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Research Article

Powdered Hierarchically Porous Silica Monoliths for the Selective
Extraction of Scandium
Aaron Brewer,* Chloé Reicher, Olivia Manatschal, Hongzhi Bai, Kazuki Nakanishi, and Freddy Kleitz*
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ABSTRACT: Scandium (Sc) is a high value Critical Material that ismost commonly used in advanced alloys. Due to current andpotential supply limitations, there has been an international effort tofind new and improved ways to extract Sc from existing and novel
resources. Solid-phase extraction (SPE) is one promising approachfor Sc recovery, particularly for use with low-grade feedstocks. Here,
unfunctionalized, powdered hierarchically porous silica monoliths
from DPS Inc. (DPS) are used for Sc extraction in batch andsemicontinuous flow systems at model conditions. The sorbent
exhibits excellent mass transfer properties, much like the wholemonoliths, which should permit Sc to be rapidly recovered fromlarge volumes of feedstock. The Sc adsorption capacity of thematerial is ∼142.7 mg/g at pH 6, dropping to ∼12.0 mg/g at pH 3, and adsorption is furthermore highly selective for Sc comparedwith the other rare earth elements (REEs). Under semicontinuous flow conditions, recovery efficiency is limited by a kinetic process.
The primary mechanism responsible for the system’s slow approach to equilibrium is the Sc adsorption reaction kinetics rather thaninter- or intraparticle diffusion. Overall, this unmodified hierarchically porous silica powder from DPS shows great promise for theselective extraction of Sc from various feedstocks.
KEYWORDS: solid-phase extraction, critical materials, scandium, hierarchically porous monoliths, silica■

INTRODUCTION
Scandium (Sc) is commonly classified as one of the 17 rareearth elements (REEs), and is one of the metals considered to
be a Critical Material (CM) by governments around the world,
particularly in North America and Europe.1,2 This Critical
Material designation is based on the importance of Sc for themodern economy combined with its concerning potential
supply risks. Scandium is used primarily for lightweight alloys,
for example in the aerospace industry, and for solid oxide fuelcells, as well as more minor applications in ceramics andelectronics.1,3 These uses have driven its price up, and Sc ispresently the most valuable of the REEs.3 Indeed, its value ishigh enough that the economic potential of some possible
future REE resources may be largely determined by thepresence of Sc alone.4 However, despite the value and practicalutility of Sc, meaningful production of this metal is limited toonly a few major sources around the world, with 66% comingfrom China in 2020.1 Since these sources have been andcontinue to be vulnerable to various geopolitical circumstances,
there has been a significant push in many regions to identify
new potential Sc feedstocks and to develop new and improved
techniques for Sc extraction and purification. Among theseemerging novel recovery systems, solid-phase extraction (SPE)is one broad class of metal recovery techniques which employsa solid material to selectively recover a specific component of© 2023 The Authors. Published byAmerican Chemical Society

an aqueous feedstock (i.e., a mineral leachate), generally
through adsorption. The purified aqueous metal is thentypically precipitated and roasted to produce a salable solid
product, such as a metal oxide. SPE can offer advantages,
particularly in the area of environmental impact, as thesesystems typically require limited or no hazardous organic
solvents for synthesis or use, unlike traditional liquid−liquid
extraction approaches. They are also highly reusable and canfunction well with low-grade feedstocks like recycled materials
or mining wastes,5 which represent valuable potential new
resources for this metal.6 Numerous studies have examined awide range of materials for the solid-phase extraction of Sc andother REEs from many different feedstocks.
Monolithic structures with both macropore and mesopore
regimes, known as hierarchically porous monoliths, have long
been an especially tempting target for use as solid-phaseextractants for specific metals, such as Sc. The monoliths haveso far been comprised of various different materials, including
Received: July 26, 2023
Revised: September 19, 2023
Published: October 11, 2023

