Browsing by Subject "Nitrate reduction"

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Catalytic nitrate reduction in drinking water using a trickle bed reactor
(2016-05) Bertoch, Madison; Werth, Charles J.; Lawler, Desmond
Palladium-based bimetallic catalysts hold promise as an alternative water treatment technology for nitrate (NO3-), but practical application requires development of a flow-through reactor that efficiently delivers hydrogen (H2) from the gas phase into water, where it serves as the electron donor for NO3- reduction. In this work, a trickle bed reactor (TBR) was fabricated and evaluated to address this challenge. A series of batch experiments with Pd-In/γ-Al2O3 catalysts were conducted in excess H2 to identify a highly active catalyst for the TBR. A 0.1wt%Pd-0.01wt%In on 1 mm γ-Al2O3 catalyst was selected due to its high activity and support size that promotes a uniform liquid distribution in a packed bed. The TBR was packed with the same catalyst, and various liquid and gas flow rates were tested to evaluate apparent catalyst activity. Influent and effluent NO3- concentrations were used to calculate apparent zero-order rate constants, and they generally increased with H2 flow rate. Above 900 mL/min, a change in flow regime from pulse to bubble flow was observed, and the calculated zero-order rate constants decreased. An optimal catalyst activity in the TBR of 19.5 mg NO3-/min∙g Pd was obtained at a liquid flow rate of 900 mL/min and H2 flow rate of 320 sccm, which is ~22% of the activity obtained in the batch reactor by the same catalyst, indicating H2 mass transfer limitations. A reactive transport model was developed and used to quantify H2 mass transfer rate coefficients from the liquid to gas phase. Mass transfer coefficients initially decrease and then stabilize as the H2 flow rate increases. At elevated H2 flow rates, the highest mass transfer coefficients were obtained at the 900 mL/min liquid flow rate, in agreement with activity trends. Evaluation of a larger range of liquid and gas flow rates is warranted to determine if H2 mass transfer in the TBR can be further enhanced.
Phenotypic traits that enhance microbial habitability of antibiotic gradients in a porous network under nitrate reducing conditions
(2021-04-06) Alcalde, Reinaldo Enrique; Werth, Charles J.; Keitz, Benjamin K; Kirisits, Mary J; Kumar, Manish
Antibiotic contamination of terrestrial and aquatic environments can promote the selection of antibiotic-resistant bacteria that threaten the efficacy of antibiotic treatment and can impact the function of nontarget native bacteria that modulate biogeochemical processes, such as nitrogen turnover. The latter is an important ecological function for the maintenance of soil and water quality. Antibiotics that enter the environment occur in spatial concentration gradients due to solute transport phenomena. However, the environmental side effects of antibiotic compounds have mostly been interpreted through laboratory models where this spatial dimension is not considered. These observations motivated us to develop a microfluidic reactor that mimics this diffusive aspect of nature to probe the microbial response to antibiotic concentration gradients under nitrate-reducing conditions. In Chapter 2, we present the microfluidic gradient chamber (MGC), a reactor that generates diffusive gradients of solutes across an interconnected porous network. We find that swimming motility and migration of Shewanella oneidensis MR-1 cells allow for habitability and metabolic activity in highly toxic regions of a ciprofloxacin gradient. Moreover, our results show that S. oneidensis MR-1 remains metabolically active for five days without observed inheritable antibiotic resistance. Chapter 3 begins to explore the underlying mechanisms that allow for such adaptive survival. We find that S. oneidensis MR-1 requires a chemotactic gene (cheA) for this habitability to occur. We then explore the role of transient adaptive resistance via resistance-nodulation-division (RND) efflux pumps; ancient elements of bacterial physiology and virulence. Contrary to expectations, we show that S. oneidensis MR-1 does not require RND efflux pumps for habitability. Lastly, in Chapter 4, we explore the role of antibiotic biodegradation on habitability in the MGC. We find that the extracellular electron transfer pathway, Mtr, enhances the degradation rate of the antibiotic sulfamethoxazole. We then provide evidence that suggests that antibiotic biodegradation is not a determinant factor for habitability in the MGC. Our work contributes to an emerging body of knowledge deciphering the effects of antibiotic spatial heterogeneity on microorganisms and highlights differences of microbial response in these systems versus well-mixed batch conditions.