Effects of Ligand Chemistry and Geometry on Rare Earth Element

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Effects of ligand chemistry and geometry on rare earth element partitioning from saline solutions to functionalized adsorbents Clinton W. Noack, Kedar Perkins, Jonathan C Callura, Newell R. Washburn, David A Dzombak, and Athanasios K Karamalidis ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01549 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Effects of ligand chemistry and geometry on rare earth element partitioning from saline solutions to functionalized adsorbents Clinton W. Noack,† Kedar M. Perkins,‡ Jonathan C. Callura,† Newell R. Washburn,‡ David A. Dzombak,† and Athanasios K. Karamalidis∗,† Department of Civil and Environmental Engineering, Carnegie Mellon University, 5000 Forbes Ave., Porter Hall 118, Pittsburgh, Pennsylvania 15213, USA, and Department of Chemistry, Carnegie Mellon University, 4400 Fifth Ave., Pittsburgh, Pennsylvania 15213, USA E-mail: [email protected]

Abstract The rare earth elements (REE) are elements that drive the development of new technologies in many sectors, including green energy. However, the supply chain of the REE is subject to a complex set of technical, environmental, and geopolitical constraints. Some of these challenges may be circumvented if the REE are recovered from naturally abundant alternative sources, such as saline waters and brines. Here we synthesized and tested aminated silica gels, functionalized with REE-reactive ligands: diethylenetriaminepentaacetic acid (DTPA), diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), and N,N-bisphosphono(methyl)glycine (BPG). ∗

To whom correspondence should be addressed Civil and Environmental Engineering ‡ Chemistry †

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A suite of characterization techniques and batch adsorption experiments were used to evaluate the properties of the functionalized silica adsorbents and test the REE-uptake chemistry of the adsorbents under environmentally relevant conditions. Results showed that BPG and DTPADA yielded the most REE-reactive adsorbents of those tested. Moreover, the DTPADA adsorbents demonstrated chemical and physical robustness as well as ease of regeneration. However, as in previous studies, amino-polycarboxylic acid adsorbents showed limited uptake at mid-range pH and low-sorbate concentrations. This work highlighted the complexity of inter-molecular interactions between even moderately sized reactive sites when developing high-capacity, high-selectivity adsorbents. Additional development is required to implement an REE recovery scheme using these materials, however it is clear that BPG- and DTPADA-based adsorbents offer a highly-reactive adsorbent warranting further study.

1

Introduction

2

The expanding range of technologies incorporating the rare earth elements (REE), and the

3

ensuing demand for these products, have established the REE as valuable global commodi-

4

ties. Domestic (US) demand in 2012 was 11,300 tons, while the global demand was more

5

than 113,000 tons (1 ). Much of that demand is a result of a booming green energies market.

6

In particular, the permanent magnets sector (which uses neodymium, praseodymium, and

7

samarium with dysprosium and terbium additives) is experiencing significant growth. The

8

U.S. Department of Energy has projected the supply risk and clean-energy demand of the

9

REE across all sectors and found a variety of criticalities for the elements (2 ).

10

Primary mining effectively constitutes 100% of the REE production worldwide (3 , 4 ).

11

Those mining activities focus exclusively on three minerals — bastnäsite (LnCO3 F), mon-

12

azite (LnPO4 ), and to a lesser extent xenotime (LnPO4 ) — and ion-adsorbed clays (5 , 6 ).

13

While bastnäsite ores are dominated by the light REE (primarily La and Ce), monazite,

14

xenotime, and ion-adsorbed clays contain significantly higher heavy REE+Y fractions (5 –

2


Page 2 of 34

Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

7 ). Rare earth-bearing minerals are primarily surface mined and comingled with a variety

16

of other products. As such, a complex, multi-step, energy- and material-intensive process is

17

required to yield REE of salable quality from conventional feed stocks. The combined effect

18

of these numerous and varied industrial production activities yields a substantial lifecycle

19

environmental impact. On a per-kg basis, REE production consumes > 10× more energy

20

and results in > 10× more greenhouse gas emissions than does steel production (8 ). While

21

this comparison will undoubtedly be different on a functional basis (i.e. per kg finished

22

product, where REE make up < 100%), Zaimes et al. (8 ) did not considered further, down-

23

stream refining processes beyond REE production in determining this lifecycle impact. Such

24

considerations would only increase the environmental burden of the REE.

25

Interest in recovery of REE from end-of-life stocks (EOL), from unconventional resources,

26

and from REE-containing industrial wastes has expanded rapidly to diminish these negative

27

impacts (9 ). High volumes of REE are deployed in permanent magnets, while high-value

28

REE are used in phosphors (10 ), making these products two primary targets for recycling

29

from EOL products along with metal hydride batteries (3 , 11 ). REE are applied in many

30

other products, but the REE content is dissipated in-use or rendered unrecyclable by current

31

designs (12 ). Ferrous shredder waste (where magnets could accumulate) is a promising

32

potential resource for REE and other critical materials. However, Bandara et al. (13 ) propose

33

that recycling of ferrous shredder-waste would need to exceed 50% in order to dampen Nd

34

price volatility from recycling alone. The conclusion from these forecasts is the need for

35

novel, alternative feedstocks (13 ).

36

Saline waters and brines contain rare earth elements (REE) and are potential sources of

37

REE (14 –17 ). Large volumes of saline waters and brines are continuously produced through

38

industrial operations including geothermal electricity generation, oil and gas production,

39

and seawater desalination. While 10−3 M total REE concentrations have been observed

40

in geothermal fluids (15 ), the vast majority of circumneutral brines contain 10−9 –10−12 M

41

concentrations, requiring significant preconcentration to achieve a valuable product (17 ).

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

42

The optimal approach for recovery from dilute streams is solid phase extraction (18 ). A

43

promising approach for recovery of REE from saline waters and brines is by adsorption

44

on engineered, highly-selective adsorbents followed by elution and then precipitation from

45

the concentrate solution. Intra-element separations from this mixed REE concentrate could

46

subsequently be achieved by conventional or novel techniques.

