Influences of redox potential on leaching behaviors of arsenic from a solidified contaminated soil
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摘要: 砷（As）的价态和迁移性与氧化还原电位（EH）密切相关，固化/稳定化后的As污染土处于复杂的氧化还原环境中，EH的变化可能导致固化体中As浸出风险增加，目前对EH变化情况下固化体中As浸出行为的研究尚不多见。通过半动态浸出试验研究了稳定化/固化修复后的高浓度As(Ⅲ)污染土在不同氧化还原条件下的As浸出行为，通过X射线衍射图谱（XRD）和X射线光电子能谱（XPS）研究了浸出试验前后土中矿物成分和As价态的变化，揭示了相关机理。结果表明，浸出液中As的浸出总量与浸提液EH密切相关，浸提液EH越低，As浸出浓度越高；当浸提液EH为0 mV时，As的扩散系数达到3.11×10−13 m2/s，可浸出指数达到了8.72，表明修复后的高浓度As(Ⅲ)污染土不适合在强还原性环境中堆放和再利用；XRD分析表明，随着浸提液EH降低，土中的铁氧化物/氢氧化物发生还原溶解，进一步导致As的解吸附；XPS分析表明，随着浸提液EH降低，土中的As(Ⅴ)被还原为高迁移性的As(Ⅲ)，从而增加了As的潜在移动风险。该研究为高浓度As(Ⅲ)污染土的长期安全处置提供了科学依据。Abstract: The valence state and mobility of arsenic (As) are closely related to redox potential (EH). Changes in EH may lead to an increasing mobility risk of As in the stabilized/solidified soils when exposed to complex redox environments. At present, the influences of EH on the leaching behaviors of As from the solidified soils have received few attention. In this study, the semi-dynamic leaching tests under different EH are carried out to investigate the leaching behaviors of As from a stabilized/solidified heavily As(Ⅲ)-contaminated soil. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to study the changes of mineral composition and the valence state of As. The results show that the total leached As in the leachate is closely related to EH of the leachant. The leached As concentration increases with a decrease in EH. When EH of the leachant is 0 mV, the diffusion coefficient of As reaches 3.11×10−13 m2/s, and its leachability index reaches 8.72, indicating that the treated heavily As(Ⅲ)-contaminated soil is not suitable for stacking and reuse under a strong reducing condition. The XRD analysis shows that as the leaching EH decreases, the reductive dissolution of iron oxides/hydroxides occurrs, which leads to the desorption of As. The XPS investigations indicate that as the leaching EH decreases, As(Ⅴ) in the soil is reduced to highly mobile As(Ⅲ), and the potential mobility risk of As increases. This study provides a scientific basis for the long-term safe disposal of heavily As(Ⅲ)-contaminated soils.
