Ex-situ chemical reduction stabilization repair method for hexavalent chromium contaminated soil

Abstract

The invention provides an ex-situ chemical reduction stabilization remediation method for hexavalent chromium contaminated soil, which comprises the following steps: collecting soil from a contaminated land parcel, airing, crushing, screening and uniformly mixing to obtain pretreated contaminated soil; testing the content of hexavalent chromium in the polluted soil, then adding solid calcium polysulfide into the polluted soil according to the molar ratio of the solid calcium polysulfide to the hexavalent chromium of (12-16): 1, and uniformly mixing; and after uniform mixing, spraying water into the polluted soil according to 15-20% of the total solid mass, and carrying out static maintenance. According to the stable remediation method, the remediation target value (the hexavalent chromium in the soil is lower than 5.7 mg / kg) is achieved on the third day of maintenance, the remediation effect is continuous and good, and rebounding does not occur.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and in particular to an ex-situ chemical reduction stabilization remediation method for hexavalent chromium contaminated soil. Background Technology

[0002] Ex-situ chemical reduction remediation technology aims to reduce the activity of pollutants in contaminated soil by mixing contaminated soil with solidifying agents, stabilizers, and other agents to induce a chemical reaction. The main characteristic of this technology is that after excavating the contaminated soil or sludge, a large-scale mixing device is used on the ground to completely mix the contaminated soil with remediation materials. The treated soil or sludge is then either landfilled or recycled.

[0003] Existing ex-situ chemical reduction remediation technologies often encounter the following problems when remediating hexavalent chromium-contaminated soil: 1. Poor sustainability of remediation effects, with a tendency for rebound; that is, after remediation, the hexavalent chromium content decreases, but it rises significantly again over time; 2. Long remediation time, which is not conducive to improving the treatment efficiency of contaminated soil; 3. Ex-situ remediation requires pretreatment of large amounts of soil, such as drying, crushing, and screening, to ensure sufficient contact between the remediation agent and the pollutants. How to quickly pretreat large amounts of soil is also a problem that needs to be solved. Summary of the Invention

[0004] This invention proposes an ex-situ chemical reduction stabilization remediation method for hexavalent chromium contaminated soil. The remediation target value was reached on the third day of maintenance, and the remediation effect remained good without any rebound.

[0005] The technical solution of this invention is implemented as follows: an ex-situ chemical reduction stabilization remediation method for hexavalent chromium contaminated soil, comprising the following steps:

[0006] (1) Soil was collected from the contaminated site, and then dried, crushed, screened and mixed to obtain pretreated contaminated soil;

[0007] (2) Measure the content of hexavalent chromium in the contaminated soil, and then add solid calcium polysulfide to the contaminated soil according to the molar ratio of solid calcium polysulfide to hexavalent chromium (12-16):1, and mix well;

[0008] (3) After mixing in step (2), spray water onto the contaminated soil at 15-20% of the total solid mass and allow it to cure statically.

[0009] Furthermore, the pH of the contaminated soil is 8.0-10.5.

[0010] Furthermore, the hexavalent chromium content in the contaminated soil ranges from 10 to 600 mg / kg, and the total chromium content ranges from 110 to 5500 mg / kg.

[0011] Furthermore, in step (3), the static curing time is more than 3 days.

[0012] Furthermore, in step (1), the soil collected from the contaminated site includes impure fill and silty clay, and the impurities in the impure fill are removed.

[0013] Furthermore, in step (1), an excavator is used to turn the soil, and then a mixer is used to break the turned soil. This process is repeated multiple times, while removing any remaining debris. Since the soil collected from the contaminated site is silty clay, it is difficult to break after losing water. Using an excavator and a mixer in combination to break the contaminated soil, and repeating the process multiple times, helps to shorten the drying and breaking time.

[0014] Further, in step (2), the method for mixing solid calcium polysulfide is as follows: the pretreated contaminated soil is laid flat and prepared, solid calcium polysulfide is evenly spread on the prepared soil surface, and it is evenly mixed with an agricultural rotary tiller. Then, the soil is turned over multiple times with an excavator. After being fully mixed, the soil is laid flat and prepared.

[0015] Furthermore, in step (3), the method for spraying water into the contaminated soil is as follows: Atomizing nozzles are evenly distributed above the leveled soil, and water is sprayed onto the soil surface through the atomizing nozzles. The water statically infiltrates downwards, and the soil is not disturbed during the spraying process. The spraying flow rate is 0.2-0.7 m³ / h. 3 / h, after which the soil moisture content reaches 15-20%.

[0016] Furthermore, in step (1), the moisture content of the pretreated contaminated soil is 0.7-3.5%.