47

Metal extraction from dilute solutions requires selective, high-capacity adsorbents in

48

order to be effective and economical. Numerous ligands exist that have been engineered for

49

their ability to selectively chelate metals in solution, however the effect of surface attachment

50

on this affinity is not always well understood. Moreover, the prior art in this subject area

51

focus on simple systems (i.e. low concentrations of background electrolytes) and high sorbate

52

concentrations. Thus, there is a need for testing functionalized adsorbents under conditions

53

of elevated salinity and with environmentally-relevant sorbate concentrations.

54

The unique chemistry of the REE — including high charge density, variable coordination

55

number, and minimally-varied atomic radii — enables design of a variety of potential ligands

56

for their separation from gangue elements as well as from each other (19 ). The REE are

57

considered hard acid cations, in the Pearson sense, and thus interact strongly with hard

58

anions (20 ). This leads to their natural speciation being dominated by carbonate, hydroxide,

59

and (where sufficient ligand is present) phosphate complexes (21 ). As a result organic

60

ligands with highly-polarizable oxygen moeities, such as phosphonates and carboxylates,

61

yield strong complexes (22 ). Moreover, the unique coordination chemistry of the REE

62

(CN∼ 8–12) yields more stable complexes with appropriate multidentate ligands, including

63

amino-polycarboxylic acids.

64

Silica gel adsorbents are commonly applied because of their low cost and ease of function-

65

alization (23 –26 ). Surface hydroxyl groups are used for the attachment of organo-silanes,

66

which can be purchased pre-functionalized with a wide variety of end groups (24 ). The

67

most common silane, because it allows for facile attachment of amino-polycarboxylic acids,

68

is aminopropyl triethoxysilane (APTES; 26 ). Additionally, silica gels are resistant to disso-

4


Page 4 of 34

Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


69

lution under acidic conditions, limiting the potential for degradation of the adsorbent with

70

repeated uptake and elution cycles.

71

The objective of this study was to validate the affinity of surface-attached ligands for

72

the REE using a model (silica) solid support and attachment schemes. This supports an

73

overriding goal to produce cost-competitive REE concentrates from abundant, but low-

74

concentration brine feedstocks. This was accomplished through: (1) functionalization of

75

silica gels with REE-reactive ligands; (2) qualitative and quantitative characterization of

76

adsorbent physical and chemical properties; and (3) determination of REE uptake behav-

77

ior under a range of conditions. The results of this study can be used to inform design of

78

functionalized adsorbents for REE extraction and recovery from dilute aqueous sources.

79

Materials and methods

80

Chemicals

81

All adsorbents were functionalized using either 3-aminopropyl silica gel (d: 75 – 150 µm; TCI

82

America) or high-purity silica gel (d: 150 – 250 µm; Sigma-Aldrich). The ligands tested were:

83

diethylenetriaminepentaacetic dianhydride (DTPADA), diethylenetriaminepentaacetic acid

84

(DTPA), phosphonoacetic acid (PAA), and N,N-bis(phosphonomethyl)gylcine (BPG); the

85

structures of the ligands (as received) are illustrated in Figure 1. These ligands were selected

86

for their combination of cost, ease of surface attachement, and demonstrated lanthanide

87

reactivity (e.g. 27 , 28 ). The related ligands DTPADA and DTPA were chosen to compare

88

the effects of forming a targeted amide tether to the surface (i.e. on the lone carboxyl

89

of the DTPADA) to the non-specific coupling with DTPA. The use of propylphosphonic

90

anhydride (T3P) and N,N’ -dicyclohexylcarbodiimide (DCC) for amide formation promotes

91

the reaction at the free carboxyl over the anhydride, though perfect selectivity is unlikely.

92

Upon introduction to an aqueous environment, the anhydride groups should hydrolyze to

93

leave four carboxyl groups free in solution. A fifth ligand, 1,4,7,10-tetraazacyclododecane5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

94

1,4,7,10-tetraacetic acid (DOTA; not pictured), was included in preliminary screening but

95

did not show meaningful uptake and was eliminated from further testing (see Supporting

96

Information for results). DTPADA

DTPA O

O

O

OH O

OH

OH

OH

O N O

N O

N

N

N

N

HO O

HO

O

O BPG

PAA O

OH

O

O HO

O

O

P

P OH

O

HO N

OH

OH HO P OH O

Figure 1: Chemical structures of the ligands studied.

97

R Plus, VWR) was used to preserve samples for ICPNitric acid (HNO3 ; BDH ARISTAR

98

R MS analysis. Hydrochloric acid (HCl; BDH ARISTAR Plus, VWR) and sodium hydroxide

99

(NaOH; Fischer Scientific) were used for pH adjustments. Background electrolyte solutions

100

were prepared by dissolving NaCl (Sigma Aldrich; ≥ 99% purity) in ultrapure water (ASTM

101

R water purification system. Type I, 18.2 MΩ/cm), prepared using a Barnstead NANOpure

102

REE spike solutions were prepared by dissolving Ln(NO3 )3 · 6 H2 O salts (Sigma Aldrich or

103

Alfa Aesar) in ultrapure water. Single element standard solutions (1000 µg/L) of the REE

104

were obtained from Inorganic Ventures and used to prepare the calibration curve for ICP-MS

105

analysis.

106

Analytical instrumentation

107

Total dissolved REE concentrations were determined using an Agilent 7700x ICP-MS with

108

HEHe-mode octopole reaction cell. Operating parameters were optimized daily via the

109

auto-tune function of the Agilent MassHunter software using 1000:1 diluted Agilent tuning

110

solutions. Concentrations were determined from a six-point calibration curve (0, 1, 5, 10, 50, 6


Page 6 of 34

Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


111

and 100 ppb) containing each of the REE. Typical instrument operating parameters and a

112

discussion of instrument operation and data analysis can be found in Supporting Information.

