Study On Phosphate Loading Performance Of Modified Sludge Biochar And Remediation Effect Of Adsorption Products On Heavy Metal Contaminated Soil

In this paper,a kind of sludge biochar was prepared by pyrolysis according to the properties of sludge,in order to meet the need of sludge recycling and safe utilization.Based on the basic properties of this sludge biochar,deashing+magnesium chloride modification was used to optimize the preparation method,and the phosphate adsorption performance of the sludge biochar was studied.At the same time,using this product of phosphate adsorption as remediation agent,this paper focused on discussing the effect of phosphorus loaded sludge biochar on the properties of heavy metal contaminated soil.In addition,the remediation effect of this adsorption product combined with phytoremediation on heavy metal contaminated soil was also studied.The specific conclusions were as follows:(1)In the pyrolysis process of magnesium modified sludge without deashing,there was an obvious endothermic peak at 450?,which was significantly different from the initial sludge pyrolysis process.The BET value of this pyrolysis product Mg-SB was 99.57m²/g,about 6 times of SB,and its surface structure was rough,showing wedge-shaped and flaky structure.The XRD patterns of Mg-SB showed obvious periclase structure,which was closely related to phosphate adsorption,and sludge impurities had a negative impact on the formation of surface MgO.Infrared analysis showed that the main functional group structures of Mg-SB were C=O,R-OH and C-OH,but these functional groups were not the main reason for Mg-SB to adsorb phosphate;(2)The concentration of MgCl₂,pyrolysis temperature and pyrolysis time had important effects on the adsorption of phosphate by Mg-DSB.The optimal preparation conditions of Mg-DSB based on response surface methodology were:MgCl₂concentration 2.15mol/L,pyrolysis temperature 522.1?,pyrolysis time 2.19h.The BET value of Mg-DSB after deashing and activation was 1.6 times that of Mg-SB.FTIR analysis showed that there was no significant change of functional groups about Mg-DSB before and after adsorption.However,compared with Mg-SB without deashing,the types of functional groups increased.SEM analysis showed that the Surface Pores of Mg-DSB was finer than that of Mg-SB,and the drusy structure of surface magnesium oxide was more obvious.EDS analysis confirmed that the ash content of Mg-DSB was significantly reduced.The XRD patterns of Mg-DSB showed obvious structure of MgO and Mg(OH)₂,and new Mg-P compounds were formed after adsorption.However,surface deposition may be the main mechanism of phosphate adsorption by Mg-DSB,and SiO₂ may provide support for magnesium oxides to enhance its phosphate adsorption performance.The results of isothermal adsorption showed that the maximum amount of phosphate adsorption by Mg-DSB was500.437g/kg,and the removal rate of low concentration phosphate was more than 95%.The bioavailable phosphorus content of the adsorption product was 70.14mg/g,and this product showed a long-term fertility characteristics;(3)According to the investigation and analysis of heavy metal pollution in sediment and coastal area of Xiawangang,the results showed that:four kinds of heavy metals(Cu,Pb,Zn,Cd)accumulated seriously in this area,and the clay content of soil was closely related to Cd,Cu,Zn,while the pH and sand content of soil were closely related to Pb.The speciation analysis showed that Cd was mainly in exchangeable and carbonate form,Cu was mainly in organic and residual form,Pb was mainly in Fe-Mn oxide and residual form,while Zn was mainly in Fe-Mn oxide form.The I_geo index showed that the pollution of four heavy metals in Xiawangang was very serious,and the non-residual content of heavy metals was linearly related to the I_geo index.Furthermore,the RAC index indicated that Cd was at high risk,Cu and Pb were at low risk,while Zn was at moderate risk.At the same time,correlation analysis showed that the main source of four heavy metal pollution was industrial emissions;(4)Based on the pollution situation of this area,four different remediation agents(SB,DSB,P-Mg-DSB,PP)were used to repair the soil polluted by heavy metals.The results showed that SB treatment significantly increased the soil pH,while DSB,P-Mg-DSB and PP treatment had no obvious effect on pH.The contents of soil organic carbon,CEC,available N,available P,available K and microbial biomass carbon were significantly increased after four treatments.Among them,3%P-Mg-DSB had the best effect on improving soil properties.The main reason for the increase of soil organic carbon content was the addition of external carbon sources.While the increase of soil available nutrients was mainly related to the synergistic effect of phosphorus and biochar.The increase of soil CEC content was mainly due to the introduction of K and Mg ions.The activities of sucrose invertase and catalase were significantly promoted by the four remediation treatments,among which 3%P-Mg-DSB was the best,but the proportion change had no significant effect on sucrose invertase.SB and DSB treatment increased the content of alkaline phosphatase,but P-Mg-DSB and PP treatment inhibited the activity of alkaline phosphatase due to the increase of soluble phosphorus content.In addition,SB treatment inhibited soil urease activity,while DSB,P-Mg-DSB and PP treatment significantly increased urease activity;(5)Based on the analysis of the effects of four remediation treatments on the properties of heavy metal contaminated soil,combined with phytoremediation,the results showed that the four remediation treatments significantly reduced the TCLP extracting amount and the acid extractable content of soil Cu,Pb,Zn,Cd,among which 3%P-Mg-DSB treatment was the best.The order of P-Mg-DSB inhibiting heavy metal extracting was Pb>Zn>Cu>Cd,in which the fixation of Pb was mainly due to the formation of insoluble precipitation between phosphate and Pb.Four treatments significantly increased the fresh and dry weight of the aboveground and underground parts of maize plants,and promoted the growth height.Among them,3%P-Mg-DSB was the best,which was mainly due to the synergistic effect of phosphorus and biochar.In addition,compared with other treatments,the treatment of 3%P-Mg-DSB increased the phosphorus uptake in maize plants most significantly,but also reduced the concentration of heavy metals in plants obviously,and effectively inhibited the transfer of heavy metals to plants.Moreover,these three kinds of biochar treatment could improve the content of phosphorus in plants better than traditional phosphate fertilizer.The method of applying 3%P-Mg-DSB remediation agent combined with maize plants repair,made the RAC risk of Cd decreased from high to moderate,Cu from moderate to the lower limit of moderate,Pb and Zn from moderate to low,which showed the remediation effect was obvious.