The impermeability of cutoff wall is one of the most important factors affecting the life of landfill. Hence, we used sodium carboxymethyl cellulose to modify bentonite and developed a modified bentonite-cement-fly ash antiseepage slurry with a low permeability coefficient. The main material of the slurry is bentonite modified by sodium carboxymethyl cellulose, cement, fly ash, and auxiliary materials such as sodium carbonate and polycarboxylate superplasticizer. The optimal ratio of the basic components of the antiseepage slurry was optimized by the orthogonal test method. The permeability coefficient of the consolidated slurry was determined by a variable head permeameter. The optimal dosage of each component of slurry is 210∼220 kg/m3 cement, 210∼220 kg/m3 bentonite, 1.5∼2.5 kg/m3 sodium carbonate, 1.5∼2.0 kg/m3 sodium carboxymethyl cellulose, 160 kg/m3 fly ash, and 3 kg/m3 polycarboxylate superplasticizer. The 28-day consolidation slurry permeability coefficient is less than 1 × 10−8 cm/s and has a good adsorption effect on the pollutants in landfill leachate.
In recent years, with the rapid development of urbanization in China, the number of urban population is expanding day by day [
If the antiseepage system of landfills cracks and penetrates, landfill leachate will invade the surrounding environment, and its main hazards include the following aspects: first, it will pollute the surface water and groundwater [
It is stipulated in the technical specification for sanitary landfill of municipal solid waste issued now that proper artificial seepage prevention measures must be established for landfill sites that do not have natural seepage prevention conditions and landfill leachate that may pollute underground water sources [
The main raw materials of antiseepage slurry are bentonite, cement, and fly ash, and the modifier is sodium carboxymethyl cellulose. Sodium carbonate and polycarboxylate water reducer were used as additives. Calcium bentonite is selected as bentonite, and the main performance parameters are shown in Table
Performance indicators of bentonite.
Colloid valence (ml/15 g±) | Granularity (mesh number) | Ion exchange capacity (meg/100±) | Initial moisture content (%) | Liquid limit (%) | Plastic limit (%) | Plasticity index (%) |
---|---|---|---|---|---|---|
90 | 180 | >70 | 10.0 | 260 | 40 | 237 |
When preparing the sample, the bentonite was first nitrified. A certain amount of bentonite was weighed and poured it into a beaker. A solution of sodium carbonate of a certain concentration is then poured into the bentonite and stirred well. The prepared carboxymethyl cellulose sodium solution was poured into the nitrified bentonite and left for more than 8 hours for modification. After the modification, a certain amount of cement and fly ash were mixed into the modified bentonite. Finally, the superplasticizer solution was added with a certain amount of water and stirred thoroughly to make the components evenly distributed. During this process, it is necessary to pay attention to the amount of water so that the slurry stirred evenly can be pumped for more than 40 min. The evenly mixed slurry was poured into a round mould to form a slurry strength sample. The size of the sample of impermeable slurry was cut with a circular ring knife of 61.8 × 40 mm. The sample of impermeable slurry was placed in water for curing. After curing for 14 days and 28 days, the samples are shown in Figure
Slurry test sample.
The permeability coefficient of impermeable slurry was measured by a variable head penetrator. The variable head penetrator is shown in Figure
The variable head permeameter.
After the test sample reaches the curing period, it is taken out for testing. Firstly, the slurry consolidated body sample was put into the permeation device, and the nut was tightened. Next, the inlet of the permeation device and the inlet pipe of the variable head permeameter were opened and the pipe clamp to let the water from the water supply bottle into the instrument was opened. Then, the exhaust valve was opened, and the air from the tube was removed until there are no bubbles in the outgoing water. Finally, water is injected into the variable manifold, and the timer is started when the outlet has water flowing out. The initial head was recorded, and then, the head position at regular intervals was recorded. The permeability coefficient of slurry consolidation body is calculated according to the following formula:
The permeability coefficient of block measured by the variable head test method is derived from the Chinese geotechnical test method standard, where
In order to study the influence of each component on the slurry experiment, a large-scale experiment is needed. It is not only difficult to operate but also can produce great errors due to experimental conditions. Therefore, combining with the previous experimental results, we decided to adopt the orthogonal experimental method. It not only greatly reduced the number of experiments but also can meet the experimental requirements.
According to the previous experiments, it is decided to carry out orthogonal experiments on the components which have a great influence on the performance of slurry: the cement content is 190∼220 kg/m3, the bentonite content is 190∼220 kg/m3, the CMC-Na content is 0.5∼2.0 kg/m3, and the sodium carbonate content is 1.0∼2.5 kg/m3. The remaining components had less effect on the experiment, taking the fixed value: the fly ash dosage was 160 kg/m3, and the amount of water reducer was 3 kg/m3.
