Effect of Hydroxypropyl Methyl Cellulose as Soil Conditioner on Solute Transport in Soil

Abstract: Hydroxypropyl methyl cellulose (HPMC) is a potential soil amendment. It has obvious seepage reduction effect after being added to the soil, which is of great significance to alleviate the loss of soil and water nutrients in the Loess Plateau. In this paper, the effect of HPMC on the transport characteristics of soil solutes was studied by adding different contents of HPMC to the soil. The results showed that: 1) When the mass fraction of HPMC was in the range of 0-0.5 g/kg, the saturated hydraulic conductivity gradually decreased with the increase of HPMC addition, and the 0.5 g/kg group decreased by 37.3% compared with the blank group without HPMC; The migration velocity of conservative solutes in the soil decreased significantly; with the increase of HPMC addition, the initial and complete penetration times of solutes were significantly delayed, and the total penetration duration was prolonged; 2) CDE equation and two-zone model can simulate well in When adding different contents of HPMC to the soil, the fitting curves of the two models can also be in good agreement with the measured curves, but the simulation accuracy of the two-region model is higher. 3) Based on the parameter fitting results of the two-zone model, with the increase of HPMC addition, the smaller the average pore water flow velocity, the higher the hydrodynamic dispersion coefficient, dispersion degree and mass exchange coefficient, and the water content ratio of soil movable water gradually reduce.

Key words: solute; model; soil; hydroxypropyl methylcellulose; saturated hydraulic conductivity; breakthrough curve; convection-dispersion equation

0.Preface

Hydroxypropyl methylcellulose (HPMC) is one of the varieties with the widest application and the best performance among all kinds of cellulose, and it is one of the nonionic cellulose mixed ethers. HPMC is safe and non-toxic, has a wide range of enzyme resistance and adhesion, and due to the hydrophilicity of hydroxyl and hydroxypropyl groups, HPMC has good water-holding properties. Wang Xinxin et al. found that the water holding capacity of HPMC has a significant effect on reducing the freezable water content in dough and improving the survival rate of yeast cells during freezing storage. Studies by Liu Haiyan and others have shown that adding an appropriate amount of HPMC can effectively improve the baking quality of bread, improve its texture properties, increase elasticity and cohesion, significantly reduce the hardness and chewiness of bread, and have a good anti-aging effect. Adding modified fiber HPMC to food packaging materials increases the practical application value of soybean protein isolate composite membrane, and has a good development and utilization prospect.

HPMC is rich in renewable sources, environmentally friendly, and biodegradable. Compared with ionic cellulose ethers, HPMC does not react with heavy metals and has acid-base stability. In agricultural production, due to its good water solubility, dispersion, thickening, water retention and film-forming properties, HPMC, as a water-soluble polymer material, forms a coating film that is colorless, odorless, tough and transparent, and is widely used as a drug coating And the rate-controlling polymer material of sustained-release preparations has obvious effects in shortening the film-forming time, reducing the shedding rate and improving the uniformity. Wang Haiyan's research found that HPMC coating can delay seed germination, which is beneficial to improve seed storage resistance, and HPMC can be effectively degraded by microorganisms in soil, with good environmental compatibility, and is a good seed coating film-forming agent material.

In recent years, soil erosion and a large number of unreasonable farming have caused a large amount of nutrient loss and water pollution. It is of far-reaching significance to study the migration mechanism of soil solutes, maintain water and soil and slow down the ineffective loss of nutrients in the soil. And the mathematical model is in In-depth study of solute transport in porous media plays an important role, in which the traditional convection-dispersion model and two-zone model are most widely used. Previous studies have shown that gel-like water-retaining agents such as HPMC can significantly improve soil moisture retention, but there are relatively few studies on the internal mechanism of soil nutrient movement and solute migration. Based on the excellent properties of HPMC such as cohesiveness, water holding capacity, pH stability and biodegradability, if HPMC is applied to the study of soil nutrient loss and heavy metal adsorption process, and the mathematical model is used to analyze its change law and internal mechanism, it can be To provide ideas and methods for improving soil water and fertilizer retention performance and improving the current situation of soil and water nutrient loss. Therefore, this paper studies the effect of HPMC on solute migration, aiming to provide a theoretical basis for the control of soil erosion.

