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石油类污染物的意外泄漏事故频繁发生,导致一定数量的非水相液体 (NAPL) 通过包气带进入地下造成污染[1]。在包气带内挥发性有机物 (VOCs) 会通过挥发作用进入土壤气体,VOCs气体在迁移过程中发生生物降解,源区轻非水相液体 (LNAPL) 由生物降解作用导致的衰减占LNAPL总质量损失的90%~99% [2-3]。因此,挥发性石油烃在包气带中的气相自然衰减和生物降解动力学规律的研究对VOCs蒸气入侵风险评估和石油场地监测自然衰减应用都具有重要意义[4-5],气相生物降解速率是蒸气入侵建模和定量风险评估的关键输入参数[6-7]。
当VOCs在污染源挥发成为气态在包气带扩散迁移过程中,吸附、生物降解等机制会导致显著的质量衰减,这被称为气相自然衰减[8-9]。在某汽油污染场地,从地下水源挥发的总碳氢化合物质量的68%在毛细管层被生物降解,石油烃蒸气潜水面上1 m内包气带内被降解殆尽[10]。HÖHENER等[11]通过微宇宙实验、柱实验及现场研究测定了包气带中13种VOCs的气相生物降解规律。VOCs气相自然衰减受到土壤类型、温度、含水率、营养水平等多种因素影响。土壤类型,特别是土壤孔隙度、含水量、土壤渗透性、有效扩散性和有机碳吸附能力是影响VOCs气相迁移归趋的重要因素[12]。YAO等[13]报道,土壤质地对VOCs浓度衰减的影响,其中土壤粒径的对底板下土壤气衰减因子的影响平均约为0.4个数量级。BEKELE等[14]研究了三氯乙烯 (TCE) 在5种不同的土壤中的气相吸附,粘土质量分数增加1倍会导致TCE蒸气吸附量增加11倍。但是,目前关于包气带VOCs气相自然衰减的研究仍然很不充分,对于不同种类石油烃蒸气在不同土壤中的吸附与生物降解规律以及我国不同区域土壤的气相自然衰减潜力尚不清楚。
本研究采用微宇宙实验,选取4种正构烷烃 (正戊烷、正己烷、正庚烷、正辛烷) 、4种环烷烃 (环戊烷、环己烷、环庚烷、环辛烷) 及4种苯系物 (苯、甲苯、乙苯、对二甲苯) ,对其在黑土、黄土、红土及石英砂等4种土壤中的气相自然衰减规律和生物降解速率进行了系统研究。
石油烃在不同土壤中的气相自然衰减规律
Natural attenuation of petroleum hydrocarbon vapors in different soils
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摘要: 挥发性有机物 (VOCs) 在包气带中的迁移扩散是土壤和地下水中可挥发污染物自然衰减的重要机制,也与蒸气入侵暴露和风险评估密切相关。采用微宇宙实验对12种挥发性石油烃 (正戊烷、正己烷、正庚烷、正辛烷、环戊烷、环己烷、环庚烷、环辛烷、苯、甲苯、乙苯、对二甲苯) 在4种土壤 (黑土、黄土、红土、石英砂) 中的气相自然衰减机制和气态生物降解动力学规律进行了研究。结果表明,正构烷烃、环烷烃和苯系物蒸气在4种土壤中的气相自然衰减去除率都遵循黑土>黄土>红土>石英砂的规律;黑土中生物降解对污染物去除率的贡献高于黄土,而红土和石英砂中的生物降解速率极低;4种苯系物的自然衰减和生物降解潜力远高于正构烷烃和环烷烃;苯系物气相生物降解速率排序为:甲苯>苯>乙苯>对二甲苯。本研究结果可为蒸气入侵定量风险评估和石油污染场地自然衰减速率定量评估提供参考。Abstract: The transport of volatile organic compounds (VOCs) in the vadose zone is an important mechanism for the natural attenuation of volatile pollutants in soil and groundwater and is also closely related to vapor intrusion and risk assessment. The natural attenuation mechanism and biodegradation kinetics of 12 volatile petroleum hydrocarbons (n-pentane, n-hexane, n-heptane, n-octane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, benzene, toluene, ethylbenzene, p-xylene) in four soils (black soil, yellow earth, lateritic red earth, and quartz sand) were studied by microcosm experiments in detail. The results showed that the natural attenuation removal rates of n-alkane, cycloalkane and monoaromatic hydrocarbon vapors in four soils followed the order: black soil > yellow earth > lateritic red earth > quartz sand; the biodegradation contribution in black soil was higher compared to yellow earth, and the biodegradation rates of petroleum hydrocarbon vapors in lateritic red earth and quartz sand were very low; the natural attenuation removal rates and biodegradation contribution of the four monoaromatic hydrocarbons were much higher than n-alkanes and cycloalkane; the biodegradation rates of four monoaromatic hydrocarbons were in the order of toluene > benzene> ethylbenzene > p-xylene. This study provided valuable insights into vapor intrusion risk assessment and natural attenuation rate quantification at petroleum contaminated sites.
