The Effect Of Aging Process On Heavy Metal In Contaminated Soil Amended With Sludge Biochar

The residual sludge of municipal sewage treatment plant can be converted into stable biochar material by slow pyrolysis technology,which can be used for the remediation of contaminated soil,providing a new way for the safe utilization and disposal of sludge.In this paper,sludge(S)and sludge biochar(SBC)are used as remediation materials.Through pot experiments,the studies were carried out to evaluate the effects of S and SBC applied to heavy metal contaminated soil on the physical and chemical properties,heavy metal morphology and bioavailability in soil,and analyze the effect of biochar aging on the remediation effect in the two-year experimental period.The main conclusions are as follows:(1)During the two-year cultivation experiment,the addition of S and SBC increased soil pH,organic matter(SOM)and cation exchange amount(CEC),and reduced exchangeable acidity.The effect of the 10% applied amount is better than the 5% applied amount.The improvement of S on CEC was superior to SBC,while the improvement of SBC on soil pH,organic matter(SOM)and exchangeable acidity were slightly less than S.(2)Compared with control,the addition of S and SBC can effectively reduce the acidextractable fraction of Cr,Cu and Zn in soil,and the decrease of acid-extractable Cr,Cu and Zn in soil amended SBC is superior to S.The acid-extractable Cr,Cu and Zn in the soil with 10% of SBC is down to 2.4%,3.6%,6.3%.The acid-soluble fractions of Cr,Cu,and Zn converted to the residual fraction,which is stable form and unavailable,based on experiment results.The leaching experiment results show that SBC can inhibit the migration of heavy metals in the soil under the condition of acid rain.(3)The results of cultivation experiments show that the uptake of Cr,Cu and Zn by radish decreased with SBC,and the reduction effect of 10% is better than 5%.At the end of the two-year experiment,compared with the control,the accumulation of Cr,Cu,and Zn with 10% of SBC reduced by 95%,43.1% and 44.1% in the shoot of radish,respectively,and reduced by 96.9%,87.1% and 70.3% in the root portion,respectively.The addition of sludge can also reduce the uptake of Cr,Cu and Zn in the root of radish compared with the control,while at the early stage of the experiment,the uptake of Cr,Cu and Zn in the shoots portion increased,and the sludge has the risk of heavy metal release.The uptake of Cr,Cu,and Zn by radish continually decreased with sludge at the application rate of 5% and 10%.At the end of the second year experiment,the uptake of Cr,Cu and Zn in the shoot of 10% of S radish decreased by 95%,43.1% and 44.1% respectively,and in the root portion decreased by 96.9%,87.1% and 70.3%,respectively.(4)Biochar was isolated from the soil,and the characterization results show that the oxygencontaining functional groups such as-OH and-COOH on the biological surface of sludge increased,the porosity decreased,and the specific surface area decreased and then increased.The pores in the scanning electron microscope line were filled,and Cr,Cu and Zn were enriched on the biochar surface.The aging of biochar in soil environment causes the change of physical and chemical properties,which lead to the decrease of the remediation effect.At the end of the second year experiment,the soil pH decreased by 9.4% and the exchangeable acid increased by 1.3%,compared to the third cultivation period(autumn in 2018)with the best results.However,the aging of biology has a positive impact on heavy metal morphology and biological effectiveness,the acid-soluble fractions of Cr,Cu,and Zn kept falling down.At the end of the second year experiment,compared with the result of the first year experiment,the acid-soluble fractions of Cr,Cu,and Zn in the soil with 10% of SBC reduced by 1%,9.6%,5.8%,the enrichment of Cr,Cu and Zn in the shoot of radish reduced by 86.9%,53.6%,37.9%,the enrichment of Cr,Cu and Zn in the root of radish reduced by 82.6%,27.8%,29.4%.