Enzyme-catalysed mineralisation experiment study to solidify desert sands

Sandstorms are meteorological phenomena common in arid and semi-arid regions and have been recognized severe natural disasters worldwide. The key problem is how to control and mitigate sandstorm natural disasters. This research aims to mitigate their development by improving surface stability and soil water retention properties through soil mineralization. The enzymatic induced carbonate precipitation (EICP) is proposed to solidify desert sands and form a hard crust layer on the surface of desert sands. In contrast to micro-induced carbonate precipitation commonly used at room temperatures, EICP had high production efficiency and productivity at a broader temperature range (10–70 °C ±) and significantly improves material water retention properties, which was more suitable to desert environment. Results demonstrate that the enzyme-catalysed mineralisation method can be better resistance to high winds as the number of spraying times increased.

www.nature.com/scientificreports/ To improve the options for mitigating sandstorm disasters, this paper explored EICP for solidifying desert sands. Experiments to solidify desert sand were conducted in the laboratory with an enzyme solution followed by a urea-calcium acetate solution for different solidification models. The objective of EICP desert sand solidification is to form a hard crust layer on the sand surface to combat wind erosion. Curing effects on solidified desert sand by EICP were evaluated by wind tunnel testing, drying-wetting cycle testing, and water retention feature testing. The effectiveness of EICP, in particular on sand water retention, wind resistance, and environment stability were closely examined. Simultaneously, EICP desert sand solidification is an environmental friend method.

Results
experimental materials. Tests reported in this paper were performed on desert sand collected from the Tengger Desert in China's Ningxia Hui Autonomous Region, which is the fourth largest desert in China. Desert sand is fine sand whose particle size distribution is shown in Fig. 1. The physical and chemical properties of Tengger Desert desert sand are listed in the Table 1. Table 2 refers to its mineral components.
Soil mineralization of eicp. X-ray diffraction (XRD) pattern and scanning electron microscope (SEM) analysis confirmed that the calcite precipitation was deposited in the desert sand samples by EICP solidification (Fig. 2). SEM showed that the calcite was deposited between desert sand particles and increased their bonding forces. Calcite was sprayed in 6 g, 14 g, and 25 g increments two, four, and six times for the EICP mixture solution, respectively, and the volume increased as spraying times increased.
Water retention capacity of EICP solidified desert sands. In deserts, the water retention capacity of natural desert sand is very low and it is easily removed by the wind. The EICP mineralisation method was able    www.nature.com/scientificreports/ to solve the blown sand problem. Figure 3 shows the soil-water retention curve (SWRC) for EICP solidified desert sand. The residual volumetric water content was 6.1% for solidified desert sand and only 2.0% for natural desert sand. The improved water retention capacity of solidified desert sand was confirmed by SEM in Fig. 2. The calcite crystals induced by EICP filled gaps between sand particles and altered the pore structure of natural desert sand. Pores may have been connected, causing water and pore-water to easily evaporate and lose most moisture at the site. The solidified desert sand formed an aggregate of desert sand bonded by calcite crystals, however, and pore structure was different from the natural desert sand. The partial pores solidified desert sand may have been closed or semi-closed. Therefore, the solidified desert sand had improved water retention capacity. Improvements using EICP could facilitate the planting of vegetation to improve the environment in arid and semi-arid areas. www.nature.com/scientificreports/ Effects of EICP solidified desert sands. This study tested solidified desert sand using EICP mineralization technology. The objective was to identify an effective and environmentally responsible technology to control sandstorm disasters. Results demonstrated that the enzyme-catalysed mineralisation method could be used for sandstorm impact reduction and improved water retention capacity in arid and semi-arid areas. (1) EICP soil mineralization can improve the surface stability of desert sand. The calcite added by EICP deposits between particles of desert sand formed an aggregate that alters the sand structure. (2) The soil-water retention curves for solidified desert sand showed that the EICP method enhanced water retention capacity. (3) EICP soil mineralization can be used to consolidate desert sand in desert areas, forming a hard crust layer on the surface that resists sand suspension and fights sandstorm disasters. (4) The calcite precipitation of EICP is stable and long lasting. Wind tunnel and wetting-drying cycle testing of solidified desert sand showed that it could endure strong winds at maximum wind speeds of 29.1 m/s. Mass loss rates for wetting-drying cycle samples of solidified desert sand at 29.1 m/s were 2.31%, 1.87%, and 0.18% after two, four, and six spraying times, respectively. These tests showed that EICP biomineralization technology clearly diminished the harmful impacts of sandstorms. (5) All tests were completed at environment temperatures of 10 °C ±, showing that EICP biomineraliztion technology has a clear obvious advantage over MICP for solidifying desert sand. EICP is environmentally-friendly and can be applied to reduce sandstorm disaster impacts in arid and semi-arid areas. Table 3 lists the mass variability for EICP solidified desert sand samples. Increases in mass were due to calcite deposited by EICP consolidation. Calcite deposition solidified the desert sand, forming a hard crust layer on the surface that resisted suspension. Figure 4 contains photos of solidified desert sand samples. It can be seen that the surface color of solidified samples became whiter as spraying time increased. Productive rates of calcite during EICP solidification were measured and calculated in Fig. 5. They were in agreement with the calculated values for calcite during EICP solidification, which decreased as spraying times increased. This was related to the balance between CO 3 2− and Ca 2+ concentrations and seepage effects of the EICP solution. For these reasons, the surface color of solidified samples appeared more white as spraying time increased in Fig. 4, which indicated that some calcium acetate remained on the surface because the productive rates of calcite did not reach 100%   www.nature.com/scientificreports/ www.nature.com/scientificreports/ during EICP solidification. This study attempted to increase the calcite efficiency in the next step to improve the EICP consolidation effects for desert sand. Table 4 shows the mass variability of natural /solidified desert sand samples after wind tunnel testing. The loss of mass by natural desert sand was large and it was clearly blown upward during wind tunnel tests. The saltation distance and suspension height for natural desert sand were measured when the wind speed reached 7.0 m/s: saltation distance was about 50-60 cm and the suspension height was about 40-45 cm. According to wind tunnel test data in Table 4, the blown mass loss rates for natural desert sand were 36.4%, 62.7%, and 80.0% at wind speeds of 14.0 m/s, 22.7 m/s, and 29.1 m/s, respectively. These tests aided the interpretation of sandstorm disasters because high amounts of desert sand were removed by strong winds in desert areas (Fig. 6).

