低温条件微生物MICP沉淀产率试验研究

低温导致微生物固化沉淀产率低,制约着该技术的应用。选取巨大芽孢杆菌,通过控制不同温度和pH值分析该菌种的生长繁殖特性和脲酶活性,并研究不同温度条件下的碳酸钙沉淀产率,通过采用营养液中添加尿素和低温驯化两种方法来提高低温条件下较低的沉淀产率,最后通过砂土固化试验,对比研究尿素添加方法和低温驯化对固化效果的影响。结果表明:温度越高,巨大芽孢杆菌的生长繁殖越快,脲酶活性越强,低温明显抑制其生长繁殖和脲酶活性;pH为8时,巨大芽孢杆菌生长繁殖最快,且脲酶活性最强;温度越高,沉淀产率越大;营养液中添加尿素和对巨大芽孢杆菌进行低温驯化都可以明显提高生长繁殖速度和沉淀产率,可以有效解决低温条件下碳酸钙沉淀不足问题,而将两者结合起来,沉淀产率提升更为明显;营养液中添加尿素和低温驯化都能提高砂土固化效果,而同时采用这两种方法固化效果提升更明显,该研究能有效解决低温条件沉淀少阻碍实际工程应用的问题,为后续低温条件微生物固化技术的应用打下基础。     

植物源脲酶诱导碳酸钙固化砂土试验研究

植物源脲酶诱导碳酸钙沉积胶结砂土是岩土工程领域的一种新型技术,相比目前广泛应用的微生物固化砂土技术具有很多优点。直接从大豆中提取脲酶,首先研究了温度及p H值对大豆脲酶活性的影响,然后控制胶凝液浓度、pH值、温度和反应时间进行了脲酶诱导碳酸钙沉积试验,在此基础上,采用循环灌注脲酶液和胶凝液的方法固化3种不同颗粒粒径范围的砂土,通过超声波测试、无侧限抗压强度测试及碳酸钙含量测试检测固化效果。结果表明:大豆脲酶最适pH值为8,15℃～75℃范围内脲酶活性随温度升高而增大。大豆脲酶诱导的沉淀产物为方解石型碳酸钙,随胶凝液浓度增大,碳酸钙产率先增大后减小,胶凝液浓度为0.75mol/L时,碳酸钙产率最大。胶凝液浓度一定时,pH值为8情况下碳酸钙产率最大,且产率随反应时间增加而增大。10℃～40℃范围内温度对碳酸钙产率影响较小。固化试样的抗压强度与碳酸钙含量呈正相关,随砂土颗粒粒径增大,试样的抗压强度先增大后减小,0.25～0.5mm砂土固化效果最好。     

营养盐对微生物加固可液化砂土效果的探讨

微生物诱导生成碳酸钙沉淀(MICP)技术是一项新兴的原位灌浆技术,通过微生物和钙盐作用形成碳酸钙沉淀可改善可液化砂土的抗液化特性。NH4+作为表征碳酸钙结晶过程的重要因子,可充分反映对可液化砂土的改良效果。选用巴氏芽孢八叠球菌,采用Ca(CH3COO)2、Ca(NO3)2和Ca Cl2三种钙盐与尿素混合溶液的营养盐,探讨采用NH4+来表征可液化砂土的微生物固化过程。结果表明:NH4+离子浓度变化能够表征MICP对可液化砂土改良的效果,其中Ca(CH3COO)2营养盐改善可液化砂土效果最佳;营养盐的用量也对可液化砂土的加固效果有明显的改善。通过对固化后试样的渗透性和超声波速的测定,也验证了加强效果。     

碳源对微生物固化砂土效果影响的试验研究

微生物固化砂土需要营养液提供所需的固化环境。营养液成分的不同,其固化效果可能也有所区别。针对营养液中的重要组成部分——碳源,选取无机碳源(碳酸氢铵和碳酸氢钠)、有机碳源(无水乙酸钠和葡萄糖)共4种碳源进行对比研究。通过砂柱试验,测定4种碳源在不同浓度下的物理力学指标,获取每种碳源固化砂土的最优浓度;然后以4种碳源的各自最优浓度,进行不同脲酶活性下的烧杯试验和砂柱试验,测定烧杯试验的SEM和砂柱试验的物理力学指标。试验结果表明:碳酸氢钠、乙酸钠和葡萄糖进行固化砂土的最优浓度为0.03 mol/L,碳酸氢铵固化砂土的最优浓度为0.02 mol/L,有机碳源的固化效果比无机碳源好;不同碳源固化砂土的效果在低脲酶活性下差异性更大;微生物诱导碳酸钙沉淀中,脲酶活性高低是整个诱导固化过程的主导因素;不同碳源诱导产生的碳酸钙沉淀的形态不同。     

