Study On Passivation Of Heavy Metals In Municipal Sludge By Composite Phosphorus-Bearing Materials

As the urbanization process has been accelerating,the number of sewage treatment plants in China are also increasing rapidly,which has led to a rapid increase in the number of sludge generated.Excessive heavy metal content is one of the main factors limiting the use of sludge for land use.In this paper,the dewatered sludge collected from sewage treatment plant in Jinan West District was studied.The combination of phosphate rock powder and superphosphate was used to passivate the heavy metals in the sludge,and stepwise extraction of heavy metal from sludge by improved BCR?Community Bureau of Reference?sequential extraction procedure,and the best passivation conditions were selected by orthogonal test.The effect of optimal passivation sludge on the growth status and bioavailability of ryegrass was analyzed;Application of toxicity characteristic leaching procedure?TCLP?and soil column leaching simulation method to evaluate the risk of heavy metals in municipal sludge under different treatments.Finally,the mechanism was analyzed.The research provided guarantee for realizing sludge utilization as a resource and effectively controlling heavy metal pollution during the process of sludge land use.The main conclusions drawn were as follows:?1?The sludge had a pH similar to that of the background soil and a higher cation exchange capacity?CEC?and conductivity?EC?than the soil.The organic matter,TN,TP,available nitrogen and available phosphorus in the sludge were much higher than those in the background soil,and it could meet the physical and chemical indexes and nutrient indexes of the sludge used for landscape.The contents of Cu?Cr?Pb?Zn in the sludge were higher than those in the background soil,but it still need the standard of sludge used in garden or parks.?2?Phosphate rock powder and superphosphate have different passivation effects on different heavy metals.The single factor test showed that the phosphate rock powder had a better passivation effect on heavy metals Cu and Pb,the passivation effect on Cr was not obvious,and the passivation effect on Zn was poor.The superphosphate had better passivation effect on Zn,Cu,Pb and Cr,which could effectively reduce the bioavailability and ecotoxicity of heavy metals.Through orthogonal test,the best passivation conditions were as follows:10%phosphorus phosphate powder,3%calcium superphosphate and 14d passivation time.The passivation time has a significant effect on the passivation effect of Zn,and the addition amount of phosphate rock has a significant effect on the passivation effect of Cu.The most effective factor for Cr immobilization was the addition amount of superphosphate.?3?The passivation agent would not only immobilize heavy metals in sewage sludge,but also affect the physico-chemical properties and fertilizer efficiency of the sludge.The fertilizer efficiency index of sewage sludge was decreased after immobilizing treatment.The treated sludge still met nutrient requirements for the sludge used in neutral alkaline soils.The ryegrass planting experiment found that it could significantly increase the plant height,tiller number,chlorophyll content and dry and wet biomass of ryegrass by adding 10%passivation sludge to the soil.After 45days of ryegrass growth,the contents of Pb,Zn,Cu and Cr in the passivation group of the underground part were 3.80 mg/kg,367.72 mg/kg,30.85 mg/kg and 58.33 mg/kg,respectively,which were much higher than the heavy metal content in the matrix and above ground.Part of the heavy metal content.The roots of ryegrass can affect the form of heavy metals in the sludge,and the order of influence on the activation of the passivation group is Zn>Cu>Cr>Pb.?4?It was found by leaching experiments that the addition of passivating agent can significantly reduce the rate of heavy metal leaching.and the migration potential of heavy metals was following the order:Zn>Cr>Cu>Pb.When the proportion of passivation sludge was 10%,the concentration of heavy metals was below class IV limit groundwater quality standard.?5?The immobilizing effect of phosphorus-containing material on Pb was mainly related to soluble P.The fixation of Cu was mainly related to pH.Three passivation mechanisms of phosphorus-containing materials on heavy metals could be deduced by BET and XRD analysis.After adding phosphorus-containing materials,sludge pH rises,causing heavy metal ions to form hydroxide precipitates,Secondly,phosphorus-based materials dissolved and the PO₄^3-reacted with heavy metal ions to form insoluble phosphate precipitation.Thirdly,heavy metal cations were passivated by adsorption and coprecipitation on the surface of phosphorus-containing minerals.These mechanisms coexisted in the passivation process of composite phosphorus-containing materials.