Effect of carbon dioxide on pyrolytic products characteristics and DOM binding heavy metals copper in sewage sludge biochar

In this study, a new method of calculation and comparison on pyrolysis products is introduced in a sealed tube furnace under different pyrolysis conditions, to explore the effects of carbon dioxide (CO2) as carrier gas on biochar characteristics. In general, the pyrolysis temperature exhibited a more significant impact on biochar characteristics than residence time. The dissolved organic carbon of biochar derived in CO2 reduced by 53.5 % and 81.6 % for 30 min pyrolysis at 500 °C and 700 °C, respectively. In addition, CO2 could reduce the total heavy metals content in biochar compared with nitrogen atmosphere under the same treatment condition. Meanwhile, the soluble microbial by-products and the humic acid-like substances were more susceptible in pyrolysis with CO2, whereas the protein-like substance containing tryptophan, tryptophan related to biological and humic-like substance involved in binding copper. Actually, it was the Fact Sage thermodynamic simulation combined with main syngas composition that identified the CO2 mainly affected the carbonaceous distribution via the dehydrogenation, methane production and Boudouard reaction, made the proportion of carbon monoxide in syngas up to 18.5 % at 700 °C. These results can provide an important path for further applying CO2 as a raw material and enhance the feasibility of sewage sludge biochar application in farmland soil.