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carbon and titania, although silica monoliths are perhaps themost common, as they offer several key advantages for SPE
systems.7 Silica is particularly promising for Sc extraction, as
silica surfaces are inherently able to selectively adsorb Sc
without prior modification.8,9 The mechanism of this selective
Sc adsorption is not yet fully understood, but it is attributed tothe silanol functional groups present on the surface ofamorphous silica, and a specific structure or conformationsof silanol groups may be required for adsorption to occur.8
Optimal Sc extraction occurs at a specific pH range (∼3−5)
because, at lower pH, the silica surface becomes positively
charged, repulsing the Sc3+ ion, while at higher pH, Sc
precipitation becomes an issue.8 Unmodified silica can adsorb
certain other metals, such as thorium,9 so this effect is notentirely exclusive to Sc, which must be taken into account forsome future feedstocks. The monolithic silica structures, inparticular, generally exhibit a high surface area for adsorption,and the silica monoliths specifically display exceptional
adaptability, due to the range of options for functionalizationand excellent tunability in terms of characteristics like poresize, pore volume, and framework structure.7,10 Perhaps mostimportantly, however, the hierarchically porous monoliths havesuperior mass transport properties compared to traditional
particulate adsorbents.9,11 This attribute would, in principle,
permit large volumes of a given feedstock to be rapidly
processed through the sorbent, greatly elevating the potential
scalability of the system and circumventing common issues,
such as pressure buildup and column clogging. That significant
advantage, combined with the other promising characteristics
of hierarchically porous silica monoliths, has generated
widespread interest in these sorbents for solid-phase extraction
applications.
Silica monoliths with the macropore−mesopore structure
were first reported in 1991 by Nakanishi and colleagues.12
Significant progress has been made since this original study;
however, even 30 years later and despite their advantages,
whole silica monoliths have, to our knowledge, not yet been
applied for selective metal extraction on anything approaching
the industrial scale. The primary obstacle to their use appears
to be scalability. To establish a point of reference, if one wereto assume a relatively low desired single-batch system output
of 100 g of Sc and a relatively high sorbent recovery capacity of
100 mg of Sc/g of monolith, the process would require 1 kg ofsorbent, which would be approximately equivalent to one 5 L
cylindrical monolith. This size is far larger than the largest silica
monoliths that have been reported thus far (∼1.1 L),13 andthere are key obstacles preventing scale-up to that level. First,
larger monoliths are highly prone to cracking during drying
due to the surface tension exerted by water evaporating in themesopores.7,13,14 This issue can be mitigated by enlarging themesopore size, for example, through the inclusion of ahydrothermal posttreatment step in the synthesis procedure;7,13,14 however, it remains to be seen whether thissolution would be sufficient to prevent breakage of even largermonoliths. There are other, more niche, methods to address
the cracking problem, such as drying the monoliths through a
coating of paraffin oil,15 but none present a clear solution forvery large monoliths without significant drawbacks. The
second issue is that the monolith’s physical characteristics
are quite sensitive to temperature during gelation andaging.7,13,14 For monoliths with a large diameter, it is difficult
to maintain a uniform temperature across the width of themold during synthesis, which could result in material

