Alvarez Pedro J J, Hunt Craig S
Department of Civil and Environmental Engineering and Iowa Institute of Hydraulic Reserch, University of Iowa, Iowa City 52242-1547, USA.
Rev Latinoam Microbiol. 2002 Apr-Jun;44(2):83-104.
The proposed replacement of the gasoline oxygenate MTBE with ethanol represents potential economic and environmental quality benefits. However, these benefits may be offset to some extent by potential detrimental effects on groundwater quality and natural attenuation of released petroleum products. The objectives of this literature review are to bound the extent to which these impacts may occur, summarize the available information on the biodegradation of ethanol in the environment, assess the potential effect that biodegradation processes may have on the fate and transport of BTEX compounds, and provide recommendations for research to enhance related risk assessment and management decisions. Ethanol that reaches groundwater aquifers is likely to be degraded at much faster rates than other gasoline constituents. If the carbon source is not limiting, a preferential degradation of ethanol over BTEX may be observed under both aerobic and anaerobic conditions. Depending on the extent of the release, ethanol may exert a high biochemical oxygen demand that would contribute to the rapid depletion of dissolved oxygen in the groundwater. Thus, ethanol will likely be degraded predominantly under anaerobic conditions. None of the potential ethanol metabolites that could accumulate in groundwater are toxic, although some potential biodegradation by-products such as butyrate could adversely affect the taste and odor of drinking water sources. In addition, acetate and other volatile fatty acids could accumulate at high concentrations, causing a pH decrease in poorly buffered systems. It is unknown, however, whether the pH would decrease to a point that inhibits natural degradative processes. Inhibition of microbial, activity near the source is likely to occur as a result of exposure to high alcohol concentrations, and bactericidal effects are likely to occur when cells are exposed to ethanol concentrations exceeding 10,000 mg/L. However, the maximum allowable ethanol content in gasoline is 10% by volume in the United States. Thus, such high ethanol concentrations are unlikely to be encountered at sites contaminated with ethanol-gasoline blends, except near the fuel/water interfaces or in the case of neat ethanol releases. Downgradient of the source area, biodegradation is unlikely to be inhibited by alcohol toxicity as concentrations decrease exponentially with distance. The preferential degradation of fuel alcohols by indigenous microorganisms and the accompanying depletion of oxygen and other electron acceptors suggest that ethanol could hinder BTEX bioremediation. This is particularly important for the fate of benzene, which is the most toxic BTEX compound and the most recalcitrant under anaerobic conditions. Alternatively, ethanol represents a carbon and energy source that is likely to stimulate the growth of a variety of aerobic and anaerobic microbial populations, including those that can degrade BTEX compounds. A higher concentration of BTEX degraders would be conducive to faster BTEX degradation rates under carbon-limiting conditions. Nevertheless, controlled studies that assess the overall effect of ethanol on BTEX bioremediation are lacking. In theory, ethanol could also contribute to longer BTEX plumes by enhancing BTEX solubilization from the fuel phase and by decreasing sorption-related retardation during transport. The overall effect of ethanol on BTEX plume length and treatment end points is likely to be system specific, and will depend largely on the release scenario and on the buffering and dilution capacity of the aquifer. Additional research is needed to understand the effect of ethanol on the stability and dimensions of co-occurring and pre-existing BTEX plumes. Future laboratory and field studies should also address response variability as a function of release scenario and site specificity, to facilitate risk assessment and remedial action decisions.
提议用乙醇替代汽油含氧化合物甲基叔丁基醚具有潜在的经济和环境质量效益。然而，这些效益可能会在一定程度上被对地下水质量的潜在不利影响以及释放的石油产品的自然衰减所抵消。本文献综述的目的是界定这些影响可能发生的程度，总结环境中乙醇生物降解的现有信息，评估生物降解过程可能对苯系物（BTEX）化合物的归宿和迁移产生的潜在影响，并为加强相关风险评估和管理决策的研究提供建议。进入地下水含水层的乙醇很可能比其他汽油成分降解速度快得多。如果碳源不限制，在好氧和厌氧条件下都可能观察到乙醇比BTEX优先降解。根据释放程度，乙醇可能会产生很高的生化需氧量，这将导致地下水中溶解氧迅速耗尽。因此，乙醇很可能主要在厌氧条件下降解。虽然一些潜在的生物降解副产物如丁酸盐可能会对饮用水源的味道和气味产生不利影响，但在地下水中可能积累的潜在乙醇代谢产物都无毒。此外，乙酸盐和其他挥发性脂肪酸可能会高浓度积累，导致缓冲能力差的系统pH值下降。然而，pH值是否会降至抑制自然降解过程的程度尚不清楚。由于暴露于高酒精浓度，在源头附近微生物活性可能受到抑制，当细胞暴露于超过10000mg/L的乙醇浓度时可能会产生杀菌作用。然而，在美国汽油中乙醇的最大允许含量为体积的10%。因此，在受乙醇 - 汽油混合物污染的场地，除了在燃料/水界面附近或纯乙醇泄漏的情况下，不太可能遇到如此高的乙醇浓度。在源区下游，随着浓度随距离呈指数下降，生物降解不太可能受到酒精毒性的抑制。本地微生物对燃料醇的优先降解以及随之而来的氧气和其他电子受体的消耗表明乙醇可能会阻碍BTEX生物修复。这对于苯的归宿尤为重要，苯是毒性最大的BTEX化合物，在厌氧条件下最难降解。另一方面，乙醇代表一种碳源和能源，可能会刺激包括那些能够降解BTEX化合物的各种好氧和厌氧微生物种群的生长。在碳限制条件下，更高浓度 的BTEX降解菌将有利于更快的BTEX降解速度。然而，缺乏评估乙醇对BTEX生物修复总体影响的对照研究。理论上，乙醇还可能通过增强BTEX从燃料相的溶解以及在运输过程中减少吸附相关的阻滞作用，导致更长的BTEX羽流。乙醇对BTEX羽流长度和处理终点的总体影响可能因系统而异，并且在很大程度上取决于释放情况以及含水层的缓冲和稀释能力。需要更多的研究来了解乙醇对同时存在和先前存在的BTEX羽流的稳定性和尺寸的影响。未来的实验室和现场研究还应解决作为释放情况和场地特异性函数的响应变异性问题，以促进风险评估和补救行动决策。
