# The Fate of MTBE and BTEX in Constructed Wetlands
## 1 Introduction  
Hydrocarbons are commonly found water contaminants due to their extensive use. They consist of simple compounds of carbon and hydrogen or complex compounds with varying physicochemical properties and are generally classified into aromatic, alicyclic, and aliphatic hydrocarbons. Light hydrocarbons (i.e., having up to 16 carbon atoms) may have higher volatility and solubility. Some hydrocarbons can be toxic; hence, they pose a risk to the environment [1]. In water, they can be free-floating, dissolved, emulsified, or adsorbed to suspended solids. Larger molecules are typically free, and smaller ones can be emulsified in water. Hydrocarbon contamination typically occurs in areas with industrial activity, for example, close to chemical/petrochemical plants, oil refineries, power plants, manufacturing industries, plastic and steel production facilities, and cooling plants. The continuous demand for oil and gas supply increases the environmental concerns, while accidental spills, partial treatment, and illegal discharge introduce these compounds to the surface and groundwater bodies. Hydrocarbons can also reach surface or groundwater resources through stormwater runoff; spills from roads, tank farms, airport surfaces, and so forth; and transportation [2, 3], even near petrol stations. Besides the risks to human health, hydrocarbons can also affect various aquatic ecosystems. BTEX (benzene, toluene, ethylbenzene, and xylenes) and MTBE (methyl _tert_-butyl ether) are fuel hydrocarbons often detected in water [4, 5]. MTBE and BTEX are highly soluble and mobile in water, and both can have toxic effects (i.e., BTEX is related to carcinogenesis); hence, they are regulated contaminants in drinking water (200 and 5 \(\upmu\)g/L, respectively [6, 7]). Another hydrocarbon compound is phenols, which are found in outflows from oil refineries and petrochemical industries; effluentsfrom tanneries [8], olive mills [9], and pulp and paper mills [10]; as well as some pesticides and disinfectants [11]. Phenols can also be toxic and a maximum allowable concentration in water has been introduced (0.001 ug/L [12]). Different physicochemical technologies are applied to remove light hydrocarbons from water, including membranes, porous media adsorption (e.g., on activated carbon and zeolites), advanced oxidation processes (e.g., UV, H\({}_{2}\)O\({}_{2}\), and O\({}_{3}\)), chemical oxidation, air stripping, and vapor extraction [3; 13; 14; 15]. These technologies are in general efficient; however, they usually require experienced and skilled personnel, are based on large and sophisticated mechanical equipment, usually demand a high initial investment, have high operation and maintenance costs, and consume large amounts of external energy, which often cannot be covered from the economic and/or technical aspect, particularly in small-scale installations [15]. Hence, the use of alternative and, at the same time, sustainable technologies is a continuously growing need. Constructed wetlands (CWs) represent an effective green technology with high treatment capacity. CWs can be classified based on the vegetation type and the flow patterns through the system to (a) surface flow constructed wetlands (SF CWs) and (b) subsurface flow constructed wetlands (SSF CWs) [5]. The typical design of SF CW systems consists of a water column (10-50 cm) above a substrate layer (usually soil). SSF CWs are typically gravel beds with vertical flow (VF CW) or horizontal subsurface flow (HSSF). According to the vegetation type, further classification includes emergent macrophyte wetlands and submerged macrophyte wetlands. The most common systems are those with rooted emergent macrophytes [5]. When more than one CW type is combined in one facility, this is called a hybrid wetland system. CWs generally possess smaller investment requirements and minimal operation and maintenance costs compared with conventional methods [1; 5].