Biochar derived from corn stalk and polyethylene co-pyrolysis: characterization and Pb(ii) removal potential

Biochar is widely used as adsorbents for gaseous or liquid pollutants due to its special pore structure. Previous studies have shown that the adsorption performance of untreated biomass pyrolysis crude carbon is poor, which can be improved by optimizing physicochemical properties such as pore structure and surface area. The study focused on the co-pyrolysis of skins, pith, and leaves with polyethylene and potassium hydroxide modification to adjust the quality of the biochar, compared with raw materials of corn stalks without separation. The physical and chemical properties of the biochar were analyzed and the adsorption effect, adsorption isotherms, and kinetics of Pb(ii) removal were investigated. Results demonstrated that co-pyrolysis of biomass and polyethylene increase the yield of biochar with an average increase of about 20%. Polyethylene brought high aromaticity, high calorific value and stable material structure to biochar. Potassium hydroxide modification increased its specific surface area and made the pore structure of biochar more uniform, mainly microporous structure. The specific surface areas of the four modified biochar were 521.07 m² g^-1, 581.85 m² g^-1, 304.99 m² g^-1, and 429.97 m² g^-1. The adsorption capacity of biochar for Pb(ii) was greatly improved with the increase of the OH functional group of biochar. The stem-pith biochar had the best adsorption effect, with an adsorption amount of 99.95 mg g^-1 and a removal efficiency of 50.35%. The Pseudo-second-order model and Langmuir adsorption isotherm model could preferably describe the adsorption process, indicating the adsorption of lead was monolayer accompanied by chemical adsorption. It can be concluded that co-pyrolysis of biomass and polyethylene and modification may be favorable to enhance the properties of biochar. In addition to syngas and bio-oil from co-pyrolysis, biochar may be a valuable by-product for commercial use, which can be used to remove heavy metals in water, especially stem-pith biochar.