The flexible and reversible preparation of columns for use in high-performance solid phase extraction chromatography by physisorption of organophosphorus acid extractants has been investigated in detail.
Trang 1Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/chroma
Meher G Sanku, Kerstin Forsberg, Michael Svärd∗
Department of Chemical Engineering, KTH Royal Institute of Technology, Teknikringen 42, SE-11428 Stockholm, Sweden
Article history:
Received 17 March 2022
Revised 18 May 2022
Accepted 24 June 2022
Available online 25 June 2022
Keywords:
Physisorption
Impregnation
Metal extraction
Column
Separation
Theflexible and reversiblepreparationofcolumns foruseinhigh-performance solidphaseextraction chromatographybyphysisorptionoforganophosphorusacidextractantshasbeeninvestigatedindetail Twoextractantshavebeenevaluated,bis(2-ethyl-1-hexyl)phosphoricacid(HDEHP)and2-ethyl-1-hexyl (2-ethyl-1-hexyl)phosphonicacid(HEHEHP),butthedevelopedprocedureshouldbebroadlyapplicable
tootherextractants.Theliquid-liquidsolubilityoftheextractantsinfeedsolventsconsistingofaqueous ethanolsolutions ofvarying compositionhasbeen determined The totalamountofadsorbed extrac-tanthasbeenquantifiedbycompletedesorptionandelutionwithethanolfollowedbyacid-base titrime-try.Columnimpregnationwithfeedsolutionsofvaryingconcentrationintheundersaturatedregionhas beensystematically evaluated,and theinfluence ofasubsequentwater washstephas beenexplored
Itisshownthattoachievearobustandreproduciblephysisorption,theadsorbedamountofextractant shouldbedeterminedafterthewashstep,andcaremustbetakenwhenusingindirectmethodsof mea-surement.EquilibriumLangmuir-typeadsorptionisothermsasafunctionoftheextractantconcentration
inthefeedsolutionhavebeendetermined.AdsorptionofHEHEHPishigherthanHDEHPforequalfeed compositions,butthesolubilityofHEHEHPislower,resultinginapproximatelyidenticalmaximum cov-eragelevels.Theabilityoftheresultingcolumnstoseparaterareearthelementshavebeenverifiedfora mixtureofeightmetalsusingacombinedisocraticandgradientelutionofnitricacid
© 2022TheAuthor(s).PublishedbyElsevierB.V ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Chromatography as a separation method has been gaining at-
tention as a promising alternative to solvent extraction – a cur-
rent cornerstone technology in hydrometallurgy – for many impor-
tant metals including but not limited to rare earths and precious
metals [1–5] Depending on the metals and the purity require-
ments, solvent extraction can require a large number of mixer-
settler units in series, which consume vast quantities of often
volatile and hazardous organic solvents [6] By attaching a suit-
able extractant onto the solid phase in a chromatographic column,
the extraction process can be made more sustainable and environ-
mentally friendly This is partly due to reduced consumption of
solvents and extractants, and improved chemical recycling possi-
bilities, but also because a single column corresponds to multiple
equilibrium stages, which eliminates the need for multiple units
In high-performance solid phase extraction chromatography [ 5, 7],
reverse-phase HPLC columns packed with particles containing ad-
∗ Corresponding author
E-mail address: micsva@kth.se (M Svärd)
sorbed extractant molecules are used The metals are typically sep- arated by elution with a gradient of a mineral acid, such as nitric acid, in a dynamic process The main challenge with a chromato- graphic process is to increase the productivity while retaining the purity of individual components [8] Although partly a multivariate optimization problem involving decisions regarding operation vari- ables and fractionation [5], attention should also be directed to- wards how to reliably and effectively supply the solid phase with
a high and stable coverage of extractant
The molecular structure of many suitable extractants is com- posed of a hydrophilic part that interacts with the metal ions and a lipophilic part that interacts with the nonpolar phase, which could
be an extraction solvent or the stationary phase of a chromato- graphic column Bis (2-ethyl-1-hexyl) phosphoric acid (HDEHP) is amongst the most extensively studied extractants [ 5, 7, 9–12] with some studies also available on 2-ethyl-1-hexyl (2-ethyl-1-hexyl) phosphonic acid (HEHEHP) [ 1, 10, 13] and other acidic as well as neutral extractants [ 1, 10, 13–16] Such extractants can be physically adsorbed (physisorption) onto the solid particles of a reverse phase column The stationary material in such columns typically consists
