Issue 10, 2020

Nanoscale observations of Fe(ii)-induced ferrihydrite transformation

Abstract

Because of its sorption properties, transformation of the nanomineral ferrihydrite (Fh) into more stable lepidocrocite (Lp) or goethite (Gt) has important impacts on the fate of metals, nutrients, and contaminants in soils/sediments. Although it is well known that the transformation rate is greatly accelerated under suboxic conditions by aqueous Fe(II), the enabling mass transfer process remains an ongoing debate among various mechanisms including dissolution/reprecipitation, solid-state recrystallization, and particle-mediated growth. Here, using electron microscopy, we examine the mineralogical evolution of 2-line Fh to Lp/Gt catalyzed by Fe(II) under strict anoxic conditions, including evaluation of Cl–SO4–HCO3 anion effects. Emergence of Lp/Gt crystallites at the nanoscale was observed at ∼20 min of reaction, earlier than previously reported. Lp is the first phase to nucleate in Cl-rich solutions without HCO3; whereas Lp and Gt concomitantly nucleate in SO42−-rich solutions, and also when co-solute HCO3 is added. Lp crystallites nucleate as quasi-2D nanosheets one-unit-cell thick that contour the Fh surface; in contrast, rod-shaped (in Cl/SO4) or acicular needle-shaped (in HCO3) Gt crystals nucleate and grow radially outward from the Fh aggregates. Stages of transformation monitored by in situ μ-XRD coupled with aqueous Fe(II) uptake/release measurements are correlated with a short initial sorption stage followed by the onset of Lp/Gt growth that then progresses to Lp loss in favor of Gt. Microscopy data overwhelmingly support dissolution/reprecipitation as the underlying mechanism, including direct evidence for classical ion-by-ion Lp/Gt growth and Lp dissolution. The collective findings imply that the iron mass transfer through solution to distal Lp/Gt growth fronts is a critical enabling process facilitating rapid transformation.

Graphical abstract: Nanoscale observations of Fe(ii)-induced ferrihydrite transformation

Supplementary files

Article information

Article type
Paper
Submitted
14 Jul 2020
Accepted
14 Aug 2020
First published
20 Aug 2020

Environ. Sci.: Nano, 2020,7, 2953-2967

Nanoscale observations of Fe(II)-induced ferrihydrite transformation

O. Qafoku, L. Kovarik, M. E. Bowden, E. Nakouzi, A. Sheng, J. Liu, C. I. Pearce and K. M. Rosso, Environ. Sci.: Nano, 2020, 7, 2953 DOI: 10.1039/D0EN00730G

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