Natural zeolites are widely used to minimise the impact of some pollutants in the environment, such as the removal of metals in various water sources as well as effluents used in the mining and metallurgical industry and in the treatment of mining acid solutions. The most common varieties of zeolites are analcime, chabazite, clinoptilolite, heroinite, ferrierite, mordenite. Researchers have studied for decades their synthesis, structure and proprieties, and as a result a large number of patents have been obtained on the preparation of different types of zeolite in conjunction with metals or metal compounds as selective adsorbents for anions and cations of water solutions. For example, iron, zirconium and magnesium hydroxide show selectivity in the adsorption of arsenic among other industrial applications of great interest.This study shows the importance of using natural pre-treated zeolites in the adsorption of arsenic in water due to the adsorption percentages that are higher than 90%, controlling the pH in the pre-treatment. Samples were treated at different pH levels, getting a better adjustment of isotherms of adsorption at pH = 9 with ZP-9.