Migration And Transformation Of Heavy Metals In Oily Sludge During Pyrolysis

Oily sludge is one of the most common solid waste in the petroleum industry production process.Due to its presence of toxic and harmful substances such as petroleum hydrocarbons and heavy metals,improper disposal can pose serious risks to the ecological environment.Many countries have classified it as hazardous waste.Among them,heavy metals are a major obstacle to the resource utilization of oily sludge.Any resource utilization approach should consider the safety of heavy metals.Research on the distribution and morphology,stabilization,accumulation in sludge,and bioavailability of heavy metals in oily sludge has received great attention.Pyrolysis can enrich heavy metals in oily sludge in the residue to varying degrees,but the mobility,bioavailability and activity of heavy metals in the ecological environment mainly depend on their existing forms.Therefore,exploring the morphological evolution of heavy metals during pyrolysis is of great significance for obtaining their environmental chemistry behavior and migration laws,It is not only conducive to scientifically evaluating the environmental effects of heavy metals in residues in order to develop relevant strategies to deal with heavy metal pollution,but also reduces the risk to the ecological environment of oily sludge in resource utilization,promoting further industrialization of pyrolysis technology.This article takes oily sludge（OS）from a certain area in northern Shaanxi as the main research object,and combines it with campus garden soil（CS）.By adding typical heavy metals（Pb,Cr,Cd）externally,two types of high heavy metal sludge（AP-OS,AP-CS）were configured for comparative analysis.Based on the analysis of the forms and basic properties of heavy metals in the sludge,it was pyrolysed using a tubular furnace,High content heavy metal sludge pyrolysis residues were prepared under different pyrolysis conditions（pyrolysis temperature,pyrolysis time,additives,sludge properties）.The enrichment status,morphological changes,environmental risks,and valence changes of heavy metals in pyrolysis residues under different pyrolysis conditions were investigated using relative enrichment coefficient,BCR stepwise extraction method,risk assessment（RAC,PERI）,and X-ray photoelectron spectroscopy.The main research contents are as follows:（1）The BCR stepwise extraction method using ultrasound instead of mechanical oscillation was used to analyze Pb,Cr,and Cd in AP-OS and AP-CS,respectively.The heavy metal content,morphology distribution,and risk assessment results in AP-OS and AP-CS were obtained,which not only ensured the extraction rate but also shortened the extraction time（16h→30min）.（2）The pyrolysis of AP-OS and AP-CS was carried out using a tube furnace,and pyrolysis residues were prepared under different pyrolysis conditions（pyrolysis temperature,pyrolysis time,additives,and sludge properties）.The Pb,Cr,and Cd in the residue were analyzed using ultrasonic BCR stepwise extraction method,and the correlation between pyrolysis conditions and heavy metal enrichment,morphology distribution,and risk assessment in the residue was obtained.In the range of pyrolysis temperature from 400℃to 1000℃,Cd and Pb are more sensitive to pyrolysis temperature,and escape occurs at 600℃and 700℃,respectively.Cr is well enriched in the pyrolysis residue within the experimental temperature range.As the temperature increases,the migration of Pb slightly increases between 400℃and 700℃.The migration of Cr in the pyrolysis residue is significantly enhanced,and the proportion of components with migration is relatively high.However,as the pyrolysis temperature increases to above 800℃,The majority of Cr exists in a stable residual state,and pyrolysis gradually stabilizes Cd in the oil sludge,with 500℃being the most suitable pyrolysis temperature;The influence of pyrolysis time on the migration of heavy metals is relatively small,but there is a synergistic effect with pyrolysis temperature;The addition of Na Cl can enhance the enrichment of Pb and Cr,making Cd more easily volatile.At the same time,it can enhance the mobility of Pb and Cd,but weaken the mobility of Cr;Ca O has little effect on the enrichment of Pb and Cd,but it can better fix Cr in the pyrolysis residue and reduce the leaching ability of Pb,Cr,and Cd heavy metals.Adding corn straw can slightly enhance the leaching ability of Pb,promote the transformation of Cr’s reducible state to other forms,and generally improve the leaching ability of Cd;Both acidic and alkaline environments are not conducive to the immobilization of Pb,and have little effect on Cr and Cd.The acidic environment has a significant activation effect on Pb,Cr,and Cd,while the alkaline environment has little effect on the morphological changes of Cr and Cd,but it can improve the mobility of Pb.（3）XPS was used to analyze the main valence states of Cr in OS and its pyrolysis residue.Representative related valence compounds were selected for pyrolysis,and the valence state changes of Cr before and after pyrolysis between OS and related valence compounds（Cr（0）,Cr（III）,Cr（VI））were compared and analyzed.Cr in OS mainly exists in valence states of 0,3,and 6.After pyrolysis treatment,low and high valence Cr tends to transition to intermediate valence states;The main sources of Cr（III）present in OS pyrolysis residue can be classified into three categories:Cr（0）,Cr（III）,and Cr（VI）,and the residues are formed by various valence Cr compounds（including but not limited to Cr、Cr（OH）₃、Cr₂O ₃、Cr O₃）.