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Key words:
- contaminated site /
- soil remediation /
- groundwater remediation /
- petroleum /
- natural attenuation /
- biodegradation /
- unsaturated zone
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表 1 实验土壤的基本理化性质
Table 1. Basic physical-chemical characteristics of experimental soils
土壤类型 pH 孔隙度 含水率 有效扩散系数 (20 ℃)/
(正戊烷,m2·d−1)密度/
(g·cm−1)有机质质量分数/% 土壤质地 机械组成/% 砂粒 粉粒 黏粒 黄土 7.8 0.5 0.02 0.28 1.3 0.66 壤土 49.6 39.6 10.7 黑土 6.1 0.8 0.09 0.80 0.6 36.70 壤质砂土 74.6 19.6 5.8 红土 5.7 0.6 0.02 0.36 1.0 0.33 粘土 11.2 5.4 83.4 石英砂 7.6 0.4 0.01 0.19 1.5 0.32 砂土 97.4 1.8 0.9 表 2 石油烃在不同土壤中自然衰减去除率
Table 2. The remove rates of petroleum hydrocarbons in different soils
% 化合物 黄土 黑土 红土 石英砂 灭菌组 非灭菌组 生物降解
的贡献*灭菌组 非灭菌组 生物降解
的贡献*灭菌组 非灭菌组 生物降解
的贡献*灭菌组 非灭菌组 生物降解
的贡献*正戊烷 24.61 27.82 3.21 42.91 68.18 25.27 21.33 23.06 1.73 3.56 5.08 1.52 正己烷 35.35 45.64 10.29 64.67 87.24 22.57 32.89 36.05 3.16 2.18 3.03 0.86 正庚烷 51.17 74.77 23.61 74.62 93.20 18.58 40.25 45.29 5.04 0.61 1.39 0.78 正辛烷 63.14 78.43 15.28 78.12 96.55 18.44 49.37 46.77 2.60 5.63 7.87 2.24 环戊烷 13.30 33.68 20.38 20.90 43.04 22.14 5.29 6.29 1.00 5.58 9.62 4.05 环己烷 12.96 28.24 15.28 46.68 70.96 24.28 1.75 7.09 5.34 4.29 9.91 5.62 环庚烷 8.23 45.91 37.68 61.22 84.85 23.63 9.88 12.89 3.01 12.67 14.29 1.62 环辛烷 57.60 66.28 8.67 71.90 85.35 13.45 9.14 17.37 8.23 12.56 21.14 8.58 苯 69.62 81.55 11.93 77.66 92.32 14.66 64.88 75.69 10.81 27.15 27.51 0.36 甲苯 72.54 80.57 8.03 82.64 91.98 9.34 67.44 75.63 8.19 26.43 27.00 0.57 乙苯 72.31 79.12 6.81 80.98 85.64 4.66 53.64 56.86 3.23 19.38 19.88 0.50 对二甲苯 75.80 82.81 7.01 77.57 83.36 5.80 51.45 55.65 4.20 19.46 20.00 0.54 注:“*”表示生物降解贡献的去除率为非灭菌组与灭菌组去除率之差。 表 3 石油烃在不同土壤中的一级衰减速率常数
Table 3. The first order attenuation rate constants of petroleum hydrocarbons in different soils
d−1 化合物 黄土 黑土 红土 石英砂 灭菌组 非灭菌组 生物降解 灭菌组 非灭菌组 生物降解 灭菌组 非灭菌组 生物降解 灭菌组 非灭菌组 生物降解 正戊烷 0.03 0.04 0.01 0.03 0.06 0.03 0.02 0.02 — — — — 正己烷 0.03 0.06 0.03 0.05 0.18 0.13 0.016 0.02 0.004 — — — 正庚烷 0.05 0.18 0.13 0.26 0.54 0.28 0.02 0.05 0.03 — — — 正辛烷 0.06 0.15 0.09 0.02 0.12 0.10 0.03 0.05 0.02 — — — 环戊烷 0.01 0.02 0.01 0.04 0.07 0.03 0.01 0.01 — — — — 环己烷 0.01 0.03 0.02 0.20 0.26 0.06 0.01 0.01 — — — — 环庚烷 0.02 0.05 0.03 0.28 0.32 0.04 0.01 0.01 — — — — 环辛烷 0.07 0.09 0.02 0.33 0.39 0.06 0.01 0.01 — — — — 苯 25.68 27.84 2.16 28.24 235.48 7.24 24.24 25.78 1.54 — — — 甲苯 30.72 38.96 8.24 29.36 39.12 9.76 25.68 28.44 2.76 — — — 乙苯 15.6 17.08 1.48 324.98 30.82 5.84 11.76 12.72 0.96 — — — 对二甲苯 14.4 15.58 1.18 23.76 27.66 3.90 11.04 11.94 0.90 — — — -
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