Investigation of U(VI) Adsorption in QuartzâChlorite Mineral Mixtures

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Investigation of U(VI) Adsorption in Quartz-Chlorite Mineral Mixtures Zheming Wang, John M Zachara, Jianying Shang, Choong Jeon, Juan Liu, and Chongxuan Liu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 14 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

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Environmental Science & Technology

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Investigation of U(VI) Adsorption in Quartz-Chlorite Mineral Mixtures Zheming Wang,* John M. Zachara, Jianying Shang, Choong Jeon†, Juan Liu and Chongxuan Liu

6

Pacific Northwest National Laboratory, Richland, WA 99352

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*

Corresponding author: Pacific Northwest National Laboratory, P.O. Box 999, MS K8-96, Richland, WA 99354; Phone: (509) 371-6349; Fax: (509) 371-6354; E-mail: [email protected] † Present Address: Department of Environmental & Applied Chemical Engineering, Gangneung-Wonju National University, Gangwon-do, Republic of Korea.

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ABSTRACT

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A batch and cryogenic laser-induced time-resolved luminescence spectroscopy

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investigation of U(VI) adsorbed on quartz-chlorite mixtures with variable mass ratios

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have been performed under field-relevant uranium concentrations (5x10-7 M and 5x10-6

43

M) in pH 8.1 synthetic groundwater. The U(VI) adsorption Kd values steadily increased

44

as the mass fraction of chlorite increased, indicating preferential sorption to chlorite. For

45

all mineral mixtures, U(VI) adsorption Kd values were lower than that calculated from the

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assumption of component additivity possibly caused by surface modifications stemming

47

from chlorite dissolution; The largest deviation occurred when the mass fractions of the

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two minerals were equal. U(VI) adsorbed on quartz and chlorite displayed characteristic

49

individual luminescence spectra that were not affected by mineral mixing. The spectra of

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U(VI) adsorbed within the mixtures could be simulated by one surface U(VI) species on

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quartz and two on chlorite. The luminescence intensity decreased in a nonlinear manner

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as the adsorbed U(VI) concentration increased with increasing chlorite mass fraction –

53

likely due to ill-defined luminescence quenching by both structural Fe/Cr in chlorite, and

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trace amounts of solubilized and re-precipitated Fe/Cr in the aqueous phase. However,

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the fractional spectral intensities of U(VI) adsorbed on quartz and chlorite followed the

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same trend of fractional adsorbed U(VI) concentration in each mineral phase with

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approximate linear correlations, offering a method to estimate of U(VI) concentration

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distribution between the mineral components with luminescence spectroscopy.


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INTRODUCTION

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The adsorption of uranium to soil minerals and subsurface sediments has been

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extensively investigated to resolve issues of environmental contamination and to support

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site selection of nuclear waste repositories.1-4 Under most environmental conditions,

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uranium exists as the hexavalent uranyl ion, UO22+. Uranyl ion adsorbs to a broad range

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of mineral phases including quartz,5, 6 feldspar,7 calcium carbonates,8, 9 iron oxides,10, 11

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and phyllosilicates.12,

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strongly on solution pH and counter anions.

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precipitate in the form of uranyl phosphate, carbonates, and oxyhydroxides mineral

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phases.14-16

13

Adsorption affinities to these phases are variable, depending At higher concentrations, uranyl may

69

Metal ions may exhibit distributed adsorption in mineral assemblages and natural

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sediments. For example, Stubbs et al. identified six mineral hosts for sorbed uranium in

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Hanford vadose zone sediment based on electron microprobe and TEM analysis.17 The

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overall adsorptivity of a composite mineral material is influenced by the adsorption

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characteristics of the contributing mineral phases and their mass fractions, and by

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interactions between the adsorbing mineral phases.18

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The description of metal ion adsorption on mineral mixtures and sediments relies

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heavily on surface complexation modeling to account for aqueous chemical and surface

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effects. Two common surface complexation modeling approaches are the generalized

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composite model (GCM) and the component additivity model (CAM).19-22 In the former,

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generic surface sites are assumed for the entire mineral assemblage. Site density and

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possible surface complexes with fitted stability constants are applied under different

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adsorption conditions to determine the likely adsorption mechanism(s) based on the



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goodness of fit. In the latter, adsorption is described as the summation of that occurring

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on individual mineral phases,19 as computed from individual surface complexation

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constants and the abundance of the mineral components in the assemblage.

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Regardless of the surface complexation modeling approach, understanding the

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adsorption process in natural materials is advanced by direct identification of surface

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complex types and their concentrations. These speciation measurements are challenging

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due to sensitivity issues and the multi-component nature of the surface complexation

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process. In a previous study, we applied time-resolved U(VI) luminescence spectroscopy

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and chemometric methods to determine the chemical forms of adsorbed U(VI) in a

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contaminated subsurface sediment.23 As U(VI) adsorbed to constituent mineral phases

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displayed characteristic luminescence spectra, we were able to determine that sediment

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adsorbed U(VI) existed as surface complexes on quartz and phyllosilicates. However, it

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was unclear if sorbent interactions influenced the luminescence spectral behavior, and if

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the spectral intensity could be used to estimate the concentrations of specific surface

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complex type. Here, we investigated U(VI) adsorption on mixtures of quartz and chlorite

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to simulate the reactive fraction of a subsurface sediment. Sorbent mass ratios were

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varied and the adsorption complex was examined by cryogenic luminescence

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spectroscopy to determine how luminescence spectral behavior evolved with changing

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mineral mass ratios in the mixtures. We assess whether it is possible to estimate the

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concentration of U(VI) on the individual phases in the mixtures based on the collective

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luminescence response.

