| Soil arsenic pollution is a global problem.There are lots of arsenic-contaminated sites in the world.China is one of developing countries that arsenic contamination is particularly serious.The existing technologies for remediation of arsenic-contaminated soils often have long remediation cycles,high costs,and difficulty in large-scale use.Soil stabilization is one of the technologies that highly meet the requirements of soil risk management and currently the most popular remediation technology due to its simplicity,short implementation period,and wide range of applications.However,it is difficult to evaluate the long-term stability of risk control after the implementation of stabilization remediation technology,and the influencing factors and mechanisms of heavy metal reactivation after stabilization need to be investigated.Therefore,in this study,the stabilization material was prepared by using a ball-mill mixture of"ferrous iron and fly ash"to come true the stimulating modification.Which was used to stabilize the soil with slightly acidic and arsenic contamination.And the test soil from a typical pharmaceutical and chemical site in Hubei province.The stabilization capacity of the"ferrous iron-fly ash"system was evaluated by soil incubation tests,and the doping amount was optimized by using ferrous iron as the stimulating modification factor,and a green,efficient and economic stabilization material system was constructed.The effects of material factors,concentration factors and environmental factors on soil stabilization morphology were investigated,and the principle of soil reactivation after stabilization was revealed to provide reference for the evaluation of soil stabilization longevity,with the following main research findings:1.The ferrous iron-fly ash(BC)technology system has a remarkable stabilization effect.Ferrous(B)is the best factor for excitation modification.A total of four raw materials,lime(A),ferrous sulfate(B),fly ash(C)and activated carbon(D),were combined and mechanically mixed and ground at a controlled speed of 120 r/min for 15 min at a ball-to-material ratio of 10:1 for stimulating modification,and a model of"ferrous iron stimulating modified fly ash"was obtained based on the screening of the soil arsenic stabilization test effect.The arsenic stabilization restoration system was obtained,and the arsenic stabilization efficiency was 90.43%after 90 days of soil maintenance.The system(BC)had been used to the stabilization remediation of the actual arsenic-contaminated site in the range of 200 m~2.Soil arsenic leaching concentration of the surface soil(0-0.5m)was reduced from 644.72μg/L to 5.8μg/L,and the arsenic stabilization efficiency of the soil at different depths was 95.37%-97.41%.2.The fly ash with ferrous component system(BC)was selected to carry out the optimization of the concentration of the stimulating modification factor.The optimum treatment system was 2.0%(w/w)ferrous sulphate+2.0%(w/w)fly ash.Stabilization mainly fixes F1 and F2 to F3 and F4 in the arsenic form.p H and water content control were the key factors affecting the stabilization efficiency.The p H of the soil after stabilization under this system ranged from 5.93 to 6.72 and the water content varied between 28%and 36%.The soil stabilization efficiency increased with increasing ferrous iron incorporation in 60 days,and the efficiency of stabilization was 88.74%at doping of 1.0%(w/w)ferrous.While its doping at 2.0%(w/w)and above,it remained above 90%.Ferrous incorporation affected the activation of heavy metals,with the lowest activation at 1.8%(w/w)ferrous.While its doping at 2.0%(w/w)and above,the activation level tends to stabilize.The p H of the soil gradually decreased with the increase of ferrous.And the rate of soil p H change was below 4.18%at 2.0%(w/w)ferrous and above,when the application of stabilizing materials had the least effect on the physicochemical properties of the soil.After the application of the optimum treatment system(BC)to the soil,exchangeable arsenic(F1)and surface adsorbed arsenic(F2)were the arsenic forms that were reduced by stabilization,amorphous and poorly-crystalline hydrous oxides of Fe/Al bound arsenic(F3)and well-crystallized hydrous oxides of Fe/Al bound arsenic(F4)were the arsenic forms that mainly increased in the soil,and the stability of the forms F4>F3.At the end of the stabilization treatment,the proportions of F1 and F2 in the soil decreased by 5.61%and 8.45%,and the proportions of F3 and F4 increased by 8.34%and7.29%with no significant difference change in the total soil arsenic content.3.Ferrous(B)doping mainly affects the distribution of F3 and F4 in the arsenic form.F3is the most prone to develop chemical desorption and F1 has the strongest soil leaching capacity.The arsenic forms F1,F2 and F3 were reduced,which lead to the distribution of F4 and F5 in the soil was increased,while ferrous salt modified doping was varied from 0%to 2.6%(w/w)of the stabilization system(BC)in test.And the distribution of the five types of arsenic forms in the soil was basically stable when the ferrous salt doping was2.0%(w/w)and above.With the increase of ferrous salt stimulate-modified content,the distribution proportion of exchangeable arsenic(F1)decreased from 0.9%to 0.2%,the distribution proportion of surface adsorbed arsenic(F2)gradually decreased from 13.2%to 10.1%,the distribution proportion of amorphous and poorly-crystalline hydrous oxides of Fe/Al bound arsenic(F3)gradually decreased from 26.7%to 11.7%,the distribution proportion of well-crystallized hydrous oxides of Fe/Al bound arsenic(F4).The distribution of arsenic in the residual state(F5)was increased from 39.6%to 46.6%.The correlation between arsenic concentration of each form and arsenic leaching concentration revealed that the exchangeable form of arsenic(F1)was the most leachable form of arsenic in the soil.And the poorly-crystalline hydrous oxides of Fe/Al bound arsenic(F3)was the most easily activated form of arsenic.The conversion of F3 to F1occurs mainly after activation.When the water stress of the soil was increased,the water content and p H in soil will be increased.It would lead to the activation of soil amorphous and poorly-crystalline hydrous oxides of Fe/Al bound arsenic(F3)and well-crystallized hydrous oxides of Fe/Al bound arsenic(F4)in the optimal system of ferrous iron-fly ash(BC)reduced their contents by 20.66%and 9.13%,causing desorption of iron oxides from the soil,and the contents of exchangeable arsenic(F1)and surface adsorbed arsenic(F2)in the soil were subsequently increased by 38.60%and 16.07%.Leaching quantity of arsenic from soil was F1>F2. |
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