U(VI) sorption during ferrihydrite formation: Underpinning radioactive effluent treatment

Ellen H. Winstanleya, Katherine Morrisa, Liam G. Abrahamsen Millsb, Richard Blackhamc, Samuel

Shawa*

aResearch Centre for Radwaste Disposal and Williamson Research Centre, School of Earth and

Environmental Sciences, The University of Manchester, M13 9PL, UK. bNational Nuclear Laboratory,

Chadwick House, Warrington, WA3 6AE, UK. cSellafield Ltd., Hinton House, Birchwood Park Avenue,

Risley, Warrington, Cheshire, WA3 6GR, UK.

*Corresponding Author: Email: sam.shaw@manchester.ac.uk; Phone: 0161 275 3826; Address:

Williamson Building, School of Earth and Environmental Sciences, The University of Manchester,

Manchester M13 9PL, UK

Abbreviations: EARP, Enhanced Actinide Removal Plant; FTIR, Fourier Transform Infrared; XAS, X-Ray Absorption Spectroscopy; EXAFS, Extended X Ray Absorption Fine Structure; ICP-MS, Inductively Coupled Mass Spectrometry; XRD, Powder X Ray Diffraction; ATEM, Analytical Transmission Electron Microscopy; EDX, Energy Dispersive X-ray Spectroscopy; DLM, Diffuse Layer Model; SIT, Specific Ion Interaction Theory; NEA, Nuclear Energy Agency; SAED, Selected Area Electron Diffraction

Abstract

Iron (oxyhydr)oxide nanoparticles are known to sorb metals, including radionuclides, from solution in

various environmental and industrial systems. Effluent treatment processes including the Enhanced

Actinide Removal Plant (EARP) (Sellafield, UK) use a neutralisation process to induce the

precipitation of iron (oxyhydr)oxides to remove radionuclides from solution. There is a paucity of

information on mechanism(s) of U(VI) removal under conditions relevant to such industrial processes.

Here, we investigated removal of U(VI) from simulated effluents containing 7.16 mM Fe(III) with

4.2x10-4-1.05 mM U(VI), during the base induced hydrolysis of Fe(III). The solid product was

ferrihydrite under all conditions. Acid dissolutions, Fourier Transform infrared spectroscopy and

thermodynamic modelling indicated that U(VI) was removed from solution by adsorption to the

ferrihydrite. The sorption mechanism was supported by X-ray Absorption Spectroscopy which showed

U(VI) was adsorbed to ferrihydrite via a bidentate edge-sharing inner-sphere species with carbonate

forming a ternary surface complex. At concentrations ≤0.42 mM U(VI) was removed entirely via

adsorption, however at 1.05 mM U(VI) there was also evidence for precipitation of a discrete U(VI)

phase. Overall these results confirm that U(VI) sequestered via adsorption to ferrihydrite over a

concentration range from 4.2x10-4-0.42mM confirming a remarkably consistent removal mechanism in

this industrially relevant system.

Keywords: Uranium; 2-line ferrihydrite; Adsorption; Spectroscopy, Co-precipitation

1. Introduction

Precipitation of metal (oxyhydr)oxide nanoparticles is utilised in various industries to sequester

dissolved contaminants from aqueous wastes, as well as to remediate contaminated ground waters in

environmental systems [1–8]. Iron (oxyhydr)oxides such as nanoparticulate ferrihydrite

(Fe5OH8•4H2O) and goethite (α-FeOOH) have high surface areas (a maximum of 700m2 g-1 and

200 m2 g-1 respectively) and reactivity, and therefore are commonly used in waste water treatment to

remove various metals including radionuclides such as lead, arsenic or uranium from solution by

processes such as coprecipitation or adsorption [4,5,9–14]. In addition, iron (oxyhydr)oxide minerals

are ubiquitous in subsurface sediments and soils, and impose a major control on the speciation and

mobility of trace metals within contaminated groundwater via various sorption processes [4,9,15]. In

order to further develop the use and efficiency of effluent treatment processes utilising iron

(oxyhydr)oxide precipitation to remove dissolved radionuclides from solution, a fundamental

understanding of the atomic and nanoscale mechanisms of radionuclide removal is required.