[0017] Furthermore, during the static curing process in step (3), the soil surface is covered with a film. Covering with a film reduces water evaporation and maintains soil moisture content.

[0018] The beneficial effects of this invention are:

[0019] The stabilization remediation method of the present invention, through pilot-scale testing, achieved the remediation target values ​​(leaching target value below 5 mg / L and soil hexavalent chromium below 5.7 mg / kg) on ​​the 3rd day of maintenance, and the remediation effect remained good. It still met the remediation target values ​​on the 40th day of maintenance, and no rebound occurred.

[0020] This invention first mixes solid calcium polysulfide with contaminated soil, and then uses atomized spraying to bring the soil moisture content to 15-20%, which avoids the problem of large soil disturbance and difficulty in uniform spraying caused by large water flow; at the same time, it also avoids the solid calcium polysulfide from corroding and clogging the spray pipes, thus affecting the remediation effect. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] An ex-situ chemical reduction stabilization remediation method for hexavalent chromium contaminated soil includes the following steps:

[0023] (1) Soil samples were collected from hexavalent chromium contaminated sites at depths of 0–1 m. In order to facilitate subsequent soil crushing and screening, the newly excavated soil needed to be dried. The soil was spread out by an excavator and dried until the moisture content of the soil was reduced to below 5% and it could be crushed. The newly excavated soil included fill soil and silty clay. The fill soil contained debris such as bricks, stones, plastic film, wire, and wooden sticks of varying sizes, which needed to be removed during the drying process. Due to the high viscosity of the silty clay, the excavated soil clumps together and gradually hardens during the drying process, so the large clumps of soil needed to be preliminarily treated.

[0024] The excavated soil was mainly silty clay, highly sticky. During air drying, as the moisture content gradually decreased, the soil hardness increased, making it more difficult to break up. To shorten the drying and breaking time, initial processing with an excavator and subsequent breaking with an agricultural rotary tiller were attempted, but the results were unsatisfactory. Due to the high soil moisture content, using a pulverizer easily caused internal blockages, requiring frequent cleaning with poor results. Finally, a construction machinery mixing plant was used for in-situ mixing and breaking up the soil, which proved to be more effective. Therefore, a combination of excavator and mixing plant was used to break up the sticky soil. After multiple rounds of turning and mixing by the excavator and mixing plant, large-diameter clay lumps were basically broken up; simultaneously, non-soil materials such as bricks, stones, and plastic film were further removed.

[0025] The following are the indicators of 6 groups of samples taken from the treated contaminated soil:

[0026] Table 1. Indicators of the 6 groups of samples before restoration

[0027] Moisture content (%) pH value Hexavalent chromium (mg / kg) Total chromium (mg / kg) 1 2.50 9.09 149 1470 2 2.70 9.10 119 1190 3 2.50 9.21 95.2 132 4 1.80 9.17 94.9 110 5 2.60 9.01 229 5330 6 3.00 8.94 221 2140

[0028] As shown in the table above, after the soil collected from the contaminated site was dried, crushed and screened, the soil moisture content decreased from the initial 21% to about 2.5%, which is a significant reduction. The pH value was about 9.0, and the content of hexavalent chromium and total chromium in the soil varied considerably.

[0029] (2) Based on laboratory-scale tests, the best remediation effect on contaminated soil was observed when the molar ratio of solid calcium polysulfide to hexavalent chromium was 12:1. The calculation process for the amount of solid calcium polysulfide added during pilot-scale testing is shown in the table below:

[0030] Table 2. Calculation process for the amount of solid calcium polysulfide added.

[0031] Mass of pilot-scale soil (kg) Hexavalent chromium (mg / kg) Total hexavalent chromium (mg) Solid calcium polysulfide (g) needs to be added. 46593 229 10669683 1265020

[0032] Considering factors such as soil water retention capacity and pesticide loss during water addition, based on the "size effect," the amount of solid calcium polysulfide added is 1.6t. The solid calcium polysulfide is evenly spread on the leveled soil surface and mixed evenly using an agricultural rotary tiller. Since the working area of ​​the rotary tiller is mainly concentrated on the surface layer, in order to ensure that the bottom soil can fully contact the solid calcium polysulfide, the mixed soil is turned over multiple times with an excavator. After thorough mixing, the soil is then leveled and spread to a thickness of approximately 50-60cm.

[0033] (3) After the solid remediation agent is mixed with the soil, it is necessary to maintain a certain moisture content. Based on laboratory tests, the best remediation effect is achieved when the moisture content is maintained between 15-20%.

[0034] Evenly distribute the atomizing nozzles above the leveled soil, spraying water onto the soil surface through the nozzles. The water infiltrates statically, without disturbing the soil during spraying. The spraying flow rate is 0.48 m³ / h. 3 / h, after the spraying is completed, the soil moisture content reaches 19%, and the total water added is 8.58m³. 3 .

