Technical Adaptation Of Iron-Manganese Oxidizing Bacteria For Remediation Of Cadmium Contaminated Calcareous Soils In Sewage Irrigation

The use of iron-manganese（Fe-Mn）oxidizing bacteria to remediate cadmium（Cd）contaminated soil is a new emerging technology in bioremediation.It has been applied in heavy metal remediation due to its high efficiency,short time,low cost,and the absence of secondary soil pollution.The presence of iron-manganese oxidizing bacteria can promote the formation of iron-manganese（hydro）oxide in the environment,and heavy metals can be stabilized or immobilizatied through biosorption,co-precipitation,and complexation.But at the same time,during the oxidation process of iron-manganese oxidizing bacteria,H⁺is produced which can activate Cd.Therefore,it is necessary to investigate whether iron-manganese oxidizing bacteria can adapt to Cd-contaminated calcareous soil and its remediation effect on calcareous soil.In this study,we investigated the mechanism of Cd remediation by Fe-Mn-oxidizing bacteria on calcareous soil using batch experiments and evaluated the effect of Fe-Mn-oxidizing bacteria on the passivation of heavy metal Cd remediation in polluted soil in the upper reaches of Dongda Ditch,Baiyin City,Gansu Province.The following main conclusions were drawn from the study:1.In calcareous soil,the addition of iron-manganese oxidizing bacteria alone can increase the soil p H,with the highest value reaching 7.66.The addition of Fe²⁺and Mn²⁺as additives alone can decrease the p H,with the treatment of only 2M Fe²⁺showing a decrease of 0.19 units compared to the control group.The combined treatment of bacteria and Fe²⁺or Mn²⁺showed a decrease in soil p H in the early stage and an increase in the later stage.The Eh range of each treatment without adding bacteria ranged from 210 to 250 m V,while adding 1%bacteria resulted in an Eh range of 190 to 240 m V.Adding 2.5%bacteria resulted in an Eh range of 160 to 235 m V and adding 5%bacteria resulted in an Eh range of 130 to 190 m V.An increase in the concentration of bacteria led to a decrease in soil Eh.In the first 30 days of cultivation,adding iron-manganese oxidizing bacteria can increase the content of available Cd and exchangeable Cd in soil.However,at day 150,some treatments showed immobilization effects,and the optimal immobilization rate reached 92%.2.Three different modes of adding iron-oxidizing bacteria before,simultaneously,and after alkaline material were applied to evaluate the remediation effect of Cd in calcareous soil.Among them,the treatment of iron-oxidizing bacteria added one week before 2.5%alkaline material showed the best immobilization effect,with a reduction of 185μg·kg^-1 in the content of available Cd compared with the control group,an 8%decrease in the proportion of exchangeable Cd（EXC-Cd）,a 7%increase in the proportion of residual Cd（RES）,and a transformation of Cd from unstable to relatively stable forms.The stabilization effect of the simultaneous addition mode was between the two modes,while the stabilization effect of the mode with iron-oxidizing bacteria added one week after alkaline material was the worst.In all treatments with the combination of iron-oxidizing bacteria and sugar beet lime,soil p H,EC,and DOC were improved.3.Iron oxidizing bacteria combined with sugar beet lime treatment was used to evaluate the remediation effect under different application modes.The experimental results showed that different modes had no significant effect on the germination rate of four vegetable seeds,and the maximum germination rate was reached on the fifth day.However,different application modes slightly inhibited the growth of seedlings of four vegetable seeds（P<0.05）.In the three modes,compared with the control group,the treatment of iron oxidizing bacteria combined with sugar beet lime significantly reduced the bioavailability of Cd（BA）（P<0.05）,with a maximum decrease of 15.37%in the stomach stage and 9.74%in the intestine stage.At the same time,using the TCLP method to determine the leaching toxicity of Cd,the maximum decrease was 51.03%.In summary,the addition of iron and manganese oxidizing bacteria alone results in a decrease in soil p H,an increase in Eh,and an elevation in the bioavailable Cd content.In contrast,the combination of iron oxidizing bacteria and alkaline material application effectively attenuates Cd in the soil,with the best effect observed when the iron oxidizing bacteria are applied one week before the alkaline material.This treatment resulted in a reduction of Cd bioavailability and leaching toxicity.Moreover,the germination rate of four vegetable seeds was not significantly affected by the three different application modes.Therefore,the application of iron oxidizing bacteria one week prior to the addition of alkaline material is deemed the optimal approach for Cd remediation.