Calcium carbonate precipitation catalyzed by soybean urease as an improvement method for fine-grained soil

Although great advances have recently been made in biological soil-improvement methods, the treatments of fine-grained soil using these methods are still challenging. In this study, we tested a new method for the improvement of silty soil. This method adopts the calcium carbonate precipitation process catalyzed by soybean urease. Crude urease is derived simply by collecting the liquid formed by soaking soybean powder in water. The activity of crude urease is linearly related to the amount of soybean powder added to the water, and is high enough to be used for soil treatment. Under batch conditions, the reactions catalyzed by crude urease can be completed, although the reaction rates are slower than those using live bacteria with the same initial activity. In triaxial consolidated undrained tests, the treated silty soil shows more dilative responses in the stress-strain curves and larger peak deviatoric stresses as compared with the untreated soil, indicating significant improvements in the mechanical behaviour. In the results of CaCO₃ measurements, it is seen that there are continuous increases in the CaCO₃ content of the samples during the treatment process, which implies that the silty soil samples are not clogged when treated by crude urease. Such results indicate that calcium carbonate precipitation catalyzed by soybean urease can be used for the improvement of fine-grained soil.     

Recycling of dredged river silt reinforced by an eco-friendly technology as microbial induced calcium carbonate precipitation (MICP)

A large amount of river silt is continuously dredged and usually dumped in landfills or oceans, resulting in land occupation and environmental pollution. Traditionally, cement-based materials are used to cement dredged river silt as building materials, which not only increases carbon dioxide emissions but also uses very little dredged silt. In order to realize the resource utilization of dredged river silt, microbial induced calcium carbonate precipitation (MICP) technology, which has the advantages of lower energy consumption, less environmental pollution and lower carbon emissions, is adopted to solidify the dredged river silt as roadbed materials in this paper. The unconfined compressive strength (UCS) test, calcium carbonate (CaCO₃) content test and microstructure test are carried out to analyze the mechanical properties of the solidified dredged river silt. The test results show that the MICP mixing method can be employed to solidify loose dredged river silt into high-strength construction materials. The concentration of the cementation solution has a significant effect on the solidification effect, and the most reasonable concentration of the cementation solution is 1.5 mol/L. With the increase of treatment times, the pores in the soil are filled with CaCO₃, and the UCS of the specimens after 10 times of treatment can reach 6.75 MPa with a relatively uniform CaCO₃ content of 27.8 %. The main crystal form of CaCO₃ is calcite, which can fill the pores and make the river silt particles cement as a whole, which is the main reason for the improvement of mechanical properties of dredged river silt.     

Mid-scale biocemented soil columns via enzyme-induced carbonate precipitation (EICP)

Biocemented soil columns were created at a reduced scale (mid-scale) using enzyme-induced carbonate precipitation (EICP) as a prelude to field scale deployment. Approximately 0.3 m diameter × 0.75 m long columns were created using a tube-à-manchette grouting technique in 0.6 m × 0.6 m × 1.2 m boxes filled with a dry washed quarry sand. Treatment solution composition and treatment protocol, including number of cycles of treatment and time interval between cycles, were established based upon laboratory testing. The urease enzyme used in the treatment solution was extracted from jack beans in a just-in-time manner on site. The biocemented soil columns were characterized in situ using shear wave velocity, needle penetrometer and pocket penetrometer testing, dimensional measurements, and by unconfined compression strength (UCS) and carbonate content measurements on specimens recovered from the columns. The in situ measurements indicated the target UCS of 500 kPa was achieved. However, the UCS tests on recovered specimens had inconsistent results, which may be attributed to sample disturbance. Overall, the results demonstrate that EICP is a viable method for creating biocemented soil columns for ground improvement.     

Soil improvement using plant-derived urease-induced calcium carbonate precipitation

This study addresses a soil improvement technique using plant-derived urease-induced calcium carbonate (CC) precipitation (PDUICCP) as an alternative to microbially induced carbonate precipitation (MICP). A crude extract of crushed watermelon (Citrullus lanatus) seeds was used as the urease source along with calcium chloride (CaCl₂) and urea (CO (NH₂)₂) for CC precipitation. Test specimens (φ?=?2.3?cm, h?=?7.1?cm) made from commercially available Mikawa sand (mean diameter, D₅₀?=?870?µm) were cemented, and estimated unconfined compressive strength (UCS) of several kPa to MPa was obtained by changing the concentration of CaCl₂- urea, urease activity, curing time, and temperature. The increase of curing time and that of the CaCl₂-urea concentration from 0.3?M to 0.7?M caused an increase in estimated UCS value. The average estimated UCS obtained after 14?days’ curing time for 0.7?M CaCl₂-urea and 3.912 U/mL urease was around 3.0?MPa and for 0.3 and 0.5?M CaCl₂-urea and 0.877 U/mL urease, it was around 1.5–2.0?MPa at 25?°C. By changing each of the abovementioned parameters, it may be possible to apply this method for strength improvement of loose sand, to mitigate the liquefaction, protection and restoration of limestone monuments and statuaries, and artificial soft rock formations. Crude urease from crushed watermelon seeds has the potential to replace commercially available urease for carbonate precipitation and for use as a low environmental impact type soil improvement method.     