of porous silica particles functionalized with e.g octadecyl (C 18) carbon chains By impregnating the particles with a feed solution https://doi.org/10.1016/j.chroma.2022.463278
0021-9673/© 2022 The Author(s) Published by Elsevier B.V This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
Trang 2containing dissolved extractant, the surface of the particles can be
coated with a layer of extractant molecules The coverage reached
is limited by the appropriate adsorption isotherm under the con-
ditions of the impregnation process Once established, the interac-
tions between the lipophilic part of the extractant and the nonpo-
lar chains on the support material are stable in aqueous solution,
even at quite low pH
The extractant can be loaded onto the solid support material
either before (batch loading) [ 7, 10, 12, 14, 15, 17, 18] or after (flow-
through loading) [ 7, 9, 13, 18–20] the column is packed The flow-
through impregnation process has several advantages; it is easy to
perform and undo without the need for specialized equipment, the
resulting column performance has been repeatedly claimed to be
stable over several repeated elutions even under harsh acidic con-
ditions, and the choice of extractant and the coverage level – and
thereby the column performance – can be tuned to specific needs
Moreover, any gradual decrease in the column performance could
easily be restored to initial levels by a re-impregnation step
Using 55 wt% methanol in water as feed solvent, extractant cov-
erages in the range 89 to 345 mol HDEHP/m 3 column have been
attained in previous studies [ 7, 9, 13, 21] For HEHEHP, a coverage of
103 mol/m 3column has been reported [13] However, the method-
ology for quantification and validation of the coverage is often not
described, or differs, ranging from analysis of breakthrough solu-
tion during impregnation [ 3, 7] to complete flushing of extractant
before analysis [13] Moreover, it is rarely shown that the impreg-
nation processes are reproducible or under which circumstances
Kifle et al evaluated impregnation of columns at a coverage of ap-
prox 120 mol/ m 3 column (approx 0.3 mmol HDEHP on the col-
umn) and reported that the process was repeatable [7] No studies
were performed at higher extractant concentrations Conversely,
Max-Hansen reported that the ligand concentration after column
impregnation did not always reach expected levels (although no
column details were reported in the study) [3]
In order to serve as the basis for a feasible separation process,
it is of crucial importance that the impregnation process can
re-producibly and reliably deliver a column with the desired extractant
coverage level, and a sufficient stability over repeated elutions un-
der the conditions required to separate the metals for which it is
designed Currently, there is ambiguity or a lack of clarity in the
available literature with respect to these matters In the present
work, the first step for metal extraction using column chromatog-
raphy, the column preparation step, has been thoroughly studied,
for two extractants (HDEHP and HEHEHP; shown in Fig.1) Data
on the liquid-liquid solubility of the extractants in the feed solvent
mixtures, crucial to avoid liquid-liquid phase separation during im-
pregnation which could lead to obstructed flow, pressure build-up
and a damaged column, has been collected Adsorption isotherms,
key to knowing how to alter the feed solution composition in order
to obtain the required extractant coverage on the stationary phase,
have been measured Particular attention is devoted to the repro-
Fig 1 Molecular structure of the two extractants
ducibility of the impregnation process Two methods of estimat- ing the extractant coverage are contrasted, shedding light on the adsorption behaviour of the extractant Reverse phase C 18-coated mesoporous silica columns have been impregnated with each ex- tractant using an ethanol-water mixture as feed solvent The re- sulting columns have been evaluated with respect to their ability
to separate eight REEs predominant in apatite ore (La, Ce, Pr, Nd,
Sm, Gd, Dy and Y) [22] However, the results of the study should be broadly applicable to other RP columns, solvents and extractants
2 Materials and methods
2.1 Materials
The different solutions used in this study are described below All solutions were prepared using the individual components as re- ceived
