Subscriber access provided by University of Sussex Library
Article 2+
Susceptibility of goethite to Fe -catalyzed recrystallization over time Prachi Joshi, Matthew Fantle, Philip Larese-Casanova, and Christopher A. Gorski Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02603 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 35
Environmental Science & Technology
Susceptibility of goethite to Fe2+-catalyzed recrystallization over time Prachi Joshi,† Matthew S. Fantle,‡ Philip Larese-Casanova,¶ and Christopher A. Gorski∗,† †Department of Civil & Environmental Engineering, Pennsylvania State University, 212 1
Sackett Building, University Park, PA 16802 ‡Department of Geosciences, Pennsylvania State University, 212 Deike Building, University Park, PA 16802 ¶Department of Civil & Environmental Engineering, Northeastern University, Snell Engineering Center, 360 Huntington Avenue, Boston, MA 02115 E-mail:
[email protected] Phone: 814-865-5673. Fax: 814-863-7304
2
Abstract
3
Recent work has shown that iron oxides, such as goethite and hematite, may recrystal-
4
lize in the presence of aqueous Fe2+ under anoxic conditions. This process, referred to
5
as Fe2+ -catalyzed recrystallization, can influence water quality by causing the incor-
6
poration/release of environmental contaminants and biological nutrients. Accounting
7
for the effects of Fe2+ -catalyzed recrystallization on water quality requires knowing
8
the time scale over which recrystallization occurs. Here, we tested the hypothesis that
9
nanoparticulate goethite becomes less susceptible to Fe2+ -catalyzed recrystallization
10
over time. We set up two batches of reactors in which
11
different time points, and tracked the
55
55
Fe2+ tracer was added at two
Fe partitioning in the aqueous and goethite
1
ACS Paragon Plus Environment
Environmental Science & Technology
55
12
phases over 60 days. Less
Fe uptake occurred between 30 and 60 days than between
13
0 and 30 days, suggesting goethite recrystallization slowed with time. Fitting the data
14
with a box model indicated that 15% of the goethite recrystallized after 30 days of
15
reaction, and an additional 2% recrystallized between 30 and 60 days. The decreasing
16
susceptibility of goethite to recrystallize as it reacted with aqueous Fe2+ suggested that
17
recrystallization is likely only an important process over short time scales.
18
Introduction
19
Iron oxides and hydroxides are ubiquitous in natural systems, where they can act as sinks for
20
and sources of nutrients, metals, and environmental contaminants. 1–13 Iron oxides also serve
21
as geological proxies, as their isotopic and elemental compositions are used to reconstruct
22
the conditions under which they formed and subsequently weathered. 14–23 Recently, several
23
studies reported evidence of a new process by which iron oxides can interact with dissolved
24
species in aqueous environments. Iron oxides, such as goethite and hematite, may recrystal-
25
lize in anoxic aqueous systems in the presence of Fe2+ , as evidenced by rapid and extensive
26
changes in their Fe and O isotopic compositions and/or the structural incorporation or release
27
of trace elements. 24–36 This process, commonly referred to as Fe2+ -catalyzed recrystalliza-
28
tion, has been observed under apparent chemical equilibrium conditions that do not result
29
in overt changes to the solution chemistry 24,26–28,32,33,36 or the mineral structure, 26,32,33,36
30
morphology, 26,32,33 or dissolution behavior. 32,33 Since Fe2+ -catalyzed recrystallization can af-
31
fect the fate and mobility of micro nutrients (e.g., Zn and Ni) 28,31,37–40 and contaminants
32
(e.g., radionuclides) 41 as well as the isotopic compositions of iron oxides, it has important
33
implications for groundwater quality, contaminant remediation, biogeochemical cycling, and
34
paleoenvironmental reconstructions. Accounting for Fe2+ -catalyzed recrystallization in these
35
research areas is currently difficult, as it remains unclear how the susceptibility of iron oxides
36
to recrystallization changes over time.
37
Constraining the susceptibility of iron oxides to Fe2+ -catalyzed recrystallization is not 2
ACS Paragon Plus Environment
Page 2 of 35
Page 3 of 35
Environmental Science & Technology
38
straightforward due to conflicting observations and difficulties in modeling data. In a typi-
39
cal experiment, an iron oxide is exposed to an aqueous solution containing dissolved Fe2+ .
