Internal nitrogen removal from sediments by the hybrid system of microbial fuel cells and submerged aquatic plants
Peng Xu, Enrong Xiao, Dan Xu, Yin Zhou +4
AI summary
82% confidenceThis sediment microbial fuel cell (SMFC) study coupled with submerged aquatic plants (V. spiralis) demonstrates that closed-circuit operation increases sediment redox potential by 474 mV, promoting mineralization of organic nitrogen and migration of NH4+ to overlying water, while simultaneously inhibiting denitrification at the sediment-water interface. Adding plants reduced power output (75 mV lower mean voltage) but enhanced internal nitrogen removal to 25.3% by day 190 through plant assimilation and stimulated nitrification and denitrification in sediments, despite accumulating NO3− in the water column during stable power generation phases.
Key insights
Trade-off Closed-circuit SMFC increased sediment Eh by 474 mV relative to open-circuit, shifting anode potential from −411 mV to +63 mV, which promoted organic nitrogen mineralization but inhibited denitrification at the sediment-water interface.
So what. Operators must accept that higher sediment redox potential trades off denitrification suppression; if N2O or NO3− accumulation is unacceptable, reduce anode potential or increase cathode distance to lower Eh.
Trade-off Plant-coupled P-SMFC-c reduced mean voltage by 75 mV and maximum power density from 4.42 to 3.16 mW/m² despite enriching electrogenic bacteria (Bacillus, Geobacter, Pseudomonas), because oxygen loss from V. spiralis roots increased anodic resistance from 167 Ω to 193 Ω.
So what. If power generation is the priority, avoid submerged plants with high radial oxygen loss; if nitrogen removal is the goal, accept the power penalty as a design trade-off.
Mechanism During phases II and III (stable and declining power), NO3− accumulated in overlying water of SMFC-c (0.93 and 1.53 mg/L) despite internal NH4+ removal, because elevated sediment Eh (550 mV above open-circuit) favored nitrification over denitrification at the sediment-water interface.
So what. Operators must monitor overlying water NO3− accumulation as a hidden cost of closed-circuit SMFC; if NO3− removal is required, couple with denitrifying cathode or reduce cathode DO by lowering air exposure.
Performance Plant assimilation removed 25.3% of internal total nitrogen by day 190 in P-SMFC-c, but this was accompanied by lower NH4+ in overlying water (0.074 mg/L vs 0.128 mg/L in SMFC-c at phase III), indicating plants competed with nitrification for NH4+.
So what. Plant-coupled systems are effective for internal nitrogen removal when overlying water quality (low NH4+) is the goal; use this configuration if eutrophication control is the priority over power.
Threshold NO2− was not detected in pore water or sediments despite strong nitrification, and cathodic denitrification was not observed in the air-cathode system even though DO at the sediment-water interface exceeded 2 mg/L, indicating the oxic environment prevented autotrophic denitrifying bacteria colonization.
So what. To enable cathodic denitrification in SMFC, use an anaerobic cathode chamber or reduce cathode DO below 1 mg/L; air-cathodes are unsuitable for NO3− reduction in this configuration.
Opportunity areas6
Specific gaps in this paper, with an experiment that would close each.
High No measurement of N2O or NO2− accumulation in overlying water or sediments despite strong nitrification and denitrification activity; N2O emissions could be a hidden cost of closed-circuit SMFC.
The paper detected no NO2− in pore water or sediments, yet nitrification was 'strong' and denitrification was 'promoted' in P-SMFC-c. N2O is a common intermediate in both pathways but was never quantified.
Try. Measure N2O in headspace gas and dissolved in water using gas chromatography or membrane inlet mass spectrometry across all four treatments during phases II and III when nitrification and denitrification are most active.
High No systematic variation of external resistance or anode/cathode potential to map the operating window for nitrogen removal vs power generation trade-off.
The study used only 100 Ω external resistance for closed-circuit operation. Polarization curves (Fig 2d) show power density peaks at 200 Ω, but nitrogen cycling was not characterized at different potentials.
Try. Operate SMFC-c and P-SMFC-c at 5–10 different external resistances (50–500 Ω) and measure anode potential, sediment Eh, overlying water NO3−/NH4+, and internal nitrogen removal for each to identify the optimal operating window.
