Study On The Regulation Mechanism Of Hydroxyapatite Assisted By Bacillus Subtilis On Cadmium Contaminated Agricultural Soil

As shown in the national compendium on the quality of the ecological environment of 2020 issued by the Ministry of ecological environment,the main pollutants affecting the environmental quality of soils in agricultural areas are heavy metals,among which cadmium(Cd)is the primary pollutant.In this paper,the passivation mechanisms of two types of passivation materials for agricultural Cd contaminated soils were systematically investigated,and microbial assisted synthesized biotype hydroxyapatite(B-HAP)as well as chemical hydroxyapatite(C-HAP)without microbial involvement were selected as passivation materials in this study,according to whether the decomposition effect of B.subtilis was utilized or not.The adsorption effect of B-HAP and C-HAP in cadmium solution and passivation effect and cadmium migration ability in actual cadmium contaminated soil were explored;The adsorption mechanisms of two types of hydroxyapatite on cadmium contaminated soils were elucidated,and the characteristics of hydroxyapatite accumulation on celery were revealed through pot experiments with actual contaminated soils.This paper provides preparation ideas for developing new passivation materials beneficial to soil health and green development,and also provides reference values for green regulatory contributions to agricultural soils contaminated with low concentrations of Cd.The main results are as follows:(1)Using Bacillus subtilis decomposition substrate disodium phenylphosphate,combined with calcium chloride again,B-HAP prepared at calcium phosphorus ratio of 1.67,p H around 10 has the characteristic functional groups of standard hydroxyapatite species.In contrast to C-HAP,B-HAP has the characteristics of large specific surface area and small pore size of single pore.This is due to the fact that the metabolites of microorganisms can improve soil nutrients,improve plant uptake of nutrient elements,promote plant growth and improve plant volume.Extracellular polymers secreted by microorganisms due to metabolism may affect soil p H,and p H changes can alter heavy metal speciation and promote interconversion among various speciation of heavy metals.(2)In the 1.00 mg/L cadmium solution adsorption experiment,the Cd concentration in the solution decreased by 94.8% and 95.8% when 0.20 mmol B-HAP and C-HAP were added.In the passivation experiment,soil p H increased from 7.08 to7.30 and from 7.08 to 7.43 when 0.021% of B-HAP and C-HAP were added.B-HAP showed little interference with soil pH.The results showed that Cd decreased by38.6% and 36.8% in the mobilizable state with 0.021% B-HAP and C-HAP in real soil samples,respectively.The structure and morphology of B-HAP were characterized by various techniques and showed that the adsorption pattern of B-HAP changed with the time of ion exchange to specific adsorption.Therefore,B-HAP can be used as an effective passivation agent for Cd removal from agricultural soils.When 0.021%B-HAP was added after 104 days,the concentration of Olsen-P decreased by 35%.(3)Extracellular polymers secreted by microorganisms due to metabolism can affect soil physicochemical properties,thereby altering heavy metal speciation and promoting interconversion among various speciation of heavy metals.Although inactive microorganisms have no ability to accumulate intracellularly,the inactive state reduces the toxic effects of heavy metals on them,thus facilitating electrostatic adsorption.Ca(Ⅱ)in hydroxyapatite is displaced from the heavy metal cadmium ion by reactions such as ion exchange,surface complexation,and release of hydroxyl groups enhances the negative charge on the soil surface and cadmium ions form precipitates,thus acting to reduce heavy metal activity.This study confirmed that hydroxyapatite synthesized with microbial assistance,a phosphate Soil Ameliorant,could exert a good passivation effect in agricultural fields contaminated with moderate and mild heavy metals.In summary,corn cob biochar in corncob biochar loaded Bacillus subtilis can be a safe habitat to provide necessary nutrients for bacteria,and Bacillus subtilis has strong immobilization effect.The combination of Bacillus subtilis and corncob biochar can overcome the disadvantages of poor stabilization effect of corncob biochar alone and short life cycle of Bacillus subtilis,to control the migration of cadmium further and achieve the purpose of synergy.