Effect of pyrolysis condition on the adsorption mechanism of heavy metals on tobacco stem biochar in competitive mode

To clarify the adsorption mechanism of multi-ions on biochars in competitive environment is very important for the decontamination of co-existed heavy metals. Herein, tobacco stem was pyrolyzed in different temperatures with selected residences to obtain biochars with various surface chemistry. Then the adsorption of co-existed typical heavy-metal ions like lead, cadmium, and copper was studied, followed with systematic analysis of surface properties of the post-adsorption biochars. After carefully examining the adsorption performance and surface property alteration of the demineralized biochars, the adsorption mechanism of multi-ions in competitive environment was discovered. Lead showed the most competitive nature with co-existence of cadmium and copper, but the adsorption capacity reduced significantly with the removal of minerals. Combined with the observation of large amount of lead containing crystals on the post-adsorption biochars, the main adsorption mechanism of lead should be precipitation. The adsorb capability of copper barely changed for biochars with and without minerals, which suggests the best affinity of copper on surface functional groups even with large content of competitors. Biochar that pyrolyzed in 700 °C for 6 h that contained more aromatic structures showed the highest sorbing capability of cadmium, which suggested the dominant position of cation-π interaction in cadmium removal.