Search

[Display omitted] • A modified geopolymer with Se oxyanions uptake capacity was synthesized. • The Se oxyanions uptake mechanism of synthetic ettringite depends on the Se concentration. • Ion exchange is the primary Se oxyanions uptake mechanism of in-situ ettringite. • A thermodynamic model was developed to describe the uptake of Se oxyanions. • The basic structure and cation uptake capacity of the modified geopolymer are retained. Studies have demonstrated the remarkable capacity of metakaolin-based geopolymers for the uptake of cationic radionuclides, such as Cs+ and Sr2+. However, the lack of uptake ability towards anionic radionuclides represents a potential avenue for improvement in utilising this material for nuclear waste disposal applications. Additionally, the prevalence of ettringite as an anion exchange agent formed in alkaline environments, such as cement hydration, is widely acknowledged in the field. However, no previous studies have been conducted to explore the utilisation of the advantageous qualities of both metakaolin-based geopolymer and ettringite in developing a comprehensive incorporation process for both cationic and anionic radionuclides. In this study, compositions and sets of preparation conditions were proposed to modify metakaolin-based geopolymer, resulting in the in-situ formation of ettringite, with the aim of enhancing its uptake capabilities for selenium oxyanions. The uptake behaviour and mechanism of ettringite, and geopolymer with in-situ ettringite, were proposed through co-precipitation experiments, binding and structural analysis. The results reveal that the ettringite can uptake SeO 3 2- and SeO 4 2- through co-precipitation; however, its effectiveness is limited to SeO 3 2- in the uptake process, where the mechanism is contingent on the concentration of Se oxyanions present. On the other hand, modified geopolymer with in-situ ettringite retained its capacity to uptake cationic radionuclides such as Cs+ and Sr2+ while evolving to exhibit an ability to uptake SeO 3 2-. Thermodynamic modelling was carried out according to the ion exchange mechanism, which effectively predicts the uptake of SeO 3 2- at low concentrations. The proposed composition and preparation conditions hold the potential for developing an effective incorporation process for cationic and anionic radionuclides. The outcomes of this research enhance the understanding of the uptake behaviour of Se oxyanions by ettringite and provide insight into the potential of metakaolin-based geopolymers to immobilise anions. [ABSTRACT FROM AUTHOR]

Inappropriate treatments for the effluents from semiconductor plants might cause the releases and wide distributions of selenium (Se) into the ecosystems. In this study, Al/Si and Fe/Si coprecipitates were selected as model adsorbents as they often formed during the wastewater coagulation process, and the removal efficiency of selenite (SeO 3 ) and selenate (SeO 4 ) onto the coprecipitates were systematically examined. The removal efficiency of SeO 3 and SeO 4 was highly related to surface properties of Al/Si and Fe/Si coprecipitates. The surface-attached Al shell of Al/Si coprecipitates shielded a portion of negative charges from the core SiO 2 , resulting in a higher point of zero charge than that of Fe/Si coprecipitates. Thus, adsorption of SeO 3 /SeO 4 was favorable on the Al/Si coprecipitates. Adsorptions of both SeO 3 and SeO 4 on Al/Si coprecipitates were exothermic reactions. On Fe/Si coprecipitates, while SeO 3 adsorption also showed the exothermic behavior, SeO 4 adsorption occurred as an endothermic reaction. The kinetic adsorption data of SeO 3 /SeO 4 on Al/Si and Fe/Si coprecipitates were described well by the pseudo-second-order kinetic model. SeO 4 and SeO 3 adsorption on Fe/Si or Al/Si were greatly inhibited by the strong PO 4 ligand, whereas the weak ligand such as SO 4 only significantly affected SeO 4 adsorption. The weakest complex between SeO 4 and Al was implied by the essentially SeO 4 desorption as SeO 4 /PO 4 molar ratios decreased from 0.5 to 0.2. These results were further confirmed by the less SeO 4 desorption (41%) from Fe/Si coprecipitates than that from Al/Si coprecipitates (78%) while PO 4 was added sequentially. [ABSTRACT FROM AUTHOR]

Biogenic formation of nano-sized particles composed of various materials (in particular, selenium) by live microorganisms is widespread in nature. This phenomenon has been increasingly attracting the attention of researchers over the last decade not only owing to a range of diverse applications of such nanoparticles (NPs) in nanobiotechnology, but also because of the specificity of methodologies and mechanisms of NPs formation related to “green synthesis”. In this mini-review, recent data are discussed on the multifaceted role of proteins in the processes of microbial reduction of selenium oxyanions and the formation of Se NPs. Besides the involvement of proteins in reducing selenites and selenates, their participation in the microbially driven growth and stabilisation of Se NPs is analysed, which results in the formation of unique nanostructured materials differing from those obtained chemically. This mini-review is thus focussed on proteins involved in microbial synthesis of Se NPs and on instrumental analysis of these processes and their products (biogenic nanostructured selenium particles functionalised by a surface-capping layer of various biomacromolecules). [ABSTRACT FROM AUTHOR]

Abstract: Removal of selenium oxyanions by the binary oxide systems, Al- or Fe-oxides mixed with X-ray noncrystalline SiO2, was previously not well understood. This study evaluates the adsorption capacity and kinetics of selenium oxyanions by different metal hydroxides onto SiO2, and uses X-ray absorption spectroscopy (XAS) to assess the interaction between selenium oxyanions and the sorbents at pH 5.0. The binary oxide systems of Al(III)- or Fe(III)-oxides mixed with SiO2 were prepared, and were characterized for their surface area, point of zero charge (PZC), pH envelopes, X-ray diffraction analysis (XRD), and then macro-scale adsorption isotherm and kinetics of selenite and selenate, micro-scale adsorption XAS. The adsorption capacity of selenite and selenate on Al(III)/SiO2 is greater than on Fe(III)/SiO2. Adsorption isothermal and kinetic data of selenium can be well fitted to the Langmuir isotherm and pseudo-second-order kinetic models. Based on simple geometrical constraints, selenite on both the binary oxide systems forms bidentate inner-sphere surface complexes, and selenate on Fe(III)/SiO2 forms stronger complexes than on Al(III)/SiO2. [Copyright &y& Elsevier]

Abstract: To understand the molecular events involved in the reduction of selenite to non-toxic elemental selenium, 4000 clones of Ralstonia metallidurans CH34 were produced by random Tn5 transposon integration mutagenesis. Eight mutants were able to resist up to 15 mM selenite while the MIC for the wild-type strain was estimated as 4–6 mM selenite. The identification of the disrupted genes was carried out by Southern blot analysis and inverse PCR. The three resistant mutants containing only one insertion were further characterized. Tn5 disrupted a gene that encoded a protein which was closely related to proteins of the DedA family. This family represents a group of integral membrane proteins with completely unknown functions. Phenotypic characterization of the dedA mutants and selenite consumption experiments strongly suggest that DedA is involved in the uptake of selenite. [Copyright &y& Elsevier]