Issue 9, 2022

Mechanism of galvanic reduction of selenate oxyanions and surface immobilization by nano zero-valent iron aggregates under anaerobic conditions: towards high electron efficiency

Abstract

Selenate(VI) – as opposed to selenite(IV) – species encountered in industrial effluent water pose challenges to their effective elimination due to poor adsorption and chemical reduction response. Nano zero-valent iron (nZVI) is a strong reducing agent that shows a lot of promise in reducing selenate to low ppb levels. However, this is achieved with a large excess of nZVI that renders its application non-sustainable. In this work the galvanic mechanism of selenate reduction on the nZVI surface is studied with the view of increasing the electron efficiency/selectivity and thereby ultimately the efficacy of the process. Under anaerobic conditions, nZVI, consisting of chains of 60 ± 20 nm size spherical particles, forms upon immersion in water with ∼3 nm thick shells of predominantly Fe(II) hydrous oxides. Experiments were conducted to determine the kinetics and nature of Se(VI) uptake on nZVI at pH 9 and 22 °C by performing nanoscale surface characterization. It is proposed that selenate (SeO42−(aq)) converts to selenite (SeO32−(surf)) via reductive adsorption on the hydrous oxide surface of nZVI by electron exchange with Fe(II) yielding inner-sphere selenite complexes anchored on the resultant mixed Fe(II/III) surface layer. The chemically adsorbed Se(IV) is reduced galvanically to Se(0) by metallic iron core electrons conducted through the Se-enriched Fe(0/II/III) oxide layer. It is postulated that semiconducting Se0 nanodots nucleating onto and into the surface oxide layer play a critical role in enhancing the selective reduction of selenium (down to a few ppb), while suppressing the parasitic evolution of hydrogen leading to record electron efficiency as high as 70%.

Graphical abstract: Mechanism of galvanic reduction of selenate oxyanions and surface immobilization by nano zero-valent iron aggregates under anaerobic conditions: towards high electron efficiency

Supplementary files

Article information

Article type
Paper
Submitted
29 Apr. 2022
Accepted
22 Jūl. 2022
First published
22 Jūl. 2022

Environ. Sci.: Water Res. Technol., 2022,8, 1910-1922

Mechanism of galvanic reduction of selenate oxyanions and surface immobilization by nano zero-valent iron aggregates under anaerobic conditions: towards high electron efficiency

K. Chalastara and G. P. Demopoulos, Environ. Sci.: Water Res. Technol., 2022, 8, 1910 DOI: 10.1039/D2EW00321J

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements