Research progress of microbial reinforcement technology
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摘要:
生物加固是工程地质领域近年来发展起来的一个新分支,微生物诱导碳酸钙沉淀(microbially induced calcite precipitation,MICP)加固技术是常用的方法之一。MICP加固技术是借助自然界广泛存在的微生物,利用其代谢活动诱导产生具有胶结作用和充填作用的碳酸钙沉淀,从而达到提高土体强度、降低土体渗透性、改善土体工程性能的目的。近20年来,MICP加固技术在理论研究、模型试验及现场试验方面已取得了许多重要成果。为推进对MICP加固技术的认识及研究,文章基于MICP加固技术目前取得的研究进展,进行了文献调研与分析,系统介绍了MICP技术的加固机理及影响因素,归纳分析了MICP加固技术的应用实例,在此基础上深刻分析了MICP技术目前存在的问题和挑战。结果如下:(1)MICP的加固机理是在微生物诱导矿化作用基础上产生的碳酸钙沉淀对土体的充填作用、覆膜作用和胶结作用;(2)MICP固化效果的影响因素主要有:菌液及胶结液的性质、pH值、温度、土体类型、注入技术等,以上影响因素均可以通过影响碳酸钙的形成及胶结效果来影响MICP的固化效果;(3)MICP加固技术在土体加固、抗裂防渗、防风抗蚀、修复污染水土等方面展现出了巨大的潜力;(4)MICP加固技术目前仍存在一些问题,包括固化均匀性、土体耐久性、经济效益、环境安全与可持续性等方面,为解决这些问题,需要进一步综合微生物学、土力学、材料科学、环境科学等多个学科进行深入研究。相关探讨有助于加深对MICP加固技术的理解,推动MICP加固技术在工程地质领域的发展及应用推广。
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关键词:
- 微生物固化 /
- 微生物诱导碳酸钙沉淀 /
- 加固机理 /
- 工程应用
Abstract:Biological reinforcement is a new branch that has developed in the field of engineering geology in recent years, among which, the microbially induced calcium carbonate precipitation (MICP) reinforcement technology is an effective method. MICP reinforcement technology utilizes the metabolic activity of widely existing microorganisms in nature to induce the precipitation of calcium carbonate with cementing and filling effects, thereby improving the strength of soil, reducing its permeability, and enhancing its engineering performance. Over the past two decades, significant progress has been made in theoretical research, model experiments, and field trials of MICP reinforcement technology. To promote a deeper understanding of MICP reinforcement technology, this paper systematically introduced the reinforcement mechanism and influencing factors of microbial-induced calcium carbonate reinforcement technology based on the current research on MICP reinforcement technology. In addition, the application of MICP reinforcement technology was discussed thoroughly. The current problems and challenges of MICP technology were analyzed in depth. The results show that the strengthening mechanism of MICP is formed based on microbial-induced mineralization, which includes the filling and cementation of soil pores by calcium carbonate precipitation. The influencing factors of the solidification effect of MICP mainly are the properties of bacterial solution and cementing solution, pH , temperature, soil type, and reinforcement method, by influencing the formation of calcium carbonate and cementing effect. The MICP reinforcement technology has shown great potential in soil reinforcement, crack resistance, impermeability, wind erosion resistance, and remediation of contaminated water/soil. At present, the MICP reinforcement technology still has some problems, including solidification uniformity, soil durability, economic benefits, environmental safety, and sustainability. To solve these problems, it is necessary to further integrate microbiology, soil mechanics, materials science, environmental science, and other disciplines to conduct in-depth research, which will help deepen the understanding of MICP reinforcement technology, promote MICP technology to achieve more comprehensive development in the engineering geological field, and further promote application of this technology.
