Application progress of superabsorbent resin in highway engineering soil remediation
DOI
10. 19817/j.cnki.issn 1006-3536 2024.06.019
Highway engineering is a linear project with a long construction period and a large amount of engineering, occupying a large amount of land and space. The ecosystem along the road will be disturbed to varying degrees and damaged to a large extent during the construction process. In addition, during the operation process, due to rainwater initializing, soil erosion, improper use of snow melting agents, etc., soil damage affects plant growth and cannot be effectively restored in the short term. Highway engineering ecological restoration is the use of effective slope protection and slope consolidation means to enhance the stability of the soil layer and restore the biological community to a self-sustaining state to improve the ability of the soil surface to resist initializing.
When carrying out ecological restoration of highway engineering, an appropriate soil restoration system should be selected according to soil texture, topography, precipitation, vegetation types, etc. Therefore, soil restoration and vegetation restoration are important contents of highway maintenance.
At present, slope restoration mainly relies on the organic combination of plants themselves and non-living materials, among which superabsorbent resin (SAP) plays a role in consolidation and stabilization. SAP is a polymer with a network structure, and its molecular chain segments are connected with hydrophilic groups such as hydroxyl, amino, and carboxyl groups, which can absorb hundreds or even thousands of times its own weight of water. SAP can regulate soil moisture, improve the stability of soil structure, and promote plant growth in soil, thereby restoring soil ecological balance.
01
Structure of SAP and Mechanism of Water Absorption and Retention
1.1 Structure of SAP
SAP has a highly cross-linked three-dimensional network structure, and its main chain or side chain contains a large number of hydrophilic groups, which has strong water absorption. Li Shuqin et al. synthesized sodium polyacrylate/humic acid SAP with dense structure and irregular network fluffy structure. Zhang Duyu et al. prepared sepiolite (ST)/polyvinyl alcohol (PVA)/polyacrylamide (PAM) water-absorbing resin, ST particles increased the specific surface area of the resin, PVA and PAM formed a semi-interpenetrating network structure, which improved the swelling ability of the resin.
Ma Li et al. prepared kaolin, montmorillonite, lithium soapstone-based composite SAP, surface wrinkles, gullies increased, pore size increased, water absorption improved.
1.2 Water absorption and retention mechanism of SAP
The carboxyl, hydroxyl, and amino groups in SAP belong to strong hydrophilic groups. When SAP encounters water, water molecules enter the interior of the resin through capillary wetting. During the process of water absorption, freely moving cations increase the osmotic pressure inside and outside the resin, and water enters the cross-linked network; negative ions are fixed on the network, resulting in repulsion and causing the expansion of the polymer network structure. A large amount of water continues to enter the molecular network until the osmotic pressure inside and outside the resin is balanced. The ionization of hydrophilic groups on the molecular surface interacts with water molecules to form hydrogen bonds, and water molecules are "locked" in SAP.
02
Performance Requirements of SAP for Soil Remediation in Highway Engineering
2.1 Salt tolerance
The salt content of the slope soil increases due to the use of a large number of snow-melting agents, and with the evaporation of soil water, the salt migrates to the soil surface. SAP will shrink and precipitate in salt water, and even lose its water absorption capacity completely, which seriously affects the soil remediation performance. Compared with other water-absorbing groups, the introduction of amide, carboxyl and sulfonic acid groups is more beneficial to improve the salt tolerance of resins.
2.2 Weather resistance
The service life of highway SAP is largely affected by the outdoor natural environment, so it is necessary to study its ability to resist UV light degradation. With the increase of cross-linking agent and acrylamide dosage, the weather resistance of SAP will gradually increase. Introducing amide bonds into the SAP structure can also improve its weather resistance.
2.3 Water and fertilizer retention
After SAP absorbs water, it can form a gel and slow down water loss. Compounding urea with SAP can slow down the diffusion of fertilizer and play a role in retaining water and fertilizer.
2.4 Degradability
The addition of synthetic SAP to soil is bound to pollute the environment due to its poor biodegradability. The degradation of polymer resins depends on its chemical structure and environment, such as air, light, heat, microorganisms, soil conditions (pH and temperature), etc. Improving the biodegradability of SAP can be achieved by introducing amides, esters, carbamates, etc. into the main chain or side groups.