Various metal (oxyhydr)oxide coprecipitation processes have been developed to treat a range of low

to medium level radioactivity waste effluents produced at various stages of the nuclear fuel cycle (e.g.

nuclear reprocessing) [16,17]. Recent studies have investigated the precipitation of Fe(III) to

remediate effluents at the Fukushima Daiichi nuclear power plant (Japan) which was shown to

effectively lower or remove 54Mn, 58/60Co, 125Sb, 90Y, and 106Ru in waste water systems [8,18]. The

Enhanced Actinide Removal Plant (EARP, Sellafield, UK) is an important treatment facility for

aqueous, radioactive effluents from reprocessing operations, which decontaminates by inducing

Fe(III) precipitation [17]. The highly acidic waste effluents contain dissolved species including

radionuclides such as 235,238U, 241Am, 237Np, 239/240/241Pu and 137Cs / 90Sr as well as transition metals

including Fe(III) and various inactive species such as PO43- and SO4

2- [17]. During the EARP process

the effluents are neutralised by the controlled addition of sodium hydroxide, raising the pH over a

rapid timescale. This induces the precipitation of Fe(III) as an iron (oxyhydr)oxide floc during which

process radionuclides such as U, Am, Pu are sequestered from solution. Recent work has shown that

under conditions relevant to the EARP process the solid product of this rapid Fe(III) hydrolysis is 2-

line ferrihydrite, a poorly ordered nanoparticulate iron (oxyhydr)oxide phase [19]. Weatherill (2016)

characterised the formation of ferrihydrite during neutralisation of Fe(III) in 1M HNO3 during the EARP

process and found that the initially aqueous Fe(III) ions at pH 0.1 form Fe13 Keggin clusters between

pH 0.12 to 0.15. Above ~pH 1 these clusters aggregate to form linear structures which densify above

~pH 2 to form mass aggregates of ferrihydrite particles [19]. However, there is a paucity of information

at the molecular scale on the interactions of U(VI) and other radionuclides during Fe(III) hydrolysis

under conditions relevant to acidic effluent treatment facilities.

U(VI) can interact with iron (oxyhydr)oxide nanoparticles by mechanisms including surface adsorption,

structural incorporation, surface precipitation and occlusion [14,20–23]. The mechanism of U(VI)

adsorption to the surface of ferrihydrite has been studied via X-ray Adsorption Spectroscopy (XAS).

Here, in carbonate-free systems, U(VI) has been shown to adsorb to ferrihydrite via a bidentate, inner

sphere, edge sharing surface complex [24,25]. However, in equilibrium with atmospheric CO2(g), U(VI)

forms a ternary uranyl carbonate surface complex at pH ≥ 5 [24,26]. Iron (oxyhydr)oxides such as

hematite (α-Fe2O3), magnetite (Fe3O4) and goethite are known, under certain conditions, to sequester

U(VI) via direct substitution for Fe in the crystal structure [14,23,26–28]. Currently, there is no

evidence for U(VI) incorporation within ferrihydrite. Previous studies of Fe(III) and U(VI)

coprecipitation suggest that U(VI) adsorbs to the iron (oxyhydr)oxide precipitate surface via inner

sphere complexation with no evidence for U(VI) surface precipitation or structural incorporation

[24,26]. However, the removal of U(VI) during such coprecipitation processes has not been examined

under conditions relevant to industrial waste treatment plants such as EARP. Here, the large change

in pH (~pH 0.1 to pH 9), rapid timescale of neutralisation (tens of minutes) and wide range of U(VI)

concentrations relevant to current and future plant operations all have the potential to impact U(VI)

speciation and the mechanism of sequestration.

In the current study, the base induced hydrolysis of Fe(III) in the presence of aqueous U(VI) was

investigated to determine the fundamental mechanisms of U(VI) removal from solution under

conditions relevant to industrial effluent treatment plants such as EARP. Experimental data and

thermodynamic modelling approaches were used to elucidate the mechanism(s) of U(VI) removal

from solution over a wide range of U(VI) (aq) concentrations (4.2 x10-4 to 1.05 mM). Extended X-ray

Absorption Fine Structure (EXAFS) and Fourier Transform Infrared (FTIR) spectroscopy were used to

determine the local coordination environment of U(VI) associated with the solid phase and to

understand the impact of carbonate on U(VI) speciation. Overall, >0.42 mM U(VI) was removed from

solution by adsorption to precipitated ferrihydrite nanoparticles. The adsorbed U(VI) formed a

bidentate edge sharing surface complex with carbonate associated as a ternary surface complex.

