## 1 Introduction  
The leather manufacturing sector is an industrial hotspot that is unfortunately known for its heavy environmental impact. It is responsible for approximately 33 million tonnes per year of solid wastes, the management of which is very challenging. The leather industry also generates massive quantities of wastewater, which contains a complex mixture of contaminants at concentration values over the established regulatory limits. This is also an environmental challenge [1]. Synthetic organic dyes commonly used in leather industry are, indeed, resistant to biodegradation and pose significant health concerns [2, 3]. It is estimated that 700,000 tonnes of colour are manufactured each year from around 100,000 commercial dyes and that the industries that produce and use dyes generate around 3000 to 4000 kilotons per year of wastewater worldwide [4]. Given the complexity of these pollutants, it is essential to put in place a reliable and efficient wastewater treatment system. This issue has been investigated by looking into physicochemical methods for the removal of organic dyes from wastewater [1]. Among these, membrane separation, osmosis, biological treatment, ion exchange, coagulation/flocculation, photocatalysis, adsorption, and chemical oxidation have proven to be effective [3, 5]. Adsorption has received particular attention as a purification technique owing to its relative simplicity and high effectiveness [5, 6]. Nevertheless, some of the disadvantagesof commonly used adsorbents include the high cost and low efficiency of the adsorption process. The preparation of adsorbents for wastewater treatment and dye elimination from residual biomass appears to be an eco-friendly and economically attractive solution. Indeed, biochars derived from different biomass feedstocks have recently been proposed as adsorbents thanks to their physical and chemical properties, such as their high surface area, surface chemistry, and morphology [7]. Biochars with a large surface area and numerous active sites often have a high capacity for the adsorption and removal of contaminants. In this respect, biochars have numerous functional groups including hydroxyls, carboxyls, carbonyls, ethers, esters, and amides, as well as trace minerals and metals, which can confer the ability to exchange electrons and a high degree of reactivity. Indeed, [PERSON] et al. [8] indicated that the carboxyl, carbonyl, lactone, and phenolic groups on the biochar surface affect its ability to eliminate dyes. [PERSON] et al. [9] have used cow hair to prepare porous biochars with different methods and operating conditions (carbonization atmospheres, the MgO template method, and KOH activation) for the removal of blue dye from wastewater in the leather industry. They reported that the adsorption capacity of the cow-hair-based char materials prepared with KOH activation was effective for blue dye adsorption, reaching a capacity of 1477 mg/g. [PERSON] et al. [10] tested biochar prepared from Musa acuminata stem for the adsorption of Congo red and Brilliant blue leather dyes. They reported a maximum adsorption capacity of 208.33 and 46.33 mg/g for Brilliant blue and Congo red, respectively. [PERSON] et al. [11] performed adsorption experiments using biochar from the pyrolysis of tannery fleshing waste for the elimination of Sella Fast Red dye. The maximum dye uptake ranged from 26 mg/g to 39.86 mg/g for biochars prepared at temperatures between 400 and 600 \({}^{\circ}\)C. [PERSON] et al. [1] also studied the adsorption of Sella Fast Grey CLL industrial dye using biochar derived from cow bone waste pyrolysis at temperatures of 500-700 \({}^{\circ}\)C. The best adsorption capacity of 61.7 mg/g was reached with the biochar produced at 700 \({}^{\circ}\)C.  
In this study, we aimed to develop new adsorbents for the removal of industrial dyes used in the leather tanning industry in Tunisia.