Analysis of Tolerance to Heavy Metal Ions and Their Oxides on Endogenous Short-Range Denitrification Systems Domesticated by Different Valence Iron
ObjectiveThe primary objective of this study was to assess the tolerance of endogenous partial denitrification systems (EPD) to heavy metal ions and their oxides, with the aim of enhancing the system's impact resistance. The research tried to domesticate the EPD system using various valence sta...
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| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Editorial Department of Journal of Sichuan University (Engineering Science Edition)
2024-01-01
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| Series: | 工程科学与技术 |
| Subjects: | |
| Online Access: | http://jsuese.scu.edu.cn/thesisDetails#10.12454/j.jsuese.202400683 |
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| Summary: | ObjectiveThe primary objective of this study was to assess the tolerance of endogenous partial denitrification systems (EPD) to heavy metal ions and their oxides, with the aim of enhancing the system's impact resistance. The research tried to domesticate the EPD system using various valence states of iron, namely nanoscale zero-valent iron (nZVI), ferrous ions (Fe<sup>2+</sup>), and ferric ions (Fe<sup>3+</sup>), and evaluated its response to different heavy metal ions and their nano-oxides, including CuSO<sub>4</sub>, ZnSO<sub>4</sub>, nano-CuO, and nano-ZnO.MethodsThe study utilized a 10-liter sequencing batch reactor (SBR) to construct the EPD system, with four distinct setups: a control system (S1), and systems supplemented with nZVI (S2), Fe<sup>2+</sup> (S3), and Fe<sup>3+</sup> (S4). The systems were operated in an anaerobic(2h)/anoxic(1h)/settling(30min)/aerobic(30min)/ static(80min)mode, with an initial mixed liquor suspended solids of 3500±500 mg/L and mixed liquor volatile suspended solids of 2500±250 mg/L. The influent C/N ratio was maintained at 4.0±0.5. The systems were dosed with nZVI, Fe<sup>2+</sup>, and Fe<sup>3+</sup> at a concentration of 5 mg/L. Following a period of over 160 days of cultivation, the systems were deemed to have reached a state of stability. The tolerance of the domesticated EPD systems to various heavy metals and their oxides was assessed by introducing varying concentrations of CuSO<sub>4</sub> and nano-CuO(0, 1, 5, 10, 20 mg/L), ZnSO<sub>4</sub> and nano-ZnO (0, 5, 10, 20, 30 mg/L) . The performance of the EPD systems was evaluated based on the accumulation rate of NO<sub>2</sub><sup>-</sup>-N, the removal rates of NO<sub>3</sub><sup>-</sup>-N and PO<sub>4</sub><sup>3-</sup>-P, and the activity of key enzymes such as peroxidase (POD) and dehydrogenase (DH). The relative abundance of denitrification functional genes including narG, napA, nirK, nirS, nosZ and norB was also assessed. Finally, the impact resistance of different iron acclimated EPD systems to heavy metal ions and their oxides was compared and analyzed.Results and Discussions The introduction of nZVI, Fe<sup>2+</sup>, and Fe<sup>3+</sup> improved the EPD system's operation performance, with maximal NO<sub>2</sub><sup>-</sup>-N accumulation rate of 91.4±3.5% under Fe<sup>3+</sup>domestication condition. The chemical oxygen demand (COD) removal rate increased from 82.69% to 85.41% and the removal rate of PO<sub>4</sub><sup>3-</sup>-P increased from 54.54% to 63.64%. Results also showed that nZVI significantly increased the activity of POD with the maximal vaule being 7.87 U/mg grot, but the Fe<sup>3+</sup> restrained the the activity of POD with the minimum value being 3.08 U/mg grot. The introduction of nZVI, Fe<sup>2+</sup> and Fe<sup>3+ </sup>inhibited the activity of DH in EPD system. The maximum DH activity in the blank control group was 2.24 EU/gVSS, while the minimum DH activity in the EPD system with nZVI addition was 1.40 EU/gVSS. The significant enrichment of the narG gene by Fe<sup>2+</sup> and its suppression