Magnesia carbonation can be adopted as a soil solidification technology for geotechnical engineering. Recent studies have shown that urea decomposition under the catalyzation of ureolytic bacteria can provide a carbon source for magnesia carbonation. Although many related studies have been reported, the mechanical behaviour of the magnesia solidified soil, especially its durability and long-term performance, still require further deep investigations. Besides, the use of plant urease instead of bacteria for magnesia carbonation is also of research interest and requires further studies. In this study, we used crude soybean urease for the catalyzation of urea decomposition in order to provide carbon source for magnesia carbonation (soybean urease intensified magnesia carbonation, SIMC). The mechanical behaviour and durability of SIMC solidified soil under drying-wetting and soaking conditions in acid rain solution were investigated. For SIMC samples, the addition of urea and urease as internal carbon sources led to a much higher strength compared with those without them. The optimum urea concentration was 2 mol/L, and higher concentrations could have negative impact on the strength. As for magnesia, the highest strengths were obtained when the addition was 8 %. During the drying-wetting cycles and soaking tests with acid rain water, there was a generally moderate decreasing trend in strength for the SIMC samples with more drying-wetting cycles or soaking durations. However, the strength reduction ratio, which was defined as the long-term strength in acid environment to that in neutral environment, was much higher compared to the PC samples, implying a much stronger resistance to acid rain water. The mineralogical analysis revealed that hydrated magnesium carbonates were the major effective cementing materials.

Tufa, a loose and porous calcium carbonate deposit, is vulnerable to weathering, which can heighten the risk of geological hazards. This study investigated the potential of microbial-induced calcite precipitation (MICP) to stabilize weathered tufa by isolating urease-producing bacteria from Jiuzhaigou, Sichuan Province. Two strains with the highest urease activity, identified as Stenotrophomonas sp. (U1) and Lysinibacillus boronitolerans (U2), were selected for mixed cultures (Mc). The physiological characteristics and calcification capacity of the strains (U1, U2, and Mc), along with the mechanical properties of treated tufa columns (SCU-1, SCU-2, and SCM), were analyzed. The findings revealed that these strains effectively induced the formation of CaCO3. Mc demonstrated strong growth dynamics (OD600 = 3.9 +/- 0.1) and urease activity (865 +/- 17 U/ml), leading to enhanced CaCO3 production. Furthermore, MICP significantly improved the compressive and shear strength of the weathered tufa, with the SCM sample showing superior results compared to SCU-1 and SCU-2. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses confirmed that Mc produced a greater quantity of CaCO3 in the crystalline form of calcite. Overall, the results indicate that MICP represents a promising environmental protection technology that can effectively enhance the engineering properties of weathered tufa.

The disposal of tailings in a safe and environmentally friendly manner has always been a challenging issue. The microbially induced carbonate precipitation (MICP) technique is used to stabilise tailings sands. MICP is an innovative soil stabilisation technology. However, its field application in tailings sands is limited due to the poor adaptability of non-native urease-producing bacteria (UPB) in different natural environments. In this study, the ultraviolet (UV) mutagenesis technology was used to improve the performance of indigenous UPB, sourced from a hot and humid area of China. Mechanical property tests and microscopic inspections were conducted to assess the feasibility and the effectiveness of the technology. The roles played by the UV-induced UPB in the processes of nucleation and crystal growth were revealed by scanning electron microscopy imaging. The impacts of elements contained in the tailings sands on the morphology of calcium carbonate crystals were studied with Raman spectroscopy and energy-dispersive X-ray spectroscopy. The precipitation pattern of calcium carbonate and the strength enhancement mechanism of bio-cemented tailings were analysed in detail. The stabilisation method of tailings sands described in this paper provides a new cost-effective approach to mitigating the environmental issues and safety risks associated with the storage of tailings.

This study investigated the effectiveness of enzyme-induced carbonate precipitation (EICP) technology in remediating Pb- and Zn-contaminated sand. The research focused on the immobilization of heavy metals and the enhancement of sand strength. Experimental results demonstrated that urease activity increased linearly with enzyme concentration, stabilizing at 100 g/L with an activity of 18 mmol/min, and reached a peak at a pH of 8. Temperature variations also positively impacted urease activity, and effective remediation levels were achieved at standard room temperature. The EICP method effectively transformed heavy metal ions from a mobile exchangeable state to a stable carbonate-bound state, and removal rates exceeded 80% for Zn2+ and 90% for Pb2+ after three treatment cycles. Furthermore, the technology significantly improved the unconfined compressive strength of contaminated sand, increasing Pb-contaminated sand strength to 0.57 MPa and Zn-contaminated sand strength to 0.439 MPa. These findings highlight the potential of EICP technology as a viable solution for the remediation of heavy metal-contaminated sand, offering both immobilization of contaminants and enhancement of sand mechanical properties.