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0. 引言
随着环境温度周期性变化,季节性冻土在反复冻融循环过程中,内部孔隙水随之发生上下迁移、相变,土中孔隙数量和大小将随着孔隙水的冻结与融化而发生变化,从而引起土体内部微观结构发生改变,导致土体出现冻胀融沉。冻融循环是一种连续不断的强风化过程,不断破坏着土的结构性,是引起土体强度参数降低,产生劣化的重要原因[1]。为克服冻融循环对土体性质的劣化影响,工程中通常采用不同的方法对公路地基土进行改良。改良方式主要可以分为物理、化学和综合改良3种[2]。物理改良法主要是在土体中加入土工织物[3]以及聚合物纤维等[4]；或是通过掺加炉渣、砂、碎石等粗颗粒来改变其颗粒级配以增强土体的物理、强度特性[5]。化学改良方法目前采用较多,在土体中外加水泥[6]、石灰[7]或是化学浆液[8]等,通过改良材料与土颗粒间发生化学反应,从而生成强度较高的新物质,以提高土体的强度和稳定性。综合法则是物理改良与化学改良的结合[9]。
可以发现,上述国内外学者关于冻融循环对改良土的研究虽多,但以往改良材料的选取大多数局限于水泥、石灰、聚丙烯高分子纤维等,并不全面,有待进一步去探索新型改良材料。众所周知,石灰、水泥通常表现出以拉伸开裂为主的脆性破坏模式；此外这种碱性化学材料的掺入,会对周围土壤环境造成严重的污染。随着国家的迅速发展,环境污染日益严重,环保问题已成为阻碍国家发展的重要因素之一,绿色、经济改良土的物理力学性质研究将成为国内外岩土工程领域的一个崭新课题。而木质素纤维作为造纸工业副产品之一,来源广泛、价格低廉,具有经济、环保以及良好耐久性的优点,因此岩土工程领域中木质素纤维改良土具有很好的应用前景。
此外研究表明,土的宏观力学性质从根本上受其微观结构的控制,通过对土微观结构的研究,可以从微观的角度上解释土的力学性质[10]。刘春等[11]基于PACS软件进行裂纹模式分析。PACS软件具有自动分割识别颗粒和孔隙的功能,可以有效地减少人为因素对处理结果的影响,因此本文选用PACS软件对SEM图片进行处理。
本文设计完成了多种掺量的木质素纤维改良土在不同冻融次数后的UU三轴试验,着重分析了不同冻融次数下改良土的应力应变特性、弹性模量、强度指标等力学性质的变化规律。探讨了改良土的抵抗冻融效果并得出最佳掺量。并对不同冻融次数后的改良土进行SEM电镜扫描试验,从微观孔隙、概率熵变化的角度解释冻融对土体宏观力学性质影响的机理。
1. 试验材料及试验方案
1.1 试验材料
试验所用土样为天津滨海新区软黏土,在105℃干燥箱中烘干24 h后,碾散并筛分,按《土工试验方法标准》（GB/T 50123—2019）测得土样基本物理指标见表1。
表 1 土样物理特性指标Table 1. Physical properties of soil sampleswop/% γ/(kN·m-3) Gs wL /% wP /% IP 19.2 18.7 2.67 42.4 22.5 20.4 试验用木质素纤维来自河北石家庄石粉厂。纤维呈絮状,长度1～3 mm,在水中具有良好的分散性。
1.2 土样制备
首先将土样晒干,然后粉碎并过筛,以除去大颗粒杂质。进行三轴试验的关键是确保制备的试样具有良好的一致性,为使纤维与土样充分均匀混合,本文试验的制样方法是：先称取一定质量干土,将不同配比（分别为干土质量的0%,0.25%,0.5%,0.75%,1%）的纤维加入一定量的水中（干土质量的20%）混合,再喷洒入干土中,充分搅拌30 min后,套上保鲜膜密封静置12 h待水分均匀分布后,再将土样分3层填入制样器,压实后削平土样表面。试样最终尺寸：ϕ39.1 mm×80 mm。
1.3 试验方案
冻融试验箱可控温度范围为-25℃～150℃,如图1（a）。将上述制得的土样用保鲜膜密封,形成封闭的冻融环境,防止冻融循环试验中水分的散失和外界补给。根据文献[1, 3, 4]可知,5次冻融后,冻融对土体的物理力学性质影响基本稳定。故本文的冻融次数分别选取0,1,3,5,7,9次,试样冻结、融化温度分别设为-20℃、20℃,冻结、融化时间均为12 h,以保证土样内部完全冻结、融化。
