Microbial reduction of selenium (Se) oxyanions to elemental Se is a promising technology for bioremediation and treatment of Se wastewaters. But a fraction of biogenic nano-Selenium (nano-Se^b) formed in bioreactors remains suspended in the treated waters, thus entering the aquatic environment. The present study investigated the toxicity of nano-Se^b formed by anaerobic granular sludge biofilms on zebrafish embryos in comparison with selenite and chemogenic nano-Se (nano-Se^c). The nano-Se^b formed by granular sludge biofilms showed a LC₅₀ value of 1.77 mg/L, which was 3.2-fold less toxic to zebrafish embryos than selenite (LC₅₀=0.55 mg/L) and 10-fold less toxic than bovine serum albumin stabilized nano-Se^c (LC₅₀=0.16 mg/L). Smaller (nano-Se^cs; particle diameter range: 25–80 nm) and larger (nano-Se^cl; particle diameter range: 50–250 nm) sized chemically synthesized nano-Se^c particles showed comparable toxicity on zebrafish embryos. The lower toxicity of nano-Se^b in comparison with nano-Se^c was analyzed in terms of the stabilizing organic layer. The results confirmed that the organic layer extracted from the nano-Se^b consisted of components of the extracellular polymeric substances (EPS) matrix, which govern the physiochemical stability and surface properties like ζ-potential of nano-Se^b. Based on the data, it is contented that the presence of humic acid like substances of EPS on the surface of nano-Se^b plays a major role in lowering the bioavailability (uptake) and toxicity of nano-Se^b by decreasing the interactions between nanoparticles and embryos.

The availability of clean water and the depletion of non-renewable resources provide challenges to modern society. The widespread use of conventional wastewater treatment necessitates significant financial and energy expenditure. Constructed Wetland Microbial Fuel Cells (CW-MFCs), a more recent alternative technology that incorporates a Microbial Fuel Cell (MFC) inside a Constructed Wetland (CW), can alleviate these problems. By utilizing a CW’s inherent redox gradient, MFC can produce electricity while also improving a CW’s capacity for wastewater treatment. Electroactive bacteria in the anaerobic zone oxidize the organic contaminants in the wastewater, releasing electrons and protons in the process. Through an external circuit, these electrons travel to the cathode and produce electricity. Researchers have demonstrated the potential of CW-MFC technology in harnessing bio-electricity from wastewater while achieving pollutant removal at the lab and pilot scales, using both domestic and industrial wastewater. However, several limitations, such as inadequate removal of nitrogen, phosphates, and toxic organic/inorganic pollutants, limits its applicability on a large scale. In addition, the whole system must be well optimized to achieve effective wastewater treatment along with energy, as the ecosystem of the CW-MFC is large, and has diverse biotic and abiotic components which interact with each other in a dynamic manner. Therefore, by modifying important components and optimizing various influencing factors, the performance of this hybrid system in terms of wastewater treatment and power generation can be improved, making CW-MFCs a cost-effective, cleaner, and more sustainable approach for wastewater treatment that can be used in real-world applications in the future.