03
Effect of SAP on Soil Physical Composition and Structure and Its Remediation Mechanism
The remediation effect of water-absorbing resin on soil is that the physical and chemical reactions occur when the water-absorbing resin is mixed with the soil. The water-absorbing resin absorbs water to form a gel, and organically combines with soil particles to form stable aggregates. The unique three-dimensional network structure of SAP also forms a large water storage space and reduces water evaporation. The effect of SAP on soil physical properties was evaluated by soil aggregates, soil swelling rate, soil bulk density and permeability coefficient.
3.1 Soil aggregates
The results show that aggregates with diameter greater than 0.25mm play a major role in soil. The carboxylic groups on the surface of SAP resin can interact with positively charged particles in the soil, causing aggregates in the soil to form aggregates, increasing the content of aggregates in the soil and improving the water stability of the aggregates.
3.2 Soil expansion rate
After the SAP absorbs water, the water molecules enter the resin, stretch the tangled polymer long chain, and increase the volume, which can increase the expansion rate of the soil and hold more water. With the increase of the soil expansion rate, the proportion of liquid phase in the soil increases, and the proportion of solid gas decreases relatively, which is conducive to improving the air permeability and water permeability of the soil and creating conditions for the growth of plants.
3.3 Soil bulk density
The smaller the soil bulk density, the more porous and loose the soil, which is conducive to the exchange of water, gas and fertilizer in the soil and the improvement of soil fertility.
3.4 Soil permeability coefficient
SAP has an impact on the permeability coefficient of soil, which can inhibit soil surface compaction, reduce surface runoff, and improve soil water resistance. After adding SAP, the content of water-stable aggregates with a particle size greater than 0.25mm in the soil increases, the soil swelling rate increases, the bulk density decreases, and the permeability coefficient increases. It improves soil permeability while increasing water absorption, effectively reduces surface runoff, improves soil erosion resistance, and plays a role in protecting soil.
04
The restoration effect of SAP on different types of soil
Our country's highways run through the north and south, and the soil types and environmental and climatic characteristics of each region are different, so the focus of soil remediation in each region is also different.
4.1 Remediation effect on loess soil
Natural loess is loose in texture, low in strength, prone to disintegration in contact with water, and weak in erosion resistance. Aggregates in loess can play a role in reducing runoff.
4.2 Remediation effect of saline-alkali soil
Saline-alkali soil is highly alkaline, and soil humus is easily leached, causing soil degradation and vegetation cannot grow normally. The improvement of saline-alkali soil is generally to passivate and fix the Na + in the soil to reduce the soil salt content. SAP can promote the formation of soil agglomerates, and can also undergo chemical reactions such as hydrolysis and adsorption with substances in the soil, thus playing a role in improvement.
4.3 Remediation effect on sandy soil
Sandy soil has large pores, loose soil, and easy evaporation of water, which is not conducive to water and fertilizer fixation. Sun Renyun sprayed different concentrations of microbial polysaccharide gum (HG), plant polysaccharide gum (GG) and MG (HG to GG mass ratio 1:1) solutions on the surface of sandy soil to form a layer of reinforcement film. It was found that the water retention capacity of sandy soil was improved, the surface hardness was enhanced, and the addition of polymer solution was also conducive to the growth of vegetation.
4.4 Remediation effect on other types of soil
Red loam soil has high acidity, low organic matter content, poor structural performance, poor water stability of granular structure, easy decomposition, and serious soil erosion. Purple soil is rich in mineral nutrients and has high fertility, but the organic matter content in the soil is low, the structure is poor and easy to disintegrate, and the water storage and water retention ability is weak.
In general, the mechanism of using SAP to improve the water absorption, water retention and fertilizer retention capacity of different types of soil can be summarized as two. First, improve the physical composition of the soil. Make more aggregates form in the soil, increase soil porosity and reduce soil bulk density; second, adjust the adsorption properties of the soil. Increase the adsorption, fixation and desorption capacity of soil for nitrogen and potassium elements, improve soil fertilizer efficiency and reduce soil nutrient leaching rate. Therefore, the purpose of improving different types of soil is to improve the structure of soil particles through the addition of adhesive materials, and ultimately improve their ability to retain water and fertilizer.
