Experimental Study On Improving Lead Contaminated Soil With Phosphogypsum-lignin

With the advancement of industrialization and urbanization,some industrial plants have been closed or relocated,exposing a large number of heavy metal contaminated sites,and the treatment of heavy metal contaminated soil has become an urgent problem to be solved.At present,the most widely used treatment method is to apply modifiers.However,traditional modifiers have problems such as strong alkalinity and high energy consumption,so it is urgent to find new environmentally friendly and economical modifiers.This paper takes the lead contaminated soil prepared manually in the laboratory as the research object,uses industrial solid waste phosphogypsum and lignin to improve the lead contaminated soil with different concentrations,tests and analyzes the conductivity,p H,unconfined compressive strength,permeability,dissolution performance and microscopic characteristics of the soil before and after the improvement,and evaluates the improvement effect,and determines the optimal mix ratio of the modified soil improver combining with the proportional optimization model.The research results are as follows:(1)The influence of lead ion concentration on the engineering properties of lead contaminated soil was investigated.The experimental results showed that the p H of lead contaminated soil significantly decreased with the increase of lead ion concentration.The electrical conductivity shows an approximate linear upward trend.The invasion of lead ions leads to a significant decrease in the unconfined compressive strength of the soil,with little change in permeability.(2)Phosphogypsum,lignin,and phosphogypsum lignin composite curing agents were used to improve lead contaminated soil with different concentrations,and the effects of the amount of curing agent and curing age on the physical,chemical,mechanical,and dissolution properties of the modified soil were analyzed.The results show that the electrical conductivity of the three improved soils increases slightly with the increase of the curing agent,and the curing age has little impact on the electrical conductivity of the soil.The p H decreases with the increase of the dosage,and increases slightly with the increase of the age.The chemical properties of the soil have little change.The unconfined compressive strength of phosphogypsum modified soil increases with the increase of its content;The strength of both lignin modified soil and composite modified soil showed a trend of first increasing and then decreasing with the increase of the dosage,and the optimal dosage was 6.5% phosphogypsum+2.0% lignin.As the curing age increases,the unconfined compressive strength of all three modified soil increases.Phosphogypsum and lignin can reduce the permeability of low concentration contaminated soil,but have little effect on the permeability of high concentration contaminated soil.After adding curing agent,the toxic leaching concentration decreases to a certain extent,and the leaching lead concentration can meet the standard by increasing the amount of curing agent or controlling the curing age.(3)According to the comprehensive scoring method,radar chart analysis method and TOPSIS comprehensive evaluation method based on entropy weight method,the optimal mix proportion of the two kinds of lead contaminated improved soil is 6.5%phosphogypsum+2.0% lignin.(4)The scanning electron microscope test results show that the curing agent can fill the pores and cement the soil particles,the hydration products make the soil structure compact,the cementitious materials form a spatial network structure in the soil,and enhance the integrity of the soil.The macroscopic performance is the enhancement of the mechanical properties of the soil.