Enzyme-induced carbonate precipitation (EICP) is an appealing bio-cementation technology for soil improvement in geotechnical engineering. This study investigated the bio-reinforcement efficacy of sword bean crude urease (SWCU)-mediated EICP and the enhancement effect of various additives on it. A set of sand column specimens with different bio-cementation levels were prepared. Magnesium chloride, sucrose, xanthan gum, sisal fiber, calcite seeds, and skim milk powder were adopted for comparison. Bio-reinforcement efficacy was evaluated by mechanical properties. SWCU possessed a similar to 127% higher specific activity than entry-level commercial urease while saving over 2000 times the enzyme cost. All specimens treated with SWCU-mediated EICP presented excellent moldability and uniformity for one-time treatment. UCS increased exponentially with bio-cementation level due to the uniformly growing CaCO3 content and crystal size. UCS of similar to 1.8 MPa was achieved in a single treatment using 60 g/L SWCU and 3.0 M urea-CaCl2. SWCU exhibited a superior bio-reinforcement efficiency over soybean crude urease, commercial urease, and bacterial urease, since higher soil strength was achieved at lower CaCO3 content. Magnesium chloride showed the most significant enhancement effect, implying an extensive application prospect of SWCU-mediated EICP in seawater environments. The absence of wet strength, markedly elevated dry strength, and notably higher stiffness and hardness at low stress (load) phase indicated that xanthan gum would be more suitable for windbreak and sand fixation in arid/semi-arid environments. Sisal fiber could also effectively improve soil mechanical properties; however, the labor and time costs caused by its premixing with soil should be considered additionally in practical applications.

This work compares the effectiveness of different biogrout agents to improve the mechanical properties (strength and stiffness) of two sandy soils. Based on the results of unconfined compressive strength tests for different curing times (3-180 days), the results obtained from microbial induced calcium carbonate precipitation using two bacteria (Sporosarcina pasteurii and Idiomarina insuliasalsae) are compared with employing urease to promote calcium carbonate precipitation and the use of the biopolymer xanthan gum (3-28 days). Additionally, the methodologies of calcite precipitation and the use of xanthan gum are also analyzed by examining scanning electron microscopy images. For a curing time of 28 days, the results show that the use of urease is the best of the three bio-based methodologies to improve sand, while the least efficient methodology is the use of the biopolymer xanthan gum. The use of both bacteria increases the unconfined compressive strength during the curing time (until 180 days); regarding the use of urease and xanthan gum, the unconfined compressive strength remains approximately constant for a curing time longer than 7-14 days. In terms of stiffness, the use of the bacteria Idiomarina insuliasalsae and urease result in a better improvement, while the use of the bacteria Sporosarcina pasteurii and xanthan gum has only a marginal effect on the stiffness.

Pisha sandstone is a kind of sandstone which is easy to collapse by water in Shanxi, Shaanxi and Inner Mongolia of China, and suffers from hydraulic erosion all the year round. In recent years, some scholars have used microbial induced calcium carbonate precipitation (MICP) technology to solidify Pisha sandstone to improve the water erosion resistance of Pisha sandstone. However, for the climate environment with low average temperature in Pisha sandstone area, the commonly used Sporosarcina pasteurii are not well adapted. The purpose of this study is to use the indigenous strainsto solidify the loose Pisha sandstone, and to compare the growth adaptability, mechanical properties and water erosion resistance of the solidified layer with Sarcina pasteurii at different temperatures, and to explore the mechanism of different temperatures and strains affecting the microbial solidification of Pisha sandstone from the micro scale. At the same time, a mixed bacterial liquid solidification test was also set up. The results showed that the solidified thickness of indigenous strains was 4.65 % higher than that of Sporosarcina pasteurii, and the thickness and strength of mixed strains were increased by 19.57 % and 36.62 %, respectively. The growth and solidification effect of indigenous strains were less affected by low temperature. Compared with Sporosarcina pasteurii, at low temperature, the bacterial concentration decrease of indigenous strains was reduced by 26.13 %, the thickness loss of solidified layer was reduced by 13.04 %, and the strength loss of solidified layer was reduced by 13.39 %. The effect of low temperature on the growth of bacteria is mainly reflected in affecting the maximum concentration of bacteria and the growth rate. The effect on MICP mainly reflected in affecting the life activities of bacteria and the crystal form and morphology of calcium carbonate. The research results provide a theoretical basis for the MICP technology application of indigenous strains and multistrains in Pisha sandstone area soil reinforcement and solidification slope.

