The following abstracts summarize previous and current research projects: 

Preliminary research has been conducted using rotating annular bioreactors for studying the influence of microbial biofilms on pyrite oxidation.  Pyrite coated coupons have been developed and installed in the systems and active microbial cultures have been established.  The online monitoring capabilities that are incorporated into the systems include pH, ORP, and dissolved oxygen and allow for continuous monitoring of changes in these parameters under varying experimental conditions.  One of the systems is being used to identify the influence of iron-oxidizing microorganisms growing as a biofilm on mineral oxidation whereas the other system is being used to identify the role of other classes of microbes on remediation or prevention of mineral oxidation.  In addition to this experimental design, our research group is also utilizing continuous flow, recirculating pyrite columns and batch flask reactors to develop data sets using other system designs to identify the influence of exogenous carbon addition on pyrite oxidation.  By utilizing these different experimental setups, we are able to identify pyrite oxidation rates at a variety of water:pyrite ratios.  We are currently in the process of evaluating several mining sites in Nevada for potential field-scale research focused on establishing the role of microbial community dynamics in organic carbon-amended sites.  It is anticipated that site location and characterization will occur during the fall term in 2005 and field work may begin during the spring of 2006. 

Another aspect of this research is focused on studying the influence of iron- and sulfur-oxidizing microorganisms on the release and speciation of arsenic from arsenic-containing minerals that may be present in the natural environment or waste tailing piles.  This aspect of the project is focusing on the rate of release and the speciation of arsenic in the pore water of mineral-microbe batch flask slurries.  These experiments have involved studying the influence of iron-oxidizing microbes on arsenic release from mixtures of minerals consisting of pyrite, arsenopyrite, and orpiment and identifying the role of heterotrophs on release and speciation of arsenic.  To date, results from the research suggest that the addition of active, heterotrophic microorganisms results in significantly lower amounts of soluble arsenic release (on the order of 5-10 times lower dissolved concentrations).  Furthermore, it has been observed that systems containing both heterotrophs and iron-oxidizing bacteria exhibited a shift in the arsenic speciation from equivalent levels of As(III) and As(V) (in iron-oxidizing-only inoculated systems) to much lower As(III):As(V) values.  Since As(III) has been reported to have a higher mobility and toxicity than As(V) in aqueous systems, this result may have important consequences for mining areas with enriched arsenic-bearing minerals.

The presence of active heterotrophic bacteria in AMD environments has been shown to have multiple effects on the acid generation process.  A summary of the important findings for this research project are included below:

(1) One significant effect is associated with the type of organic carbon substrate included to stimulate heterotrophic growth.  Yeast extract alone was found to have a marked influence on pyrite oxidation by A. ferrooxidans at concentrations as low as 300 mg/L.  Lower concentrations of yeast extract resulted in a lag before pyrite oxidation was observed (about 50 days compared to 5 days in the absence of yeast extract).  Furthermore, the pyrite oxidation rate, as measured by accumulation of sulfate in the media, was significantly lower in the presence of yeast extract.  The rate of sulfate accumulation was 125 ± 17 mg/L-day without yeast extract and only 2.2 ± 0.9 mg/L-day in the presence of 300 mg/L yeast extract during the lag period.  Following the lag period in systems containing pyrite, iron-oxidizing bacteria, and yeast extract, pyrite oxidation occurred at a rate similar to the systems without yeast extract.  This result may indicate that the yeast extract temporarily inhibited growth of A. ferrooxidans; however, after a sufficient acclimation period, growth on pyrite could occur.  Thus, yeast extract alone resulted in a temporary cessation of pyrite oxidation.

(2) When active heterotrophic cultures were added in addition to the yeast extract, a different phenomenon was observed.  Pyrite oxidation did not occur at an appreciable rate in systems with the active heterotroph (sulfate accumulation was about 30 times lower than a system without the heterotrophic culture).  This phenomenon was observed in a variety of system configurations including batch reactors, continuous flow pyrite columns, and annular biofilm reactors. 

(3) In other experimental systems containing a combination of pyrite and arsenic-bearing minerals (orpiment and arsenopyrite), the growth of heterotrophic bacteria was found to have a profound influence on the release and speciation of arsenic.  Compared to similar systems with the minerals and iron-oxidizing bacteria, the heterotroph-containing systems had 5-10 times lower concentrations of dissolved arsenic after more than 3 months of incubation.  Further, while mineral systems lacking the heterotroph had approximately equal concentrations of As(III) and As(V), the heterotroph-amended samples had close to 90% of the dissolved arsenic present in the +5 valence state.  This is significant on many fronts: (a) transport of As(V) in the environment is usually more limited than As(III) due to increased sorption to mineral solids, and (b) As(III) is considered to exert a more toxic influence on organisms in the environment than As(V).  Currently, efforts are being focused on identifying the specific factors associated with heterotrophic bacterial growth that influence arsenic release and speciation.

