Field Test on Soybean-Urease Induced Calcite Precipitation (SICP) for Desert Sand Stabilization against the Wind-Induced Erosion

Abstract

Soybean-urease induced calcite precipitation (SICP) is an effective method for the improvement of sand, which forms a biocemented layer on the desert sand surface to resist erosion induced by the wind. Under this study, field tests were carried out to determine how the SICP approach may enhance the resistance of the desert to wind-induced erosion and the durability of SICP treatment in southeastern margin of Tengger Desert, Ningxia Hui Autonomous Region, China. The experimental results demonstrated that the erosion resistance of desert sand was significantly enhanced due to the SICP treatment, and the improvement effect was enhanced with the increase of the biocement solution concentration and dosage and the number of treatment cycles. Furthermore, it was also found that the resistance of SICP-treated sand to erosion induced by the wind reduced as the development of time reduced. Based on the test results in this paper, larger biocement solution concentration and dosage and multiple treatment cycles are proposed in the areas where severe wind-induced erosion takes place in order to improve the ductility of SICP treatment.

1. Introduction

One of the major environmental issues globally is desertification. It is a great hazard to national economic development, ecological balance, and social stability [1]. The development of social industry, the extreme climate, and human activities have led to increasing desertification. The major reason for desertification and sandstorms in desert regions is wind-induced erosion. This erosion reduces the contents of fine grains and nutrients in the soil and thereby it reduces soil fertility and water-retention ability [2]. As the wind blows, the dust that is already there in the atmosphere is blown away. This reduces the visibility, causes air pollution, and is a threat to the human health [3]. Thus, regulating the wind-induced erosion of desert regions is vital to maintain human health, the environment, and agriculture.

In the literature, different methods, e.g., vegetation and chemical solidification, were used in order to reduce the erosion induced by the wind. One of the most effective approaches is the vegetation cover on the surface of the land [4]. However, vegetation may not be possible in some areas where the soil is unfertile and not suitable for agriculture. In such places, strong winds uproot the plants or cover them under the quicksand [5]. In such conditions, a chemical solidification approach is employed [6,7]. In the chemical modification method, different organic and inorganic materials, namely cements, products of petroleum, and synthetic polymers as stabilizers, are employed [8,9,10,11]. These stabilizers avoid wind-induced soil erosion by immobilizing sandy soils. However, some chemical soil stabilizers are harmful to the environment and may be toxic [12,13].

Recently, biocementation, based on a urea hydrolysis process treated using urease, was utilized to improve the mechanical properties of sand. The reaction includes two steps. First, urea is hydrolyzed into carbonate and ammonium by soybean urease. Second, calcium carbonate is produced in the presence of calcium source. The steps of the reactions are shown in Equations (1) and (2):

$${\mathrm{NH}}_2{-\mathrm{CO}-\mathrm{NH}}_2{+2\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{urease}}{\to }{2\mathrm{NH}}_4{}^{+}{+\mathrm{CO}}_3{}^{2-}$$

(1)

$${\mathrm{Ca}}^{2+}{+\mathrm{CO}}_3{}^{2-}\to {\mathrm{CaCO}}_3\downarrow$$

(2)

The calcite that is produced and the calcium carbonate crystals in stable form act as a cementing agent to fill soil pores and bind soil particles, thereby increasing the stiffness and strength of soils significantly [14,15,16,17,18,19,20]. Urease is very important in the biocementation process and is naturally found in microorganisms and plants. The biocementation techniques are called microbially induced calcium carbonate precipitation (MICP) and soybean-urease induced calcium carbonate precipitation (SICP), respectively, utilizing ureolytic bacteria or soybean urease for calcite precipitation [13,21]. The MICP and SICP methods are widely investigated for various civil engineering uses, including soft grounds improvement, slope stability, and bearing capacity of foundation enhancement, etc., [22,23,24,25,26,27,28,29].

Further, some literature also reports the efficiency of MICP and SICP methods in increasing wind-induced erosion resistance [30,31,32]. However, most have focused on investigations of the efficiency of biocementation under laboratory conditions. They have not considered that the biocemented sand is exposed to complex environmental challenges under natural conditions. Besides, SICP and MICP also have some major differences. Bacteria cultivation is more complicated and has more technical challenges than the soybean urease approach under practical conditions for practical engineers. In this work, field tests were carried out to exploit the feasibility of using SICP method to enhance the wind-induced erosion resistance of desert sand and evaluate the durability of SICP treatment in southeastern margin of Tengger Desert, Ningxia Hui Autonomous Region, China.

2. Materials and Methods

2.1. Biocementation

As the biocementation relied on a urea hydrolyzing process treated using soybean urease, the microbial treatment was conducted with soybean urease, urea, and calcium salt.

