Effect Of Strong Acid And High Salt Environment And Multi-physical Field On The Performance Of Solidified Heavy Metal Contaminated Soil

The heavy metal contaminated soil around the tailings landfill presents the chemical characteristics of strong acidity and high salt content,which poses a challenge to the solidification/stabilization（S/S）of heavy metals.Therefore,it is necessary to develop an agent with acid and salt resistant characteristics for the S/S of the contaminated soil.Furthermore,the amend effectiveness of the agent on heavy metal contaminated soil with strong acid and high salt content characteristics should be evaluation and the immobilization mechanism need to be investigated.Financial support for this project was provided by the National Key Research and Development Program of China（No.2019YFC1806000）.A sustainable P-based binder was developed to amend the contaminated soil,the S/S effectiveness was evaluated through various laboratory experiments,and the immobilization meachanism was also investigated.The main results and conclusions are as follows:（1）The S/S effectiveness of the mixture of bone meal-calcium carbide residue and bone charcoal-calcium carbide residue on Pb,Zn and Cu co-contaminated soil was evaluated by the pure water leaching test.The highest S/S efficiency for Pb and the better S/S efficiency for Cu were concluded when the agent with the P-Ca molar ratio of 1.24:1.Furthermore,the p H value of solidified soil significantly affects the S/S efficiency of Zn.（2）The influence of agent dosage,curing time,leaching solution,moisture content on leaching toxicity of solidified contaminated soil with different salt content were investigated.With the increase of agent dosage and curing time,the leached concentration of Zn decreased significantly,while the continuous increaseing of dosage showed little effect on the Cu leached concentration.The Zn leached concentration conducted by SPLP leaching test was much higher than that extracted by deionized water while the Cu leached concentrations were keeped in the same level.The Zn leached concentration decreased with the increasing of moisture content.The observed diffusion coefficients(D^obs)for the releases of Zn and Cu from solidified contaminated soil ranged from 10^-11 to 10^-12 cm² s^-1 and were 4 orders of magnitude lower than that of untreated contaminated soil.After S/S,the water soluble and weak acid extractable fractions of Cu and Zn in solidified soil decreased significantly while the proportion of reducible,oxidizable and residual fractions increased significantly,and the increaseing of residual fraction for Cu was greater than that of Zn.（3）The immobilization mechanism of SS-H for Zn and Cu was investigated by X-ray diffraction（XRD）,scanning electron microscopy and energy spectrum（SEM-EDS）analysis.The SS-H mainly fixes Cu by dissolving-precipitation and adsorption,and fixes Zn by adsorption and ion exchange.The results of adsorption experiment showed that the affinity of Cu to bone meal was stronger than that of Zn,and Fe had a synergistic effect on the adsorption of Zn while a competitive effect on the adsorption of Cu.（4）The influence of temperature and hydraulic conductivity（k_w）on the migration of heavy metals was clarified by numerical simulation using TOUGHREACT software.Under the simulated conditions in this study,the inhibition effect of evaporation on downward migration of heavy metals and the promotion effect of temperature rise on migration of heavy metals offset each other,and temperature had slightly impact on the migration of heavy metals.The migration of heavy metals in solidified soil is dominated by diffusion when the k_w of solidified soil is lower than 1×10^-11 m s^-1.