1. Materials and methods

1.1 Test material

1.1.1 Soil samples for testing

The sampling time of the test soil was September 2017. The sampling site was the Changwu Agricultural Ecological Experimental Station of the Chinese Academy of Sciences (35°12'N, 107°40'E). The Mastersizer 2000 laser particle size analyzer was used for particle analysis of the tested soil. The mass fraction of clay (<0.002 mm) in the tested soil was 6.83%, silt (0.002-<0.02 mm) was 93.08%, and sand (0.02-<2 mm) was 93.08%. mm) is 0.09%. According to the international soil texture classification standard, the tested soil belongs to silty loam. Using ring knife method to measure the bulk density, the measured soil bulk density is 1.31 g/cm³. The saturated moisture content of the soil is 42%, and the residual moisture content is 1.2% obtained by using the van Genuchten model to fit the parameters of the soil moisture characteristic curve. The soil to be tested was crushed to remove impurities such as gravel, dead grass and root residues, and passed through a 2 mm sieve after air drying.

1.1.2 Physical and chemical properties of HPMC

HPMC is a semi-synthetic cellulose ether polymer, soluble in a certain concentration of alcohol, propanol, dichloroethane solution, relatively stable in nature, and the colloidal solution has a certain viscoelasticity. HPMC is a solid particle or fibrous white powder at room temperature, incompatible with strong oxidants, solid and flammable. The apparent density of HPMC (also known as apparent density, which refers to the dry mass per unit volume under natural conditions) is usually about 0.5 g/cm³, and the relative density is 1.3. It is easy to carbonize inside. At 22 ℃, the viscosity of HPMC with a concentration of 2% in aqueous solution is in the range of 5-2×105 mPa·s, and the surface tension is 0.042-0.056 N/m. The methoxyl value and hydroxypropyl value of HPMC are 19%-30% and 4%-12%, respectively. Different production processes of HPMC have slight differences in properties, and HPMC produced by KIMA CHEMICAL CO., LTD was selected for this test.

1.2 Test method

The experiment was carried out at the Soil Physics Laboratory of Xi'an University of Technology in April 2018. The infiltration tests with different HPMC contents showed that: when the application amount of HPMC in the soil is 0.1-1.0 g/kg, the infiltration state can form a relatively sharp contrast, but when the application amount is more than 0.5 g/kg, the infiltration speed is relatively slow, The time required for the soil column to reach saturation is too long, so the addition ratio of HPMC in this experiment design is 0 (CK), 0.1, 0.2, 0.3, 0.4, 0.5 g/kg, a total of 6 treatments. In order to keep the viscosity of HPMC constant, the middle viscosity level of HPMC is selected: 100 Pa·s.

Before the test, the HPMC and the spare soil sample were uniformly mixed according to the predetermined ratio, and a cylindrical plexiglass soil column with a height of 50 cm and a diameter of 5 cm was used as the test device, and a layer of fibrous qualitative filter paper was laid on the bottom of the soil column to prevent soil loading. Uneven soil loading caused by loss of soil particles. Soil samples with different application amounts of HPMC were loaded into the soil column in layers of 5 cm per layer. The cumulative height of 4 layers was 20 cm, and the bulk density was 1.31 g/cm³. After each layer was installed, the topsoil was scraped to make the soil The layers are fully combined, and the overall filling is more uniform. The experimental water supply system is a Marsh flask with a height of 40 cm and a diameter of 5 cm, and the control head height is 4.5 cm.

After the fully infiltrated soil is saturated, the Martens bottle continues to supply water at a constant head, connects to the outflow liquid below the soil column, and measures the volume of the outflow liquid every 120 min with a graduated cylinder to calculate the soil saturated hydraulic conductivity Ks. Then stop the water supply to the soil column, and immediately absorb the accumulated water on the surface of the soil column; replace the water supply device with a Martens flask filled with 0.2 mol/L CaCl2 solution, and keep the water head at 4.5 cm. Connect the effluent from the lower end, once every 10 mL is full, and measure the chloride ion concentration with the potassium chromate-silver nitrate method until the Cl- concentration in the effluent is close to the Cl- concentration in the Martens flask and is relatively stable for a period of time ; After collecting the effluent, quickly take about 5 g of soil samples from the center of each layer of the soil column (every 5 cm is a layer), mix the soil samples with pure water at a mass ratio of 1:5, and use a shaker to medium Shake at a rotating speed for 30 min, take the filtrate and measure its Cl- concentration.