HNO 3( >69.9%), and ethanol ( >99%) were purchased from VWR, acetic acid ( >96%) from Merck, HDEHP (D 2EHPA, bis (2-ethyl-1- hexyl) phosphoric acid; >97%), Arsenazo III (2,7-bis (2-arsono- phenylazo) chromotropic acid) and urea ( >99.5%) from Sigma- Aldrich, HEHEHP (EHEHPA; PC-88A; 2-ethyl-1-hexyl (2-ethyl-1- hexyl) phosphonic acid; >95%) from Daihachi Chemical Industry Co., and NaOH (2.5 M) from J.T.Baker Single-element REE standard solutions (10,0 0 0 mg/L) were purchased from Teknolab Sorbent All chemicals were used as received Milli-Q grade water was used to prepare all the solutions
Column conditioner A solution of ethanol and water with a
concentration matching the feed solution: 62 wt% ethanol in water
Feed solution Acidic organophosphorus solutions of HDEHP or
HEHEHP dissolved in 62 wt% ethanol in water The amount of ex- tractant in these solutions was decided based on the solubility, and the resulting solution was verified to be a homogeneous single- phase liquid
NaOH solution A 0.25 M solution of NaOH in water was used
for titrations
REE solution A solution of eight REEs (La, Ce, Pr, Nd, Sm, Gd,
Dy and Y), with a concentration of 37.5 mg/L (with respect to each metal) or 300 mg/L (with respect to the total REE content), pre- pared from standard solutions mixed in equal amounts The HNO 3 concentration in the solution was maintained at 0.59 M
HNO 3 solutions 2.0 M and 5.0 M HNO 3 solutions prepared by dilution of concentrated HNO 3 (69.9%)
Arsenazo III solution A 0.15 mM aqueous Arsenazo III solu-
tion, containing 0.10 M acetic acid and 10 mM urea, used for post- column reaction
2.2 Experimental setup
A modified Thermo Scientific Dionex ICS-50 0 0 + Ion Chro- matography System, shown in Fig.2, has been used in the present work Solutions, kept in a cryostatic water bath (Julabo FP-50,
25 ±1 °C) for temperature control, were pumped via a degasser through the column using a quaternary gradient pump The col- umn temperature was maintained at 25 ±2 °C by means of a col- umn thermostat (BioTek Instruments HPLC 582) and the tubing between the solution bottles and the column was thermally in- sulated A dedicated pump was used for the post-column reac- tion solution, which was mixed with the eluting solution down- stream of the column A 750 μL knitted reaction coil was used to provide the necessary reaction time for Arsenazo III-REE complex formation A Dionex UV–Vis variable wavelength detector (VWD), placed downstream of the column, was used to detect extrac- tant breakthrough signals at 288 nm and chromatograms (Arse- nazo III-REE complexes) at 658 nm An automatic fraction col- lector module was used to collect samples for NaOH titration A
Trang 3Fig 2 Schematic of the HPLC setup used in this work The solutions used at the different channels in the water bath vary with the purpose For column preparation: A –
column conditioner, B – acidic organophosphorus solution, C – ethanol, and D – Milli-Q water For REE separation: A – Milli-Q water, B – 2 M HNO 3 , C – 5 M HNO 3 and E – Arsenazo III solution
150 mm x 4.6 mm (i.d.) column packed with Kromasil
(Nouryon) C 18-functionalized mesoporous spherical particles (di-
ameter = 10 μm; pore size = 100 ˚A; pore volume = 0.9 mL/g;
BET-surface = 320 m 2/g; packed density = 0.66 g/mL; carbon con-
tent = 20% or 3.5 μmol/m 2) was used The volume of the column
available for the liquid flow (henceforth CV) as well as contribu-
tions to the dead volume was measured by tracer analysis using
uracil
2.3 Column preparation
2.3.1 Solubility of organophosphorus compounds
The (liquid-liquid) solubility of the organophosphorus extrac-
tant compounds (HDEHP and HEHEHP) in aqueous ethanol solu-
tions has been determined using an iterative process Initially, 0.4
to 5 g of the respective organophosphorus compound was added to
3.6 to 15 g of ethanol to form a homogeneous solution Water was
added to this solution dropwise until the solution turned turbid,
indicating liquid-liquid phase separation Ethanol was again added
until the clear point was reached The process was then repeated
several times The cloud points (onset of liquid-liquid separation)
and the clear points (homogeneous solution) thus form two curves,
which flank the true solubility curve Experiments were performed
in a total of 20 vials to produce 104 and 114 data points (counting
both cloud and clear points) for HDEHP and HEHEHP, respectively
2.3.2 Resin impregnation
The retention of ligands on the column is a result of hydropho-
bic interactions between the C 18chains and the aliphatic moieties
of the HDEHP and HEHEHP molecules Before column impregna-
tion, any retained acid from previous runs was eluted with ethanol