Enzymatically induced carbonate precipitation (EICP) is an emerging and eco-friendly technology, which is considered a green alternative to traditional cement in soil stabilization. When stabilizing soil use one-phase grouting method, the activity of urease is often adjusted, leads to changes in the composition and cementitious characteristics of CaCO3. Previous studies primarily focused on the mechanical properties of solidified soil samples, while the production and cementitious characteristics of CaCO3 influenced by the control of urease activity is rarely discussed. This study investigated production and cementitious characteristics of CaCO3 under different reaction environment (pH adjustment, temperature adjustment, and addition of inhibitors), during which the urease activity tests, pH tests, CaCO3 production tests and ultrasonic oscillation tests are conducted. Meanwhile, the morphological characteristics and mineral composition of CaCO3 are revealed through Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS) tests and X-ray Diffraction (XRD) test. The results demonstrate that all three one-phase grouting methods can delay the production of CaCO3 at early-stage of EICP, while the pH should be maintained above 4 to prevent significant urease deactivation. The CaCO3 generated in EICP mainly consists of calcite and vaterite, and the size of CaCO3 increases with the urease activity increased. The cementitious characteristics of CaCO3 is mainly determined by the percentage composition of calcite and vaterite, where higher vaterite content results in weaker cementitious characteristics. This study provides insights for evaluating the cementitious characteristics of CaCO3, which is beneficial for guiding the promotion and application of one-phase grouting method.

This study aimed to enhance the efficiency of microbial-induced carbonate precipitation (MICP) for reinforcing sandy soil by inspiring natural processes involving microbial-induced carbon cycling and carbonation. The experiment focused on enhancing MICP curing of sandy soil using carbonic anhydrase (CA), which significantly increases the reaction rate of CO2 hydration (10(8) times faster) and facilitates the rapid hydration of CO2 (produced by urease (UA) decomposition of urea) to form a substantial amount of carbonate. The effect of carbonic anhydrase on MICP-reinforced sandy soil and its underlying mechanism were systematically examined through a combination of macroscopic physical and mechanical tests and microfabrication tests. The results showed that: (1) CA significantly increases the production of cement during the microbial consolidation of sandy soils, and the optimum dose of carbonic anhydrase producing bacteria is reached at about 4%, which increases the production of cement by 105.3%, compared with conventional MICP. (2) The incorporation of CA improves the compressive strength and resistance of the cured body. In the range 0.25-4.00%, the unconfined compressive strength of the solidified soil sample increases with the increase of the CA bacteria content. The strength of the cured soil sample reaches 1.915 MPa when the content is 4%, which is 8.54 times the strength of the conventional MICP cured sample. (3) CA does not change the product of the MICP process, it is still calcite, but after adding CA, the grain size of the calcite is larger, the shape of the hexahedron is more standardised, and the mechanical properties are improved. (4) In the process of MICP, urease and CA co-precipitate calcium carbonate-cured sandy soil. CA can significantly accelerate the rate of urea-generated CO2 hydrate and form HCO3- and CO32-, providing more favourable conditions for mineralisation.

Enzyme-induced carbonate precipitation (EICP) is an attractive bio-geotechnical technique for soil improvement. As promising alternatives to commercial ureases, legume ureases crudely extracted from primary agricultural products can provide remarkable cost savings. This study investigated the bio-cementation effect of legume ureases with different protein contents on pore-scale, mechanical, and hydraulic properties of EICP-treated sand and revealed the causes, mechanisms, and effects of the bio-clogging induced by high protein level-legume urease. Urease centrifugal liquids of sword bean (JU), pigeon pea (PU), and soybean (SU) were prepared at equal activity of 10 mM/min for sand bio-cementation. Mechanical properties were analyzed based on CaCO3 content and soil strength. Pore-features were revealed by mercury intrusion porosimetry and scanning electron microscopy, and permeability was measured to evaluate the hydraulic properties. Results showed that JU and PU with lower protein content were more effective in multi-cycle EICP-treatments, since denser bio-cemented sands with higher strengths were obtained while being vertically uniform in CaCO3 distribution and pore structure. Conversely, the high protein level of SU induced uneven bio-cementation and surface bio-clogging, resulting in bad mechanical properties, such as low strength and a destruction pattern of bottom collapse. Bio-clogging virtually eliminated the effectiveness of subsequent EICP-treatments. SU exhibited an advantage over JU and PU in reducing soil permeability, as a dramatically lower permeability was achieved at a lower treatment cycle. Comprehensive analysis concluded that the high protein level, salting-out, different precipitation rate between protein and CaCO3, and limited soil filtration capacity were the key reasons for bio-clogging induced by SU.