Experiments and predictions of physical properties of sand cemented by enzymatically-induced carbonate precipitation

A grouting technique that utilizes precipitated calcium carbonate as a cementing material is presented. The enzyme urease is used to enhance the rate and the magnitude of the calcium carbonate precipitation. Evolutions in the mechanical and the hydraulic properties of treated sand samples are examined through unconfined compression and permeability tests, respectively. The grout is mainly composed of urease, which bio-catalyzes the hydrolysis of urea into carbon dioxide and ammonia, urea, and calcium chloride solutions. This method employs chemical reactions catalyzed by the enzyme, and ultimately acquires precipitated calcium carbonate within soils. The mechanical test results show that even a small percentage of calcium carbonate, precipitated within soils of interest, brings about a drastic improvement in the strength of the soils compared to that of untreated soils—the unconfined compressive strength of the samples treated with <10 vol% calcium carbonate precipitation against the initial pore volume ranges from ∼400 kPa to 1.6 MPa. Likewise, the hydraulic test results indicate the significant impervious effects of the grouting technique—the permeability of the improved samples shows more than one order of magnitude smaller than that of the untreated soils. Evolutions in the measured hydraulic conductivity and porosity are followed by a flow simulator that accounts for the solute transport process of the injected solutions and the chemical reaction of the calcite precipitation. Predictions of the changes in permeability with time overestimate the test measurements, but those of the changes in porosity show a good agreement with the actual measurements, indicating that such simulations should become a significant supplementary tool when considering real site applications.     

Assessment of the bio-cementation effect on shale soil using ultrasound measurement

The assessment of bio-cementation effect using nondestructive testing methods is important for the application of microbial-induced carbonate precipitation (MICP) technique. In this study, a special grouting system was designed to conduct the bio-grouting of shale soils on a testing model. The ultrasound technique was used to evaluate the effectiveness of bio-cementation. A method was derived to estimate the distribution of ultrasonic wave velocity values within the testing model based on the average values measured on the model surfaces. From the changes of isosurfaces of velocity values, the evolution of soil improvement within the model corresponding to the different number of grouting cycles was analyzed numerically and visually. The SEM imaging technique was used to illustrate the mechanism of influence of the calcium carbonate precipitation on the wave velocity of the bio-cemented specimen. The linear relationships were established among the ultrasonic wave velocity, unconfined compressive strength (UCS), and the amount of CaCO₃ precipitations formed in the MICP process. In combination with the application of the ultrasound measurement technique as demonstrated in this work, the relationship provides a very useful nondestructive testing tool to assess the effectiveness of bio-cementation within a large soil body.     

微生物诱导碳酸镁沉淀试验研究

碳酸镁和碳酸钙一样也具有胶结作用,且镁矿强度远大于钙矿,因此研究碳酸镁固化技术具有重要意义。本文测量了巴氏芽孢杆菌在培养过程中的吸光度和脲酶活性,并计算得到单位脲酶活性;研究了尿素、氯化钠、醋酸根、Ca2+和Mg2+浓度对脲酶活性的影响;控制温度、pH值和离子浓度对比了钙和镁沉淀效率和不同温度不同尿素浓度下碳酸镁沉淀产率;对比研究了碳酸钙和碳酸镁沉淀下砂土固化效果。结果表明,48 h培养过程中,吸光度和脲酶活性的增长都先缓慢后迅速增长再减小最后停止,单位脲酶活性则是先增加后减小。适当增加尿素或Mg2+浓度可增强细菌脲酶活性,氯化钠和醋酸根浓度对酶活性无明显影响,Ca2+浓度对脲酶活性有明显抑制作用。同一温度、pH值和离子浓度条件下,碳酸镁产率明显小于碳酸钙。而在菌液中添加尿素可促进碳酸镁沉淀生成,且温度相同,尿素浓度越高,碳酸镁产率越大。菌液中添加尿素可使得碳酸镁固化砂土成型并具有一定强度,因此,该方法可解决砂土固化碳酸镁沉淀不足的问题,为后续碳酸镁固化试验奠定基础。     

