Stabilization Of Heavy Metals Of River And Lake Sediment By Hydrothermal Treatment

The release of heavy metals from sediment to water is one of the key problems in water restoration.When the water environment changes,the release of heavy metals enriched in the sediment of water can pollute the overlying water body,cause secondary pollution of water body,and there is a risk of entering the human body through the food chain.Therefore,the stabilization of heavy metals in sediment is an important measure to control the risk of heavy metals in sediment.Due to the great differences in the properties of sediment in different water bodies,this study took sediment in rivers and lakes as the research object and took hydrothermal treatment as the treatment method.By changing the hydrothermal reaction conditions,the physical and chemical properties of sediment in rivers and lakes before and after heat treatment as well as the migration and transformation behavior of heavy metals were explored.The main studies are as follows:(1)Occurrence form and risk assessment of heavy metals in sediments.This study conducted environmental risk assessment and analysis of heavy metals in river and lake sediments from five aspects: heavy metal concentration,chemical speciation,RAC risk assessment,heavy metal ecological risk assessment and leaching toxicity.The similarities and differences of heavy metals and environmental risks in river and lake sediment were analyzed.The results showed that the heavy metals in the sediment of the river and lake used have certain environmental risks,and due to the different sources of pollution,the river sediment is more polluted than the lake bottom mud and requires harmless treatment.The analysis of the form of heavy metals showed that Cd in lake sediment,Zn and Cd in river sediment mainly exist in direct risk state,Pb in lake sediment,Cu and Pb in river sediment mainly exist in potential risk state,and Zn,Cu,Cr,Ni and Cr and Ni in river sediment mainly exist in stable state.The RAC evaluation showed that Zn and Cd in the sediment are high-level environmental risks,Cd in the lake sediment is high-level environmental risks,Ni in the river sediment is a medium-level environmental risk,and Zn,Cu,Cu,Ni,Pb and Cu,Cr,and Pb in the sediment of the river show low environmental risk,and the Cr in the sediment of the lake shows no environmental risk.The ecological risk assessment of heavy metals shows that the sediment of rivers and lakes have certain potential ecological risks,that is,the sediment of lakes have slight ecological hazards,and the sediments of rivers have moderate ecological hazards.The analysis of leaching toxicity shows that the river and lake sediment all show a certain potential leaching toxicity risk.(2)The influence of hydrothermal treatment on the physical and chemical properties of heavy metals in sediments.This research adopts hydrothermal method to treat river and lake sediment,and studies the effects of hydrothermal temperature and hydrothermal time on the organic matter reduction,dehydration performance,nutrients(N,P)and organics leaching of river and lake sediment,and passed the element analysis,surface characteristics analysis and infrared spectrum analysis of river and lake sediment have explored the main reasons for the reduction and stabilization of river and lake sediment.The research results show that after hydrothermal treatment of river and lake sediment,its organics reduction,dehydration performance improvement,nutrient(N,P)release,and organics leaching concentration are reduced,achieving the reduction and stabilization of river and lake bottom mud.And the higher the hydrothermal temperature or the longer the hydrothermal time,the more significant the reduction and stabilization effect.At the same time,hydrothermal treatment causes dehydration,decarboxylation,deamination,aromatization and other reactions in the river and lake sediment,resulting in an increase in the ash content of the river and lake sediment,the proportion of C,H,O,and N,the H/C ratio,and the O/C ratio and p H value decrease,the Zeta potential,porosity,specific surface area,and contact angle all increase,and the enhancement of dehydration performance.(3)The influence of hydrothermal treatment on the toxicity and migration and transformation behavior of heavy metals in sediments.The migration and transformation of heavy metals in the river and lake sediment hydrothermal treatment process are studied,and the environmental risks of heavy metals in the river and lake sediment after hydrothermal treatment are evaluated to ensure the safety of the river and lake sediment after hydrothermal treatment.The research results show that after hydrothermal treatment,the heavy metals in the river and lake sediment migrate and transform from the weakly bound state to the relatively stable state and stable state.At the same time,the content of heavy metal in the leachable part is also reduced,thus realizing the stabilization of the heavy metals in the river and lake sediment.The RAC evaluation showed that the environmental risks of heavy metals in lake sediment are reduced to the lowest level after hydrothermal treatment at 280 ℃ for 3 h.After hydrothermal treatment at 280 ℃ for 3 h,the environmental risk of heavy metals in river sediment is also reduced to the lowest level,and the environmental risk of Zn and Cd is reduced from high risk to medium risk,the environmental risk of Ni and Pb is reduced to low risk,and the environmental risk of Cu and Cr is reduced to no risk.Moreover,the higher the hydrothermal temperature or the longer the hydrothermal time,the lower the environmental risk of heavy metals in the sediment of rivers and lakes.The ecological risk assessment of heavy metals showed that hydrothermal treatment can significantly reduce the potential ecological hazards of heavy metals in sediment S1 and S2.And the higher the hydrothermal temperature or the longer the hydrothermal time,the lower the potential ecological risk of heavy metals in river and lake sediments.After hydrothermal treatment at 240 ℃for 7.5 h,the potential ecological risk index RI value of lake sediment S1 decreased from 144.26 to32.80,and the potential ecological risk index RI value of river sediment S2 decreased from 237.68 to 113.87,reducing the slight ecological hazard from moderate ecological damage harm.The leaching toxicity analysis showed that after hydrothermal treatment at 280 ℃ for 7.5 h,the leaching concentration of Zn,Cu,Cr,Cd,Ni,and Pb in the lake sediment decreased by 77.36%,84.41%,65.62%,25%,58.12% and 59.62% respectively,and the leaching concentration of Zn,Cu,Cr,Cd,Ni,Pb in the river sediment decreased by 88.89%,80.13%,55.17%,51.52%,89.52%,85.09%respectively,and the leaching concentrations of heavy metals were all lower than the international standard limits.In addition,the leaching rate of heavy metals in the lake sediment reached the lowest value after hydrothermal treatment at 240 ℃ for 7.5 h,and the leaching rate of heavy metals in the river sediment reached the lowest value after hydrothermal treatment at 280 ℃ for 3 h.After hydrothermal treatment,the heavy metals in river and lake sediment tend to be stabilized.On the one hand,it may be because some heavy metals existing in weak bond state in river and lake sediment dissolve into liquid phase and form more stable minerals through precipitation and complexation with anions in liquid phase.On the other hand,it may be due to the hydrothermal treatment that some organics(alkanes,cyclic hydrocarbons,etc.)in the sediment take place aromatization reaction to generate aromatic groups,which combine with some dissolved heavy metals or heavy metals in the sediment to generate strong bond compounds,and finally be fixed in the minerals.