Lactic acid bacteria (LAB) such as Enterococcus spp. have an advantage over several bacteria because of their ability to easily adapt to extreme conditions which include high temperatures, highly acidic or alkaline conditions and toxic metals. Although many microorganisms have been shown to reduce selenite (SeO₃²⁻) to elemental selenium (Se⁰), not much work has been done on the combined effect of Enterococcus spp. In this study, aerobic batch reduction of different selenite concentrations (1, 3 and 5 mM) was conducted using Enterococcus hermanniensis sp. and Enterococcus gallinarum sp. (3.5 h, 35 ± 2 °C, starting pH > 8.5). Results from the experiments showed that the average reductions rates were 0.608, 1.921 and 3.238 mmol·(L·h)⁻¹, for the 1, 3 and 5 mM SeO₃²⁻ concentrations respectively. In addition, more selenite was reduced for the 5 mM concentration compared to the 1 and 3 mM concentrations albeit constant biomass being used for all experiments. Other parameters which were monitored were the glucose consumption rate, protein variation, pH and ORP (oxidation reduction potential). TEM analysis was also conducted and it showed the location of electron-dense selenium nanoparticles (SeNPs). From the results obtained in this study, the authors concluded that Enterococcus species’s high adaptability makes it suitable for rapid selenium reduction and biosynthesis of elemental selenium.

Selenite (SeO₃²⁻), the most toxic and most reactive selenium (Se) oxyanion, can be reduced to elemental selenium (Se⁰) nanoparticles by a variety of bacteria, including Enterococcus spp. Previously, the orthodox view held that the reduction of SeO₃²⁻ to Se⁰ by a wide range of bacteria was solely accomplished by biological processes; however, recent studies have shown that various bacterial strains secrete metal-reducing metabolites, thereby indirectly catalysing the reduction of these metal species. In the current study, selenium nanoparticles were synthesised from the abiotic reduction of selenite with the use of Enterococcus spp. cell-free extract. Once separated from the cell-free extract, the particles were analysed using Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Transmission electron microscopy (TEM) and a Zetasizer. The results revealed that the SeNPs were spherical in shape, containing both amorphous and crystalline properties, and the sizes with the highest frequency ranged close to 200 nm. Additionally, the obtained nanoparticles exhibited antimicrobial properties by directly inhibiting the viability of an E. coli bacterial strain. The results demonstrate not only the potential of abiotic production of SeNPs, but also the potential for these particles as microbial inhibitors in medical or similar fields.

The presence of toxic and hazardous chemical species in municipal wastewater poses a significant environmental and public health challenge, necessitating innovative, sustainable, and cost-effective treatment solutions. This study pioneers the recovery and valorisation of polycationic metals from real acid mine drainage (AMD) for municipal wastewater treatment, demonstrating a novel approach that integrates resource recovery with wastewater remediation. A key strength of this study is the use of real municipal wastewater (authentic MWW) in the treatment phase, ensuring that the findings accurately reflect real-world conditions. Advanced analytical techniques were employed to characterise both aqueous and solid samples, and batch experiments were conducted to assess the removal efficiency of polycationic metals for key contaminants: ammonium (NH₄⁺), sulphate (SO₄²⁻), phosphate (PO₄³⁻), and nitrate (NO₃⁻). The optimised conditions are 2 g of polycationic metals per 100 mL, 90 min of contact time, and 35 °C. The yielded exceptional removal efficiencies are PO₄³⁻ (>99.9%), NH₄⁺ (>99.7%), NO₃⁻ (>99%), and SO₄²⁻ (>96%), achieving final concentrations of <0.5 mg/L for PO₄³⁻ and NH₄⁺, 2.1 mg/L for NO₃⁻, and 9.1 mg/L for SO₄²⁻. Adsorption kinetics followed a pseudo-first-order model, indicating physisorption, while the Two-Surface Langmuir model suggested a combination of homogeneous and heterogeneous adsorption mechanisms. FTIR, SEM-EDX mapping, and XRF analyses confirmed the retention of P, S, and N in the product sludge, validating the adsorption process. This study is the first of its kind to recover Al-rich Fe species from real AMD and activate them for municipal wastewater remediation using authentic MWW, bridging the gap between laboratory-scale research and real-world applications. By simultaneously addressing AMD pollution and municipal wastewater treatment, this research advances circular economy principles, promotes sustainable water management, and contributes to national and global efforts toward water security and environmental protection.

This study investigates the use of a UASB (Upflow Anaerobic Sludge Blanket) reactor operating under continuous anoxic conditions to remediate Pb(II) contamination in aqueous environments. Two experimental runs were conducted to evaluate the microbiome’s performance in removing Pb(II) at varying concentrations, ranging from 80 to 2000 ppm, while monitoring nitrate and Pb(II) levels. Metabarcoding of the 16S rRNA gene was done to understand the detoxification mechanisms utilised by the microbial community in Pb(II) removal. The system demonstrated high robustness, achieving up to 99% Pb(II) removal efficiency with sufficient nutrient availability, particularly at 15 g/L yeast extract (YE), compared to lower nutrient levels of 5 g/L YE. Denitrification was identified as the dominant mechanism of detoxification, supported by additional processes such as biosorption, sulfur-reducing bacterial activity, bioprecipitation, and bioremoval. Analysis of the precipitate recovered from the reactor indicated the presence of elemental lead, PbS, and PbO, highlighting the potential for lead recovery. These findings suggest that the system not only effectively removes Pb(II) from contaminated environments but also offers a sustainable pathway for lead recovery through smelting, making it a promising circular bioremediation strategy. The results indicate that this biological approach is a viable solution for lead pollution and recovery in industrial applications.