103 104 105

EXPERIMENTAL


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Materials

Quartz (Min-U-Sil 30, U.S. Silica Company, Mill Creek,

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Oklahoma) was pretreated to remove organic contaminants and reactive fines following

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the procedures of Kohler et al24 and rinsed with DI water until the supernatant

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conductivity was 18 MΩ cm or lower. The material was then oven-dried at 40 °C for 3−4

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days. Specific surface area was determined by the N2 BET method (Quantachrome

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Autosorb 6-B gas adsorption system) to be 1.63 m2/g.

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[(Mg2.95Al0.05)(Si2.99Al1.01)O10(OH)2.(Mg1.97Al0.66 Cr0.25Fe3+0.06Fe2+0.06) (OH)6;25] was

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hand ground to yield 20-53 µm fraction with a BET surface area of 2.6 m2/g. The

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properties of smaller size fractions of the same mineral specimen (2 to 5 µm and 5 – 20

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µm) have been previously reported.23 Calcite-saturated synthetic groundwater (SGW2;

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pH 8.1) was prepared according to Bond et al.26 A stock solution of uranium nitrate (1.06

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x 10-3 M, pH 1) was prepared by dissolving a weighed amount of uranyl nitrate (Fluka) in

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0.1 M nitric acid. All other chemicals were reagent grade.

California chlinochlore

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U(VI) Adsorption Two identical series of eleven quartz-chlorite mixtures were

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prepared by mixing weighed amounts of the solids in 15 ml polypropylene centrifuge

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tubes to quartz:chlorite mass ratios of 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9 and

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0:10. The total mineral weight was 0.2 g. The sorbents were twice equilibrated with

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SGW2 for 24 h and separated from the supernatants by centrifugation. The resulting

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solids were then re-suspended in 10 ml of SGW2 and spiked with the U(VI) stock

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solution to yield 5x10-6 M U(VI) in one series and 5x10-7 M U(VI) in the other. The

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suspension pH was maintained at pH 8.1 with dilute NaOH. The U(VI)-bearing mineral

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suspensions were placed on an orbital shaker and slowly agitated for 48 h to allow U(VI)

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adsorption equilibrium.

After 48 h, the solid and solution were separated by



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centrifugation. Both the solid paste and the supernatant were sampled into 2 mm x 4 mm

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x 25 mm (ID) quartz cuvettes for luminescence analysis, while the remainder of the

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supernatants was acidified for U(VI) (KPA), Mg, Al, Si and Fe analysis (ICP/MS).

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Aqueous U(VI) speciation calculation The speciation of U(VI) in SGW2 at pH

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8.1 under ambient conditions was calculated using the MINTEQA227 software with the

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most current, critically reviewed thermodynamic stability constants for the U(VI)

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complexes.28

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Cryogenic Laser Luminescence Spectroscopy

The instrumentation and

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experimental procedures for luminescence spectroscopic measurements at LHeT were

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described previously.15, 29 The quartz sample cuvettes were attached to the cold finger of

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a CRYO Industries RC152 cryostat with liquid helium vaporizing beneath the sample to

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reach a sample temperature of 8 ± 2 ̊K. The sample was excited with a Spectra-Physics

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Nd:YAG laser pumped Lasertechnik-GWU MOPO laser at 415 nm and the emitted light

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was collected at 85˚ to the excitation beam and detected with a thermoelectrically cooled

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Princeton Instruments PIMAX intensified CCD camera after spectral dispersion through

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an Acton SpectroPro 300i double monochromator spectrograph.

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analyzed using a commercial software, IGOR®, from Wavematrix, Inc. All spectra were

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corrected for the mass ratio-weighted background emission observed for quartz and

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chlorite prepared under the same conditions as those of the U(VI)-bearing samples except

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that no U(VI) was added.

149 150

RESULTS AND DISCUSSION


The spectra were

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Adsorption of U(VI) on quartz-chlorite mixtures The magnitude of U(VI)

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adsorption on the quartz-chlorite mixtures varied with the mineral mass ratio and U(VI)

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concentration (Table 1 and Figure 1). The highest adsorption was seen on pure chlorite

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and the lowest on pure quartz. At a U(VI) concentration of 5×10-6 M, U(VI) adsorption

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was 36.2% on quartz and 84.9% on chlorite, corresponding to adsorption Kd values of

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28.3 ml/g and 281 ml/g, respectively. Lowering the U(VI) concentration to 5×10-7 M

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increased U(VI) adsorption to 46.7% on quartz and 92.9% on chlorite, corresponding to

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Kd values of 43.7 and 870 ml/g, respectively. These measured Kd values were consistent

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with ones measured previously,23 when differences in surface areas are taken into

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account.

