| As the cornerstone of human survival and development and the material basis of food crop production,soil has been seriously polluted by heavy metals such as As and Cd due to human activities,which poses a threat to the safe production of food crops and human health.Immobilization remediation has attracted wide attention in the remediation of heavy metal contaminated soil in farmland due to its advantages of wide remediation area and good effect.The amendments used in immobilization are usually solid powders,which naturally diffuse slowly in soil and produce slow effects.However,in soil,As and Cd exist in the form of anions and cations respectively,and their forms and migration ability are significantly different in the soil with negative charges,which makes the immobilization remediation of As and Cd contaminated soil difficult.Therefore,it is of great environmental significance to develop new adsorptive immobilization technology and reagents for As and Cd contaminated soil.Zero-valent iron has strong adsorption and precipitation capacity for As,and also have a certain immobilization remediation capacity for Cd.Sodium thiosulfate has the function of complexing and extracting Cd.It converts to other sulfides in soil and immobilizes Cd in soil.In this study,the idea of simultaneous remediation of As and Cd by in situ extraction with sodium thiosulfate and adsorption immobilization with zero-valent iron was proposed.Taking the actual farmland soil as the research object,the immobilization remediation effects of sodium thiosulfate and zero valent iron on As and Cd contaminated soil were studied by soil culture and pot experiments,and the chemicals with immobilization remediation effects on As and Cd contaminated soil.Finally,field experiments were conducted to verify the effectiveness of soil remediation agents.(1)Soil adsorption experiment and soil culture experiment were used to study the soil remediation effect of sodium thiosulfate on Cd and As.The results of isothermal adsorption experiment showed that sodium thiosulfate could reduce the adsorption capacity of Cd ion by complexing with Cd ion,and promote Cd ion from solid soil to soil solution.The residual Cd ion concentration in the solution was 1.55-6.18 times higher than that in the control.28-day soil culture experiment showed that the degradation rate of sodium thiosulfate is 83.22%-98.26%,and the content of available Cd in soil decreased by 2.04%-3.20%.The ratios of Cd in soil changed from Fe-Mn oxide bound Cd and exchangeable Cd>residual Cd and carbonate bound Cd to residual Cd and iron-manganese oxide bound Cd>exchangeable Cd and carbonate bound Cd.The ratios of residual Cd increased by 11.93%-35.76%compared with the control.Sodium thiosulfate with 0.1%and 0.5%additions has good remediation effect.Similarly,sodium thiosulfate treatment reduced the available As content by 0.83%-13.37%and increased the residual As content by 5.98%-9.93%.(2)The adsorption and fixation effects of zero-valent iron on As and Cd were studied by adsorption experiments and soil culture experiments.The results show that zero-valent iron can effectively remove Roxarsone and p-arsanilic acid,and the removal efficiency reaches 100%at the addition of 0.50 g/L.The inorganic As content in the solution increases first and then decreases.XPS analysis showed that trivalent As and pentavalent As were detected on the adsorbent surface,indicating that zero-valent iron could remove Roxarsone and p-arsanilic acid through Fenton-like oxidation,coprecipitation and adsorption.After 28 days of incubation,the residual contents of Roxarsone and p-arsanilic acid in soil decreased by 87.08%-93.89%,67.28%-79.23%respectively,and the available As content in soil was-15.80%-23.72%lower than that in control.The content of As in soil changed from residual As>organic bound As>carbonate bound As>iron-manganese oxide bound As>exchangeable As to residual As>organic bound As>iron-manganese oxide bound As>carbonate bound As and exchangeable As.The content of residual As increased by 13.33%-41.10%.Zero-valent iron with 0.1%and0.5%additions has good remediation effect.Zero-valent iron can also effectively remove Cd ions.XPS analysis showed that some Cd ions were reduced to Cd(0)during the removal process.After 28 days of soil culture,available Cd decreased by 5.7%-22.78%,while residual Cd increased by 7.07%-24.25%compared with the control.(3)Pot experiments were conducted to study the effects of the combination of zero-valent iron and sodium thiosulfate on the forms of As and Cd in soil and the changes of As and Cd uptake by wheat.The results showed that the ratio of zero-valent iron and sodium thiosulfate(3:1-1:3)decreased the available As by 20.06%-38.34%compared with the control,promoted the transformation of exchangeable and carbonate-bound As to residual As,reduced the available Cd by 2.28%-19.28%,and promoted the transformation of exchangeable Cd to residual Cd.The immobilization effect of As and Cd in soil increased with the addition of iron-sulfur complex.The contents of total As and Cd in the above-ground parts of wheat treated with different chemicals were 45.89%-64.96%and 42.92%-66.13%lower than those of the control.Sodium humate,sodium lignosulfonate and modified magnesite powder were added into the iron-sulfur compound to treat the soil.The total As in the above-ground part of wheat decreased by 36.48-63.40%compared with the non-application treatment,and the change was not significant compared with the compound treatment.The total Cd in the above ground parts of wheat decreased by 19.70-78.19%compared with the control,and the effect of 0.5%Iron-sulfur complex reagent on reducing Cd uptake by wheat increased was 44.44%higher than that non-application treatment.The application depth of iron and sulphur chemicals had obvious effects on the forms of As and Cd in soil,decreased the total arsenic content in the above ground part of wheat by 21.22%-46.59%,but had no significant effect on the total cadmium content.The correlation between As and Cd in wheat and As and Cd in soil was analyzed.The results showed that there was a significant positive correlation(correlation coefficient was 0.76)between total Cd in the above-ground parts of wheat and total Cd in wheat roots,a significant positive correlation(correlation coefficient was 0.77)with exchangeable Cd in soil,and a significant negative correlation(correlation coefficient was-0.69)with carbonate bound Cd.There was no significant relationship between total As in the above-ground parts wheat and total As in wheat roots,nor between total As in wheat roots and various forms of As in soil.(4)Field experiments were conducted to further verify the efficacy of the complex reagents.Field experiment results showed that under natural environment,the decrease range of soil available As content was increased from 49.50%to 51.16%-53.58%and that of residual As was reduced from 5.94%to 1.47%-4.19%.Soil available Cd content decreased from 6.92%to10.15%-13.00%and residual Cd increased from 103.54%to 130.38%-159.20%.The content of total As and Cd in the above-ground part of wheat was also decreased by the complex reagent.The soil treated with 0.9 kg/m~2 had better effect.At 152 days,the content of total As and Cd in the above-ground part of wheat was 49.60%and 15.03%lower than that of the control wheat.In summary,this paper used soil adsorption experiment,soil culture experiment and pot experiment to screen the compound reagent with zero-valent iron and sodium thiosulfate as the center activator.The optimum dosage of the compound reagent in soil was 0.5%.Field experiment proved that the compound reagent had the effect of reducing the transmission of As and Cd from wheat root to above ground,and provided reference for the safe production of As and Cd contaminated farmland soil. |
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