Immobilization And Transformation Of As By Fe–Mn Minerals Mediated By Microorganisms In Groundwater

Primary arsenic（As）-rich groundwater is widely distributed in the world,which is a worldwide environmental geological problem of great concern to the international community and seriously threatens human life and health.In natural groundwater environment,As mainly exists in the form of arsenate[As（V）]and arsenite[As（III）].The coupling technology of As（III）oxidation and As（V）adsorption can be an ideal strategy for in-situ As immobilization in As-rich groundwater.In the As-rich groundwater environment,Fe–Mn binary oxide（Fe–Mn oxide）minerals are widely present and have both the physical and chemical characteristics of Mn and Fe oxides,so they can rapidly oxidize As（III）and effectively adsorb As（V）at the same time.Fe–Mn oxide minerals play an important role in the natural remediation of As-rich groundwater.However,the respective contributions and interactions of Fe and Mn oxide components remain to be further studied.In addition,the As cycle in groundwater is closely related to microbial activities,and functional microorganisms such as metal-oxidizing and metal-reducing bacteria play an important role in controlling As adsorption and oxidation induced by Fe/Mn oxides.Existing studies mainly focus on the retention of As by single Fe/Mn oxides mediated by microorganisms.However,in the presence of oxidizing and reducing bacteria,the immobilization mechanism of Fe–Mn oxides on As in As-rich groundwater and the interaction between bacteria and minerals are still lack of in in-depth research.This study focused on the key scientific problem of“the immobilization and transformation mechanism of As by Fe–Mn minerals mediated by microorganisms in groundwater”.Based on Mn（II）oxidizing bacteria（Comamonas sp.RM6）,the immobilization of As by biogenic Fe–Mn oxides（BFMO）in groundwater was investigated through microcosm simulation experiment and modern instrumental analytical techniques,and the simultaneous immobilization of As（III）and their coupling mechanism in groundwater during the in-situ formation of Fe–Mn minerals driven by oxygen perturbation and Mn（II）oxidizing bacteria was revealed.Based on metal-reducing bacteria（Bacillus sp.FMR）,the immobilization of As by Fe–Mn oxide minerals mediated by metal-reducing bacteria in groundwater in anaerobic environment were elucidated,and the influencing mechanism of metal-reducing bacteria on the in-situ immobilization of As by Fe–Mn minerals in groundwater was discussed.Based on As（V）-reducing bacteria（Shewanella putrefaciens strain CN32）,the biotransformation and release of As in As-containing Fe–Mn minerals mediated by As（V）-reducing bacteria was revealed.The immobilization and transformation of As via Fe–Mn minerals mediated by microorganisms in groundwater was studied,which is helpful to further understand the in-situ immobilization of As by Fe–Mn minerals in As-rich groundwater,has important environmental implications,and provides theoretical support for the green in-situ remediation of As-rich groundwater.The main results are as follows:（1）Mn-oxidizing bacteria（Comamonas sp.RM6）were newly isolated from As-rich groundwater sediments.The immobilization of As（III/V）by BFMO mediated by strain RM6 in simulated As-rich groundwater was investigated,and the respective contributions of Fe oxide and biogenic Mn oxide components and their interactions were revealed.In the formation of BFMO mediated by strain RM6,strain RM6 showed the optimal Mn（II）oxidation activity（45%）at an initial Mn（II）concentration of 15 mg L^-1.Fe（II）rapidly formed Fe oxides（2-line ferrihydrite）under the oxidation of oxygen,and then combined with biogenic Mn oxides（δ-Mn O₂）induced by strain RM6 to form BFMO through co-precipitation.BFMO minerals can effectively immobilized As（III）and As（V）in groundwater,and the immobilization efficiency of As（T）was 83%and 82%,respectively.The immobilization mechanism of As（V）by BFMO was adsorption,and the immobilization mechanism of As（III）was the coupling of oxidation and adsorption.Fe and Mn oxide components synergistically promoted the immobilization of As（III/V）,in which the Fe oxide component dominated the adsorption of As and the Mn oxide component dominated the oxidation of As（III）.As adsorption on BFMO minerals was a heterogeneous multilayer adsorption process,which was an inner sphere complexation mechanism.（2）Based on the initial Mn（II）concentration（about 0.3 m M）of the optimal Mn（II）bio-oxidation activity and the significant As immobilization performance of BFMO minerals,the transformation of Fe（II）,Mn（II）,and As（III）mediated by oxygen perturbation and strain RM6 at different Fe（II）/Mn（II）molar ratios（1:1,2:1,and 3:1）were investigated through microcosm simulation experiments.The simultaneous immobilization and coupling mechanism of Fe,Mn,and As were revealed during the in-situ formation of BFMO driven by oxygen perturbation and strain RM6 in As-rich groundwater coexisting with Fe（II）,Mn（II）,and As（III）.During the in-situ formation of BFMO driven by oxygen perturbation and strain RM6,the bio-oxidation proportions of Mn（II）increased by 1.33,1.67,and 1.44 times at initial Fe（II）concentration of 0.3,0.6,and 0.9 m M,respectively,compared with the system without Fe（II）.The coexistence of Fe（II）significantly promoted the bio-oxidation of Mn（II）and the formation of mineral precipitation,enhancing the in-situ immobilization of As in groundwater.The As（III）immobilization