2.1.1. Soybean Urease

Soybeans were bought from a farmer market in Zhongwei, Ningxia Hui Autonomous Region. The soybeans were crushed using a crushing machine, and sieved using a 60-mesh sieve. The soybean powder and calcium chloride were soaked in tap water at 60 g/L and 0.008 mol/L, respectively, and stirred until they were evenly dispersed in the tap water. The homogeneous solution was allowed to stand for 60 min before the supernatants were collected to get the soybean urease solution. In this way, the urease activity of soybean urease solution determined was around 4.44 mM-urea/min (roughly 0.4 mS/cm/min in terms of the electric conductivity change rate [33]). The soybean urease solution was used immediately after preparation.

2.1.2. Biocement Solution

The biocement solution was obtained by mixed an equimolar urea-calcium chloride solution with a soybean urease solution at 1:1 volume ratio. Three different concentrations of the biocement solution namely 0.3 M, 0.4 M, and 0.5 M were used. The indicated concentration values denote the concentrations of urea and calcium chloride in the biocement solution. The biocement solution was used instantly after preparation.

2.2. Test Sites

The flied tests were carried out in southeastern margin of Tengger Desert (105°0′ E, 37°28′ N), Ningxia Hui Autonomous Region, China. This region had a yearly evapotranspiration rate of 1744.6 mm and a continental dry and semi-arid climate zone with a yearly precipitation of 150.0 mm. 11.0 °C was the typical annual temperature. The highest temperature recorded was 37.0 °C, while the lowest was −21.0 °C. The highest wind speed exceeded 17.2 m/s, and the average wind speed was roughly 2.8 m/s. The properties and grain size distribution of the local sand are displayed in

Table 1. Basic properties of desert sand.

Property	Value
Specific Gravity, Gs	2.66
Uniformity coefficient, Cu	1.73
Coefficient of curvature, Cc	1.04
Maximum void ratio, emax	0.847
Minimum void ratio, emin	0.546

and Figure 1, respectively.

2.3. Microbial Treatment

Figure 2 presents the test area. Twenty-five test plots were established in the sandy land, where an irregularly shaped plot was untreated as the reference. The residual twenty-four were treated by SICP process, each with an area of 3 m × 3 m. To avoid nearby quicksand from being blown into test sites during testing, grass barriers were created around the perimeter of every test site. Before microbial treatment, nine erosion pins were evenly arranged in each test plot for subsequent measurement of wind erosion depth. In the microbial treatment, eight different treatment types with various treatment cycles, biocement solution concentrations and dosages were used, and each treatment type with three parallel sets of tests. The testing parameters and the results are illustrated in

Table 2. Testing parameters and results.

Test No.	Biocement Solution Concentration, (mol/L)	Biocement Solution Dosage, (L/m2)	Treatment Cycles	Initial Penetration Resistance, (kPa)	Initial Thickness of Biocemented Layer, (mm)	Calcite Content, (%)
T-0	0	0	0	11	0	0
T-1	0.3	4	1	178	4	0.34
T-2	0.4	4	1	203	4	0.47
T-3	0.5	4	1	273	6	0.51
T-4	0.5	6	1	439	9	0.48
T-5	0.5	6	2	613	11	0.83
T-6	0.5	6	4	1065	11	1.36
T-7	0.5	12	1	593	17	0.49
T-8	0.5	18	1	824	21	0.54

Note: the name of a test comprises 2 parts: ‘T’ for test, and the following number for test ID.

.

Field tests were conducted in May 2019. The equimolar urea-calcium chloride solution and the soybean urease solution were made separately in two containers for the field test and then mixed to produce the biocement solution. After that, the biocement solution was sprayed uniformly on the test plot surfaces using an agricultural sprayer. To reduce the biochemical reaction in the sprayer, the short retention period (less than 10 min) was investigated. Furthermore, the spray treatment was conducted near sundown to decrease evaporation brought on by intense solar exposure and hot sand’s surface temperature, thereby enhances the effectiveness of the biochemical reaction. When the test plots need more than one treatment cycle, each treatment time interval is 24 h. After the treatment, all the test sites were exposed to the local weather conditions 3 d. Then, the further studies were conducted. During the 3 d, the total precipitation was zero and the field temperature ranged from 6 to 25 °C.

2.4. Surface Penetration Resistance Test and Erosion Pin Test

The penetrometer test is an indirect method used to evaluate the wind-induced erosion resistance of soil surface [34,35,36]. This research used a micro-penetrometer (HP-50, Aidebao, China) with a 3 mm radius to measure the surface penetration resistance of test plots (Figure 3). Three arbitrarily chosen points were measured on each test plot. The average surface penetration resistance was determined at the three points of every test plot to further obtain the average surface penetration resistance of each treatment type.

