Here, microbially induced calcite precipitation (MICP) technology is presented to improve soil properties by immersion.
The goal of this article is to develop an immersion method to improve the microbially induced calcite precipitation (MICP) treated samples. A batch reactor was assembled to immerse soil samples into cementation media. The cementation media can freely diffuse into the soil samples in the batch reactor instead of cementation media being injected. A full contact flexible mold, a rigid full contact mold, and a cored brick mold were used to prepare different soil sample holders. Synthetic fibers and natural fibers were selected to reinforce the MICP-treated soil samples. The precipitated CaCO3 in different areas of the MICP-treated samples was measured. The CaCO3 distribution results demonstrated that the precipitated CaCO3 was distributed uniformly in the soil sample by the immersion method.
As a biological ground improvement technology, microbially induced calcite precipitation (MICP) is capable of improving engineering properties of soil. It has been used to enhance the strength, stiffness, and permeability of soil. The MICP technique has gained much attention for soil improvement worldwide1,2,3,4. Carbonate precipitation naturally happens and can be induced by nonpathogenic organisms that are native to the soil environment5. The MICP biogeochemical reaction is driven by the existence of ureolytic bacteria, urea and a calcium-rich solution5,6. Sporosarcina pasteurii is a highly active urease enzyme that catalyzes the reaction network towards precipitation of calcite7,8. The urea hydrolysis process produces dissolved ammonium (NH4+) and inorganic carbonate (CO32-). The carbonate ions react with calcium ions to precipitate as calcium carbonate crystals. The urea hydrolysis reactions are shown here:
The precipitated CaCO3 can bond the sand particles together to improve the engineering properties of MICP-treated soil. The MICP technique has been applied in various applications, such as improvement of strength and stiffness of soil, repair of concrete, and environmental remediation9,10,11,12,13,14,15.
Zhao et al.16 developed an immersion method to prepare MICP-treated samples. A full contact flexible mold made of geotextile was used in this method. The precipitated CaCO3 distributed uniformly throughout their MICP-treated samples. Bu et al.17 developed a rigid full contact mold to prepare MICP-treated beam samples by an immersion method. The MICP-treated sample prepared by this method using a rigid full contact mold can form the suitable beam shape. The MICP-treated sample was divided into four and the CaCO3 contents were measured. The CaCO3 content ranged from 8.4 ± 1.5% to 9.4 ± 1.2% by weight, which indicated that the CaCO3 distributed uniformly in the MICP-treated samples by the immersion method. These MICP-treated samples also achieved better mechanical properties. These MICP-treated bio-specimens reached a 950 kPa flexure strength, which was similar to that of 20- 25% cement-treated samples (600- 1300 kPa). Li et al.10 added randomly distributed discrete fiber into the sandy soil and treated the soil by the MICP immersion method. They found that the shear strength, ductility, and failure strain of MICP-treated soil were enhanced obviously by adding appropriate fiber.
The immersion method for MICP has been continually improved10,16,17. This method can be used to prepare MICP-treated soil samples and MICP-treated prefabricated building materials, such as bricks and beams. Different geometry dimensions of sample preparation mold were developed. Fibers were added in the MICP-treated samples to enhance their properties. This detailed protocol was intended to document the immersion methods for MICP treatment.
NOTE: All relevant material used in the following procedures are non-hazardous. Personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes) are still needed.
1. Preparation of bacteria solution
2. Preparation of cementation media
NOTE: Cementation media is used to provide chemicals to induce the calcite precipitation during the MICP treatment. The urea-Ca2+ molar ratio is 1:1. The chemical components of cementation media is shown in Table 1. The following procedure is for 20 L of cementation media with 0.5 M Ca.
