Effects Of Different Reaction Conditions On Characteristics Of Sludge Hydrothermal Carbonization Products And Behavior Of Heavy Metals

In recent years,faster urbanization in China has resulted in an increasing amount of sewage sludge.Therefore,it’s an urgent task to treat the sludge and convert it from pollutants to biomass,the so-called "turning waste into treasure".Hydrothermal carbonization as a moderate yet efficient method for the transformation,is increasingly favored by researchers.It can not only increase production capacity,but also stabilize sludge and reduce its ecological hazard.This paper studied the respective effects of hydrothermal reaction temperature(180-300℃),hydrothermal reaction time(90min-180min)and biomass excipients(rice hulls,bamboo shavings)on the quality of hydrothermal carbonization in reducing heavy metal risk,with remnant sludge from Lin’an Wastewater Treatment Plant as the raw material,and the results show that:As the temperature increases,the carbon yield of hydrothermal carbon decreases,the specific surface area-4ncreases,the porosity increases,the pH value decreases,and the acidity of hydrothermal carbon increases.The content of C,H,N and S elements decreased,the molar ratio of H/C and O/C decreased,and the aromaticity of hydrothermal carbon increased.Hydrothermal carbonization promoted the enrichment of heavy metals Cu,Zn,Pb,Cr and Ni in hydrothermal charcoal,and the enrichment level was the highest at 240℃.As the temperature rises,the effective state of the heavy metals and the leaching toxicity content decrease,and are at the minimum at 300 ℃.The continuous extraction results of BCR shows that the unstable state of heavy metals is converted to a stable state,and the mobility of each heavy metal element is lowered.The RAC ecological risk assessment showed that the increase in temperature promoted the ecological risk of hydrothermal charcoal,and the heavy metal elements were reduced to no risk or low risk at 300℃.Treatment time(90 min,180 min,360 min)has a significant effect on the physical and chemical properties of hydrothermal charcoal and the ecological risk of heavy metals.With the prolongation of hydrothermal carbonization time,the yield of hydrothermal carbon is slightly increased,the specific surface area is increased,the pores are increased,and the pH value is decreased.The contents of C,H and N decreased as the reaction time prolonged.The molar ratio of H/C and(O+N)/C atoms decreases,the aromaticity of hydrothermal carbon increases,the polarity decreases,and the stability is enhanced.The hydrothermal carbonization reaction promoted the enrichment of heavy metals in hydrothermal charcoal,and the enrichment was highest at 180 min.The continuous extraction results of BCR show that hydrothermal carbonization is beneficial to reduce the effective content of heavy metals and promote the conversion of heavy metals to stable state.The toxicity and effective content of heavy metal leaching decreased with the prolongation of treatment time,and reached the lowest value at 180 min,except Pb.The RAC risk assessment showed that the amount of Cu,Zn℃ Pb,Cr,and Ni were all at low-risk or risk-free level when the reaction time was 180 min.Based on the physical and chemical properties of hydrothermal carbon and the risk assessment of heavy metals,180 min is the best treatment time for hydrothermal carbonization.The addition of rice hulls and bamboo shavings has a positive effect on carbon quality and can reduce the effectiveness of heavy metals.With the addition of rice hulls and bamboo shavings,the ash content and specific surface area of the sludge biomass carbon increased,the yield increased,and the H/C,(O+N)/C,O/C atomic molar ratio decreased,while Fourier infrared spectrum showed that the addition and hydrothermal carbonization facilitated the aromatization of the sludge and thus promoted the production of aromatized substances.The hydrothermal carbonization reaction promotes the enrichment of heavy metals in the sludge,but the addition of biomass excipients can produce a certain dilution effect on heavy metals.The results of BCR continuous extraction and heavy metal leaching toxicity analysis showed that the addition of rice hulls and bamboo shavings could significantly reduce the percentage of heavy metals in biologically active state and leaching toxicity,and increase their content in oxidizable and residual states.It shows that the addition of biomass excipients can effectively reduce the potential ecological risk level of heavy metals in sludge and promote the synergistic passivation of heavy metals.