Effects Of Pyrolysis Temperature And Agro-forestry Biowastes Addition On Immobilization And Environmental Risk Of Heavy Metals In Biochars Derived From Municipal Sewage Sludge

To investigate the effects of pyrolysis and addition of external raw materials(bamboo sawdust(BS),rice husk(RH))on the characteristics and ultrastructure of biochars derived from municipal sewage sludge(SSB)and content and fractionation of heavy metals in SSB,the sludge-based pyrolytic biochars were obtained in a self-made furnace under 400-600 ? using sludge or its mixture with bamboo sawdust and rice husk as raw material.Advanced instruments including FTIR and SEM were employed to reveal the change of elemental components,functional groups and surface characteristic before and after pyrolysis process.The variation of the heavy metals speciation was also illustrated via BCR sequential extraction technique.The effects of pyrolysis temperature and addition of external raw materials on immobilization and the potential ecological risk of heavy metals in the sludge-based biochars were critically assessed based on the outcomes mentioned above,which were also essential for providing with sufficient scientific evidence to evaluate the feasibility of biochar in real application.The results showed that the H/C ratio in sludge-based biochars was dramatically decreased with the increase of temperature during pyrolysis while the pH value and ash content as well as surface area were increased.Meanwhile,the higher temperature in pyrolysis process tended to promote the aromatization of carbon in accordance with the FTIR analysis,although the contents of H and N was decreased accordingly.The result of BCR sequence extraction revealed that the bioavailable fractions of heavy metals(F1+F2)were significantly reduced while the oxidisable(F3)and residual(F4)fractions were increased in the pyrolytic process.It indicated that the higher temperature of pyrolysis may effectively mitigate the potential ecological risk caused by the heavy metal in the sludge.In comparison with sludge only,the co-pyrolysis of the mixture of sludge and bamboo sawdust/rice hulk had a lower production rate of end product,H/C ratio and ash content.The total heavy metals concentrations showed a decreasing pattern due to the dilution effects by the addition of external raw materials.More importantly,the bioavailable fractions of heavy metals(F1+F2)were further reduced as the oxidisable and residual fractions were increased after the pyrolytic process.It implied that the co-pyrolysis endowed the pyrolytic biochar with a much lower ecological risk.In addition,the rice hulk had a better performance than bamboo sawdust in immobilization of heavy metals.