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As a result of the disparate U(VI) adsorption on the two phases, the total adsorbed

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U(VI) concentration, CU,Ads, increased with the mass fraction of chlorite (Table 1 and SI

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Figure 1). The concentration of adsorbed U(VI) contributed by quartz and chlorite in the

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solid mixtures at each initial U(VI) concentration were calculated through the following

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relationship, f Q × K d ,Q f Q × K d ,Q + f CC × K d , CC

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CU,Q = CU,Ads ×

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CU,CC = CU,Ads - CU,Q

(1)

(2)

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Where CU,Q and CU,CC were adsorbed U(VI) concentrations contributed by quartz and

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chlorite in the mineral mixture. Kd,Q and Kd,CC were the U(VI) adsorption Kd values on

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pure quartz and chlorite at the corresponding initial U(VI) concentration, and fQ and fCC

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were the respective mass fractions. The fractional concentration contribution to the

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adsorbed U(VI) by quartz (FU,Q) and chlorite (FU,CC) in the mixtures were defined as: 7 ACS Paragon Plus Environment


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FU,Q = CU,Q/CU,Ads and FU,CC = CU,CC/CU,Ads

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(3)

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Reflecting the lower affinity of quartz as compared to chlorite, the calculated U(VI)

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concentration contributions from quartz were consistently lower in all quartz-chlorite

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mixtures (SI Figure 2). For all mineral mixtures in which fCC > 0.5, the fractional

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concentration contribution of quartz to adsorbed U(VI) in the mineral mixtures was ~ 5%

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or less (Figure 2).

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With the component additivity model, the distribution coefficient for adsorbed

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U(VI) in the quartz-chlorite mixtures equals the mass-weighted sum of the adsorption Kd

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values on the individual sorbents,18, 30, 31

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Kd, mixture = fQ Kd, Q + fCC Kd, CC

(4)

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Accordingly, the calculated Kd values displayed straight lines as a function of chlorite

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mass fraction at both U(VI) concentrations (dashed lines in Figure 1). The calculated Kd

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values were all above the measured ones for the mineral mixtures, and the largest

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differences were observed when both minerals were at equal mass. Such deviations

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suggest that either there is interaction between the quartz and chlorite, or that surface

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modifications occurred as a result of mineral mixing and co-equilibration. Apparently,

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the component additivity approach is not valid for quantitatively modeling U(VI)

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adsorption in such mixtures without properly taking into account the causes for the

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deviation.

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Previous studies on the applicability of adsorption additivity for metal ion

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adsorption in binary mineral mixtures18, 32, 33 revealed dependence on the specific metal

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ion and sorbent identity. The adsorption of both Cs+ and Sr2+ followed the additivity rule

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on bentonite-sand and silica-montmorillonite mixtures.18 However, significant negative 8 ACS Paragon Plus Environment

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deviations from adsorption additivity were observed for Sr2+ adsorption on alumina-illite

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mixtures, while large positive deviations were observed for Sr2+ adsorption on alumina-

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montmorillonite mixtures.18 The dissolution of alumina and the formation of unidentified

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Sr-bearing precipitates were speculated to cause the noted effect.

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Anderson and Benjamin systematically investigated the adsorption of Ag+, Cd2+,

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Zn2+ and several inorganic anions on Fe(OH)3, Al(OH)3, and Fe(OH)3-Al(OH)3 mixtures

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and co-precipitates under various chemical conditions.32,

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caused small enhancements in Zn2+ adsorption by Fe(OH)3 but inhibited the adsorption of

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Ag+ and Cd2+. Constant-capacitance surface complexation modeling suggested that a

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majority of the Fe(OH)3 surface sites were covered and replaced by Al(OH)3 upon

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mixing, leading to reduced metal ion adsorption. Surface and bulk analyses clearly

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indicated that particle interactions in the mixtures, including dissolution and precipitation,

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resulting in changes in particle size distribution, surface charge, and surface site

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distribution.

33

The presence of Al(OH)3

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SEM images of the post-reaction quartz-chlorite mixture (SI Figure 3) revealed

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that the individual phases generally remained separate except that fine-grained quartz

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associated with larger chlorite platelets.

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aggregation of the sorbents was an unlikely cause of the observed lower Kd values, which

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in some samples was as much as 50% as compared to the pure phases (Figure 1). We

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hypothesize that the decrease of Kd values for the quartz-chlorite mixtures was due to

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trace amounts of the dissolved solids, particularly the brucite layer of chlorite, releasing

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Mg2+, Al3+, Cr3+, Fe3+ and Fe2+.

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concentration in the presence of chlorite was between 17 µM to 48 µM higher than that in

Thus, surface area loss due to physical

As indicated by the results of ICP analysis, Mg



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SGW2 and increases along with the mass fraction of chlorite (SI Figure 4), equivalent to

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between 0.01% to 0.04% chlorite dissolution.

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stoichiometric concentrations of other metal ions are also released into the solution.

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Some of the leached metal ions may occupy surface sites that would otherwise be

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available for uranyl adsorption, especially on quartz which is mostly negatively charged

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at the present pH. As will be shown in the next section, a major surface U(VI) complex

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on chlorite involved the negatively charged uranyl-tricarbonate anion complex,

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UO2(CO3)34-. Therefore, the loss of highly-charged cation sites on chlorite would also

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reduce U(VI) sorption on chlorite. In addition, the attachment of negatively charged fine

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quartz particle might also adversely impact the adsorption U(VI) on some of the

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positively charged surface sites on chlorite.

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If the dissolution is congruent,

Luminescence spectroscopy of adsorbed U(VI) The luminescence spectra of

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U(VI) adsorbed on the quartz-chlorite mixtures displayed characteristic features (Figure

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3). The vibronic band maxima were located at 499.7 nm, 520.4 nm and 541.3 nm for

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U(VI) adsorbed on quartz at an initial U(VI) concentration of 5x10-6 M, while those for

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U(VI) on chlorite were at 480.3 nm, 500.8 nm, 521.4 nm and 543.2 nm (Figure 3A and SI

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Table 1). These results were in good agreement with previously reported values.23 The

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uranyl ion formed inner-sphere surface complexes with deprotonated silanol sites on

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quartz, while the U(VI) spectra on chlorite resembled those of uranyl-

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tricarbonate/dicarbonate or calcium-uranyl-tricarbonate complexes.29, 34, 35 The spectral

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features of U(VI) adsorbed on quartz decreased and eventually disappeared as the mass

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fraction of chlorite (fCC) increased. The spectral pattern for chlorite, defined here as UCa,

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became dominant over fCC = 0.5.