Effect of ureolytic bacteria on concrete properties

Bacteria are incredibly diverse. Numerous diverse bacterial species participate in the precipitation of mineral carbonates in various natural environments, including soils, geological formations, freshwater biofilms, oceans and saline lakes.Bacteria are believed to affect carbonate precipitation both through affecting local geochemical conditions and by serving as potential, nucleation sites for mineral formation. A novel technique for the remediation of damaged structural formations has been developed by employing a selective microbial plugging process, in which metabolic activities promote precipitation of calcium carbonate in the form of calcite. Recently, microbial mineral precipitation resulting from metabolic activities of some specific microorganisms in concrete to improve the overall behavior of concrete has become an important area of research. It has been hypothesized that almost all bacteria are capable of CaCO₃ production because precipitation occurs as a byproduct of common metabolic processes such as photosynthesis, sulfate reduction, and urea hydrolysis.In this review paper, an overview of bacteria, their types based on the classification has been studied. Even the effect of bacteria on various parameters in concrete proves to be beneficial development. Based on the studied properties like compressive strength, permeability, water absorption, chloride ingression, the microbial mineral precipitation appears to be a promising technique at this state of development.     

Regeneration of anion exchange resins by calcium carbonate and carbon dioxide

Partial demineralization of water can be achieved by bicarbonate from anion exchange resins. The exchangers can be converted into bicarbonate form by carbon dioxide and calcium carbonate. The advantage of this method lies in these chemicals not being detrimental to health and not polluting the environment. The equilibrium of the CO₂ and CaCO₃ treatment has been investigated and compared with selectivity measurements. It is obvious from the results that the regeneration efficiency will be small but sufficient for technical purposes     

Influence of clogging substances on pore characteristics and permeability of geotextile envelopes of subsurface drainage pipes in arid areas

This study investigates the influence of clogging substances on pore characteristics and permeability of geotextile envelopes that were used for 3, 7 and 15 years in irrigated farmlands in Xinjiang region, which is arid and suffers from the soil salinity problem. Results show that the macropores (above 125 μm) of envelopes are evidently clogged, whereas the smaller pores less than 100 μm are still unblocked after operation. The permeability coefficients of geotextile envelopes after serving for 3 and 15 years are smaller than the minimum required permeability coefficients after clogging. The main chemical components of clogging substances in the geotextile envelope are silicon dioxide and calcium carbonate. Calcium carbonate content of the geotextile envelope is consistent with calcium carbonate content of soil. Chemical clogging susceptibility increases with the operation time of the subsurface drainage pipes. The ratio of O₉₀ size of envelope material over d₉₀ of soils (O₉₀/d₉₀) and saturation index (SI) can be used to assess the susceptibility of physical and chemical clogging respectively. This study provides a preliminary reference for estimating the clogging susceptibility of geotextile envelopes in arid areas.     

Assessment of groundwater quality and its suitability for agricultural usage in and around Rangampeta area,Andhra Pradesh,south India

The aim of this study is to assess the groundwater quality of Rangampeta area for irrigation and domestic purposes. The groundwater samples were analyzed for distribution of chemical elements Ca, Mg, Na, K, Si, HCO₃, CO₃, Cl and SO₄. It also includes pH, electrical conductivity, total hardness, non carbonate hardness and total alkalinity. The parameters like sodium absorption ratio, adjusted sodium absorption ratio, sodium percentage, potential salinity, permeability index and residual sodium carbonate were calculated. The dominant hydrochemical facies of groundwater are Ca–Mg–Cl and Na–Cl Water Types. The Gibbs’s diagram plots suggest that the chemical weathering of rock forming minerals is the major driving force controlling water chemistry in this area. The positive chloro-alkaline indices revealed that the groundwater has suffered ion exchange between Na and K of water with Ca and Mg of soil during its flow.     

