## 1 Introduction  
There has been a great deal of concern about the detection of pharmaceuticals and personal care products in soil, sediments, surface and groundwater [1] as they have become major pollutants due to their low biodegradability, high persistence, and facile bioaccumulation [2]. Among these products, methylthioninium chloride (MC), which is part of a type of medication derived from phenothiazzines, has been widely used in a variety of medical treatments, such as methemoglobinitia [3] and lymphatic mapping/sentinel lymphadenectomy in staging melanoma and breast cancer [4]. It has also been used as a therapeutic agent in neurodegenerative diseases [5; 6]. This product is not only used in the pharmaceutical field, but it is also used in several other industrial fields including textiles, leather, cosmetics, paper, printing, plastic and food [7; 8; 9].Therefore, MC is often present in the various effluents that are discharged into the environment. Thus, the continuous release of this pollutant into the environment significantly affects human health and aquatic systems [9; 10].  
Therefore, the treatment of these effluents by appropriate techniques is very necessary [9; 11]. For this purpose, several physical, chemical and biological methods have been developed including coagulation and flocculation, membrane separation, reverse osmosis, oxidation and co-precipitation [9; 12]. The use of an inexpensive and effective method is very important [11; 13]. The adsorption technique has certain advantages; it is a simple and easy technique, it costs less and it is respectful of the environment [14; 15]. Activated carbon is the most commonly used adsorbent due to its very high adsorption capacity [16; 17; 18], but the disadvantage is that it is expensive [16]. For this reason, several studies have been carried out to examine the use of low-cost adsorbents [19]. For example, various agricultural byproducts, clays [20], biomass [15], synthetic and natural polymers [21], have been evaluated. Thus, it is useful to find effective adsorbent materials, that are available locally and especially, low cost [7; 22]. Indeed, sunflower seed shells, an agricultural byproduct of biomass, could be exploited in this area as they are a low-cost product, widely used and can be used without any treatment.  
However, several factors greatly influence the adsorption process, such as the initial pollutant concentration, adsorbent dosage, pH and temperature of the solution, etc. These are important because they affect the adsorption process, particularly the adsorption capacity.  
Accordingly, it is necessary to use methods based on statistical calculations like the Box-Behnken design (BBD), which is a useful tool because it is fast, efficient and economical in comparison to the classic univariate methods where only one factor is studied at a time. Indeed, the advantage of this technique is the collection of a large amount of information from a minimum of experimental trials, without sacrificing the accuracy of the results. The main aim of this study was to find the best operating conditions for the adsorption of MC on a low-cost adsorbent prepared from sunflower seed shells (SSS). Thus, a study was carried out to determine the influence of the operating parameters on the adsorption efficiency. The adsorption mechanism of the SSS is shown through the study of adsorption isotherms, kinetics and thermodynamics. The efficiency of the adsorption of SSS for the elimination of MC was optimized by evaluating the effects of three parameters, namely, the initial concentration of the drug, the dose of adsorbent and the pH of the solution. Modeling and optimization were carried out by the Box-Behnken design, which allowed us to determine the optimal values of these parameters and to ensure the high adsorption efficiency of MC on the SSS powder.