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The spectrum of U(VI) adsorbed on quartz was unchanged at 5x10-7 M U(VI) as

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compared with that at 5x10-7 M U(VI) (Figure 3B). However, the spectra of U(VI)

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adsorbed on chlorite developed more than one spectral pattern at this concentration. The

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spectrum was identical to UCa when fCC < 0.5 while a broad, poorly resolved spectra

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with an emission maximum of 521.7 nm (SI Table 1), defined here as pattern UCb,

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became dominantt at fCC > 0.5 (Figure 3B lower traces), indicating the presence of

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another U(VI) surface complex on chlorite.

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U(VI) luminescence spectra were also recorded for the supernatants and for

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SGW2 that was spiked with U(VI) nitrate (data not shown). Consistent with previous

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measurements, U(VI) in the supernatants showed luminescence spectra that closely

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resembled those of Ca2UO2(CO3)3(aq) and CaUO2(CO3)32- complexes as expected from

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aqueous speciation calculation. The same type of spectra were displayed on the time-

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resolved spectra of both of the wet paste samples of U(VI) adsorbed on quartz and

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chlorite at 5x10-6 M U(VI) at long delay times, consistent with the presumption that the

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wet paste samples entrained a small fraction of the supernatant. Such spectra were absent

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in samples with large fCC values at 5x10-7 M U(VI) as a results of nearly complete U(VI)

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adsorption.

259

Time-resolved luminescence spectra at delay times up to 4 ms showed that while

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there was a single dominant species of U(VI) adsorbed on quartz (data not shown), both

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U(VI) surface species on chlorite represented by the UCa and UCb spectral patterns were

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present at 5x10-6 M U(VI), as expected, and UCb became more pronounced at longer

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delay times (SI Figure 5). For all the samples with U(VI) adsorbed on quartz-chlorite

264

mixtures, the time-resolved spectra reflected the variation of the relative intensities of the



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three surface U(VI) species as governed by their luminescence lifetimes. For example,

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for U(VI) adsorbed on quartz-chlorite mixture with fCC = 0.4 at 5x10-6 M U(VI), the

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spectra were primarily composed of those of U(VI) adsorbed on quartz and on chlorite

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with a spectral pattern UCa at shorter delay times (Figure 4A). As the delay time

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increased, the relative intensity of UCa decreased gradually as indicated by the slight

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drop of intensity in the 480.3 nm band where the spectra of U(VI) adsorbed on quartz

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displayed only weak emission, pointing to increased character of U(VI) adsorbed on

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quartz. At the same time, the relative intensity in the region from ~ 510 nm to 550 nm

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was noticeably elevated, a clear indication of the growing contribution of the species

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UCb.

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spectral pattern UCa had a shorter luminescence lifetime than the U(VI) surface species

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on quartz and the U(VI) species with a spectral pattern UCb had the longest luminescence

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lifetime.

Such spectral changes implied that the surface U(VI) species corresponding to

278

As shown for the sample with fCC = 0.3, the time-resolved emission spectra at

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5x10-7 M U(VI), again, indicated that at shorter delay times, the spectra represented both

280

U(VI) adsorbed on quartz and chlorite (UCa), and the latter decreased as delay time

281

increased to 0.7 ms based on the gradual drop of the intensity of the 480.3 nm band

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(Figure 4B). As delay time further increased to 2.8 ms and 4.0 ms the spectral character

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of U(VI) adsorbed on quartz also disappeared and the spectra became similar to those

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observed at large fCC values (≥ 0.6) with dominant spectral pattern UCb at the same

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U(VI) concentration, confirming the presence of two U(VI) surface complexes on

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chlorite: one with spectral pattern UCa at lower fCC values and shorter delay times, and

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the other with spectral pattern UCb that was present primarily at higher fCC values and


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longer delay times at low U(VI) concentration (i.e. 5x10-7 M)(Figures 3B, 4B). Thus the

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second surface U(VI) species on chlorite (UCb) likely formed on a subset of surface sites

290

with the highest U(VI) affinities. The site density for surface U(VI) complex with

291

spectral pattern UCa is much higher and thus this complex dominated at higher U(VI)

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concentrations. The UCb spectrum was red-shifted by over 20 nm as compared to UCa,

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which reflected the formation of strong, inner-sphere surface complex, consistent with

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U(VI) adsorption at sites of higher U(VI) affinity.

295

Further spectral analyses verified that the luminescence spectra of adsorbed U(VI)

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on all of the quartz-chlorite mixtures could be simulated by a linear combination of the

297

three species described above: one U(VI) surface complex on quartz and two on chlorite

298

with spectral profiles of UCa and UCb (Figure 5 as an example).

299

characteristics of U(VI) surface complexes on the individual phases were not affected by

300

mixing of the mineral components.