Orthogonal experimental scheme.
Number | Cement (kg/m3) | Bentonite (kg/m3) | Sodium carbonate (kg/m3) | CMC-Na (kg/m3) | Permeability coefficient (cm/s) | |
---|---|---|---|---|---|---|
14 d | 28 d | |||||
1 | 190 | 190 | 1.00 | 0.50 | 7.30 | 2.20 |
2 | 190 | 200 | 1.50 | 1.00 | 5.60 | 1.50 |
3 | 190 | 210 | 2.00 | 1.50 | 4.50 | 7.40 |
4 | 190 | 220 | 2.50 | 2.00 | 3.90 | 5.70 |
5 | 200 | 190 | 1.50 | 1.50 | 5.60 | 1.40 |
6 | 200 | 200 | 1.00 | 2.00 | 4.70 | 9.00 |
7 | 200 | 210 | 2.50 | 0.50 | 4.30 | 7.40 |
8 | 200 | 220 | 2.00 | 1.00 | 3.90 | 5.90 |
9 | 210 | 190 | 2.00 | 2.00 | 1.90 | 6.40 |
10 | 210 | 200 | 2.50 | 1.50 | 1.80 | 5.00 |
11 | 210 | 210 | 1.00 | 1.00 | 2.60 | 5.30 |
12 | 210 | 220 | 1.50 | 0.50 | 4.30 | 3.80 |
13 | 220 | 190 | 2.50 | 1.00 | 2.50 | 7.30 |
14 | 220 | 200 | 2.00 | 0.50 | 3.70 | 7.20 |
15 | 220 | 210 | 1.50 | 2.00 | 7.20 | 2.70 |
16 | 220 | 220 | 1.00 | 1.50 | 9.10 | 2.90 |
In order to find out the optimum level of each component and to make the primary and secondary sequential mining of each component which affects the permeability coefficient of slurry consolidation body, the orthogonal experimental data were analyzed by the method of range analysis. The analysis of permeability coefficient range difference of slurry consolidation body at 14 days and 28 days is shown in Tables
Range analysis of 14-day permeability coefficient of samples.
Number | |||||
---|---|---|---|---|---|
Cement | Bentonite | Sodium carbonate | CMC-Na | ||
Content (kg/m3) | 1 | 190 | 190 | 1.00 | 0.50 |
2 | 200 | 200 | 1.50 | 1.00 | |
3 | 210 | 210 | 2.00 | 1.50 | |
4 | 220 | 220 | 2.50 | 2.00 | |
Permeability coefficient (cm/s) | 5.33 | 4.33 | 3.88 | 4.90 | |
4.63 | 3.68 | 4.06 | 3.65 | ||
2.65 | 3.03 | 3.50 | 3.20 | ||
1.96 | 3.25 | 3.13 | 2.81 | ||
Excellent level range | |||||
3.37 | 1.30 | 9.30 | 2.10 | ||
Primary and secondary order |
Range analysis of 28-day permeability coefficient of samples.
Number | |||||
---|---|---|---|---|---|
Cement | Bentonite | Sodium carbonate | CMC-Na | ||
Content (kg/m3) | 1 | 190 | 190 | 1.00 | 0.50 |
2 | 200 | 200 | 1.50 | 1.00 | |
3 | 210 | 210 | 2.00 | 1.50 | |
4 | 220 | 220 | 2.50 | 2.00 | |
Permeability coefficient (cm/s) | 1.25 | 1.24 | 9.80 | 1.01 | |
9.08 | 9.05 | 8.88 | 8.38 | ||
5.13 | 5.70 | 6.73 | 7.33 | ||
5.03 | 4.58 | 6.35 | 5.95 | ||
Excellent level range | |||||
7.50 | 7.85 | 3.08 | 4.15 | ||
Primary and secondary order | |||||
According to the range analysis table, the influence trend curve of each component on the permeability coefficient of samples in different periods is shown in Figure
(a) Curve of influence of cement content on sample permeability coefficient; (b) curve of influence of bentonite content on sample permeability coefficient; (c) curve of influence of sodium carbonate content on sample permeability coefficient; (d) curve of influence of CMC-Na content on sample permeability coefficient.
Tables
At 14 days, the influence of each factor content on the permeability coefficient of the slurry consolidation body was ranked from large to small: cement, sodium carboxymethyl cellulose, bentonite, and sodium carbonate. At 28 days, the influence of various factors on the permeability coefficient of slurry consolidation body was as follows: bentonite, cement, sodium carboxymethyl cellulose, and sodium carbonate. At 28 days, bentonite replaced cement as the main factor affecting the permeability coefficient of the slurry consolidation body.