The erosion pins were used as they are convenient to measure sand erosion and deposition due to their negligible obstruction towards sand movement [37]. For each test plot, nine erosion pins with 30 cm long were evenly inserted midway into the soil before SICP treatment (15 cm above the ground). The difference in erosion pin heights was measured to obtain the wind erosion depth, and the average wind erosion depth of each treatment type was computed and displayed.

2.5. Calcium Carbonate Content Measurement

For each test plot, three small samples of sand in the crust were collected from various positions at the soil surface to calculate the CaCO₃ content in the sand. A small amount of deionized water-rinsed sand was combined with the right amount of acidic liquid during this phenomenon to help to dissolve calcite. Further, the EDTA titrimetric method was adopted to evaluate the calcium concentration in the liquid [38]. Then, the calcium carbonate content in every test plot with various treatment types was evaluated (Table 2).

3. Results and Discussion

3.1. Effect of SICP Treatment on the Surface Penetration Resistance of Desert Sand

3.1.1. The Influence of the Biocement Solution Concentration

The differences in the surface penetration resistances with different biocement solution concentrations are displayed in Figure 3a. Generally, the surface penetration resistance of the SICP-treated sand increases as the biocement solution concentration rises, indicating that the surface stabilization impact of SICP on desert sand increases as biocement solution concentration rises. This is good agreement with the observations of Hang et al. (2022) [39]. The laboratory test results showed that the resistance of wind erosion of biocemented soil increased with the biocement solution concentration (0.1 mol/L, 0.2 mol/L, and 0.4 mol/L). The average resistance of the sand is increased from 178 to 273 kPa after SICP treatment, as shown in Table 2, while the average resistance of the pure sand is zero. The increase occurs when the biocement solution concentration is increased from 0.3 mol/L to 0.4 mol/L at the dosage of 4 L/m². The vibration of the calcium carbonate quantity in the SICP-treated layers and thickness biocemented layer with biocement solution concentration explains the average trend of the resistance of SICP-treated sand-biocement solution concentration behavior. From Figure 3b and Table 2, it is observed that as the biocement solution concentration increases from 0.3 mol/L to 0.4 mol/L, the average calcium carbonate content in the biocemented layers enhances from 0.34% to 0.51%, and the average thickness of biocemented layer increases from 4 mm to 6 mm. Furthermore, the higher concentration provides more reactants used in the reaction, which produces more calcium carbonate and thicker biocement layer generated in the soils. The calcium carbonate generated in biocemented soils can fill soil pores and bind soil particles to improve the strength and integrity of the soil. This enhancement effect could be improved when the content of calcium carbonate and biocemented layer thickness increases. Therefore, the higher amount of calcium carbonate and thicker biocemented layer result in a higher surface penetration resistance of the biocemented sand.

3.1.2. The Influence of Treatment Cycles

The surface penetration resistance of the SICP-treated sand using different treatment cycles is shown in Figure 4a. For the SICP-treated desert sand with 6 L/m² biocement solution dosage, the average surface penetration resistance enhances from 439 kPa to 613 kPa when the number of treatment rounds increases from 1 to 2, and it increases to 1065 kPa as the number of treatment rounds keeps increasing to 4. From Figure 4b and Table 2, it can be found that the variation of treatment cycles has a great influence on the calcium carbonate content, while a small influence on the thickness of biocemented layer. Specifically, as the number of treatment rounds increases from 1 to 2, the average calcium carbonate content in the biocemented layers increases from 0.48% to 0.83%. Further, when the number of treatment rounds increases to 4, the average calcium carbonate content reaches 1.36. However, the thickness of biocemented layer only increases from 9 mm to 11 mm when the number of treatment rounds increases from 1 to 4. In the process of multiple treatment cycles, most of the biocement solution spayed onto the sand surface reacts in the top layer of a certain thickness. Thus, the produced calcium carbonate in each treatment cycle is continuously accumulated in this soil layer, while the thickness of the biocemented layer basically has no increase. The increased amount of calcium carbonate in the biocemented layer effectively enhances the surface penetration resistance of desert sand.