Antiscalant removal in accelerated desupersaturation of RO concentrate via chemically-enhanced seeded precipitation (CESP)

An experimental study was carried out to demonstrate and quantify the feasibility of antiscalant (AS) removal from brackish water RO concentrate of high gypsum scaling propensity via lime treatment prior to seeded gypsum precipitation. Based on studies with model solutions, it was shown that sufficient AS removal (up to ∼90%) from RO concentrate is feasible via a lime treatment step (at a dose significantly lower than that required for conventional lime softening) to enable effective subsequent seeded gypsum precipitation. This two-step chemically-enhanced seeded precipitation (CESP) treatment of primary RO concentrate is suitable as an intermediate concentrate demineralization (ICD) stage for high recovery desalting employing secondary RO desalination. Analysis of gypsum precipitation and lime treatment kinetic data suggests that, after adequate CaCO₃ precipitation has been induced for effective AS scavenging, CaSO₄ desupersaturation can be achieved via seeded gypsum precipitation without retardation due to seed poisoning by AS. Also, the lime dose required to prevent seed poisoning during subsequent gypsum desupersaturation via seeded gypsum precipitation can be adequately assessed with a precipitation kinetics model that considers AS seed poisoning based on a Langmuir adsorption isotherm. The degree of AS removal after lime treatment increased linearly with the logarithm of the single lime dose additions. Staged lime dosing (i.e., multiple lime additions), however, removed a higher degree of AS relative to an equivalent single lime dose addition since a higher driving force for CaCO₃ precipitation could be maintained over the course of the lime treatment period.     

Traitement des effluents urbains ou industriels par entrainement des polluants a l'occasion d'une precipitation de CaCO3

Studies carried out in our laboratories on calcium carbonate precipitations have led to the conclusion than substantial amounts of organic matter could be entrained by the crystals that are produced and in particular when the precipitate is in the form of vaterite. Moreover entrainments of this type are used currently in sugar-refining for the purification of sugar juices. Such CaCO₃ precipitation may be easily induced by means of the reaction of lime with a current of CO₂-enriched gas which is bubbled through the solution. Given also the well-known coagulating properties of lime, it has been considered worthwhile investigating the treatment of certain wastes by means of this method. The initial trials were sufficiently conclusive for the SOAF Company to taken an interest in the project and for a decision to be made to construct a pilot plant of 10 cm h⁻¹ capacity. Concurrently with this, a 301 h⁻¹ laboratory unit was also built, and it is this which has provided the initial results that we propose to set out here.     

青海强盐渍粉砂土MICP的有效性探索

在极端环境中发现原生高产脲酶微生物并开展岩土体的固强研究,是岩土体微生物矿化研究的一个热点和难点.该文在青海柴达木地区的强盐渍土中发现一种新型原生高产脲酶微生物,在强盐环境中,试验该微生物的盐耐受性和矿化性能,开展被加固土体力学强度试验.结果表明:在强盐渍环境下,该新型微生物脲酶活性保持在3.02U～5.03U.在强盐...     

Cementation of Sands Due to Microbiologically-Induced Carbonate Precipitation

In this study, microbial precipitation of carbonate was observed using high microbial urease activity, and it was found that the ratio of Mg/Ca affected the types of crystals produced. Without Mg²⁺, calcite was produced using only CaCl₂, while the presence of Mg produced Mg-calcite, magnesite and/or possibly dolomite of round, spherical or fibrous shapes, depending on reaction time, pH and Mg/Ca ratio. The carbonate produced contributed to the development of cementation for sands. The presence of Mg showed a relatively strong cementation of the carbonate.     

Calcium carbonate hexahydrate: Its properties and formation in lime-soda softening

The properties and the conditions of formation of a metastable hydrated form of calcium carbonate (CaCO₃6H₂O) are described. This compound often crystallises when Essex river waters are softened by the lime/soda process. Formation of the hydrated salt is favoured by low water temperatures and by high orthophosphate levels. Calcium carbonate hexahydrate forms pure well-defined crystals which are considerably more soluble in water than calcite, the usual product of lime/soda softening. The efficiency of the softening process is very much reduced when the hydrated compound crystallises, and increased quantities of chemicals are required both for softening and for subsequent stabilisation of the softened water.Considerable success has been achieved on the practical scale in reducing the incidence of formation of the hexahydrate by precipitating phosphate before softening using ferric salts and by addition of pre-formed calcite nuclei in the form of commercial Whiting; this has led to substantial economies in chemical consumption.     

Improving settlement and reinforcement uniformity of marine clay in electro-osmotic consolidation using microbially induced carbonate precipitation