(14 CVs) Then 20 CVs of column conditioner was run through the
column at 1 mL/min This was followed by equilibration of the col-
umn with organophosphorus feed solution at varying flow rates
(0.85 to 1 mL/min for HEHEHP and 0.61 to 1 mL/min for HDEHP)
chosen to ensure constant inlet pressure (72.5 ± 4 bar) Prelimi-
nary runs were performed to measure the amount of feed solu-
tion required to achieve an equilibrium coverage level Finally, the
column was washed with Milli-Q water (at least 20 CVs) For im-
proved reproducibility, special attention has been paid to transition
steps that can lead to formation of two phases For example, the
ethanol wash and the water wash steps are performed at low flow
rates of 0.1 or 0.2 mL/min for at least 1.2 CVs followed by gradual
increase of flow rate and flushing the column with ethanol/water
at a higher flow rate The higher flow rate was set to 1 mL/min for
ethanol and 2 mL/min for water unless otherwise specified
2.3.3 Titration
HDEHP and HEHEHP can be detected at a wavelength of
288 nm However, because of the wide range of extractant concen- trations evaluated, a linear relationship between the intensity and concentration according to the Beer-Lambert law is not applicable, and the use of the detector was restricted to qualitative analysis In this work, the amount of adsorbed organophosphorus compound was calculated using NaOH titration Two methods were used: the
indirect method ( Eq (1)), by titration of the feed collected after passing through the column, and the direct method ( Eq (2)), by titration of the ethanol eluate collected after washing the column with water
where q is the estimated amount of adsorbed acid (in mmol), 1is the inlet concentration (in M) of acid feed solution to the column,
F is the feed flow rate (in mL/min), t is the feed duration (in min),
V is the volume of NaOH solution consumed (in mL) and 2is the concentration of NaOH solution (in M)
The difference between the acid amount obtained by both methods should correspond to the difference between the amounts
of extractant adsorbed strongly and weakly (weakly adsorbed acid
is removed in the water wash step) All samples were titrated with 0.25 M NaOH solution using phenolphthalein as indicator To establish the accuracy and validity of the process, titration was also performed using solutions of known amounts of HDEHP and HEHEHP Titration of samples from the water wash step was not done due to the presence of two liquid phases
2.3.4 Data presentation
The performance indicator of interest in this study is the amount of organophosphorus acid adsorbed on the stationary phase (the extractant coverage) Presenting coverage values only
as amount of acid adsorbed on the column ( q; in mmol) restricts the comparison of data to a single column For the results to be comparable across different columns, it is beneficial to also present them in a more generalized form Coverage values given in units of e.g mmol/m 3of internal column volume, or mmol/m 2of available stationary phase surface area, could be scaled with the column di- mension, provided that the columns are identically packed with the same stationary phase particles Kifle et al compared the re- sults of an impregnation process on different columns, including C 8 and C 18columns as well as two C 18columns with different surface area [7] This study clearly shows that the amount of adsorbed acid (presented as mmol/g silica) is affected by both the hydrophobic- ity (length of the carbon chain) of the column material and by the
Trang 4physical properties of the material such as surface area and pore
size The results of a C 18column are hardly transferable to a C 8col-
umn, but more studies are required even to compare two C 18ma-
terials (of different pore size and surface area) and to know under
what conditions they are comparable In the present study, since
only one type of column was used for all experiments, extractant
coverage is presented in terms of mmol adsorbed acid per entire
column Corresponding values of mmol adsorbed acid per mmol of
carbon on the stationary phase surface (as estimated by the sup-
plier) are given in Table A.1 and A.2 of the Appendix The data can
be converted to other forms based on the information provided in
Section 2.2according to Eq.(3) Since most literature data is only
presented in terms of mmol per entire column with little details
about the properties of the material used, in the introduction sec-
tion, data reported in literature is presented in terms of mmol/m 3
of internal column volume for crude comparison
