Calcined pyrite accelerates sulfur metabolic and electron transfer in driving targeted microbial fuel cell denitrification.

The sulfur powder as electron donor in driving dual-chamber microbial fuel cell denitrification (S) process has the advantages in economy and pollution-free to treat nitrate-contained groundwater. However, the low efficiency of electron utilization in sulfur oxidation (ACE) is the bottleneck to this method. In this study, the addition of calcined pyrite to the S system (SCP) accelerated electron generation and intra/extracellular transfer efficiency, thereby improving ACE and denitrification performance. The highest nitrate removal rate reached to 3.55 ± 0.01 mg N/L/h in SCP system, and the ACE was 103 % higher than that in S system. More importantly, calcined pyrite enhanced the enrichment of functional bacteria (Burkholderiales, Thiomonas and Sulfurovum) and functional genes which related to sulfur metabolism and electron transfer. This study was more effective in removing nitrate from groundwater without compromising the water quality.

Granular activated carbon assisted biocathode for effective electrotrophic denitrification in microbial fuel cells.

Granular activated carbon (GAC) has been widely used at the anode of a microbial fuel cell (MFC) to enhance anode performance due to its outstanding capacitance property. To the best of our knowledge, there haven't been any studies on GAC in the cathode for biofilm development and nitrate reduction in MFC. In this study, by adding GAC to biocathode, we investigated the impact of different GAC amounts and stirring speeds on power generation and nitrate reduction rate in MFC. The denitrification rate was found to be nearly two-times higher in MFCs with GAC (0.046 ± 0.0016 kg m-3 d-1) compared to that deprived of GAC (0.024 ± 0.0012 kg m-3 d-1). The electrotrophic denitrification has produced a maximum power density of 37.6 ± 4.8 mW m-2, which was further increased to 79.2 ± 7.4 mW m-2 with the amount of GAC in the biocathode. A comparative study performed with chemical catalyst (Pt carbon with air sparging) cathode and GAC biocathode showed that power densities produced with GAC biocathode were close to that with Pt cathode. Cyclic voltammetry analysis conducted at 10 mV s-1 between -0.9 V and +0.3 V (vs. Ag/AgCl) showed consistent reduction peaks at -0.6V (Ag/AgCl) confirming the reduction reaction in the biocathode. This demonstrates that the GAC biocathode used in this research is effective at producing power density and denitrification in MFC. Our belief that the nitrate reduction was caused by the GAC biocathode in MFC was further strengthened when SEM analysis showing bacterial aggregation and biofilm formation on the surface of GAC. The GAC biocathode system described in this research may be an excellent substitute for MFC's dual functions of current generation and nitrate reduction.

Aerobic electrotrophic denitrification coupled with biologically induced phosphate precipitation for nitrogen and phosphorus removal from high-salinity wastewater: Performance, mechanism, and microbial community.

Electrotrophic denitrification (ED) is a promising nitrogen removal technique; however, the potential of ED coupled with biologically induced phosphate precipitation (BIPP) has not been fully explored. In this study, the performances, mechanisms, and microbial communities of the coupled system were investigated. The results showed that excellent nitrogen and phosphorus removal (both exceeding 92 %) was achieved in the salinity range of 20-60 g/L. ED contributed to approximately 83.4 % of nitrogen removal. BIPP removed approximately 63.5 % of the phosphorus. Batch activity tests confirmed that aerobic/anoxic bio-electrochemical and autotrophic/heterotrophic denitrification worked together for nitrate removal. Sulfate reduction had a negative impact on denitrification. Moreover, phosphorus removal was controlled by ED and calcium ions. The alkaline solution environment created by denitrification may greatly promote the formation of hydroxyapatite. Microbial community analyses indicated that the key bacteria involved in aerobic ED was Arcobacter. These findings will aid in the advanced treatment of high-salinity wastewater.

The potential of electrotrophic denitrification coupled with sulfur recycle in MFC and its responses to COD/SO42- ratios.

Electrotrophic denitrification is a promising novel nitrogen removal technique. In this study, the performance and the mechanism of electrotrophic denitrification coupled with sulfate-sulfide cycle were investigated under different anodic influent COD/SO42- ratios. The results showed that electrotrophic denitrification contributed to more than 22% total nitrogen removal in cathode chamber. Higher COD/SO42- ratios would deteriorate the sulfate reduction but enhance methane production. Further mass balance indicated that the electron flow utilized by methanogenic archaea (MA) increased while that utilized by sulfate-reducing bacteria (SRB) decreased as the COD/SO42- ratio increased from 0.44 to 1.11. However, higher COD/SO42- ratios would produce more electrons to strengthen electrotrophic denitrification. Microbial community analysis showed that the biocathode was predominantly covered by Thiobacillus that encoded with narG gene. These findings collectively suggest that electrotrophic denitrification could be a sustainable approach to simultaneously remove COD and nitrogen under suitable COD/SO42- ratio based on sulfur cycle in wastewater.

Highly efficient nitrate reduction driven by an electrocoagulation system: An electrochemical and molecular mechanism.

Electrotrophic denitrification is suitable for nitrate removal in aqueous environments where bioavailable electron donors are limited such as in urban polluted water. Herein, a novel microbial denitrifying electrocoagulation cell (MDECC) with an Fe anode and an electrotrophic denitrifying biocathode was constructed. Nitrate reduction was verified relying solely on the electrons originated from the electrolysis process at the Fe anode. In situ generated coagulant at the anode was utilized to effectively flocculate and precipitate pollutants as well as naturally occurring components. Nitrate reduction by the biocathode showed pseudo-first-order kinetics with a maximum NO3--N removal rate of 67 ± 7 g m-3 d-1 and a total nitrogen (TN) removal rate of 39 ± 6 g m-3 d-1. Mechanistic research demonstrated that the system achieved highest current efficiency and denitrification enzyme activity at an initial NO3--N concentration (IC-N) of 100 mg L-1. Further pyrosequencing evidenced that higher initial NO3--N concentration increased the abundance of denitrifiers on biocathode. Correlation analysis indicated that nitrate reductase (NAR) and nitrite reductase (NIR) activities were crucial for NO3--N and TN removal. The metal anode was a promising alternative for providing electrons for electrotrophic denitrification and pollutant elimination.

Impacts of electron donor and acceptor on the performance of electrotrophic denitrification.

Electrotrophic denitrification is a novel nitrogen removal technique. In this study, the performance and the mechanism of electrotrophic denitrification were investigated at different nitrate concentrations and current intensities. The results showed that the performance of electrotrophic denitrification was good with a sludge loading of 0.39 kg N/kg VSS day. The half-saturation constant for nitrate-N was 1894.03 mg/L. The optimal nitrate-N concentration and current intensity were 1500 mg/L and 20 µA, respectively. Electrotrophic denitrification was defined as the process of direct use of electron for nitrate reduction, and electrotrophic denitrifier was proposed to be the microbe of using electricity as energy source directly. The present work will benefit the development and application of electrotrophic denitrification. Graphical abstract ᅟ.