## 1 Introduction  
The use of hydroxyapatite (HAp) with carbon materials has become very common nowadays, as it can be considered a novel material that can be used in diverse applications. [PERSON] et al., in 2019, obtained oxidized carbon nanotubes modified with HAp particles that showed great potential for bone regeneration applications [1]. [PERSON] et al., in 2018, obtained a bonding between activated carbon fibers (ACFs) with HAp by electrochemical deposition, resulting in an ACF/HAp composite to be used as a biofilm for support in bioreactors because of its strong ability to immobilize activated sludge and bacteria [2]. [PERSON] et al., in 2018, conducted studies with microorganisms on HAp-modified carbon-based materials demonstrating antimicrobial and metal ion removal properties [3].  
The combination of carbon-based materials with HAp has also been widely studied in the environmental field, where the most frequent application is the removal of pollutants in aqueous solutions. [PERSON] et al., in 2015, studied the adsorption of Pb\({}^{2+}\) by a composite prepared with granular activated carbon and HAp [4]. [PERSON] et al., in 2019, synthesizeda granular activated carbon-hydroxapatite composite by the chemical coprecipitation of CaCl\({}_{2}\) on the activated carbon in a (NH\({}_{4}\))\({}_{2}\)HPO\({}_{4}\) solution for Pb\({}^{2+}\) removal [5]. [PERSON] et al., in 2018, investigated the adsorption of Pb\({}^{2+}\) ions with a composite material obtained from the carbonized organic material of sugarcane with HAp [6].  
These types of composite materials improve adsorption capacity due to the ion exchange properties of HAp and the carboxylic functional groups (-COOH) present in the carbon material surface; so, increasing these groups on the surface is of great importance. Several studies have focused on increasing the oxygenated functional groups. [PERSON] et al., in 2020, showed the roles of the different surface functional groups in graphene-based nanomaterials [7]. [PERSON] et al., in 2018, prepared biochar from rice straw and modified the surface with the carboxyl and phenol groups for Cd\({}^{2+}\) adsorption [8]. [PERSON] et al., 2018, studied the influence of functional groups on activated carbon (AC) and the effects of acetone adsorption [9]. [PERSON] et al., in 2022, investigated the effect of functional groups on VOCs' adsorption [10].  
The adsorption of organic molecules by AC/HAp composites is one potential application. [PERSON] et al., 2021, reported the sorption of organic/inorganic molecules by hydroxyapatite/carbon composites [11]. [PERSON] et al., 2019, reported the adsorption of methylene blue (MB) by AC/HAp composites [12]; [PERSON] et al., 2018, reported the obtention of biochar-supported HAp and MB adsorption [13]. [PERSON] et al., in 2015, obtained a magnetic carbon with HAp for the removal of organic compounds [14].  
The purpose of this work was to obtain a composite of granular activated carbon with hydroxyapatite grown by the hydrothermal method, modifying parameters, such as the AC surface oxidation and the addition of CaSiO\({}_{3}\), to improve the adsorptive properties of the materials. The obtained materials were tested for the adsorption and kinetics removal of methylene blue as a potential application.