Low-pH Fe(?) Oxidation And Microbial Diversity In Acid Mine Drainage Environments

For acidic,Fe(II)-rich sources of coal mine drainage,microbial low-p H Fe(II)oxidation can be an effective component of an acid mine drainage(AMD)treatment system.However,the use of microbial low-p H Fe(II)oxidation for AMD treatment is limited because of uncertainties associated with its Fe(II)rate and solubility of Fe(III)minerals.?Based on previous field research of geochemical gradient across terrace iron formations(TIFs),we selected two sites in the Appalachian Bituminous Coal Basin that displayed the fastest(Scalp Level)and regional-average(Brubaker Run)rates of Fe(II)oxidation.We enriched Fe(II)-oxidizing microbes from both sites and then used chemostatic bioreactors to measure Fe(II)oxidation and total Fe removal kinetics as a function of hydro-geochemical conditions.We also measured the microbial community compositions in both field and these bioreactors as determined by high-throughput 16S r RNA gene sequencing.The objectives of this research were to develop generalized rate laws for biological low-p H Fe(II)oxidation,optimize the hydrogeochemical conditions for both Fe(II)oxidation and total Fe removal in active treatment bioreactors,developed a thermodynamic-based framework to describe the kinetics of microbial low-p H Fe(II)oxidation and developed geochemical niches model for the distribution of Fe(II)-oxidizing bacteria(Fe OB).Hydrogeochemical conditions control the low-p H Fe(II)oxidation.As the hydraulic residence time(HRT)was incrementally decreased,both rates of Fe(II)oxidation and total Fe removal decreased,however,the extent of Fe(II)oxidation increased.Rates of Fe(II)oxidation increased at the lower p H and the higher influent Fe(II)concentration.Rates of Fe(II)oxidation in these laboratory systems were slower compared to rates measured at the corresponding field sites,because of different hydrogeochemical conditions.Laboratory-based rates of Fe(II)oxidation were more similar for these reactors as compared to their corresponding field-based rates.These bioreactors effectively removed total Fe(up to 400 g/d/m²,GDM),at rates 7 to 20 times better than passive treatment settling ponds(5-20 GDM).The maximum removal of total Fe occurred between p H 2.90 to 3.30.Schwertmannite was the predominant Fe(III)mineral formed in these systems under varying geochemical controls.A dual-Monod rate formulation of three parameters(k^*_bio,K_H+,K_Fe(II))could effectively model Fe(II)oxidation kinetics in the bioreactor systems.These results point to the incorporation of low-p H Fe(II)oxidation into both active and passive AMD treatments.Thermodynamic calculations of the Gibbs free energy of Fe(II)oxidation(?G_oxidation)was used to confirm and explain how fastest Fe(II)oxidation rates occurred at the lowest p H values.The?G_oxidation was more negative at the environments with the lower p H values,thus the energy for the microbes enhanced.Hydrogeochemistry of sites and bioreactors control the thermodynamic favorability,which greatly influenced the Fe(II)oxidation kinetics.In addition to AMD treatments,this simplified model that describes the relationship between free energy and microbial kinetics should be broadly applicable to many biogeochemical systems with Fe-cycles.Bacterial phyla Betaproteobacteria,Gammaproteobacteria,Alphaproteobacteria,Nitrospirae and Actinobacteria were predominant in both field sediments and labotoray biofilm samples,accountting for more than 95%of the relative abundance of the total 16S r RNA gene sequences across all bioreactor samples.The distributions of these dominant phyla were greatly influenced by varying geochemical conditions.Alpha diversity decreased as p H decreased and as the Fe(II)concentration increased,coincident with conditions that attained the fastest rates of Fe(II)oxidation.The distribution of the seven most abundant bacterial genera could be explained by a combination of p H and Fe(II)concentration.Bacterial genera Acidithiobacillus,Ferrovum,Gallionella,Leptospirillum,Ferrimicrobium,Acidiphilium,and Acidocella were all found to be restricted within specific bounds of p H and Fe(II)concentration,and a simple geochemical niche model can predict their distributions.While these microbial communities were enriched from sites that displayed markedly different field rates of Fe(II)oxidation,rates of Fe(II)oxidation measured in laboratory bioreactors were essentially the same,indicating that the performance of suspended-growth bioreactors for AMD treatment may not be strongly dependent on the inoculum used for reactor startup.In addition to geochemical selection,temporal distance could also be a simple explanation for the differences between these evolving microbial assemblages.This research can help us to select the most adaptive and efficient Fe(II)-oxidizers for different influents in passive treatments and optimize the abundance of the high-efficient Fe OB under controlled geochemical niches to maximize removal efficiency in active treatments,and effectively predict the distribution of Fe(II)-oxidizing microbial communities.