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Keywords:
- microbial reinforcement /
- MICP /
- strengthening mechanism /
- engineering application
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随着社会经济的快速发展,城市中大型基础设施不断增加,工程地质问题也随之发生,如边坡失稳[1]、砂土液化[2]、土体污染[3]、结构侵蚀[4]、固废处置[5]等。针对这些问题,学者们进行了大量土体加固方面的研究。传统的治理方法包括物理加固(如夯实[6]、预压[7]、换填[8]、排水[9]等)和化学加固(如添加水泥[10]、石灰[11]、粉煤灰[12]、高分子材料[13]等)。然而,这些方法往往存在工作量大、工期长、环境友好性差、通用性低等局限性。因此,需要深入分析传统方法中存在的问题,寻找新的解决方法,以兼顾工程需求、降低成本、减少对环境的影响。
近年来,关于微生物诱导碳酸钙沉淀(microbially induced calcite precipitation,MICP)加固技术的研究在全球工程地质领域引起了广泛的关注。李俊等[14] 通过研究冻融循环对MICP加固土性能的影响,发现MICP加固技术能够有效减少雨水渗透,提高土体的抗侵蚀性和强度,从而提高边坡的稳定性。Arpajirakul等[15]利用MICP加固技术来改性黄土,发现MICP能够明显改善黄土的工程性质。Liu等[16]研究了MICP加固技术在古黏土墙面侵蚀控制方面的适用性,结果表明MICP保护层能够有效减缓古黏土墙面的风化。MICP作为一种微生物矿化技术,与传统的物理/化学加固技术相比,具有加固效率高、通用性好、能耗低和对环境影响小的优点。此外,MICP加固技术所使用的菌液和胶结液具有较低的黏度[17],相较于传统注浆浆液更易在土体中渗透,也更易于处理较大厚度和较深层次的土体。
目前,MICP加固技术在理论研究、模型试验及现场试验方面已取得了许多重要成果。为推进对MICP加固技术的进一步认识与研究,本文基于MICP加固技术目前取得的研究进展,归纳总结了目前国内外学者对MICP技术加固机理及影响因素的研究成果,概述了MICP加固技术在工程中的应用实例,同时,对当前存在的问题进行了分析,并展望了MICP加固技术未来的研究方向,以期推动MICP加固技术在工程地质领域的发展及应用。
1. MICP加固机理
自然界中存在大量的微生物,其中约66%的微生物均能够通过矿化作用产出碳酸钙晶体[18]。常见的可参与矿化的微生物有产脲酶细菌、硫酸盐还原细菌、反硝化细菌等。其中产脲酶细菌具有成本低、反应机制简单、矿化效率高、反应过程可控等优点[19]。以产脲酶细菌为例,MICP的矿化机理是利用尿素水解产生的碱性环境诱导产生碳酸钙沉淀,其矿化过程如下。
首先,产脲酶细菌将尿素水解成氨基甲酸和氨:
CO(NH2)2+H2O→NH2COOH+NH3 然后,氨基甲酸与水反应,分解成了氨和碳酸。一方面,产生的氨与水反应得到铵离子和氢氧根离子,提高了整个体系的pH值,使得整个体系产生碱性环境。另一方面,碳酸分解为水溶性的碳酸氢根离子,并释放出氢离子:
NH2COOH+H2O→NH3+H2CO3 NH3+H2O→NH+4+OH− H2CO3→HCO−3+H+ 紧接着,碳酸氢根离子会与体系中的氢离子和氢氧根离子反应得到碳酸根离子和水:
