Biochars, serving as natural fertilizers and agents improving water retention, have been used in agriculture for years. Although obtained via biomass pyrolysis, the CO2 released during their production had been absorbed during the plant growth. Due to the risk of contaminants dissemination in the environment, biochars produced from biomass harvested on contaminated areas lose the agricultural usefulness. However, even when buried deep underground, they maintain their CO2-sequestering properties. Converting contaminated biomass into unreactive biochar later safely stored underground is a concept that might result in both improved biosphere purity and reduced greenhouse gases levels. The passivation effect can be achieved by creating a low- or non-reactive coke layer on the biochar surface. Within this research, passivation of contaminated biochars by tar conversion over their surface was attempted. The examined biochars were prepared at different temperatures (500°C, 700°C, 900°C) from birch wood samples sourced from an area polluted with Pb and Zn. Moreover, a portion of the biochar produced at 700°C was submitted to further activation via gasification using CO2 and CO2/steam mixtures. Conversion of three model tars (toluene, eugenol, acetic acid) was analysed in a quartz-tube reactor. In parallel, tests with tars generated in-situ by gasification of cellulose, lignin and beech wood chips were performed. The amount of model tar converted as well as the conversion products were assessed by means of GC-FID. The highest tar conversion was observed in case of eugenol. SEM-EDS and XPS analyses confirmed that the contamination of the passivated biochars’ surface was lower than for the respective non-passivated samples. Additionally, the differences in graphitization degree of both pristine and passivated biochars were assessed by Raman spectroscopy. The influence of pyrolysis temperature and activation conditions on metal retention within the carbon matrix was examined by ICP. To examine the morphology and surface area of the pores, dual 2D-NLDFT model was fitted to the measured N2 and CO2 adsorption isotherms. Both N2 and CO2 adsorption isotherms for passivated biochars were significantly lower than in case of pristine biochars, which corresponds to the decay of micro- and mesopores during the passivation process. For samples passivated with eugenol, the decline was observed even at the ultramicroporous level.