Discussions
However, Table 4 shows that the solidified desert sand resisted the strong winds and limited sandstorm impacts. During wind tunnel testing, the solidified desert sand samples did not exhibit the suspension and saltation phenomena of natural desert sand, even if winds were low. Higher EICP spraying times also resulted in better solidification effects of desert sand. According to wind tunnel test results in Table 4, the blown mass loss rates of solidified desert sand were less than 3.0% after two spraying times at wind speeds of 14.0 m/s, 22.7 m/s, and 29.1 m/s. Mass loss rates at a wind speed of 29.1 m/s were 2.23%, 1.61%, and 0.11% after two, four, and six spraying times, respectively. Figures 7, 8, and 9 show photos of solidified desert sand under strong Table 4. The mass variations of solidified desert sand samples of EICP after blowing at different wind speeds during the wind tunnel tests. www.nature.com/scientificreports/ winds at different intensities. They demonstrate that EICP was a good method for solidifying desert sand and can be used to limit sandstorm disaster impacts in desert areas. In addition, EICP tests were completed at low environmental temperatures (10 °C ±). This means that EICP can acclimate to broader field conditions and may be more widely applied. Table 5 lists the mass variability of solidified desert sand after wetting-drying cycles at different wind speeds. The lack of mass variability in the samples illustrates that the calcite deposition of EICP was not hydrolyzed and was stable over long time periods. The stability and duration of EICP solidification were also demonstrated by wind tunnel tests on wetting-drying cycle samples. Figure 10 shows that wetting-drying cycle samples resisted the impacts of strong winds and did not experience the same saltation and suspension phenomena at a wind speed of 29.1 m/s. During wind tunnel testing, mass losses were less than 3.0% for wetting-drying cycle samples of solidified sand, and the lost components were partial calcium acetate crystals that remained on the surface. The mass loss rates of wetting-drying cycle samples of solidified desert sand at a wind speed of 29.1 m/s were 2.31%, 1.87%, and 0.18% after two, four, and six spraying times, respectively. Therefore, EICP had long-term application potential for consolidating desert sand and limit sandstorm disaster and combat desertification.

Mass changes (g)
In addition, the cost of consolidating desert sand of EICP is about the same as the grass square method and the cost estimate is based on EICP test effects in the laboratory and field tests 30 . (1) temperatures and pH values. The temperature was 10 °C ± during the EICP reaction and the pH of the solution was 7. Based on the desired engineering application of the enzyme-catalysed mineralization method, the low temperature was controlled during solidification desert sand tests. Wind tunnel testing for solidified desert sand. To evaluate the solidification effects, wind tunnel and drying-wetting cycle testing was conducted on solidified desert sand. The following procedure for wind testing was followed:  Wetting-drying cycle testing for solidified desert sand. The following procedure was used for drying-wetting testing:

Methods
1. Wetting phase: The solidified samples of desert sand on plates B, C, and D were weighed and then sprayed with 200 mL of tap water, then retained for 24 h in the laboratory. 2. Drying phase: The wet solidified samples of desert sand on plates B, C, and D were kiln-dried in an oven for 8 h at 60 °C in the laboratory, and then weighed. The dried samples of desert sand on plates B, C, and D were stored in the laboratory for a second day.  www.nature.com/scientificreports/ 3. The above procedure was repeated for three wetting-drying cycles.