由于软黏土透水性较差,故本文对其进行不固结不排水三轴试验（UU）,试验采用双联三轴应变控制式系统,如图1（b）所示,试验机轴向加载量程选择0～1 kN,三轴围压、反压压力范围0～1 MPa。对经过不同冻融次数后的土样进行UU三轴试验,围压分别选取100,200,300 kPa,剪切速率选取0.5 mm/min,剪切应变达到20%时停止试验。
微观试验采用型号为Quanta250的电镜扫描仪,如图1（c）。电子发射电压为5 kV,接口连接到计算机,可以将扫描图像直接传送到计算机。分别对不同冻融次数后木质素纤维掺量为0%,0.25%,0.75%的改良土进行电镜扫描,扫描倍数设为100,500,1000。
2. 试验结果与分析
2.1 应力-应变曲线
图2为0.75%掺量木质素纤维改良土的应力应变曲线。可以看出,在100 kPa围压下,改良土的应力应变曲线均表现为应变软化型,而在200,300 kPa的围压下,表现为应变硬化型。这体现了围压大小对冻融循环后土体应力应变曲线具有重要的影响。在较高围压下,由于侧限的存在,随机分布在土体中的纤维构成了三维网架结构,加强了土中潜在的薄弱面,减小土中裂隙的延伸和发展,提高土样塑性。同时高围压还会增强了土颗粒间的联锁力,增加了相对滑移面（即剪切面）上的正应力,从而减小了冻融过程中产生的裂缝和裂隙,围压的压实作用增强了土体的变形抗力。
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图 2 冻融次数对0.75%纤维改良土应力应变曲线影响Figure 2. Effects of freeze-thaw cycles on stress-strain curves of 0.75% lignin fiber-reinforced soils此外未经过冻融的土体体现出更强的应变软化特性,随着冻融次数的增加逐渐由应变软化型向硬化型转变,应力应变曲线的平面位置随冻融循环次数的增加而不断下降。这主要是冻融对土体有一个结构重塑效应,土体在冻结时,内部水结冰形成的冰晶体在土体内部产生冻胀力,当冻胀力超过土颗粒间的黏结力时,增大土颗粒间的微孔隙,导致土颗粒结构发生破坏,重新排列,从而在宏观上应力幅值随冻融循环次数的增加表现出逐渐劣化衰减的趋势。
图3绘制了不同木质素纤维掺量改良土在100,300 kPa两种围压下的应力应变曲线。在100 kPa下,随着木质素纤维掺量的增加,应力应变曲线逐渐呈现出应变软化特性。这是由于絮状的木质素纤维在加入水中时,生成的浆体有一定的胶结固化特性,改良土产生了一定的脆性,表现出应变软化的特点。而在300 kPa围压下,由于高围压对土体的塑性提升大于胶结固化产生的脆性,所以表现出更强的应变硬化特性。
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图 3 不同木质素纤维掺量对改良土应力应变曲线的影响Figure 3. Effects of fiber content on stress-strain curves of reinforced soils2.2 弹性模量
土体弹性模量的确定通常是取1%轴向应变时偏应力增量与轴向应变增量之比[12]：
E=ΔσΔε=σ1.0%−σ0ε1.0%−ε0, (1) 式中 E为弹性模量；
Δσ 为应力增量；Δε 为应变增量,σ0 和ε0 分别对应初始应力和初始应变。根据式（1）,计算得到不同木质素纤维掺量在不同冻融次数后弹性模量的变化曲线,由于篇幅有限,仅列出100 kPa围压下的结果,如图4。冻融循环会使弹性模量大幅度减小,第1次冻融循环后的弹性模量降低幅度较大。而木质素纤维的掺入使弹性模量产生了不同程度的提高,其中,木质素纤维掺量为0.75%时改良土的弹性模量最大。
为探究在同一冻融次数下不同木质素纤维掺量抵抗冻融劣化的效果,定义弹性模量的衰减幅度
DE 为DE=EN−E0E0×100%, (2) 式中,
EN 为N次冻融后的弹性模量,E0为未冻融的弹性模量。经过上式计算,冻融9次后,不同木质素纤维掺量改良土弹性模量的衰减幅度见表2。在200,300 kPa围压下,0.75%掺量的改良土弹性模量衰减幅度分别为53.55%,47.33%,均为同一围压下的最低值。在100 kPa围压时,衰减幅度为58.74%,仅比0.25%掺量的改良土大,因此综合来看,可以说明0.75%木质素纤维改良土抵抗冻融对弹模劣化影响的效果最好。