113

An OrionTM 8165BNWP ROSSTM Sure-FlowTM pH electrode (Thermo Scientific), cou-

114

pled to an accumetTM XL600 meter (Fisher Scientific), was used for pH measurements of

115

high-total dissolved solid (TDS) solutions. The pH meter was calibrated with pH 2.0, 4.0,

116

and 7.0 standards daily.

117

All samples were prepared gravimetrically using an analytical balance with 0.01 mg pre-

118

cision (Adam Equipment).

119

Functionalization

120

Two schemes for functionalization of silica supports were used in this study, which are unique

121

compared to the common approaches of materials reviewed by Repo et al. (26 ). Both

122

approaches make use of initiators which have become common practice in peptide synthesis

123

(29 , 30 ), allowing for direct formation of amides from carboxylic acids and primary amines.

124

Pre-aminated silica gels were functionalized via a “bottom-up” scheme, building functional

125

moieties piece by piece from the surface. Conversely, a “top-down” strategy was employed

126

by first forming a ligand-functionalized silane, which was subsequently attached to silica gel

127

supports. This approach was only tested for DTPADA. The procedure for each scheme is

128

illustrated schematically Figure 2.

129

Bottom-up functionalization

130

A solution of the desired ligands (128 mM), 4-dimethylaminopyridine (4-DMAP; 154 mM),

131

3-aminopropyl functionalized silica (25.7 mM amine), and T3P (77.1 mM) in dimethyl-

132

formamide (DMF; 35 mL total volume) was stirred at room temperature overnight. The

133

suspension was then transferred to a centrifuge tube and centrifuged for 15 mins at 25◦ C

134

and 4.4 rpm. The supernatant was removed and then the pellet was resuspended in 25 mL of

135

DMF. The suspension was centrifuged for 10 mins, the supernatant removed and the pellet 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

136

resuspended four additional times in DMF. The suspension was then transferred to a glass

137

vial and the solvent removed under vacuum and heat.

138

Top-down functionalization

139

Diethylenetriaminepentaacetic dianhydride (2.5 g, 7.0 mmol) and DCC (1.6 g, 7.7 mmol)

140

were added to a flask under a N2 atmosphere. The flask was then filled with dichloromethane

141

(25 mL) and then APTES (1.8 mL, 7.7 mmol) and stirred overnight. The reaction solution

142

was then filtered and concentrated under vacuum. This product was then added to 1.62 g

143

of SiO2 gel in dry toluene (25 mL) and stirred overnight under N2 . The final product was

144

washed five times with toluene, five times with tetrahydrofuran, and three times with warm

145

water and dried in a vacuum oven (65◦ C). All results related to samples prepared via this

146

top-down functionalization will be referred to as TD-DTPADA.

147

Adsorbent Characterization

148

The primary objective of adsorbent characterization was to confirm the functionalization

149

of the silica surface by the various ligands. The formation of amide bonds between the

150

surface amines and the desired ligand should result in a shift of surface acid-base chemistry

151

from highly basic (amine pKa ∼ 9 − 10) to acidic (ligand pKa1 ∼ 2). This shift was

152

investigated by rapid acid base titrations of particle suspensions. This shift was also tested by

153

inferring surface charge from electrophoretic mobility measurements. The site concentration

154

and grafting efficiency were estimated by thermogravimetric analysis (TGA). Finally, the

155

presence of the desired amide tether was investigated by attenuated total reflectance - Fourier

156

transform infrared spectroscopy (ATR-FTIR). The details of these methods are described

157

subsequently.

8


Page 8 of 34

Page 9 of 34


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

158

Rapid titrations of particle suspensions

159

Acid- and base-neutralizing capacities of the functionalized adsorbents were investigated by

160

rapid titrations of 5 g solid/L suspensions in 0.5 m NaCl. The suspensions were mixed,

161

without any acid or base addition, for 1 minute or until a stable pH reading was achieved

162

(defined here as unchanging at 0.01 pH unit precision over 5 seconds). Small doses (10–

163

20 µL) of either 0.5 N NaOH or 0.5 N HCl were added to the suspensions and a pH was

164

recorded once stable. This process was repeated until the pH exceeded 8.5, for initially acidic

165

suspensions, or was under 5, for initially basic suspensions. The titrant was then switched

166

(i.e. acid for base) and the curve reversed to examine hysteresis effects. Results are presented

167

normalized to the solids concentration of each suspension. Rapid titrations were chosen to

168

avoid the slow kinetics of long-term protonation reactions (as described by Dzombak and

169

Morel (31 )) and to avoid complications associated with the high porosity of the silica gel

170

(24 ).

171

Electrophoretic mobility

172

Measurements of the electrophoretic mobility (EPM) of each adsorbent were made using a

173

Malvern Zetasizer NanoZS with DI water suspensions of particles (100 g solid/L). The pH of

174

the particle suspension was verified just prior to removal of an aliquot for the EPM measure-

175

ment. The applied voltage was 150 V after a sample equilibration time of 2 min. A minimum

176

of 10 runs and a maximum of 100 runs were performed for each EPM measurement. The

177

runs were averaged to determine the EPM value. Triplicate measurements were performed

178

at each pH.

179

Quantification of surface sites

180

The efficiency of ligand attachment was estimated by TGA. Adapting from the methods of

181

Mansfield et al. (32 ), approximately 10 mg of sample was heated in a slow temperature ramp

182

first to 105◦ C (dT /dt = 20◦ C/min), which is maintained for 20 minutes to eliminate moisture, 10


Page 10 of 34

Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


183

and subsequently to 550◦ C (dT /dt = 10◦ C/min), which is maintained for 20 minutes. Change

184

in mass after drying was attributed to the removal of organic material, however it does

185

not guarantee that all removed organics were surface groups as non-specific binding (e.g.

186

adsorption or polymerization) could have also occurred.