efficiency of the coexisting system of Fe（II）and Mn（II）（73%,91%,92%）was significantly higher than that of corresponding single Fe（II）（31%,59%,74%）and Mn（II）system（12%）at initial Fe（II）concentration of0.3,0.6,and 0.9 m M,indicating that the oxidation of Fe（II）and Mn（II）synergistically promoted the immobilization of As（III）.The BFMO formed in situ significantly promoted the simultaneous immobilization of Fe/Mn/As in poor-quality groundwater through biological,physical,and chemical coupling mechanisms such as oxidation,adsorption,and precipitation/co-precipitation.The fixation mechanism of Fe（II）was chemical oxidation and precipitation,the fixation mechanism of Mn（II）was bio-oxidation,adsorption,and precipitation,and the fixation mechanism of As（III）was the coupling of oxidation and adsorption/co-precipitation.（3）Metal-reducing bacteria（Bacillus sp.FMR）were newly isolated from As-rich aquifer sediments.Based on the microcosm simulation experiments,the transformation and immobilization of As（III/V）by Fe–Mn oxide minerals were studied in As-rich groundwater with strain FMR in anaerobic environment,and the influencing mechanism of strain FMR on in-situ immobilization of As by Fe–Mn oxide minerals was revealed.Strain FMR showed high tolerance to As（III/V）and high reduction capacity to Fe（III）and Mn（IV）,but no significant oxidation,reduction,and adsorption capacity to As（III/V）.In anaerobic environment,strain FMR induced significant reduction and release of Fe/Mn from single Fe and Mn oxide minerals,resulting in the adsorption–desorption conversion of As during As immobilization by minerals.Compared with the system without bacteria,the removal efficiency of As（T）was decreased by 22.9%and 23.2%in the presence of strain FMR in the fixation of As（III）and As（V）by Fe oxides,respectively,and 52.9%and52.6%in the fixation of As（III）and As（V）by Mn oxides.However,for the Fe–Mn oxide minerals,the Mn oxide component significantly inhibited the bio-reduction and dissolution of the Fe oxide component,preventing the desorption of As on the mineral,resulting in the synergistic promotion of As（III/V）fixation by Fe and Mn oxide components.However,the Fe and Mn oxide components only slightly promoted the fixation of As（III/V）through their interaction in the absence of strain FMR.In the fixation of As（III）and As（V）by Fe–Mn minerals,the removal efficiency of As（T）with strain FMR increased by 17%and 16%,respectively,compared with the system without bacteria,which was attributed to the increase of the specific surface area and reactive active sites of the minerals induced by strain FMR.（4）As（V）-bearing Fe–Mn oxide minerals with different Mn/Fe molar ratios（0:1,1:5,1:3,1:1）were prepared.Based on As（V）-reducing bacteria（Shewanella putrefaciens strain CN32）isolated from anaerobic aquifer cores,the biotransformation and release characteristics of As/Fe/Mn in As-containing Fe–Mn minerals in the presence of strain CN32 were revealed by microcosm simulation experiments,modern characterization techniques,linear models,multiple linear stepwise regression analysis,and structural equation models,and the influencing mechanism of Mn oxide components on As bio-transformation and release from As（V）-containing Fe–Mn minerals was investigated.Strain CN32 induced significant bio-reduction of As（V）/Fe（III）/Mn（IV）in each As（V）-containing Fe–Mn mineral system.However,the increase of the molar proportion of Mn oxide component significantly inhibited the formation and release of Fe（II）/As（III）,which was attributed to the combined effects of the bio-reduction priority of the Mn oxide component and its reoxidation of the generated Fe（II）/As（III）,as well as the adsorption of As（III）by the newly formed Fe oxide precipitate.Compared with As（V）-containing pure Fe oxides with Mn/Fe molar ratio of 0:1,the bio-reduction of As in As（V）-containing Fe–Mn minerals with Mn/Fe molar ratio of 1:5,1:3,and 1:1 was decreased by 28%,34%,and 47%,respectively.The structural equation model showed that Mn（II）release,Fe reduction,Fe（II）release,and As reduction mediated by strain CN32 had significant positive direct effects on As（T）release,and As reduction was a key control factor for bio-release of As.The reduction and dissolution of Fe（III）/Mn（IV）oxides and the bio-reduction of As（V）were the main mechanisms for the bio-release of As from As（V）-containing Fe–Mn oxide minerals induced by strain CN32.Compared with As（V）-containing pure Fe oxides with Mn/Fe molar ratio of 0:1,the bio-release of As was reduced by 24%,41%,and 59%in As（V）-containing Fe–Mn minerals with Mn/Fe molar ratios of 1:5,1:3,and 1:1,respectively.This is attributed to the combined effect of the inhibition of As/Fe reduction by Mn（IV）oxide component and the fixation of As（III/V）by the newly formed Fe oxide precipitate.The characteristics and innovations of this thesis are as follows:（1）The internal mechanism of As immobilization by Fe–Mn oxide minerals mediated by microorganisms was revealed,which provided new insights into the simultaneous immobilization of pollutants in poor-quality groundwater coexisting with Fe（II）,Mn（II）,and As（III）,and also provided a potential strategy for green in-situ remediation of As-rich groundwater.（2）The inhibition mechanism of the Mn component on the release of As from As（V）-containing Fe–Mn oxide minerals was clarified,which provided new insights for the reduction of natural arsenic ecological risk and the green in-situ remediation of primary high As groundwater.