Effect of hydraulic retention time, ZVI concentration, and Fe2+ concentration on autotrophic denitrification efficiency with iron cycle bacterium strain CC76

The immobilized reactor of iron-reducing bacteria and zero-valent iron (ZVI) integrated system was established. This study has shown that the effects of hydraulic retention times (9, 11, 13 h), ZVI concentrations (2, 4, 6, 8 mg/L), and Fe²⁺ concentrations (5, 10, 15 mg/L) on the denitrification characteristics of iron cycle bacterium strain CC76. The results show that the longer the HRT is, the stronger ability of bacteria to remove nitrate. When ZVI concentration was 4 mg/L and the Fe²⁺ concentration is 15 mg/L, the removal efficiency of nitrate was the highest, reaching the maximum value of 93.02% (1.07 mg/L/h). Since increasing ZVI concentration in a certain range can not only promote chemical reduction but also make use of strain CC76 as an electron donor. Also, the abundance of strain CC76 decreased with the increase of ZVI concentration, which indicated that adding a low concentration of ZVI could reduce the inhibitory effect on bacteria. Hypothesis analysis of principal components showed that a low concentration of ZVI is beneficial to increase nitrate removal rate. Community structure analysis indicated that strain CC76 and related bacteria were the most abundant bacteria in the reactor.

Application of a novel Polydopamine@EDTA@Fe3O4 material for efficient simultaneous nitrogen and nickel removal in an immobilized biofilm reactor

High NO3− and Ni(II) concentrations in mining wastewater pose a risk to public health. In this study, the NO3− and Ni(II) removal process was performed using a novel PDA@EDTA@Fe3O4 immobilization carrier. The effects of hydraulic retention time (HRT; 6, 8, and 10 h), along with Fe(II) (10, 15, and 20 mg/L) and Ni(II) (10, 20, and 30 mg/L) influent concentrations on the simultaneous removal of NO3− and Ni(II) were investigated in immobilized biofilm reactors. Results showed that the highest NO3− removal efficiency (97.78%) and Ni(II) removal efficiency (91.21%) were obtained in the immobilized biofilm reactor with PDA@EDTA@Fe3O4 under the conditions of 10 h HRT, influent Fe(II) concentrations of 20 mg/L and Ni(II) concentrations of 10 mg/L. High-throughput sequencing results confirmed that Cupriavidus sp.CC1 plays a major role in the functioning of the immobilized reactor. This process provides the potential for effective treatment of NO3− and Ni(II) polluted water.

Effects of Low Frequency-Low Voltage Alternating Electric Current on Apoptosis Progression in Bioelectrical Reactor Biofilm

Bioelectrochemical systems have undergone several modifications to promote the enzymes or pathways used to reduce the energy required for microbial metabolism. Changes in dominant bacteria, population, and growth rates occur when an electric current is applied intermittently. Applying electricity to bioelectrical reactor (BER) biofilms can either stimulate cells or lead to cell death; therefore, determining the applied voltage range that leads to viable and stimulated bacteria is crucial. We investigated the progression of apoptosis induced by a low frequency-low voltage alternating electric current (AC) in a BER biofilm and found that biofilms on carbon cloth (CC) and stainless steel (SS) 304 electrodes had pH_zpc values of 8.67. The pH_zpc of the biofilms increased by two compared to that of the inoculant bacteria mass. Furthermore, the Henderson–Hasselbalch equation reveals that the compositions of cell walls of the biofilms that formed on the CC and SS304 electrodes are very similar. In contrast, the CC and SS304 biofilms differ from the inoculant biomass without the influence of an AC field; this indicates that there are differences in the compositions of the cell walls in the present bacteria. Fourier transform infrared spectroscopy was used to compare spectra of the biofilms with that of the inoculation mass, and there were differences in shape and absorbance intensity, indicating variability in the composition, and quantity of each individual biofilm component. In addition, the dehydrogenase activity (DHA) content varied under different applied voltages; the highest DHA was obtained at 8 Vpp. A flow cytometry analysis showed a relatively low number of apoptotic cells (10.93 ± 5.19%) for the AC amplitudes studied. Thus, a low voltage-low frequency AC likely induces significant changes in bacterial metabolic activity but causes no significant change in their viability.

Performance of the dominant bacterial species and microbial community in autotrophic denitrification coupled with iron cycle in immobilized systems

We used three stable reactors to investigate the rates of nitrate removal coupled with iron cycle and the subsequent influence of the reaction on bacterial communities. The iron-reducing bacterium Klebsiella sp. FC61 was immobilized on the reactor columns of the experimental groups B (only Klebsiella) and C (Klebsiella+magnetite). With the fluctuation of Fe²⁺ to Fe3⁺ (iron cycle), the average nitrate removal efficiency increased from 73.22% to 93.91% and 86.92% to 97.84% in groups B and C, respectively, as the influent nitrate concentration decreased from 40 to 10mg/L. However, the average rate of nitrate removal showed the opposite trend (from 2.08mg/L/h to 0.67mg/L/h and 2.41mg/L/h to 0.69mg/L/h in groups B and C, respectively) as the influent nitrate concentration decreased. Analysis of microbial distribution and community structures indicated that the population of Klebsiella sp. increased in groups B (from 18.21% to 41.21%) and C (from 25.43% to 46.80%) and contributed to the effective removal of nitrate in the reactors.