187

The resulting mass-loss curve from TGA was interpreted by considering the progres-

188

sive, stoichiometric attachment of the ligand to surface amine groups (assuming 1:1 binding

189

between amine and ligand; Figure 2). By this method, the mass gained during functional-

190

ization is a function of the amine conversion efficiency. Considering the exchange of surface

191

amines (as propyl-amine, M WAP = 59 g/mol) for the amide-bonded ligand with the loss

192

of H2 O (M Wsurf = (M WL − M WOH ) + (M WAP − M WH )) the expected maximal TGA

193

loss can be determined based on the attachment efficiency. Inverting this relationship allows

194

for estimation of attachment efficiency from measured TGA loss. For the poly-carboxylate

195

ligands studied, the model was also developed to consider higher stoichiometric ratios of

196

amines reacted with ligand. While the coupling chemistry is well understood, TGA results

197

do not prove the desired functionalization has occurred as the results may be obscured by

198

non-specific binding. Spectroscopic investigation was used to identify the expected surface

199

amide bonds.

200

Confirmation of surface tethering

201

The formation of the surface amide tethers was examined with ATR-FTIR (νC−O, amide ≈

202

1690 − 1630 cm−1 ). ATR-FTIR spectra were obtained using a PerkinElmer Frontier FT-IR

203

spectrometer equipped with a germanium ATR crystal. Scans were performed from 4000

204

cm−1 to 700 cm−1 at a rate of 0.2 cm−1 /s. The traces are the average of 4 scans with a 4

205

cm−1 resolution. Spectra were generated using PerkinElmer Spectrum software.

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

206

REE uptake experiments

207

Batch experiments were used to study the reactivity of the functionalized adsorbents under

208

a variety of conditions and to probe a variety of adsorbent properties including: uptake

209

kinetics, pH dependance, and affinity. For analytical and experimental simplicity, as well as

210

conciseness of data visualization, a mixture of three REE, which roughly span the range of

211

the REE and can be representative of light-, middle-, and heavy-REE — Nd, Gd, and Ho

212

respectively — was used in all adsorption experiments.

213

Uptake kinetics were studied by equilibrating separate batches of 10 g/L adsorbent sus-

214

pensions in 15 mL polypropylene tubes with 0.5 m NaCl background to the desired pH,

215

dosing each reactor to 100 ppb of each REE investigated, and mixing end-over-end at 30

216

rpm for the desired amount of time. After the specified mixing time, the reactor was removed,

217

centrifuged for 10 min at 6000×g, and an aliquot of the supernatant removed and acidified

218

for ICP-MS analysis. Final solution pH was measured after completion of all experiments.

219

Adsorption edge and isotherm were performed in 12 mL screw-cap teflon digestion vessels

220

(Savillex). The suspensions were manually agitated and then mixed, end over end, for 3 hours

221

at 30 rpm. After mixing, the suspensions were allowed to settle by gravity, at which point

222

an aliquot of the supernatant was removed and analyzed by ICP-MS and pH was measured.

223

The chemical and physical stability properties of the adsorbents were also investigated

224

via adsorption isotherms. Aliquots of the DTPADA adsorbent were washed with either 1

225

N HCl or 1 N NaOH overnight. After centrifugation, the supernatant was decanted and

226

the solids rinsed three times with DI water. After decanting the final DI wash, the slurries

227

were dried overnight at 98◦ C in polypropylene centrifuge tubes and used in an adsorption

228

isotherm experiment. Solid masses were not tracked during these wash protocols to account

229

for dissolution. However, since all adsorbents were weighed prior to use, any silica dissolution

230

would not have an impact on interpreting results.

12


Page 12 of 34

Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


231

Models used to evaluate adsorbent performance

232

All data analysis and visualization was performed in the R language for statistical computing

233

(33 ), utilizing additional packages plyr, dplyr, tidyr, broom, and ggplot2 where

234

necessary (34 –38 ).

235

In all experiments, changes in the supernatant concentration (initial, Ci , and final, Ce )

236

were used to determine the relevant quantities, either the sorbed fraction (Eq. 1a) or the

237

mass adsorbed (Eq. 1b; V is solution volume, m is adsorbent mass). Raw data for the results

238

presented here are found in Supporting Information.

Ce Ci

(1a)

V (Ci − Ce ) m

(1b)

fads = 1 − qe =

239

Isotherms were evaluated using a Freundlich model as the data were log-log linear in all

240

experiments (i.e. adsorbate doses were insufficient to approach saturation of the active sites).

241

This relationship is defined by Equation 2, where qe is defined previously by Eq. 1b. The

242

relevant parameters are the partition coefficient (or chemical affinity), Kd , and the degree

243

of linearity, n. Parameters were fit to experimental isotherms by first log-transforming the

244

data and using ordinary least squares regression.

1

qe = Kd · Cen

13


(2)



Page 14 of 34

Page 15 of 34




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

274

tantly, the yields reported by Wissmann and Kleiner (29 ) were for solution-phase coupling,

275

and not surface tethering. By considering the possible formation of multiple amide bonds to

276

individual DTPADA species, the observed mass loss may indicate more complete conversion

277

of the surface amines. Namely, if two amide moeities are formed per DTPADA ligand, the

278

observed mass loss would correspond to 74% amine conversion, though this would not have

279

an impact on the moles of DTPADA grafted. Similarly, the observed mass loss would indi-

280

cate > 100% conversion in a 3:1 model. The theoretical curves and observed data are given

281

in the Supporting Information. Conversely, the DTPA-functionalized solid had > 100% ef-

282

ficiency. This excess of ligand suggests non-specific binding of the ligand to the surface,

283

and may also explain the significant base neutralizing capacity demonstrated in Figure 3.