Therefore, the

301

Luminescence decay analysis indicated that the decay curves of U(VI) adsorbed

302

on chlorite could be fit by two exponential functions, resulting in luminescence lifetimes

303

of 155 ± 52 µs and 1328 ± 396 µs, respectively, consistent with the assignment to the

304

U(VI) species with spectral patterns of UCa and UCb. Unexpectedly, fit of the decay

305

curves of U(VI) adsorbed on quartz also required two exponential functions with

306

luminescence lifetime of 796 ± 142 µs and 233 ± 135 µs, respectively (SI Table 1). This

307

could be due to many reasons, such as two U(VI) surface complexes on quartz with

308

similar profiles, same surface species with different hydration waters or surface cite

309

inhomogeneity 2, 36, 37. The trend of the measured luminescence lifetimes were consistent

310

with that expected from the variations of the time-resolved spectra (Figure 4). However,



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it should be cautioned that large deviations were observed in the measured luminescence

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lifetimes data, possibly due to sample heterogeneity, and luminescence decay fitting with

313

multiple exponential functions are prone to errors.

314

Variation of U(VI) spectral intensities with mineral compositions Countering

315

the trend of the increasing Kd values (Table 1) and adsorbed U(VI) concentration with

316

increasing fCC values (SI Figure 1), the overall spectral intensity decreased with

317

increasing fCC (Figure 6). The sharpest intensity reduction occurred at fCC ≤ 0.2. The

318

spectral area for U(VI) adsorbed on quartz (fCC = 0) was 461 times larger than U(VI)

319

adsorbed on chlorite (fCC = 1) at 5x10-7 M U(VI), and 775 times larger at 5x10-6 M

320

U(VI). Given that the adsorbed U(VI) concentration on chlorite was more than twice that

321

on quartz, these intensity values indicated that that the luminescence quantum yield of

322

U(VI) adsorbed on quartz was about three orders of magnitude larger than that on

323

chlorite, assuming that the luminescence spectra of U(VI) adsorbed on quartz and chlorite

324

have similar molar absorptivity.

325

Evaluation of both the deconvoluted luminescence spectra and the corresponding

326

normalization factors indicated that the sharp intensity drop from fCC = 0 to fCC = 0.2 was

327

more significant for U(VI) adsorbed on quartz. The subsequent intensity drop at fCC > 0.2

328

applied to U(VI) adsorbed on both quartz and chlorite (SI Figure 6).

329

concentration of 5×10-7 M, the concentration of U(VI) adsorbed on quartz decreased from

330

1.17×10-8 to 0.32×10-8 mole/g from fCC =0 to fCC = 0.2. However, the corresponding

331

luminescence spectral area dropped from 1550 to 32 based on the results of spectral

332

deconvolution. Such large disproportionality between the emission intensity and the

333

concentration of adsorbed U(VI) indicated that there was pronounced luminescence


At a U(VI)

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quenching,38, 39 and/or self-absorption of the emitted light by Fe and Cr in chlorite.40, 41 If

335

self-absorption were the primary factor, it would be expected that adsorbed U(VI) on

336

quartz and chlorite would behave equally and that the decrease of luminescence intensity

337

would be proportional to fCC. Neither of these expectations was observed (Figure 6 and

338

SI Figure 6). Thus, the luminescence decrease with increasing fCC was likely caused by

339

luminescence quenching by Fe, Cr and other transition metal impurities in the chlorite.

340

The exact quenching mechanisms of adsorbed U(VI) luminescence by the mineral

341

mixtures was not determined. The chlorite contained both Fe (1.2%) and Cr (2.3%). The

342

total available Fe and Cr concentrations (3.8x10-3 M Fe and 8.7x10-3 M Cr) were three or

343

four orders of magnitude higher than that of U(VI) (5x10-6 M or 5x10-7 M, respectively)

344

at the given the solid-solution ratios. Assuming congruent dissolution of chlorite, the

345

dissolved Cr3+, Fe3+, and Fe2+ concentrations would be in the ranges of 0.86x10-6 M –

346

2.44x10-6 M (Cr3+) and 0.21x10-6 M – 0.59x10-6 M (both Fe2+ and Fe3+), respectively,

347

based on the measured Mg concentrations.

348

dissolved oxygen concentration could be as high as 1.4x10-3 M,42 and thus the solubilized

349

Fe2+ would be oxidized to Fe3+ by oxygen. The low Fe2+ concentration and the presence

350

of dissolved oxygen would also minimize any reduction of uranyl ion by Fe2+ in the

351

present system, such reductions had been observed with Fe-rich chlorite under anaerobic

352

conditions.43 Limited by low solubility, both Fe3+ and Cr3+ ions could exist as surface-

353

sorbed species, precipitated clusters, or ferrihydrite-like nano-particulates,44, 45 leading to

354

enhanced luminescence quenching. Indeed, trace Fe was observed in association with

355

both chlorite and quartz in the EDS elemental map of the U-bearing quartz-chlorite

Considering the aerobic conditions, the



Page 16 of 33

356

mixtures (SI Figure 3). It is also possible that exposed structural and surface-adsorbed Fe

357

were active U(VI) adsorption sites in the mineral mixtures.43, 44

358

Comparison of Figures 2 and 3 showed that the increasing spectral characters of

359

U(VI) adsorbed on chlorite with increasing fCC values on the normalized luminescence

360

spectra followed the same trend as that of the fractional concentration of adsorbed U(VI)

361

in the mineral mixtures. In order to quantify the correlations between the fractional

362

U(VI) concentration in each mineral phase in the solid mixture with the corresponding

363

fractional intensity in the measured spectra, the fractional spectral intensity of U(VI)

364

adsorbed on quartz and chlorite, fS,Q and fS,CC, were calculated at all fCC values as follows,

365

f S ,Q =

AQ AQ + AUCa + AUCb

and

f S ,CC =

AUCa + AUCb AQ + AUCa + AUCb

(5)

366

where AQ, AUCa and AUCb are the integrated areas under the corresponding deconvoluted

367

luminescence spectra of U(VI) associated with quartz and chlorite (both UCa and UCb).

