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Removal of selenate (SeO42-) from selenate-contaminated wastewater is challenging due to the commonly coexisting and competing anions of sulfate (SO42-) and nitrate (NO3-). This study investigates SeO42- reduction to elemental selenium (Se0) in a cathode-based bioelectrochemical (BEC) reactor and a conventional biofilm reactor (i.e., an upflow anaerobic reactor). The simulated wastewater contained SeO42- at a typical concentration of 5 mg Se/L, SO42- at a typical concentration of 1000 mg S/L, and NO3- at concentrations that varied from 0 to 10 mg N/L. The impact of sulfate on the BEC reactor was much lower than that on the conventional reactor: The selenium removal, defined as (selenate in influent – dissolved selenium in effluent)/selenate in influent, was 99 % in the BEC reactor versus 69 % in the conventional biofilm reactor. The lower selenium removal in the conventional reactor was mainly due to the >10 times higher reduction of sulfate, which directly caused competition between sulfate and selenate for the common resources such as electrons. The more reduction of sulfate in the conventional reactor further led to 45 times higher production of selenide. Selenide is usually assumed to be minimal and therefore not measured in the literature. This simplification may significantly overestimate selenium removal when the influent sulfate concentration is very high. NO3- in the influent of the BEC reactor promoted selenium removal when it was less than 5.0 mg N/L but inhibited selenate removal when it was more than 7.5 mg N/L. This was supported by the microbial community analysis and intermediate (nitrite) analysis. 

Tellurium is a critical mineral for the foreseeable future due to its scarcity and importance in future energy technology. A biocathode of a bioelectrochemical reactor (BEC) was used for the first time to extracellularly reduce TeO32- in simulated wastewater to elemental Te0 nanorods, which could potentially be recovered. Scanning transmission electron microscopy revealed that only 2% of the cells on the biocathode contained intracellular Te0 nanorods. In contrast, in the conventional bioreactor (CBR), 40% of the cells contained intracellular Te0 nanorods. Raman spectroscopy determined that the Te0 nanorods were trigonal and amorphous Te0. Microbial community analysis showed the dominance of Pseudomonas, Stenotrophomonas, and Azospira phylotypes in the cathode chamber, despite being < 8% in the inoculum. They were all putative TeO32- reducers due to their known ability to reduce tellurite and transfer extracellular electrons. The TeO32- removal efficiency in the BEC reactor reached 97% when the influent TeO32- was 5 mg Te/L. The reactor operating conditions, including the flow rate, the external resistor, and the cation exchange membrane, were optimized. This work demonstrates the potential of BEC reactors for continuous and green synthesis of Te0 nanorods. 

Microbial processes are crucial in the redox transformations of toxic selenium oxyanions. This study focused on isolating an efficient selenate-reducing strain, Azospira sp. A9D-23B, and evaluating its capability to recover extracellular selenium nanoparticles (SeNPs) from selenium-laden wastewater in different reactor setups. Analysis using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) revealed significantly higher extracellular SeNPs production (99%) on the biocathode of the bioelectrochemical (BEC) reactor compared to the conventional bioreactor (65%). Further investigations into the selenate reductase activity of strain A9D-23B revealed distinct mechanisms of selenate reduction in BEC and conventional bioreactor settings. Notably, selenate reductases associated with the outer membrane and periplasm displayed higher activity (18.31 ± 3.8 µmol/mg-min) on the BEC reactor's biocathode compared to the upflow anaerobic conventional bioreactor (3.24 ± 2.9 µmol/mg-min). Conversely, the selenate reductases associated with the inner membrane and cytoplasm exhibited lower activity (5.82 ± 2.2 µmol/mg-min) on the BEC reactor's biocathode compared to the conventional bioreactor (9.18 ± 1.6 µmol/mg-min). However, the comparable kinetic parameter (K_m) across cellular fractions in both reactors suggest that SeNPs localization was influenced by enzyme activity rather than selenate affinity. Overall, the mechanism involved in selenate reduction to SeNPs and the strain's efficiency in detoxifying selenate below levels regulated by U.S. Environmental Protection Agency have broad implications for sustainable environmental remediation strategies. 

Abstract A continuous gas–liquid flowing film reactor with a nanosecond‐pulsed power supply was utilized for the degradation of perfluorooctanoic acid (PFOA) as assessed by fluoride (F⁻) formation. PFOA, 50 mg/L, dissolved in deionized water was supplied at 2 ml/min with an argon carrier gas. The liquid phase was analyzed for F⁻using ion chromatography. The power supply pulse frequency (f) was varied between 0.25‐ and 10‐kHz using a constant 16‐kV input voltage and 40‐ns pulse width. The highest F⁻production rate (), 1.57 × 10⁻⁸ mol/s, occurred at 5 kHz whereas the highest efficiency of F⁻production (), 9.12 × 10⁻⁹ mol/J, was found at 0.25 kHz.