Research On The Stability Of Cadmium Passivation In Weakly Alkaline And Polluted Soil By Phosphate Powder And Bentonite

Soil heavy metal pollution is one of the global ecological and environmental problems affecting human and environmental health.Excess cadmium（Cd）in soil can be transferred to edible parts of crops and enter the human body through the food chain,endangering human health.Phosphate powder and bentonite performed well in Cd passivation remediation of acidic soil,but less research has been reported on passivation remediation of weakly alkaline Cdcontaminated soil.Moreover,there is a risk of re-release of passivated heavy metals in contaminated soil as environmental conditions changed,so it is important to investigate the stability of passivation remediation in heavy metal contaminated soil.In present study,two passivation materials,phosphate powder and bentonite were selected and three addition levels（low,medium and high）were set for field experiment,the available Cd content in soil and Cd contents in grain were evaluated by phosphate powder and bentonite application.Combined with the analysis of soil Cd form,Cd absorption and transportion characteristics of wheat,the passivation effect was clarified.Meanwhile,the physical and chemical characteristics of soil after passivation materials application were also analyzed,and the correlation between soil environmental factors and soil Cd form were established.Furthermore,two wheat varieties with different Cd accumulation characteristics were cultivated to study their effects on Cd passivation effect of phosphate powder and bentonite.These results would provide sound theoretical basis and technical support for the safety utilization of weakly alkaline Cdcontaminated soil.The main research results were as follows.（1）Available Cd in soil and Cd content in grain were significantly reduced by phosphate powder and bentonite application,and this passivation effect by bentonite was stronger than that by phosphate powder.Meanwhile,the reduction effect on available Cd in soil and Cd content in grain by phosphate powder and bentonite were varied depending on the application level,and the reduction effect on available Cd in soil and Cd content in grain by phosphate powder and bentonite behaved gradually increasing trend as the application level increased,soil available Cd content was reduced by 11.52%-26.65% and 11.08%-35.00%,,and Cd content in wheat grain was reduced by 7.28%-49.94% and 14.14%-57.61%,respectively.Moreover,the soil Cd forms could be changed by two passivation materials application,the exchangeable Cd content was significantly reduced by medium and high levels of bentonite application（11.46%and 17.10%,respectively）;the carbonate bound Cd content was significantly reduced by the medium and high levels of phosphate powder and bentonite application（11.87%,18.92% and9.82%,21.87%,respectively）;and Fe-Mn O-bound Cd was significantly reduced by high level of bentonite application（11.77%）.（2）The soil p H and the stability of soil aggregates were enhanced by phosphate powder and bentonite application,and the increased scope was positively correlated with the application level.No significant effects on soil enzyme activity,available potassium content,soil CEC,and soil nutrients by phosphate powder and bentonite application.Soil p H was elevated by 0.06-0.15 and 0.01-0.18 units;soil aggregate GMD was elevated by 6.28%-12.11% and 12.46%-18.37% by phosphate powder and bentonite application.Furthermore,the soil available phosphorus content was increased by phosphate powder application,and the soil available phosphorus content was increased with 32.70%-40.56% by medium and high levels of phosphate powder application relative to CK.Soil available Cd is negatively correlated with soil p H,available phosphorus content,and there is also a high negative correlation between soil Cd form and physic-chemical properties.Soil p H,soil water content and soil aggregate stability are negatively correlated with exchangeable Cd,carbonate-bound Cd,and positively correlated with the Fe-Mn oxides-bound Cd;and soil available phosphorus content was also negatively correlated with carbonate-bound Cd.（3）The absorption and transportion abilities of Cd in Zhoumai 18 and Jimai 22 were reduced by phosphate powder and bentonite application,and the absorption and transport capacity of Zhoumai 18 was higher than that of Jimai 22.Compared with Jimai 22,the shoot Cd content and enrichment coefficient of Zhoumai 18 were enhanced with phosphate powder application,and the shoot Cd content of Zhoumai 18 was increased by 15.41%-40.00%,and enrichment coefficient of Zhoumai 18 was increased by 12.15%-14.79%,respectively.Moreover,compared with Jimai 22,the shoot Cd content and transport coefficient of Zhoumai18 were increased by 33.33%-43.35% and 26.86%-48.69% with bentonite application,respectively.The growth of Zhoumai 18 and Jimai 22 were promoted by phosphate powder and bentonite application,and the root system configuration,biomass accumulation,photosynthesis,crop yield and components were improved by phosphate powder and bentonite application,but the Zhoumai 18 was lower than Jimai 22 in passivating treatment.The total root length of Zhoumai 18 was 7.20% lower than that of Jimai 22 with medium levels of bentonite application.Compared with Jimai 22,the shoot biomass of Zhoumai 18 was decreased by 27.54%-29.33%;the root biomass of Zhoumai 18 was decreased by 17.14%-27.51% by phosphate powder application.Moreover,the net photosynthetic rate of Zhoumai 18 was 30.39%-59.24% lower than that of Jimai 22 by bentonite application.（4）Compared with Jimai 22,Zhoumai 18 planting reduced the soil p H.Compared with Jimai22,the soil p H was significant reduced 0.11 units by Zhoumai 18 in soil without passivation material addition.The decreasing effect of Zhoumai 18 on soil p H was slowed down by passivation materials.No significant difference in soil p H by the passivation application in two wheat in the same treatment.Furthermore,no significant effect on CEC,soil water content,available phosphorus and available potassium of soil by planting two wheat cultivars.Compared with Jimai 22,the FEA value of zhoumai 18 root soil was higher,and the exchangeable Cd in Zhoumai 18 root soil under high applied phosphate powder and medium applied bentonite treatments were significantly increased by 11.03% and 21.62%,and the FeMn oxides-bound Cd of Zhoumai 18 under high applied phosphate powder treatment was significantly lowered by 3.02%.The organic-bound Cd of Jimai 22 was significantly improved by 17.09% and 14.71% in medium and high applied bentonite treatments than that of Zhoumai18.（5）Phosphate powder and bentonite showed good passivation stability,and no significant differences in grain Cd content between two wheat cultivars,available Cd,CEC,available phosphorus and available potassium between years after the cessation of passivation application,however,the soil p H was reduced.The Cd content in wheat grains were significantly elevated by 28.66% with low level phosphate powder application indicated weak stability.The effect of passivation was not significantly different between the different applications of phosphate powder.However,the continuous application of bentonite was beneficial to the reduction of soil available Cd.Compared with single application,the soil available Cd was significantly reduced by 19.21% for applying three times in a medium application with medium level bentonite.The passivation effect of phosphate powder and bentonite was significantly affected by different Cd accumulation wheat varieties,and jimai 22 was more favorable to maintain the passivation stability than Zhoumai 18.