Cr(III) adsorption by cluster formation on boehmite nanoplates in

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Energy and the Environment

Cr(III) adsorption by cluster formation on boehmite nanoplates in highly alkaline solution Wenwen Cui, Xin Zhang, Carolyn I. Pearce, Ying Chen, Shuai zhang, Wen Liu, Mark H. Engelhard, Libor Kovarik, Meirong Zong, Hailin Zhang, Eric D. Walter, Zihua Zhu, Steve Heald, Micah P Prange, James J. De Yoreo, Shili Zheng, Yi Zhang, Sue B. Clark, Ping Li, Zheming Wang, and Kevin M. Rosso Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02693 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Environmental Science & Technology

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Cr(III) adsorption by cluster formation on boehmite nanoplates in

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highly alkaline solution

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Wenwen Cui1,2,3, Xin Zhang1,*, Carolyn I. Pearce4, Ying Chen1, Shuai Zhang1,5, Wen

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Liu6, Mark H. Engelhard6, Libor Kovarik6, Meirong Zong1,7, Hailin Zhang1,2,3, Eric D.

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Walter6, Zihua Zhu6, Steve Heald8, Micah P. Prange1, James J. De Yoreo1,5, Shili

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Zheng2, Yi Zhang2, Sue B. Clark1,9, Ping Li2,*, Zheming Wang1,*, and Kevin M.

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Rosso1,*

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1 – Physical & Computational Science Directorate, Pacific Northwest National

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Laboratory, Richland, Washington 99354, USA

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2 –National Engineering Laboratory for Hydrometallurgical Cleaner Production

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Technology, Key Laboratory of Green Process and Engineering, Institute of Process

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Engineering, Chinese Academy of Sciences, Beijing, 100190, China

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3 – University of Chinese Academy of Sciences, Beijing, 100049, China

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4 – Energy & Environment Directorate, Pacific Northwest National Laboratory,

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Richland, WA, USA

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5 – Department of Materials Science and Engineering, University of Washington,

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Seattle, Washington 98195, USA

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6 – Environmental Molecular Sciences Laboratory, Pacific Northwest National

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Laboratory, Richland, Washington 99354, USA

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7 – School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 1

ACS Paragon Plus Environment


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Province, 210023.

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8 – Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA

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9 – The Voiland School of Chemical and Biological Engineering, Washington State

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University, Pullman, Washington, USA

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Abstract.

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The development of advanced functional nanomaterials for selective adsorption in

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complex chemical environments requires partner studies of binding mechanisms.

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Motivated by observations of selective Cr(III) adsorption on boehmite nanoplates (-

29

AlOOH) in highly caustic multicomponent solutions of nuclear tank waste, here we

30

unravel the adsorption mechanism in molecular detail. We examined Cr(III) adsorption

31

to synthetic boehmite nanoplates in sodium hydroxide solutions up to 3 M, using a

32

combination of XRD, Raman, XPS, STEM, EELS, HR-AFM, TOF-SIMS, Cr K-edge

33

XANES/EXAFS, and EPR. Adsorption isotherms and kinetics were successfully fit to

34

Langmuir and pseudo-second-order kinetic models, respectively, consistent with

35

monotonic uptake of Cr(OH)4- monomers until saturation coverage of approximately

36

half the aluminum surface site density. High resolution AFM revealed monolayer

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cluster self-assembly on the (010) basal surfaces with increasing Cr(III) loading,

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possessing a structural motif similar to guyanaite (β-CrOOH), stabilized by corner-

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sharing Cr-O-Cr bonds and attached to the surface with edge-sharing Cr-O-Al bonds.

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The selective uptake appears related to short-range surface templating effects, with

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bridging metal connections likely enabled by hydroxyl anion ligand exchange reactions

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at the surface. Such a cluster formation mechanism, which stops short of more laterally

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extensive heteroepitaxy, could be a metal uptake discrimination mechanism more

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prevalent than currently recognized.

45

INTRODUCTION

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Given the growing preponderance of functional nanomaterials intended for unique 3



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applications, it is increasingly important to fundamentally understand the basis for their

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selective interaction with other species in complex chemical environments1-6. One such

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category of interaction having diverse impacts is the adsorption of metals. For example,

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the persistence of toxic heavy metals in the environment poses major risks to

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ecosystems and human health7-11. Most nanomaterials of a given composition can now

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be specifically tailored in terms of particle size, shape, and surface functionalization,

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suggesting the potential for new metal-selective sorbents for the protection or

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remediation of water resources7, 12-15. Such a strategy offers flexibility in design and

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operation, better sustainability and cost-effectiveness relative to standard methods such

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as chemical precipitation16, 17, ion exchange18, 19, photocalysis20, membrane filtration7,

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21-24

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inexpensive, simple binary metal (oxyhydr)oxides, such as those of iron14,

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aluminum29-32, titanium7, 33, and magnesium7, 34 show great promise for this purpose.

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Their high density of surface OH groups and their high attainable surface area in

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nanomaterial forms enables a high selectivity and capacity for heavy metal adsorption35.

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In particular, nanosized boehmite (γ-AlOOH) is one sorbent of interest for a variety

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of selective metal uptake applications including Cr,36 V,32, 37 Hg,38 As,39 Pb,40 Cd,41 and

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even organic pollutants39, 42. Boehmite is a layered structure material crystallizing in

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the orthorhombic space group Cmcm and typically possessing a tabular

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pseudohexagonal or rhombic habit. The dominant basal (010) surface of layers contain

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alternating oxygen and hydroxyl groups that aggregate the layers via hydrogen bonding.

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The resulting platelet habit depends on the interplay between relatively stable (100) and

and use of electrochemical techniques24. Because they are chemically stabile and

4


25-28,

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(101) edge surfaces, with both edges present in hexagonal nanoplates, whereas only

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(101) edges are present on rhombic nanoplates43-45. Hence this structure type offers a

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variety of oxo and hydroxo functional groups for selective metal binding on both basal

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and edge surfaces.

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The fact that boehmite can tightly and selectively bind metals, even in extremely

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complex chemical environments, has no better demonstration than through its known

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chemical associations found in caustic nuclear waste. For example, at the U.S.

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Department of Energy’s Hanford site, where vast quantities of nuclear fuel reprocessing

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waste containing high concentrations of aluminum and sodium hydroxide are stored46-

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51,

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dictate its overall chemical behavior46, 47. In these caustic multicomponent solutions the

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adsorption mechanism is not well known, nor is the basis for its selectivity for Cr(III).

