## 1 Introduction  
To fulfill the different aspects of green chemistry and to achieve full use of lignin from its primitive resources, the necessity of developing green methods for the fractionation of lignocellulosic materials is obvious. Ionic liquids (ILs) are comprised of large organic cations and inorganic or organic anions that exist as liquids at relatively low temperatures (lower than 100 \({}^{\circ}\)C) [1; 2]. Compared to traditional organic solvents, ILs have attractive properties, such as thermal stability, versatility, self-organized structure, high solvent capacity, insignificant vapor pressure (reduction in volatile organic compound emissions), thermal and chemical stability, recyclability and often non-flammability, which make them suitable to be considered as green solvents in lignocellulosic material dissolution [3; 4; 5]. However, ILshave also several weaknesses, such as high cost, low moisture tolerance, biodegradability that is not very notable, and the inclusion of synthesis mostly from oil-based chemicals [5; 6; 7]. Moreover, the possible toxicity of ILs is a factor of particular importance that should be taken into account in the case of practical applications in a large scale [7; 8]. These downsides, together with the requirement of a large amount of salts and solvents in their synthesis in order to exchange the anions, entirely hinder their industrial inception [9]. Deep eutectic solvents (DES) as a new generation of ILs have emerged as a renewable and biodegradable green solvent with almost all the advantages of ILs in the dissociation of lignocellulosic material, as well as their considerably lower toxicity and price [9]. In general, the interactions between their hydrogen bonds of an HBD (hydrogen bond donor) and an HBA (hydrogen bond acceptor) mixture results in the formation of DESs. Consequently, the melting temperature of the formed eutectic mixture is lower compared to that of its constituents [10]. Deep eutectic solvents can play a substantial role in pulp and paper and recycling industries. The treatment of wood chips or annual plants with DESs in delignification or pulping processes can remarkably reduce the required energy for pulp production. Furthermore, there is a potential to utilize deep eutectic solvents in hydrolysis or dissolution of certain components of lignocellulosic biomass, such as lignin, under mild conditions, which in turn can minimize the risk of further degradation of these components [11]. In addition, higher solubility of lignin in deep eutectic solvents (DES delignification) compared to the other delignification processes (e.g., oxygen delignification) has been reported in different studies [12; 13]. Polyethersulfone (PES) is considered as one of the most favorable polymeric materials in the fabrication of ultrafiltration membrane, due to its outstanding chemical and thermal stability, appropriate mechanical strength, and film-forming properties [14; 15; 16]. Furthermore, it has been implemented as the main base membrane material in various substantial separation processes, such as concentration, extraction, purification, and hemodialysis [15]. Even with its attractive properties, a PES membrane is susceptible to fouling due to its relatively low hydrophilic nature, e.g., in the treatment of biorefinery streams and pulp and paper industry process waters [17; 18]. Thus, the incorporation of different organic or inorganic additives, such as polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), and polyethylene glycol (PEG), in the dope solution before phase inversion can be considered a promising approach to eliminate this downside, which in turn can enhance the membrane performance [16]. The existing literature on the application of lignin and its derivatives in different fields of science is extensive and focuses particularly on the utilization of this natural adhesion promoter in water treatment for removing different pollutants [19] or as an additive, such as binder, dispersant, and emulsifier [10].