2. Methods

Fe(III) and U(VI) coprecipitation experiments were performed at 35°C using simulated acidic effluent

consisting of 1 M HNO3 with 7.16 mM Fe(III) and between 4.2 x10-4 and 1.05 mM U(VI). This resulted

in approximately 0.012 to 30.3 weight % uranium associated with the ferrihydrite product

(Fe5OH8•4H2O). Two ‘control’ precipitation experiments were also performed, a ‘pure’ Fe(III) system

where no U(VI) was added to the effluent, in order to determine baseline Fe(III) hydrolysis

independent of the presence of U(VI) (referred to as the Fe(III) only ‘control’ system), and a ‘pure’

U(VI) system (0.42 mM) with no Fe(III) present, to determine the U(VI) baseline precipitation

behaviour (the U(VI) only ‘control’ system). Alkaline dosing was achieved using a small scale

automated reactor (Applikon MiniBio) following the method of Weatherill et al., (2016) [19] under

conditions representative of the EARP process. To neutralise the simulated effluent and initiate Fe(III)

precipitation 7 M NaOH was added at 1.5 mL/min to raise the pH to 2.3, then at 0.3 mL/min to pH 3

and finally 0.2 M NaOH was added at 1.5 mL/min to raise the pH to the endpoint of pH 9, over

approximately 45 minutes. Once the desired pH was reached, the effluent was left to stir under pH

control for a further 30 minutes, during which time up to 1.2 mL 0.2 M NaOH was required to maintain

the pH at 9. Throughout the neutralisation process, selected samples were taken for both aqueous

and solid phase analyses. An additional ‘surface adsorption’ experiment was also performed where

U(VI) (0.042 mM) was added directly into the experiment after neutralisation of the inactive effluent to

pH 9. The U(VI) was then allowed to equilibrate with the ferrihydrite floc at pH 9 for 30 minutes, to

create a system where U(VI) was removed from solution by adsorption only. To determine the

reversibility of U(VI) sorption to the ferrihydrite at pH 9, acidification experiments were performed

under controlled conditions in the small scale automated reactor [23,25,29]. Acidification was

achieved by the addition of 2 M HNO3 at a rate of 0.07 mL/min to lower the pH to 3.5 then a rate of

0.21 mL/min until pH 2 whereupon 5 M HNO3 was added at a rate of 0.7 mL/min to lower the pH to

0.2, the total acidification process taking ca. 45 minutes.

Solution samples were extracted throughout the neutralisation and acidification processes and were

filtered (<0.22 µm polyethersulfone filters). Filtered solution samples were then analysed using either

inductively coupled mass spectrometry (ICP-MS, Agilent 7500cx) for total U(VI) concentration or by

the ferrozine method to determine dissolved Fe concentrations [30]. Solution data were corrected for

dilution effects prior to plotting. The solid product at pH 5, pH 7 or pH 9 was isolated by centrifugation

then analysed using X Ray Diffraction (XRD) (Bruker D8 Advance), Analytical Transmission Electron

Microscopy (ATEM) (FEI Tecnai 20 or FEI Tecnai TF30), and Fourier Transform Infrared

spectroscopy (FTIR) (Perkin Elmer Frontier ATR FTIR spectrometer) (Supplementary Material). U(VI)

LIII edge EXAFS spectra were also collected on the solid products using beamlines B18 or I20 at the

Diamond Light Source Synchrotron [31]. Data were collected on beamline B18 in either fluorescence

or transmission mode and beamline I20 in fluorescence mode, using multi element Ge detectors. In

line yttrium foil reference spectra were collected to allow energy calibration for each sample.

Background subtraction, data normalization, and fitting of the EXAFS spectra was performed using

the software packages Athena and Artemis [32].

