Effect of drying-wetting cycles on pore characteristics and mechanical properties of enzyme-induced carbonate precipitation-reinforced sea sand

Enzyme-induced carbonate precipitation(EICP)is an emanating,eco-friendly and potentially sound technique that has presented promise in various geotechnical applications.However,the durability and microscopic characteristics of EICP-treated specimens against the impact of drying-wetting(D-W)cycles is under-explored yet.This study investigates the evolution of mechanical behavior and pore charac-teristics of EICP-treated sea sand subjected to D-W cycles.The uniaxial compressive strength(UCS)tests,synchrotron radiation micro-computed tomography(micro-CT),and three-dimensional(3D)recon-struction of CT images were performed to study the multiscale evolution characteristics of EICP-reinforced sea sand under the effect of D-W cycles.The potential correlations between microstructure characteristics and macro-mechanical property deterioration were investigated using gray relational analysis(GRA).Results showed that the UCS of EICP-treated specimens decreases by 63.7% after 15 D-W cycles.The proportion of mesopores gradually decreases whereas the proportion of macropores in-creases due to the exfoliated calcium carbonate with increasing number of D-W cycles.The micro-structure in EICP-reinforced sea sand was gradually disintegrated,resulting in increasing pore size and development of pore shape from ellipsoidal to columnar and branched.The gray relational degree suggested that the weight loss rate and UCS deterioration were attributed to the development of branched pores with a size of 100-1000 m m under the action of D-W cycles.Overall,the results in this study provide a useful guidancee for the long-term stability and evolution characteristics of EICP-reinforced sea sand under D-W weathering conditions.

Biomineralization of soil with crude soybean urease using different calcium salts

Calcium salt is an important contributing factor for calcium-based biomineralization.To study the effect of calcium salt on soil biomineralization using crude soybean urease,the calcium salts,including the calcium chloride (CaCl_(2)),calcium acetate ((CH_(3)COO)_(2)Ca) and calcium nitrate (Ca(NO_(3))_(2)),were used to prepare the biotreatment solution to carry out the biomineralization tests in this paper.Two series of biomineralization tests in solution and sand column,respectively,were conducted.Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were performed to determine the microscopic characteristics of the precipitated calcium carbonate (CaCO_(3)) crystals.The experimental results indicate that the biomineralization effect is the best for the CaCl2 case,followed by (CH_(3)COO)_(2)Ca,and worst for Ca(NO_(3))_(2) under the test conditions of this study (i.e.1 mol/L of calcium salt-urea).The mechanism for the effect of the calcium salt on the biomineralization of crude soybean urease mainly involves: (1) inhibition of urease activity,and (2) influence on the crystal size and morphology of CaCO_(3).Besides Ca^(2+) ,the anions in solution can inhibit the activity of crude soybean urease,and NO_(3)− has a stronger inhibitory effect on the urease activity compared with both CH_(3)COO^(−) and Cl^(−) .The co-inhibition of Ca^(2+) and NO_(3)− on the activity of urease is the key reason for the worst biomineralization of the Ca(NO_(3))_(2) case in this study.The difference in biomineralization between the CaCl_(2) and (CH_(3)COO)_(2) Ca cases is strongly correlated with the crystal morphology of the precipitated CaCO_(3).

Factors affecting the effectiveness of biocementation of soil

Microbially or enzyme induced carbonate precipitation has emerged to be a new type of soil improvement method.However,it appears that the biocementation process is affected by many factors and a common understanding on the control factors on the biocement effect has not been reached.This paper attempts to identify the main factors that controlling the MICP or EICP effect through an in-depth discussion on the fundamentals of biocementation process.Similar to other cemented granular materials,biocemented soil is a structural soil composite consisting of soil skeleton and biocement force chain or biocement network.The strength and stiffness of the biocemented soil is controlled by the reinforcement effect of the biocement network on the soil skeleton or the interplay of the soil skeleton and precipitates.The contribution of the strength by soil skeleton is affected by the soil types and soil properties,while the contribution of the precipitates is through the distribution of the biocement network and the properties of the precipitates.