40
Initially, either the solid or aqueous Fe phase is enriched with an isotopic tracer (typically
41
55
Fe or
57
Fe) and isotopic mixing between the aqueous and solid phases is measured over
42
time. 24,26–29,31–33,36 Prior studies have observed that (1) isotopic mixing stopped within the
43
experimental period (400) in four samples: (i) unreacted goethite (ii) goethite reacted with aqueous
404
Fe2+ for 30 days, (iii) goethite reacted with aqueous Fe2+ for 60 days, and (iv) goethite in
405
Fe2+ -free buffer for 60 days. The results were consistent with our previous work that looked
406
at goethite particles over 30 days, 36 and therefore we only summarize the key findings here
407
(more details can be found in SI Section S3). In all four samples, the goethite consisted
408
of acicular nanoparticles. The particles became significantly wider between Day 0 (mean
409
width = 7.9±0.2 nm, ±standard error) and Day 60 (mean width = 9.1±0.1 nm). The length
410
of the particles was statistically similar at Day 0 (mean length = 60.3±1.4 nm) and Day
411
30 (mean length = 56.6±1.2 nm), but slightly lower at Day 60 (mean length = 51.3±1.3
412
nm). Overall, the anisotropic changes in the dimensions of the nanoparticles suggested that
413
recrystallization may have occurred through preferential growth on the edges (leading to the
414
increase in width) coupled to preferential dissolution from the tips.
415
We used mediated potentiometry to identify if ∆G0f,goethite changed over the course of 60
416
days when reacted with aqueous Fe2+ using conditions identical to those used for the
417
tracer experiments (details in SI Section S6). In an initial set of experiments, we determined
418
the ∆G0f,goethite of the original goethite by measuring reduction potential (EH ) values of
419
goethite and aqueous Fe2+ suspensions at pH 6.0 and 7.5 as a function of aqueous Fe2+
20
ACS Paragon Plus Environment
55
Fe
Page 21 of 35
420
Environmental Science & Technology
activity (SI Section S6.4). The relevant half reaction is: 55 α−FeOOH(s) + e− + 3 H+ ⇀ ↽ Fe2+ (aq) + 2 H2 O
(6)
421
The measured EH values depend on solution pH and aqueous Fe2+ activity, as described by
422
the following Nernst equation (SI Section S6.4): EH = EH0 −
RT RT ln{Fe2+ ln{H+ } (aq) } + 3 F F
(7)
423
where R is the ideal gas constant, T is absolute temperature, F is the Faraday constant, EH0
424
is the standard reduction potential, and curly brackets denote activity. We fit the data by
425
0 floating the EH0 and the slopes of the {Fe2+ aq } activity and pH. The best fit resulted in an EH
426
value of 897 mV, a slope of –53.9 for the Fe2+ aq term, and a slope of –183.8 for the pH term
427
(χ2 =94.2, SI Section S6.4). The slope values for Fe2+ aq and pH are in good agreement with
428
◦ the theoretical values (-59 mV for the Fe2+ aq term and –177 mV for the pH term at 25 C),
429
consistent with our previous work on micron-scale goethite. 55 The deviations between the
430
measured and theoretical slopes were most likely due to experimental error; therefore, we
431
fit the data assuming using the theoretical slopes, yielding a EH0 value of 861±1 mV for the
432
original nanoparticulate goethite and a ∆G0f,goethite value of –481.2±0.1 kJ/mol (SI Section
433
S6.5).
434
The ∆G0f,goethite of nanoparticulate goethite in this study was more positive than reported
435
values for micron-scale goethite (–490.6±1.5 kJ/mol, 69 –490.2±0.1 kJ/mol 55 ), which was
436
consistent with the fact that nanoparticles have higher surface energies. 69–72 We calculated
437
a theoretical ∆G0f,goethite that accounted for surface enthalpy using reference values from
438
the literature, 69 which yielded a value of –485.9±1.7 kJ/mol based on the BET measured
439
specific surface area and a value of –475.4±1.6 kJ/mol based on TEM estimated specific
440
surface area (SI Section S6.7). The relatively good agreement between the theoretical and
441
measured ∆G0f,goethite values coupled with the ability of the Nernst equation to describe the 21
ACS Paragon Plus Environment
Environmental Science & Technology
442
EH data suggested that the goethite-Fe2+ suspensions were at a metastable equilibrium state.
443
We then measured how the ∆G0f,goethite changed over the course of the 60 day experiment
444
to identify if recrystallization induced any detectable thermodynamic changes. The measured
445
∆G0f,goethite on Day 1 (–481.1±0.1 kJ/mol) and Day 60 (–480.9±0.2 kJ/mol) were the same
446
within error, but the ∆G0f,goethite at intermediate time points slightly differed (