In this test, the potassium chromate-silver nitrate method is used to determine the chloride ion concentration. The specific steps are: put the solution to be tested into a 150 mL Erlenmeyer flask, add 8 drops of 5% potassium chromate indicator, and use 0.01 mol/L nitric acid Titrate with silver standard solution. Shake the Erlenmeyer flask continuously during the titration process until brick red precipitate appears and does not disappear, and record the volume of standard solution consumed. Data were analyzed in June 2018 after the trial ended.

1.3 Basic theory and index calculation

1.3.1 Saturated hydraulic conductivity formula

Soil saturated hydraulic conductivity Ks refers to the water flux through the saturated soil under the unit water potential gradient, which is closely related to the pore structure, bulk density and soil texture of the soil, and is an important index reflecting the movement of soil water and affecting the migration of solutes. The saturated hydraulic conductivity measured by the fixed head method can be calculated by the formula Ks=QL/AtsH.

1.3.2 Solute transport model

This experiment uses Cl as a tracer ion to study the one-dimensional solute penetration of saturated soil. The test results show that the final penetration concentration of the solution is basically the same as the initial concentration, and the Cl concentration at the soil column section with different contents of HPMC has no obvious The difference proves that the influence of HPMC on the concentration distribution of Cl in the soil profile is not obvious enough. Therefore, the convection-dispersion equation (CDE) of conservative solute transport under the condition of one-dimensional saturated steady flow and the two-region model (TRM) under the condition of steady flow were selected.

1.3.3 Diffusion under one-dimensional condition

1.3.4 Other formulas

1.4 Statistical analysis

In this paper, the CXTFIT2.1 software is used to simulate and analyze the CDE and two-zone model of six groups of solute breakthrough curves with different contents of HPMC. The model parameters v, D, β, ω and the coefficient of determination R2 and Residual squares and residual squares (sum of squared residuals, SSQ), where β=θm/θ, is the ratio of movable water body content. The application software WPS 2017 was used to process the test data, Origin 2017 was used for drawing, and SPSS19.0 was used for analysis of variance and empirical parameter fitting.

2. Results and Analysis

2.1 Effect of HPMC content on saturated hydraulic conductivity

When the mass fraction of HPMC was in the range of 0-0.5 g/kg, Ks gradually decreased with the increase of HPMC content, and Ks decreased by 37.3% in the 0.5 g/kg group compared with that without HPMC (P<0.05). This is because HPMC can swell into a three-dimensional network-like gel structure between solid and liquid when it meets water, filling the soil pores to a certain extent. Gao Jie's research found that with the increase of HPMC addition, the number and size of the internal pores of the dough showed a decreasing trend. Wang Yanru found that adding HPMC with a mass fraction of 0.02% can significantly reduce the pore size of the foamed cement insulation board and increase the surface tension. The formation of this structure makes the soil water movement channel more tortuous and complex, and the soil outflow decreases in the same period of time, and the higher the HPMC content, the denser the gel network structure and the smaller the saturated hydraulic conductivity.

2.2 The effect of HPMC content on the characteristics of solute breakthrough curve

Soil solute breakthrough curves (BTCs) are used to reflect the relationship between the relative concentration of solutes in the flowing fluid (Cr) and the pore volume T, which is an important way to study the mechanism of soil solute transport. It can be seen from the solute breakthrough curves of the soil after adding different proportions of HPMC that under the same hydraulic gradient, the shape of the solute breakthrough curves of the soil columns with different contents of HPMC is similar to that of the CK group, and they are all S-shaped smooth curves. Under different treatment conditions, the S-type breakthrough curves have a gradual change trend, that is, with the increase of HPMC content, the gradual increase of relative concentration with the increase of effluent liquid slows down, the tailing characteristics are obvious, and the complete penetration (ie When the relative concentration is 1), the pore volume increases, so the addition of HPMC has a certain delay effect on the soil solute migration process. This is because the tracer ion Cl is a conservative solute ion and does not react with HPMC. The three-dimensional interpenetrating gel network formed by the combination of HPMC and soil water fills the soil pores to a certain extent, making the pore structure of saturated soil more complex. The tortuosity of the solute penetration path increases, so that with the increase of HPMC content in the soil, the Fick migration of solutes is weakened, the mechanical dispersion is enhanced, and the volume of pores that completely penetrate the soil column increases. Unevenly sized soil pores lead to unbalanced water flow velocity, which forms an unbalanced solute front in the soil profile, so the increase in HPMC content causes the solute breakthrough curve to prolong the tail.