284

This is supported by the dramatically different mass-loss pattern observed for these solids

285

compared to the rest of the materials (Supporting Information). Finally, the top-down func-

286

tionalization approach appears to yield a greater ligand loading, however the precise form

287

and stoichiometry of the surface groups cannot be determined by TGA. Table 1: Organic content, amine conversion efficiency, and ligand loading for functionalized adsorbents determined by TGA. The range of values for DTPADA represents the range of outcomes for 1:1 or greater stoichiometry of ligand attachment; see text for details. Loading value for TD-DTPADA represents a maximum bound, assuming all organic content occurs as the desired functional group (Figure 2). Solid Organic content (%) AP-SiO2 5.69 BPG 7.54 DTPA 50.1 DTPADA 14.4 PAA 7.39 TD-DTPADA 20.2

Amine conversion (%) — 9 >100 28 − 100 16 —

Ligand loading (mmol/g) 0.96 0.086 >0.96 0.268 0.15 ≤ 0.463

288

Infrared spectroscopy was used, via ATR-FTIR, to investigate the chemistry of the surface

289

functionalization. Figure 5 shows the presence of the expected amide bond in all samples

290

studied (νC−O, amide ≈ 1650 cm−1 ). These spectra, along with the previously presented

291

characterization data, indicate that the functionalization was successful via the proposed

292

attachment scheme. 16


Page 16 of 34

Page 17 of 34




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

293

Adsorption kinetics

294

For all equilibrium testing (adsorption edges and isotherms) a 3-hour contact time was chosen

295

to approximate equilibrium conditions. However, process design considerations (primarily

296

packed bed sizing) would require shorter contact times to be economically viable. Figure 6

297

shows the rapid uptake (within 5 minutes) of the REE by both the DTPADA and PAA

298

adsorbents. A pseudo equilibrium was reached in each of these tests in under one hour,

299

indicating that the desirable, short contact times could be feasible with these materials.

300

The mono-dentate PAA adsorbent had notably more rapid kinetics than the more complex,

301

multi-dentate DTPADA adsorbent, reaching a steady removal after just 5 minutes of mixing.

302

pH dependence of REE uptake

303

As with metal-oxide surface complexation, adsorption on the functionalized SiO2 was ob-

304

served to exhibit a strong dependence on pH. A discussion of the predicted thermodynamic

305

trends for the carboxylate-based adsorbents is found in the Supporting Information. Ad-

306

sorption pH edges for all materials studied are shown in Figure 7.

307

Carboxylate adsorbents

308

Near-complete uptake was observed under acidic conditions (pH < 3) for DTPADA and

309

TD-DTPADA adsorbents. However, contrary to the thermodynamic predictions (see SI),

310

no metal removal was observed at mid-range and basic pH values for DTPA or DTPADA.

311

While DTPA acid-form material showed little uptake at any pH, there was a similar trend

312

to DTPADA of decreasing uptake from pH 2–4, though the effect was subtle, decreasing

313

from a maximum of 15–20% removal to a minimum of 3–7%. Ionic strength effects were

314

ruled out as the cause of the decrease by observing the same effect under three separate

315

electrolyte conditions (see Supporting Information). A similar phenomenon was observed by

316

Shiraishi et al. (23 ), where Cu2+ adsorption was diminished with increasing pH. Repo et al.

317

(39 ) reported a traditional adsorption edge onto DTPA-functionalized chitosan for Ni2+ and 18


Page 18 of 34

Page 19 of 34





Page 20 of 34

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


318

Co2+ at high sorbate concentrations (100 ppm), while exhibiting a similar decline with pH

319

under lower sorbate loading (1 ppm).

320

The lack of REE uptake at mid-range pH may be explained by an electrostatic interaction

321

between the COO– moieties of the DTPA molecules and either (1) the positively-charged,

322

unreacted surface amines or (2) the tertiary amine moieties of adjacent surface DTPA. At

323

low pH, the carboxyl groups are sufficiently protonated (and therefore neutral) as to im-

324

pede interaction with any positively charged groups; here the adsorption is effectively an

325

exchange of protons for the sorbing species. At higher pH, increased sorbate concentrations

326

could provide sufficient electrostatic attraction between the surface groups and the free ion

327

to overcome these effects. Our preliminary data (Supporting Information) in the same ex-

328

periments, but with Gd at 1 mM, showed removal at pH 5–7 by DTPA and pH 3.5 for

329

DTPADA (higher pH not tested).

330

We have employed the top-down functionalization strategy in an attempt to assess the

331

influence of excess surface amines as this scheme should limit the number amount of un-

332

reacted amines on the particle surfaces. As seen in Figure 7, REE removal does decrease

333

with increasing pH, however some functionality is retained through mid-ranged pH. These

334

results indicate that excess surface amines were likely playing a role in limiting adsorption

335

by bottom-up functionalized materials. In addition to DTPA, both Shiraishi et al. (23 ) and

336

Repo et al. (39 ) also studied EDTA-functionalized solids, which did not produce the de-

337

creased uptake trend; this suggests that the additional, central amine of DTPA contributes

338

to the observed effects via the proposed cross-linking mechanism. The persistence of a de-

339

crease in the TD-DTPADA material appears to corroborate this mechanism after controlling

340

for surface amines.

341

Phosphonate adsorbents

342

The PAA-functionalized adsorbent exhibited more typical pH edge adsorption behavior for

343

cations, increasing as a function of pH, however there was a noticeable dip in the adsorption

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

344

edge, decreasing above pH 7. Greater removal of the heavier REE suggests some form of

345

differential complexation; the trend appears to match closely the predicted speciation of the

346

REE in the experimental solutions (Supporting Information), where the free Ln3+ ion persists

347

to more alkaline conditions for Ho vs. Nd (for which hydroxy-complexes begin to dominate

348

near pH 8). In addition to results from the top-down materials, uptake by PAA of the

349

REE at mid-ranged pH supports the hypothesis of the DTPA carboxyl groups interacting

350

electrostatically with surface amines, because, while negatively charged, the phosphonate

351

group has little to no freedom of movement, owing to the short single carbon “chain” between

352

the active group and the amide surface-attachment.