q
n = q
where n denotes the amount of C 18groups in the packed column
(in mmol), V c the hollow column volume (in mL), and where ρp
is the packing density (in g/mL), a the specific surface area (in
m 2/g), and ϕ the carbon content (in mmol C 18/m 2) of the station-
ary phase
2.4 REE separation
Two columns were prepared, with coverages of 0.5 mmol of
HDEHP and HEHEHP, respectively, according to the method de-
scribed in Section2.3.2, and evaluated for separation of eight REEs
(La, Ce, Nd, Pr, Sm, Gd, Dy and Y) under different elution condi-
tions Before each run, remaining traces of metals were initially re-
moved from the column by eluting with 5 CVs of 5 M HNO 3 solu-
tion, after which the column was conditioned with at least 5 CVs
of HNO 3solution of the same concentration as the elution solution
A 50 μL sample of REE solution was then injected and elution was
performed under isocratic conditions for 30 min A 10 min HNO 3
gradient up to 5 M HNO 3 was appended after the isocratic step to
ensure all REEs in the column were completely eluted The HNO 3
concentration was controlled using the quaternary pump by means
of mixing water with 2 M or 5 M HNO 3solution The column and
solution temperatures were kept constant at 25 °C A constant flow
rate of 1 mL/min was used throughout the experiments
3 Results and discussion
3.1 Validation of titration method
Table 1 shows the concentrations of HDEHP and HEHEHP ob-
tained by titration ( t) for solutions of known extractant concen-
trations ( f) as well as for pure water and ethanol Between 1 –
3 repeat analyses were carried out for each solution Negligible
amounts of acid were detected in the pure solvents as expected
The deviation between repeat experiments is consistently below
1.4%, indicating good reproducibility irrespective of differences in
concentration and extractant However, the relative error with re-
spect to the known concentration is generally higher (mean 4.8%,
ranging up to 13%) Most of the errors obtained are positive, and
a part of this can be attributed to the small but systematic er-
ror involved in using phenolphthalein as indicator There is a trend
of larger relative errors obtained for lower acid concentrations, as
should be expected
Fig 3 Experimental data and regressed exponential curves of the liquid-liquid sol-
ubility of HDEHP in ethanol-water solutions Black and white symbols - experimen- tal data; black line - regressed curve; pink bands - 95% confidence bands; solid symbols – cloud points; hollow symbols – clear points; red triangle – maximum feed concentration used for impregnation experiments
Fig 4 Experimental data and regressed exponential curves of the liquid-liquid sol-
ubility of HEHEHP in ethanol-water solutions Black and white symbols - experi- mental data; black line - regressed curve; pink bands - 95% confidence bands; solid symbols – cloud points; hollow symbols – clear points; red triangle – maximum feed concentration used for impregnation experiments
3.2 Column preparation 3.2.1 Solubility of organophosphorus compounds
The liquid-liquid solubility of HDEHP and HEHEHP in ethanol- water solutions is shown in Figs 3 and 4 as sets of experimen- tally determined cloud- and clear points The experimental data has been regressed to fit an exponential function, Eq.(4), shown as solid black lines in the graphs together with associated 95% con- fidence bands The corresponding fitting parameters are given in Table2together with goodness of fit values ( R2)
c s=A· expx
B
Trang 5
Table 1
Validation of the titration method against solutions of known concentration
Sample
cf
(mmol)
ct
(mmol)
RE ∗
(%)
ct
(mmol)
RE ∗
(%)
ct
(mmol)
RE ∗
(%)
Dev #
(%)
HDEHP
(167 mM)
HDEHP
(239 mM)
HDEHP
(300 mM)
HDEHP
HEHEHP
(21 mM)
HEHEHP
(83 mM)
HEHEHP
(207 mM)
4.288 2.144
4.463 2.153
4%
∗ Relative error = 100 ·|ct−cf |
cf
# Deviation = 100
ct
( ct−ct)2
n where c ̅t is the mean of the values obtained by repeated titration and n is the total number of measurements
Table 2
Solubility regression parameters ( Eq (4) )
Table 3
Effect of amount of feed solution on the amount of acid retained in the column, estimated using the indirect method ( Eq (1) )
Run 1
q
(mmol)
Run 2
q
Run 1
q
(mmol)
Run 2
q
Table 4
The effect of flow rate during the water wash step on the loss of adsorbed acid
F (mL/min)
q
(CVs) #
VH2O,2
(CVs) §
∗ Calculated by the direct method ( Eq (2) )
# The amount of water required to reach an approximately flat detector signal (around 6 mins)
in Fig 4
