochemical methods. The heavy metal removal rate of group T1 was the lowest, with anode area removal rate of 69.14%, middle area removal rate of 64.29%, and cathode area removal rate of 61.89%; group T2 had a significant improvement over group T1, with anode area removal rate of 84.79%, middle area removal rate of 84.28%, and cathode area removal rate of 83.04%; the heavy metal removal rates of the anode, middle and cathode areas of group T3 were 90.03%, 88.76% and 87.41%, respectively; the heavy metal removal rate of group T4 was the highest, which was increased by more than 25% compared with group T1, among which the cathode area had the largest increase, increasing by 31.74% to 93.63%, followed by the middle area, which increased by 30.67%; the heavy metal removal rates of each area of ​​group T5 were 92.28%, 91.76% and 89.6%, respectively. In summary, the comprehensive removal rate of group T1, which only used electrochemical reinforcement and repair, was the lowest, while the comprehensive removal rates of heavy metals in each region of groups T2-T5, which used vacuum combined with electrochemical reinforcement and repair, were all above 80%, indicating that electrochemical combined with vacuum preloading can greatly improve the removal rate of heavy metal ions in the soil. The comprehensive removal rate of heavy metal nickel in group T5 was slightly lower by 3.3% than that in group T4, which may be related to the shorter effective power-on time in the test method using vacuum combined with intermittent step-by-step loading.

[0148] 4.3 Discharge, moisture content and shear strength

[0149] (1) Displacement: Fig.16 It was found that the drainage volumes collected from groups T1 to T6 were 1990mL, 6040mL, 6690mL, 7050mL, and 7500mL, respectively.

[0150] from Fig.17It was found that the drainage volume of group T1 was much different from that of the other four groups because the dry soil mass and the added chelating agent were only half of those of the other four groups, and part of the water evaporated during the test. The vacuum-combined electrochemical group T2-T5 used a vacuum membrane to seal the model box to avoid the evaporation of water during the test. The drainage volume of group T1 was much different from that of the other four groups. The group T2 used a higher initial voltage, which enabled it to obtain a higher drainage efficiency at the beginning of the test. However, as the test progressed, the soil cracked too quickly, resulting in a continuous reduction in the contact area and an increase in the interface resistance, which resulted in a lower final drainage volume of group T2. The group T3-T4 used a vacuum-combined step-by-step loading voltage loading mode, which effectively slowed down the current decay and increased the drainage capacity in the middle and late stages of the test. In addition, by comparing the test data of group T3 and group T4, it was found that the final drainage volume of group T4 was higher than that of group T3, which indicates that under the same conditions, the more the number of vacuum-combined step-by-step loading voltages, the more the drainage capacity of the soil can be improved. By comparing the water discharge of group T4 and group T5, it was found that the water discharge of group T5 was lower than that of group T4 at the beginning of the experiment, while the final water discharge of group T5 was 6.4% higher than that of group T2. This may be because group T5 adopted the method of vacuum combined with intermittent step-by-step loading, which further reduced the degree of current attenuation on the basis of step-by-step loading, thereby improving the water discharge capacity in the middle and late stages of the experiment.

[0151] (2) Moisture content and shear strength

[0152] Fig.17 The results show that the average moisture content of groups T1 to T5 are 38.56%, 38.85%, 36.12%, 35.44% and 33.04% respectively. The average values ​​of the cross-plate shear strength are 50.33kPa, 60kPa, 76.83kPa, 78.83kPa and 86.33kPa. The moisture content of group T1 in the cathode area is relatively high, while the shear strength of groups T2 to T5 in the cathode area using vacuum combined electrochemistry has been significantly improved, indicating that vacuum preloading effectively solves the problem of excessive moisture content in the cathode of the electrochemical test. Among them, group T5 has the lowest average moisture content and the highest corresponding shear strength, followed by group T4, whose average moisture content is slightly higher than that of group T5 by 2.4% and whose shear strength is reduced by 7.5kPa. In summary, compared with the T1 group without vacuum preloading, the uneven consolidation phenomenon of the soil in the T2-T5 groups using vacuum combined with electrochemistry was alleviated to a great extent, the water content in each area was significantly reduced, and the shear strength was also improved accordingly.

