Six biochars derived from olive mill solid waste were prepared by varying the temperature and treatment with FeCl3. The adsorption capacity of the biochar increased with the pyrolysis temperature and was in the order pre-treated>post-treated>untreated. The Langmuir isotherm and the pseudo-second-order kinetic models best fit the experimental results. The 550°C pre-treated biochar had the highest adsorption capacity from aqueous solutions (103.9 and 73.9 mg.g−1 at pH = 2 and 5, respectively). Chemisorption was the dominating mechanism. The adsorption capacity using olive mill wastewater was 51.3 mg.g−1. This work demonstrates that waste can be reused within the process to achieve cleaner production.