Thermodynamic modelling of the experiments was performed using PHREEQC version 3.3.5 with

thermodynamic data derived from the Dzombak and Morel diffuse layer model (DLM) and a modified

specific ion interaction theory (SIT) database [33,34]. The equilibrium constants for relevant aqueous

iron species were replaced with more recent constants recommended by the NEA [35]. Relevant

U(VI) and carbonate surface complexation reaction constants were also included [25,34–36]. The

surface area of the ferrihydrite solid was defined as 290 ± 15 m2 g−1 [19]. The model defined two types

of ferrihydrite surface site, a ‘strong’ surface site (site density of 0.005 mol/mol Fe) and a ‘weak’ site

(0.2 mol/mol Fe) [36]. To explore Fe(III) and U(VI) precipitation as distinct phases the model allowed

the precipitation of a ferrihydrite phase (Fe(OH)3) [37] and a metaschoepite phase (UO3:0.9H2O)

respectively. The model was equilibrated with carbonate at atmospheric pressure reflecting the

experimental system and EARP operations.

3. Results and Discussion

3.1. U(VI) sorption during ferrihydrite precipitation

Solution data from the ‘control’ Fe(III) only system showed that 100% of Fe(III) precipitated from

solution between pH 1.5 to pH 4 when U(VI) was not present (Fig. S1a). XRD of the solid product at

pH 9 confirmed the characteristic broad peaks of 2-line ferrihydrite (Fig. S2) [19]. The U(VI) only

‘control’ experiment showed that when there was no Fe(III) present, approximately 85% of U(VI)

precipitated from solution between pH 4.5 - 7 whilst the remaining 15% U(VI) remained in solution up

to pH 9 (Fig. S1b). XRD of the solid product at pH 9 confirmed the formation of metaschoepite (Fig.

S3). In the coprecipitation experiments containing both Fe(III) and U(VI), regardless of initial U(VI)

concentration, 100% of the Fe(III) was removed from solution between pH 1.5 to pH 4, mirroring the

Fe(III) only ‘control’ experiment (Fig. 1a). XRD confirmed the solid product was 2-line ferrihydrite

independent of the concentration of U(VI) present (Fig. S2). In the coprecipitation system, the U(VI)

removal profile from solution deviated from that of the ‘control’ U(VI) only experiment as 100% of

U(VI) was removed by pH 6. This shows that for the entire range of U(VI) concentrations studied,

coprecipitation is a more effective mechanism for U(VI) removal. In addition, the pH range of U(VI)

removal during the coprecipitation was between ~pH 3 and pH 6, suggesting that U(VI) did not

precipitate as a discrete phase when Fe(III) was present (Fig. 1a). Here, between 60 - 80% of Fe(III)

had precipitated by the point at which U(VI) removal started, indicating U(VI) was interacting with the

newly formed nanoparticulate ferrihydrite phase. In addition, the U(VI) concentration as a function of

pH mirrored previous work showing U(VI) uptake via adsorption to preformed ferrihydrite as a function

of pH, further suggesting that U(VI) sequestration occurred via surface adsorption [25]. TEM shows

that the solid phase consists of nanoparticulate 2-line ferrihydrite, which is supported by selected area

electron diffraction (SAED), and confirmed that U(VI) was associated with the ferrihydrite solid phase

at pH 9 (Fig. S6) showing an increase in intensity of the diagnostic Energy Dispersive X-ray (EDX)

spectroscopy uranium peaks as the experimental U(VI) concentration increased (Fig. S7). In the

coprecipitation experiments as the pH increased above 7 there was an increase in the concentration

of U(VI) in solution up to a maximum of approximately 11% by pH 9 (Fig. 1a). This release of U(VI)

under alkaline pH conditions has been observed by Waite et al., (1994) [25] and was attributed to the

formation of aqueous U(VI) carbonate complexes.

Figure 1: (a) Fe and U concentrations in solution (< 0.22 µm) during coprecipitation with 7.16 mM Fe

(red) and U(VI) (blue) at 4.2 x10-3 (●), 0.042 (▲), or 0.42 mM (♦). (b) Acid dissolutions of systems

containing 7.16 mM Fe (red) and U(VI) (blue) at 4.2 x10-3 (●), 0.042 (▲), or 0.42 mM (♦) , also showing

the 100% adsorbed system with 7.16 mM Fe(III) (red line) with 0.042 mM U(VI) (blue line). All data

are corrected for dilution during base/acid addition. Error bars show the relative standard deviation,

some error bars are concealed by symbols.