2.3 Effect of HPMC content on breakthrough time

The initial breakthrough time, complete breakthrough time, and total breakthrough time are all important characteristic parameters of solute breakthrough, which are jointly determined by the pore water flow rate and the hydrodynamic diffusion coefficient of the soil. From the Cl breakthrough time table of adding different proportions of HPMC, it can be obtained that: when the amount of HPMC added is in the range of 0-0.5 g/kg, t1 and t2 are proportional to the content of HPMC. The later the penetration time, further observation can be obtained: 0-0.2 g/kg, the increase of t1 and t2 is larger, and when 0.2-0.5 g/kg, the increase of t1 and t2 slows down. In the range of 0.1-0.4 g/kg mass fraction of HPMC, t total increases gradually with the increase of HPMC content, and t total when HPMC is 0.5 g/kg is slightly decreased compared with t total when 0.4 g/kg , the total duration of solute breakthrough was the longest when HPMC was 0.4 g/kg. This shows that the increase of HPMC content can prolong the initial penetration time, complete penetration time and total penetration time to a certain extent, which indirectly shows that the application of HPMC has a certain impact on the average pore flow velocity of soil.

2.4 Comparison and analysis of CDE equation and two-zone model fitting

In order to further study the Cl-transport characteristics of HPMC-applied soils, and to compare and analyze the applicability of different solute transport mathematical models, this paper fits the main parameters of the CDE equation and the two-zone model. From the fitting results of the convection-dispersion equation and the two-zone model parameters, it can be seen that the fitting values of the CDE equation and the two-zone model for v, D, and λ tend to be consistent with the change of HPMC content, and the coefficient of determination of the six groups of experimental parameters fitted by the CDE equation R2 was higher than 0.98, and the minimum value of SSQ was 0.009, while the R2 of the two-region model fitting of the six experimental parameters were all greater than 0.999 3, and the SSQ were all less than 0.002. This shows that both the CDE equation and the two-zone model can better fit the solute migration conditions under different HPMC additions, and both have high fitting accuracy, but the two-zone model is more accurate than the CDE equation. Further observation shows that the average pore water velocity v, hydrodynamic diffusion coefficient D, and diffusion coefficient λ fitted by the CDE equation are all somewhat larger, because the traditional CDE model only considers the convective and diffusive effects of solutes, and does not consider the Solute diffusion in the immovable zone of the soil, and both convective dispersion and diffusion can cause the dispersion of soil solutes, and the two together constitute the hydrodynamic dispersion effect.

In the Cl penetration test by Liu Yanli et al., the clay mass fraction of the tested soil sample was 5.53%, silt was 19.32%, and sand was 75.15%. The soil type was sandy loam, and the water head height was 7 cm. The v, D and The fitting values of λ are 13.64 cm/h and 0.264 cm² respectively

/h and 0.019. The soil sample used in this test is disturbed soil, the total mass fraction of clay and silt is as high as 91%, and sand only accounts for 0.09%. It is a silty loam soil with a water head height of 4.5 cm. The greater the proportion of silt and clay in the soil particles, the lower the permeability of the soil, and the structure of the disturbed soil is destroyed, and the permeability coefficient is smaller compared with the original soil; the lower the water head height, the slower the infiltration rate; The addition of HPMC can reduce the infiltration rate in the soil, so the fitted values of v, D and λ under the test conditions are small. The same analogy analysis can also be carried out on the research data of the ion migration process by Lu Jinbang et al.

In order to more intuitively compare the differences and connections between the measured, CDE equation and the solute breakthrough curves simulated by the two-zone model, the three solute breakthrough curves were plotted when the mass fraction of HPMC was 0-0.5 g/kg. It can be seen from the observation that the fitting curves of the CDE equation and the two-zone model can be well matched with the measured curves, but each group of curves fitted by the CDE equation has different degrees of alienation from the measured curves, especially when the mass fraction of HPMC is 0-0.2 g/kg, the alienation phenomenon of the curve is more obvious; use tm to represent the average breakthrough time (the average value of t1 and t2), in the range of t1 ~ tm, the CDE fitting curve is higher than the measured curve, tm ~ t2 Within the range, the CDE fitting curve is lower than the measured curve first and then slightly higher than the measured curve; while the fitting results of the two-region model are in good agreement with the measured curve, there is no obvious alienation phenomenon, and both in the early penetration and near complete penetration. reached a higher level of fit. This shows that the solute transport mode in the soil added with HPMC includes both the convective-dispersive form in the movable area and the solute diffusion form in the immovable area where the soil water is relatively immovable. The solute must be in the mobile and immobile soil Penetration is complete when both areas are balanced. The above analysis further shows that, compared with the CDE equation, the two-zone model can better simulate the migration of soil solutes when different contents of HPMC are added, and the fitting results are more reliable.