3. Preparation of molds
4. Preparation of the batch reactor
NOTE: The reactor shown in Figure 4 consists of a plastic box, cementation media, a sample supported shelf, and air pumps. The soil samples can fully immerse into the cementation media while the cementation media can freely diffuse into the soil samples by this method. The air pump in the reactor provides oxygen for bacteria. To determine the effects of different oxygen supply on MICP treatment catalyzed by Sporosarcina pasteurii, Li et al. 201718 conducted contrast tests under three different conditions: an aerated condition, an air restricted condition, and an open-air condition. They found that a well-oxygenated condition is essential to improve MICP processes catalyzed by aerobic bacteria.
5. Preparation of soil samples
Figure 7 shows the distribution of precipitated CaCO3 throughout the MICP-treated sample. The MICP-treated sample was divided into three different areas. The CaCO3 content in each area was tested by the acid washing method. To dissolve precipitated carbonates, the dry MICP-treated samples were washed in a HCl solution (0.1 M), then rinsed, drained, and oven-dried for 48 hours. The difference value between the masses of samples before and after acid washing was considered to be the mass of the carbonates precipitated in the MICP-treated samples. The CaCO3 content is indicated as percentage of sample weight. The CaCO3 content of the MICP-treated sample by the immersion method ranged from 9.0% to 9.5%. The results indicated that the precipitated CaCO3 was distributed uniformly throughout the soil sample. While Martinez et al. 201319 conducted experiments on 50 cm long sand columns by an injection method in the laboratory, they found that the calcite distributed nonuniformly along the MICP-treated sand column. Most of the calcite precipitated near the influent column and hindered the cementation reaction in the deeper section of the column.
The stress-strain curves of bio-brick reinforced with three layers of palm fiber and unreinforced bio-brick obtained using a four-point test is shown in Figure 8. The flexure strength of unreinforced bio-brick was 1,150 kPa, while that of reinforced bio-brick was 980 kPa. Their flexure strengths were similar, but the flexure strain was improved significantly by addition of the palm fiber. These results indicate that palm fiber can contribute to the improvement of ductility.
Figure 1: Full contact flexible mold for direct shear tests.
The full contact flexible molds were made of geotextile. The geotextile was a polypropylene, staple fiber and needle punched nonwoven material. The cylinder-shaped mold had a diameter of 62 mm and a height of 26 mm. Please click here to view a larger version of this figure.
Figure 2: Sample preparation of bio-bricks.
Assembled mold for brick;
Sand added into the mold;
Flexible cover added on the top of sand sample. The rigid full contact mold consists of a flexible layer and a rigid holder. The flexible layer was made of geotextile, and the rigid holder was made of a polypropylene perforated sheet. The mold consisted of three chambers and the size of each chamber was 177.8 mm in length, 76.2 mm in width and 38.1 mm in height. Please click here to view a larger version of this figure.
Figure 3: Cored brick molds.
(a) Holes distribution on one chamber of mold; (b) Sample preparation of bio-cored bricks Assembled mold for cored brick;
Sand added into the mold;
Flexible cover added on the top of sand sample. The cored brick mold included a rigid holder, a flexible layer, and cardboard tubes. The size of cardboard tube was 60 mm x 140 mm x 60 mm. Three chambers were included in one mold and the size of each chamber of mold was 177.8 mm in length, 76.2 mm in width and 38.1 mm in height. Please click here to view a larger version of this figure.
Figure 4: Sketch of batch reactor for MICP.
All samples were prepared in a completely stirred tank reactor. The batch reactor included a plastic box to contain soil samples and cementation media, a magnetic mixer to keep the solution uniform, and an air pump to provide oxygen for bacteria. A major feature of this method is to allow soil samples fully immerse into the cementation media and to allow the cementation media to freely penetrate into the soil samples. Please click here to view a larger version of this figure.
Figure 5: Soil samples placed on the supported shelf.
(a) before the MICP reactions; (b) after the MICP reactions. The bio-brick samples were prepared with the full contact mold. A geotextile cover was applied on the top of the mold. Each bio-brick had a size of 177.8 mm in length, 76.2 mm in width and 38.1 mm in height. Please click here to view a larger version of this figure.