heterogeneity within the monolith, which is not ideal forhighly controlled metal extraction operations.13 A theoreticalworkaround would be to produce many smaller monoliths to
use in parallel for Sc extraction. However, this solution has its
own drawbacks, namely monolith containment, which is itself anontrivial obstacle to monolith scale-up. Each individual
monolith, regardless of size, must be strictly contained withinsome type of external sleeve (e.g., a glass or metal cylinder,
resin coating, or heat shrink tubing)16,17 to retain the liquid
feedstock within the system without leaking. If the containment system is not precisely fitted to the monolith, therecovery process will suffer from wall effects and/or channelingand exhibit reduced extraction efficiency. The costs and risks
associated with monolith containment are expected to increase
with the use of many small monoliths compared with fewer,
larger monoliths. Despite decades of research, these scalability
issues continue to prevent the use of hierarchically porous
silica monoliths for selective metal recovery on the industrial
scale.
Powdered silica monoliths may offer an alternative
monolith-based sorbent, largely retaining the advantages ofsingle-piece monoliths while avoiding many of their disadvantages. Essentially, a whole monolith is ground and passed
through a series of sieves to achieve a powder with ahomogeneous particle size. These powders have the same
surface chemistry and macropore−mesopore framework (e.g.,high surface area) as the whole monoliths, so they are readilyfunctionalized, and their structure can still be easily tuned
during the normal monolith synthesis procedure prior topowdering. Furthermore, since the particles still retain thehierarchically porous framework of the whole monoliths, they
are also expected to exhibit improved mass transport properties
compared to more traditional particulate silica sorbents. The
use of powdered monoliths makes the possibility of monolithcracking a nonissue, given that the monoliths will be broken up
into small particles anyway. It also minimizes the impact of any
monolith heterogeneity given that the fine powder will bemixed and homogenized during grinding. Adsorbent containment is also much more straightforward since the powder canjust be packed into a column of any size and shape like thetraditional SPE sorbents that have been used for decades. With
all of these expected advantages, powdered silica monoliths
present a promising material for the recovery of Sc and other
Critical Materials.
In this study, the characteristics and selective Sc extraction
performance of powdered silica monoliths were assessed. First,
the physical attributes of the material are reported and its masstransport behavior is analyzed. Then, the Sc recovery
capabilities of the powder are discussed in the context of aseries of batch and semicontinuous flow column extraction
tests at various environmental conditions. Throughout, thesefindings are evaluated in the context of the potential of thisSPE sorbent for future large-scale metal recovery operations.

■

Research Article

MATERIALS AND METHODS

Hierarchically porous silica monolith powders were obtained fromDPS Inc. in Kyoto, Japan. For the experiment testing column pressure
as a function of flow rate, 106−212 μm diameter particles were used,
and for all other experiments, 63−105 μm diameter particles wereused. There are many options for tuning the physical parameters ofthese powders, and the characteristics of the selected powder are
listed in Table 1. Mesoporous silica spheres (150 μm diameter;
SL12SA5) were obtained from YMC Co., Ltd. Aqueous scandium(10000 ppm) and bromide (1000 ppm) standard solutions were15433

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ACS Sustainable Chemistry & Engineeringpubs.acs.org/journal/ascecg

Table 1. Physiochemical Parameters of the Unmodified DPS
Monolith Powder and YMC Silica Spheres Derived from N2
Physisorption (−196 °C) and Mercury Porosimetrya

The results for the adsorption isotherm, the adsorption kinetics,
and the breakthrough columns were fitted with various models using
nonlinear least-squares curve fitting in Matlab. The Sc adsorption
isotherm data were fitted using the standard Langmuir and Freundlich
isotherm models. The Sc adsorption kinetics data were fitted withpseudo-first-order, pseudo-second-order, and Weber and Morrismodels.18 The Sc and Br breakthrough behaviors were fitted using
a standard and/or fractal-like Bohart−Adams model.19 These models
are discussed in more detail in Results and Discussion.

DPS
YMC

SBET
(m2/g)

dmeso
(nm)

Vmeso
(cm3/g)dmacro(μm)

Vmacro(cm3/g)bulk density(g/cm3)

490
400

8.2
12

0.58
n/a

1
n/a

112.5
1.1

0.20
0.44

a

Source data by DPS Inc. and YMC Co., Ltd., Kyoto, Japan.