HCO−3+H++2OH−→CO2−3+2H2O 最后,伴随着钙离子的加入,带负电荷的微生物细胞将钙离子吸附聚集在细胞壁上,与产脲酶作用产生的碳酸根离子反应,生成碳酸钙晶体沉淀:
CO2−3+Ca2+→CaCO3↓ 碳酸钙晶体的沉淀导致土体孔隙结构发生变化。Wang等[20]通过微流控技术开展了MICP矿化试验,结果表明随着胶结液注入次数的增加,生成碳酸钙晶体面积与孔隙面积的比值逐渐增加,见图1(a)左。Xiao等[21]也通过该技术探究了MICP动态矿化过程,表征了碳酸钙晶体在多孔介质中的沉淀特征,见图1(a)右。此外,Tobler等[22]通过对花岗岩预制裂隙进行MICP注浆修复,发现经MICP技术处理后的试样裂隙空间减少了67%。进一步研究表明,在应用MICP技术修复裂隙的过程中,碳酸钙晶体首先在裂隙表面形成黏结层,然后通过碳酸钙晶体持续生长逐渐桥接完成修复,见图1(b)。MICP技术通过生成的碳酸钙沉淀,填充了土体的孔隙,减小了孔隙度,密实了土体。
碳酸钙晶体的空间分布直接决定了MICP加固技术的有效性。碳酸钙晶体在孔隙中的分布如图1(c)所示,理论上碳酸钙晶体在土颗粒间可能呈等厚度沉淀的均匀分布模式,理想状态下有可能呈现只在颗粒接触处沉淀的优势分布模式,而实际上碳酸钙晶体沉淀是介于均匀分布和优势分布之间的状态:既有覆膜包裹,也在颗粒接触处沉积胶结。这是由微生物的运动特点以及颗粒间孔喉对碳酸钙的过滤作用决定的[23]。MICP的加固机理实际是在微生物诱导矿化作用基础上产生的碳酸钙沉淀对土体的填充作用、覆膜作用和胶结作用。
2. MICP加固效果的影响因素
MICP加固技术受到多种因素的制约,这些因素直接影响了MICP固化效果。影响MICP固化效果的因素主要有:菌液及胶结液的性质、pH值、温度、土体类型、固化方法等,以上影响因素均可以通过影响碳酸钙晶体的沉淀量、晶体形貌、分布均匀性及胶结特性来影响加固土体工程性质。
2.1 菌液及胶结液的性质
菌液的性质涉及到细菌的类型和浓度等方面。可以产生脲酶的微生物主要包括球形芽孢杆菌、巴氏芽孢八叠球菌、巨大芽孢杆菌、迟缓芽孢杆菌和巴氏芽孢杆菌等[24 − 25]。在相同条件下,使用不同种类的菌株尿素水解速率会有差异,进而导致在一定时间内的碳酸钙产出量不同。在MICP加固土体方面,巴氏芽孢杆菌被认为是最具有潜力的尿素水解细菌,具有产酶能力高、比表面积高、环境适应性强等优点[26 − 27]。此外,细菌浓度对MICP固化效果也具有重要影响。Okwadha等[28]的研究结果表明,细菌浓度与尿素分解速率、碳酸钙生成量分别呈正相关。多数研究认为,菌液浓度增大,碳酸钙生成量增加,固化效果显著提升[29 − 30]。然而,高浓度的菌液也可能会导致有限的注/渗入距离,从而影响加固土体的均匀性,进而导致加固效果不显著[31]。此外,菌液浓度还对碳酸钙晶体大小及形态具有调控作用。成亮等[32]研究了细菌浓度与碳酸钙形貌的联系。发现在低细菌浓度下,碳酸钙晶体尺寸较大且呈菱面体或长方体,而在高细菌浓度下,碳酸钙晶体尺寸较小且呈球形(图2)。Wang等[33]进一步研究发现,在低细菌浓度下有助于在早期便形成稳定的碳酸钙晶体(方解石),但需要较长的沉淀时间;而在高细菌浓度下形成的碳酸钙晶体会很快形成不稳定的无定形碳酸钙,再由不稳定状态缓慢过渡至稳定状态,即遵循无定形碳酸钙—球霰石—文石—方解石序列。因此,可以根据细菌浓度与碳酸钙晶体产量及性质的相关性,对不同工况进行相应的MICP加固设计。
