This study characterized a natural adsorbent material, a dacitic tuff, from Aguas Blancas in Zimapan Hgo. by emission spectroscopy of inductive coupling plasma, X-ray diffraction and scanning electronic microscopy. Also, the influences of concentration, contact time, and pH on the adsorption capacity of arsenic (As) on a dacitic tuff doped with MgO by batch technique were assessed. The results show that the treated material can increase adsorption capacity by 95% compared to natural, untreated material. The pH range for better As adsorption is 3–7 at ambient temperature. The data were evaluated using Langmuir and Freundlich models; these were better suited to the Langmuir model with a capacity of adsorption of 1,897.1 μg g−1. Finally, the mechanism proposed between adsorbent and As compounds is an anion exchange, according to the Kaganer–Dubinin–Radushkevich model, which is provided by functional groups MgOH2+ that were fixed on the material.