■

acquired from LabKings and Inorganic Ventures, respectively. MES(2-(N-morpholino)ethanesulfonic acid) buffer and HomoPIPES(homopiperazine-1,4-bis(2-ethanesulfonic acid)) buffer were acquired
from Sigma-Aldrich, and ammonium oxalate and reagent-grade nitricacid were acquired from Alfa Aesar. For column studies, the powder
was packed in empty flash columns from Santai Science. The
dimensions of the metal extraction columns were 2.2 cm × 12.6 cm,
and the dimensions of the mass transport test columns were 4.6 mm
× 100 mm.
To determine the mass transport characteristics of the monolith
powders compared to traditional spherical sorbents, isopropanol was
passed through the fixed-bed columns at 0−6 mm/s. The pump used
was a Shimazu LC-20AD. The pressure was measured at the inlet andoutlet of the column, and the difference between them was reported
as a pressure drop. The reported values are the median of 120
measurements made with the pressure gauge (SMC PSE560-01).
Throughout these tests, the temperature was kept at 25 °C and thehumidity was kept at 50%. Scanning electron microscopy (SEM)
images of the DPS and YMC sorbents were taken on a JEOL Ltd.
JSM-6510 instrument.
Batch experiments for this study included an adsorption capacity
test at various feedstock concentrations, a kinetics test, and a REE
selectivity test, as well as tests at different pH and temperature
conditions. To construct an adsorption isotherm, 10 mg aliquots of
the DPS powder were exposed to 14.8 mL of a Sc solution from 0 to
100 ppm. The feedstock solutions were all buffered to pH 4 using a 10mM HomoPIPES buffer and kept at room temperature. For the pH
test, 10 mg aliquots of the DPS powder were exposed to 14.8 mL of
Sc solution (100 ppm) at room temperature and the pH was varied
from 3 to 6. At pH 3, the feedstock was composed of 1 mM nitric acidand was not buffered. At pH 4 and 5, the feedstock was buffered with
10 mM HomoPIPES buffer, and at pH 6, the feedstock was bufferedwith 10 mM MES buffer. For the temperature test, 10 mg aliquots of
the DPS powder were exposed to 14.8 mL of Sc solution (100 ppm)
at pH 4 (10 mM HomoPIPES buffer) and placed in an oven or
freezer at 5, 25, 40, 50, or 60 °C. Each of these assays was left for 1
week before being sampled for analysis. For the kinetics assay, 10 mg
aliquots of the DPS powder were exposed to 14.8 mL of a Sc solution(20 ppm) at pH 4 (10 mM HomoPIPES buffer). The samples wereshaken at room temperature for a set period of time from 10 to 10080
min (7 days) before the experiment was concluded. For the REE
selectivity test, 10 mg aliquots of the DPS powder were exposed for 1
week to 14.8 mL of a mixed REE solution containing 5 ppm of each
REE (except radioactive Pm) at pH 4 and room temperature. In every
case, samples of the liquid phase were diluted in 3% nitric acid, andtheir Sc or REE contents were analyzed on an Agilent 7800 ICP-MS.
All experiments were conducted in triplicate.
For the Sc and Br column tests, 8.0 g of monolith powder (dryweight) was suspended in isopropanol (Sigma-Aldrich) and carefully
packed into an empty flash column. Following packing, the sorbent
beds were first conditioned with ∼2 L of 0.1 mM nitric acid (pH 4).
Then, ∼4.5 L of Sc3+ solution (50 ppm of Sc, 0.1 mM nitric acid) or∼0.6 L of bromide (Br−) tracer solution (50 ppm Br, 0.1 mM nitricacid) were passed through the column and collected in 7 mL
increments. In both experiments, the fluid flow rate through thecolumn was 0.5 mL/min for every step. Prior to analysis, every sixth
sample (for the Sc column) or every second sample (for the Br
column) was diluted in 3% nitric acid. The Sc and Br contents of thesamples were analyzed on an Agilent 7800 ICP-MS.

