Study On The Influence Of Passivation Materials On The Stability Of Soil Cadmium And The Dynamic Changes Of Base Ions

Hebei province is an important production base of vegetable facilities in Beijing-Tianjin-Hebei.Facility vegetable plots are continuously and intensively planted all year round,and the blind application of various fertilizers has caused heavy metal pollution in the vegetable plot soil and the accumulation of base ions gradually increased.Therefore,targeted remediation and improvement measures for heavy metal pollution in the vegetable fields with medium and alkaline facilities in the north,taking into account the accumulation of soil salt base ions,is the“golden key”to improve heavy metal pollution in the farmland with facilities and ensure the quality of agricultural products.In this paper,a township facility agriculture concentrated planting area in Hebei Province was taken as the research area.Six representative heavy metals,soil physical and chemical properties and nutrient indexes of 57greenhouse soils in this region were analyzed.Based on the properties of alkaline soil in northern China and previous results,the effect and stability of Cd²⁺passivation remediation were evaluated,and the rules of soil base ion change during the remediation process was explored.The main findings are as follows:（1）With regard to heavy metals,referring to China’s"Environmental Quality Evaluation Standards for Greenhouse Vegetable Producing Areas",the content of Cd in some sample points exceeded the standard to varying degrees.The proportion of greenhouses is 26.32%.The p H of the facility greenhouse is alkaline,even strongly alkaline.Facility greenhouses with nitrogen supersaturation（above the second-level level）accounted for about 22.81%of the survey area,and the overall phosphorus and available potassium content were relatively high,of which the total phosphorus and available potassium content reached the first-level level.There is a positive correlation between total phosphors,total nitrogen and heavy metals,especially Zinc.（2）The results of the follow-up study on the passivation effect of effective Cd in the soil within three years show that the application of each passivation material the soil p H remained stable and had no significant effect on soil EC.The combined application of hydroxyapatite and biochar satisfies the dual indexes of heavy metal Cd passivation ability and long-term stability.Among all the treatments,Hydroxyapatite+1.2%wheat straw biochar has the best passivation effect,with a monthly reduction rate of 58.44%compared to CK.（3）The application of passivation materials will not increase the soil base ion content of the facility,and the combined application of hydroxyapatite and biochar can alleviate the accumulation of soil base ions in a short period of time.In addition,soil p H,EC and base ions were significantly negatively correlated and positively correlated at P<0.01 level.（4）When the soil available Cd content was in the range of 0.05～0.16 mg/kg,0.16～0.25mg/kg,0.25～0.35 mg/kg,the base cations and sulfate ions showed positive correlation,negative correlation and no obvious correlation with the available Cd.Cl^-and effective Cd showed a negative correlation in the range of 0.05～0.32 mg/kg Cd²⁺concentration.In summary,the application of biochar and hydroxyapatite in combination not only can significantly reduce the soil available Cd content,but also alleviate the accumulation of base ions.Therefore,it can be used as a soil conditioner for Cd contaminated facilities in Beijing-Tianjin-Hebei region.