## 1 Introduction  
Water contamination by pharmaceutical residues has been widely recognized as an emerging environmental issue, necessitating effective solutions to mitigate its impacts. The increasing concentration of these substances in aquatic effluents has led to their classification as Contaminants of Emerging Concern (CECs) [1]. Among these contaminants, the residues of tetracycline hydrochloride (TC), an antibiotic known for its high persistence and low biodegradability, are of particular concern. These properties allow for the accumulation of TC in aquatic environments, promoting the spread of antimicrobial resistance [1]. Additionally, the absorption rate in animals is low, even at high dosages, with 50% to 80% of the drug entering the environment, contributing significantly to the rise of bacteria resistant to these antibiotics [2]. In Brazil, the pharmaceutical industry reported revenues of approximately BRL 20 billion in 2021, which resulted in the improper disposal of significant volumes of medications. It is estimated that 14,000 tons of expired drugs are discarded annually, exacerbating the contamination of water bodies [3; 4]. The inclusion of pharmaceutical waste in the class of Contaminants of Emerging Concer (CECs) has led to an increased demand for new methods to mitigate these pollutants while minimizing environmental impact. Most pharmaceuticals are not completely metabolized by the body and are excreted through urine and feces. Even in minimal concentrations, these substances have been shown to have harmful effects on biota and pose significant risks to public health. Conventional water treatment techniques, such as coagulation-flocculation, membrane filtration, electrochemical degradation, photodegradation, and advanced oxidation, have been studied to remove pharmaceutical residues but exhibit significant limitations, particularly because many of these compounds are recalcitrant in nature [5]. Additionally, these techniques often come with high operational costs and can generate potentially toxic byproducts, which reduce their economic and environmental feasibility on a large scale [6]. Among the various treatment options, adsorption has been extensively studied as a promising alternative due to its efficiency, simplicity, and low cost [7]. In particular, bentonite, a natural clay mineral, has gained attention for its favorable structural properties, such as high surface area and cation exchange capacity [8]. Bentonite is a clay mineral composed primarily of montmorillonite, a hydrated aluminum silicate with the structural formula (Na,Ca)\({}_{0.33}\)(Al,Mg)\({}_{2}\)(Si\({}_{4}\)O\({}_{10}\))(OH)\({}_{2}\cdot\)nH\({}_{2}\)O. Belonging to the smectite group, bentonite has a layered structure consisting of silica tetrahedral sheets and aluminum octahedral sheets. These layers are separated by an interlayer space filled with water molecules and exchangeable cations, which confer its swelling ability and high specific surface area. When treated with acid, bentonite's adsorptive properties are further enhanced by increasing its porosity and removing surface impurities, making it more efficient in removing pharmaceutical contaminants [9]. The use of natural adsorbents like bentonite for the removal of drugs has demonstrated satisfactory results in recent years, highlighting the potential of such materials for sustainable water treatment. However, there is still a lack of comprehensive studies on the interaction mechanisms between modified bentonite and pharmaceuticals, as well as the feasibility of large-scale application. To address this gap, this study investigates the effectiveness of both natural (BN) and acid-treated (BA1) bentonite in removing TC from aqueous solutions.