§ The amount of water required until no more peaks can be observed in the zoomed-in detector signal (calculated from the beginning of water wash)
Table 5
Langmuir isotherm parameters ( Eq (5) ), together with standard errors and R 2
Trang 6Fig 5 The amount of organophosphorus acid retained in the column estimated
using the indirect method ( Eq (1) ) of titration of feed solutions (solid symbols)
and the direct method ( Eq (2) ) of titration of ethanol eluate after washing (hollow
symbols) Circles – HDEHP; triangles – HEHEHP Error bars represent the propagated
error The corresponding data is shown in Table A.1 and A.2 in the appendix
In Eq.(4), s is the solubility of the extractant, x is the pro-
portion (in wt-%) of ethanol in the ethanol-water mixture (solvent
basis) and A, B and C are fitting parameters
As can be seen from a comparison of Figs.3and 4, HDEHP has
higher solubility in aqueous ethanol solutions than HEHEHP, for all
evaluated compositions Based on the measured solubilities, max-
imum concentration limits of HDEHP and HEHEHP feed solutions
were chosen (indicated by red diamonds in the graphs) as 0.75 and
0.26 mmol/g solvent mixture, respectively
3.2.2 Column impregnation
Preliminary experiments were performed to investigate the
amount of feed solution required for the extractant adsorption to
reach equilibrium between the solid phase and the solution, shown
in Table3 Increasing the amount of feed from 10 CVs to 30 CVs
did not lead to a statistically significant increase in the amount of ligand retained in the column It can thus be established that 10 CVs is a sufficient amount of feed solution, and this amount was used in all subsequent experiments
In total 38 column impregnation experiments (20 of which have been analysed both by the direct and the indirect method), consisting of several repeats using a range of feed concentra- tions of the two extractants, have been performed The results in terms of amount of extractant retained in the column are given
in Fig.5 The amount of extractant retained in the column during the impregnation step has been calculated by the indirect method ( Eq (1)), and the amount of adsorbed extractant by the direct method ( Eq.(2)) The two sets of values are compared in Fig.5 As seen in the figure and Table3, the values obtained by the indirect method ( Eq.(1)) show low repeatability, with a deviation between repeat experiments as high as 33% The variability is particularly pronounced at higher concentrations, and for HDEHP (as shown
in Fig.5) However, the values of the adsorbed amount obtained
by Eq.(2)show a high repeatability, with deviations between re- peat experiments lower than 7% in all cases There is a marked difference between the sets of values obtained with the two meth- ods, with adsorbed amount consistently lower than the amount re- tained during the feed step This shows that a significant fraction
of the retained extractant is loosely bound to the column after the impregnation step, and can be washed out with water The large difference between the values obtained with the two methods em- phasizes that care must be exercised when using indirect meth- ods to quantify the amount of extractant adsorbed during column impregnation At least, any indirect measurement method should specifically account for extractant lost in the water wash step The influence of the flow rate of the water wash step has been studied for column impregnation with a feed concentration
of 447 mM HDEHP The final flow rate of the water wash step was changed and the adsorbed acid on the column was measured by the direct method ( Eq.(2)) after 40 CVs of water wash The results
of the changed flow rate, shown in Table4, suggest that the flow rate did not have any noticeable effect on the amount of adsorbed acid for the range of flow rates and duration considered in these studies Additionally, the UV signal at 288 nm was used as a quali- tative indication of the loss of acid during the water wash step As
Fig 6 Loss of organophosphorus acid during the water wash step detected with an in-line UV detector Black – Signal value (mAU); Red – Flow rate (mL/min) The data
shown corresponds to F = 2 mL/min in Table 4 Inset shows a magnified part of the detector signal
Trang 7Fig 7 Column adsorption isotherms of HDEHP and HEHEHP at 25 °C Symbols –
experimental data; lines – regressed Langmuir isotherms; pink bands - 95% confi-
dence bands; circles – HDEHP; triangles – HEHEHP Error bars represent the error
in experimental data calculated as shown in the Appendix The corresponding data
