| Iron?Fe?is the fourth most abundant element in the earth's crust.Fe occurs in two main redox states in the environment:ferric Fe?Fe????and ferrous Fe?Fe????.The redox processes of Fe are linked to the cycling of elements including oxygen,carbon,nitrogen,and sulfur,and influence the fate and transport of environmental pollutants in natural systems.At circumneutral conditions,because Fe???is easily oxidized to Fe???by oxygen?O2?under oxic conditions,Fe???is the main Fe species.Fe is usually in the form of Fe???due to microbial reduction of Fe???under anoxic condtions.Although the subsurface environment is commonly under anoxic or hypoxic condtions,the redox conditions is disturbed by O2 during the natural processes?such as the seasonal alternation of wetting and drying,and the interaction of groundwater and surface water?and anthropic activities?such as artificial recharge of groundwater,and remediation of contaminated groundwater by aeration?.Consequently,the redox condition alternately transforms between oxic and anoxic condition,and iron cycles between Fe???and Fe???forms under subsurface environment.There are plentiful researches about the individual processes of Fe???oxidation and Fe???bio-reduction,however it is little knowledge about the effect of Fe???oxidation on the subsequent Fe???bio-reduction.According to Haber-Weiss mechanisms,Fe???oxidation by O2 can produce reactive oxidants at circumneutral conditions.The reactive oxidants produced from Fe???chemical oxidation have generally bactericidal activity,which can kill microorganisms including Escherichia coli,Peseudomonas fluorescens,MS2 coliphage,and so on.However,it is not clear whether the reactive oxidants produced from Fe???chemical oxidation can kill Fe???reducing Fe???-reducing microorganisms,because different types of microorganisms have different resistance to the active oxidants,and the reductive substances?such as Fe???and organic matter?that widely existed in the underground environment can consume the reactive oxidants.Hypothetically,Fe???reducing-reducing microorganisms can be killed by the reactive oxidants produced from Fe???chemical oxidation,then the reactive oxidants produced from Fe???chemical oxidation will kill some Fe???reduction microorganisms when O2 is introduced into subsurface environments.The microbial reduction of Fe???will be inhibited,because only the surviving Fe???-reducing microorganisms can reduce Fe???to Fe???when O2 is depleted.Because different types of Fe???-reducing microorganisms have different resistance to the active oxidants and fecundity,and the widespread reducing substances?such as Fe???and organic matters?can consume reactive oxidants,the anoxic-oxic cycle will facilitate the enrichment of Fe???-reducing microorganisms with strong reproductive capacity and strong resistance to the active oxidants.Consequently,the community structure of Fe???-reducing microorganisms will be changed.To verify the above hypothesis,the combination of static simulation and dynamic column was applied to explore the influencing mechanism of Fe???chemical oxidation on the subsequent Fe???reduction when the redox conditions altered alternately.The content and results of this paper were divided into the following four aspects.?1?Effect and mechansims of Fe???chemical oxidation on the survival of Fe???-reducing microorganismsTo unravel the impact of Fe???oxidation by O2 on the survival of Fe???reducing bacteria,mixed solution of Fe????0.1?0.5 mM?and Shewanella oneidensis strain MR-1?MR-1,2.0×107 CFU/mL?at pH 6.6?7.1 were first exposed to laboratory air for Fe???oxidation and bacterial inactivation.The MR-1 was used as a representative of Fe???-reducing microorganisms.The results showed that Fe???at 0.1 mM was oxidized 85.9±1.1%and at 0.2 and 0.5 mM was oxidized completely within 60 min.Meanwhile,the coexisting MR-1 was inactivated by 0.84?1.71 orders of magnitude?log?within 60 min.The images of transmission electron microscopy showed that the cell membrane was disrupted and intracellular materials leaked out.The resultant Fe precipitates were attached to the damaged membrane.To explore the bactericidal mechanisms of Fe???chemical oxidation on Fe???-reducing microorganiss,the quenching experiment and the detection of intracellular active oxidants were carried out.The hydroxyl radical and tetravalent iron that was produced in solution and hydroxyl radical that was produced intracellularly may countribute to the death of MR-1.?2?Bactericidal activity on Fe???-reducing bacteria during table fluctuation of Fe???