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Cr(III) has been proposed to adhere to the boehmite surface either as a nano-to-micron

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sized precipitate or as an adsorbed complex, the latter of which may or may not then

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ultimately lead to its incorporation as a substituent for Al in the boehmite structure47.

nanosized boehmite appears to selectively bind metals such as Cr(III) that then

84

Using this system as inspiration, the present work examines Cr(III) adsorption on

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nanoboehmite in concentrated sodium hydroxide solution, to help advance a more

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detailed understanding of selective uptake mechanisms in complex environments. In

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caustic sodium hydroxide both the surface structure of nanoboehmite and the speciation

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of Cr(III), beyond the expectedly dominant tetrahydroxyanion Cr(OH)4

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comprehensively known, although recent modeling work has begun to examine the

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structure at boehmite surfaces in concentrated electrolytes44. It is thus not clear whether 5


52-54,

are not


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Cr(III) adsorption at alkaline conditions will bear any mechanistic correspondence with

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the precedent already established for acidic to circumneutral pH regime. For example,

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using the iron oxyhydroxide goethite (-FeOOH), an X-ray absorption spectroscopic

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investigation of Cr(III) at pH 4 suggested the importance of inner-sphere surface

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complex formation and polymerization catalyzed by the oxide surface55. It was

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proposed that the surface structure acted as a template to precipitate Cr(III)

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oxyhydroxide as isostructural α-CrOOH, which at higher coverages converted to

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growth of the γ-CrOOH polymorph55. But the extent to which such a mechanism is also

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relevant at highly alkaline conditions is unknown. In particular, this study seeks to

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answer to the following two questions:

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(1)

What is the loading capacity and kinetics of Cr(III) adsorption on

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nanoboehmite in sodium hydroxide solutions?

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(2)

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is there a catalytic templating role of the nanoboehmite surface)?

Does Cr(III) adsorb as isolated ions, clusters or epitaxial overlayers (i.e.,

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To do so we examined the adsorption behavior of Cr(III) on ~38 nm boehmite

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nanocrystals of thickness ~6 nm in caustic sodium hydroxide solutions from pH 13 to

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3 M NaOH, the latter being similar to the alkalinity in Hanford nuclear waste.

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Macroscopic adsorption isotherms and kinetics were determined using inductively

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coupled plasma mass spectrometry (ICP-MS). Microscopic adsorption mechanisms

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were explored at the nanoscale using a combination of X-ray diffraction (XRD), Raman

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spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy

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(XAS), scanning transmission electron microscopy (STEM) with electron energy loss 6


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spectroscopy (EELS), high resolution-atomic force microscopy (HR-AFM), time-of-

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flight secondary ion mass spectrometry (TOF-SIMS) and electron paramagnetic

115

resonance (EPR).

116

The integrated findings reveal that the adsorption of Cr(III) on nanoboehmite at

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alkaline conditions initially proceeds via hydroxyl ligand exchange enabling inner-

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sphere binding of Cr(OH)4- monomers. At higher loadings when the Cr(III)

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concentration in solution was higher than 20 ppm these monomers polymerize into

120

clusters incipient to the guyanaite (β-CrOOH) structure. Hence, similar to observations

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at acidic conditions, at alkaline conditions the adsorption selectivity and capacity of

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nanoboehmite for Cr(III) appears linked to the catalytic effects of specific boehmite

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surfaces.

124 125

EXPERIMENTAL METHODS

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Chemicals and Materials

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Aluminum nitrate (Al(NO3)3·9H2O, ≥98%), sodium hydroxide (NaOH, ≥98%), and

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chromium nitrate (Cr(NO3)3·9H2O, ≥99%) were purchased from Sigma-Aldrich

129

Chemical Reagent Co. Ltd., USA. All chemicals were of analytical grade and used as

130

received without further treatment and purification. Deionized water was used

131

throughout the experiments.

132

Preparation of Boehmite Nanoplates

133

Boehmite nanoplates were synthesized by our previously reported hydrothermal 7



134

method.56 Briefly, a 0.25 M aluminum nitrate solution was prepared by dissolving an

135

weighed amount of Al(NO3)3·9H2O in deionized water under stirring at room

136

temperature, and titrated with 1 M NaOH to adjust the pH of the solution to ~10.0. After

137

continuous stirring for 1 h, gel-like Al(OH)3 precipitates formed. The solid was

138

separated from the suspension by centrifugation and washed with deionized water three

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times to remove all soluble salts.

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In a 100 ml Teflon liner, a weighed amount of the Al(OH)3 gel solids were dispersed

141

into deionized water and the suspension pH was adjusted to ~12 using NaOH solution.

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The concentration of gels (defined as the concentration of Al3+) was 0.5 M and the

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volume of the solution was 80 mL. The Teflon container was sealed into a Parr steel

144

vessel and then was heated in an electric oven with a rotation device (10 rpm) at 200

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oC

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deionized water three times and dried at 80 oC overnight, and then was characterized by

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XRD, Raman spectroscopy, scanning electron microscopy (SEM), and transmission

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electron microscopy (TEM) to ensure phase purity.

149

Cr(III) Adsorption Measurements

for 48 h. The resulting white product was recovered by centrifugation, washed with

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Batch Cr(III) ion adsorption experiments were performed in 50 mL centrifuge tubes.

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Cr(NO3)3·9H2O was dissolved in NaOH solution with different concentrations (0.1, 0.2,

152

0.4, 0.5, 0.8, 1, 2, 3 M) to investigate the effect of NaOH concentration on Cr(III)

153

adsorption on boehmite. Cr(III) solutions with concentrations (1, 5, 10, 20, 50, 100, 200

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mg·L-1) at pH of 13 were then prepared to measure the adsorption isotherm. Boehmite

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(0.01 g) was added into 10 mL Cr(III) solution of various concentrations and shaken at 8


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room temperature, with specific time intervals for measurement. After agitation, the

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supernatants were filtered through a syringe filter (0.45 m) and the residual Cr(III)

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concentration in the solution was determined by inductively coupled plasma - optical

159

emission spectrometry (ICP-OES). The solids of Cr(III)-adsorbed boehmite were

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washed with deionized water three times, dried at 80 oC and characterized to study the

161

mechanism of Cr(III) adsorption. The Cr(III) adsorbed boehmite samples with the

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initial Cr concentration of 50 and 200 mg·L-1 were defined as Cr-B-1 and Cr-B-2,

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respectively.