368

Even though the calculations employed nontrivial approximations in the absence of

369

precise determination of the quantum yields of the surface complexes, plots of the

370

calculated fractional spectral intensity as a function of fCC (Figure 7) displayed

371

approximate resemblance to the fractional adsorbed U(VI) concentrations on quartz and

372

chlorite (Figure 2).

373

function of the spectral fractions of U(VI) adsorbed on quartz and chlorite (SI Figure 7)

374

only displayed qualitative linear correlations in the fCC value range of 0.2 – 1.0 at both

375

U(VI) concentrations, and the concentration fractions of U(VI) adsorbed on quartz were

376

underestimated while those on chlorite were overestimated in all the mixtures. The latter

377

inferred that the observed negative deviation of the measured Kd values from the

378

calculated ones based on component additivity (Figure 1) had a larger contribution from

However, correlation plots of the concentration fractions as a


Page 17 of 33


379

decreased U(VI) adsorption on chlorite in the mixtures. By assuming that the deviation

380

was solely due to decreased U(VI) adsorption on chlorite, the Kd contributions from

381

quartz and chlorite, Kd,Q’ and Kd,CC’ were calculated from their mass fractions as the

382

following,

383

Kd,Q’ = Kd,Q × fQ

(6)

384

Kd,CC’ = Kd - Kd,Q’

(7)

385

The resulting Kd,Q’ and Kd,CC’ values were used to calculate the concentration

386

contributions and fractional concentrations of U(VI) adsorbed on quartz and chlorite in

387

the mixtures using Eqs. (1) – (3).

388

Plots of the calculated concentration fractions as a function of fCC (SI Figiure 8)

389

displayed improved resemblance to those of the spectral fractions (Figure 7) and

390

correlation plots of the concentration fractions as a function of the spectral fractions

391

became linear across the entire fCC range with correlation coefficients of 0.95 and 0.92 at

392

U(VI) concentrations of 5 × 10-6 M and 5 × 10-7 M, respectively (SI Figure 9). These

393

results indicated that mixing quartz and chlorite primarily affected U(VI) adsorption on

394

chlorite with little impact on U(VI) adsorption on quartz even though larger U(VI)

395

luminescence quenching was observed on the latter. The approximate linear correlations

396

between the concentration fractions and the spectral fractions suggest that it is possible to

397

estimate U(VI) concentration in individual mineral phases based on the mineralogical

398

composition, total U(VI) concentration and the measured luminescence spectra, thus

399

offering credible empirical inputs for model simulation of U(VI) adsorption in mineral

400

mixtures when precise measurement is not feasible.



Page 18 of 33

401

Implications Quartz is the most abundant mineral phase among the fine size

402

fraction (< 125 µm) vadose zone sediments at both the US DOE Hanford Site and

403

Naturita Site.

404

abundance, possess high surface area and strong affinities to U(VI). Their common

405

association with larger crystallites as surface coatings further highlight their role in U(VI)

406

adsorption in subsurface sediments. The present results indicate that mixing quartz and

407

chlorite leads to deviation of U(VI) adsorption properties (e.g. Kd) from what is expected

408

from component additivity model, particularly for more reactive minerals such as

409

chlorite. This implies that for natural sediments, which are aged mineral mixtures,

410

adequate corrections taking into consideration of possible alteration of the mineral

411

constituents are required to simulate U(VI) adsorption in the mineral mixture based

412

adsorption parameters obtained on the individual mineralogical components.

413

invariant spectral profiles of the primary U(VI) surface species and approximate linear

414

correlations between the fractional U(VI) concentration and spectra in the binary mineral

415

mixtures, on the other hand, suggest that with precise knowledge of the mineralogical

416

composition and overall U(VI) concentration, it is possible to predict U(VI) distribution

417

empirically in such mineral mixtures using U(VI) luminescence spectroscopy

418

measurements.

Chlorite, along with other phyllosilicates, though present with low

The

419 420

ASSOCIATED CONTENT

421

Supporting Information

422

Additional information as noted in the text. This material is available free of charge via

423

the Internet at http://pubs.acs.org.


Page 19 of 33

424 425


ACKNOWLEDGEMENTS

426

The authors thank Dr. James Amonette for providing the chlorite mineral

427

specimen and Mr. Bruce Arey for SEM and EDS sample analysis. The authors are also

428

indebted to four anonymous reviewers whose insightful comments greatly helped to

429

improve the quality of the manuscript. This project was supported by the Hanford site

430

SFA Program managed by the U.S. DOE Office of Biological and Environmental

431

Research (OBER). Part of this research was performed at EMSL, a national scientific

432

user facility at PNNL managed by the Department of Energy’s Office of Biological and

433

Environmental Research. Pacific Northwest National Laboratory is operated for the U.S.

434

Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.