3.1.3. The Influence of Biocement Solution Dosage

The biocement solution dosage denotes to the biocement solution quantity utilized per m² in once treatment. Figure 5a illustrates the influence of biocement solution dosage on the surface penetration resistance of SICP-treated sand. Within the range of tested biocement solution dosage, the surface penetration resistance improved as the dosage raised. With the increase in the biocement solution dosage from 4 L/m² to 18 L/m², the surface penetration resistance of biocemented sand is found to increase from 273 kPa to 824 kPa. The results show that when the biocement solution dosage is increased, it provides an obvious enhancement in the surface penetration resistance of biocemented desert sand. As shown in Figure 5b and Table 2, when the biocement solution dosage increases from 4 L/m² to 18 L/m², the thickness of biocemented layer effectively increases from 6 mm to 21 mm, while the calcium carbonate almost has no change. This is related to the capillary action of the desert sand. As the biocement solution dosage exceeds the capillary action of the desert sand, the biocement solution flows into deeper soil. Therefore, the increase of biocement solution dosage has a great effect on increasing the thickness of biocemented layer, but no effect on increasing the calcium carbonate content in the biocemented layer. Compared with the increase of biocement solution concentration and treatment cycles, the increase of biocement solution dosage is the most effective method to increase the thickness of biocemented layer. The increase in surface penetration resistance with the increase in biocement solution dosage is attributed to the increased thickness of biocemented layer.

3.2. Durability of SICP Treated Desert Sand against Wind-Induced Erosion

The surface penetration resistance, thickness of biocemented layer and soil erosion depth in five months are shown in Figure 6, Figure 7 and Figure 8, respectively.

Table 3. The specific values of measured objects.

Test No.	Penetration Resistance after One Month of SICP Treatment, (kPa)	Penetration Resistance after Five Months of SICP Treatment, (kPa)	Thickness of Biocemented Layer after One Month of SICP Treatment, (mm)	Thickness of Biocemented Layer after Five Months of SICP Treatment, (mm)	Erosion Depth after One Month of SICP Treatment, (mm)	Erosion Depth after Five Months of SICP Treatment, (mm)
T-0	13	10	0	0	6	29
T-1	168	12	3	0	1	16
T-2	191	11	3	0	1	13
T-3	259	11	5	0	1	18
T-4	419	67	9	0	0	10
T-5	590	94	11	2	0	9
T-6	1050	784	11	9	0	2
T-7	571	101	17	7	0	10
T-8	798	290	21	12	0	9

Note: the name of a test comprises 2 parts: ‘T’ for test, and the following number for test ID.

lists the specific values of these physical quantities. The SICP-treated test plots have a similar change during the five months. Generally, the surface penetration resistance and thickness of biocemented layer decrease with the development of time, and the soil erosion depth has the opposite trend. In the first month after SICP treatment, the surface penetration resistance, thickness of biocemented layer and soil erosion depth change slightly. However, after five months of SICP treatment, the surface penetration resistance and the thickness of biocemented layer decreases greatly, and the soil erosion depth also has a great increase. For the SICP-treated test plots, the test plot with higher initial surface penetration resistance has better durability against wind-induced erosion. Compared with untreated test plot, it is obvious that the wind-induced erosion resistance of the desert sand land is greatly improved using the SICP treatment over five months. Environmental deterioration including rains, winds, wet-dry cycles, and ultraviolet rays, may produce disturbance to biocemented layer, leading to the decrease of wind-induced erosion resistance in the long term [12,16]. The climate change in one month and five months after SICP treatment is presented in Figure 9 and Figure 10, respectively. According to the above analysis and the climate change conditions, wind is the main reason for the attenuation of physical properties of SICP-treated soil in this study. For instance, larger biocement solution concentration and dosage and multiple treatment cycles are proposed in the regions affected by critical wind-induced erosion to improve the ductility of SICP treatment.

4. Conclusions

In this research, field tests were carried out to investigate the effect of the soybean-urease induced calcite precipitation (SICP) method on enhancing the wind-induced erosion resistance of desert sand and the durability of SICP treatment. Based on the data, the following findings are achieved:

• The SICP method could successfully improve the wind-induced erosion resistance of desert sand by forming a biocemented layer on the desert sand surface. The results showed that the surface penetration resistance of the SICP-treated test plot was much higher than that of the untreated test plot.
• The surface penetration resistance of SICP-treated desert sand was enhanced for increasing the biocement solution concentration and dosage and number of treatment cycles. The higher biocement solution concentration and dosage and the increase in the number of treatment cycles could result in a higher quantity of calcium carbonate in the biocemented layer or a thicker thickness of biocemented layer, which could more effectively strengthen the integrity of the desert sand, thereby improving the wind-induced erosion resistance.
• The wind-induced erosion resistance of SICP-treated desert sand decreased with the development of time. The wind is the main reason for the attenuation of wind-induced erosion resistance. According to the test results, the increase of biocement solution concentration and dosage and number of treatment cycles is the productive method to enhance the durability of SICP-treated desert sand.
• Overall, the data presented in this study could be used as a reference for the application of the SICP method in desert windbreak and sand fixation.