胶结液的性质包括胶结液成分和浓度等方面,直接影响着碳酸钙晶体的生成过程。胶结液的基本成分是尿素-Ca2+的混合液。胶结液中钙盐的类型影响着碳酸钙晶体的类型和形态,从而影响固化效果(图3)。在室内试验研究中,通常钙源有氯化钙、乳酸钙、醋酸钙、硝酸钙等。Zhang等[34]发现不同的钙盐可以诱导产生不同大小的晶体,氯化钙处理后形成的碳酸钙晶体类型为表面光滑的六面体方解石,硝酸钙处理后形成的碳酸钙晶体类型为表面粗糙的正六面体方解石,而醋酸钙处理后形成的碳酸钙晶体类型主要是针状文石。Gorospe等[35]研究了氯化钙、醋酸钙、乳酸钙和葡萄糖酸钙为钙源的MICP成矿效果,结果表明,使用氯化钙作为钙源时,形成的碳酸钙晶体形貌与Zhang等[34]的研究结果相一致。当以醋酸钙为钙源时,碳酸钙晶体为片状球霰石,而当以乳酸钙和葡萄糖酸钙为钙源时,生成的碳酸钙晶体为球状球霰石。在一定条件下,以氯化钙为钙源的固化试样稳定性较高,因此也通常被认为是MICP的最佳钙源[36 − 37]。此外,胶结液的浓度也影响着碳酸钙晶体的形成,进而对加固土体的工程性质产生影响。Al Qabany等[38]使用了0.1,0.25,0.5 ,1.0 mol/L 4个浓度的尿素-氯化钙混合溶液固化砂样,探究了不同胶结液浓度影响下的固结砂样强度及渗透特征,结果表明使用低浓度胶结液加固的土样具有更高的强度,另外,使用高浓度胶结液加固的土样,其渗透性仅在试验前期能快速降低,相较之下使用低浓度胶结液加固的土样渗透性降低速率更均匀,这也与Soon等[39]的研究结果相一致。Mujah等[40]发现,较低的胶结液浓度(0.25 mol/L)和较高的细菌浓度配比,或较高的胶结液浓度(0.5 mol/L或1 mol/L)和较低的细菌浓度配比,对土体强度的影响更大。因此,选择合适的胶结液浓度及胶结液/菌液的配比对MICP固化效果有重要影响。
2.2 pH和温度
以pH、温度为代表的环境因素对细菌的生命活动、碳酸钙沉淀及胶结过程具有重要影响。把pH值设定在合适范围内,可以有效地提高脲酶活性,促进尿素水解反应。Yi等[41]研究发现,在pH值为7~9时,巴氏芽孢杆菌脲酶的活性随着pH值的升高而增大,脲酶呈高活性,但在pH≥9时,脲酶活性下降。Zheng等[42]的研究表明,初始pH值为9~10时,细菌生长良好,当pH值达到13时,细菌几乎不能生长。Lai 等[43]探究初始pH值为4~8时,MICP固化砂柱中碳酸钙晶体的沉积及分布情况,结果表明当pH值为4时,不会发生尿素水解;当pH值控制在4~7时,pH值对碳酸钙产量及纵向的分布情况影响不大(图4)。pH值的变化还会对碳酸钙的沉淀量产生影响。袁亮[44]的研究发现,当初始pH值为8.5时,碳酸钙晶体的生成效率较高。Cheng等[45]研究了pH值在中性、酸性、碱性条件下形成的固化样中碳酸钙生成量及无侧限抗压强度特征,结果发现相比于中性条件下,酸性和碱性条件下生成的固化样虽有较高的碳酸钙生成量,但强度性能低,固化效果差。考虑常用的产脲细菌适宜在碱性环境中生长,MICP加固技术最佳pH值应控制在7~9之间的偏碱性环境。
温度也是影响MICP加固效果的关键因素之一。适宜的温度有助于提高微生物的代谢和脲酶的活性,从而促进碳酸钙的形成。Sun等[46]研究了温度在15~30 °C范围下巴氏芽孢杆菌和巨大芽孢杆菌对碳酸钙生长方式、酶活性及沉淀速率的影响,研究表明温度升高会促进细菌生长,提高脲酶活性、增大碳酸钙沉淀率。彭劼等[47]对10~25 °C环境下MICP加固效果进行了研究,结果发现温度越高,加固土体强度越高,渗透性越低。