Effect Of Butyl Xanthate On Release And Migration Of Pb And Cd In Mining Soil And The Mechanisms

Flotation agents can enter the soil and water environment around the mining area through beneficiation wastewater discharge and overflow from tailings pond.Lead and cadmium are the main heavy metal pollution elements produced in the process of sulfide mining,which seriously harm the ecological environment of mining area and human health safety.Flotation agents and heavy metals are easy to form compound pollution in soil,which changes the migration behavior and bioavailability of heavy metals in the environment.However,the effect mechanism of flotation reagents on the release and migration of heavy metals in mining soil is unclear.Butyl xanthate（PBX）was used as the research object in this study,and the soil around a lead-zinc tailings pond was used as the test soil.Through batch adsorption,static release and constant temperature and humidity culture experiments,the adsorption of Pb and Cd in soil were investigated in the presence of PBX under different p H and initial PBX,Pb and Cd concentrations;The release characteristics of Pb and Cd in contaminated soil under different concentrations of PBX and its UV degradation solution were investigated;The speciation changes of Pb and Cd in soil under different water conditions were studied when PBX was present.The main results are described as follows:（1）PBX significantly inhibited the adsorption of Pb and Cd,and increased the migration capacity of Pb and Cd in soil.When the PBX concentration was 40 mg·L^-1,the adsorption capacity of Pb and Cd decreased from 3540 and 387 mg·kg^-1 to 3085 and 100 mg·kg^-1,respectively,and the adsorption capacity decreased by 12.9%and 74.2%.The adsorption kinetics of Pb and Cd on soil can be fitted by a quasi-second-order kinetic model,which indicates that the adsorption of Pb and Cd on soil is dominated by chemical adsorption.The degree of impact improved with increasing initial concentration of PBX and p H,but decreased with increasing initial concentration of Pb and Cd,and the adsorption isotherms conformed to the Freundlich isotherm.The formation of hydrophobic and insoluble complex and competitive adsorption between PBX,Pb,and Cd on soil surface was the main reason for reducing the adsorption capacity,and PBX could increase the potential ecological risks of Pb and Cd in the environment.（2）PBX and its UV degradation solution can increase the release potential of Pb,Cd and Zn in contaminated soil,but have no obvious effect on the dissolution of As in soil.Compared with the control,Pb,Cd and Zn release increased by 252%,207%and 402%when PBX concentration was25 mg·L^-1,respectively.With the increase of PBX concentration,the promoting effect gradually decreased.When the concentration of PBX in the solution exceeds 400 mg·L^-1,it showed a slight inhibition on the release of heavy metals in soil.Release kinetics indicated that PBX may increase the concentration of Pb,Cd and Zn in the leached solution by decrease the readsorption of heavy metal ions in soil.Fourier transform infrared analysis showed that the absorption peaks of C₄H₉OCSS^-and-C₄H₉perssad in soil increased after PBX leaching.It is possible that the high concentration of PBX and heavy metal ions in soil form complex to wrap soil particles,which affects the release of Pb,Cd and Zn in soil pores.The promoting effect of PBX on the release of heavy metals in soil increased with the decrease of solution p H value.When p H=2.0,the contents of Pb,Cd and Zn in 50 mg·L^-1 PBX solution increased by 135%,79.6%and 201%,respectively,compared with the control solution.The photodegraded PBX solution promoted the dissolution of heavy metals in the soil more strongly than the original solution.Ultraviolet spectrum analysis showed that the dissolution of Pb,Cd and Zn in the soil was further enhanced by the acidic substances such as CS₂,HS^-and CO₃^2-after degradation.Under natural light conditions,the degradation products of PBX can significantly increase the migration risk of heavy metals in soil.（3）PBX can promote the transformation of lead to a more stable form,but increase the content of available cadmium in soil.At the 56th day of culture,compared with the control,the residue lead content in soil with different PBX concentration of 100 and 200 mg·kg^-1 increased by 7.84%and 10.0%,while the reducible cadmium content in soil increased by 3.46%and 6.08%,respectively.The complex structure of Pb（C₄H₉OCS₂）₂ was denser than that of Cd（C₄H₉OCS₂）₂,and the complex capacity of Pb（C₄H₉OCS₂）₂ was stronger than that of Cd（C₄H₉OCS₂）₂,which might be the main factor causing the difference effect of PBX on Pb and Cd speciation in soil.Compared with 30%moisture content culture,exchangeable and reducible lead content in soil decreased by 2.86%～6.03%and 13.2%～14.0%,respectively,and exchangeable Lead content in flooded soil increased by 3.05%～5.88%.Different humidity treatments can improve the stability of cadmium in soil.The proportion of active lead content in soil of mining area may increase in rainy season,leading to the aggravation of lead pollution.The study showed that PBX could improve the migration capacity of Pb and Cd by reducing the equilibrium adsorption capacity of Pb and Cd in soil.Ore dressing wastewater containing low concentration PBX and its secondary pollutants can greatly increase the release risk of Pb,Cd and other heavy metals in contaminated soil.Under combined pollution,PBX can reduce the content of available lead in soil by forming stable complex with lead ions,but the formation of cadmium xanthate can enhance the activity of cadmium in soil.This study provides a scientific basis for the potential ecological risk of heavy metals such as Pb and Cd in soil and pollution prevention and control.