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多环麝香(Polycyclic musks,PCMs)作为重要的人工合成香料被广泛应用于日用品中,在土壤、水、沉积物和生物体等环境介质中均有检出[1]。其中,佳乐麝香(Galaxolide,HHCB)是土壤中检出率和检出浓度最高的典型PCMs之一,在长江三角洲、天津和东北三省土壤中已检测出,检出质量分数最高为7.22 μg·kg−1 [2]。HHCB被用于治疗心肌梗塞、调配香水香皂等,是一类低水溶性、高脂溶性的半挥发持久性有机污染物,在环境中难降解,容易在生物体内富集,具有内分泌干扰性和引起癌细胞增殖和胎畸[3]。由于对人体有雌激素作用与抗雌激素效应,该物质被欧盟列为“潜在人类致癌物”[3-4]。
目前,可用于原位修复半挥发性有机污染土壤方法主要有热脱附、气相抽提、电动修复和超临界流体修复等[5-7]。热脱附中的原位电阻加热技术(electrical resistance heating,ERH)在土壤中安装三相、六相电极后将土壤加热,其热量产生于土壤内部,相较于常规修复技术具有加热相对均匀、地质条件适应性强、对土壤扰动小、修复彻底和不引入外源性污染等优点[8-9]。ERH早期被应用于采油技术,随着应用的需要逐渐被引入到土壤和地下水污染修复中[10]。ERH是基于欧姆定律,将电能转化为热能,提升土壤温度,使部分挥发和半挥发性有机污染物与水溶液发生共沸,最终通过气相抽提将污染物转移并处置[11-12]。影响土壤ERH的因素包括土壤含水率、电场强度、土壤粒径以及加热时间等[12-15]。HAN等[13]和HAN等[16]发现,水分是ERH关键因素;没有足够的水分,即使电场强度在8 V·cm−1时,土壤也难以被加热到水沸点。田垚等[17]提出,在初始含水量充足的条件下,高电场强度使土壤快速升温,8 V·cm−1的电场强度能将15 g 土壤于5 min 内加热至水沸点,而2 V·cm−1电场无法使土壤加热,在8 V·cm−1电场强度下加6 mL0.1%NaCl,每30 min补水6 mL,苯并(a)芘去除率为51.56%。FU等[18]的研究表明,土壤中污染物脱附效率随粒径增加而增大,粒径为小于75 μm、75~125 μm、125~250 μm、250~425 μm土壤颗粒的多溴联苯醚脱附效率分别为49.53%、73.88%、83.56%和87.09%。李晓雅等[19]采用Design-Expert响应曲面法优化热强化土壤气相抽提技术的影响参数,考察通气速率、土量和水蒸气浓度单独变量和交互作用,发现单因素变量、通气速率与土量的交互项均对烃类污染物的去除速率有显著影响。目前,已发表的文献中仅对ERH处理PAHs污染土壤进行了初步探索,对性质相似的半挥发性有机污染物PCMs的研究鲜有报道,而且,ERH去除过程中的影响因素及脱附规律等尚未明晰。
因此,本研究使用自主研制的电阻加热装置,研究电场强度、含水率和土壤粒径对人工模拟HHCB污染土壤去除效果的影响,并采用Box-Behnken响应曲面法建立各因素与土壤HHCB去除率之间的回归模型,通过对响应曲面图和等高线图的分析,研究ERH修复HHCB污染土壤中各影响因素单独及交互作用,得到最优工艺参数,以期为电阻加热修复技术的工程应用提供理论和实践指导。
电阻加热修复佳乐麝香污染土壤的工艺优化
Optimization of Response Surface Process for Remediation of Galaxolide Contaminated Soil by Electric Resistance Heating
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摘要: 佳乐麝香(Galaxolide,HHCB)是土壤中的一种新兴的半挥发性有机污染物,具有较高的毒性,需修复治理。电阻加热技术(electrical resistance heating,ERH)因加热均匀、效果好,逐渐被应用于有机污染土壤修复工程。采用自主研制的电阻加热装置,研究了加热过程中电场强度、土壤含水率和土壤粒径等因素对土壤HHCB去除效果的影响,并采用响应面法对工艺参数进行了优化。结果表明,土壤 HHCB去除率随电场强度和含水率的增大先增加后减小,随土壤粒径的增大而增大;在土壤含水率为40%、土壤粒径1~2 mm和电场强度12 V·cm−1的条件下,加热6 h,HHCB去除效果最好,去除率达到了81.35%。通过响应曲面模拟优化得到的最佳工艺参数条件为,含水率39.07%、土壤粒径2.92 mm、电场强度12.64 V·cm−1,在此条件下土壤HHCB的去除率为86.43%;在本实验条件下,3个因素对土壤HHCB去除率的影响为:含水率>土壤粒径>电场强度。本研究结果可为多环麝香污染土壤的修复提供参考。Abstract: Galaxolide(HHCB) is an emerging semi-volatile organic pollutant in soil, it has high toxicity and therefore needs remediation. Due to its high efficiency in achieving homogeneous soil heating, electrical resistance heating (ERH) technology has been more and more applied in organic polluted soil remediation. In this paper, the influence of electric field intensity, water content, and soil particle size on the ERH removal efficiency of HHCB was investigated using a self-made device, the operation parameters were optimized following the response surface