Study On The Effect Of Phosphorus-rich Biochar On The Passivation,Migration,and Transformation Of Heavy Metals In Soil

Biochar has vast application possibilities in the field of soil heavy metal pollution prevention because to its low cost,"negative carbon emission,"and adequate scale promotion.The application of phosphate-modified biochar can not only increase its heavy metal adsorption ability,but also minimize phosphorus leaching and promote phosphorus usage.This study systematically studied the effects of pyrolysis temperature,biomass raw material types,and phosphate precursor types on the physicochemical properties of phosphorus biochar and the influence mechanism of heavy metal adsorption characteristics.In addition,an in-depth discussion of the adsorption mechanism of phosphorus-rich biochar on heavy metals and the influence mechanism of soil heavy metal migration and transformation were conducted.Firstly,the influence of preparation temperature on the physicochemical properties of phosphorus-rich biochar and its effect on the adsorption performance of heavy metals was studied.With the increase of pyrolysis temperature,the yield,pH value,and surface oxygen-containing functional group content of K₃PO₄ modified bamboo biochar all decrease,and the specific surface area and total pore volume first increase and then decrease.Phosphorus-rich biochars generated at various temperatures primarily fix heavy metals in water by organic complexation with surface-OH/-COOH,precipitation with PO₄^3-,or inorganic complexation with P₂O₇^4-.550℃ is the best temperature for biochar preparation,and the maximum adsorption capacity of Cd（II）at this temperature is 289.32 mg/g.Then,the influence mechanism of the types of biomass raw materials on the physical and chemical properties of phosphorus-rich biochar,soil heavy metal forms,and bioavailability was explored.The results found that phosphorus-rich biochar derived from agricultural waste has higher pH,CEC content,BET specific surface area,total pore volume,and stronger phosphorus loading capacity than forestry waste.In comparison,the phosphorus-rich biochar derived from agricultural waste has better passivation performance for Cd（II）and Cu（II）,which are 14～24%and 19～33%higher,respectively.Compared with the unmodified biochar,the passivation efficiency of Cu（II）and Cd（II）using phosphorus-rich biochar increased by at least 2 to 3 times.Next,the influence of phosphorus precursors on the conversion path of phosphorus in the phosphorus-rich biochar and the effect on the passivation mechanism of heavy metals was explored.The passivation performance of several kinds of biochar on Cd（II）follows the order,K₃PO₄-modified biochar>H₃PO₄-modified biochar>（NH₄）₃PO₄-modified biochar>unmodified biochar.The passivation mechanism of H₃PO₄-modified biochar and（NH₄）₃PO₄-modified biochar on Cd（II）includes the complexation by surface organic phosphorus-containing groups（orthophosphate monoester）,oxygen-containing functional groups（hydroxyl,carboxyl）,and pyrophosphate.The immobilization mechanism of K₃PO₄-modified biochar on Cd（II）is dominated by the（co）precipitation with orthophosphate.Adding 2%potassium phosphate-modified biochar reduced the content of avaliable Cd（II）by 81.03%.Based on the previous chapters,the passivation mechanism of carboxymethyl cellulose sodium-coated phosphorus-rich biochar on soil heavy metals and the slow release characteristics of phosphorus were explored.The adsorption capacity of coated phosphorus-rich biochar for Pb（II）reached the maximum value of 1120 mg/g.The adsorption process of heavy metals by the coated phosphorus-rich biochar involves three stages.The cumulative release rate of phosphorus after 1 day（ω₁）and 28 days(ω₂₈)of coated phosphorus-rich biochar in still water was 6.21 wt.%and 74.95 wt.%,respectively.Finally,based on experiments,a model of solute transport in multi-layer coupling paddy soil was established and calibrated.It simulated the advection-diffusion,dual-zone diffusion,preferential flow,cation exchange,complexation,and precipitation of Cd（II）in the soil-biochar system.When there is a higher concentration（0.1 mmol/L）of cadmium input,only2%phosphorus-rich biochar can make the cadmium concentration lower than the regulated soil cadmium limit（0.3 mg/kg）.Under a 360-day irrigation cycle,adding 2%of unmodified biochar is of little significance,while adding 2%of phosphorus-rich biochar can ensure that the cadmium concentration in paddy soil is within the specified limit during the 360-day irrigation cycle.