表 2 改良土9次冻融后的弹性模量衰减幅度Table 2. Attenuation amplitudes of elastic modulus of reinforced soils after 9 freeze-thaw cycles(%) 纤维掺量/% 围压 100 kPa 200 kPa 300 kPa 0 -58.96 -54.38 -62.47 0.25 -52.13 -56.02 -58.48 0.50 -64.86 -60.64 -68.72 0.75 -58.74 -53.55 -47.33 1.00 -59.59 -56.73 -47.69 2.3 破坏强度
对于应变硬化型,取15%轴向应变对应的偏应力作为其破坏强度,对于应变软化型,取其峰值应力作为破坏强度。据此可以得到围压100 kPa下木质素纤维改良土的破坏强度,如图5所示,将改良土经9次冻融循环后的破坏强度衰减值列于表3。
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图 5 冻融次数对不同纤维掺量改良土破坏强度的影响Figure 5. Effects of freeze-thaw cycles on failure strength of soils reinforced by lignin fiber表 3 改良土9次冻融后的强度衰减幅度Table 3. Attenuation amplitudes of strength of reinforced soils after 9 freeze-thaw cycles(%) 纤维掺量/% 围压 100 kPa 200 kPa 300 kPa 0 -34.90 -26.32 -33.02 0.25 -34.35 -21.67 -32.26 0.50 -25.50 -22.83 -22.96 0.75 -26.51 -16.92 -17.16 1.00 -26.33 -17.64 -19.82 由图5可以看出,在同一冻融次数下,当木质素纤维掺量小于0.75%时,强度随掺量的增加而增大,超过0.75%时,强度随掺量的增加反而大幅度减小,呈现出这种现象的原因主要是木质素纤维掺量较多,分布比较集中,在土样内部形成了薄弱面,从而降低了土样强度。0.5%和0.75%掺量的木质素纤维改良土性能最优,二者强度接近,而超过5次冻融后,0.75%掺量的改良土在强度方面的优势开始扩大。
另外,由表3可知,随着冻融次数的增加,土样强度均出现不同幅度的降低。9次冻融后,素土的强度衰减幅度在100 kPa围压下达到最大值34.9%,而0.75%木质素纤维改良土的衰减幅度在200,300 kPa围压下分别为16.92%,17.16%,均为同围压下的最小值,故综合来看,可以确定木质素纤维最佳掺量在0.75%附近。
2.4 抗剪强度指标
根据UU试验得到的应力应变曲线,对3种围压所形成的破坏包络线进行直线拟合,得到抗剪强度指标随冻融循环的变化规律,如图6所示。整体来看,改良土表现出更高的黏聚力和内摩擦角。素土的黏聚力随着冻融次数的增加,呈现出先增加后降低的趋势,内摩擦角随冻融次数的增加而减小7°,变化幅度较大。0.75%木质素纤维改良土的黏聚力和内摩擦角均为最大；且0.75%掺量的改良土随冻融次数的增加,抗剪强度指标变化幅度较小、相对更加稳定。因此也可以说明木质素纤维最佳掺量在0.75%附近。
3. 微观结构分析
100倍SEM图像如图7所示,其中黑色代表孔隙,白色代表土体。可以看出9次冻融后的孔隙明显大于1次冻融后的孔隙,且9次冻融后颗粒有明显的抱团聚拢现象。0.75%木质素纤维改良土的裂隙明显比素土更少。对改良土样局部放大1000倍后得到图8,可以发现纤维在土样内部主要起到了三维网架结构或是“桥梁”搭接作用,加强了土体之间的联结力,从而在宏观上增加了力学性能指标。
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图 7 冻融1,9次后,100倍下改良土SEM照片(左边为原图,右边为二值化处理后图像)Figure 7. SEM photos of reinforced soils under 100 times after 1 and 9 freeze-thaw cycles![]()
图 8 冻融9次,0.75%纤维改良土1000倍的SEM照片Figure 8. SEM photos of 0.75% lignin fiber-reinforced soils 1000 times after 9 freeze-thaw cycles通过PACS图像处理软件对电镜扫描后的图像进行定量分析,可以得到孔隙度、概率熵等参数。
对100倍下的图像进行二值化处理,并降噪分割,可以得到不同冻融次数后的孔隙大小。如图9所示,随着冻融次数的增加,土样内部孔隙率明显增大,在宏观上体现出冻融对力学性质的劣化影响。另外,0.75%木质素纤维改良土相比于其他土样有更小的孔隙度,从而提高了土体的破坏强度,说明了改良效果最佳,这也与前文分析得到的数据结果一致。