[0035] After spraying, the soil was allowed to cure naturally, and samples were collected and monitored on days 3, 8, 16, and 40 after curing. Vertical sampling was performed using a Luoyang shovel, with multiple collection points, and the collected soil samples ranged from 2.5 to 10.0 kg.

[0036] Table 3. Soil hexavalent chromium content before and after remediation (mg / kg)

[0037] Before repair Day 3 of maintenance Day 8 of maintenance Day 16 of maintenance Day 40 of maintenance 1 149 Not detected Not detected Not detected 1.5 2 119 Not detected Not detected Not detected 2.4 3 95.2 Not detected 3.0 Not detected 2.1 4 94.9 Not detected 2.9 Not detected 1.4

[0038] Table 4. Total chromium concentration leached from soil before and after remediation (mg / L)

[0039] Before repair Day 3 of maintenance Day 8 of maintenance Day 16 of maintenance Day 40 of maintenance 1 20.0 1.0 1.2 2.2 0.72 2 17.9 1.8 0.2 1.4 0.85 3 17.5 1.8 0.6 2.1 0.58 4 19.1 2.7 1.8 1.1 0.43

[0040] Table 5. Soil leaching concentration of hexavalent chromium before and after remediation (mg / L)

[0041] Before repair Day 3 of maintenance Day 8 of maintenance Day 16 of maintenance Day 40 of maintenance 1 13.15 0.02 0.31 0.02 0.18 2 12.41 0.10 0.10 0.24 0.06 3 11.86 0.06 0.31 0.04 0.06 4 14.45 0.18 0.36 0.04 0.17

[0042] As can be seen from the above, after adding solid calcium polysulfide once, the remediation target values ​​(leaching target value below 5 mg / L and soil hexavalent chromium below 5.7 mg / kg) can be fully achieved, and the remediation effect is good and sustainable with no rebound.

[0043] In addition, if citric acid is added to adjust the pH during the remediation of contaminated soil, the remediation effect will be less stable and sustainable, and there will be a significant rebound (the hexavalent chromium content will decrease and then increase again), as shown in Table 6. Therefore, citric acid is not suitable for addition.

[0044] Table 6. Soil hexavalent chromium content (mg / kg) before and after citric acid addition for remediation.

[0045] Before repair Day 3 of maintenance Day 8 of maintenance Day 16 of maintenance Day 40 of maintenance 95.7 2.2 Not detected 57.2 76.9 94.1 Not detected Not detected 8.5 28.7

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil, characterized in that, Includes the following steps: (1) Soil was collected from the contaminated site, and then dried, crushed, screened and mixed to obtain pretreated contaminated soil; (2) Measure the content of hexavalent chromium in the contaminated soil, and then add solid calcium polysulfide to the contaminated soil according to the molar ratio of solid calcium polysulfide to hexavalent chromium (12-16):1, and mix well; (3) After mixing in step (2), spray water onto the contaminated soil at 15-20% of the total solid mass and allow it to cure statically.

2. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, The pH of the contaminated soil is 8.0-10.

5.

3. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, The hexavalent chromium content in the contaminated soil is 10-600 mg / kg, and the total chromium content is 110-5500 mg / kg.

4. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, In step (3), the static curing time is more than 3 days.

5. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, In step (1), the soil collected from the contaminated site includes impure fill and silty clay, and the impurities in the impure fill are removed.

6. A method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1 or 5, characterized in that, In step (1), an excavator is used to turn the soil, and then a mixer is used to break up the turned soil. This process is repeated multiple times, while removing any remaining debris.

7. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, In step (2), the method for mixing solid calcium polysulfide is as follows: the pretreated contaminated soil is laid flat and prepared, solid calcium polysulfide is evenly spread on the prepared soil surface, and it is evenly mixed with an agricultural rotary tiller. Then, the soil is turned over multiple times with an excavator. After being fully mixed, the soil is laid flat and prepared.

8. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, In step (3), the method for spraying water into the contaminated soil is as follows: Atomizing nozzles are evenly distributed above the leveled soil, and water is sprayed onto the soil surface through the atomizing nozzles. The water seeps statically downwards, and the soil is not disturbed during the spraying process. The spraying flow rate is 0.2-0.7 m³ / h. 3 / h, after which the soil moisture content reaches 15-20%.

9. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, In step (1), the moisture content of the pretreated contaminated soil is 0.7-3.5%.

10. The method for ex-situ chemical reduction stabilization remediation of hexavalent chromium contaminated soil according to claim 1, characterized in that, During the static curing process in step (3), the soil surface is covered with a film.