Wang等[48]探究了在4 ,20 ,35 ,50 °C条件下MICP的固化效果,结果表明从4 °C升高到35 °C时,碳酸钙晶体产出量及尺寸逐渐增加,但在50 °C时,碳酸钙晶体产出量及尺寸显著下降。此外,在35 °C条件下处理的样品的峰值强度和残余强度最高,其次是20 °C,而较低和较高温度下处理的样品强度较低(图5)。Cheng等[45]在25,50 °C 2 个温度条件下进行了MICP固化试验,结果表明25 °C下的固化试样具有更高的无侧限抗压强度值。综上所述,MICP加固技术最适宜的温度范围应控制在20~35 °C之间,这个范围接近自然气候的温度范围,也显示了MICP在自然环境工程中应用的巨大潜力。
2.3 土的性质
土的性质是影响MICP固化效果的关键因素。MICP加固过程中需要考虑土的性质,特别是土颗粒粒径、分布、相对密度、饱和度等因素。Rebata-Landa[49]使用MICP方法对11种不同颗粒粒径的土进行了固化试验,研究了碳酸钙产出量随颗粒粒径的变化规律,结果表明在粒径约为100 μm的细砂样品中,产生了碳酸钙沉淀量的最高峰值,而高岭土、粗砂以及砾石的样品出现了未固结或固结效果较差的现象(图6)。Liang等[50]对4种不同级配的砂进行了MICP固化试验,探究了不同级配砂样固化后的力学性能,结果显示级配良好的砂样具有较高的抗压强度和较低的渗透系数,在碳酸钙用量相同的情况下,细砂的单轴抗压强度高于粗砂。研究表明细菌直径约0.5~3 μm,过小的土颗粒会限制细菌的自由迁移,导致孔隙堵塞,进而引起碳酸钙的不均匀分布,而较大的土颗粒(如粗砂、砾石)导致孔隙过大,会减弱土颗粒与碳酸钙之间的结合[51]。Mahawish等[52]研究发现,向粗颗粒中加入细颗粒可以增加颗粒间的有效胶结,进而影响固化试样的力学性能。因此,在实际工程中可以通过优化土的粒径级配来提高MICP固化效果。
此外,土的相对密度和饱和度也是影响MICP固化效果的重要因素。Qabany等[53]探究了石英砂的相对密度与MICP固化试样无侧限抗压强度的关系,发现石英砂的固化强度与其相对密度呈正相关关系。Rowshanbakht等[54]的研究结果表明,随着压实度的增加,试样单轴抗压强度增加,渗透系数降低,但是较高的压实度也可能由于土体孔喉变小,造成加固效果的不均匀。Cheng等[55]研究了不同饱和度条件下生物胶结砂土的土体特性(图7),结果显示在完全饱和(Sr=100%,Sr为土体饱和度)条件下,产生的碳酸钙晶体比低饱和(Sr=20%)条件下更多;然而,由于在较低饱和度条件下颗粒间接触点形成的碳酸钙晶体更为密集,因此低饱和度条件下生成的胶结试样具有更高的强度。
2.4 固化方法
固化方法是指将选定的菌液和胶结液通过一定方式加入到土体的方法。根据不同的加固目的采用合适的固化方法,不仅有助于提高碳酸钙晶体的产出量,还能增加加固试样的均匀性,对MICP技术固化的效果具有重要影响。目前,常用的固化方法主要有:浸泡法、表面喷淋法、拌和法、注入法等。
浸泡法是指将样品完全浸泡在微生物培养基或胶结液中,利用自然渗透作用逐渐在试样中生成碳酸钙沉淀的方法。Li等[56]对比了不同浸没次数对MICP技术加固试样抗压强度的影响,结果表明多次浸没的试样具有更高的抗压强度。喷淋法是在土体表面喷洒细菌溶液和胶结溶液,溶液通过重力和毛细管力渗入。Dagliya 等[57]采用喷淋法进行了MICP固化钙质砂研究,试验结果表明喷淋法可以有效地促进沙漠土颗粒间碳酸钙晶体的形成,缓解风蚀作用。