Research Article

RESULTS AND DISCUSSION

Mass Transfer Properties. The macroporous structure of
the hierarchically porous silica monoliths is known to grant
these materials excellent mass transport characteristics,9,13 andthis behavior is expected to be retained in the monolith
powders. To test the mass transport characteristics of thepowders, the effect of increasing flow rate on internal pressure
was observed for two columns, one packed with powderedmonolith material (DPS), and the other packed withrepresentative mesoporous silica spheres (YMC). The particles
in both columns had a similar size (DPS = 106−212 μm and
YMC = 150 μm) and mesopore diameter (DPS = 8.19 nm and
YMC = 12 nm). The DPS powder exhibited a typicalmacroporous structure (1 μm diameter macropores), while
the YMC spheres did not. At flow rates between 0 and 6 mm/s, the column pressure was higher for the silica spheres than forthe monolith powders, and the pressure difference between thetwo columns became more exaggerated as the flow rateincreased (Figure 1). At a flow rate of 6 mm/s, for example,

Figure 1. (a) SEM image of a powdered DPS monolith particle
exhibiting the characteristic macroporous structure of the material.
(b) SEM image of a YMC silica sphere for comparison. (c) Increase
in internal column pressure as feedstock flow rate increases for acolumn packed with the DPS monolith powder vs YMC silica spheres.

the pressure of the silica sphere column (0.120 MPa) was morethan twice that of the monolith powder column (0.051 MPa;
Figure 1). The monolith powders clearly do, at least in largepart, retain the advantageous mass transport characteristics of
the whole monoliths. This ability to operate at high influent
flow rates with substantially decreased column pressure is asignificant benefit for industrial-scale metal extraction operations as it would allow large volumes of feedstock to beprocessed much more rapidly. It is therefore worthwhile toexplore the possibilities of using these monolith powders forthe selective extraction of critical materials.
Adsorption Capacity and Selectivity. Unmodified silica
surfaces are known to have an inherent affinity for Sc
adsorption due to the abundance of silanol groups,8,9,20 andthe silica DPS powder also exhibits this useful property. When
exposed to a monoelement Sc solution at pH 4, the powder has
a Sc adsorption capacity of ∼24.9 mg/g (Figure 2a). This
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Figure 2. (a) Scandium adsorption isotherm results for the DPS
powder fitted with the Langmuir and Freundlich models. Qe
represents the milligrams of Sc adsorbed per gram of powder. Ce
represents the concentration of Sc remaining in solution following
adsorption. The feedstock in each case was kept at pH 4 and 25 °C.
(b) REE adsorption results from a solution containing 5 ppm of eachindividual REE. Error bars represent the standard deviation of
triplicates.

Research Article

Figure 3. (a) Scandium adsorption results for the DPS powder at pH
3−6. (b) Scandium adsorption results for the DPS powder at 5−60
°C. Qe represents the mg of Sc adsorbed per g of powder. Error bars
represent the standard deviation of triplicates.

In the context of actual industrial-scale Sc extraction, theadsorption behavior of DPS powder presents certain
advantages. That strong selectivity for Sc, which is characteristic of unmodified silica, is the first clear benefit. The DPS
powder did not adsorb any of the other REEs, and based on
the adsorption behavior of other unmodified silica materials, it
is not expected to adsorb significant amounts of most abundant
non-REE metals.8,9 At certain conditions for certain feedstocks,
some amount of aluminum and/or thorium may becoadsorbed with the Sc,8,9 requiring additional purification
steps before or after the SPE step using the DPS powder. At∼12.0 mg/g at pH 3, the DPS powder also has a high Sc
adsorption capacity compared to similar unmodified silica
sorbents, such as SBA-15 and KIT-6 at 1.14 mg/g and 1.03
mg/g, respectively.8 This performance at pH 3 is important
given that the majority of Sc feedstocks are likely to be metalor mineral leachates at very low pH. If the leachate requires
less pH adjustment prior to the SPE step, then that is anadvantage for the overall extraction system, likely decreasing
costs. The material also operates at its peak at a reasonabletemperature range for large-scale extraction (25 °C to morethan 60 °C), so temperature will typically not be a factor that
presents an issue. The performance of the DPS powder inmore realistic, semicontinuous flow systems will be assessed in
the following sections; however, the fundamental sorption
characteristics are promising for practical Sc extraction.
Fixed-Bed Column Extraction and Adsorption Kinetics. Larger-scale SPE operations frequently involve
continuous or semicontinuous flow of the feedstock througha fixed bed column in order to maximize the volume offeedstock exposed to a given mass of adsorbent and the rate ofthat exposure, so the DPS powder was tested in a fixed-bed
column setup. Eight grams of powder was packed into a 2.2 cm

