The textile industry uses large amount of water and organic dyes to tint their products. These synthetic dyes are toxic and cause water coloring. This results in colored wastewater that must be treated before being discharged. In addition, most dyes are not biodegradable, and once released they alter the aquatic environment. Among the processes for treating these effluents, adsorption is an easy and a cost-effective process to implement. The objective of this study is the preparation of biochars from common reed applicable for the discoloration of effluents used in the textile industry. Common reed (Phragmites australis) has been collected from the Sed-Ksob river of M'sila (Algeria) and used as precursor for the production of biochars activated by phosphoric acid. The surface chemistry of the prepared materials has been characterized using selective titration (Boehm assay), TGA analysis and pH of point zero charge (pHpzc) measurements. The porous texture has been investigated by using methylene blue adsorption and iodine number measurements. The results of the TGA analysis and the Boehm assay show the presence of many superficial functional groups on the surface of the biochars. The amount of oxygenated functional groups on all the prepared biochars depends on the impregnation ratio of phosphoric acid, except for the one obtained at 150% impregnation ratio. The adsorption of methylene blue and the iodine test indicate an increase in the specific surface area with an increase in the impregnation ratio due to the development of the microporosity and the mesoporosity. The adsorption properties of methyl orange, an anionic dye, were studied on the prepared biochars as function of concentration, contact time, and temperature. The Langmuir model describes well the equilibrium adsorption of methyl orange on the prepared biochars. The adsorption kinetics are described by the pseudo-second order and the liquid film diffusion models. An endothermic physical adsorption of methyl orange on the prepared biochars has been highlighted.