The present study explores the use of carbon dots coated with Iron (II, III) oxide (Fe₃O₄) for its application as an anode in microbial fuel cells (MFC). Fe₃O₄@PSA-C was synthesized using a hydrothermal-assisted probe sonication method. Nanoparticles were characterized with XRD, SEM, FTIR, and RAMAN Spectroscopy. Different concentrations of Fe₃O₄- carbon dots (0.25, 0.5, 0.75, and 1 mg/cm²) were coated onto the graphite sheets (Fe₃O₄@PSA-C), and their performance in MFC was evaluated. Cyclic voltammetry (CV) of Fe₃O₄@PSA-C (1 mg/cm²) modified anode indicated oxidation peaks at −0.26 mV and +0.16 mV, respectively, with peak currents of 7.7 mA and 8.1 mA. The fluxes of these anodes were much higher than those of other low-concentration Fe₃O₄@PSA-C modified anodes and the bare graphite sheet anode. The maximum power density (Pmax) was observed in MFC with a 1 mg/cm² concentration of Fe₃O₄@PSA-C was 440.01 mW/m², 1.54 times higher than MFCs using bare graphite sheet anode (285.01 mW/m²). The elevated interaction area of carbon dots permits pervasive Fe₃O₄ crystallization providing enhanced cell attachment capability of the anode, boosting the biocompatibility of Fe₃O₄@PSA-C. This significantly improved the performance of the MFC, making Fe₃O₄@PSA-C modified graphite sheets a good choice as an anode for its application in MFC.

Waste streams, leachates, and wastewater often contain high-strength ammonia, which can be challenging to manage. Microbial fuel cells (MFCs) offer a promising solution for treating such a nuisance of high-strength ammonia. However, optimizing MFC operating conditions, at lower technology readiness levels, is crucial to achieve a sustainable and economically viable application. This study investigates the factors affecting ammonia nitrogen removal in MFCs. MFCs with a cation exchange membrane (CEM) exhibit a higher diffusion rate of ammonium ions from the anode to the cathode compared to those with a proton exchange membrane (PEM). In close circuit mode (CCM), MFCs with a Pt-coated cathode electrode achieved an ammonium removal efficiency of 96% in the cathode chamber. Moreover, a plain carbon cathode electrode yielded an 87.1% removal efficiency. These results indicate that the combination of a catalyst (Pt) and oxygen in the cathode chamber can effectively remove or recover ammonia nitrogen from wastewater. Simultaneously, the removal of ammonia nitrogen in a microbial electrolysis cell (MEC) was studied. At an applied potential of 1.0 V, an ammonium removal efficiency of 87.5% was achieved. It was concluded that ammonium losses in MFCs can occur through electron migration, volatilization, and biological processes such as nitrification and denitrification.

Aquaponic systems are the integration of aquaculture and hydroponic systems to enhance productivity, reduce land use, and improve sustainability. This review focused on commonly used life cycle assessment (LCA) methodologies, system boundaries, and functional units used in aquaponics, standard impact categories, and identified hotspots. The scope is worldwide and encompasses a variety of aquaponic designs, fish species, and crops, illustrating the diversity of the systems examined. The analysis indicates that aquaponics provides the considerable environmental advantages of decreased fertilizer consumption and water conservation in comparison with aquaculture and hydroponic system. However, aquaponics systems are characterized by high energy consumption and may produce greater greenhouse gas (GHG) emissions compared to traditional farming methods when reliant on fossil fuel energy sources. Studies show that fish feed production, system infrastructure, and electricity usage for pumps, lights, heating, and other controls are hotspots. Harmonized comparisons of previous studies show methodological differences, especially in fish–plant co-production. Despite these variations, most believe that energy efficiency, renewable energy, feed optimization, and waste reuse may make aquaponics more sustainable. The study recommends the inclusion of broader environmental and social impacts. Also, future focus might be on making a standard functional unit or specifying system boundaries which might provide different accurate outcomes.

Aquaponics (AP) is the combination of aquaculture and hydroponic systems, developed based on waste to wealth theory. This study compared the plant growth and overall productivity of an aquaponic system (AP) with a controlled hydroponic system (HP) to assess the AP system’s performance and identification of the performance-limiting factors. This comparative study spanned over a 35-day period, supported by batch tests for the nutrient accumulation rate in plants and the NH₄⁺-N excretion rate by fish as a baseline for the system design. HP performed better in terms of plant growth, showing a mean plant fresh weight (g) of 165.6 ± 3.01 while AP showed 147.0 ± 4.6. Nutrient accumulation was better in HP for K and P; however, Ca²⁺, Mg²⁺, and Fe accumulation was higher in AP plants. The AP system supported a better fish growth of 31.95 ± 3.21% (FCR 1.29 ± 0.1, SGR 0.79 ± 0.06, and PER 2.24 ± 0.18) and a moderate plant biomass production. Further system design modifications and integrations are required to optimize the nutrient availability and sustainability of the AP systems.

Nutrient deficiencies and pH fluctuations are among the major issues identified in most aquaponic studies. Studies have mainly used chemical fertilizers and pH adjusters to resolve these issues; however, a sustainable and environment friendly permanent solution is needed. In this novel pilot study, we incorporated coral carbonate media into a koi carp-lettuce aquaponic system-A (APA) to provide a continuous release of Ca²⁺, Mg²⁺, Fe, and P and to support accumulation in plant and fish biomass while maintaining neutral pH, in order to compare with the control aquaponic-B (APB). In APA, mean FCR, SGR, and PER were recorded as 1.57 kg feed·kg fish⁻¹, 0.58%·day⁻¹, and 1.77 kg feed·kg fish⁻¹ protein, respectively, in comparison with the control APB which showed mean FCR, SGR, and PER as 3.45 kg feed·kg fish⁻¹, 0.27%·day⁻¹, and 0.80 kg fish·kg protein⁻¹, respectively. Also, the mean plant mass obtained during this 28-day study was 5.28 and 4.40 kg with a fish weight gain of 0.51 and 0.22 kg for APA and APB, respectively. In this pilot-scale study (n = 2 independent systems per treatment), the observed plant and fish biomass in APA were 20.9% and 10.1% higher respectively than APB; these values show descriptive differences observed in this study. Nutrient analysis was found to show higher release and accumulation of key nutrients in APA. This study examined a low-chemical, sustainable approach for aquaponics by using reusable coral carbonate media to maintain neutral pH and to improve nutrient availability and productivity.