概率熵是一个描述颗粒排列的结构参数,可用来分析冻融循环作用后的土颗粒排列情况。从图10中可以看出,随着冻融次数的增加,三种土样的概率熵都逐渐减小。这说明冻融作用对土颗粒排列的定向性有一定的影响,根据概率熵的定义可知,在冻融作用下,颗粒排列由较混乱状态向有序状态发展,也就是说颗粒排列的定向性越来越好。
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图 10 概率熵随冻融次数的变化规律Figure 10. Variation of probability entropy with freeze-thaw cycles这是因为,在冻融循环过程中,外界温度条件的改变使得孔隙水发生相变,形成一定体积的冰晶结构,相邻土颗粒在冰晶体的楔形力挤压作用下,颗粒发生相对位移。此外在低温冻结过程中,由于冻结锋面的存在,土体内部水分有向冻结锋面定向迁移的趋势,为土颗粒的位移提供条件,故而在冻融循环作用下,土颗粒间的相对位移改变了土颗粒的排列方式,使土颗粒排列的定向性越来越好。
此外,还可以发现,0.75%纤维改良土的概率熵要比另外两种土样的小。说明了0.75%纤维改良土中的土颗粒排列在经历冻融循环后更加有序,抵抗冻融的劣化影响更加明显。
4. 结论
本文以木质素纤维改良土为研究对象,开展了一系列冻融循环试验、UU三轴试验和SEM电镜扫描试验,分析总结了冻融次数和木质素纤维掺量对该改良土力学性质变化的影响规律,探讨了木质素纤维改良土的抵抗冻融效果以及微观机制。主要得到以下结论：
（1）随着围压的增大,冻融次数的增加,应力应变曲线逐渐由应变软化型过渡到应变硬化型。随着木质素纤维掺量的增加,应力应变曲线呈现出软化特性；高围压下,此现象不明显。
（2）首次冻融后,弹性模量降低幅度较大,之后降低幅度逐渐放缓。木质素纤维掺量为0.75%时,改良土的弹性模量、破坏强度和黏聚力均达到最大值,内摩擦角变化最为稳定,抵抗冻融劣化能力最强。在实际工程应用中,建议采用0.75%的纤维掺量来改良季节性冻土地区路基土。
（3）从微观图像上可以看出,冻融作用使土体内部微观孔隙增大,导致宏观上力学性能的衰减,概率熵随着冻融的增加而减小。纤维在改良土样内部主要起到了三维网架结构或是“桥梁”搭接作用,减小了冻融对土体孔隙损伤的影响,从而增强了土体的强度。
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图 1 自动控制pH和EH的半动态浸出试验系统
Figure 1. pH and EH automatic control system for semi-dynamic leaching tests
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图 4 半动态浸出试验前后土样的XRD图谱
Figure 4. XRD patterns of specimens before and after semi-dynamic leaching tests
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图 5 半动态浸出试验前后土样的XPS图谱
Figure 5. XPS spectra before and after semi-dynamic leaching tests
表 1 试验用土的金属含量
Table 1 Metal contents in test soil
金属元素 As Ca Fe Mn Al Mg 含量/(mg·kg-1) 8.9 9650 13100 697 7700 2470 
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表 2 EH对As有效扩散系数De和可浸出指数LI的影响
Table 2 Effects of EH on effective diffusion coefficient De and leachability index LI
参数 De(0~6 d)/(m2·s-1) De(> 6 d)/(m2·s-1) LI 0 mV 3.11×10-13 1.18×10-13 8.72 200 mV 4.59×10-14 1.95×10-14 9.52 400 mV 2.10×10-15 1.58×10-15 10.74
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