## 1 Introduction  
Palladium (Pd) is one of the precious metals of great economic importance. Its specific physical and chemical properties make it useful in many fields such as electrical and electronic industries, corrosion-resistant materials, jewelry or as catalysts in various chemical processes [1, 2]. Due to its importance and the rare ore deposits which make a high price for this metal, it is essential to develop efficient recovery methods for this element, especially from water containing trace amounts of Pd. There are numerous methods for palladium recovery from different waste, such as solvent extraction [3, 4, 5], ion exchange [6, 7, 8], electrometallurgy [9] and adsorption [10, 11, 12]. From all the mentioned methods, adsorption proved to be the most efficient for the recovery of precious metals from aqueous solutionsdue to easy operation, the existence of versatile adsorbents (such as activated carbon, chitosan, calcium alginate, graphene oxide, titanate nanotubes, etc.) and high efficiency even in case of solutions containing trace amounts of metal ions [11; 12; 13; 14; 15; 16; 17]. Since the ionic liquids (IL) were successfully used in the liquid-liquid extraction of precious metals [18; 19], in the last years, new attention has been given to the use of ionic liquids for obtaining different adsorbents for palladium removal from aqueous solutions [20; 21; 22]. The immobilization of the ionic liquids in suitable solid support prevents the drawbacks of the liquid-liquid extraction, such as the loss of the ionic liquid in water and a possible occurrence of an ion-exchange between the metal ion and the cation of the ionic liquid. Through impregnation of the ionic liquid onto a suitable solid support, higher efficiency in the removal process of metal ions from aqueous solutions is achieved because it favorably combines the properties of ILs with the advantages of the solid supports [23; 24]. In a previous study, the adsorption performance of the Florisil impregnated with Cyphos IL 101 (trihexyl tetradecyl phosphonium chloride) in the removal process of Pd(II) ions from residual solutions was studied [25]. It was observed that the impregnated Florisil with Cyphos IL-101 develops a maximum adsorption capacity in the removal process of Pd(II) ions from an aqueous solution of 126 mg/g, which is two times higher than the adsorption capacity developed by the raw Florisil [25]. The Circular Economy Action Plan of EU intends to close the loop of product lifecycles through sustainable consumption and production by improving waste management with greater recycling and reuse in order to create benefits for the environment, the economy and the society [26]. In all the adsorption studies, the major problem is, \"What happened with the exhausted adsorbent?\". Very few papers answer this question. In order to respond to the European Union targets and to create benefits for the environment, economy and society and to respond to this frequently asked question in adsorption processes treatment, the aim of this paper is to study the possible use as a photocatalyst of an exhausted adsorbent from the removal process of Pd(II) ions from aqueous solutions. Many Pd-based composites are synthesized and studied as photocatalysts in various degradation processes [27; 28; 29; 30]. However, in all cases, the obtaining method is very complicated, involving several steps of synthesis and using expensive resources. By using a waste resulted from another process as a photocatalyst, the Pd ions from secondary sources were capitalized. Consequently, the use of Pd ions from prime materials is avoided, which led to a minimization of the cost of treatment. The method proposed in this paper represents a closed cycle technology, and it is advantageous from both economic and environmental protection perspectives. The Pd-based photocatalysts obtained through Pd adsorption onto Cyphos IL 101 impregnated Florisil have been used in the degradation process of phenol from aqueous solutions.