Field implementation of enzyme-induced carbonate precipitation technology for reinforcing a bedding layer beneath an underground cable duct

A suitable bearing capacity of foundation is critical for the safety of civil structures.Sometimes foundation reinforcement is necessary and an effective and environmentally friendly method would be the preferred choice.In this study,the potential application of enzyme-induced carbonate precipitation(EICP)was investigated for reinforcing a 0.6 m bedding layer on top of clay to improve the bearing capacity of the foundation underneath an underground cable duct.Laboratory experiments were conducted to determine the optimal operational parameters for the extraction of crude urease liquid and optimal grain size range of sea sands to be used to construct the bedding layer.Field tests were planned based on orthogonal experimental design to study the factors that would significantly affect the biocementation effect on site.The dynamic deformation modulus,calcium carbonate content and longterm ground stress variations were used to evaluate the bio-cementation effect and the long-term performance of the EICP-treated bedding layer.The laboratory test results showed that the optimal duration for the extraction of crude urease liquid is 1 h and the optimal usage of soybean husk powder in urease extraction solution is 100 g/L.The calcium carbonate production rate decreases significantly when the concentration of cementation solution exceeds 0.5 mol/L.The results of site trial showed that the number of EICP treatments has the most significant impact on the effectiveness of EICP treatment and the highest dynamic deformation modulus(Evd)of EICP-treated bedding layer reached 50.55 MPa.The area with better bio-cementation effect was found to take higher ground stress which validates that the EICP treatment could improve the bearing capacity of foundation by reinforcing the bedding layer.The field trial described and the analysis introduced in this paper can provide a practical basis for applying EICP technology to the reinforcement of bedding layer in poor ground conditions.

Mechanical properties and disintegration behavior of EICP-reinforced sea sand subjected to drying-wetting cycles

Enzyme-induced carbonate precipitation(EICP)has emerged promising in various geotechnical applications,and has been presented as an alternative to the traditional cementitious materials-based ground improvement method.However,the study on mechanical properties and disintegration behavior of EICP-reinforced sea sand subjected to drying-wetting cycles are limited.This study investigated the mechanical properties and disintegration behavior of EICP-reinforced sea sand against the impact of drying-wetting(D-W)cycles.The uniaxial compressive strength(UCS)tests were performed to discuss the effect of drying-wetting cycles on the mechanical behavior of EICP-treated sea sand.The disintegration tests were conducted on EICP-treated sea sand to investigate the disintegration resistance of bio-cemented samples with various cementation levels.The microstructures of samples before and after disintegration were examined to disclose the disintegration mechanisms of EICP-reinforced sea sand.D-W cycles significantly affect the mechanical properties of EICP-reinforced sea sand,with UCS decreasing by 63.7%after undergoing 15 D-W cycles.The disintegration resistance index of specimens with a lower cementation level decreases significantly under the effect of D-W treatment.The higher disintegration resistance of specimens with higher cementation can be attributed to more crystals with better crystallinity formed in the contact point between sand particles within specimen.The crystals formed by soybean husk urease are mainly calcite and the crystallinity of spherical calcites would gradually change into larger rhombic calcite with further bio-grouting.The crystal with poor crystallinity is susceptible to the effect of D-W treatment,resulting in the obvious disintegration of EICP-reinforced sea sand.Overall,this study is expected to provide useful guidance on the long-term stability and drying-wetting disintegration mechanisms of EICP-reinforced sea sand.

Thermal and mechanical properties of bio-cemented quartz sand mixed with steel slag

