Construction And Application Of Evaluation Model For Immobilization Remediation Of Heavy Metal Contaminated Farmland Soil

Soil is contaminated by heavy metals due to industrial wastewater discharge,agricultural sewage irrigation,and excessive application of chemical fertilizers.Heavy metals cannot be degraded automatically after entering the soil and accumulate continuously,which brings great risks to agricultural production.According to the characteristics of heavy metal pollution in Chinese farmland soil,immobilization remediation technology has become a mainstream technology.However,this technique involves a wide range of amendments,complex immobilization mechanisms,and a wide range of influencing factors,resulting in large differences in immobilization remediation effects.Therefore,a scientific evaluation of immobilization remediation effect of heavy metal contaminated soil is the key to the successful application of this technology.According to the literature review,there is still a lack of evaluation system for the immobilization remediation effect of heavy metal contaminated farmland soil,which has become a technical bottleneck for the application of this technology in China.Therefore,this thesis addresses the hotspots and difficulties in the immobilization remediation of heavy metal contaminated soil in China,and takes the major national needs as the guide.Firstly,based on the bibliometric method and the Delphi method,the hierarchical analysis method is used to construct an evaluation indicator system for the immobilization remediation effect.Then,according to the classification of the attributes of the indicators,an evaluation model of the effect was constructed and the evaluation system was studied.Finally,the feasibility of the evaluation system was verified through pot experiments and field trials.It provides a theoretical basis and technical support for the evaluation of the immobilization remediation effect of heavy metal contaminated soil.The main conclusions are as follows:(1)Literature from the CNKI database and Web of Science core database for the past 20 years was used as the data source,and the soil-plant system was used as the research object.The evaluation system of immobilization remediation effect of heavy metal contaminated farmland soil was constructed and index weights were calculated using bibliometric,theoretical analysis,hierarchical analysis,Delphi method and other research methods.The principles of comprehensiveness,objectivity,stability,ease of measurement and evaluability were taken into account in the construction process.A total of one target layer,three criterion layers,six sub-criterion layers,and 16 indicator layers were constructed.The immobilization remediation effect is evaluated in six aspects: soil fertility,soil heavy metals,crop yield,accumulation of heavy metal in crop,the economic benefits of amendment,and ecological benefits of amendment,respectively.The weights for the 3 criterion layers of soil indicator,crop indicator and amendment indicator were 0.544,0.316,and 0.104 respectively.Among the 16 indicator layers,the available heavier metals(0.294)and the heavy metal content of edible parts(0.150)are given the greatest weight.This is in line with the current assessment status that the immobilization remediation is mainly to reduce the available heavy metals in soil and reach the standard of heavy metal content in the edible parts of crops.(2)According to the different attributes of the indicators,all indicators in the evaluation system are innovatively divided into three categories.The first category is indicators with reference standards(Indicator I),which mainly include total soil heavy metals,heavy metal content in edible parts,and heavy metal content in amendment.The second category of indicators can be graded without reference standards(Indicator II),which mainly include soil p H,soil organic matter(SOM),cation exchange capacity(CEC),available nitrogen(A-N),available phosphorus(A-P),available potassium(A-K),crop yield and biomass,and cost of amendment.The third category is the indicators without reference standards but dynamic changes(Indicator III),including the available heavy metals,heavy metal content in above-ground part,root heavy metal content in roots,and stability of amendment.Referring to current national standards,a single factor index model was used for Indicator I.Referring to the graded evaluation system,a comprehensive weighted scoring mode was used for Indicator II.In Indicator III,bioconcentration factors(BCF)was applied for heavy metal in above-ground part and heavy metal in roots,and the useful life was used for stability of amendment.Complex evaluation models and cumbersome evaluation processes was avoided.(3)Multiple regression analysis was performed using days of immobilization,soil type,different concentrations of amendment,amendment type,and crop type as independent variables.The results showed that the available Cd was influenced in descending order by 2%hydroxyapatite(HAP)> 1% HAP > 2% reed biochar(RBC)> 1% RBC > ryegrass(R)> soil type > Baby Bok Choy(W)> days of immobilization.(4)Depending on the evaluation standard and basis of the indicators,evaluation procedures and evaluation protocols are obtained.In the first step,indicator I is evaluated.If it conforms to the national standard,the remediation effect meets the minimum requirements,and can be evaluated in the next step.If not,the immobilization remediation fails.In the second step,Indicator II is evaluated,which represents the soil fertility level.If the soil quality improves or remains unchanged after the immobilization remediation,then the final step is evaluated.If not,the immobilization remediation fails.In the third step,a normalization function and a multi-objective integrated weighting model are used to evaluate all the indicators in order to obtain a comprehensive immobilization remediation effect.(5)In the soil culture experiment,the immobilization remediation effect of reed biochar(RBC)was rated as grade III and hydroxyapatite(HAP)was evaluated as grade II.Compared with the group without amendment(CK),the RBC improved the general soil quality by one grade and the immobilization remediation effect was good,while the HAP improved the general soil quality by two grades and the immobilization remediation effect was excellent.This indicates that the remediation of Cd-contaminated soil using HAP is better than RBC.In the pot experiment,the immobilization remediation effect of the treatment group of imposing reed biochar and planting ryegrass(RBC+R)was evaluated at grade III,and the treatment group of imposing hydroxyapatite and planting ryegrass(HPA+R)was evaluated at grade II.Compared with treatment group of ryegrass(R),the grade of RBC+R did not change,indicating that the immobilization remediation effect of RBC was average when ryegrass was planted.HPA+R improved by one grade compared with R,and the evaluation result of HAP was good.This indicates that the application of HAP could better improve the general soil quality.The experimental results obtained by the evaluation system and the evaluation model under laboratory simulation conditions was generally consistent with the results of the current study.This proves that the evaluation system and evaluation model constructed are feasible.(6)In this study,immobilization remediation was carried out on farmland where Cd contamination was above the risk screening value.The ranking of the immobilization remediation effect was in the following order: The treatment group of applying HAP with RBC and planting the wheat with low accumulation of heavy metals Jimai 22(RHJ),the treatment group of applying HAP and planting the heavy metal accumulating wheat Lunxuan 103(HL),the treatment group of applying HAP with RBC and planting Lunxuan 103(RHL),the treatment group of applying HAP and planting Jimai 22(HJ),the treatment group of applying RBC and planting Jimai 22(RJ),the treatment group of applying RBC and planting Lunxuan 103(RL).RHJ had the best immobilization remediation effect,RJ and RL failed.In terms of the comprehensive remediation effect,the mixed application is better than the single application,and HAP is better than RBC.In terms of Cd accumulation,Jimai 22 was better than Lunxuan 103.For grain safety reasons,it is recommended to plant low accumulation varieties in areas with slight or light Cd contamination.The evaluation results are basically consistent with the theoretical study and further validate the feasibility of the evaluation system and evaluation model.It provides a scientific basis and technical support for the evaluation of the immobilization remediation effect on heavy metal contaminated soil,and provides ideas and methodological references for further research in this field.