Research On Stabilization Of Heavy Metals And High-value Utilization Of Bio-oil During Thermal Treatment Of Hyperaccumulators

Phytoremediation technology has a broad application prospect in the treatment of heavy metal pollution in soil.However,the plants which contain high concentration of heavy metals need to be properly disposed to realize harmlessness and resource utilization.Thermal treatment technology has been widely concerned because of its economic and environmental advantages.In this paper,the pyrolysis and combustion experiments of P.Vittata and Sedum spectabile were carried out in a tube furnace to study the migration characteristics of heavy metals in the process of thermal treatment and the stability of heavy metals in the solid residues.And the composition of fast pyrolysis product bio-oil of Sedum spectabile and the selectivity of phosphoric acid to the target product levoglucosenone(LGO)were further studied by Py-GC/MS technology.The main contents are as follows:1.The pyrolysis and combustion experiments of P.Vittata were carried out in a tube furnace to study the concentrations and speciation distributions of As and Pb in the solid residues obtained from different thermal treatment conditions.Results showed that the concentrations of As in the biochars and bio-ashes are about 350 mg/kg and1100 mg/kg,respectively which are almost unaffected by temperature while significantly affected by oxygen ratio.When the oxygen ratio was increased from 0 to7%at 700℃,the concentration of As more than doubled.The concentrations of Pb in the solid residues varied from 34 mg/kg to 1050 mg/kg which are affected by temperature and oxygen ratio.Based on BCR sequence extraction,As is more stable in the biochars with higher pyrolysis temperature while Pb is more stable in the combustion ashes,oppositely.However,the ecological risk assessment showed that the correlation index between the C_f of As(0.995)and the risk index is much bigger than that of Pb(0.117),which means pyrolysis method should be chosen at priority.2.The pyrolysis experiments with or without phosphoric acid pretreatment of Sedum spectabile were conducted at different temperatures in a tube furnace to study the migration characteristics and speciation distributions of As and Pb.The results showed that the recovery rate of As fluctuated and the recovery rate of Pb was increased firstly and then decreased with the increase of temperature.The recovery rate of As and Pb both reached maximum at 500℃with 66.2%and 73.08%respectively.The recovery of As and Pb was increased after adding 8%phosphoric acid in a certain temperature range and reached maximum at 300℃with 83.75%and 92.78%respectively.When the pyrolysis temperature was increased from 300℃to 600℃,the stable form(F4+F5)of As in biochars was increased from less than 20%to about 70%,and the most stable form(F5)of Pb was increased from 3%to 32%.After the pyrolysis with 8%phosphoric acid pretreatment,the stability of As in biochar was increased slightly with F5increasing by 20%and 5%respectively at 500℃and 600℃;and the stability of Pb was increased remarkably with(F4+F5)reaching more than 90%.The dosage of phosphoric acid had no significant effect on the distribution of heavy metals.These results indicated that phosphoric acid pretreatment could be used for pyrolysis of phytoremediation plants to increase the recovery and stability of heavy metals in the biochars.In the meantime,the potential ecological risk index was also reduced.3.The composition of bio-oil from biomass pyrolysis is complex.By choosing suitable additives and reaction conditions,the content of specific components can be increased.Py-GC/MS technology was used to study the effects of temperature and catalyst ratio on the relative content of LGO and other important products.The results showed that LGO was not detected at all temperatures under the condition of non-catalytic pyrolysis;the relative content of LGO was decreased with the increase of pyrolysis temperature after adding 16%phosphoric acid,and the maximum relative content of LGO was 24.44%at 300℃;the relative content of LGO was increased first and then decreased with the increase of pyrolysis temperature after adding 32%phosphoric acid,and the maximum relative content of LGO was 31.61%at 400℃.LGO was not detected after adding over 32%phosphoric acid.