The Immobilization Mechanism Of Heavy Metals In Soil By Phosphate Fertilizer Combined With Biochar And Phosphorous Modified Biochar

Heavy metal has become the main factor of farmland soil pollution,which seriously threatens crop growth and food safety.Biochar combined with phosphate（P）fertilizer can promote complexation,ion exchange and coprecipitation to reduce the availability of Cd ions in soil.However,the effect of plant feedback regulation is often overshadowed by the effect of biochar and P fertilizer.The response of organic acids and other root exudates to fertilization and the potential of Cd transformation in soil deserve further study.The possibility of producing phosphorous（P）modified biochar instead of application biochar and P fertilizer was further explored.In this study,Cd contaminated farmland soil was used as the research object,P fertilizer,biochar and modified biochar were used as the influencing factors.The experiment was designed as（1）pot experiment(biochar was 0（C₀）and 20 g kg^-1(C₂₀).P fertilizer was 0（P₀）,20(P₂₀)and 40 mg P kg^-1(P₄₀).（2）rhizosphere soil:CK（the control）;PF(40 mg P kg^-1);BC(20 g kg^-1 biochar);BC+PF(40 mg P kg^-1+20 g kg^-1 biochar).（3）Preparation of phosphorus modified biochar:BC（maize straw）;PBC（maize straw+phosphorylation）;PBC-CS（maize straw+phosphorylation+Calcium superphosphate）;PBC-AP（maize straw+phosphorylation+Ammonium polyphosphate）.We aimed to（1）explore the synergistic effect of biochar and different P fertilizer application rates on Cd transformation and plant uptake in Cd contaminated soil;（2）explore the transformation mechanism between soil phosphorus supply and Cd fixation under the combined application of biochar and P fertilizer,and to explore the competitive relationship between Cd dissolved by organic acids and phosphate ion co-precipitation;（3）explore the potential of co-pyrolysis biochar of phosphate fertilizer in Cd fixation and as slow-release fertilizer.The results are as follows:（1）A pot experiment was conducted to investigate the effect of combined with biochar and P fertilizer on soil Cd availability and translocation.Results showed that,compared with C₀ level,the content of DTPA-Cd in soil was significantly decreased with biochar adition after 60 days of cultivation,under C₂₀ level,soil DTPA-Cd in C₂₀P₄₀ treatment were significantly increased.Under both C levels,the percentage of exchangeable Cd fraction at P₄₀ rate was significantly lower than that at P₂₀ rate,bacauce the excess P in soil could precipitate Cd.The percentage of residual-Cd fraction was significantly increased with the combined addition of biochar and P fertilizer,particularly in C₂₀P₄₀ treatment,which was 75.95%,while it was only61.65%in C₀P₀ treatment.The Cd translocation factor（TF）and bio-concentration factor（BCF）were also significantly reduced in C₂₀P₂₀ and C₂₀P₄₀ treatments compared with C₀P₀ treatment.Therefore,the combined high P and biochar application was a good choice in inhibiting soil Cd availability and plant Cd uptake,which benefited to the safe utility of the Cd contaminated soil.（2）The Cd speciations in rhizosphere soil were investigated by pot experiment and adsorption experiment.After 60 days of cultivation,the content of DTPA（diethylenetriamine pentaacetate acid）-Cd in PF,BC and PF+BC treatments were significantly reduced by 17.3%,36.2%,23.9%compared with CK treatment,respectively.We found that the p H of rhizosphere soil was significantly lower than that of non-rhizosphere soil in all treatments.The order of p H in rhizosphere soil was BC>PF>PF+BC>CK.The content of organic acid in PF,BC and PF+BC treatments were significantly reduced by 39.8%,71.1%,25.3%compared with CK treatment,respectively.The rhizosphere soil in PF+BC treatment had the largest available P(36.32 mg kg^-1)and the largest adsorption capacity(7409 mg kg^-1).The results of Energy dispersive X-ray（EDX）mapping confirmed that Cd²⁺and PO₄^3-to form bond bridges or precipitates such as Cd₃（PO₄）₂ on the surface of soil or biochar in PF+BC treatment.When P is sufficient,although it increases the availability of Cd,the high adsorption capacity produced by the addition of biochar can re-precipitate the Cd dissolved by organic acids.（3）Biochar with high porosity and good carbon structure and rich in phosphorus functional groups can be obtained from the co-pyrolysis of superphosphate and ammonium polyphosphate soaked with phosphoric acid.The addition of phosphorus fertilizer is also an important factor to protect carbon loss and form stable carbon structure in the process of co-pyrolysis.Similarly,the stability of carbon structure promotes the bonding of phosphorus functional groups on the surface of biochar,which is also an important way to retain phosphorus nutrients.In conclusion,biochar combined with high phosphorus can achieve stable and long-term fixation of heavy metal Cd in soil.Superphosphate has a good effect on the modification of biochar,and modified biochar has great potential in immobilization of heavy metals in soil and as slow release fertilizer.