[0153] 4.4 Current intensity and unit drainage energy consumption

[0154] like Fig.18As shown in the figure, by comparing the initial current intensity of group T2 and group T3-T5, it is found that the initial current intensity of group T2 is significantly higher than that of group T3-T5. This difference is mainly attributed to the voltage applied to group T2 at the beginning of the test, which is 60V, much higher than the 20V of group T3-T5. In addition, at the beginning of the test, the current intensity of group T1 was higher than that of group T3-T5, which indicates that vacuum preloading affects the current, which may be related to the enhanced drainage capacity near the drainage board (i.e., the cathode area of ​​the soil) at the beginning of the test.

[0155] By comparing the groups, it was found that the current intensity of the T1 group without vacuum was slightly lower than that of the T5 group with vacuum preloading in the middle period (60-96h), and was higher than that of the T5 group at other times; and the moment when the T1 current decayed was earlier than that of the T2 group and the T5 group. The main reason for this phenomenon was that the initial loading voltage of the T1 group was too high, which caused the soil to shrink and increased the interface resistance. Since the T3 group and the T4 group adopted the loading mode of step-by-step loading voltage, the current decay in the middle and late stages of electroosmosis was effectively suppressed, making the decay amplitude much smaller than that of T2. And by comparing the decay amplitude of the T3 group and the T4 group, it can be seen that the more the number of loading levels, the smaller the decay amplitude of the current circuit. Similarly, since electrolysis reaction occurs at the electrode, gas that can hinder the contact between the electrode and the soil is generated, and the T5 group using vacuum combined with intermittent step-by-step loading can fully discharge these gases during the power outage, the current intensity will increase after the soil is powered off and then powered on.

[0156] In summary, too high current intensity will cause the soil to consolidate too quickly, causing the soil to crack quickly, resulting in an increase in soil resistance and a sharp drop in current intensity. Ultimately, the consolidation and decontamination effects of the soil are not ideal, while too low current will extend the test cycle and fail to achieve the desired results. Therefore, the current intensity needs to be stable in a certain range for a long time to obtain a good soil treatment effect. Here, according to the test results, it is concluded that maintaining the current intensity at 1000mA-2000mA is the best choice.

[0157] like Fig.19As shown in the figure, the average energy consumption coefficient of group T1 was the highest, which was 1.52W / mL. This was because group T1 did not use the combined vacuum test method, resulting in a lower final drainage volume. The average energy consumption coefficients of groups T2-T5 were 5.9%, 38.8%, 42.1%, and 57.9% lower than those of group T1, respectively, due to the combined vacuum preloading method. By comparing groups T2, T3, and T4, it was found that the energy consumption of group T4 decreased by 35% and 38.5% respectively compared with groups T2 and T3, which used constant voltage power supply. The use of vacuum combined with step-by-step voltage loading can effectively reduce the energy consumption during the test, and the more loading levels, the more obvious the effect of reducing electroosmotic energy consumption. In addition, compared with group T4, which only used vacuum combined with step-by-step voltage loading, group T5, which used vacuum combined with intermittent step-by-step loading, decreased by 27.3%, indicating that the loading mode of vacuum combined with intermittent step-by-step loading effectively reduced energy consumption.

[0158] In summary, although the T4 group had the highest heavy metal removal rate and the lowest soil moisture content after the test, its energy consumption was higher than that of the T5 group. Therefore, in general, the T5 group using vacuum combined with intermittent multi-level step-by-step loading voltage is the best choice for repairing and reinforcing contaminated foundation soil.