methodology. The results showed that HHCB removal rate first increased and then decreased with the increment of electric field intensity and water content, and increased with the increase of soil particle size. Best removal rate 81.35% was achieved after ERH treatment for 6 h, under the conditions of soil water content 40%, soil particle size of 1~2 mm, and electric field intensity of 12 V·cm-1. The utilized parameters obtained by response surface simulation were as following: water content 39.07%, soil particle size 2.92 mm, and electric field intensity 12.64 V·cm-1, the estimated removal rate of HHCB was 86.43%. Under the experimental conditions, the effects of the three factors on the ERH removal rate of HHCB were as following: water content > soil particle size > electric field intensity. These results provide a new approach for polycyclic musk contaminated soil remediation.
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表 1 实验土壤基本理化性质
Table 1. Basic properties of experimental soil
pH TOC 有机质/% 总N/% 总P/% 总K/% 粒径分布 黏粒(<0.002 mm)/% 粉粒(0.002~0.02 mm)/% 砂粒(0.02~2 mm)/% 9.01 1.75 1.38 0.11 0.68 20.1 2.99 14.52 82.49 表 2 单因素实验方案
Table 2. Scheme of single factor experiment
序号 电场强度/(V·cm−1) 含水率/% 土壤粒径/mm 1 8 30 1~2 2 10 30 1~2 3 12 30 1~2 4 14 30 1~2 5 16 30 1~2 6 12 10 1~2 7 12 20 1~2 8 12 30 1~2 9 12 40 1~2 10 12 50 1~2 11 12 30 <0.25 12 12 30 0.25~0.5 13 12 30 0.5~1 14 12 30 1~2 15 12 30 2~3 表 3 响应曲面法的影响因子编码和水平
Table 3. Level and code of experimental variables based on response surface methodology
因素 编码 水平 −1 0 +1 电场强度 X1 10 v·cm−1 12 v·cm−1 14 v·cm−1 含水率 X2 20% 30% 40% 土壤粒径 X3 <1 mm 1~2 mm 2~3 mm 表 4 响应曲面实验工况
Table 4. Response surface experimental conditions
序号 X1/ (V·cm−1) X2/% X3/mm Y1/% 1 14 40 2 78.9 2 10 20 2 55.0 3 14 30 3 71.5 4 12 30 2 66.9 5 12 20 3 47.7 6 12 30 2 69.5 7 14 30 1 56.0 8 12 20 1 46.3 9 10 30 1 49.2 10 12 40 1 58.4 11 12 40 3 86.0 12 12 30 2 68.0 13 12 30 2 68.7 14 14 20 2 43.9 15 10 40 2 66.1 16 12 30 2 69.0 17 10 30 3 68.5 表 5 响应曲面二次模型方差分析
Table 5. Response surface quadric model analysis of variance
来源 平方和 自由度 均方和 F值 Prob>F 回归模型 2228.36 9 247.60 92.26 < 0.000 1** X1 16.53 1 16.53 6.16 0.042 1* X2 1164.03 1 1164.03 433.75 < 0.000 1** X3 508.80 1 508.80 189.59 < 0.000 1** X1 X2 142.80 1 142.80 53.21 0.000 2** X1 X3 3.61 1 3.61 1.35 0.284 1 X2 X3 171.61 1 171.61 63.95 < 0.000 1** X12 34.74 1 34.74 12.95 0.008 8** X22 88.03 1 88.03 32.80 0.000 7** X32 75.96 1 75.96 28.31 0.001 1** 残差 18.79 7 2.68 − − 失拟项 14.72 3 4.91 4.82 0.081 3 纯误差 4.07 4 1.02 − − 总离差 2247.14 16 − − − 注:“**”表示该项极显著(P<0.01);“*”表示该项显著(P<0.05);“−”表示此项无数值。 -
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