This study explored the effect of microbially induced carbonate precipitation (MICP), a natural bio-geotechnical process, on mitigating the uneven settlement in electro-osmotic consolidation of soft soil. The real-time drainage and settlement were measured, and the control behavior of drainage to settlement was discussed. MICP solution components were also selected as the different additives to determine the control mechanism of MICP in improving settlement and reinforcement uniformity of clay. After the tests, the chemical properties and microstructure were analyzed according to pH, conductivity, and SEM. The addition of MICP solution in clay significantly even reduced the coefficient of settlement variation by 53.2%, and the upper surface profile tended to be uniform. Contrary to control, the coefficient of settlement variation of MICP-treated soil decreased gradually with drainage volume, mainly due to the filling of solid substances such as calcium carbonate, biofilm, and/or calcium hydroxide produced within soil pores. MICP significantly improved the uneven soil reinforcement generated during the electro-osmotic consolidation but resulted in the lower strength near the anode due to the less drainage. The contribution of MICP solution components to the improvement of settlement and strength uniformity obviously varies. Bacterial cells improved the settlement uniformity but had no effect on the strength improvement of soil. The co-existence of Ca²⁺ and bacterial cells maximized the modification effect, which determined the production of mineral precipitation. Microstructure observation proved the formation of calcium carbonate. The results demonstrated that MICP is an effective technique to improve the settlement and reinforcement uniformity of marine clay in electro-osmotic consolidation.

     

Hydrogeochemie von Eisenausfällungen an Grundwasseraustritten der Abraumkippe Berrenrath (Rheinisches Braunkohlenrevier)

Groundwater at the abandoned lignite mine in Berrenrath (Germany) is acidic and highly mineralised. Average iron concentrations of 588 mg/l indicate a high potential for acidification. Furthermore, investigations revealed a complete depletion of the carbonate buffer capacity (CaCO₃ and $\mathrm{HCO}_{3}^{-}$ ) during chemical groundwater evolution. Over the next few decades, no decrease of dissolved pyrite oxidation products is expected. On the contrary, within the drainage ditches at the edges of the dump, an intense precipitation of iron oxides is occurring. However, despite oxygen saturation, precipitation of iron oxides is incomplete due to strong acidification of the surface water (pH<3.0). The main contamination of the iron sludge is caused by high nickel concentrations. This element is only loosely bound to the iron oxides and is easily mobilised. Because of high nickel- and sulphate-concentrations, the excavated material has to be deposited in a controlled landfill (German dump category of 2 to 3).     

Water stability mechanism of silicification grouted loess

Silicification is one of the chemical stabilisation methods used in the treatment of collapsible loess soils. The water stability therein is a key parameter in the silicification of grouted loess. Based on slaking tests, permeability measurement, X-ray diffraction spectra, X-ray energy dispersive spectroscopy, and scanning electron microscopy, the water stability mechanism inherent in the CO₂-silicification grouted loess was investigated. Samples of original, compacted, and CO₂-silicification grouted loess in 30 days curing were tested. To assess the long-term water stability, CO₂-silicification grouted loess samples in 13, 19, and 24 years of curing were analysed. The study showed that the CO₂-silicification grouted loess had good water erosion resistance, no disintegration, and good water stability over time. The water stability of CO₂-silicification grouted loess depended on the strong bond strength of the grains and a low permeability. The complex physicochemical reactions among CO₂, water, alkali earth metal salts, clay minerals, and organic matter in loess produced hydrate calcium (and magnesium) silicate gels, which were mainly coated on the surface of the soil skeleton grains and original cements. A few filled in the trellis pores. The gels coated on the soil skeleton limit the hydrophilicity of clay minerals and organic matter and improve water resistance, and if coated on original cements reinforce bond strength, consequently, the water stability of CO₂-silicification grouted loess was improved.     

pH adjustment in anaerobic digestion

In the pH range 6·0–7·5, the pH in anaerobic processes is controlled by the interaction of the carbonic system and a net strong base. The acid-base state of a digestor can be monitored by only measuring pH and CO₂ partial pressure. Shock doses of strong bases and carbonates causes temporary undersaturated CO₂ conditions and excessively high pH. Bicarbonate dosing leaves the CO₂ solubility equilibrium unchanged. In the absence of a CaCO₃ precipitation inhibiting agent. CaCO₃ solubility limits the pH, and Ca(OH)₂ dosing is unable to raise the pH significantly. Orthophosphates inhibit CaCO₃ precipitation. With [PO₄] > 1·0 × 10⁻³mole·1⁻¹. CaCO₃. precipitation is partially inhibited. Ca(OH)₂ dosing being approximately 45 per cent effective for doses up to 15000 mg 1⁻¹ as CaCO₃. At [PO₄] < 1·0 × 10⁻³moles·1⁻¹ orthophosphates eventually precipitate out during Ca(OH)₂ dosing, thus removing the inhibition mechanism: pH is then limited by the CaCO₃ solubility. Most wastes contain [PO₄] > 2·0 × 10⁻³moles·1⁻¹ making pH adjustment with Ca(OH)₂ possible to a pH of about 7·2 although the dosages will be very high. The pH changes in a process following dosing can be predicted by the graphical representation of the carbonic and net strong base systems.