164

Solids Characterization

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X-ray diffraction (XRD)

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XRD patterns were recorded on a Philips X’pert Multi-Purpose Diffractometer (MPD)

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(PANAlytical, Almelo, The Netherlands) equipped with a fixed Cu anode operating at

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50 kV and 40 mA. XRD patterns were collected in the 5−80° 2θ range. Phase

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identification was performed using JADE 9.5.1 from Materials Data Inc., and the 2012

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PDF4+ database from the International Center for Diffraction Data (ICDD) database.

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Raman spectroscopy

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Raman spectra were collected using a Horiba LabRam HR spectrometer coupled with

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an inverted optical microscope (Nikon Ti-E) with a 40X objective and a 632.8 nm HeNe

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laser light source. Spectra were collected in the 150−4000 cm−1 range using three 60 s

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exposure times.

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Nitrogen Adsorption/Desorption Isotherms 9



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Nitrogen adsorption-desorption isotherms were collected by a surface area and porosity

178

analyzer (Micromeritics, ASAP 2020), and the Brunauer-Emmett-Teller (BET) surface

179

area was calculated from the linear part of the BET plot.

180

Scanning Electron Microscopy (SEM)

181

Morphology measurements were carried out using a Helios NanoLab 600i SEM (FEI,

182

Hillsboro, OR). All samples were sputter-coated with a thin layer of carbon prior to

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analysis (∼5 nm) to ensure good conductivity and imaging.

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Transmission electron microscopy (TEM)

185

As-prepared samples were dispersed in water with sonication for 5 min. Samples were

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prepared by placing drops of solution onto the copper grid (Lacey Carbon, 300 mesh,

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Copper grid, Ted Pella, Inc.). After drying under ambient conditions, the samples were

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introduced into the FEI Titan TEM. The samples were imaged using an acceleration

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voltage of 300 kV. SEM and TEM were also used to evaluate the size distribution of

190

as-synthesized boehmite samples. The width and thickness of the particles were

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determined based on average values for more than 20 particles.

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X-ray photoelectron spectroscopy (XPS)

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Physical Electronics Quantera Scanning X-ray Microprobe was used to perform the

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XPS measurements. Monochromatic Al Kα X-ray (1486.7 eV) was used as source for

195

excitation and a spherical section was used as the analyzer. The equipment has a

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detection system with a 32-element multichannel. The sample was probed by directing

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the X-ray beam perpendicular to the sample and the detector was at 45°. The spectra 10


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were collected using a pass-energy of 69.0 eV with a step size of 0.125 eV. The Ag3d5/2

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line, showed a FWHM of 1.0 eV ± 0.05 eV using these conditions. The Cu 2p3/2 at

200

932.62 ± 0.05 eV and the Au 4f7/2 at 83.96 ± 0.05 eV features were used to calibrate the

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binding energy scale. Charging was observed during the experiment which was

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minimized by using low energy electrons at 1.5 eV, 20μA and low energy Ar+ ions. All

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samples were prepared as pressed powders supported on a metal bar for the XPS

204

measurements. Scans were recorded from 0 – 1350 eV, with subsequent high-resolution

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scans of the Al 2p, O 1s and Cr 2p regions obtained.

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X-ray absorption spectroscopy (XAS)

207

Extended X-ray absorption fine structure (EXAFS) data at the Cr K-edge were acquired

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in fluorescence mode on beamline 20-BM-B at the Advanced Photon Source. The

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incident beam energy was selected using a Si (111) monochromator, and the X-ray

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beam was focused to spot size of ~400 microns using a toroidal mirror. The

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fluorescence signal was monitored using a multi-element, energy dispersive germanium

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detector. Initial energy calibration was performed using a chromium foil. Slight changes

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in monochromator energy were monitored and accounted for by simultaneously

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measuring the spectra of the chromium foil as a reference standard. Al K-edge EXAFS

215

spectra were acquired at the Advanced Light Source (Berkeley, CA) at beamline 6.2.1.2.

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To mount samples, powder was lightly pressed into indium foil and attached to a copper

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sample holder using silver paint. A reference spectrum collected on corundum (α-Al2O3)

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was used to calibrate the energy scale. Al K-edge EXAFS data were collected in total

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electron yield (TEY) mode over the scan range from 1520 to 1850 eV. Both the Cr and 11



220

Al K-edge EXAFS data were collected at room temperature and analyzed using the

221

Athena interface to the IFEFFIT program package57.

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Scanning transmission electron microscopy (STEM) and electron energy loss

223

spectroscopy (EELS)

224

Microstructural investigation was performed with an aberration-corrected electron

225

microscope (FEI, Model Titan 80-300) in STEM mode. STEM images were taken at

226

300 keV in STEM mode with high angle annular dark field (HAADF) detector. Data

227

collection and analysis was performed on Gatan’s Digital Micrograph 1.9.4. The STEM

228

sample preparation was same as for TEM. The microscope is equipped with Gatan

229

Quantum ER energy filter for electron energy loss spectroscopy (EELS) analysis. EELS

230

measurements were performed at 300 kV in dual EELS mode and with the dispersion

231

of 0.1eV. The EELS data were recorded in the STEM mode with a probe size of 10 nm

232

to minimize the dose density and electron dose effects. All EELS spectra were recorded

233

in dual EELS mode, allowing to establish a reference zero-loss peak (ZLP) with high

234

accuracy. The high energy loss spectrum was recorded from 480 eV to 684.8 eV, to

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cover the ionization energy ranges for the oxygen K-edge and the chromium L-edge.

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High-resolution atomic force microscopy (HRAFM)

237

HRAFM images were captured by Cypher-ESTM AFM (Asylum Research, CA) with

238

amplitude-modulate mode within nuclease-free water (Ambion). ArrowTM-UHFAuD

239

(NanoWorld) cantilever was used to image Boehmite Cr-B-1. The offline data

240

processing was done with software SPIPTM (Image Metrology, Denmark). 12


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Time-of-fight secondary ion mass spectrometry (ToF-SIMS)

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ToF-SIMS measurement was performed at Environmental Molecular Sciences

243

Laboratory (EMSL), which located at Pacific Northwest National Laboratory. A

244

TOF.SIMS5 instrument (IONTOF GmbH, Münster, Germany) was used. A 25 keV

245

pulsed Bi3+ beam was used as the analysis beam to collect SIMS spectra. The Bi3+ beam

246

was focused to be ~5 µm diameter and scanned over a 200  200 µm2 area.

247

current of the pulsed Bi3+ beam (10 kHz) was about 0.56 pA, and data collection time

248

is ~96 s per spectrum. Mass resolution was in a range of 5000-7000, varying from

249

sample to sample due to sample roughness. A low energy (10 eV) electron flood gun

250

was used for charge compensation in all measurement. The ToF-SIMS sample

251

preparation was as follows: as-prepared samples were dispersed in water by a sonicator

252

for 5 min, and then the dispersed suspension was dropped on a silicon wafer, which

253

was then dried under ambient conditions prior to being introduced into the chamber.