[0159] 4.5 Evaluation of contaminated sludge remediation

[0160] According to the soil environmental quality construction land pollution risk control standard GB 36600-2018, heavy metal nickel contaminated soil is divided into three categories: no risk contaminated sludge (≤150mg / kg), possible risk contaminated sludge (>150mg / kg, ≤600mg / kg), and unacceptable risk contaminated sludge (>600mg / kg). 2+ The average value of the concentration determines the change in soil contamination level during the remediation process, such as Fig. 20 As shown. Compared with the T1 group and the T2 group, under the condition of other constant conditions, the soil pollution degree of the T2 group after the intermittent combined step-by-step loading test was lighter, with an average concentration of 586.29 mg / kg, a decrease of 7.07% compared with the T1 group. The heavy metal nickel ions in the T3 group changed significantly by changing different electrode materials and complexing agents, with an average concentration of 279.15 mg / kg, which shows that the selection of complexing agents is particularly important for the grouting electrochemical treatment of nickel-contaminated sludge. The T4-T7 groups adopted a vacuum combined with electroosmosis method, which greatly reduced the heavy metal nickel ions in the soil, from the initial unacceptable risk pollution to a risk-free pollution level.

[0161] The remaining nickel ion content in the soil can be calculated from the final nickel ion concentration in the filtrate, such as Fig. 20 As shown by the red line in the figure, and compared with the measured results, i.e. the bar chart in the figure, it is found that the trends of the two are roughly the same. This is because Ni in the soil 2+The uneven distribution of nickel ions and the adsorption of some nickel ions on the electrode materials will lead to errors between the calculated results and the actual measurements, but they are all within a reasonable range. In summary, the soil treated by grouting electrochemical treatment reduces the concentration of heavy metals, and most of the soil will be reduced to soil with possible risk of contamination. The vacuum combined with electroosmosis method can greatly improve the comprehensive removal rate and obtain better soil reinforcement and repair effects.

[0162] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for repairing heavy metal contaminated sludge, characterized in that: Using an iridium-tantalum coated titanium electrode as an anode and a conductive electrode as a cathode, a chelating agent is added to the soft soil contaminated by heavy metals, one end of the anode and cathode is placed in the soft soil contaminated by heavy metals, and the other end is electrically connected to a power source. Then, a vacuum combined intermittent multi-stage step-by-step pressurized electroosmosis technology is used to repair and reinforce the soft soil contaminated by heavy metals. The vacuum combined intermittent multi-stage step-by-step pressurized electroosmosis technology is as follows: under vacuum, voltage is applied step by step to the soft soil contaminated by heavy metals through a power supply, an anode and a cathode, and electroosmosis is carried out by intermittent treatment of power on and power off.

2. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: The operation of intermittent multi-stage step-by-step pressurized electroosmosis technology is: repeat the power-on for 3-4 hours and power-off for 1-2 hours at a voltage of 12-20V, for a total of 48 hours; continue to repeat the power-on for 3-4 hours and power-off for 1-2 hours at a voltage of 24-40V, for a total of 48 hours; then repeat the power-on for 6-8 hours and power-off for 2-4 hours at a voltage of 36-60V, for a total of 48 hours.

3. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: The vacuum pressure is 20-80KPa.

4. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: Several pipes are inserted into the soft soil contaminated by heavy metals, and the complexing agent is discharged into the soft soil through the pipes.

5. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: The complexing agent is selected from carboxymethyl cellulose, chitosan or sodium alginate.

6. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: The mass ratio of the chelating agent to the solid components in the soft soil contaminated by heavy metals is 1-2:

100.

7. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: In the intermittent multi-stage step-by-step pressurized electroosmosis technology, the current intensity of the soft soil contaminated by heavy metals is maintained at 1000-2000mA.

8. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: The moisture content of soft soil is 75-85%.

9. A method for repairing heavy metal contaminated sludge according to claim 1, characterized in that: The conductive electrode is a conductive metal electrode.

10. A method for repairing heavy metal contaminated sludge according to claim 9, characterized in that: The conductive metal is selected from platinum, gold, silver, copper, titanium or nickel.

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