of the napA gene were observed, whereas the effects of Fe<sup>3+</sup> on these two genes were opposite to those of Fe<sup>2+</sup>. The abundance of the nirK gene increased in the presence of all iron species, while the abundance of the nirS, norB, and nosZ genes was suppressed. These findings indicate that the valence state of iron ions significantly affects the microbial nitrogen transformation capacity, thereby influencing the denitrification process and the accumulation of nitrogen forms.Proteobacteria, known for its dominant role in denitrification processes, was significantly enriched in systems supplemented with nZVI, Fe2+, and Fe3+, with its abundance increasing from 28.75% (S1) to 45.76%, 30.30%, and 32.24% respectively. This enrichment is crucial for the accumulation of NO2--N and suggests a shift in the microbial community that favors the denitrification process. Additionally, Bacteroidota, Actinobacteria, and Chlorobacteria, all implicated in denitrification, were detected in all EPD systems with varying abundances. The systems also showed an enrichment of Candidatus_Competibacter, a typical denitrifying glycogen-accumulating organism (DGAO), and Thauera, a denitrifying phosphorus-accumulating organism (DPAO), which are essential for NO2--N accumulation and PO43--P removal. The introduction of iron ions led to a significant enrichment of DGAOs and DPAOs, which is beneficial for improving the performance of the EPD system. The nZVI-amended EPD system demonstrated superior resistance to CuSO4, maintaining PO<sub>4</sub><sup>3-</sup>-P removal rates and NO<sub>2</sub><sup>-</sup>-N accumulation of 60.24% and 21.91 mg/L, with a notable increase in catalase activity and the secretion of extracellular polymeric substances (EPS) that potentially formed a protective layer around microbial cells, enhancing their resistance to copper stress. This aligns with findings that EPS can shield microbes from external stressors and improve survival rates . In contrast, EPD systems domesticated with Fe<sup>2+</sup> and Fe<sup>3+</sup> exhibited greater resilience to ZnSO4, with NO<sub>2</sub><sup>-</sup>-N accumulation of 22.28 mg/L and 26.83 mg/L, and PO<sub>4</sub><sup>3-</sup>-P removal rates of 84.03% and 90.60%,possibly due to the formation of insoluble zinc phosphates, which mitigated the toxic effects of zinc ions . However, the tolerance of domesticated EPD systems to nano-CuO and nano-ZnO was lower than expected, potentially due to the high reactivity and oxidative stress induced by these nanoparticles .ConclusionsThe study indicated that the EPD system exhibited well tolerance to different valence states of iron, and the operation performance of EPD was significantly improved with higher accumulating rate of NO<sub>2</sub><sup>-</sup>-N and removing rate of COD, NO<sub>3</sub><sup>-</sup>-N and PO<sub>4</sub><sup>3-</sup>-P. The nZVI and Fe<sup>2+</sup> enhanced POD activity but inhibited DH activity. The introduction of three types of iron ions increased the relative abundance of narG, but decreased nirK, norB, and nosZ, which was beneficial for the stable operation of the EPD system. The domesticated EPD systems with nZVI showed well shock resistance to CuSO<sub>4</sub>, while domesticated EPD systems with Fe<sup>2+</sup> and Fe<sup>3+</sup> showed resilience to ZnSO<sub>4</sub>. However, the domesticated EPD systems did not show improved shock resistance to nano-CuO and nano-ZnO. This research provided theoretical support for domesticating EPD system and improving its shock resistance through technical means, which was crucial for the promotion of Anammox technology. This study contributed to the understanding of the tolerance mechanisms of EPD systems to heavy metals and their oxides and provided practical implications for enhancing the resilience of wastewater treatment systems to heavy metal contamination. |
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| ISSN: | 2096-3246 |