435



436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462

Table 1. Experimental data for U(VI) adsorption on quartz (Qtz)-chlorite (Chl) mixtures in pH 8.1 synthetic groundwater at 25°C. Total mineral weight = 0.2 g. Solution volume: 10 ml. Sample Qtz:Chl Surface [U]total U UAds Kd U Adsorption No. Ratio Area Adsorbed Concentration Density (m2) (M) (%) (10-8 Mole/g) (ml/g) (Mole/m2·108) 36.2 9.04 28.3 5.55 1 10:0 0.326 5.0·10-6 2. 9:1 0.346 5.0·10-6 35.6 8.90 27.6 5.15 -6 41.0 10.3 34.8 5.62 3 8:2 0.364 5.0·10 4 7:3 0.384 5.0·10-6 53.2 13.3 56.9 6.93 57.5 14.4 67.8 7.13 5 6:4 0.404 5.0·10-6 6 5:5 0.424 5.0·10-6 62.2 15.6 82.3 7.35 7 4:6 0.442 5.0·10-6 72.5 18.1 132 8.19 -6 8 3:7 0.462 5.0·10 76.1 19.0 160 8.24 80.2 20.0 202 8.33 9 2:8 0.482 5.0·10-6 10 1:9 0.500 5.0·10-6 83.7 20.9 258 8.36 11 0:10 0.520 5.0·10-6 84.9 21.2 281 8.16 -7 12 10:0 0.326 5.0·10 46.7 1.17 43.7 0.72 13 9:1 0.346 5.0·10-7 69.5 1.74 114 1.01 75.1 1.88 150 1.03 14 8:2 0.364 5.0·10-7 15 7:3 0.384 5.0·10-7 82.6 2.07 237 1.08 -7 16 6:4 0.404 5.0·10 85.8 2.14 301 1.06 17 5:5 0.424 5.0·10-7 88.8 2.22 397 1.05 18 4:6 0.442 5.0·10-7 90.4 2.26 470 1.02 -7 91.8 2.30 562 0.99 19 3:7 0.462 5.0·10 20 2:8 0.482 5.0·10-7 93.0 2.32 663 0.97 -7 21 1:9 0.500 5.0·10 94.1 2.35 793 0.94 94.6 2.36 870 0.91 22 0:10 0.520 5.0·10-7

20

ACS Paragon Plus Environment

Page 20 of 33

Page 21 of 33

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508


Figure Captions Figure 1.

Plot of measured Kd values of U(VI) adsorption in quartz-chlorite mixtures as a function of the mass fraction of chlorite in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (filled circle) and 5·10-7 M (open circle), respectively. Solid lines are polynomial fit of the measured Kd values. Dashed lines are calculated Kd values based on the component additivity model.

Figure 2.

Simulated fractional contribution to the concentration of adsorbed U(VI) by quartz (open circle) and chlorite (filled circle) in the mineral mixtures as a function of the mass fraction of chlorite in quartz:chlorite mixtures in pH 8.1 synthetic groundwater. Total U(VI) concentrations are 5·10-6 M (A) and 5·10-7 M (B), respectively.

Figure 3.

LHeT Luminescence spectra of U(VI) adsorbed on quartz:chlorite mixtures in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively. All spectra were normalized to the same maximum intensity and offset along the Y-axis for clarity. λex = 415 nm.

Figure 4.

Time-resolved LHeT Luminescence spectra of U(VI) adsorbed on quartz:chlorite mixtures in synthetic groundwater, pH 8.1 at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively. In A, fCC = 0.4. In B, fCC = 0.3. The delay times are: a) 0.0 ms; b) 0.2 ms; c) 0.7 ms; d) 1.1 ms; e) 2.8 ms; and f) 4.0 ms. All spectra were normalized to the same maximum intensity and were offset along the Y-axis for clarity. λex = 415 nm.

Figure 5.

Deconvoluted LHeT luminescence spectra (circles) of U(VI) adsorbed on quartz:chlorite mixtures in pH 8.1 synthetic groundwater, pH 8.1 at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively, as the combination of U(VI) adsorbed on quartz (blue) and chlorite (green and purple). In A: fCC = 0.6. In B: fCC = 0.3. Both measured spectra were normalized to the same maximum intensity. λex = 415 nm.

Figure 6.

Luminescence intensity of U(VI) adsorbed on quartz-chlorite mixtures as a function of the mass fraction of chlorite in the mineral mixtures in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (filled circle) and 5·10-7 M (open circle), respectively. The solid and dashed lines are fitted trend lines of the intensity data.

Figure 7.

Simulated spectral fractions of U(VI) adsorbed on quartz (filled circle) and chlorite (open circle) in the mineral mixtures as a function of the mass fraction of chlorite in pH 8.1 synthetic groundwater at U(VI) concentrations of 5·10-6 M (A) and 5·10-7 M (B), respectively.

