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页岩气是一种比传统化石燃料更清洁的非常规天然气,其开发利用可缓解天然气的供需矛盾,提高国家能源安全,促进能源战略转型[1]. 中国拥有丰富的页岩气资源,储量大约32万亿 m3[2],随着自主开发技术的发展,中国已成为世界上第三个页岩气开发商业化的国家[3],然而快速发展的页岩气开采行业引起了人们对其潜在环境影响的忧虑[4]. 国内外对页岩气开采环境风险的研究主要集中在压裂返排液的潜在影响,发现压裂返排液可能对周边水环境产生不利影响,也可能污染周边农田,影响农作物生长[5 − 14]. 中国超过65%的页岩气资源埋藏深度大于
3500 m,地层环境复杂,为降低开采事故发生率,降低开采成本,在压裂液外,也广泛使用高稳定性、耐高温性、润滑性的油基钻井液[15 − 17]. 油基钻井液含高浓度的烷烃、多环芳烃(polycyclic aromatic hydrocarbons, PAHs)、重金属等[18 − 20],可能泄露进入周边土壤环境,王兵等[21]指出页岩气开采场地中石油烃污染最严重的土壤往往在钻井液堆放区附近,但是相关土壤毒性数据目前相当匮乏. 弹尾目生物跳虫是一种小型节肢动物,分布广泛,是陆地生态系统中最丰富的无脊椎动物,在土壤生态功能中发挥着至关重要的作用. 跳虫以土壤微生物为食物,是最早接触土壤污染物的生物之一,因此常用作土壤污染的预警[22]. 剖析油基钻井液污染土壤对跳虫的不良效应,有助于评估页岩气开采对周边土壤生态系统的潜在威胁.土壤中污染物的生物有效性是其风险的决定因素之一[23]. 污染物在土壤中主要以结合态、可解吸态和游离态的形式存在,而只有可解吸态及游离态才可被生物所吸收利用并产生效应,研究发现仅少部分PAHs(0.7%—41.2%)能真正能被生物所用,ISO-
17402 -2008[24]明确指出生物效应与土壤中污染物的总浓度无关,而应考虑生物有效性,即污染物到达生物体或靶标器官的净通量[25]. 影响生物有效性的因素众多,老化是一个重要原因,即污染物和土壤成分间的相互作用随时间推移而变化,通常污染物与土壤接触时间越长,污染物生物有效性会降低. 虽然生物测试能清楚直观获取土壤生物效应信息,但实验耗时耗力,成本高且重现性较差,难以广泛运用,而且单纯生物测试无法提供关键致毒物信息[26]. 因此,有必要结合生物测试和化学分析,评估土壤中主要污染物的生物有效性,明晰老化作用对毒性的影响,提高环境风险评估的准确性[27].为预测油基钻井液意外泄露对周边土壤的长期潜在风险,本研究分析了270 d期间油基钻井液污染土壤对跳虫的毒性变化规律,并结合耗竭式和Tenax萃取探明了主要污染物的生物有效性的变化趋势,揭示了其与毒性的关联.
页岩气钻井液污染土壤中石油烃类污染物的生物有效性及毒性的时间变化趋势
Temporal variation in bioavailability and toxicity of petroleum hydrocarbons in soil contaminated by shale gas drilling fluids
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摘要: 页岩气开发有助于促进能源结构转型,了解页岩气开采过程的潜在风险对行业的可持续发展至关重要. 油基钻井液含高浓度的石油烃类物质,可造成周边土壤污染,但目前对污染土壤的毒性效应知之甚少. 为考察老化对页岩气开采污染土壤中典型污染物的生物有效性和毒性的影响,本研究将油基钻井液污染土壤老化至270 d,利用耗竭性萃取和Tenax萃取分析了不同时间点土壤中多环芳烃(PAHs)和烷烃总浓度和生物有效浓度,同时测试了土壤对白符跳虫的96-h致死性以及存活跳虫中富集的污染物. 结果表明,油基钻井液污染土壤中PAHs主要以低环形式存在,污染物生物有效性随老化时间延长显著下降,且高环PAHs生物有效性降低更快. 场地环境可检出浓度下,即使老化270 d,钻井液污染土壤仍对跳虫表现高毒性(96-h LC50为4.00%),但毒性随老化时间延长,逐步下降,且与土壤中多环芳烃生物有效浓度以及生物体内富集有机物浓度密切相关. 本研究揭示了老化时间对钻井液污染土壤中主要污染物生物有效性和毒性的影响,有利于全面认识页岩气开采过程对周边土壤的潜在风险.Abstract: The development of shale gas is conducive to energy structure transformation. Understanding potential risks related to shale gas exploitation is crucial to the sustainable development of this industry. Oil-based drilling fluids contain high concentrations of petroleum hydrocarbons, which may pollute surrounding soils, but the toxicity of contaminated soil is largely unknown currently. To investigate the impact of aging on the bioavailability and toxicity of typical pollutants in soil contaminated by shale gas extraction, oil-based drilling fluid contaminated soil was aged until 270 d, and the total and bioavailable concentrations of polycyclic aromatic hydrocarbons (PAHs) and alkanes were analyzed in soils with different aging time using exhaustive and Tenax extraction. In addition, 96-h lethality of the contaminated soil to springtails was examined and the pollutants accumulated in surviving springtails were also assessed. Results showed that PAHs in oil-based drilling fluid-contaminated soil mainly occurred in low-ring forms. The bioavailability of pollutants decreased with increasing aging time, and the bioavailability of high-ring PAHs decreased faster than the low-rings. At the concentrations similar to those in the shale gas extraction sites, even if aged for 270 d, soil contaminated by drilling fluid was still highly toxic to the springtails (96-h LC50 of 4.00%), but the toxicity gradually reduced with aging time. The toxicity was related to the bioavailable concentration of PAHs in the soil and body residues in organisms. The present study revealed the impact of aging time on the bioavailability and toxicity of main pollutants in drilling fluid-contaminated soil, which is beneficial to a comprehensive understanding of potential risks of the shale gas extraction process to surrounding soil.
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Key words:
- Polycyclic aromatic hydrocarbons /
- bioavailability /
- soil toxicity /
- aging /
- Collembola
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