353

The adsorbent with bisphosphonate ligand, BPG, showed excellent uptake (> 95%) across

354

the whole pH range tested, except for a dip near pH 5. This minimum uptake corresponds

355

closely to the maximum observed zeta potential for the unfunctionalized, aminopropyl-silica

356

and the PZC of the BPG-functionalized adsorbent (Figure 4). The correlation of this max-

357

imum positive charge (corresponding to unreacted surface amines) to the pH of decreasing

358

REE removal by the BPG- and DTPADA-adsorbents lends further evidence to the electro-

359

static mechanisms proposed above. Conversely, the subsequent increase in REE removal by

360

the BPG-adsorbent (as compared to both DTPADA and TD-DTPADA) seems to reinforce

361

the hypothesis that the long nitrogen backbone of the DTPA molecules adversely affects

362

their performance, as BPG is geometrically more similar to the EDTA adsorbents studied

363

by Shiraishi et al. (23 ) and Repo et al. (39 ). In the absence of strong intra-molecular forces

364

between the surface groups, electrostatic interactions between the charge-dense REE and

365

the reactive sites may be sufficient to overcome the influence of unreacted surface amines.

366

Constant pH isotherms

367

Adsorption isotherms were used to quantify the affinity of the surface-attached ligands for

368

the REE under a range of conditions. The performance of the adsorbents with the four

369

ligands are compared in Figure 8 on the uptake of Nd, Gd, and Ho from 0.5 m NaCl. In this 22


Page 22 of 34

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


370

figure, a more reactive adsorbent plots further to the left. The adsorbents functionalized with

371

DTPADA (at pH 2) and BPG (at pH 1.6) were the clearly superior adsorbents among the

372

group, followed by PAA and TD-DTPA, and lastly by the adsorbent with acid-form DTPA.

373

Isotherms were fit using the Freundlich model (Eq. 2), with the estimates of Kd shown in

374

the lower portion of Figure 8. These results further illustrate the large disparity between

375

the DTPADA and BPG adsorbents and the alternatives. The distribution coefficients follow

376

an opposite trend to the stability constants for the aqueous complexes (except for with TD-

377

DTPADA), decreasing with increasing atomic number for the REE. The data were insufficient

378

to determine the cause of this difference.

379

While highly dependent upon experimental conditions and analytical precision, these Kd

380

values compare favorably to those published for other novel adsorbents. For example, the

381

self assembled monolayers on mesoporous supports (SAMMS) developed by Fryxell et al.

382

(40 ) demonstrated > 99.9% removal of REE from 0.1 M NaNO3 at pH 2 for a variety of

383

phosphonate ligands (i.e. Kd ∼ 104 mL/g). Similarly, Kd values on the order of 104 mL/g

384

were observed for EDTA-based SAMMS studied in natural water samples (41 ). Finally,

385

the Kd values observed here exceed those measured by Roosen et al. (42 ) using DTPA-

386

functionalized chitosan-silica hybrid materials at pH 2 (∼ 100 − 1000 mL/g depending on

387

element).

388

As was concluded from the adsorption edge data, it is apparent that the ability to attach

389

the DTPA at a single carboxyl group (by performing the synthesis with the dianhydride

390

form) offers significant benefits over a functionalization scheme using the acid-form. This

391

could result from two factors. First, the high affinity for DTPA towards the REE in solution

392

is based on the ability of the ligand to form a highly-coordinated “cage” around the metal.

393

This mechanism utilizes all five of the carboxyl groups in solution to maximally coordinate

394

the ion. The attachment of the ligand to a surface at any of the carboxyl groups will alter the

395

ability to coordinate the REE ions. We propose that these results suggest this “penalty” is

396

limited by attaching at the lone carboxyl group, emanating from the central, tertiary amine

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

397

of the DTPA molecule. Stated differently, the potential for the attachment of the acid-form

398

DTPA at any of its carboxyl groups results in surface groups that are sterically hindered

399

from forming the desired coordination geometry, and thus relies primarily on electro-static

400

interactions with the ion. Alternatively, because there are no protected carboxyl groups

401

in the acid-form molecule, there is a potential for multiple carboxyl groups from the same

402

molecule to attach to the surface via amide bonds.

403

In order to be economically feasible, the adsorbents must be simple to regenerate (e.g.

404

with an acid elution) and withstand the chemical stresses of the eluent. In addition, the

405

high temperatures of brines from the subsurface necessitate the demonstration of a material

406

resistant to thermal decomposition. Elution of the sorbed REE was tested for the constant-

407

pH isotherms on the DTPADA and BPG adsorbents by mixing the water-washed solids with

408

10 mL of 5wt% HNO3 (≈ 0.8 N) for 3 hours. In a single step, 83 – 94 % of the adsorbed mass

409

was eluted for DTPADA and 89 – 104% for BPG (Figure 9), with the order of elution being

410

Nd > Gd > Ho. The eluted mass was uncorrelated with the adsorbed mass (not shown).

411

Contrary to the isotherms themselves, the elution data do follow the trend predicted from

412

thermodynamics, where the order of βM L values is Ho > Gd > Nd (28 ).

413

Figure 10 shows the results of adsorption isotherms for the DTPADA adsorbent following

414

either HCl or NaOH wash and drying at 98◦ C. While the test conditions were slightly different

415

than tests with the untreated materials (i.e. 5 g solid/L vs. 10 g/L, higher sorbate loading

416

up to 3 ppm), the uptake remained high. These results speak to the chemical and thermal

417

stability of the support as well as the functionalization chemistry.

