Modified Potassium Feldspar Passivation Of Heavy Metals In Municipal Sludge And Its Land Use Safety Evaluation

With the development of society,the safe discharge,storage,treatment and disposal of sludge has become one of the major problems that need to be solved urgently in China,and has also become a bottleneck limiting the development of the wastewater treatment industry.Land application of sludge can utilize the organic matter and nitrogen and phosphorus nutrients in it to improve the stability of soil and effectively solve the sludge treatment problem.However,heavy metals in sludge are the main factor limiting the land application of sludge.The use of clay minerals to passivate heavy metals from sludge is a low-cost and efficient method and can improve soil properties.Therefore,in this thesis,potassium feldspar was modified to passivate sludge heavy metals with modified potassium feldspar,and the effect of passivated sludge on plant growth was investigated using germination and potting experiments.The environmental risk of sludge land use was evaluated by studying the enrichment and transport properties of ryegrass for sludge heavy metals and determining the transport characteristics of sludge heavy metals in combination with leaching experiments.The results of the study are as follows:1.The optimum modification conditions of potassium feldspar were determined:potassium feldspar,limestone and water were mixed in the ratio of 2:1:3 by mass,dried and ground,and calcined at 900 degrees Celsius for 30 minutes.After modification,the adsorption performance of potassium feldspar was enhanced,and the adsorption rate of Cu²⁺increased from 5.05%to 99.75%,which was 18.8 times higher.After modification,the lattice defects of potassium feldspar increased,the integrity of crystals decreased,the specific surface area increased 13.5 times of that before modification,and the cation exchange amount increased 5.8 times.2.In the sludge heavy metal passivation experiments,the stability coefficients of Cu,Cr and Zn in the sludge decreased and the proportion of the residue state increased as the addition of modified potassium feldspar increased.The stability of sludge heavy metals increased with the duration of passivation,with the maximum effect being achieved at a passivation time of 10 days.The stability coefficients of sludge did not significantly change if the passivation time was longer than 10 days.The ideal passivation conditions of potassium feldspar for heavy metals were established as 30%potassium feldspar addition and 10 days passivation time,taking into account the passivation effect of the four heavy metals.The passivated sludge’s p H and conductivity met the requirements for land use.3.While the unpassivated sludge extracts significantly inhibited seed germination（the relative seed germination was 68.3%）and radicle growth（the relative root elongation was 45.2%）,the passivated sludge extracts had no adverse effects on ryegrass seed germination and promoted radicle growth.Passivated sludge had a growth promoting effect on ryegrass after applied to soil.The addition of 10%passivated sludge to the soil had the best effect on promoting the growth of ryegrass,with plant height,tillering number,wet weight and chlorophyll content increasing by 22.0%,12.3%,111.2%and 17.7%,respectively,compared to the control treatment.All these indicators decreased compared to W10 treatment,when 20%of passivated sludge was added,indicating that the optimum amount of passivated sludge is 10%.The application of unpassivated sludge had a significant inhibitory effect on plants growth.The addition of 10%passivated sludge did not increase the risk of heavy metal transfer to ryegrass,while the addition of unpassivated sludge increased the risk of transfer.The transfer coefficients of all four metals were less than 1.The enrichment factors and transfer coefficients of Cu and Zn,elements required for plant growth,were higher than those of Pb and Cr.4.With the addition of 10%passivated sludge,the concentrations of Cu and Cr in the leachate met the Class II index of the groundwater quality standard and the concentration of Zn met the Class I index of the groundwater quality standard.This indicates that the addition of 10%passivated sludge to the surface soil does not pose a risk of heavy metal contamination in groundwater.The kinetic analysis of the cumulative release of heavy metals showed that both the double constant rate equation and the parabolic diffusion equation were well fitted for Cu,Cr and Zn,while the first order kinetic equation and the modified Elovich equation were poorly fitted,indicating that the release process of heavy metals in sludge was a complex process involving multiple factors.