The aim of this study is to disclose the feasibility of improving the thermal conductivity and mechanical strength of quartz sand steel slag mixtures treated by enzyme-induced carbonate precipitation(EICP).In this work,the effects of steel slag content(SSC)and number of treatment cycle(N)on the thermal conductivity and mechanical strength of EICP-treated specimens were investigated.The immersion method was adopted for specimen preparation.The thermal conductivity was measured by transient plane source method(TPS)and the unconfined compressive strength(UCS)was obtained through a uniaxial compression test.Moreover,the SEM test was conducted to obtain the morphology and deposition characteristics of calcium carbonate crystals.The result shows that the thermal conductivity and UCS of EICP-treated sands increase before decreasing as the SSC increases.Consequently,the maximum values of thermal conductivity and UCS are 1.28 W/(m⊡K)and 6.31 MPa,respectively,corresponding to the optimal parameter of 20%SSC at 12 N.The optimal thermal conductivity and UCS increase by 367%and 137%,respectively,compared to that of EICP-treated sand with no addition of steel slag.The SEM analysis indicates that the spherical calcium carbonate exists in the range of 0-20%SSC,whereas there is mainly amorphous calcium carbonate when the SSC varies from 40%to 80%.It also demonstrates that the UCS is more sensitive to the variation of calcium carbonate content than that of thermal conductivity.

Bio-grouting technologies for enhancing uniformity of biocementation:A review

Biocementation-based soil improvement is an emerging ground treatment method in geotechnical engineering that has garnered extensive attention over the past two decades.One of the challenges associated with this method revolves around the uniformity of biocementation,a crucial factor closely tied to bio-grouting technology.The traditional biotreatment methods,the two-phase method and the one-phase method,suffer from the issue of non-uniform biocementation.Consequently,in recent years,various improved grouting technologies have been proposed to address this concern by aiding bacterial adsorption and controlling carbonate precipitation.This paper reviews the mechanisms and grouting processes employed in these enhanced bio-grouting technologies.Additionally,the challenges of implementing these grouting technologies in real-world applications are also thoroughly discussed.

Mechanisms and influencing factors of biomineralization based heavy metal remediation:A review

Heavy metal contamination of soil and water is one of the most prominent environmental issues worldwide.Through bioaccumulation and biomagnification of the food chain,heavy metals can be enriched hundreds of times and eventually enter the human body,posing a major threat to human health.Biomineralization has the greatest potential to become an efficient and environmentally friendly heavy metal remediation technology and has received much attention in recent decades.This review summarizes the latest progress of biomineralization technology on carbonate precipitation and phosphate precipitation in heavy metal remediation.Both microorganisms(including bacteria and fungi)and enzymes can induce carbonate and phosphate precipitation,converting the free heavy metal ions into insoluble salts.However,the mechanisms of the heavy metal remediation are significantly different.For example,urea hydrolysis,which occurs intracellularly when urease-producing bacteria(UPB)are used,is the most commonly used mechanism for carbonate precipitation based bioremediation.In contrast,phosphate solubilization by either enzymes or organic acids secreted by phosphate solubilizing bacteria(PSB)is extracellular,and both soluble and insoluble phosphorus can be decomposed by PSB.Moreover,some influencing factors such as the different species of microorganism,heavy metals and some environmental conditions that may affect the bioremediation of heavy metals were also summarized in this paper.The challenges of biomineralization based heavy metal remediation are also discussed.Based on the reviews of previous studies,a comprehensive understanding of heavy metal removal through microorganism can be increased,and thus promotes the applications of biomineralization technology in the treatment of large-scale heavy metal contaminated sites.

Effect of acid type on biomineralization of soil using crude soybean urease solution

The one-phase-low-pH method is a simple,efficient,and user-friendly biogrouting technique that can effectively improve the biomineralization of enzyme-induced carbonate precipitation(EICP)using free urease enzyme.One of the most significant advantages of this method is its capacity to effectively delay calcium carbonate(CaCO3)precipitation by reducing the pH of the solution through the addition of acid.This prevents bioclogging during the biogrouting process and improves the biomineralization effect.However,the biomineralization of the one-phase-low-pH based EICP method may be influenced by the specific acid used.To investigate the impact of acid type on the one-phase-low-pH EICP method using crude soybean urease solution(CSUS),four types of acids,including hydrochloric acid(HCl),nitric acid(HNO_(3)),acetic acid(CH_(3)COOH),and lactic acid(C_(3)H_(6)O_(3)),were used to adjust the pH of CSUS.A series of macroscopic and microscopic experiments were conducted to evaluate the effect of acid type on the onephase-low-pH EICP method.The results indicate that the acid has an inhibition on the urease activity(UA)of CSUS.Among the acids tested,HNO_(3)exhibits the most pronounced inhibitory effect on the UA of CSUS,followed by HCl,and the least pronounced inhibitory effect for CH_(3)COOH and C3H6O_(3)under the same pH conditions.Meanwhile,CH_(3)COOH and C_(3)H_(6)O_(3)could provide a longer delay duration of CaCO_(3)precipitation than HNO_(3)and HCl.Therefore,the one-phase-low-pH EICP method based on CH_(3)COOH and C_(3)H_(6)O_(3)can significantly improve the effective biocementation depth compared to that based on HNO_(3)and HCl.Nevertheless,the different types of acids appear to have no obvious effect on the polymorph and crystalline of the precipitated CaCO_(3)crystals.