To determine the reversibility of U(VI) adsorbed to the ferrihydrite floc, acidification experiments were

conducted to determine the congruency of Fe and U dissolution from both the coprecipitation systems

containing Fe(III) and U(VI) (4.2 x10-3 to 0.42 mM) and the 100% surface adsorbed U(VI) system

(0.042 mM U(VI)). As the pH was lowered from 9, the dissolution/ desorption behaviour of the

coprecipitation systems was identical to that of the surface adsorbed U(VI) system (Fig. 1b). In all

systems as the pH was lowered from pH 9 to pH 3, between 70 - 80% of U(VI) was released into

solution whilst 100% of Fe(III) remained associated with the solid phase. By between pH 2 - 1.5,

100% of U(VI) had resolubilised along with ~80% of the Fe(III). The uniformity of the dissolution in

coprecipitation systems independent of the U(VI) concentration, and the consistency of results with

the 100% adsorbed U(VI) system indicated that U(VI) adsorbs to the ferrihydrite surface during

coprecipitation across the range of U(VI) concentrations studied.

3.2. PHREEQC modelling of U(VI) removal during ferrihydrite precipitation

Modelling of the coprecipitation system showed that during the base induced Fe(III) hydrolysis

process, Fe(III) is expected to be removed from solution via precipitation of ferrihydrite between pH

2.3 and pH 3.5, whilst U(VI) (≤0.42 mM) is predicted to be removed from solution between pH 3 and

pH 5.4 via adsorption to the surface of the newly formed ferrihydrite (Fig. 2, Fig. S4a). A comparison

of the experimental 0.042 mM U(VI) coprecipitation system with the equivalent thermodynamic model

shows a good model prediction of Fe(III) precipitation above pH 2.2 (Fig. 2) whereas between pH 0.1

and 2 the Fe(III) behaviour is not as well represented. The model predicts that ferrihydrite is

undersaturated until >pH 2.2, however experimental results show the Fe(III) concentration decreasing

between pH 0.1 and 2, possibly due to the formation of ferrihydrite particles because of local mixing

effects, explaining the difference between experimental and predicted precipitation profiles [19,38].

The experimentally derived data show U(VI) is removed from solution between pH 2.5 and pH 6 and

again the model predicts a similar removal between pH 3 and pH 5.4. Above pH 8 the model shows a

rise in aqueous U(VI) which is linked to the dissolution of atmospheric CO2 at high pH. The release of

U(VI) under these conditions is due to the formation of soluble and stable aqueous U(VI) carbonate

complexes such as UO2(CO3)3-4

(aq). This is in agreement with the experimental results and previous

literature (Fig. 1a, Fig. S4a,b) [25,39]. The amount of U(VI) predicted to desorb at pH 9 in the model is

much higher than seen experimentally, for example the experimental 0.42 mM U(VI) system releases

1.2% U(VI) whereas the modelled 0.42 mM U(VI) system predicts 30.9% of the U(VI) will release into

solution at pH 9. This difference is likely due to the rate of U(VI) desorption from ferrihydrite in the

experimental system compared to the model reaching equilibrium with atmospheric carbonate, neither

of which would be apparent during the rapid experimental process. Again this is a function of the rapid

base neutralisation during the experiment which was selected as it is relevant to plant conditions

within the EARP facility.

Thermodynamic modelling of the highest, 1.05 mM U(VI) coprecipitation system predicted that a

small, but significant, fraction (0.185 mM, ~18%) of U(VI) was expected to precipitate as

metaschoepite as the ferrihydrite surface binding sites became oversaturated with adsorbed U(VI).

Indeed, metaschoepite was the reaction product in the U(VI) only ‘control’ system indicating that this

is the likely product under conditions where the concentration of U(VI) is high enough to oversaturate

the ferrihydrite surface. Overall thermodynamic modelling predicted the removal of U(VI) from solution

over 4.2 x10-3 to 1.05 mM U(VI) predominantly by adsorption, under conditions reflecting the EARP

process. The minor variations between modelling and experimental results can be explained by the

rapid neutralisation process and the use of intrinsic constants in the model from different systems

previously described in the literature. Nonetheless, the thermodynamic modelling approach provides a

powerful predictive tool to define the fate of uranium in these dynamic systems.

Figure 2: Modelled data (black) showing 0.042 mM U(VI) (▲) and 7.16 mM Fe(III) (■) in solution

during the acid neutralisation process overlaid with experimental data 0.042 mM U(VI) (blue) and 7.16

mM Fe(III) (red). Error bars show the relative standard deviation for the experimental data.

