# The Fate of MTBE and BTEX in Constructed Wetlands
## 2 Fate of Benzene in Constructed Wetlands  
Benzene (C\({}_{6}\)H\({}_{6}\)), a BTEX compound generated from gasoline production, is frequently detected in groundwater [17; 18]. It has high water solubility (1780 mg/L at 20 \({}^{\circ}\)C) and very high mobility [19]. Benzene is the most toxic BTEX compound and can be biologically degraded [20] under oxic [21] or hypoxic environments [22]; however, it is highly recalcitrant under anoxic conditions [23; 24]. Different physical, chemical, and biological processes are applied for benzene transformation and/or removal (e.g., sorption, volatilization, and dilution) that change its phase. Plants also assimilate benzene in their tissues and through transpiration transfer and release it to the atmosphere. Microbial degradation is believed to be the most effective process, allowing even for its complete removal. Biodegradation strongly depends on various physicochemical factors (e.g., temperature; oxygen levels; salinity; nutrients; pH; and the chemical composition, physical state, and concentration of the contaminant) and on biological factors, including the composition and adjustability of microorganisms [25]. CWs have also been successfully implemented for the remediation of waters containing benzene. As a nature-based technology, CWs generally remove organic contaminants through different processes initiated by the interactions between plants and microorganisms in the root system [26; 27; 28; 29]. CWs create the appropriate environment for the growth of microorganisms that biodegrade organic compounds [30]. Bacteria that degrade volatile organic compounds (e.g., benzene, toluene, ethylbenzene, and p-xylene) have been detected in the plant rhizosphere [31; 32]. The most abundant species are those of _Pseudomonas_, representing more than 85% of the degrading bacteria in polluted aquifers [33]. Additionally, isotope studies have revealed that most benzene fractions can be degraded under aerobic conditions [18; 34]. The plants in CWs can also uptake compounds such as benzene (phytotransformation), concentrate it in their tissues (phytoextraction), and release it to the atmosphere through their transpiration. Plant uptake is closely related to the compound's lipophilicity; the low octanol-water partition coefficient logKOW of benzene is between 0.5 and 3 (i.e., 2.13), implying plant uptake [35; 36; 37]. Benzene can also be released directly from the substrate media surface, while phytovolatilization via plant stems and leaves also occurs. This process relates to substances being rapidly translocated (logKOW \(<\) 3.5) and to high vapor pressure (Vp \(>\) 1.01 kPa) [38] or high Henry's law constant (KAW \(>\) 10-5) [39]. The values for benzene (KAW = 0.22, Vp = 12.7 kPa at 25 \({}^{\circ}\)C) [35] imply that phytovolatilization is a possible benzene removal process. Studies have shown that this process is responsible for a major part of both benzene and MTBE emission [40]. It has been found that under a hydraulic residence time exceeding 1 day, the role of volatilization is most important in VF CWs [41], although lower rates (\(<\)5%) have also been estimated [42; 43]. On the other hand, the contribution of volatilization in benzene removal was assessed at around 1% in HSSF CWs [44]. Table 1 shows studies that can be found in the international literature on benzene removal with different CW types. As can be seen, though all wetland designs have been tested and applied, most of them are with subsurface flow, both horizontal and vertical flow. The reported results clearly indicate the high treatment and removal capacity of CWs for benzene (or BTEX) from water. As Table 1 shows, various benzene influent concentrations have been tested in CWs. High performance is in general achieved, while even complete benzene removal is reported in some systems. Considering that benzene is mainly removed via biodegradation under aerobic conditions, the VF CW design is preferable due to the good aerobic conditions in this wetland type caused by the