Heavy metals (HMs) are the most common and dangerous pollutants present in water. Among the different methods used for HMs removal, adsorption is one of the best options since its cost-effectivity and easy operation. In this regard, Metal Organic Frameworks (MOFs) adsorbents are gaining interest in that field due to their unique characteristics such high porosity, easy tunability and high chemical stability. In order to increase the adsorption capacity and selectivity of that adsorbents different modifications of the structure are carried out. In that work, six different UiO66 Zr-crystal frameworks have been used to study the effect of incorporating hydroxyl and amine groups within the organic linkers, as well as of the presence of defective positions on the adsorption capacity of Cr(VI) and Cr(III) species. Results (Figs. 1-7) shows that, the generation of defective points increase the density of chemisorption points for chromate anions (Cr(VI)) while for cationic Cr(III) species the density of hydroxyl or amine groups is the fact that governs the adsorption capacity.