-containing groundwaterBeing based on the static experiment above,the effect of Fe???oxidation on the survival of co-existing MR-1 in the system during water level fluctuation were investigated in dynamic column experiments.Deoxygenated solution containing 20mg/L Fe???and 2×107 CFU/mL MR-1 at pH 6.5 was pumped into the sand column.Subsequently,water table fluctuations were manipulated.Results showed that Fe???was oxidized by O2 that was entrapped by the deoxygenated solution,meanwhile the number of viable MR-1 cells decreased by 1.4?2.42 orders of magnitude.Multiple cycle fluctuations can further enhance the decrease in viable MR-1 cells on sandy column profile.The reactive oxidants that were produced from Fe???oxidation may account for the inactivation of MR-1.?3?Impact mechanism of chemical oxidation of Fe???on subsequent Fe???reductionTo explore the impact of Fe???oxidation by O2 in the presence of iron-reducing bacteria on subsequent Fe???bio-reduction,Fe????0.1?0.5 mM?in the presence of 2.0×107 CFU/mL MR-1 was aerated at pH 6.6?7.1.When Fe???was fully oxidized,nitrogen was poured into the system to remove O2 and lactate was added.The resultant Fe???was bio-reduced by the surviving MR-1.For comparison,the Fe???that was produced from Fe????0.1?0.5 mM?oxidation in the absence of MR-1 was bio-reduced by fresh MR-1 at 2.0×107 CFU/mL under anoxic conditions.Bio-reduction of the resultant Fe???by the surviving MR-1 was 1.8?2.5 times faster than that of the Fe???that produced from Fe???oxidation without MR-1 by fresh MR-1 cells at 2.0×107CFU/mL,which indicated that Fe???oxidation in the presence of Fe???-reducing microorganism could promote subsequent Fe???reduction.To explore the mechanism of Fe???chemical oxidation on subsequent Fe???reduction,the composition of Fe???,the effect of Fe???oxidation on the release of electron shuttle,and the reactivity of the dead Fe???-reducing bacteria were analyzed.M?ssbauer spectra reveal that lepidocrocite was the sole Fe???mineral in pure Fe???,while 19%ferrihydrite occurred in Fe???-MR-1.The formation of low-crystallinity ferrihydrite accounts for the increase in bio-availability of the Fe???minerals.On one hand,subsequent bio-reduction is inhibited due to bacteria attenuation by Fe???oxidation.On the other hand,subsequent bio-reduction is enhanced by the increased Fe???bio-availability,increased release of electron shuttle,and the residual reactivity of dead bacteria due to Fe???oxidation.As the enhancement outcompeted the inhibition,an apparent acceleration in Fe???-MR-1 bio-reduction was observed.?4?Response of the cycling of Fe???/Fe???and Fe???-reducing functional groups in sediment during anoxic-oxic alternationTo explore the cycling of Fe???/Fe???and the variation of community structure of Fe???reducing bacteria during the redox fluctuation in sediment.Basing on the simple system above,the sediment was explored in the process of the redox fluctuation by two turns of alternating anoxic-oxic treatment.The content and composition of Fe???and Fe???,the content of organic carbon,the abundance and community structure of Fe???reducing bacteria were measured during the experiment.During two anoxic-oxic cycle,the concentrations of Fe???in dissolved state,adsorbed state and easily reducible iron oxides rapidly decreased,while Fe???in difficultly reducible iron oxides and clay slightly varied under oxic conditions.During the first anoxic-oxic cycle,Fe???production was mainly attributed from the reduction of Fe???in easily and difficultly reducible iron oxides under anoxic conditions.During the second anoxic-oxic cycle,Fe???production was mainly attributed from the reduction of Fe???in easily reducible iron oxides and clay under anoxic conditions.Results of chemical analysis and characterization showed that Fe???content on sediment surface was higher than total Fe???under anoxic conditions,and the result was similar for Fe???under oxic conditions,indicating that Fe???oxidation and Fe???reduction took place preferentially on the surface of mineral.During first anoxic-oxic cycle,the abundance of Fe???reducing bacteria decreased from 2.28×108 to 9.34×106 copies/g after oxic treatment,while the subsequnt Fe???bio-reduction was enhanced under anoxic stage.The rate constant of Fe???production increased from 100.09 to 128.58 mg/?kg.d?.The decrease in the abundance of Fe???reducing bacteria indicated that sediments containing Fe???exposure to oxygen can lead to the inactivation of Fe???reducing bacteria.The increase in the rate of Fe???reduction may be attributed to the enhancement by the decrease in the crystallinity of iron-bearing mineral,which can outcompete the inhibition by the decrease in abundance of Fe???reducing bacteria.During the second anoxic-oxic cycle,the abundance of reducing bacteria decreased from 8.83×107 to 1.57×107 copies/g after oxic treatment,then the rate constant of Fe???production decreased from 128.58 to 49.05 mg/?kg.d?under anoxic stage.The decrease in the rate of Fe???reduction may be ascribed to the collective inhibition of the increased crystallinity of iron-bearing mineral and the decreased abundance of Fe???reducing bacteria on the subsequent Fe???reduction.The innovative pionts of this study is to reveal the interaction and mechanisms between Fe???chemical oxidation and microbial reduction of Fe???during the fluctuation of redox conditions.It can supplemente the basic understanding of the relationship between Fe???/Fe???cycle and the community evolution of Fe???-reduing microorganisms at the oxic-anoxic interface.The results provide a new knowledge for the biochemical cycle and transformation of iron when the redox conditions fluctuate. |
PDF Full Text Request