254

Electron paramagnetic resonance (EPR) spectroscopy

255

All EPR measurements were performed on Bruker ELEXSYS E580 spectrometer. The

256

typical settings for the static spectra were microwave frequency = 9.32 GHz, sweep

257

width = 5000 G, time constant = 40.96 ms, sweep time = 82 s, power = 0.2 mW, field

258

modulation amplitude = 0.5 G. Temperature-dependent spectra were recorded after a

259

sample was equilibrated at a temperature for ten minutes. Absolute Cr3+ spin

260

concentration absorbed on boehmite was determined by comparing its double

261

integration to that of the spin standard at 125 K.

13


The


262 263

RESULTS AND DISCUSSION

264

Characterization of Boehmite Nanoparticles

265

The XRD pattern of the as-prepared boehmite (Figure 1a) agreed well with the

266

powder diffraction file of pure boehmite (ICDD PDF # 00-74-1895)56; the strong

267

diffraction peak at the 2θ angle of 14.5° was assigned to (020) diffraction. As shown in

268

Figure 1b, the sample displayed Raman bands in the low wavenumber region (150-800

269

cm-1) at 343, 366, 455, 498 and 677 cm-1, which agree well with literature data for

270

boehmite and correspond to structural hydroxyl translational modes43, 58. Two broad

271

bands at 3078 and 3221 cm-1 in Figure 1c are attributed to the symmetric and

272

asymmetric stretches of these hydroxyl groups43, 47, 58.

273

SEM and TEM images (Figure 1d-f) show that the boehmite samples were all

274

rhombic nanoplates, which includes two dominant (010) basal surfaces terminated with

275

four (101) edge surfaces43, 44, 56. TEM images indicated that the mean size parallel to

276

[101] and average thickness along [010] (Figure 1g and 1h) were 38.3 and 5.7 nm,

277

respectively. It is noteworthy that this morphology and size of our synthetic material

278

are similar to boehmite particles found in radioactive tank waste at the Hanford site 59.

279

The results of nitrogen adsorption-desorption isotherms (Figure S1) were used to

280

calculate a BET surface area of 52.22 m2·g-1 for our material.

14


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281 282 283

Figure 1. Characterization of as-prepared boehmite: (a) XRD pattern, (b) and (c) Raman spectra, (d) SEM image, (e) and (f) TEM images; (g) size distribution and (h) thickness distribution.

284

Adsorption of Cr(III)

285

Overview of Cr(III) speciation and adsorption

286

Here we briefly summarize known relationships between Cr(III) speciation and its

287

adsorption mechanism, which are dominated by studies at low pH. Clearly critical is

288

the aqueous speciation of Cr(III), which is strongly pH dependent. At pH less than 2 it

289

exists as the bulky hydrated species Cr(H2O)63+ 60. Between pH 2 to 6.3 CrOH2+ is the

290

dominant hydroxo species52,

291

Cr2(OH)24+ and Cr3(OH)45+ also form in this pH range52, 61, 62. Cr(OH)3 solids precipitate

292

in the pH range from 6.3 to 11.552,

293

dominant52, 53. In our experiments, we could visually observe a trace undissolved Cr(III)

294

solid phase component when NaOH < 0.1 M. Above 0.1 M the solution is optically

295

very clear, consistent with complete dissolution of the Cr(III) precursor.

53,

but Cr(OH)2+ and polynuclear species such as

53,

and above this pH the Cr(OH)4 species is

15


Thus, to


296

avoid precipitation, Cr(III) adsorption at alkaline conditions was performed in the

297

current study with its focus on pH 13 and above.

298

Previous adsorption work in the low pH range, where Cr(III) exists as cationic

299

species, shows that its adsorption can be viewed as an ion-exchange process with

300

protons33. For example, Cr(III) adsorption on lignin in the pH range 1.5~5.5 was well

301

described by a Langmuir isotherm and pseudo-second-order kinetics suggesting

302

monolayer chemisorption via cation exchange with surface H+ 63. Likewise, Cr(III)

303

adsorption on activated carbon at pH 3.7 was proposed to be a simple ion-exchange of

304

Cr(OH)2+ with –OH groups on the surface followed by formation of inner-sphere

305

complexes64. Adsorption of Cr(III) on titanate nanotubes at pH=5 also involved ion-

306

exchange with H+/Na+ in the interlayer33. Site-specific electrostatic attraction and

307

coordination of Cr(III) ions to material surfaces is an important control over its

308

adsorption behavior60, 61, 65. Thus, at highly alkaline conditions, it is possible that similar

309

ion exchange concepts, except in this case anionic ones, pertain to adsorption of

310

Cr(OH)4 on nanoboehmite.

311

Effect of NaOH concentration

312

Adsorption experiments were performed with varying NaOH concentration from

313

0.1 M to 3 M at an initial Cr(III) concentration of 50 mg·L-1. Aqueous Cr(III) speciation

314

as monomeric Cr(OH)4- is expected across this entire range. As shown in Figure 2, the

315

adsorption capacity for Cr(III) on boehmite nanocrystals was strongly affected by initial

316

solution pH, decreasing sharply with increasing NaOH concentration. The highest

317

adsorption capacity of Cr(III) was ~18 mg·g-1 at 0.1 M NaOH. 16


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318

To better understand the adsorption behavior, we selected pH 13 as the optimal pH

319

value for investigating the effects of other system variables such as the Cr(III)

320

concentration.

321 322 323

Figure 2. Effect of NaOH concentration on the Cr(III) adsorption on boehmite Adsorption isotherms

324

Adsorption experiments at pH 13 (0.1 M NaOH) with a series of Cr(III) solutions

325

at concentrations ranging from 1 to 200 mg·L-1, and a boehmite concentration of 1 g·L‑1

326

(10 mg boehmite in 10 mL Cr(III) solutions), were conducted at room temperature for

327

3 h. Both the Freundlich isotherm model and the Langmuir isotherm model were used

328

for the analysis of the obtained adsorption data as follows30, 41, 65: 1

329

The Freundlich isotherm model: ln 𝑄𝑒 = 𝑛ln 𝐶𝑒 + ln 𝐾𝐹

330

The Langmuir isotherm model: Qe = 1 + KLCeQmax

KLCe

(1) (2)

331

Where Ce is the equilibrium concentration of Cr(III) in solution (mg·L-1), Qe is the

332

equilibrium adsorption capacity (mg·g-1), KF (mg·g-1(mg·L-1)1/n) and n is the Freundlich

333

adsorption constant related to adsorption capacity and adsorption intensity, KL is the 17



Page 18 of 43

334

Langmuir adsorption constant (L·mg-1), and Qmax is the maximum adsorption capacity

335

of the boehmite adsorbent (mg·g-1), respectively.

