Study On Migration Of Heavy Metals And Utilization Of Products During The Thermal Treatment Of Sedum Alfredii

Phytoremediation,one of remediation technology of heavy metal contaminated soil,has attracted increased attention due to its environmental friendliness and economic feasibility.However,how to realize the harmless treatment of heavy metal hyperaccumulator and avoid secondary pollution is one of the bottlenecks in the promotion of its application.In this paper,the effects of different reaction conditions on the migration of heavy metals and the characteristics of biochar were studied by pyrolysis and hydrothermal conversion of Sedum alfredii,a Cd/Zn-co-hyperaccumulator.The differences between pyrolysis and hydrothermal conversion were further compared,and the approaches of harmless treatment of S.alfredii and resource utilization of products were put forward.The pyrolysis of S.alfredii and the co-pyrolysis of S.alfredii and polyvinyl chloride（PVC）were performed to study the effects of temperature（300-900℃）and PVC addition（5-25 wt.%）on the migration of heavy metals and the characteristics of pyrochar.The results showed that:（1）Increased temperature facilitated the volatilization of Cd and Zn and improved the stability of Cd and Zn retained in pyrochar,which reduced the toxicity leaching risk.The derived pyrochar showed considerable Cd sorption capacity（87.6-198.3 mg/g）.（2）The actual pyrochar yield was 5.18-37.19%higher than the theoretical value with the addition of PVC.Low PVC amount（5 wt.%）promoted the volatilization of Cd and Zn,whereas high PVC amount（15-25 wt.%）inhibited the volatilization of Cd and Zn at insufficient temperature.With the increase of PVC addition,the leaching risk of Cd and Zn in pyrochar increased and the Cd sorption capacity declined.PVC has a double-edge effect during the co-pyrolysis process.Hydrothermal conversion of S.alfredii was carried out to investigate the migration of heavy metals and the characteristics of hydrochar at different temperatures（210-300℃）and reaction medium（H₂O and HCl）.The results showed that:（1）Low temperature and acidic medium could promote to the removal of heavy metals.（2）Temperature and reaction medium comprehensively affected the characteristics of hydrochar.High temperature reduced the leaching toxicity risk of Cd and enhanced the leaching toxicity risk of Zn in hydrochar.Acidic reaction medium facilitated the migration characteristics of Cd and promoted the stability of Zn in hydrochar.（3）Hydrochar derived from low temperature showed better Cu adsorption capacity（10.14mg/g）,while acidic reaction medium improved the higher heating value of hydrochar（19.97-20.79 MJ/kg）.Therefore,the hydrothermal conversion under low temperature and acidic medium could simultaneously realize the harmless treatment of S.alfredii and the resource utilization of hydrochar.The difference between pyrolysis and hydrothermal conversion technology leads to the difference of heavy metal recovery and environmental application potential between pyrochar and hydrochar.Low temperature thermochemical conversion technology has a promising future in the field of biochar energy recovery.The pyrolysis and hydrothermal conversion of S.alfredii can provide practical guidance for the harmless treatment of heavy metal-enriched hyperaccumulators and the resource utilization of products,which is of great significance to realize the green sustainable development of phytoremediation.