is shown in Table A.1 and A.2 in the appendix
seen in Fig.6, the loss of adsorbed acid occurs in two steps First,
there is a period of significant loss of acid (in this case until about
6 mins) followed by a period of more subtle and intermittent loss
seen as peaks in the inset The total number of CVs of water wash
required for these respective losses are noted in Table4for com-
parison Again, the effect of flow rate was negligible For all feed
concentrations between 80 and 500 mM, for both extractants, re-
peat experiments were carried out using different amounts of wa-
ter, in the range 20 – 50 CVs In all cases, increasing the amount
of water in the wash step did not lead to a detectable change in
the amount of extractant adsorbed This shows that any loss of
loosely bound extractant occurring after the initial loss did not sig-
nificantly affect the adsorbed acid amount However, long term ex-
posure, e.g over several months, might lead to a significant loss of
extractant and should be investigated to analyse the need for col- umn regeneration steps, which would affect the economic viability
of the process
3.2.3 Adsorption isotherms
The experimentally determined values for the adsorbed amount
of extractant after water washing ( Eq.(2)) are plotted vs feed con- centration in Fig.7, for both extractants For identical molar ex- tractant concentrations in the feed, significantly more HEHEHP is adsorbed compared to HDEHP However, the solubility of HEHEHP
in a given ethanol-water solution is much lower than the solubil- ity of HDEHP Consequently, the maximum amount of extractant that can be adsorbed on the column under the evaluated con- ditions (column properties, temperature and solvent composition)
is quite similar for both compounds (slightly more than 1 mmol, corresponding to approximately 400 mol/m 3 column or 0.6 mmol acid/mmol C 18) Nevertheless, the amount of extractant required in the solution for a given coverage level is lower in case of HEHEHP compared to HDEHP The experimental data for both extractants is well described by a Langmuir isotherm, Eq.(5), where q denotes the amount adsorbed on the column, the feed concentration, and
qmax(the maximum adsorbed amount) and K(the equilibrium con- stant) are parameters determined in the fit The fitted isotherms are shown in Fig.7with parameters given in Table5
q=qmaxKc
3.3 REE separation
For the columns used for separation experiments, the amount
of adsorbed extractant was found to be 0.52 and 0.56 mmol of HDEHP and HEHEHP, respectively, as obtained after titration of ethanol elution Fig.8shows chromatograms obtained in three REE separation experiments On the HDEHP column, isocratic elution using 0.12 M of nitric acid solution as eluent led to visible sep- aration of the lighter REEs (La, Ce, Pr and Nd) while the middle
to heavy REEs (Sm, Gd, Dy and Y) could be separated using gra- dient elution, in well separated peaks over the approximate nitric
Fig 8 Chromatograms showing separation of REE using a C18 column functionalized with 0.5 mmol of HDEHP (top) and HEHEHP (middle and bottom), with 30 min isocratic
elution at the concentration specified in the legend followed by 10 min gradient elution to 5 M HNO 3 (elution profiles shown as black lines on second axis)
Trang 8acid concentration range 1 – 3 M On the HEHEHP column, iso-
cratic elution at an HNO 3concentration of 0.12 M led to co-elution
of La, Ce, Pr and Nd as one peak Sm eluted directly following the
peak of the light REEs, with Gd following The heavy REEs (Dy and
Y) remained to be eluted by gradient elution (at approx 2 – 3 M
HNO 3) A significantly lower HNO 3concentration of 0.05 M during
the isocratic stage provided for a much better separation of the
lighter REEs on the HEHEHP column, and also resulted in Sm and
Gd being retained in the column until the gradient step
Overall, the results show that RP-HPLC columns with either
HDEHP or HEHEHP as adsorbed extractants, prepared using the
method described in this work, can be used for separation of REEs
It should be noted that these chromatographic experiments are not
optimized for either resolution or productivity Proper optimization
is a major undertaking, and would require a substantial amount
of experimental data, and – for preparative chromatographic
purposes – significantly increased sample loads into the so-called
overloaded range
It is interesting to compare the performance of the two ex-
tractants for REE separation, relative to each other in a chro-
matographic process, and relative to their reported performance in