3.3. U(VI) adsorption mechanism: X-ray Absorption and FTIR Spectroscopy

U LIII edge EXAFS spectra from coprecipitation samples across all experimental conditions were

remarkably consistent. Here for all samples, the modelled fits showed U(VI) with two axial oxygen

backscatterers (Oax) at 1.81 to 1.82 Å and 5 equatorial oxygen backscatterers, confirming a uranyl like

coordination environment (Table 1, Fig. 3). For pH 9 samples containing 0.042 mM or higher U(VI),

which had data over a k range >12, fitting showed the uranyl equatorial oxygens were split into two

shells with 3 and 2 oxygens backscatterers respectively, indicative of inner sphere adsorption of

uranyl [25,40]. For this split shell model, two of the equatorial oxygen atoms (Oeq1) were fitted to

distances of 2.44 – 2.52 Å, which are the oxygen which are bound to the surface of the ferrihydrite

[25,41]. The remaining equatorial oxygen backscatterers (Oeq2) were modelled between 2.31 – 2.37 Å

[40]. Additionally for pH 9 samples at concentrations ≥0.042mM U(VI), between 1.6 to 2.2 carbon

atoms were fitted at a distance of 2.89 to 2.93 Å, confirming the presence of ternary U(VI) carbonate

surface complexes adsorbed to the ferrihydrite surface [40,41]. Finally, it was possible to fit one Fe(III)

backscatterer at a distance of between 3.42 to 3.49 Å, consistent with the U(VI) forming a

monodentate edge sharing inner sphere surface complex, as previously observed with ferrihydrite

[25,42]. The Debye Waller factor for each Fe shell is relatively high, presumably due to the disordered

structure of the ferrihydrite surface, and is consistent with equivalent Debye Waller factors in the

literature [24,25,43–45].

Figure 3: (a) U LIII edge k3 weighted EXAFS spectra and (b) corresponding Fourier Transforms for the

EXAFS data, showing comparison of ferrihydrite with (A) 4.2 x10 -4, (B) 2.1 x10-3, (C) 4.2 x10-3, (D)

0.042, (E) 0.42 and (F) 1.05 mM U(VI) samples withdrawn at pH 9. Black lines are k 3 weighted data

and red lines are modelled fits. Fit parameters are shown in Table 1.

At the highest U(VI) loading of 1.05 mM U(VI) thermodynamic modelling suggested that 18% of the

U(VI) would be present as metaschoepite due to oversaturation of U(VI) at the ferrihydrite surface.

Interestingly, there was no evidence for this species in the EXAFS data, although the U-U shell of

metaschoepite is unlikely to be observed at these concentrations due to the poorly ordered nature of

this phase [46]. For pH 9 samples containing very low concentrations of U(VI) (4.2 x10 -4, 2.1 x10-3, 4.2

x10-3 mM U(VI)) the data were fitted over a restricted k-range of between 10 and 12 and therefore it

was not possible to split the equatorial oxygen shell within the constraints of fitting (Fig. 3). This led to

an increase in apparent disorder (i.e. increased Debye-Waller factors) of the Oeq shell, an effect which

has been reported in EXAFS fitting for similar systems [40]. Finally, solid samples withdrawn at pH 5

containing 4.2 x10-3, 0.042, or 0.42 mM U(VI) were also analysed using EXAFS. In each case the

modelled fit confirmed the presence of the edge sharing adsorbed surface complex, even under these

mildly acidic conditions. Again, due to the lower k range of between 10 and 12 for all pH 5 samples, it

was not possible to split the equatorial oxygen shell or to account for a possible carbonate shell within

the constraints of fitting (Table S1, Fig. S5). Here, the EXAFS data from a broad range of samples

containing between 4.2 x10-4 to 1.05 mM U withdrawn at pH 5 or pH 9 can all be fit with the same

model, showing U(VI) is removed from solution via adsorption to the surface of ferrihydrite as an inner

sphere, bidentate edge-sharing surface complex with carbonate present as a ternary associated

complex.