336

As shown in Figure 3 and Table 1, the Cr(III) adsorption data was better described

337

by the Langmuir isotherm model, with a substantially higher correlation coefficient (R2

338

value) of 0.998. Fits of this model indicate a maximum adsorption capacity (Qmax) of

339

19.85 mg·g-1. Based on the measured BET surface area 55.22 m2/g and measured mean

340

size and average thickness of the boehmite samples, and assuming adsorption site

341

densities equivalent to Al on the exposed surface, the corresponding theoretical

342

monolayer Qmax would be 40.46 mg Cr(III)/g AlOOH. Given that our measured Qmax is

343

less than half of this value, this implies lower Cr(III) sorption site densities, such as via

344

a sparse monolayer that is not structurally commensurate with the underlying boehmite

345

surface over long range.

346 347 348 349

Figure 3. Cr(III) adsorption on boehmite with different initial concentration (a), Langmuir isothermal fitting model (b) and Freundlich isothermal fitting model (c) (boehmite 1 g·L-1, p H=13, room temperature, 3 h)

350 351 352

Table 1 Isotherm adsorption parameters for the Langmuir and Freundlich isothermal model fitting of Cr(III) adsorption on boehmite. (boehmite (1 g·L-1), pH=13, room temperature, 3 h) Qmax(mg·g-1)

KL(L·mg-1)

R2

19.85

0.317

0.998

n

KF

R2

Langmuir

Freundlich

18


Page 19 of 43


2.425

3.715

0.850

353 354

Adsorption kinetics

355

Similarly at pH 13 (0.1 M NaOH), the adsorption kinetics were investigated with

356

an initial Cr(III) concentration of 100 mg·L-1 and boehmite loading of 1 g·L‑1, using

357

different equilibration times. Both pseudo-first-order and pseudo-second-order kinetic

358

models were used to fit the experimental data, according to35, 60, 66 log (𝑄𝑒 ― 𝑄𝑡) = log 𝑄𝑒 ―

359

The pseudo-first-order model:

360

The pseudo-second-order model:

t Qt

1

𝑘1

( )𝑡 2.303

1

= k Q2 + Qet 2 e

(3) (4)

361

where t (min) is time, Qe and Qt (mg·g-1) are the adsorption capacity at equilibrium and

362

at time t, respectively. k1 (min-1) is the second-order rate model adsorption constant, k2

363

(mg·g-1·min-1) is the second-order rate model adsorption constant, and V0 (mg·g-1·min-1)

364

is the initial adsorption rate which can be calculated by k2 and Qe.

365

The kinetic data for Cr(III) adsorption on boehmite is shown in Figure 4, along

366

with the pseudo-first-order and pseudo-second-order model fitting. The results suggest

367

that the pseudo-second-order kinetic model gave a better fit to the adsorption data and

368

consequently suggest that a chemisorption process34 adequately describes the measured

369

Cr(III) adsorption behavior (R2＞0.998, Figure 4b, Table 2). Parameters obtained from

370

the pseudo-second-order kinetic model by plotting t versus t/Qt are given in Table 2.

19



Page 20 of 43

371 372 373 374

Figure 4. Cr(III) adsorption on boehmite with different times (a), the Pseudo- Second -Order fitting model (b) and the Pseudo-First-Order fitting model (c) (boehmite 1 g ·L-1, pH=13, room temperature, initial concentration of Cr(III)=100 mg·L-1)

375 376 377

Table 2 Kinetics parameters of the pseudo-second-order kinetic model fitting of of Cr(III) adsorption on boehmite. (boehmite 1 g·L-1, pH=13, room temperature, initial concentration of Cr(III)=100 mg·L-1) The pseudo-firstorder kinetic model

The pseudo-secondorder kinetic model

378

Qe(mg·g-1)

k1

R2

9.643

0.0211

0.972

Qe(mg·g-1)

k2(mg·g-1·min-1)

R2

19.59

8.71×10-3

0.998

Nanoboehmite Characterization after Cr(III) Adsorption

379

Figure 5 shows that no new phases were detected in the XRD patterns of Cr(III)

380

adsorbed boehmite nanocrystals Cr-B-1 (50 mg·L-1) and Cr-B-2 (200 mg·L-1), and no

381

peaks for Cr(III)-based species were observed in both low and high wavenumber

382

regions of the Raman spectra. Therefore, any bulk crystalline Cr(III) species present,

383

such as by crystal nucleation and growth on the nanoboehmite surfaces, were

384

negligible.58

20


Page 21 of 43


385 386 387 388

Figure 5. XRD patterns (a) and Raman spectra (b, c) of Cr(III)-adsorbed boehmite (Cr-B-1, Cr-B-2, boehmite 1 g·L-1, pH=13, room temperature, 3 h, initial concentration of Cr(III)=50, 200 mg·L-1, respectively)

389

However, surface-sensitive XPS spectra clearly showed that Cr adsorption had

390

indeed occurred, which were also used to provide information on surface binding sites

391

(Figure 6 and Table 3). Survey spectra (Figure 6a) show the presence of Cr on the

392

surface after adsorption. The Al 2p binding energy for boehmite both before and after

393

Cr adsorption was 73.9 eV, indicating that the local bonding environment of Al did not

394

change as a result of interaction with Cr (Figure 6b)48. The O1s binding energy for

395

boehmite (531eV) also did not change significantly upon Cr adsorption, which is

396

anticipated given that the oxygen binding environment in AlOOH and CrOOH is

397

similar67. The binding energies for the Cr 2p3/2 and Cr 2p1/2 peaks (Figure 6d) were

398

576.7 eV and 586.4 eV for Cr-B-1, and 576.8 eV, and 586.3 eV for Cr-B-2, respectively.

399

These binding energies confirm that Cr is present on the boehmite surface as Cr(III).

400

The atomic concentrations of Cr at the surface increased concurrently with decreasing

401

Al during the adsorption process (Table 3). The amount of Cr(III) on the boehmite

402

surface, as determined by XPS, is in qualitative agreement with the adsorption capacity

403

predicted by the Langmuir isotherm model in Table 1; given that in our XPS 21



Page 22 of 43

404

measurement geometry the information depth was several Ångstroms into the surface,

405

quantitative comparison was not practical.

