# Cleaning Phenolic Compounds Present in Water Using Salting-Out Effect with DCA-Based Ionic Liquids
## 1 Introduction  
Water is an essential natural resource, and due to over-consumption and its scarcity, it is necessary to establish systems that allow the recovery of industrial wastewater with the aim of reusing it and developing more sustainable and environmentally friendly processes. The World Health Organization (WHO) and the European Union (EU) have established a list of priority pollutant substances in water, in which some phenolic compounds were included, and the Environmental Protection Agency (EPA) has fixed their limit concentration in wastewater as 1 mg:L\({}^{-1}\)[1; 2]. Due to their high toxicity, it is necessary to clean this water by removing these compounds before they reach the environment, with the aim of complying with the legislation [3]. Phenolic compounds are organic substances that have at least one aromatic ring with a hydroxyl group in their structure. They are naturally present in fruits and vegetables, but they can also be found in industrial wastewater, such as petrochemical, coal, paper, pharmaceutical industries, or coke plants, as pollutant substances (Table 1). A large part of industrial waste ends up polluting water, and therefore, it must be treated to obtain reusable water or water that can be discharged into the environment. The presence of these compounds in water, even at low concentrations, causes contamination because some types of phenols are very toxic to human health and aquatic life [4; 5; 6; 7]. Liquid-liquid extraction (LLE) is a common technique that is used to remove phenols from water due to its effectiveness. It presents some advantages over other techniques, such as distillation, chemical oxidation, and adsorption, among others (Table 2). Since its consumption of energy is lower, it is a simple process and allows the removal of contaminants even at low concentrations [6; 7; 8]. The most commonly used solvents to remove substances from aqueous systems by liquid-liquid extraction are volatile organic compounds (VOCs); these solvents is volatile, flammable, and toxic--all undesirable properties--therefore, they must be replaced by other solvents with more suitable characteristics [13; 14]. Due to their versatility, ionic liquids (ILs) have been widely applied as extraction agents in recent years since they can be used as solvents for the dissolution of both organic and inorganic compounds, as well as chemical or electrochemical synthesis [15; 16; 17]. ILs are organic salts formed by bulky organic cations and small organic or inorganic anions, which remain in a liquid state below 373 K [18; 19; 20]. They have suitable properties such as high thermal and chemical stability and negligible vapor pressure, and it is possible to design them for a specific task, combining the cations and anions used in their obtention [21; 22; 23]. Hence, they are presented as a great option to replace VOCs as extraction agents for the removal of phenolic compounds from aqueous media. ILs can be obtained by mixing different cations (imidazolium, pyridinium, piperidinium, pyrrolinium) and anions ((bis(trifluoromethylsulfonyl)imide [NTf\({}_{2}\)], bis(fluorosulfini)imide [Nf\({}_{2}\)], acetate [OAc], dimethylphosphate [DMP], thiocyanate [SCN], methylsulfate [MeSO\({}_{4}\)], triflate [Tfo], cicyanamide [DCA]). The nature of the anion has a greater influence on its properties, being more hydrophilic than those based on [Tfo] or [DMP] and is more hydrophobic than those based on [NTf\({}_{2}\)] or [Nf\({}_{2}\)], among others. Different studies on their application to the extraction of phenolic compounds have been published. [PERSON] et al. [24] studied the extraction of chlorophenols,nitrophenyl, and methylphenols, among others, using hydrophobic phosphonium-based ILs with good extraction results. The extraction efficiencies of phenolic compounds using [NTf\({}_{2}\)] and [Nf\({}_{2}\)]-based ILs were published by our research group.