This paper presents the results of characterization, thermal, and chemical treatments made over natural eggshells (ES) and olive stones (OS) samples. The main goal was to perform an analysis to assess the potential of treated ES and OS to remediate heavy metals from polluted soils with mining tailings (PSMT). Sample powders of ES, OS, and PSMT were mechanically treated and characterized with chemical analysis, X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric, and differential thermogravimetric techniques (TGA & DTG). ES was subjected to a calcination test. OS samples were thermally treated and chemically modified with thiol. PSMT was characterized using specific gravity, average particle size, and environmental indicators. FT-IR analysis indicated the presence of CaCO3, Li2CO3, K2CO3, and KNO3 functional groups in ES and OS. The XRD analysis of ES samples showed the presence of one mineralogical phase (CaCO3). Diffraction patterns in the OS could not be identified due the amorphous condition of the sample. ES calcination tests presented a mass loss near 47% (w/w). The process to obtain chemically modified pre-oxidized biochar from OS reported pyrolysis mass loss of 80.59%, pre-oxidized mass loss of 35.43%, and an addition of 0.14 g during the chemical treatment. Equilibrium diagrams showed that Cd+2 and Hg+2 are most likely in the PSMT soil solution (aqueous phase), whose cations could be removed via cation exchange of the calcinated ES. The mercaptoethanol content in the chemically modified pre-oxidized biochar could be effective in removing the Hg content in PSMT.