21



509

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(28) Guillaumount, R., Fanghänel, T., Neck, V., Fuger, J., Palmer, D.A., Grenthe, I., and Rand, M.H., Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium. 2003, Amsterdam: Elsevier. (29) Wang, Z., Zachara, J.M., Yantasee, W., Gassman, P.L., Liu, C.X., and Joly, A.G. Cryogenic laser induced fluorescence characterization of U(VI) in Hanford vadose zone pore waters. Environ. Sci. Technol. 2004, 38(21), 5591-5597. (30) Jacquier, P., Meier, P., and Ly, J. Adsorption of radioelements on mixtures of minerals - experimental study. Appl. Geochem. 2001, 16(1), 85-93. (31) Tao, Z.Y. and Dong, W.M. Additivity rule and its application to the sorption of radionuclides on soils. Radiochim. Acta 2003, 91(5), 299-303. (32) Anderson, P.R. and Benjamin, M.M. Modeling adsorption in aluminum-iron binary oxide suspensions. Environ. Sci. Technol. 1990, 24(10), 1586-1592. (33) Anderson, P.R. and Benjamin, M.M. Surface and bulk characteristics of binary oxide suspensions. Environ. Sci. Technol. 1990, 24(5), 692-698. (34) Ilton, E.S., Wang, Z., Boily, J.-F., Qafoku, O., Rosso, K.M., and Smith, S.C. The Effect of pH and Time on the Extractability and Speciation of Uranium(VI) Sorbed to SiO2. Environ. Sci. Technol. 2012, 46(12), 6604-6611. (35) Wang, Z., Zachara, J.M., Liu, C., Gassman, P.L., Felmy, A.R., and Clark, S.B. A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals. Radiochim. Acta 2008, 96(9-11), 591-598. (36) Del Nero, M., Galindo, C., Barillon, R., and Made, B. TRLFS Evidence for Precipitation of Uranyl Phosphate on the Surface of Alumina: Environmental Implications. Environ. Sci. Technol. 2011, 45(9), 3982-3988. (37) Gabriel, U., Charlet, L., Schlapfer, C.W., Vial, J.C., Brachmann, A., and Gabriel, G. Uranyl surface speciation in silica particles studied by time-resolved laserinduced fluorescence spectroscopy. J. Colloid Interf. Sci. 2001, 239(2), 358-368. (38) Bodenant, B., Fages, F., and Delville, M.H. Metal-induced self-assembly of a pyrene-tethered hydroxamate ligand for the generation of multichromophoric supramolecular systems. The pyrene excimer as switch for iron(III)-driven intramolecular fluorescence quenching. J. Am. Chem. Soc. 1998, 120(30), 75117519. (39) Varnes, A.W., Dodson, R.B., and Wehry, E.L. Interactions of transition-metal ions with photoexcited states of flavines. Fluorescence quenching studies. J. Am. Chem. Soc. 1972, 94(3), 946-950. (40) Maksimovic, Z., White, J.L., and Logar, M. Chromium-bearing dickite and chromium-bearing kaolinite from Teslic, Yugoslavia. Clay Clay Miner. 1981, 29(3), 213-218. (41) White, W.B. and Keester, K.L. Optical absorption spectra of iron in rock-forming silicates. Am. Mineral. 1966, 51(5-6), 774-790. (42) Lide, D.R., ed. CRC Handbook of Chemistry and Physics. 72nd ed. 1991, CRC Press: Boston. (43) Singer, D.M., Maher, K., and Brown Jr, G.E. Uranyl-chlorite sorption/desorption: Evaluation of different U(VI) sequestration processes. Geochim. Cosmochim. Acta 2009, 73(20), 5989-6007.

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(44) Arnold, T., Zorn, T., Zanker, H., Bernhard, G., and Nitsche, H. Sorption behavior of U(VI) on phyllite: experiments and modeling. J. Contam. Hydrol. 2001, 47(2-4), 219-231. (45) Banfield, J.F. and Murakami, T. Atomic-resolution transmission electron microscope evidence for the mechanism by which chlorite weathers to 1 : 1 semiregular chlorite-vermiculite. Am. Mineral. 1998, 83(3-4), 348-357.

25



Page 26 of 33

1000

Kd(ml/g)

800

600

400

200

0 0.0

0.5

1.0

fCC

650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673

Figure 1

26


674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715


fSolid U Conc

Page 27 of 33

1.0

1.0

0.8

0.8

0.6

0.6

A

0.4 0.2

0.2

0.0

0.0 0.0

0.2

0.4

0.6

0.8

B

0.4

1.0

0.0

0.2

fCC

0.4

0.6

0.8

1.0

fCC

Figure 2

27



6

A

6

B FCC =

FCC = 0.0

5

Relative Intensity

716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760

Page 28 of 33

0.0

5 0.2

0.2

4

4 0.4

0.4

3

3 0.6

0.6

2

2 0.8

0.8

1

1 1.0

1.0

0

0 500

550

600

Wavelength (nm)

500

550

600

Wavelength (nm)

Figure 3

28


Page 29 of 33

6

A

6

B a

a 5

Relative Intensity

761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805


5

b

b 4

4

c

c 3

3

d

d 2

2

e

e 1

1

f

f 0

0 500

550

600

Wavelength (nm)

500

550

Wavelength (nm)

Figure 4

29


600


1.0

Intensity

806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851

A

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 500

550

600

Wavelength (nm)

Page 30 of 33

B

500

550

Wavelength (nm)

Figure 5

30


600

Page 31 of 33


Log(Peak Area)

4

3

2

1

0 0.0

852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877

0.2

0.4

0.6

0.8

1.0

fCC

Figure 6

31



1.0

fNormalized Spectra

878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923

1.0

A

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0.0

0.5

1.0

Page 32 of 33

B

0.0

0.2

fWeight of Chlorite

0.4

0.6

0.8

1.0

fWeight of Chlorite

Figure 7

32


Page 33 of 33

TOC Figure

1.0

0.8

U in Chlorite

0.6 0.4

U in Quartz

0.2 0.0

Fractional U(VI) Concentration

1.0

Fractional Normalized Spectra

924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943


U in Chlorite 0.8 0.6 0.4 0.2

U in Quartz

0.0 0.0

0.5 1.0 Chlorite Mass Fraction

33

0.0


0.5 1.0 Chlorite Mass Fraction

Investigation of U(VI) Adsorption in QuartzâChlorite Mineral Mixtures

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