Qian等[58]对浸泡法、喷淋法、刷层法3种方法进行MICP处理后的水泥砂浆体表面进行了扫描电子显微镜分析,结果表明3种方法均产生了碳酸钙晶体的析出,但是每个方法形成碳酸钙晶体的形貌存在差别:浸泡法生成的碳酸钙晶体呈不规则球体,尺寸较大(直径可达100 μm);喷涂法生成的碳酸钙晶体尺寸相对较小,平均直径为10 μm左右;刷层法生成的碳酸钙晶体尺寸最小且基本集中在表层(图8)。拌和法是先将菌液与土颗粒预混合,然后通过浸泡或者注入的方式将胶结液加入到试样中的一种固化方法,该方法一般适用于细粒土。郭红仙等[59]采用拌和法对钙质砂进行固化,探究了固化样的一维固结特征,结果表明固化后钙质砂样的压缩性显著降低。Lu等[60]通过一系列试验研究了制样方法和注浆工艺对MICP型尾砂加固效果的影响,结果表明将菌液与土颗粒预混合,能够避免土体内部细菌分布不均匀的情况,从而强化加固效果。注入法是将细菌溶液和胶结液以一定方式注入土体中形成固化体的方法。常用的注入法有单相法、两相法、分段多次注入法等。Shahrokhi-Shahraki等[61]对单相注入、两相注入和分段多次注入3种方法处理后的石英砂固化样进行了比较分析,结果表明分段多次注入法固化效果最好。崔明娟等[62]研究了单一浓度和多浓度相结合的注入方法对砂土无侧限抗压强度的影响,结果表明多浓度相结合的处理方式能够以较少的灌浆次数获得较高强度的试样。此外,注浆压力[63]、注浆流速[64]、处理时间[65]及次数[66]等因素也会对MICP固化效果产生影响。
通过对MICP加固技术影响因素进行总结分析,大致得到如下认识:(1)不同种类的菌株,矿化能力不同,巴氏芽孢杆菌被认为是最具有潜力的尿素水解细菌。胶结液中钙盐的类型影响着碳酸钙晶体的形貌特征,其中氯化钙被认为是MICP的最佳钙源。菌液及胶结液的浓度对脲酶活性、碳酸钙生成量及性质具有调控作用,将菌液/胶结液进行多浓度搭配能够取得较好的加固效果。(2)环境因素的影响主要体现在脲酶活性及碳酸钙生成量及晶体大小方面,对碳酸钙晶体类型的影响不大。MICP加固技术最佳的pH值应控制在7~9之间的偏碱性环境,最适宜的温度范围应控制在20~35 °C之间。(3)MICP加固土体的有效粒径为10~
1000 μm之间,可以通过优化颗粒级配,适当地增加压实度及降低饱和度来增强MICP的加固效果。(4)根据不同固化目的,可以选择不同的固化方法。表面喷淋法和刷层法形成的碳酸钙晶体多集中在表层位置,因此多用于文物遗迹修复及防风固沙。拌和法和浸泡法通常被结合起来使用,固化样均匀性比较好,但由于现场操作难度较大,目前多用于室内试验研究。注入法是目前室内及现场试验中使用最普遍的方法,使用分段多次注入法和多浓度相结合注入法能够获得较好的加固效果。3. MICP加固技术的应用
目前,MICP加固技术在工程地质领域的研究主要集中在土体固化、抗裂防渗、防风抗蚀、污染水/土修复等方面。
3.1 土体固化
MICP加固技术利用微生物诱导碳酸钙沉淀,通过充填胶结土体孔隙,增强土体的力学性质及稳定性,在地基、边坡等土体的加固等方面具有显著潜力。目前,MICP固化土体类型多以砂土、砾石居多。Star 等[67]前后通过开展室内单元试验、模型试验、现场试验,利用MICP加固技术对砾石进行了固化,结果表明MICP固化后的砾石具有足够的强度和稳定性,证实了MICP用于实际工程的可行性。Cui等[68]通过排水三轴试验和扫描电子显微镜观察,多方面分析了MICP固化砂土的强度特性,试验结果表明,沉淀的碳酸钙能有效填充粒间孔隙,黏结相邻颗粒,从而提高钙质砂的剪切强度。Sharma等[69]对在不同冻融循环条件下,MICP固化河砂的剪切强度和剪切模量进行了研究,结果发现即使在发生20次冻融循环的情况下,所有试样均能保持较高的抗液化能力。此外,对黄土[70]、红黏土[71]、膨胀土[72]等特殊性土的MICP固化研究也逐渐开展起来。