406 407 408 409

Figure 6. XPS full spectra (a), XPS spectra of Al 2p (b), O 1s (c) and Cr 2p (d) of pure boehmite and Cr(III) adsorbed boehmite (B, pure boehmite; Cr-B-1, Cr-B-2, boehmite 1 g·L-1, pH=13, room temperature, 3 h, initial concentration of Cr(III)=50, 200 mg·L-1, respectively).

410 411 412 413

Table 3 Atomic concentration and weight content of pure boehmite and Cr(III) adsorbed boehmite (B, pure boehmite; Cr-B-1, Cr-B-2, boehmite 1 g·L-1, pH=13, room temperature, 3 h, initial concentration of Cr(III)=50, 200 mg·L-1, respectively) Atomic concentration(%)

Weight content(%)

Samples Al

O

Cr

Al

O

Cr

B

26.4

68.6

0.0

38.0

58.8

0.0

Cr-B-1

24.2

67.1

2.0

33.6

54.1

5.7

Cr-B-2

22.3

65.6

3.5

30.8

53.7

9.3

414 415

We now turn our attention to high-resolution microscopic characterization of the

416

nanoboehmites after Cr adsorption. HAADF STEM images (Figure 7a) performed on

417

the Cr-B-1 sample indicate the presence of small Cr clusters on the boehmite surface.

418

Corresponding EELS data compared with that for Cr(III)2O3 and Na2Cr(VI)O4 22


Page 23 of 43


419

standards confirm the trivalent oxidation state of Cr on the boehmite surface. For

420

example, the average L3 ionization energy loss was measured to be at 576.8 eV, which

421

matches the energy of the Cr(III)2O3 standard (Figure 7b), and is much lower than the

422

energy of Na2Cr(VI)O4 at 579.5 eV (Figure 7b). Thus, consistent with the XPS results,

423

EELS also shows that the adsorbed Cr species is present as Cr(III), not Cr(VI) for

424

example.

425 426 427

Figure 7. STEM HAADF image of Cr-B-1 (a) and EELS spectra of Cr-B-1, Cr2O3 and Na2CrO4 (b). Red circles indicate example locations of adsorbed Cr(III) species on the boehmite surface.

428

Given the expectations from the adsorption isotherm and XPS-based surface

429

analyses for approximately 50% surface coverage at maximum loading, attempts were

430

made to directly visualize the Cr distribution on the nanoboehmite surface. HRAFM

431

was performed before and after Cr adsorption. The observed size (40 nm) and thickness

432

(5 nm) of the as-prepared boehmite agreed well with the TEM and SEM measurements

433

(Figure 8a). The dominant basal (010) surface consistently showed minimal initial

434

microtopography and was therefore ideal for imaging at atomic resolution. The (010)

435

lattice was readily resolved (Figure 8b and 8c), revealing a rectangular surface unit cell

436

0.29 nm by 0.36 nm in dimension, which agrees well with expected values. In contrast, 23



437

Cr(III)-reacted samples routinely showed additional features at the nanoscale. The (010)

438

plane for the Cr-B-1 sample was rougher, revealing small adsorbed patchy domains

439

(Fig. 8d). Based on the consistently uniform indications from XPS, STEM and EELS

440

that Cr(III) is on the surface, we thus make this assignment to these patches observed

441

in HRAFM. At high resolution, although the atomic structure of the surrounding (010)

442

surface could still be resolved (Figure 8e and 8f), no such ordered structure was readily

443

apparent while imaging the adsorbed patches. We thus assume that this indicates a lack

444

of substantial crystalline order within the adsorbed Cr(III) domains. Topographically

445

higher ‘brighter’ spots on the (010) surface at the margins of the patches are

446

conceptually consistent with adatom monomeric Cr(III)(OH)4 ions52, 53; their average

447

height, along with that of the patches themselves, is around 0.2 nm (arrows in Figure

448

8e and 8f). Collectively, the HRAFM images suggest that on nanoboehmite (010) Cr(III)

449

supplied in solution concentration of 50 ppm tends to adsorb as monomers that

450

segregate into poorly ordered single-layer clusters on the surface. It is also noteworthy

451

that surface coverages estimated by HRAFM agree well with coverages estimated both

452

by the measured adsorption isotherm and XPS-based surface concentrations, albeit here

453

limited to observations on just the (010) surface.

454 455

Figure 8. AFM images of as-prepared boehmite: (a) low resolution and (b) high resolution. The inset in 24


Page 24 of 43

Page 25 of 43


456 457 458

panel (a) is zoomed-in on a single particle. (c) FFT of high-resolution image shown in panel (b), with arrows indicating boehmite lattice vectors. (d) Low-resolution image of Cr-B-1, (e) and (f) high-resolution image and corresponding phase image, respectively, of Cr-B-1.

459

To gain more insight into the structure of these adsorbed Cr(III) domains we

460

performed ToF-SIMS surface analytical characterization. ToF-SIMS spectra of Cr(III)

461

adsorbed boehmite with the initial Cr(III) concentration of 1, 5, 10, 20, 50, 100, 200

462

mg·L-1 were obtained at the positive ion mode (Figure 9). Figure 9(a) shows the most

463

consistently intense Cr(III) ion peaks were

464

and Cr3O3+(m/z=203.81) in the m/z ranging from 45 to 210. The ToF-SIMS

465

measurement is based on an energetic sputtering process that more often tends to

466

deconstruct rather than construct metal clusters68.

467

Cr(III)-oxo fragments were consistently observed along with monomer fragments

468

suggests the likelihood of Cr-O-Cr clusters in the patchy domains on the surface as the

469

source material. More to this point, the relative abundances for the Cr2O2+ and Cr3O3+

470

multi-center secondary ions showed a systematic trend with the initial Cr(III) solution

471

concentration (Figure S2 and Figure 9b). Importantly, no Cr(III)-based clusters were

472

indicated to be present on the boehmite surface when the initial concentration of Cr(III)

473

was 1 ppm. Small amounts of Cr(III)-based dimers and trimers were detected when the

474

initial concentration of Cr(III) was 5 and 10 ppm. Many clusters were detected when

475

the initial concentration of Cr(III) was > 20 ppm. Hence the ToF-SIMS based analyses

476

suggest an uptake mechanism consistent with both the isotherm data and the HRAFM

477

images that entails Cr(III) monomer adsorption that assemble into surface clusters

478

systematically with increasingly higher surface Cr(III) loadings.

