Empty oil palm bunches (EFB) can be converted by hydrothermal carbonation process into biochar that can be used as low-cost adsorbent. This study aims to identify the effects of pretreatment and post-treatment using alkaline activators on the biochar characteristics produced from EFB. The activation process was carried out before and after pyrolysis by heating it using an autoclave at 121 oC for 90 minutes. Biochar was then soaked using NaOH or KOH with a concentration of 0%, 4%, 8% and 12% for 3 h. The ability of biochar as an adsorbent was analyzed for its ability to absorb iodine and methylene blue. Iodine absorption analysis was carried out using the Titrimetric method, while the methylene blue absorption test was carried out using the Spectrophotometric method. Results of the analysis showed that the absorption capacity of the resulting biochar for iodine ranged from 208.86-616.32 mg/g, and the absorption capacity of biochar for methylene blue ranged from 62.53-81.11 mg/g.