3.2 防渗与补裂
微生物诱导碳酸钙沉淀作用会使碳酸钙晶体填充土体孔隙,从而改善岩土体的孔隙率和渗透性。目前,MICP加固技术在防渗与补裂研究方面主要涉及了砂土渗透性处理,细粒土、岩体、混凝土裂隙的修复等方面。Yang等[73]利用MICP方法在砂土中建造了一个蓄水池,砂土经MICP表面处理后,形成一层具有低渗透性、高抗弯强度的不透水层。Wu等[74]研究了生物浆液对岩石裂隙的抗渗效果,研究表明MICP处理后2 d 试样的渗透率降低了3个数量级。Liu等[75]开展了MICP技术修复黏性土裂隙的试验研究,结果表明黏性土裂隙参数随着MICP处理次数的增加而显著减小。Sun等[76]利用MICP 方法修复了混凝土裂隙(0.05~0.15 mm),使试样的无侧限抗压强度提升了20%以上,且裂隙修复效果受入渗深度影响,裂隙越宽入渗越深,修复效果越好。
3.3 抗侵蚀处理
侵蚀作用是指波浪、潮流、风等外营力对地表或建筑物表面造成的磨损和破坏。这种作用可以导致河海岸的破坏、沙漠的扩展、建筑物的腐蚀和损坏等。微生物诱导碳酸钙沉淀形成的致密结构,使MICP固化体能够更有效地抵御外部冲击力,表现出较强的抗侵蚀性。Devrani等[77]通过风洞试验对比研究MICP加固土体的抗风蚀能力,风蚀试验结果表明,MICP处理后的土体质量损失率大幅下降,证明了MICP固化对抗风蚀的高效性,证实了MICP 技术在防风固沙方面的应用前景。Clarà Saracho等[78]对MICP处理后的细砂进行了冲蚀试验,结果表明MICP固化能够有效改善细砂的抗冲刷性能,且抗冲刷的有效性与碳酸钙的产出量及微观结构特征关系密切。Wang等[79]通过渗透试验和降雨侵蚀试验对MICP技术处理后生成的土体结壳的侵蚀情况进行了评价,结果表明MICP处理后使土体渗透率和侵蚀率分别降低了90.9%和95.2%。
3.4 污染水/土修复
重金属是存在于土、水和空气环境中的微量元素。随着社会经济的发展,由工业活动产生的重金属污染问题越来越严峻。MICP技术能够有效修复自然环境中的重金属污染,并且在修复土体污染的同时改善土体性质。Zhao等[80]比较了MICP技术和生物吸附法去除镉的效果,发现MICP技术去除镉的效率比生物吸附法高11%,这表明MICP技术是一种高效、环保的原位修复技术。Kim等[81]从重金属污染土和尾矿中分离出3株本地菌株,通过一系列试验,证明了这些菌株可以有效通过水解尿素酶沉淀土体和尾矿中的重金属,并在重金属的稳定化过程发挥显著作用。Zeng等[82]开展了MICP处置污水污泥的试验研究,发现MICP不仅阻止了离子态镉从污泥向上清液的转移,还使可交换态铅、镉的溶解态含量分别减少了100%和48.54%,残留态含量分别增加了22.54%和81.77%,证实了产脲酶芽孢杆菌ML-2在污泥处理中的优良性能。
4. MICP加固技术存在的问题
近些年来,基于MICP加固技术的研究已经积累了大量的研究成果,显示了MICP技术的广泛应用前景。尽管如此,MICP技术到成熟应用尚存在一些问题需要进一步研究讨论。
4.1 固化均匀性问题
目前,MICP技术的加固效果受到多种因素的制约,其中固化效果在空间上分布不均匀是普遍存在的问题。微生物矿化生成的碳酸钙总是倾向在注浆口附近优先沉淀,因此对深部土体加固效果不理想。目前也有较多研究是从降低pH、降低温度、调整灌浆方式等手段,探索提高MICP矿化均匀性的方法。尽管如此,但这些研究多局限于室内试验,在大尺度及现场试验中能否适用是MICP加固技术面临的一大挑战。