微生物注浆固化砂土若干因素的影响机制研究

微生物诱导碳酸钙沉淀(MICP)由于其良好的粘结性和环境友好性,近年来已广泛应用于砂土固化领域。通过测试2种嗜碱性产脲酶菌的生长曲线以及通气和不通气情况下的脲酶活性,探索其与固化砂土的无侧限抗压强度、碳酸钙含量及均匀性之间的联系,并尝试从微观层面探讨微生物注浆固化砂土的主要影响机制。结果表明,通过此方法制得的砂柱强度最高可达586 k Pa。脲酶活性差异导致碳酸钙含量差异,碳酸钙含量则与砂土固化强度相关。而在碳酸钙含量相近的情况下,固化均匀性将进一步对强度造成影响。     

砂土微生物固化过程中尿素的影响研究

砂土固化技术可改善砂土力学特性,被广泛应用于工程中,而在菌种培养液中添加尿素可大大加快固化反应提高砂土固化效率。开展了在菌种培养液中添加尿素分析其对砂土固化影响的试验研究。首先将尿素添加方式分为灭菌前加尿素、灭菌后加尿素和不加尿素3种,研究不同方式对菌种生长和脲酶活性的影响;再控制不同尿素添加量对比分析得到适合的尿素添加量以便后续研究;然后在砂土固化试验中采用灭菌后加尿素的方式研究其对砂土固化的影响;最后通过控制砂柱长度、胶凝液灌注速度和砂土颗粒粒径等因素,得到各因素对砂柱固化效果的影响。结果表明培养液中添加尿素可提高脲酶活性,但稍微抑制菌种生长;适合的尿素添加量为5~20 g/L;灭菌后添加尿素能显著提高灌注部位的强度,但长砂柱却因强度不均而整体强度较低;灌注速度越快,整体强度越高;添加20 g/L尿素时,整体强度随颗粒粒径增大而增大,因此,该方法适用于粗砂。研究成果对后期砂土固化技术的应用具有重要指导意义。     

多浓度营养盐处理对微生物胶结砂土均匀性与强度的影响

采用二次注入菌液方式,制备不同浓度营养盐处理的MICP(微生物诱导碳酸钙沉淀)胶结砂样。选用巴氏芽孢杆菌作为固化细菌,采用单一浓度(0.5、1.0mol/L)和多浓度相结合(前期采用0.5mol/L,后期采用1.0mol/L)的处理方式注射营养盐(尿素/氯化钙混合液),研究多浓度营养盐结合处理方式对微生物固化砂土强度及均匀性的影响。基于试验测试分析了固化砂土试样不同区间段的强度、弹性模量以及碳酸钙含量。试验结果表明,多浓度营养盐处理方式对固化砂土试样的强度及碳酸钙含量有明显影响;多浓度营养盐结合处理方式能够保证试样有较好的均匀性条件下获得较高强度及弹性模量。基于多浓度营养盐处理方式,探讨分析了影响试样强度和均匀性的基本因素。     

环境变化对微生物活性及矿化的影响

微生物诱导碳酸钙沉积（MICP）技术因具有低能耗、低污染等环境友好性优势成为文物保护、水处理、混凝土修补加固等领域的研究热点。MICP的矿化效果受多因素的制约与影响。本文开展了环境温度、pH值等因素对细菌生长和微生物矿化的影响研究。结果表明,环境温度不超过40℃时对菌液脲酶活性影响不大,42.5~45℃时6h后脲酶活性急剧降低,超过45℃时30min后菌液浓度及脲酶活性均大幅下降,直至失活;pH值越大,脲酶活性衰减越明显,pH≥11时脲酶活性急剧下降,大量的细菌不能承受高碱性环境进一步分解死亡。MICP技术在30℃时碳酸钙产量最高,在pH值为11~13的环境中细菌很难生长繁殖,碳酸钙生成量较低。     

