Elemental selenium (Se ⁰ ) nanomaterials undergo allotropic transition from thermodynamically‐unstable to more stable phases. This process is significantly different when Se ⁰ nanoparticles (NPs) are produced via physico‐chemical and biological pathways. While the allotropic transition of physico‐chemically synthesized Se ⁰ is fast (minutes to hours), the biogenic Se ⁰ takes months to complete. The biopolymer layer covering biogenic Se ⁰ NPs might be the main factor controlling this retardation, but this still remains an open question. Phylogenetically‐diverse bacteria reduce selenium oxyanions to red amorphous Se ⁰ allotrope, which has low market value. Then, red Se ⁰ undergoes allotropic transition to trigonal (metallic grey) allotrope, the end product having important industrial applications (e.g. semiconductors, alloys). Is it not yet clear whether biogenic Se ⁰ presents any biological function, or it is mainly a detoxification and respiratory by‐product. The better understanding of this transition would benefit the recovery of Se ⁰ NPs from secondary resources and its targeted utilization with respect to each allotropic stage. This review article presents and critically discusses the main physico‐chemical methods and biosynthetic pathways of Se ⁰ (bio)mineralization. In addition, the article proposes a conceptual model for the resource recovery potential of trigonal selenium nanomaterials in the context of circular economy.

Bacteria play crucial roles in the biogeochemical cycle of arsenic (As) and selenium (Se) as these elements are metabolized via detoxification, energy generation (anaerobic respiration) and biosynthesis (e.g. selenocysteine) strategies. To date, arsenic and selenium biomineralization in bacteria were studied separately. In this study, the anaerobic metabolism of As and Se in Shewanella sp. O23S was investigated separately and mixed, with an emphasis put on the biomineralization products of this process. Multiple analytical techniques including ICP-MS, TEM-EDS, XRD, Micro-Raman, spectrophotometry and surface charge (zeta potential) were employed. Shewanella sp. O23S is capable of reducing selenate (SeO₄^2-) and selenite (SeO₃^2-) to red Se(-S)⁰, and arsenate (AsO₄^3-) to arsenite (AsO₃^3-). The release of H₂S from cysteine led to the precipitation of AsS minerals: nanorod AsS and granular As₂S₃. When As and Se oxyanions were mixed, both As-S and Se(-S)⁰ biominerals were synthesized. All biominerals were extracellular, amorphous and presented a negative surface charge (-24 to -38 mV). Kinetic analysis indicated the following reduction yields: SeO₃^2- (90%), AsO₄^3- (60%), and SeO₄^2- (<10%). The mix of SeO₃^2- with AsO₄^3- led to a decrease in As removal to 30%, while Se reduction yield was unaffected (88%). Interestingly, SeO₄^2- incubated with AsO₄^3- boosted the Se removal (71%). The exclusive extracellular formation of As and Se biominerals might indicate an extracellular respiratory process characteristic of various Shewanella species and strains. This is the first study documenting a complex interplay between As and Se oxyanions: selenite decreased arsenate reduction, whereas arsenate stimulated selenate reduction. Further investigation needs to clarify whether Shewanella sp. O23S employs multi-substrate respiratory enzymes or separate, high affinity enzymes for As and Se oxyanion respiration.

Bacillus sp. Abq, belonging to Bacillus cereus sensu lato, was isolated from an aquifer in New Mexico, USA and phylogenetically classified. The isolate possesses the unusual property of precipitating Pb(II) by using cysteine, which is degraded intracellularly to hydrogen sulfide (H₂S). H₂S is then exported to the extracellular environment to react with Pb(II), yielding PbS (galena). Biochemical and growth tests showed that other sulfur sources tested (sulfate, thiosulfate, and methionine) were not reduced to hydrogen sulfide. Using equimolar concentration of cysteine, 1 mM of soluble Pb(II) was removed from Lysogeny Broth (LB) medium within 120 h of aerobic incubation forming black, solid PbS, with a removal rate of 2.03 µg L⁻¹ h⁻¹ (∼8.7 µM L⁻¹ h⁻¹). The mineralogy of biogenic PbS was characterized and confirmed by XRD, HRTEM and EDX. Electron microscopy and electron diffraction identified crystalline PbS nanoparticles with a diameter <10 nm, localized in the extracellular matrix and on the surface of the cells. This is the first study demonstrating the use of cysteine in Pb(II) precipitation as insoluble PbS and it may pave the way to PbS recovery from secondary resources, such as Pb-laden industrial effluents.

Coal-fired power facilities generate a polymetallic effluent (Flue Gas Desulfurization—FGD) rich in sulfate. FGD effluents may be considered an important secondary resource. This paper investigates the recovery of sulfate as barite (BaSO₄), a mineral with high commercial value and a critical raw material. Using equimolar BaCl₂, >99% desulfurization of an FGD effluent produced by a coal-fired power plant operating in central Poland was achieved, yielding up to 16.5 kg high purity barite m⁻³. The recovered barite was characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric (TGA), scanning electron microscopy analysis (SEM), surface properties (PZC), density, and chemical stability (TCLP), and was compared with a commercial reference material. Barite recovery also led to the reduction in concentration of Al (86%), Cu (52%), K (69%), Mo (62%), Se (40%), Sr (91%), and U (75%) initially present in the FGD effluent. TCLP results indicate the entrapment and the stabilization of ~70% Se and ~90% Al in the barite structure. Based on this dataset, an in-depth characterization of the recovered barite is presented, and the removal mechanism of the elements is discussed. The study also provides a preliminary cost benefit analysis of the process. To our best knowledge, this is the first work showing barite recovery and metal removal from FGD effluents using a one-step process.