Table 1: Details of EXAFS fit parameters from U(VI) adsorbed to ferrihydrite labelled with initial U

concentration and final U weight % on the solid, samples withdrawn at an end point of pH 9.a

Sample Path CN R (Å) σ2 (Å2) ΔE0 (eV) SO2 Χν2 R

4.2 x10-4 mM U#

(0.012 wt% U)Oax 2 1.82(1) 0.0016(8) 6.1±2.1 0.95* 36.8 0.018Oeq 5 2.38(2) 0.0103(9)Fe 1 3.46(9) 0.0122(7)

2.1 x10-3 mM U(0.06 wt% U)

Oax 2 1.83(1) 0.0023(9) 8.6±2.2 0.997 110.7 0.012Oeq 5 2.39(3) 0.0110(9)Fe 1 3.45(4) 0.0047(2)

4.2 x10-3 mM U#

(0.12 wt% U)Oax 2 1.82(1) 0.0017(1) 7.3±1.4 0.933 112.9 0.015Oeq 5 2.38(2) 0.0104(4)Fe 1 3.42(3) 0.0056(2)

0.042 mM U#

(1.2 wt% U)Oax 2 1.82(1) 0.0022(4) 9.6±1.0 0.90* 104.2 0.004Oeq1 2 2.51(1) 0.0022(5)Oeq2 3 2.37(1) 0.0046(3)C 1.6 2.92(2) 0.0042(3)Fe 1 3.46(3) 0.0094(8)

0.42 mM U#

(12 wt% U)Oax 2 1.83(1) 0.0031(1) 10.3±2.0 1.00* 4248.9 0.017Oeq1 2 2.52(3) 0.0026(3)Oeq2 3 2.36(2) 0.0045(9)C 2.2 2.89(5) 0.0093(4)Fe 1 3.49(8) 0.0131(2)

1.05 mM U#

(30.3 wt% U)Oax 2 1.82(1) 0.0026(8) 8.3±1.8 0.949 1097.2 0.006Oeq1 2 2.5(3) 0.0040(3)Oeq2 3 2.35(2) 0.0058(4)C 2 2.93(5) 0.0122(1)Fe 1 3.49(8) 0.0156(6)

a CN denotes coordination number; R denotes atomic distance; σ2 denotes Debye−Waller factor; ΔE0

denotes the shift in energy from the calculated Fermi level; S02 denotes the amplitude factor; X2

denotes the reduced Χ square value; R denotes “goodness of fit”; Oax refers to axial oxygen

backscatterers, Oeq are equatorial oxygen. # indicates that the fit contains a forward through

absorber multiple scattering path in the axial O-U-O unit. *indicates a single fixed parameter. Numbers

in parentheses are the standard deviation on the last decimal place.

As well as EXAFS analyses, FTIR spectra were also taken on solid products from selected

experiments. Here, in the Fe(III) only ‘control’ the presence of ferrihydrite at pH 5 and 9 was indicated

in the FTIR spectra via a peak at ~900 cm -1 corresponding to the ferrihydrite δ(OH) bending vibration

(Fig.4a) [24]. Carbonate and nitrate surface species can be distinguished with characteristic peaks at

700 cm-1 (v2(CO32-)), 1060 cm-1 (v1(CO3

2-), v1(NO3-)), 1340 cm-1 (v4(CO3

2-), v4(NO3-)), 1475 cm-1

(v3(CO32-), v3(NO3

-)) [47–49]. The coprecipitation samples containing U(VI) (4.2 x10-3 to 1.05 mM) also

showed the ferrihydrite δ(OH) band at ~900 cm-1 along with the peaks for the corresponding carbonate

and nitrate surface species. As the concentration of U(VI) was increased, a sharp ν3(UO22+) peak

around 880 cm-1 became more prominent but was partially masked by the broad signal from

ferrihydrite at ~900 cm-1.

Figure 4: (a) FTIR spectra of the ‘pure’ Fe system at pH 5 (black) and pH 9 (green) and the 0.42 mM

U(VI) coprecipitated system at pH 5 (red) pH 7 (blue) and pH 9 (pink). Major peaks v 3(CO32-) ~1475

cm-1, δ-Fh(OH) ~900 cm-1 (dashed box) and ν3(UO22+) ~880 cm-1 are highlighted. (b) Difference

spectra from coprecipitation systems containing 4.2 x10-3 mM U(VI) (green), 0.042 mM U(VI) (red),

0.42mM U(VI) (blue) and 1.05 mM U(VI) (pink).