In order to obtain a lower permeability coefficient, the content of each component of slurry is controlled in the following range: cement 210∼220 kg/m3, swelling 210∼220 kg/m3, sodium carbonate 2.0∼2.5 kg/m3, and sodium carboxymethyl cellulose 1.5∼2.0 kg/m3.
Sodium carbonate and sodium carboxymethyl cellulose, as modifiers of bentonite, affect the expansibility of bentonite by modifying the bentonite, thus affecting the permeability coefficient of the sample. The coupling effect among its factors is not considered in the orthogonal experiment. Therefore, it is necessary to carry out factor influence analysis to further analyze the relationship between factors and slurry permeability coefficient.
First, the influence of cement and bentonite content on the permeability coefficient of samples was analyzed. According to the orthogonal experiment results, with 210 kg/m3 cement, 210 kg/m3 bentonite, 2.0 kg/m3 sodium carbonate, and 1.5 kg/m3 sodium carboxymethyl cellulose, except for water, the content of other materials is not changed into the benchmark ratio. The control cement content is 210∼250 kg/m3, and bentonite content is 200∼240 kg/m3. The permeability coefficients of samples with different cement and bentonite contents were tested, respectively, and the specific proportions are shown in Table
Effects of cement and bentonite content on permeability coefficient of test sample.
Number | Cement (kg/m3) | Bentonite (kg/m3) | Permeability coefficient (cm/s) | ||
---|---|---|---|---|---|
14 d | 28 d | 60 d | |||
210 | 210 | 2.10 | 4.90 | 3.10 | |
220 | 210 | 1.20 | 2.30 | 1.80 | |
230 | 210 | 1.00 | 1.70 | 1.60 | |
240 | 210 | 9.10 | 1.60 | 1.50 | |
250 | 210 | 8.30 | 1.50 | 1.40 | |
210 | 200 | 1.70 | 5.30 | 3.20 | |
210 | 210 | 1.10 | 3.80 | 2.60 | |
210 | 220 | 7.60 | 2.20 | 1.90 | |
210 | 230 | 7.90 | 1.60 | 1.40 | |
210 | 240 | 8.20 | 1.40 | 1.30 |
The influence curves of cement with different contents on the permeability coefficient of samples drawn from Table
Curve of influence of different cement contents on permeability coefficient of samples.
Curve of influence of different bentonite contents on permeability coefficient of samples.
It can be seen from Table
Then, the influence of sodium carbonate and sodium carboxymethyl cellulose on the permeability coefficient of the sample was analyzed. Sodium carbonate is the nitrifier of bentonite and sodium carboxymethyl cellulose is the modifier of bentonite. By acting on bentonite, they changed the water absorption and expansion coefficient of bentonite so as to influence the permeability coefficient of the sample. According to the orthogonal experiment results, with 210 kg/m3 cement, 210 kg/m3 bentonite, 2.0 kg/m3 sodium carbonate, and 1.5 kg/m3 sodium carboxymethyl cellulose, except for water, the content of other materials does not change into the benchmark ratio. The sodium carbonate content was controlled to be 1∼3 kg/m3, and the sodium carboxymethyl cellulose content was 0.5∼2.5 kg/m3. The permeability coefficients of samples with different sodium carbonate and sodium carboxymethyl cellulose contents were tested, respectively, and the specific proportions are shown in Table
Effects of sodium carbonate and CMC-Na content on permeability coefficient of test samples.
Number | Sodium carbonate (kg/m3) | CMC-Na (kg/m3) | Permeability coefficient (cm/s) | ||
---|---|---|---|---|---|
14 d | 28 d | 60 d | |||
1 | 1.5 | 3.40 | 6.00 | 4.80 | |
1.5 | 1.5 | 3.90 | 5.70 | 3.50 | |
2 | 1.5 | 3.10 | 2.60 | 2.20 | |
2.5 | 1.5 | 2.70 | 3.30 | 2.90 | |
3 | 1.5 | 2.50 | 2.90 | 2.70 | |
2.0 | 0.5 | 4.10 | 8.80 | 4.10 | |
2.0 | 1 | 3.30 | 6.20 | 3.30 | |
2.0 | 1.5 | 2.70 | 4.70 | 2.70 | |
2.0 | 2 | 2.30 | 3.40 | 2.20 | |
2.0 | 2.5 | 2.20 | 2.90 | 2.20 |
The influence curves of sodium carbonate with different contents on the permeability coefficient of samples drawn from Table
Curve of influence of different sodium carbonate contents on permeability coefficient of samples.