solvent extraction This has been studied for analytical application
by Horwitz et al [10]using a chromatographic setup where extrac-
tant is adsorbed on large (50–100 μm) polymer beads and slurry-
packed in glass column, and where REEs are eluted by gravity-
induced flow In their comparison, the selectivities of the extrac-
tants was shown to be virtually identical for the two techniques
HDEHP (p Ka = 3.24) [23] is known to be an efficient extractant
for the separation of REEs in traditional solvent extraction, but the
difficulty in stripping the loaded metals leads to a limited use for
extraction of heavy REEs HEHEHP (p Ka = 4.51) [23] has a some-
what lower affinity for REE overall, but compensates with a some-
what increased selectivity, and allows stripping of REEs at lower
acid concentrations [ 6, 24, 25] For the HPLC-type process used in
the present work, a comparison of the extractants ( Fig.8) adsorbed
at approximately equal molar coverage levels shows that elution
with comparable separation can be achieved at significantly lower
acidity for HEHEHP compared to HDEHP, mirroring their solvent
extraction behaviour A reduced HNO 3consumption would be ben-
eficial in an industrial process [ 6, 26]
4 Conclusions
A process has been developed for reliably preparing an RP-HPLC
column with organophosphorus acid extractants by physisorption
through impregnation by elution with aqueous alcoholic solutions,
as demonstrated for HDEHP and HEHEHP By making use of the
solubility curves and adsorption isotherms, appropriate feed con-
centrations can be obtained, allowing columns with a required ex-
tractant concentration to be prepared The resulting columns con-
taining either HDEHP or HEHEHP as adsorbed extractants, prepared
using the method described in this work, are verified to be able to
separate the REEs in mixture of eight elements using a combined
isocratic and gradient elution with nitric acid
The solubility of both extractants increases non-linearly with
ethanol content in the solvent mixture, with the solubility of
HEHEHP in any given solvent composition being lower than
HDEHP For solutions of equal extractant concentrations, however,
the amount of extractant adsorbed on the column at equilibrium is
significantly higher for HEHEHP than for HDEHP This indicates that
a lower extractant consumption is required for HEHEHP compared
to HDEHP, in order to reach the same coverage on the column It
is established that elution with 10 CVs of feed solution is sufficient
to reach equilibrium with respect to the adsorbed amount of ex-
tractant
It is also shown that a significant and variable amount of the extractant is loosely bound to the column and readily removed by water in a subsequent washing step A significant loss of extractant
is shown to occur at the beginning of the washing step (requiring
in total about 2.4 CVs of water) with subsequent loss being inter- mittent and lasting for approx 16–23 CVs Additional washing did not show a detectable difference in the amount of extractant ad- sorbed on the column The influence of the flow rate during the washing step was found to be negligible Overall, this stresses the importance of basing the estimation of the amount of extractant adsorbed on the column on data obtained after washing with suf- ficient amounts of water It should be mentioned that phenomena such as pore dewetting and phase collapse [27], which constitute potential problems for scale-up of chromatographic processes, have not been studied specifically in this work, and should be investi- gated in more detail
Funding sources
This work was carried out within the REEform project, for which the authors gratefully acknowledge funding by Formas (grant no 2019–01150)
Declaration of Competing Interest
The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper
CRediT authorship contribution statement Meher G Sanku: Formal analysis, Investigation, Methodology, Validation, Writing – original draft, Writing – review & edit- ing Kerstin Forsberg: Conceptualization, Funding acquisition, Su- pervision, Writing – original draft, Writing – review & editing
Michael Svärd: Conceptualization, Funding acquisition, Methodol-
ogy, Project administration, Supervision, Writing – original draft, Writing – review & editing
Acknowledgement
Nouryon Pulp and Performance Chemicals, Bohus, Sweden, are gratefully acknowledged for supplying Kromasil C 18columns
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.chroma.2022.463278
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