Study On Passivation Effect Of Cadmium In Greenhouse Vegetable Soil

The scale of greenhouse vegetable was increased continuously because of the vegetable varieties and quality changed with the improvement of living standards.Therefore,the chemical fertilizers,pesticides,and livestock wastes were needed to ensure the quality and yield of vegetables in the greenhouse facility.However,these agricultural products often contain heavy metals,and that concentration of heavy metals in the vegetable fields would be accumulated to exceed the soil pollutant standard after a certain years.In many soil remediation methods of heavy metal pollution,it is easy,economical and efficient to apply the natural material based passivator without secondary pollution.In this article,various of passivators of effective cadmium were studied such as montmorillonite?Mnt?,natural zeolite?Zeo?,hydroxyapatite?HAP?,nano-hydroxyapatite?n-HAP?,fly ash?FA?,wheat straw biochar?WBC?,rice straw biochar?RBC?,peanut shell biochar?PBC?,and corn biochar?CBC?.The passivation agents were selected through the evaluation of cadmium removal effect in aqueous solution.Furthermore,the passivating effect was investigated by applying different combined passivators in greenhouse vegetable soil.The sulfhydryl group modification of montmorillonite was proposed in order to reduce the cost,increase the application range and ensure the passivation effect of cadmium.Then sulfhydryl montmorillonite was investigated by indoor soil incubation experiments.Among minerals,the removal rate of cadmium was 97.63%by using n-HAP in aqueous solutions.In biochars,the removal efficiency of WBC and RBC was better than others.The influencing factors of cadmium passivation were ranked by orthogonal test:pH value>dosage of passivator>type of passivator>reaction time.The removal order of cadmium was Mnt>RBC>FA with pH 3-7.In the vegetable field trials,the soil pH value was all increased after the combined passivators were applied,and the effective cadmium levels were reduced.The available cadmium removal rates of three better combinations were 39.05%,36.09%,and34.93%,respectively.Those combimations were wheat straw biochar?0.3%?+nano-hydroxyapatite?225kg/km²??W+H?,montmorillonite?0.3%?+nano-hydroxyapatite?225kg/km²??M+H?and rice straw biochar?0.3%?+nano-hydroxyapatite?225kg/km²??R+H?.In order to ensure the passivation effect of cadmium in the soil from greenhouse vegetable field,the microwave-ultrasonic method was used to prepare sulfhydryl montmorillonite?Mnt-SH?with a widely application range,low cost,and no secondary pollution.The removal rate of cadmium reached 97.47%in the aqueous solution,and that was verified by indoor oil incubation experiments.The sulfhydryl montmorillonite is suitable for a wide range of pH,especially in acidic soil environment,which will significantly better than other passivators.The results of orthogonal test indicated the order of influencing factors of Mnt-SH,n-HAP and potassium dihydrogen phosphate?KH₂PO₄?was dosage>ratio of passivator and WBC>type of passivator.The better cadmium passivation effect of Mnt-SH had been proved after applied 40 days in soil incubation experiments.Mnt-SH had not advantage compared with n-HAP in alkaline soil environment,while that was more suitable for cadmium passivation in acidic soils.In the passivation process of effective cadmium in greenhouse vegetable fields,the mineral-based passivator was the key;the biochars could provide a suitable passivation environment for the former.Combined application of the two will significantly increase the passivation effect,reduce the cost and maintain the fertility of soil.In addition,the soil pH and the surface active groups on passivator were also the main influencing factors.The useing of compound passivator as sulfhydryl modified montmorillonite+biochar will significantly reduce the cost,avoid soil salinization,and provide new select and technical support for the heavy metals passivation in different soil with different pH values.