Effect of crystal morphology on cementability and micromechanical properties of calcium carbonate precipitate induced by crude soybean enzyme

Enzyme-induced carbonate precipitation(EICP)has emerged as an innovative soil stabilization technology to precipitate CaCO_(3)by catalyzing urea decomposition.Although extensive efforts have been made to increase the calcium carbonate content(CCC)formed in the EICP process for the better biocementation effect,the cementability and micromechanical properties of CaCO_(3)are rarely known.A study of the cementitious characteristics and micromechanical properties of CaCO_(3)precipitates with different mixing percentages of crystal morphology is essential for soil improvement.In the present study,ultrasonic oscillation tests and nanoindentation tests were performed to investigate the cementability and micromechanical properties of CaCO_(3)precipitate.The results show that the cementability and micromechanical properties of CaCO_(3)precipitate are related to the composition of the crystal morphology.A high content of calcite is beneficial to improve the adhesion of calcium carbonate precipitate.Calcite has better mechanical properties(elastic modulus,hardness and ductility)than vaterite,and the presence of vaterite can significantly affect the measured value of mechanical properties in nanoindentation tests.The ductility of CaCO_(3)precipitate induced by crude soybean urease(CSU)is higher than that of CaCO_(3)precipitate induced by commercially available pure enzyme,suggesting that commercially available pure enzyme can be replaced by CSU for cost-effective field-scale engineering applications.This work can provide insight into optimizing the properties of CaCO_(3)precipitate from the micro-scale.

A new bacterial concentration method for large-scale applications of biomineralization

Bacterial suspension is an essential component of microbially induced carbonate precipitation(MICP)-based biocement and a large-scale production is required for field applications.In this study,a new bacterial concentration method is proposed to enable high concentration bacterial suspension to be produced to facilitate field work.By adding low concentration calcium to bacterial suspension,flocs are formed and bacterial cells are adsorbed on the flocs to achieve bacterial concentration.Compared to the traditional bacterial concentration method using centrifugation and freezing-drying method,the proposed method can concentrate a large volume of bacterial suspension without using special equipment.The feasibility of this method is verified by bacterial concentration tests,solution tests and sand column treatment tests.The results of both the solution test and the sand column treatment test show that the bacterial suspension concentrated by the proposed method can be effectively used for soil biocementation.There is a threshold calcium concentration that allows a complete bacterial concentration for the proposed method,and this threshold calcium concentration tends to increase linearly with the optical density of the cell suspension at a wavelength of 600 nm(OD_(600)).

Recent development in biogeotechnology and its engineering applications

Microbial geotechnology or biogeotechnology is a new branch of geotechnical engineering.It involves the use of microbiology for traditional geotechnical applications.Many new innovative soil improvement methods have been developed in recent years based on this approach.A proper understanding of the various approaches and the performances of different methods can help researchers and engineers to develop the most appropriate geotechnical solutions.At present,most of the methods can be categorized into three major types,biocementation,bioclogging,and biogas desaturation.Similarities and differences of different approaches and their potential applications are reviewed.Factors affecting the different processes are also discussed.Examples of up-scaled model tests and pilot trials are presented to show the emerging applications.The challenges and problems of biogeotechnology are also discussed.