Figure 6: (a) Synthetic fiber; (b) natural palm fiber.
The synthetic fiber was a homopolymer polypropylene multifilament fiber with a specific gravity of 0.91. It is chemically inert with high acid salt resistance. The length and thickness of the fibers used in this study were 12 and 0.1 mm, respectively, with an aspect ratio of 120 between the length and thickness of the fiber. Please click here to view a larger version of this figure.
Figure 7: CaCO3 distribution in three areas of MICP-treated sample.
Three zones were divided in the sample. In each zone, the amount of precipitated CaCO3 was measured and calculated as a percentage by weight. Please click here to view a larger version of this figure.
Figure 8: Flexure stress as a function of flexure strain for unreinforced bio-brick and palm fiber reinforced bio-brick with MICP treatment.
The flexure strength of unreinforced bio-brick was 1,150 kPa, while that of reinforced bio-brick was 980 kPa. The flexure strain was improved significantly by addition of palm fiber. These results indicate that palm fiber can contribute to the improvement of ductility. Please click here to view a larger version of this figure.
Chemical | Concentration of cementation media (g/L) | |||
0.25 M Ca | 0.5 M Ca | 1 M Ca | 1.5 M Ca | |
NH4Cl | 10 | 10 | 10 | 10 |
Nutrient broth | 3 | 3 | 3 | 3 |
NaHCO3 | 2.12 | 2.12 | 2.12 | 2.12 |
Urea | 15 | 30 | 60 | 90 |
CaCl2∙2H2O | 36.8 | 73.5 | 147 | 220.5 |
Table 1: Chemical components of cementation media. The chemicals were used to prepare four concentrations of cementation media in 0.25 M Ca, 0.5 M Ca, 1 M Ca, and 1.5 M Ca. The urea-Ca2+ molar ratio was fixed as 1:1.
The MICP technique by immersion was presented in this paper. Soil samples were immersed into the batch reactor to get fully penetrated by cementation media in the MICP process. In this method, a full contact flexible mold, a rigid full contact mold, and a cored brick mold were applied to prepare MICP-treated samples.
Different molds can be designed for different geometry requirements. The fibrous structure of geotextile increased the contact area between sand and cementation media, which effectively increased the penetration of cementation media into soil samples. The large amounts of pores of geotextile also allowed more precipitation occurring inside the mold to improve the strength of MICP-treated samples. The soil properties of MICP-treated samples, such as strength and calcite content, were greatly improved by using these molds in the immersion method. The immersion method showed an advantage in preparing prefabricated building materials, such as bio-bricks and bio hollow bricks. Synthetic fiber and natural fiber can be added in the soil to enhance the MICP-treated samples. Fiber addition is an appropriate way to improve prefabricated MICP-treated materials. MICP technique with immersion method can be applied to perform surface treatment for cement-treated bricks to improve their properties, such as enhancing the durability of cement-treated materials by reducing their permeability. However, this immersion method is difficult to implement in the field due to the limitation of its operation, future research on how to use this method on site is needed to apply this method in the field.
The authors have nothing to disclose.
This work was supported by the National Science Foundation Grant No. 1531382 and MarTREC.
Ammonium Chloride, >99% | Bio-world | 40100196-3 (705033) | |
Ammonium Sulfate | Bio-world | 30635330-3 | |
Calcium Chloride Dihydrate, >99% | Bio-world | 40300016-3 (705111) | |
Nutrient Broth | Bio-world | 30620056-3 | |
Sodium Bicarbonate, >99% | Bio-world | 41900068-3 (705727) | |
Sporosarcina pasteurii | American Type Culture Collection | ATCC 11859 | |
Synthetic fiber | FIBERMESH | Fibermesh 150e3 | |
Tris-Base, Biotechnology Grade, >99.7% | Bio-world | 42020309-2 (730205) | |
Urea, USP Grade, >99% | Bio-world | 42100008-2 (705986) | |
Yeast Extract | Bio-world | 30620096-3 (760095) |