Investigators:  Pat Plumb, Chris Ore, David Levine, Derek Ridenoure, and Eric Marchand  

Recent research has focused on the use of ion exchange or membrane treatment methods for removing perchlorate  from drinking water.  While these processes have been shown to be effective in removing perchlorate, they result in concentrate or brine solutions that contain elevated concentrations of perchlorate and other salts.  The combination of these technologies with biological perchlorate reduction is a novel concept that has not been studied and can have the potential of treating drinking water to low perchlorate levels while minimizing the production of concentrated perchlorate solutions.  Many different types of microorganisms from a variety of environments have been identified that are capable of biologically reducing perchlorate; however, a major limitation has been the difficulty in acclimating cultures to treat high concentrations of perchlorate in high-salinity wastewaters.  This severely limits the application of biological treatment technologies for removing perchlorate from concentrated wastes.  

An investigation into salt toxicity has been conducted using freshwater cultures that have been acclimated to hyper saline conditions (up to 8% w/v salt) and high concentrations of perchlorate (up to 1,000 mg/L).  Preliminary results have indicated that while the kinetics of perchlorate degradation decrease markedly with increasing salinity, microorganisms isolated from a sewage treatment plant can still degrade perchlorate at favorable rates.

Results have indicated that the highest rates of perchlorate degradation occurred at the 0% salinity condition with 0-order specific perchlorate degradation rates of approximately 9.7 mg perchlorate/g MLVSS-hr.  At a salinity of 3%, the specific perchlorate removal rate was observed to decrease to 5.9 mg perchlorate/g MLVSS-hr.

Results from this work suggest that the perchlorate-reducing microbes that are dominant in low salinity environments, in particular strict anaerobes, may not be well suited for treating high-strength wastewaters due to their inhibition by high salt concentrations.  Further, this research shows that microorganisms can be acclimated to high-salinity wastewaters and effective at reducing perchlorate.  Application of these findings in water treatment practice where ion exchange or membrane technologies are used to remove perchlorate has the potential to increase the overall attractiveness of these processes by eliminating the need to dispose of a concentrated perchlorate solution.

Investigators:  Chanjae Park and Eric Marchand

Wastewater treatment systems for space missions are often constrained by several factors including a limited supply of electron-donating compounds required to sustain biological growth and limitations regarding gas transfer of required gasses.  The goal of this research project is to identify whether these constraints can be overcome by operating a hydrogen gas bioreactor equipped with bubble-less membrane modules to efficiently transfer the gas to the liquid wastewater stream.  

The bioreactor system has been designed as a two-phase process – the first stage is an anaerobic reactor where hydrogen gas serves as a supplemental electron donor with the second reactor serving as an aerobic reactor, where the biomass oxidize nitrogen-containing compounds.  Both reactors are fixed film systems where the biomass grows attached to support media with the liquid solution flowing through the media.  The coupled bioreactor system is being  operated with a well-defined simulated space mission wastewater and both contaminant removal and biomass growth are being monitored during the experiments.  

To establish the influence of adding hydrogen gas to a bioprocess treating simulated space mission wastewater, the performance of the 2-phase hydrogen gas bioreactor system will be compared to a process where the hydrogen gas source is not provided.  Since the microbial consortia contained in the bioreactors contains a heterogeneous mixture of organisms, the project team will also study how the biomass composition changes between a hydrogen-fed anaerobic bioreactor and an anaerobic reactor without the hydrogen gas amendment.  To address how the system handles variable wastewater feed conditions, the feed components will be varied in strength and contaminant degradation will be assessed.  It is anticipated that this research will identify the optimum operating conditions for a two-phase hydrogen gas bioreactor system and demonstrate its value for efficiently treating space mission wastewater streams.

Investigators:  Rob Dyer and Eric Marchand

This research project is investigating the interaction between microorganisms and membrane surfaces using state-of-the-art atomic force microscope (AFM) techniques to measure the interaction forces between microorganisms, their extracellular products, and membrane surfaces.  Although interaction forces have been studied between microbes and mineral surfaces and between microbes and other surfaces, there have been no previous AFM investigations of microbe-membrane interactions.

Investigators:  Nichole Whisman, Jesse Adams, Amy Childress, and Eric Marchand