For loess, it is mainly improved in terms of improving soil stability and resistance to disintegration; for sandy soil, it is mainly through aggregating fine particles, adjusting the three-phase ratio of soil solid, liquid and gas, and improving soil viscosity.
05
Research Progress of SAP in Highway Engineering Ecological Restoration
5.1 Research Status of Soil Remediation in Highway Engineering
Developed countries such as Europe and the United States have started early in the ecological restoration of highway projects, from early measures such as resin cuttings and hedges, to today's hydraulic spraying and grass planting technologies. The United States has formulated technical standards for highway greening, emphasizing the coordination of highways and surrounding landscapes, and paying attention to the aesthetics of ecological environment landscapes. The United Kingdom combines ecological restoration with reinforced soil technology to strengthen slopes, and at the same time establish vegetation protection. Germany and Switzerland have fully considered the maintenance of biodiversity and biological migration paths, and also built "ecological bridges" to ensure animal migration during restoration.
Japan's highway construction is late, but road area environmental restoration technology is in a leading position, paying great attention to new ecological restoration technologies.
The research on soil remediation technology of highway engineering in our country started late, and the soil types are diverse, so it presents the characteristics of uneven regional development and complex technical needs. At present, the planting bag slope protection, geocell slope protection, mesh bag grass planting technology, three-dimensional vegetation network technology, hanging net spraying slope protection, etc. are widely used. After years of development and accumulated rich experience, but there is a problem of neglecting the combination of restoration scheme and local ecological environment, such as the short survival period of vegetation due to the introduction of foreign vegetation. In addition, the current restoration mainly focuses on the short-term vegetation restoration effect, and cannot fundamentally improve the soil structure, and the long-term restoration effect is difficult to evaluate.
5.2 Key Points and Difficulties of Slope Repair Technology in Highway Engineering
The main points of slope restoration in highway engineering include restoring the stability of the slope by reconstructing the slope structure; restoring the slope biomes to achieve a state of self-repair. The current restoration points mainly focus on the instability of the slope. However, during the operation of the highway, the influence of human factors such as vehicle exhaust emissions, snowmelt salt dispersion, etc., affects the growth of surface vegetation on both sides of the highway to varying degrees, reducing the ability of plants to consolidate the soil; natural disasters such as freeze-thaw erosion and wind erosion increase the exposed area of the slope surface, resulting in increased sensitivity to wind and hydraulic forces, decreased stability, and even landslides.
In severe climatic conditions, it is extremely difficult to carry out efficient slope soil remediation, so higher requirements are also placed on SAP.
06
conclusion
SAP has become one of the main materials for soil remediation in highway engineering due to its excellent water absorption and water retention, as well as the advantages of improving soil structure and promoting plant growth. However, there are still the following problems in practical application:
(1) There are relatively few studies on SAP in soil remediation of highway engineering slopes, especially the differences between it and the natural environment conditions of traditional agricultural soil remediation, and the mechanism of slope soil remediation is still unclear. How to combine SAP soil improvement with the needs of local pioneer vegetation for soil nutrients needs to be further studied to adapt it to the complex climate environment.
(2) There are various macro-evaluation system models for slope restoration effect, but the specific evaluation method of SAP in slope restoration is relatively simple, limited to the determination of SAP water absorption rate, soil trace elements and soil basic physical and chemical properties. Synthetic SAP is a non-degradable material, and long-term excessive use may pose risks to plant growth and soil fertility, and cause secondary damage to soil.
(4) Our country needs to consume a lot of manpower, material and financial resources for road maintenance every year, but it does not pay enough attention to the maintenance of highway engineering soil. Failure to choose appropriate restoration vegetation to carry out engineering practice in combination with highway geology and geomorphology and climatic conditions leads to unsatisfactory long-term restoration results. Therefore, it is of great significance to develop SAP and composite restoration systems with strong adaptability, stability and low cost to conduct deeper theoretical research on the soil remediation mechanism of SAP.
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