Kim, Inyoung2019-05-282019-05-282018http://udspace.udel.edu/handle/19716/24199Microbial denitrification is an environmentally beneficial process that removes nitrate to innocuous nitrogen gas (N2) from water environment. In this study, we examined the feasibility of zero-valent iron (ZVI) as the sustained source of electron donors to support autotrophic denitrification. It was hypothesized that ZVI granules can serve as support media for the enrichment of autotrophic denitrifying population by (1) continuously supplying the electron donors via anaerobic corrosion of ZVI (e.g., cathodic hydrogen (H2) gas and ferrous (Fe2+) ion), and (2) providing a nutrientand substrate-rich solid-liquid interface for bacterial colonization. ☐ The objectives of this study were 1) to investigate the synergetic effect of denitrifying mixed cultures on nitrate reduction by ZVI, 2) to evaluate the nitrate reduction by ZVI system at low temperatures, and 3) to investigate the potential occurrence of anammox-like process in microbial-ZVI systems. ☐ A series of batch denitrification tests were conducted with ZVI granules or its corrosion products (H2 and Fe2+) as the source of electron donor to elucidate the effects of these secondary electron donors on the rate and extent of nitrate reduction in the ZVI-supported mixed cultures. The ZVI-supported mixed cultures completely removed 40 mg/L of nitrate in batch reactors in 24 hours. Slower removal rates were observed in H2-cell and Fe+2-cell reactors as the complete removal occurred in 3 and 4 days, respectively. Repeated spiking of nitrate to batch reactors containing ZVI granules and microorganisms showed that complete nitrate reduction by the ZVIsupported cultures was sustained over a long period. In order to understand the major microbial reaction in the ZVI- supported cultures, bacterial 16S ribosomal RNA (16S rRNA) gene sequencing and analysis were performed for denitrifying cultures. Analysis of microbial distribution patterns and subsequent principal component analysis (PCA) showed clear distinctions not only between ZVI-supported denitrifying culture and seed bacteria, but also among denitrifying cultures receiving different electron donors. On the other hand, the microbial composition of H2-fed cultures was similar to that of ZVI-supported cultures, suggesting that even though the hydrogen sources are different between two cultures, the same populations of bacteria may be involved in denitrification process. ☐ The effects of temperatures on abiotic and biotic nitrate reduction by zerovalent iron were examined at temperatures below 25 ℃. Under anoxic conditions, NO3- reduction rates in both ZVI-only and ZVI-cell reactors declined as temperature decreased. In ZVI-only reactor, 61.3% and 17.3% of initial nitrate concentration were reduced in 6 days at 25 and 3.5 ℃, respectively. The reduced nitrate was completely recovered as ammonium ions (NH4+) at both temperatures. Nitrate in the ZVI-cell reactors was completely removed within 1 – 2 days at 25 and 10 ℃, and 67 % of reduction was achieved at 3.5 ℃. Less than 25% of the reduced nitrate was recovered as NH4+ in all ZVI-cell reactor. Soluble iron concentrations (Fe2+ and Fe3+) in the ZVI reactors were also measured as the indicators of anaerobic corrosion. In ZVI-cell reactors, the detected soluble iron concentrations were 1.7 times higher than that in ZVI-only reactors at 25 ℃, suggesting that the enhanced nitrate reduction in the ZVIcell reactors may be partly due to increased redox activity (i.e., corrosion) on iron surfaces. Anaerobic corrosion of ZVI was also temperature-dependent as substantially lower concentrations of corrosion products were detected at lower incubation temperatures; however, microbially induced corrosion (MIC) of ZVI was much less impacted at lower temperatures than abiotic ZVI corrosion. This study demonstrated that ZVI-supported microbial process (i.e., denitrification) is not only more sustainable at lower temperatures, but it becomes more dominant reaction for nitrate removal in microbial-ZVI systems at low temperatures. ☐ Electron donor (ammonia) and acceptor (nitrite) for the anammox reaction are typically present in microbial-ZVI systems. In this study, it was hypothesized that ZVI granules could serve as support media for the enrichment of anammox biofilm. The feasibility of ZVI-supported anammox process was demonstrated in a preliminary batch study. An anaerobic fluidized bioreactor (AFBR), using ZVI granules as the solid support materials for the biomass was operated for 50 days to study the potential colonization and stable biofilm formation of ZVI surfaces by anammox-performing cultures. During the 50-day experimental period, nitrate was completely removed with minimal production of ammonium ions. The confocal images showed that microbial colonies were formed not only at the ZVI surface, but also within the crevices of ZVI particles. Scanning electron micrograph of the ZVI granule from the AFBR shows that the ZVI surface was completely covered by the deposition of microorganisms and minerals. Genomic analysis of biofilm on the ZVI surface was conducted to examine the diversity and abundance of microbial communities in the ZVI-supported biofilm. Even though the relative abundance of Planctomycetes in the ZVI culture was small, the detection of Planctomycetes combined with the chemical data supporting anammox-like process suggested that low ammonia accumulation in the AFBR may be due to anammox activity.Thesis1102593363https://doi.org/10.58088/775t-dz662019-02-14en