418

Summary and Conclusions

419

The overall objective of this work was to validate the affinity of surface-attached ligands

420

for the REE using a model (silica) solid support and attachment schemes. Silica gel ad-

421

sorbents functionalized with REE-reactive ligands were synthesized and characterizated by

24


Page 24 of 34

Page 25 of 34





Page 26 of 34

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


422

a suite of techniques to confirm successful synthesis. Batch adsorption testing was done to

423

determine the performance of the adsorbents under environmentally relevant sorbate concen-

424

trations. Multi-dentate ligands, DTPADA and BPG yielded the most reactive functionalized

425

adsorbents (average Kd = 2413 and 3829 mL/g for DTPADA and BPG respectively), were

426

chemically and physically robust (no loss of performance after aggressive acid and base

427

washes followed by heated dessication), and regenerateable (> 80% elution of REE from

428

DTPADA and BPG in a single step). However, as in previous studies, amino-polycarboxylic

429

acid adsorbents showed limited uptake at mid-range pH and low-sorbate concentrations. As

430

a potential remedy to this observation an alternative functionalization scheme was employed

431

to limit excess surface amines; an improvement was observed with respect to mid-ranged

432

pH uptake, however it was accompanied by a decrease in reactivity. This work highlighted

433

the complexity of inter-molecular interactions between even moderately sized reactive sites

434

when developing high-capacity, high-selectivity adsorbents. Further optimization of material

435

synthesis was not the focus of this work (e.g. increasing ligand load), but is possible and

436

could alter the resulting performance of these materials.

437

This study has not considered the additional complexities of natural brines — such as

438

competing cations and anions as well as dissolved organic carbon — which would be required

439

for use of these adsorbents in an REE recovery scheme. Future studies of these materials

440

should focus on determining their selectivity for uptake of the REE over non-target cations

441

in more complex solutions. Further study of amino-polycarboxylic acid adsorbents should

442

also seek to resolve possible issues of electrostatic surface site cross-linking; this may be

443

accomplished by decreasing amine density (and therefore increasing functional group spacing)

444

to limit these interactions.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

445

Acknowledgements

446

This research was funded by the DOE Office of Energy Efficiency and Renewable Energy

447

under contract DE-EE0006749. J. Callura was also supported by the CMU College of Engi-

448

neering Dean’s Fellowship; D. Dzombak was also supported by the Hamerschlag University

449

Professorship.

450

Supporting Information

451

The Supporting Information contains: ICP-MS operational parameters; full ATR-FTIR spec-

452

tra; a discussion of thermodynamic predictions of REE speciation and adsorption; ancillary

453

adsorption pH edge data (including the effects of ionic strength and high REE concentration).

454

In addition, the raw data for all figures presented in this manuscript are available.

455

For Table of Contents Use Only

456

Title

457

Effects of ligand chemistry and geometry on rare earth element partitioning from saline

458

solutions to functionalized adsorbents

459

Authors

460

Clinton W. Noack, Kedar M. Perkins, Jonathan C. Callura, Newell R. Washburn, David A.

461

Dzombak, and Athanasios K. Karamalidis

462

Sustainability synopsis

463

Exploitation of abundant low-value saline waters for recovery of energy-critical materials will

464

help achieve global green-energy development goals.

28


Page 28 of 34

Page 29 of 34




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

467

References

468

(1) Frost & Sullivan, Supply/Demand Dynamics of Rare Earth Elements; Report, 2013.

469

(2) Bauer, D.; Diamond, D.; Li, J.; Sandalow, D.; Telleen, P.; Wanner, B. US Department

470

of Energy Critical Materials Strategy; Report, 2010.

471

(3) Binnemans, K.; Jones, P. T.; Blanpain, B.; Van Gerven, T.; Yang, Y.; Walton, A.;

472

Buchert, M. Recycling of rare earths: a critical review. Journal of Cleaner Production

473

2013, 51, 1–22.

474

(4) U.S. Geological Survey, Mineral commodity summaries 2015 ; Report, 2015.

475

(5) Gupta, C.; Krishnamurthy, N. Extractive metallurgy of rare earths. International Ma-

476

477

478

479

480

terials Reviews 1992, 37, 197–248. (6) Peiró, L. T.; Méndez, G. V. Material and energy requirement for rare earth production. JOM 2013, 65, 1327–1340. (7) Chakhmouradian, A. R.; Wall, F. Rare Earth Elements: Minerals, Mines, Magnets (and More). Elements 2012, 8, 333–340.

481

(8) Zaimes, G. G.; Hubler, B. J.; Wang, S.; Khanna, V. Environmental Life Cycle Per-

482

spective on Rare Earth Oxide Production. ACS Sustainable Chemistry & Engineering

483

2015, 3, 237–244.

484

(9) Binnemans, K.; Jones, P. T.; Blanpain, B.; Van Gerven, T.; Pontikes, Y. Towards zero-

485

waste valorisation of rare-earth-containing industrial process residues: a critical review.

486

Journal of Cleaner Production 2015, 99, 17–38.

487

488

(10) Hatch, G. P. Dynamics in the Global Market for Rare Earths. Elements 2012, 8, 341– 346.

30


Page 30 of 34

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


489

(11) Tunsu, C.; Petranikova, M.; Gergorić, M.; Ekberg, C.; Retegan, T. Reclaiming rare

490

earth elements from end-of-life products: A review of the perspectives for urban mining

491

using hydrometallurgical unit operations. Hydrometallurgy 2015, In press.

492

493

(12) Ciacci, L.; Reck, B. K.; Nassar, N. T.; Graedel, T. E. Lost by Design. Environmental Science & Technology 2015, 49, 9443–51.

494

(13) Bandara, H. M. D.; Mantell, M. A.; Darcy, J. W.; Emmert, M. H. Rare Earth Recycling:

495

Forecast of Recoverable Nd from Shredder Scrap and Influence of Recycling Rates on

496

Price Volatility. Journal of Sustainable Metallurgy 2015, 1–10.

497

498

(14) Michard, A. Rare earth element systematics in hydrothermal fluids. Geochimica et Cosmochimica Acta 1989, 53, 745–750.

499

(15) Lewis, A. J.; Palmer, M. R.; Sturchio, N. C.; Kemp, A. J. The rare earth element

500

geochemistry of acid-sulphate and acid-sulphate-chloride geothermal systems from Yel-

501

lowstone National Park, Wyoming, USA. Geochimica et Cosmochimica Acta 1997, 61,

502

695–706.

503

(16) Lewis, A.; Komninou, A.; Yardley, B.; Palmer, M. Rare earth element speciation in

504

geothermal fluids from Yellowstone National Park, Wyoming, USA. Geochimica et Cos-

505

mochimica Acta 1998, 62, 657–663.

