This study aims to evaluate the possibility of using raw silica extracted from Bouillante geothermal waters (Guadeloupe, FWI) as an adsorbent for the remediation of natural waters contaminated by industrial pollutants. The raw silica SiO2 was tested as well as a modified silica called PEI/SiO2 prepared by impregnation polyethyleneimine (PEI) onto the raw silica surface by a simple adsorption process. The impregnation of the PEI molecules onto SiO2 surface was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analysis. The changes in porous structures were evaluated by N2 adsorption at 77 K. The adsorption capacities of SiO2 and PEI/SiO2 were investigated for two contaminants: methylene blue (MB) and lead (Pb(II)). The influence of various parameters (pH, temperature, initial concentration, adsorbent dosage) was studied. Adsorption isotherms were fitted with the Langmuir, Freundlich, Redlich-Peterson and Brouers-Sotolongo models and the kinetic data with the pseudo-first-order and pseudo-second-order models. The results show that raw geothermal SiO2 has an interesting adsorption capacity towards methylene blue (qe = 108 mg g−1), with no significant enhancement of the adsorption capacity with the PEI impregnated silica. On the other hand, the PEI impregnation significantly improved the adsorption capacity of geothermal silica for lead since qe increases from 5.3 to 18 mg g−1. For both pollutants, the data modeling reveals that the adsorption kinetics corresponds well with pseudo-second order kinetic model suggesting a chemisorption, and the adsorption isotherms are in agreement with Redlich-Peterson model.