Study On Passivation Mechanism And Remediation Experiment Of Cadmium And Lead In Contaminated Soil

Heavy metal pollution of soil is one of the most pressing soil environmental problems that need to be addressed today.Until now,the best option for solving the problem of soil heavy metal pollution by using passivators.In this paper,the first-principle are applied to the calculation of heavy metal system in order to find a more efficient,reasonable and inexpensive passivators.On this basis,the selections and experiments of passivators were carried out.According to the first-principle calculation of the stability about the selected lead-containing system compounds,the comparison of their degree of stability shows thatPb₃Fe₂?PO₄?₄>Pb₃?PO₄?₂>PbSO₄>PbCO₃>PbAl₂O₄>PbCl₂.Combined with theanalytical work of various ionic groups,such as CO3^2-,SO4^2-,Cl-,PO4^3-,Al3+,Fe3+,etc.,it is determined that SO4^2-and PO4^3-can be used as the passivator functional group in cases of serious pollutions to reduce the lead pollution.In case of especially serious pollution which is difficult to passivation and precipitation,Fe³⁺can be added to improve the efficiency of passivation.Then,calculation of the stability about the cadmium-containing compounds shows that the sequential stability is following asCdSiO₃>Cd₃?PO₄?₂>CdSO₄>CdCO₃>CdCl₂ decreased sequentially.In combinationwith the analysis of various ionic groups,such as CO3^2-,SO₄^2-,Cl^-,PO₄^3-and SiO₃^2-,it was determined that SiO₃^2-was optimized to be the passivator functional group for controlling cadmium pollution,and so do the ionic groups of PO₄^3-and SO₄^2-.On the basis of the first-principle calculations,sodium dihydrogen phosphate?Na₂HPO₄·12H₂O?,anhydrous sodium sulfate?Na₂SO₄?,sodium silicate?Na₂SiO₃·9H₂O?were selected as the preferred passivator materials.There were three improved passivators as following:a ratio of pulverized fuel ash to Na₂HPO₄·12H₂O of 1:2.5,a ratio of pulverized fuel ash to Na₂SO₄ of 1:1,a ratio of pulverized fuel ash to Na₂SiO₃·9H₂O of 1:2,they were mixed in proportion,burned and then grinded.In addition,desulfurization gypsum?CaSO₄·2H₂O?was milled to used as passivator.The results of experiments on the original passivator materials and the improved passivator materials on self-made samples of heavy metal-contaminated soil and natural contamination soil,are consistent with the calculated conclusions.Na₂HPO₄·12H₂O,Na₂SO₄ and Na₂SiO₃·9H₂O were indeed effective in treating the soil pollution,and the improved passivator sodium silicate(SiO3^2-)was more effective.Taking the different mass passivators with the quality 1%or 2%of the soil quality as the experimental comparison group,passivation treatments were carried out.The passivation effect was increased but not significantly increased with twice the mass of the passivator.Once the passivator was added,the passivation reaction was rapidly reversed,with the most severe reaction occurring during the first 3 days,after which the rate of passivation slowly slowed down,the passivation effect was stable until 15?90 days.Considering the economic factors of heavy metal pollution control,it is proved that that CaSO₄·2H₂O has a certain passivation effect can also be used as a passivator by experiments.When the cadmium and lead pollution of soil is not serious,CaSO₄·2H₂O?its mass with a quality of 2%of soil?can be considered as a passivator,which is economical and can ensure certain efficiency.This will ensure the efficiency of treatment,meanwhile control the cost of pollution cleaning.If the soil was polluted by cadmium and lead seriously,Na₂SiO₃·9H₂O mixed with pulverized fuel ash?their mass with a quality 2%of soil?can be used as passivator,which has the best passivation effect and moderate costs.Selected the passivators by the theoretical calculation,then verified thepassivation efficiency in experiments,it was confirmed that PO₄^3-and SO₄^2-can be used as passivation functional groups for dealing with the cadmium and lead soil pollution,and the improved SiO3^2-is also the preferred material for passivation of cadmium and lead pollution.