Curve of influence of different CMC-Na contents on permeability coefficient of samples.
When the content of sodium carbonate increases from 1.0 g to 2.0 g during the curing period of 14 and 28 days, the permeability coefficient of the sample decreases significantly. When the content of sodium carbonate increased from 2.0 g to 3.0 g, the permeability coefficient curve of the sample was relatively flat and hardly changed. In each period of sample curing, with the increase in sodium carboxymethyl cellulose content, the permeability coefficient of the sample showed a gradually decreasing trend. However, the decreasing trend in the later period became more and more gradual, with almost no change. In addition, during the curing period of 60 days, the permeability coefficient of the sample decreased more smoothly than that at 14 days and 28 days. The general variation trend is consistent with the orthogonal experiment.
The adsorption performance of the slurry consolidation body was tested by the self-made adsorption percolation apparatus, as shown in Figure
Self-made adsorption infiltrator and its working principle.
The adsorption and retarding properties of the slurry were tested with artificial leachate in the laboratory. The main components of leachate are shown in Table
Composition of landfill leachate prepared in laboratory.
Main components of leachate | Content (mg/L) |
---|---|
Pb | 25.02 |
Cr | 5.83 |
Hg | 2.68 |
NH4-N | 703.73 |
C16H22O4 | 8.67 |
The results of the adsorption retardation test on the percolate by the cementing body of impermeable slurry were compared with the discharge standards of various pollutants to evaluate its adsorption retardation performance. The adsorption retardation test results of slurry consolidation body to leachate are shown in Table
Results of leachate adsorption by slurry consolidation.
Main components of leachate | Concentration before the test (mg/L) | Concentration after the test (mg/L) | Emission standards (mg/L) | Adsorption rate (%) |
---|---|---|---|---|
Pb | 25.02 | 3.25 × 10−2 | 1.0 | 99.87 |
Cr | 5.83 | 5.83 × 10−4 | 1.5 | 99.99 |
Hg | 2.68 | 1.87 × 10−4 | 0.05 | 99.93 |
NH4-N | 703.73 | 1.12 | 15 | 99.84 |
C16H22O4 | 8.67 | 0.163 | 0.2 | 98.12 |
Emission standard is the latest pollutant emission standard of China.
After modified by sodium carboxymethyl cellulose, the adsorption effect of bentonite to various pollutants has been improved to some extent. The adsorption effect of modified bentonite slurry on heavy metal ions and ammonia nitrogen ions reached over 99.80%, and the adsorption effect of dibutyl phthalate was significantly improved, which could meet the specified emission requirements.
The critical pore size reflects the connectivity degree of pores and the tortuous degree of permeation path in the material, which is the essence of permeability. The permeability coefficient is determined by the critical pore size of the slurry. With the increase in critical pore size, the antipermeability of slurry consolidation decreases gradually [
The crystal structure unit of bentonite is composed of two layers of SiO4 tetrahedron and an intermediate layer of AlO2(OH)4 octahedron. The interlaminar contact is weak; water is easy to enter and make its volume expand [
The nitrification of bentonite is achieved by increasing the Na+ concentration in bentonite. The principle is that Na+ has higher bonding strength than Si4+ and Al3+ in montmorillonite structure and can be substituted with other larger ions. The original calcium-based bentonite was changed to sodium-based bentonite after nitrification, which further increased the expansion coefficient and water absorption. This makes the bentonite to fill the cement skeleton more compact, the critical pore size decreases, the permeability coefficient is smaller, and the deposition of pollutants in leachate is more obvious. The exchangeable cation content of bentonite increased from 600 mL/kg to 750∼1000 mL/kg [
It can be seen from Tables
In order to further explore the principle of strength formation of consolidated body,
SEM photograph of (a) sample surface and (b) sample internal.
In this paper, sodium carboxymethyl cellulose was used to modify bentonite to make a new material for the cutoff wall. The orthogonal experiment was used to optimize the slurry composition. The influence of each component on the permeability coefficient of slurry in different periods was studied by factor influence analysis. The microscopic mechanism of slurry seepage prevention was explained by the SEM experiment. The following conclusions can be drawn: The permeability coefficient of the consolidated slurry decreased with the increase in the content of cement, sodium carbonate, and sodium carboxymethyl cellulose. The permeability coefficient is controlled by the critical pore size of the consolidated material. With the increase in cement and bentonite content, the critical pore size of the consolidated body decreases, and the permeability coefficient decreases. Slurry consolidation body has a good adsorption effect on heavy metal ions in landfill leachate and has a certain adsorption effect on organic pollutants, which meets the emission requirements.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The support by National Natural Science Foundation of China (no. 51678083) is gratefully acknowledged.