Immobilization Of Cadmium By Secondary Minerals Formed By The Reaction Of Sulfate Reducing Bacteria With Clay Minerals In Purple Soil

Cadmium,as a non-essential trace metal element for human body,will cause toxicity and carcinogenesis when ingested.Soil cadmium pollution caused by human activities and natural background value in soil rock seriously endangers food safety and life health in China.Soil heavy metal remediation is mainly carried out by adding passivating agents,modifying remediation materials,planting highly enriched plants,and adding earthworms and other organisms.However,China has a vast territory,large topographic differences and high soil heterogeneity.The purple soil in Southwest China is rich in minerals,moreover the background values of cadmium are higher than the national average.Clay minerals formed by rock weathering are components of soil,and the content of heavy metals in rock parent material affects the background value of elements in soil.Therefore,it is necessary to study the migration and activation of heavy metal cadmium in clay components in soil.The microbial-clay mineral system can reduce the mobility of heavy metals through biological mineralization,thus reducing the env Femental problems caused by heavy metals.Therefore,sulfate reducing bacteria were extracted from natural soil to react with purple soil clay system under different cadmium concentration stress,and then reacted with purple soil,clay minerals,clay minerals removing amorphous Fe oxides,clay particles removing crystalline Fe oxides and residual clay particles after hydrochloric acid treatment.The effects of mineral differences in different systems on the migration and transformation of cadmium were studied.Finally,the microbial mineralization mechanism is explored in combination with different stratum samples in Xiushan area of Chongqing.The specific research results are as follows:（1）The Fe-containing substances mainly exist in clay minerals.The Fe content in natural soil is 34.37g·kg^-1,and the content in clay minerals is 62.84g·kg^-1.The simulated cadmium contaminated soil(0,2,5,10,20 mg·kg^-1)formed a reaction system with microorganisms,in which the transformation of Fe was obvious.After 2mg·kg^-1clay reacted with microorganisms,the Fe content of Fe manganese oxide also increased from 9.25%to 23.45%.After the reaction of 20mg·kg^-1 cosmids with sulfate-reducing bacteria,the cadmium of the indefinite conjugate changed from35.02%to 18.38%,and the Fe content of the Fe-manganese oxide also changed from8.35%to 19.31%.The microbe promoted the transfer of Fe from unstable state to crystallized state,and cadmium was mainly transferred to the oxidized state of Fe and manganese,which confirmed the solidification of crystallized Fe oxide on cadmium under the microbe action.（2）The cadmium in the purple soil is mainly accumulated in the clay minerals,according to the cadmium content in the soil is 0.25 mg·kg^-1 and 0.35 mg·kg^-1 in the clay.While the organic matter was removed,the cadmium in the clay minerals decreased by 25.3%,and the cadmium was 0.26 mg·kg^-1.After removing the infinitive Fe oxides,cadmium is 0.09 mg·kg^-1,which is 66.7%less than the clay minerals after removing the organic matter.It can be seen that cadmium is mainly adsorbent on the infinitive Fe oxides.According to X-ray diffraction analysis,Clay_-ox and Clay_-CBD are rich in minerals in clay minerals,especially Clay_-CBD after removing crystalline Fe oxide is activated.The pearson correlation analysis of cadmium and soluble Fe content（r=0.944 P<0.05）showed that cadmium is closely related to Fe oxides in clay minerals.（3）Fe-bearing minerals are very important for the adsorption of cadmium in soil and clay.X-ray diffraction analysis shows that Fe and cadmium substances are dissociated from Fe manganese ore after removal of indefinite Fe oxides.When crystalline Fe and manganese are removed oxide’s Clay_-CBD that removed fibroferrite.Siderite is formed in both microbial and clay systems after inoculation,and the clay minerals can promote the formation of Fe bearing minerals under the action of microorganisms,and the migration of cadmium is affected by Fe reduction and Fe mineral phase.Clay_-ox reacts with microorganisms to regenerate ferromanganese and Cd Fe O₄,Clay_-CBD reacts with bacteria to regenerate Fe containing minerals such as fibroferrite,and Clay_-HCl reacts with bacteria to generate Cd SO₄ and siderite.In addition,during the removal of chemical clay particles,Clay_-ox turned black after reacting with sulfate reducing bacteria,which was detected by XRD as Fe-S compound.The secondary minerals formed under microorganisms after sulfate reduction and Fe reduction are consistent with the clay minerals removed from the original soil clay,indicating that the strain can participate in the formation of soil minerals.（4）The clay samples from different strata in Xiushan were analyzed.The Fe bearing minerals in the clay were found by XRD analysis.The types of Fe minerals in different strata are different,and most of them are kornelite,coquimbite,and bilinite as the main Fe-bearing minerals.When the above clay minerals containing Fe minerals are reacted with sulfate-reducing bacteria,the microorganisms carry out Fe reduction and Fe oxide reduction and dissolution in anoxic env Fement,forming mixed valence Fe（II）/Fe（III）minerals,suchas:Bilinite（Fe Fe₂（SO₄）₄·22H₂O）,copiapite（Fe Fe₄（SO₄）₆（OH）₂·20H₂O）,or siderite（Fe CO₃）,Bulterite（Fe（OH）SO₄·2H₂O）.There was a significant negative correlation between exchangeable state and Fe-Mn binding state（r=-0.650 P<0.05）,and between carbonate binding state and Fe-Mn binding state（r=-0.533 P<0.05）.The exchangeable state and the carbonate-bound state are transformed into the Fe-manganese-bound state and the residue state,which is beneficial to the fixation of cadmium species.The above analysis shows that the solidification of cadmium by clay Fe minerals after the reaction between Xiushan layer samples and microorganisms is the same as that of purple soil,which further confirms the migration and activation mechanism of secondary Fe minerals to heavy metal cadmium.