Difference spectra were created to remove the masking influence of the ferrihydrite δ(OH) peak from

the antisymmetric U(VI) stretch (ν3(UO22+)) (Fig. 4b) [24,50–52]. Foerstendorf (2012) stated that the

ν3(UO22+) vibration from an inner sphere uranyl surface complex resulted in a peak around 903 cm -1

whereas outer sphere U(VI) adsorption would be expected to produce a peak closer to 912 cm -1

[50,51]. Wazne et al., (2003) [52] found that the ν3(UO22+) peak shifted to lower wavenumbers as the

number of carbonate ligands increased, reaching 880 cm-1 at a carbonate concentration of 1.26 × 10-3

M. Difference spectra for the 0.042, 0.42 and 1.05 mM U(VI) systems at pH 9 clearly show the sharp

ν3(UO22+) band at 876, 879 and 888 cm-1, respectively, indicating U(VI) inner sphere sorption with

carbonate ligands, however the peak could not be distinguished in the 4.2 x10 -3 mM U(VI) sample,

presumably as it was below the detection limit for the (Fig. 4b). The difference spectrum for the

system containing Fe(III) with 1.05 mM U(VI) revealed a sharp peak at 929 cm -1 and a shoulder at 910

cm-1 (Fig. 4b). This is likely to be due to the presence of metaschoepite as precipitated from the

control U(VI) only system and described by Chukanov et al. (2016) [53] with distinct FTIR peaks at

910 and 958 cm-1. No U(VI) phase was observed from EXAFS analyses, but these additional FTIR

peaks indicate that in this system the U(VI) oversaturated the ferrihydrite surface sites and formed a

distinct U(VI) solid phase as predicted by thermodynamic modelling. FTIR analyses showed that at pH

9 the ν3(UO22+) peak was shifted to higher wavenumbers as [U(VI)] was increased, presumably due to

the corresponding increase of associated ternary carbonate ligands [52,54,55]. Also, the v3(CO32-)

contribution at pH 5 is much more intense for the 0.42 mM U(VI) system (1502 cm -1) than the ‘pure’

ferrihydrite sample (1494 cm-1), indicating that the presence of U(VI) causes an increase in carbonate

interactions with the ferrihydrite surface at pH 5, and indicating the presence of ternary uranyl

carbonate complexes (Fig. 4a) [51].

Conclusions

Here we demonstrate the effective removal of U(VI) over a wide concentration range (4.2 x10 -4 mM to

1.05 mM U(VI)) during base hydrolysis of synthetic acidic effluent containing Fe(III) and U(VI) under

previously unstudied conditions relevant to industrial effluent treatments, such as the EARP process.

Thermodynamic modelling, spectroscopic analysis and chemical data all showed that the mechanism

of U(VI) removal from solution was by U(VI) adsorption to ferrihydrite following the rapid Fe(III)

hydrolysis and precipitation as 2-line ferrihydrite. Uranium speciation was dominated by a bidentate,

edge sharing, inner sphere ternary carbonate surface complexes at all U(VI) concentrations between

pH 5 and 9. There was no evidence for incorporation or occlusion of U(VI) within the iron

(oxyhydr)oxide phase at any concentration. The use of Fe(III) hydrolysis to remediate EARP effluent

waste streams shows a consistent and effective mechanism of U(VI) removal. Importantly the U(VI)

removed from solution by adsorption is still labile, so perturbations in chemical conditions such as

changes in pH, carbonation, or the addition of competing ions may favour U(VI) desorption.

Conversely, with aging any adsorbed U(VI) could potentially become incorporated within the aged iron

(oxyhydr)oxide structure, which could be important when considering disposal of the solid floc waste

[23].

Supplementary data

Additional details about ATEM and FTIR sample preparation, as well as figures and tables showing

solution data, XRD, ATEM, and modelling results, and pH 5 EXAFS spectra and fit details are all

available as supporting information.

Acknowledgements

This work was co-funded by The University of Manchester and Sellafield Ltd. via the Effluents and

Decontamination Centre of Expertise. This work was also supported by Env Rad Net (ST/K001787/1

and ST/N002474/1). Diamond Light Source provided beamtime awards (SP9621-3, SP13559-3,

SP17243-1 and SP13559-4), and we thank Stephen Parry and Shusaku Hayama for beamline

assistance. We also thank Joshua Weatherill, Paul Lythgoe, John Waters, Roy Wogelius and Heath

Bagshaw for assistance during analyses.

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