Montmorillonites modified with lanthanum(III) and iron(III) were prepared from two commercial clays. These materials were characterized by powder X-ray diffraction analysis, X-ray fluorescence, infrared spectroscopy, N2 adsorption, and scanning electron microscopy. The incorporation of lanthanum(III) and iron(III) varied from 0 to 4.00%. The amount of these cations governed the adsorption capacity of the material since an increase in the percentage of La by 1.7 times improved the adsorption capacity by 3.3 times. The structural properties of the precursor clay minerals, such as pore size, may have influenced the extension of the modification process. Kinetics and equilibrium studies indicated that adsorption occurs mainly within 24 h, and the pseudo-second order and Lagmuir models described the process. The phosphate adsorption capacity ranged from 2.86 to 17.86 mg g-1 for the modified materials and was mainly dependent on the La content. A laboratory test with a natural water sample from the Jacarepaguá Lagoon showed reduction of the soluble reactive phosphorus concentration from 0.41 to 0.04 mg L-1, for the material with 4.00% La2O3. The materials described in this work were effective for the reduction of the soluble reactive phosphate, contributing to the research of a national product destined to the restoration of eutrophic aquatic environments.