506

(17) Noack, C. W.; Dzombak, D. A.; Karamalidis, A. K. Rare Earth Element Distributions

507

and Trends in Natural Waters with a Focus on Groundwater. Environmental Science

508

& Technology 2014, 48, 4317–4326.

509

510

511

512

(18) Lightfoot, E.; Cockrem, M. What are dilute solutions? Separation Science and Technology 1987, 22, 165–189. (19) Nash, K. L. A review of the basic chemistry and recent developments in trivalent felements separations. Solvent Extraction and Ion Exchange 1993, 11, 729–768. 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

513

514

(20) Pearson, R. G. Hard and soft acids and bases. Journal of the American Chemical Society 1963, 85, 3533–3539.

515

(21) Johannesson, K. H.; Lyons, W. B.; Stetzenbach, K. J.; Byrne, R. H. The solubility con-

516

trol of rare earth elements in natural terrestrial waters and the significance of PO3– 4 and

517

CO2– 3 in limiting dissolved rare earth concentrations: A review of recent information.

518

Aquatic Geochemistry 1995, 1, 157–173.

519

520

(22) Xie, F.; Zhang, T. A.; Dreisinger, D.; Doyle, F. A critical review on solvent extraction of rare earths from aqueous solutions. Minerals Engineering 2014, 56, 10–28.

521

(23) Shiraishi, Y.; Nishimura, G.; Hirai, T.; Komasawa, I. Separation of transition metals

522

using inorganic adsorbents modified with chelating ligands. Industrial & Engineering

523

Chemistry Research 2002, 41, 5065–5070.

524

(24) Walcarius, A.; Etienne, M.; Bessière, J. Rate of access to the binding sites in organ-

525

ically modified silicates. 1. Amorphous silica gels grafted with amine or thiol groups.

526

Chemistry of materials 2002, 14, 2757–2766.

527

(25) Repo, E.; Kurniawan, T. A.; Warchol, J. K.; Sillanpää, M. E. Removal of Co (II) and

528

Ni (II) ions from contaminated water using silica gel functionalized with EDTA and/or

529

DTPA as chelating agents. Journal of Hazardous Materials 2009, 171, 1071–1080.

530

(26) Repo, E.; Warchoł, J. K.; Bhatnagar, A.; Mudhoo, A.; Sillanpää, M. Aminopolycar-

531

boxylic acid functionalized adsorbents for heavy metals removal from water. Water

532

research 2013, 47, 4812–4832.

533

(27) Popov, K.; Rönkkömäki, H.; Lajunen, L. H. Critical evaluation of stability constants

534

of phosphonic acids (IUPAC technical report). Pure and applied chemistry 2001, 73,

535

1641–1677.

32


Page 32 of 34

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

536

537

538

539

540

541

542

543


(28) Grimes, T. S.; Nash, K. L. Acid dissociation constants and rare earth stability constants for DTPA. Journal of Solution Chemistry 2014, 43, 298–313. (29) Wissmann, H.; Kleiner, H.-J. New peptide synthesis. Angewandte Chemie International Edition in English 1980, 19, 133–134. (30) Montalbetti, C. A. G. N.; Falque, V. Amide bond formation and peptide coupling. Tetrahedron 2005, 61, 10827–10852. (31) Dzombak, D. A.; Morel, F. M. Surface complexation modeling: hydrous ferric oxide; John Wiley & Sons, 1990.

544

(32) Mansfield, E.; Tyner, K. M.; Poling, C. M.; Blacklock, J. L. Determination of nanopar-

545

ticle surface coatings and nanoparticle purity using microscale thermogravimetric anal-

546

ysis. Analytical chemistry 2014, 86, 1478–1484.

547

548

549

550

551

552

553

554

555

556

557

(33) R Core Team, R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing: Vienna, Austria, 2014. (34) Wickham, H. The Split-Apply-Combine Strategy for Data Analysis. Journal of Statistical Software 2011, 40, 1–29. (35) Wickham, H.; Francois, R. dplyr: A Grammar of Data Manipulation. 2014; R package version 0.3.0.2. (36) Wickham, H. tidyr: Easily tidy data with spread and gather functions. 2014; R package version 0.1. (37) Robinson, D. broom: Convert Statistical Analysis Objects into Tidy Data Frames. 2015; R package version 0.3.6. (38) Wickham, H. ggplot2: elegant graphics for data analysis; Springer New York, 2009.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

558

(39) Repo, E.; Warchol, J. K.; Kurniawan, T. A.; Sillanpää, M. E. Adsorption of Co (II) and

559

Ni (II) by EDTA-and/or DTPA-modified chitosan: kinetic and equilibrium modeling.

560

Chemical Engineering Journal 2010, 161, 73–82.

561

(40) Fryxell, G. E.; Wu, H.; Lin, Y.; Shaw, W. J.; Birnbaum, J. C.; Linehan, J. C.; Nie, Z.;

562

Kemner, K.; Kelly, S. Lanthanide selective sorbents: self-assembled monolayers on

563

mesoporous supports (SAMMS). Journal of Materials Chemistry 2004, 14, 3356–3363.

564

(41) Johnson, B. E.; Santschi, P. H.; Chuang, C.-Y.; Otosaka, S.; Addleman, R. S.;

565

Douglas, M.; Rutledge, R. D.; Chouyyok, W.; Davidson, J. D.; Fryxell, G. E.;

566

Schwantes, J. M. Collection of Lanthanides and Actinides from Natural Waters with

567

Conventional and Nanoporous Sorbents. Environmental Science & Technology 2012,

568

46, 11251–11258.

569

(42) Roosen, J.; Spooren, J.; Binnemans, K. Adsorption performance of functionalized

570

chitosan–silica hybrid materials toward rare earths. Journal of Materials Chemistry

571

A 2014, 2, 19415–19426.

34


Page 34 of 34

Effects of Ligand Chemistry and Geometry on Rare Earth Element

Recommend Documents