4.2 土体耐久性问题
MICP加固技术的耐久性也是需要关注的问题。一方面,MICP固化土体存在脆性破坏特征。尽管已有学者将MICP加固技术与不同技术手段(如添加纤维[83]、聚合物[84]等)结合来改善固化体脆性破坏特征,但相关研究仍显不足,需要更多的试验验证和理论探讨,以找到最有效的改进方法。另一方面,在实际工程中冻融循环、干湿循环、加载循环等因素会影响MICP加固体的耐久性。然而,关于这些环境因素对MICP固化土耐久性的影响机理和长期稳定性影响的研究还相对不足。因此,有必要加强对MICP加固体耐久性试验及机理研究,以全面评估其长期稳定性和耐久性。
4.3 经济效益问题
与传统的土体加固方式相比,MICP加固技术在成本上并未表现出明显的优势。这是因为固化反应物(包括碳源、钙源和微生物)在培养制备、储存和施工工艺等方面需要昂贵的成本,这也是限制MICP加固技术大规模应用的主要原因。基于经济成本和时间成本的考虑,一些学者已经开始探索解决方案,例如探索寻找低成本的营养液[85]、碳源[86]和钙源[87],激发土体中原有细菌的活性[88]等方法。因此,降低原料成本,寻求经济环保的MICP加固方法,也是未来需要改进和突破的方向之一。
4.4 环境安全与可持续性问题
环境安全与可持续性问题主要体现在以下几个方面:首先,MICP加固技术大多数的研究方法是向土体中引入特定的细菌菌株,这可能会导致本地微生物种群发生重大变化,影响土体生态系统的平衡和稳定性;其次,MICP加固技术还会改变土体的孔隙结构和渗透性,从而影响土体的持水性和透气性,进而影响土体生态系统的功能性;最后,MICP过程中使用的尿素水解会产生氨副产物,这可能对环境造成伤害;为解决这些问题,还需要进行大量的室内和现场试验来检验和优化MICP加固技术。
5. 结语
(1) 基于脲酶水解的MICP加固技术的研究获得了较广泛的关注,其矿化机制是尿素水解产生的碳酸根离子与溶液中的钙离子发生反应,生成碳酸钙沉淀的过程。而MICP的加固机理实际是在矿化作用基础上产生的碳酸钙沉淀对岩土体的填充作用、覆膜作用和胶结作用。
(2) MICP固化效果的影响因素主要有:菌液及胶结液的性质、pH值、温度、土体类型、固化方法等。以上影响因素均可以通过影响碳酸钙晶体的沉淀量、晶体形貌、分布均匀性及胶结特性对加固土体工程性质产生影响。目前,巴氏芽孢杆菌和氯化钙被广泛认为是最佳的尿素水解细菌和钙源。一般认为,MICP的最佳环境条件为:pH值7~9,温度20~35 °C,有效土体粒径为10~
1000 μm。通过多种方法,如菌液/胶结液的多浓度搭配、颗粒级配的优化、压实度的增加、饱和度的降低以及分段多次处理,可以提高MICP加固效果。(3) MICP技术在岩土工程和环境地质领域的研究非常广泛,特别是在土体加固、抗裂防渗、防风抗蚀、污染水/土修复等方面,MICP技术都展现出了巨大的潜力。MICP加固可以提高土体的强度和稳定性,控制裂隙发展,降低土体渗透性,增强土体抗侵蚀性,修复受污染的水土,为工程地质领域提供了新的技术手段。
(4) MICP技术在成熟应用过程中仍存在一些问题,包括固化均匀性、土体耐久性、经济效益、环境安全与可持续性等方面。这些问题需要未来进一步综合微生物学、土力学、材料科学、环境科学等多个学科进行深入研究。
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