With rapid industrialization and population growth, sewage sludge generation has increased worldwide, and it needs to be treated properly. The pyrolysis of sewage sludge into biochar provides sustainable benefits for concomitant pollutant adsorption and waste treatment. Sulfamethoxazole (SMX) antibiotics are highly prevalent in wastewater owing to their widespread utilization and low metabolic rate and removal efficiency during conventional wastewater treatment. Biochar is known to effectively remove pollutants from wastewater. However, the adsorption capacity and mechanism of SMX adsorption onto sludge-based biochar are currently unclear. Therefore, the adsorption behavior of SMX on sludge-based biochar from three sources (raw sludge, compost sludge, and digested sludge) and ZnCl2- modified biochar was investigated. Among the unmodified biochars, raw sludge-based biochar exhibited the highest adsorption capacity, followed by compost sludge-based and digested sludge-based biochar. The pore-forming effect of ZnCl2 application significantly increased the biochar specific surface area, which increased the equilibrium adsorption of SMX from 6.1mg/g to 49.3mg/g. The adsorption mechanisms involved electrostatic interactions, pore filling, hydrophobic interactions, hydrogen bonding, and π-π interactions. The findings of this study demonstrate the development of sewage sludge biochar and its effectiveness for the treatment of antibiotics containing wastewater.