钙源对微生物沉积碳酸钙固化砂土的试验研究

微生物诱导碳酸钙沉淀是一种环保的地基处理方法,通过微生物和钙盐作用形成碳酸钙沉淀加固土体。钙盐种类的不同对生物固化砂土的效果也不尽相同。选取氯化钙、硝酸钙、乙酸钙和乳酸钙4种钙盐,探究营养液中钙盐种类对微生物固化砂土效果的影响。基于渗透试验、干密度试验、吸水率试验、无侧限抗压强度试验和碳酸钙含量试验测定微生物固化砂土的物理力学指标,从宏观角度分析钙源对微生物固化效果的影响;结合电镜扫描测试,从微观角度分析了钙源对碳酸钙沉淀晶体形态的影响。试验结果表明:硝酸钙的固化效果最好,其次是乙酸钙、氯化钙和乳酸钙;乙酸钙为钙源固化后的砂柱强度达到2.884 MPa;固化后砂柱受压发生破坏主要源于固化薄弱区;无机钙源获取的碳酸钙沉淀颗粒要大于有机钙源。     

紫外诱变产脲酶菌株加固粉土的试验研究

为提高微生物诱导碳酸钙沉淀(MICP)技术的固化效果,采用紫外诱变技术对产脲酶菌株进行改良,筛选出优良菌株。随后结合先拌和菌液后滴注胶结液(尿素和氯化钙)的方式,运用改良后菌株来固化粉土。通过无侧限抗压强度试验、碳酸钙含量测定和微观试验分析,来对比诱变前后菌株固化粉土的效果。结果表明:紫外诱变技术可以有效改良产脲酶菌株的性能,使菌株的脲酶活性、矿化生成的碳酸钙含量得到提高;使用紫外诱变后菌株来固化粉土,可显著提高土体的无侧限抗压强度。该研究从源头来选育优良菌株,有效提高了MICP技术的固化效果。     

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.     

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.     

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.     

Effect of a magnetic water treatment on homogeneous and heterogeneous precipitation of calcium carbonate

In this paper are reported experimental results on the effect of a magnetic field on the precipitation process of calcium carbonate scale from a hard water. Carbonically pure water was circulated at a constant flow rate in a magnetic field. After this treatment, calcium carbonate precipitation was induced by degassing dissolved carbonic gas. The nucleation time was identified from the variations of the pH and the Ca(2+) concentration. The ratio between homogeneous and heterogeneous nucleation was determined from the measurement of the mass of precipitated calcium carbonate. It is shown that the magnetic treatment increases the total amount of precipitate. This effect depends on the solution pH, the flow rate and the duration of the treatment. In addition, the magnetic treatment modifies the ratio between homogeneous/heterogeneous nucleation. Homogeneous nucleation is promoted by an increasing the pH of water, the flow rate as well as the residence time. The magnetic treatment enhances these effects with a maximum for a 15 min treatment time. It is shown that the presence of calcium carbonate colloid particles is not necessary. It is advanced that the main magnetic effects concern the associations of ionic species which are present in the solution and which are involved in the nucleation process of calcium carbonate precipitation.     

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...     

The effect of antiscalant addition on calcium carbonate precipitation for a simplified synthetic brackish water reverse osmosis concentrate

The primary limitations to inland brackish water reverse osmosis (RO) desalination are the cost and technical feasibility of concentrate disposal. To decrease concentrate volume, a side-stream process can be used to precipitate problematic scaling salts and remove the precipitate with a solid/liquid separation step. The treated concentrate can then be purified through a secondary reverse osmosis stage to increase overall recovery and decrease the volume of waste requiring disposal. Antiscalants are used in an RO system to prevent salt precipitation but might affect side-stream concentrate treatment. Precipitation experiments were performed on a synthetic RO concentrate with and without antiscalant; of particular interest was the precipitation of calcium carbonate. Particle size distributions, calcium precipitation, microfiltration flux, and scanning electron microscopy were used to evaluate the effects of antiscalant type, antiscalant concentration, and precipitation pH on calcium carbonate precipitation and filtration. Results show that antiscalants can decrease precipitate particle size and change the shape of the particles; smaller particles can cause an increase in microfiltration flux decline during the solid/liquid separation step. The presence of antiscalant during precipitation can also decrease the mass of precipitated calcium carbonate.     

Effect of pH and temperature on calcium carbonate precipitation by CO2 removal from iron‐rich water

In the present work, the effect of temperature and solution pH on calcium carbonate precipitation from iron‐rich waters was investigated. Calcium carbonate was precipitated by CO₂ removal. The increase in the temperature or the solution pH leads to the acceleration of calcium carbonate nucleation and crystal growth. Iron addition retards the formation of calcium carbonate crystals and enhanced the precipitation in the bulk solution. At high supersaturations, the inhibition effectiveness of iron is small and it could be improved by lowering the solution pH. The results of the present work show that it is possible to reduce or completely prevent scale formation in different water treatment processes by controlling the operating parameters which favourably affects the water treatment costs, increases the equipment life and allows increased product water recovery.     

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.