# The Fate of MTBE and BTEX in Constructed Wetlands
## 3 Fate of MTBE in Constructed Wetlands  
MTBE (methyl _tert_-butyl ether; C\({}_{5}\)H\({}_{12}\)O) is a fuel oxygenate that is typically used as an alternative to lead additives. MTBE has high water solubility (48 g/L at 25 \({}^{\circ}\)C [58]) and volatility ([PERSON]'s law coefficient of 0.026 at 25 \({}^{\circ}\)C [59]). The global production of MTBE was 21 million t in 1999 [60], but dropped to 12 million tons in 2011 when MTBE was banned in the United States [61]; however, the increasing demand in Asia over the last years is again gradually increasing its global production. Refineries are major release points of MTBE. Significant volumes are discharged into surface water or injected into deep reservoirs [13; 62]. Storage tank spills, pipeline leakages, spills from various industrial and fueling facilities, and transportation accidents represent other point sources [13]. Nonpoint pollution sources, such as from urban stormwater and atmospheric deposition [59], are of less importance. MTBE is highly persistent in the environment [3; 13; 59] and is frequently found in contaminated groundwaters in Europe and North America [19; 63; 64; 65; 66]. MTBE affects water taste and odor and may affect human health [13]; therefore, it is classified as a health hazard by the US EPA and has a maximum allowed concentration in drinking water (200 ug/L) in the United States and Germany [44]. Thus, MTBE removal from industrial effluents is required in order to avoid further environmental contamination before it reaches drinking water resources. MTBE can be removed from water with different abiotic processes [3; 13; 58]. Activated carbon adsorption is widely used, though other media are also in use, such as resins, diatomic, or silica [66; 67; 68; 69]. Although sorption does not produce any harmful by-products, it does not also completely destruct MTBE. Air stripping also shows limited efficiency [66; 68], since MTBE has lower volatility than other volatile organic compounds. Membrane technologies are often used to address this limitation [61], while advanced oxidation processes lead to complete MTBE mineralization. However, these processes require expensive reactants and take place under specific conditions, and they may also result in the formation of undesired by-products. In addition, cost efficiency is also an issue for the various abiotic methods [66; 68], especially when large volumes are considered for treatment. Biological processes represent an alternative solution to MTBE removal. Microbial degradation (i.e., biodegradation) has been extensively studied in the laboratory [3; 58; 70; 71; 72], which confirmed its degradation potential under an aerobic environment. However, site-specific parameters affect the occurrence of aerobic MTBE biodegradation [73; 74], while only a few microorganisms can utilize MTBE as their sole carbon source [75; 76; 77]. Moreover, microbial MTBE degradation is also feasible but slower under anaerobic conditions [58] and production of _tert_-butyl alcohol or other intermediates may take place. Although the number of field studies is smaller, it is generally indicated that in situ biodegradation occurs under aerobic [78] and anaerobic conditions [79]. Nevertheless, anaerobic conditions do not often result in high biodegradation rates, and increased oxygen supply may be required to enhance the process [80; 81; 82]. Phytoremediation studies have shown that various plant species, such as poplars [83; 84], alfalfa [85], weeping willows [86], or conifers [87], remove MTBE from groundwater through their roots, but no evidence has been provided that MTBE is degraded within plant tissue [84; 85; 86; 87; 88; 89]; rather, it is transpired, resulting in potentially harmful emissions. However, no plant toxicity effects have been reported [86]. Considering that aerobic biodegradation is an established removal process, CWs can be a potential treatment option. To date, only a few relevant studies are available in the international literature.