capacity is well within the range of other publishedexperimental REE sorbents,21 and is especially promising
given that it was accomplished with only bare silica, without
any grafted ligands or other modification. This performance isprimarily due to the high surface area of the hierarchicallyporous powder (490 m2/g; Table 1), which results in the
exposure of a high number of silanol groups to the aqueous
feedstock for a given mass of sorbent.8,9,20 The adsorption
isotherm (at 25 °C) could be modeled using the typicalLangmuir and Freundlich isotherms, with the Langmuir model
providing a better fit (R2 of 0.9631 compared to 0.9468; Figure2a). The agreement between the Langmuir isotherm and theexperimental data suggests that the sorption sites on the silica
surfaces are finite and homogeneous, which makes sense giventhat the sorbent is unmodified silica, and also that sorption ismonolayer.22 Furthermore, the high adsorption capacity isaccompanied by high Sc adsorption selectivity. In an
adsorption test in which equal concentrations (∼5 ppm) of
each REE were exposed to the DPS powder at pH 4, only Sc
was adsorbed (Figure 2b), showing that the powder has astrong preference for Sc adsorption, even compared with theother REEs, which typically exhibit similar behavior to Sc.
Adsorption is strongly influenced by pH, with capacity fallingto ∼12.0 mg/g at pH 3 and rising to ∼74.6 and ∼142.7 mg/g
at pH 5 and 6, respectively (Figure 3a). Temperature plays asmaller role in performance, unlike some other materials,23,24
with adsorption capacity appearing to plateau above ∼25 °C,
though the capacity is reduced at 5 °C (Figure 3b). These
basic Sc adsorption characteristics demonstrate the potential of
DPS powder for Sc recovery operations.
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Research Article

Figure 4. Scandium breakthrough column. The influent was 50 ppm of Sc (mg/L) at pH 4 in 10 mM HomoPIPES buffer. One bed volume for thecolumn was ∼38.8 mL. The solid line is a fractal-like Bohart−Adams model, which is designed to account for intraparticle diffusion limitations.

Figure 5. (a) Scandium adsorption kinetics of the DPS powder. The dashed and solid lines represent pseudo-first-order and pseudo-second orderkinetics models, respectively. (b) Weber and Morris kinetics model for the same kinetics data. Error bars represent the standard deviation of
triplicates.

× 12.6 cm column, and ∼4.5 L of 50 ppm of Sc solution (pH4) was passed through that column at a flow rate of 0.5 mL/
min. Scandium breakthrough was observed after ∼17 bed
volumes (∼680 mL; Figure 4). The initial breakthroughoccurred with a sharp transition, but the approach toequilibrium at 100% breakthrough occurred only gradually,
not reflecting the traditional idealized behavior of an extraction
column (Figure 4). A total of ∼77.7 mg of Sc (or ∼9.7 mg Sc
per g powder) was adsorbed by the extractant over theduration of the experiment. Approximately 38% of that total
capacity was reached prior to initial breakthrough, and ∼62%
occurred after breakthrough during the gradual approach toequilibrium. The system had not yet reached equilibrium at100% breakthrough by the conclusion of this test, so strictly
speaking, the full adsorption capacity is higher than ∼9.7 mg/g,
and a higher fraction of the adsorption occurs after
breakthrough. However, this additional capacity would likely
be irrelevant as extraction would be stopped prior to that point
in order to avoid wasting feedstock when minimal additional
extraction is occurring. A practical extraction scheme would

