Adsorption is an effective method for the treatment of wastewater containing low concentrations of heavy metals. This kind of metals such as Chromium and Lead could affect health and the ecosystem. In this work, biomass of avocado seed was used as adsorbent. It was tested as adsorbent in natural form (NB), as a chemically activated (AB) form and as activated carbon (AC). Batch reactors were used to investigate the adsorbent efficiency. Concentration of metal ions was measured using Total Reflection X-Ray Fluorescence. Operational conditions influencing adsorption, such as: pH, adsorbent dose, initial concentration and contact time, were measured and controlled. The 80% of adsorption was reached, at pH: 5 and 25 °C, when were used 50 mL of: a 20 mg L of Cr (VI) solution with a dose of 1.25 g of NB, a 30 mg L of Pb (II) solution with a dose of 0.15 g of NB, a 50 mg L Pb (II) solution with a dose of 0.15 g of AB, a 30 mg L Cr (VI) solution with a dose of 0.35 g of AB, a 30 mg L of both metals, with a dose of 0.15 g of AC for Pb (II) and 0.7 g of AC for Cr (VI). In all cases, the pH value before and during the experiments remained constant, indicating the lack of acid/base reactions during the processes. The Langmuir adsorption isotherm model best fitted to the experimental data. The experimental results from kinetic studies best correlated using the pseudo-second order model. An increase in the remotion of both ions (Pb (II) and Cr (VI)), was observed when comparing the results obtained using the activated biomass. However, considering the loss of biomass that the pre-treatment causes, the remotion per gram of initial biomass does not vary significantly.