50Cr+(m/z=49.95),

Cr2O2+ (m/z=135.88),

Hence, the fact that multi-center

25



479 480 481

Figure 9. ToF-SIMS (+) spectra (a) and relative abundance of CrxOy+ secondary ions (b) of Cr(III) adsorbed boehmite with different initial concentration of Cr(III) from 1 ppm to 200 ppm.

482

Given that the formation of Cr(III) clusters is likely, but that these clusters lack

483

long-range structural order, we used Cr K-edge XANES and EXAFS to obtain more

484

specific information on the local molecular environment around both Cr and Al at the

485

nanoboehmite surface. Cr K-edge XANES spectra for Cr-B-1 and Cr-B-2 were very

486

similar (Figure 10a). The spectra closely resemble those measured by Frommer et al.69

487

for a series of Fe-Cr-oxyhydroxides. The low energy contributions in the pre-edge

488

spectra are assigned to local transitions and the intensity of the first peak is the same

489

for Cr-B-1 and Cr-B-2, indicating similar distortion of the Cr coordination site. The

490

only difference between the XANES spectra for Cr-B-1 and Cr-B-2 is a slight decrease

491

in intensity of the higher energy pre-edge peak, likely related to non-local transitions to

492

neighboring Al in Al-Cr-oxyhydroxides (~5992.6 eV, shown in inset in Figure 10a).

493

This change can be explained by substituting some of the neighboring Al in Cr-B-1 by

494

Cr in Cr-B-2. Analysis of these non-local transitions suggests a lower amount of Cr26


Page 26 of 43

Page 27 of 43

495


clustering in Cr-B-1.

496

For corresponding EXAFS analyses, Figure 10b shows the k3-weighted χ spectra

497

of Cr-B-1 and Cr-B-2, and the magnitude of the Fourier transformed spectra are shown

498

in Figure 10c. The first peak in the Fourier transform, which is the contribution of the

499

nearest-neighbor O atoms, is of similar height and position for both samples. Beyond

500

this intense Cr-O contribution, four weak but well-defined Cr-Me peaks are clearly

501

shown between 2 and 4 Å. The first and third Cr-Me distances are very close to the Me

502

shell distance in boehmite, as shown by the Al K-edge Fourier transform in Figure

503

10c43. Analysis of the data suggests that the key set of Cr-(Cr, Al) bond distances

504

pertaining to the adsorbed clusters are 3.01 Å, 3.45 Å, and 3.95 Å. It is noteworthy that

505

this set resembles but is distinct from those observed by Charlet and Manceau for small

506

surface clusters of Cr(III) templated onto goethite surfaces.55 (Charlet and Manceau

507

data shown in blue translated above the other Fourier transforms in Figure 10c). In that

508

study, α-FeOOH-like CrOOH cluster formation (Cr…Cr distance 3.45 Å) was enabled

509

by Cr(III) substitution into Fe(III) lattice positions on the surface (Fe…Fe distance 3.43

510

Å). In our case, however, boehmite Al…Al distances are 2.87 and 3.69 Å,70 suggesting

511

a Cr(III) cluster formation mechanism other than surface site substitution.

512

Comparison of our set of three key Cr-(Cr, Al) distances with those reported for

513

various bulk CrOOH polymorphs (Fig. S4), which are generally known to adopt bulk

514

topologies distinct from (Fe,Al)OOH polymorphs,71 revealed one prospect. Cr…Cr

515

distances reported for γ-AlOOH-like CrOOH are 3.05 and 3.98 Å55,

516

preclude our 3.45 Å observation. The γ-CrOOH mineral bracewellite entails Cr…Cr 27


72

but these


517

distances of 2.97, 3.26, and 3.40 Å (Fig. S4c),73 and the α-CrOOH mineral grimaldiite

518

entails 2.97 and 4.77 Å (Fig. S4a),74 both generally incompatible with our observations.

519

The β-CrOOH mineral guyanaite, however, is based on a set of Cr…Cr distances (3.00,

520

3.51, and 4.30 Å) 75, 76 (Fig. S4b) sufficiently similar to those indicated by our collective

521

XAS information to suggest that a relaxed version of this CrOOH topology could be

522

representative of clusters on the boehmite surface.

523 524 525 526 527 528

Figure 10. (a) Cr K-edge XANES spectra with inset showing pre-edge region; (b) Cr K-edge k3weighted χ(k) spectra; (c) Fourier transform (FT) magnitude; for Cr K-edge Cr-B-1 (black solid line), Cr K-edge Cr-B-2 (red solid line), Al K-edge Cr-B-1 (black dotted line), Al K-edge Cr-B-2 (red dotted line) and Cr K-edge for Cr(III) sorbed on goethite from Charlet and Manceau55, translated above the other FTs for clarity.

529

Electron paramagnetic resonance (EPR), which is not surface specific but is highly

530

sensitive to Cr(III), was performed due to its ability to provide further information on

531

Cr-Cr interactions and interatomic distances within the adsorbed clusters. Figure 11

532

shows EPR spectra of pure boehmite versus Cr(III) adsorbed boehmite with varying

533

amount of chromium, with the broad Cr3+ signal located at a center of g = 1.984. The

534

weight percentage (wt%) of absorbed Cr(III) quantified by comparing to the EPR signal

535

of a standard solution 0.5 mM Cr(NO3)3 was plotted against the concentration of Cr(III)

536

in solution (Fig. 11b). A dramatic rise in Cr(III) uptake was observed for the solutions

537

between 1 and 20 ppm, followed by a slower increase between 20 and 100 ppm and a 28


Page 28 of 43

Page 29 of 43


538

plateau between 100 and 200 ppm; consistent with the adsorption isotherm

539

investigation (Fig. 3a).

540

The linewidth of the EPR signal of regular crystals usually comes from the

541

interactions between the magnetic dipoles77, but for the paramagnetic species absorbed

542

inside regular crystals, the additional broadening effect may be caused by the

543

orientational and structural disorder of paramagnetic centers as well as the site-to-site

544

variation of the magnetic environment78. The EPR signal of absorbed Cr(III) on

545

nanoboehmite is relatively broad with a linewidth of 600 gauss at 0.1 wt% load,

546

suggesting the remarkable orientational, structural and magnetic variability of the

547

absorption sites. As shown in Figure 11c, the linewidth shows little difference when

548

the Cr(III) load increases from 0.1 wt% to 1.4 wt%.

