# The Fate of MTBE and BTEX in Constructed Wetlands
## 2 Fate of Benzene in Constructed Wetlands
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Weland Type** & **Plant Species** & **Substrate** & **Flow (m)\({}^{2}\)/day** & **HRT \({}^{2}\) (days)** & **Influent** & **Removal Rate** & **Reference** \\ \hline HSSF \({}^{3}\) CW & _Phosphmiles_ & gravel (67\%) & & & & & \\ (6 x 1 x 0.5) & _Australis_ & sand (25\%) & 0.113 & 6 & 0.026 & 93 & [34] \\ HSSF CW & _P. australis_ & gravel & 0.528 & 10 & 10.2 & 72-82 & [1] \\ HSSF CW & _P. australis_ & fine gravel, charcoal & & & & & \\ Plant root mat & & 0.144 & & 20 & 81–43 (summer– & [29] \\ (water depth 0.15 m) & _P. australis_ & – & 0.166 & & 20 & 99–18 (summer– & [29] \\ HSSF CW pilot (? \(\times\) 1.7 x & _Salix, Pragmites,_ & & & & 0.17 & \\ 1.1) & _Scholarpedus,_ & gravel, sand & 5.4 and & (benzene) & 100 (below & [45] \\ full scale HSSF + SF CW & _Iuros, Commas,_ & 6000 & 1 & (0.47 & detection limit) & [45] \\ (1.9 h, d = 0.9) & _Typhaapsifolia_ & & & & (HTEX) & \\ HSSF CW & _P. australis,_ & clay soil, stones, & & & & \\ (4.8 x 7 x 2.0 k) & _Typhaifolia_ & gravel & 1 & 1.5 & 0.6 & 57 & [46] \\ HSSF CW & _P. australis_ & & 0.144 & & up to 24 & 1900 mg/day & [47] \\ (5 x 1.1 x 0.6) & _P. australis_ & gravel & 0.144 & 6 & 13 & 100 (summer) & [44] \\ Plant root mat & _P. australis_ & – & 0.144 & & & 100 (summer) & \\ (water depth 0.15) & _Pragmites,_ & washed soil, & & & & \\ (7 x 1.7 x 1.1) & _Cornus_ & gravel & 5.5 & & 0.395 & 61–81 & [48] \\ VF CW \({}^{4}\) & _P. australis_ & stones, gravel, & & & & \\ (H = 0.75, d = 0.1) & _P. australis_ & sand & & 1 & 1.3 & 85–95 & [41] \\ VF CW & & stones, gravel, & & & & \\ (0.75, d = 0.1) & _P. australis_ & sand & & & & \\ VF CW & & stones, gravel, & & & & \\ (0.75, d = 0.1) & _P. australis_ & sand & & & & \\ VF CW & & granular media, & clay, zeolite & up to 1.9 & & 13.9 & 100 & [43] \\ (2 x 3 x 1.75 x 1.75) & _Pragmites Iarka_ & gravel, soil & & & & & \\ SF CW \({}^{5}\) & _P. australis T._ & & & & & & \\ (4 x 12 x 0.45) & _Latifolia_ & soil & 3 & & & & \\  & & & & & & & \\  & & & & & & & \\ \end{tabular}  
* \({}^{1}\) L x \(\times\) W x \(\times\) D: Length \(\times\) Width \(\times\) Depth, \({}^{2}\) HRT \(=\) hydraulic residence time, \({}^{3}\) HSSF \(=\) horizontal subsurface flow,
* \({}^{4}\) VF \(=\) vertical flow, \({}^{5}\) SF \(=\) surface flow.  
\end{table} 
Table 1: Studies on constructed wetland (CW) systems treating water contaminated with benzene. feeding method, that is, the wastewater is applied on the gravel surface and then drains vertically with gravity [5]. Aerated ponds and plant root mats are also two systems that have been evaluated [29]. Such natural treatment systems are also efficient, and sometimes the reported efficiency exceeds that of HSSF CWs, reaching almost up to 100% removal for an influent of 20 mg/L [24]. In general, it is reported that only 1% of benzene removal was attributed to volatilization in plant root mats, while benzene removal takes place mostly through aerobic biodegradation. Seasonal variations also have an effect on benzene removal, with higher removal rates occurring at high temperatures. For values above 15 \({}^{\circ}\)C and during the warm season, the performance of both HSSF CWs [1; 29; 42] and VF CWs [41; 52] is enhanced. Benzene removal in VF CWs is reported to be negatively correlated to dissolved oxygen variations and positively correlated to the pH and redox variations related to the season [52]. Considering that oxygen availability is a critical parameter [29], high temperatures favor plant growth and promote the establishment of microbial biofilm, increasing the biodegradation rate. SF CWs have also been found effective, with reported BTEX removal rates exceeding 90% [51]. Plants are another important parameter in benzene removal. Higher removal rates are generally found in planted than unplanted CWs [1; 29; 42; 53; 54; 55]. Increased monochlorobenzene removal by 33% was reported in an HSSF CW that was planted with common needs (_Phragmites australis_) compared with an unplanted unit [27; 56].