| Heavy metals of farmland soil in our country are severely exceeded.At present,although there are many known methods for the remediation of contaminated soil,but most of them are pot experiments in a greenhouse,and there are few studies in the field.This study firstly investigated the status of heavy metal pollution and basic physico-chemical properties of soil in the experimental farmland in Datun Town,Gejiu City,Yunnan Province.The experimental site was divided into three areas with different degrees of heavy metal pollution according to the Nemero’s comprehensive pollution index.Passivation remediation was applied to the high-polluted area,hyperaccumulators intercropped with corn were used for moderate pollution areas,and low-metal-accumulative vegetables were used for low pollution areas.For the passivation remediation of high-pollution areas,single application of 1% ferrous sulfate(1% FS),2% lime(2% LM),2% sepiolite(2% SP)and their composite application were conducted to investigate the changes of bioavailable heavy metals and basic physico-chemical properties of soil,as well as the effects of passivators on corn biomass and heavy metal uptake.For the medium-polluted area,the cadmium hyperaccumulator Mirabilis jalapa L.,lead hyperaccumulator Chenopodium ambrosioides L.,arsenic hyperaccumulator Pteris vittata L.and inter-cropping corn(Zea mays)were used for its remediation.The biomass and heavy metal concentrations in different parts of hyperaccumulators and corn were determined,and the remediation efficiency and effects of relay intercropping on corn were also analyzed.The low-metal-accumulative vegetables were directly planted in low pollution areas,and the biomass and heavy metal concentrations in their edible parts were measured to analyze the effect of low pollution soil on vegetable growth indexes and food safety.Finally,a comparative analysis of the remediation benefits of the three remediation methods was conducted to achieve an ideal remediation scheme for the remediation of farmland soil contaminated by combined heavy metals.The results showed as follows:1.The concentrations of Cd,Pb and As in the farmland soil exceeded the screening values in "Soil environmental quality Risk control standards for soil contamination of agricultual land(tentative standard,GB15618-2018).The multiple factor potential ecological risk indexes(RI)of Cd,Pb and As in each polluted area were over 150 and belonged to high risk.The degree of risk followed the order of Cd> As> Pb.The bioavailable Cd and Pb accounted for 50% and 30% of their total concentrations.The soil p H was weakly acidic,the contents of total nitrogen and available nitrogen were low,and the overall soil fertility was inferior.2.The single and combined application of three passivation agents in high pollution areas had no significant effect on growth of corn,and the concentrations of Cd and As in grains were significantly reduced at the combined application of 2% LM+ 2% SP,which met the Cd and As limit values in "National Food Safety Standards Limits of Contaminants in Food"(GB2762-2017,Cd: 0.1 mg/kg,As: 0.5 mg/kg).The bioavailable concentrations of Cd and Pb in soil were significantly decreased at the single application of 2% LM,with a decreasing rate of 6.4% and 21.6%,respectively.When 2% LM and 2% SP were combinedly applied,the concentrations of bioavailable Cd and Pb were decreased by 10.9% and 9.6%,respectively.The best As passivation was observed at 1% FS application,with a decreasing rate of 28.9% in bioavailable As.The p H value of soil was over 7.0 and the soil acidity was neutral when the passivation agents were added in high pollution soil.3.In the relay intercropping of three hyperaccumulators with corn under medium heavy metal pollution condition,the growth and yield of corn were not significantly affected.Furthermore,the concentrations of Cd,Pb and As in grains met the limiting values in “Hygienical Standard for Feeds”(GB13078-2017,Cd: 2 mg/kg,Pb: 10 mg/ kg,As: 2 mg/kg).It should be noted that the concentrations of Cd Pb and As in leaves and roots of corn were significantly increased when the corn was respectively intercropped with M.jalapa,C.ambrosioides and P.vittata.A possible reason was that the heavy metals in soil were activated by roots of hyperaccumulators and easily absorbed by corn.However,the activated heavy metals were not translocated to the grains.4.The concentrations of Pb and Cd(excluding radish)in the edible parts of 4vegetables grown in the low pollution area exceeded the limiting values of food safety standards and posed a food safety risk.However,the As concentration and Cd concentration in radish met the requirement of food safety standard.5.The cost of three remediation methods was calculated according to the labor cost,material cost and maintenance cost.The result showed that the total cost of passivation remediation,intercropping remediation and low-metal-accumulative vegetable plantation was 3472,5324 and 2315 yuan/mu.Therefore,for the Cd-Pb-As combined pollution farmland soil in this area containing high bioavailable metal concentrations,passivation remediation could achieve a good remediation result in a short time,and the cost was relatively low.It can be used for the soil remediation in a larger scale in such areas.In conclusion,the soil in the study area was combinedly polluted by Cd,Pb and As.The concentrations of bioavailable Cd and Pb,as well as the ecological risk index,were high.The concentrations of bioavailable Cd and Pb were significantly decreased with addition of 2% LM and 2% LM + 2% SP,and the best As passivation was shown at 1% FS application.At the same time,the Cd and As concentrations in the grain of corn were significantly decreased with addition of 2% LM combined with 2% SP.Therefore,passivation remediaiton was an ideal method for the remediation of such farmland soil.It should be noted that low-metal-accumulative varieties are not suitable for the remediation in the soil containing high concentrations of Cd and Pb but low concentrations of heavy metals.Passivation remediation combined with cultivation of low-metal-accumulative varieties was a better alternative under such condition. |
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