2.5 Variation characteristics and analysis of parameters in the two-zone model

Correlation analysis was carried out between the HPMC quality fraction and the parameters v, D, λ, β and ω of the two-zone model fitting, and the correlation between the HPMC content and D, λ, β and ω reached a significant level. The HPMC mass fraction is denoted by A.

Combining the convection-dispersion equation and the two-zone model parameter fitting results, the two-zone model parameter fitting results, the change characteristics of the corresponding parameters are analyzed as follows:

1) Average pore water velocity v

v refers to the effective flow of water in the soil, that is, the volume of liquid flowing through the unit flow section per unit time. From the Cl breakthrough time of different proportions of HPMC, it can be obtained that in the range of HPMC mass fraction of 0-0.5 g/kg, the average pore water velocity v obtained by the two-zone model is in the range of 0.904 cm/h, and v The values all decreased with the increase of HPMC addition. This may be due to the combination of HPMC and soil water to form a three-dimensional gel network, resulting in reduced soil pore size and narrowed water flow channels.

2) Hydrodynamic dispersion coefficient D

The solute flux due to hydrodynamic dispersion is known as the hydrodynamic dispersion coefficient. Two-zone model fitting under different HPMC content D increases with the increase of HPMC addition. Since the size of the hydrodynamic dispersion is determined by the water content and the pore water flow rate, and the addition of HPMC reduces the average pore water flow rate of the soil and increases the tortuosity of the water flow channel, so the mechanical dispersion of Cl in the penetration process Enhanced, the hydrodynamic diffusivity tends to increase significantly with the increase of HPMC content.

3) Diffusion λ

λ is used to characterize the dispersion ability of solute in the porous medium. Its size is inseparable from the average particle size and uniformity of the porous medium. It is equal to the ratio of D and v in quantity. The larger the λ, the better the solute diffusion ability of the porous medium. powerful. λ increases with the addition of HPMC, which indicates that the greater the addition of HPMC, the greater the dispersion of soil solutes, and the stronger the ability of solutes to fully diffuse in the soil.

4) Moisture content ratio in the movable zone β

Moisture ratio β in the movable region represents the percentage of solute in the total soil concentration in the movable region under equilibrium conditions. The closer the β value is to 1, the less affected the solute is by the physical non-equilibrium mechanism during the migration process, which can be considered as a physical equilibrium process. The simulation results of the two-zone model can be obtained: with the increase of HPMC content, β decreases from 0.946 to 0.925. The decrease of β indicates that the water content of the immovable zone increases, and the physical process of solute penetration tends to be more balanced.

5) Mass exchange coefficient ω

ω is a parameter characterizing the degree of solute exchange between the movable and immovable regions. The ω of the tested soil increased with the addition of HPMC, and the value of ω increased from 1.43 to 5.63 with the addition of HPMC from 0 to 0.5 g/kg. It can be seen that the addition of HPMC promotes the degree of solute exchange between the movable region and the immovable region, and promotes the diffusion of Cl to the soil micropores and dead pores where water can hardly flow, which proves that HPMC can promote the solute ion in the soil. Potential for storage and diffusion.

3. Conclusion

Through the CaCl2 solute penetration test of HPMC soil, the CDE equation and the two-zone model were used for simulation and comparative analysis. The results show that:

1) When the mass fraction of HPMC is in the range of 0-0.5 g/kg, the saturated hydraulic conductivity gradually decreases with the increase of HPMC addition, which is at most 37.3% lower than that of the group without HPMC; the migration speed of conservative solutes in soil significantly decreased; with the increase of HPMC content, the initial and complete penetration time of the solute was significantly delayed, and the total penetration duration was prolonged; 2) Both the CDE equation and the two-zone model could well simulate the transport of solutes when different amounts of HPMC were added to the soil. However, there is a slight distance between the fitted curves of the CDE equation and the measured curves, and the fitting accuracy of the two-zone model is higher, and the effect is better; 3) Based on the parameter fitting results of the two-zone model, with the increase of HPMC addition, The smaller the average pore water flow rate, the higher the hydrodynamic diffusion coefficient, the diffusivity and the mass exchange coefficient, and the lower the water content ratio of the soil movable water body.