most likely stop the recovery process at ∼16 bed volumes andreplace the fixed-bed column with a fresh one, which would
minimize Sc loss. The adsorbed Sc would then be eluted fromthe loaded column in a separate step, and the sorbent could bereused. Such a scheme would maximize the advantages of thesystem (i.e., rapid mass transport, high initial Sc sorption
capacity), while minimizing the effects of the slow sorption
reaction kinetics.
Given the extent to which the breakthrough behavior
exhibited by this system departs from the expected, theoreticalbehavior, it is clear that Sc recovery was limited by one ormore kinetic process. The rate of Sc adsorption onto the silica
surfaces could be limited by (1) external diffusion, which inthis system would include Sc ion diffusion in the spaces
between particles and diffusion within the particle macropores(1 μm in diameter), (2) internal diffusion occurring within theparticle mesopores (<10 nm in diameter), and (3) Sc
adsorption reaction kinetics on the silanol functional groups.18
External diffusion limits are unlikely to be the main cause of
the observed behavior, given that the majority of the particle
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surface area and therefore functional groups for adsorption are
expected to be contained within the mesopore structure.
Internal diffusion limitations, however, are known to produce
similarly asymmetrical breakthrough patterns in othersystems.25 To investigate this possibility, the breakthroughdata was modeled using a fractal-like Bohart−Adams model(eq 1), which has been modified from the traditional Bohart−
Adams model specifically in an effort to account for internal
diffusion limitations within the sorbent.19

the slow Sc adsorption reaction kinetics appears to be primarily
responsible for this limitation.
The DPS powder did function as a fixed-bed sorbent for Sc
recovery under semicontinuous flow, though the breakthroughbehavior may present certain difficulties for real-world SPE
operations. The equilibrium adsorption capacity at idealized
conditions is quite high (∼142.7 mg/g at pH 6), and theselectivity against other REEs is promising. However, if during
extraction, the operation must be halted at the breakthroughpoint, then a large fraction (>68%) of the total adsorption
capacity is not used, reducing the practical capacity of thecolumn. If the operation is halted at a later point to takeadvantage of this additional capacity, significant Sc will haveescaped through the column as waste, reducing the extraction
efficiency of the system. That being said, the adsorption
capacity up to breakthrough (∼3.7 mg/g at pH 4) may still beadequate for extraction operations in some scenarios, especiallygiven the other advantages of this simple system. Typically,
highly engineered sorbents such as the DPS powder are
functionalized with ligands designed to target specific metals.
Eliminating this modification step decreases costs, minimizes
the use of potentially hazardous solvents during production,and likely increases material reusability since there is no ligandthat can be leached out of the sorbent over time. These
promising characteristics may in some cases outweigh the
reduced practical adsorption capacity due to the slow reaction
kinetics.
Looking into the future, the basic physicochemical properties of the DPS powder provide a solid foundation to build
upon for a highly adaptable CM sorbent. In the case of Sc, theDPS powder is expected to function well with feedstocks suchas red mud, bauxite residue from aluminum mining that isenriched in Sc, and manufacturing waste from the productionof Sc-bearing alloys and fuel cells, just to name two examples.
Additional forthcoming testing will assess the extraction
performance at more realistic conditions that more accuratelyreflect the conditions found in leachates from these real-world
feedstocks. Further testing to elucidate the nature of theinteractions between silanol groups and Sc would also be
useful in optimizing the Sc recovery process. To target otherdesirable metals beyond Sc, the DPS powder can, of course, befunctionalized with a wide range of targeted ligands, which
could both increase adsorption capacity for a given metal andcompletely circumvent the kinetics issue by changing thespecies relevant for the adsorption reactions, though at anincreased cost. Other REEs, noble metals, and various other
Critical Materials could potentially be extracted and purified
with this approach. With or without functionalization, the DPS
powder does show great promise for future CM extraction
applications.