549

According to a simplified treatment for the line broadening effect caused by dipolar

550

interactions between electron spins, the linewidth (full width at half-maximum height)

551

𝐵 can be calculated from the distance 𝑟𝑖𝑗 between the two spins i and j:79

552

𝐵 = (1.95 × 104)

〈∑

〉

𝑟𝑖𝑗 ―6

1/2

553

where B is in gauss and 𝑟𝑖𝑗 in Ångstroms. Therefore, for a base linewidth of 600 gauss

554

caused by site-to-site variation, we do not expect to observe a notable difference in

555

linewidth if the additional dipolar line broadening is much smaller than 600 gauss,

556

which in this case corresponds to an averaged spin-spin distance of 3.2 Å. This average

557

minimum distance of 3.2 Å between Cr(III) atoms provides additional insight regarding

558

the apparent structure of the sorbed Cr(III) clusters. Among the key Cr-(Cr,Al) 29



559

distances inferred by EXAFS, the shortest (3.01 Å) may thus represent Cr-O-Al

560

interactions, likely an edge-sharing connection between Cr(III) in clusters to Al on the

561

nanoboehmite surface. To also achieve Cr-O-Cr bond lengths on the order of 3.45 Å,

562

such connections are more likely to be double corner-sharing. Both conditions are

563

compatible with the prospect of a β-CrOOH motif for the clusters directly bonded in an

564

inner-sphere fashion to the boehmite surface. Finally, it is noteworthy that the invariant

565

linewidth shown in Figure S3 indicates that the average minimum distance between

566

Cr(III) absorbed on nanoboehmite does not change with increasing Cr(III) loading.

567 568 569 570 571

Figure 11. EPR spectra of pure boehmite and Cr(III) adsorbed boehmite at different initial concentration of Cr(III) measured at 125 K (a); the weight percentage (wt%) of chromium absorbed in boehmite surface calculated as a function of the solution concentration (b) , and the Cr(III) EPR linewidth of different Cr(III)-boehmite samples (c).

572 573

The integrated findings converge upon a Cr(III) adsorption mechanism that

574

involves the surface-catalyzed self-assembly of adsorbed Cr(OH)4- monomers to form

575

clusters as a sparse monolayer at saturation coverages on the nanoboehmite surface.

576

Although the clusters are poorly ordered, the collective data allow for some conclusions

577

to be made regarding their structure. The local structure around Cr(III) is octahedral

578

and coordinated to both oxy and hydroxo groups. Both the EXAFS data and the ToF30


Page 30 of 43

Page 31 of 43


579

SIMS results confirm the presence of Cr-O-Cr moieties. In addition, because both the

580

EPR data and the EXAFS data strongly suggest average nearest-neighbor Me…Me

581

distances greater than 3 Å, the local structure around the Cr atoms on boehmite is likely

582

not the same as that implied by the measured Cr2On secondary ions which, near their

583

energetic minimum, would contain Cr…Cr distances less than 2.3 Å80. The shortest Cr-

584

Cr bond distance (RCr…Cr) according to EPR is 3.2 Å, thus, the shorter EXAFS bond

585

distance (3.05 Å) likely corresponds to RCr-Al in edge-sharing Cr-O-Al octahedra that

586

connect the Cr(III) clusters to the nanobohemite surface. The observed Cr(III)-

587

(oxyhydr)oxide clusters appear dominated by corner-sharing Cr-O-Cr octahedral

588

linkages RCr-Cr of 3.45 Å and 3.95 Å.55, 72

589

Although it is difficult to posit a definite structure for the adsorbed Cr based on the

590

data assembled so far, analysis of the topologies of guyanaite and the boehmite (010)

591

surface showed a nearly heteroepitaxial lattice match in which two successive Cr atoms

592

along [100] share edges with two successive Al sites in a [101] column on the surface.

593

Such a structure, based on a strained guyanaite motif bound to the surface as depicted

594

in Figure 12, is compatible with the collective data set. Ongoing work entails molecular

595

simulations to test the veracity of this model.

31



596 597

Figure 12. Proposed structure model for Cr(III) adsorption on boehmite (010).

598

The cluster formation mechanism is likely specific to these highly alkaline The high activity of the OH- anion is such that it is readily available as a

599

conditions.

600

metal-coordinating ligand both at the boehmite surface and in solution. We suggest that

601

the adsorption mechanism is based on an OH- ligand exchange process at the interface,

602

resulting in: (i) stabilization of inner-sphere Cr(OH)4- monomers on the boehmite

603

surface; and then (ii) their self-assembly polymerization into corner-sharing Cr-O-Cr

604

linkages that then stabilize the observed β-Cr(III)OOH-like clusters. Analogous to the

605

H+ ion exchange mechanism proposed for Cr(III) cation adsorption at low pH,

606

adsorption and polymerization of Cr(III) tetrahydroxyanions on the surface at high pH

607

may be mediated by OH- anion exchange processes.

608

In addition to answering our stated science questions, the insights obtained here

609

contribute more generally to the understanding of heavy metal adsorption on metal

610

(oxyhydroxide) nanomaterials in caustic environments, including in complex

611

radioactive liquid wastes, currently stored in tanks and awaiting processing at DOE 32


Page 32 of 43

Page 33 of 43


612

nuclear legacy sites, such as Hanford.

613

ASSOCIATED CONTENT

614

Supporting Information

615

The Supporting Information is available free of charge on the ACS Publications website

616

at XXX.

617

Nitrogen adsorption-desorption isotherms curves of boehmite, ToF-SIMS and EPR

618

spectra of various Cr(III) adsorbed boehmite samples.

619

AUTHOR INFORMATION

620

Corresponding Authors

621

* Email: [email protected](X. Z.), [email protected](P. L.),

622

[email protected] (Z. W.), and [email protected] (K. M. R.)

623

ORCID

624

Xin Zhang: 0000-0003-2000-858X

625

Zihua Zhu: 0000-0001-5770-8462

626

Kevin Rosso: 0000-0002-8474-7720

627

Notes

628

The authors declare no competing financial interest.

629

ACKNOWLEDGMENTS

630

The authors thank PNNL scientists Odeta Qafoku and Charles T. Resch for the help on 33



631

the BET and ICP-MS measurements. This work was supported by IDREAM

632

(Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier

633

Research Center funded by the U.S. Department of Energy (DOE), Office of Science,

634

Basic Energy Sciences (BES). A portion of this research was performed using EMSL,

635

a national scientific user facility sponsored by the DOE Office of Biological and

636

Environmental Research and located at PNNL. PNNL is a multiprogram national

637

laboratory operated for DOE by Battelle Memorial Institute under Contract DE-AC05-

638

76RL0-1830. WWC thanks the support from the China Scholarship Council.

639

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640

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641

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characterized lithium cobalt double aryloxides and the nanoparticles and thin films

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For Table of Contents Use Only

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43


Cr(III) adsorption by cluster formation on boehmite nanoplates in

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