Medium No direct measurement of ammonia volatilization rate or pH-dependent NH4+/NH3 speciation in overlying water to confirm the proposed removal mechanism.
The authors propose that 'elevated pH near the cathode drove the ammonium volatilization with the conversion of ammonium ion to the more volatile ammonia species' but provide no direct evidence (e.g., ammonia gas measurement, pH gradient mapping).
Try. Measure pH at multiple heights in overlying water and near cathode; quantify NH3 gas flux using closed-chamber headspace analysis; calculate NH4+/NH3 speciation using Henderson-Hasselbalch equation at measured pH values.
Medium No variation of hydraulic retention time (HRT) or water exchange rate; all tanks were static with only evaporation/sampling water loss replaced.
The paper states 'Water lost due to evaporation and sampling was replenished with distilled water' but does not test whether HRT affects nitrogen removal efficiency or NO3− accumulation in overlying water.
Try. Operate replicate SMFC-c and P-SMFC-c systems with controlled water exchange rates (0%, 10%, 25%, 50% per week) and measure overlying water NO3−, NH4+, and internal nitrogen removal to determine if flushing mitigates NO3− accumulation.
Medium No measurement of cathode potential or dissolved oxygen profile in sediments; cathode DO is assumed to be high but not quantified at the sediment-water interface or within the oxygenated layer.
The paper states 'even at the sediment-water interface, the DO level was higher than 2 mg/L' but provides only one measurement point (5 cm above interface). Vertical DO profile would clarify the thickness and stability of the oxygenated layer.
Try. Deploy oxygen microelectrodes at 0, 2, 5, 10, 20 cm above and below the sediment-water interface in SMFC-c and P-SMFC-c to map the oxic/anoxic boundary and its temporal variation across the three phases.
Low No test of C/N ratio or organic matter quality on nitrogen cycling outcomes; sediment organic matter composition was not characterized beyond total organic matter (13.33 dt%).
The paper measures total OM but not C/N ratio, labile vs refractory carbon, or carbohydrate/protein/lipid composition, which could explain differences in mineralization rates between treatments.
Try. Analyze sediment OM composition using elemental analysis (C, N, H), pyrolysis-GC-MS, or FTIR spectroscopy; correlate C/N ratio and OM quality with mineralization rate and nitrification/denitrification activity.
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What they did
- System
- MFC
What worked
Abstract
Sediment internal nitrogen release is a significant pollution source in the overlying water of aquatic ecosystems. This study aims to remove internal nitrogen in sediment-water microcosms by coupling sediment microbial fuel cells (SMFCs) with submerged aquatic plants. Twelve tanks including four treatments in triplicates were designed: open-circuit (SMFC-o), closed-circuit (SMFC-c), aquatic plants with open-circuit (P-SMFC-o) and aquatic plants with closed-circuit (P-SMFC-c). The changes in the bio-electrochemical characteristics of the nitrogen levels in overlying water, pore water, sediments, and aquatic plants were documented to explain the migration and transformation pathways of internal nitrogen. The results showed that both electrogenesis and aquatic plants could facilitate the mineralization of organic nitrogen in sediments. In SMFC, electrogenesis promoted the release of ammonium from the pore water, followed by the accumulation of ammonium and nitrate in the overlying water. The increased redox potential of sediments due to electrogenesis also contributed to higher levels of nitrate in overlying water when nitrification in pore water was facilitated and denitrification at the sediment-water interface was inhibited. When the aquatic plants were introduced into the closed-circuit SMFC, the internal ammonium assimilation by aquatic plants was advanced by electrogenesis; nitrification in pore water and denitrification in sediments were also promoted. These processes might result in the maximum decrease of internal nitrogen with low nitrogen levels in the overlying water despite the lower power production. The P-SMFC-c reduced 8.1%, 16.2%, 24.7%, and 25.3% of internal total nitrogen compared to SMFC-o on the 55th, 82th, 136th, and 190th days, respectively. The smaller number of Nitrospira and the larger number of Bacillus and Pseudomonas on the anodes via high throughput sequencing may account for strong mineralization and denitrification in the sediments under closed-circuit. The coupled P-SMFC system has shown good potential for the efficient removal of internal nitrogen.
Keywords
Identifiers
- PubMed
- 28241072
- Journal
- PLoS ONE
- Year
- 2017
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