C
=
C0

1
ÄÅ
ÅÅ
ax
1 + expÅÅÅkBA,0t hC0 uC0
0
ÅÇ

(

ÉÑ
Ñ
t ÑÑÑÑ
ÑÖ)(1)Here, C is the Sc concentration of the effluent, C0 is the Sc
concentration of the influent, kBA,0 is the fractal-like Bohart−
Adams rate constant, t is time, h is the fractal-like exponent, a0
is the final adsorption capacity, x is the bed height, and u is thelinear flow velocity. The traditional Bohart−Adams model was
unable to be fitted to these breakthrough results at all (data notshown). Even the fractal-like model that is well-suited toaccount for intraparticle diffusion effects could not provide a
precise fit to this highly asymmetrical breakthrough pattern(Figure 4). Interestingly, bromide (Br−), a tracer ion that is notadsorbed by the silica, exhibited close to ideal breakthroughbehavior (Supporting Information, Figure S1), suggesting that
diffusion of the Br− ions through the available pore volumes
did not strongly limit the transport of Br through the system.
Taken together, these modeling results appear to show thatinternal diffusion may not be the primary limiting factor in thissystem.
To further explore the breakthrough behavior exhibited by
the DPS sorbent, the Sc adsorption kinetics of the material
were analyzed in batch. Aliquots of the DPS powder (10 mg)
were gently shaken in ∼15 mL of 20 ppm of Sc solution (pH4) for a set period of time from 10 min to 7 days (Figure 5a,b).
The system reached equilibrium after ∼3 h, following a rapid
initial increase in adsorption. Both a pseudo-first-order and apseudo-second-order kinetics model were applied to the data
(Figure 5a).18 The pseudo-second-order model provided abetter fit (R2 of 0.9653 compared to 0.8195), which suggeststhat the adsorption reaction kinetics are likely the limiting
factor rather than intraparticle diffusion (Figure 5a).18,26 A
Weber and Morris model was also applied to the data (Figure5b), and the intercept of the initial rapid phase of adsorption iswell above zero, which again indicates that intraparticle
diffusion is not the sole factor influencing extraction
kinetics.18,26 The batch reaction data and the three models
applied to it, in combination with the results of the fractal-like
Bohart−Adams model for the continuous-flow system, showthat the adsorption reaction kinetics rather than internal
diffusion is most likely the primary process controlling the rateof Sc extraction, although small effects from internal andexternal diffusion may also be present. The mechanism(s) of
Sc adsorption onto the silica surfaces are complex, potentiallyinvolving van der Waals forces, electrostatic interactions
between Sc3+ cations and the negatively charged silica, andthe formation of covalent Si−O−Sc bonds.8,27 While theformer two processes occur rapidly, the Si−O−Sc covalentbonds form more slowly,8,27 which could explain the observed
behavior of the DPS powder in column and in batch. Overall,
Sc extraction from the fixed-bed column is limited to an extent
by one or more rate-limiting processes within the system, andResearch Article■

CONCLUSIONSThe performance of DPS powder as an extractant for batchand semicontinuous flow recovery of scandium was analyzed atconditions relevant for future real-world metal recovery
operations. The sorbent exhibited the following characteristics:
1. Mass transfer of fluid feedstocks through a fixed-bed
column could occur rapidly without producing significant pressure increases, unlike many powdered SPE
sorbents, due to the hierarchical pore structure of the
DPS powder.
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ACS Sustainable Chemistry & Engineeringpubs.acs.org/journal/ascecg

Dr. Freddy Kleitz and by a KU-UNIVIE Joint Grant Program
Grant to Dr. Brewer, Prof. Kleitz, and Prof. Kazuki Nakanishi.
The authors also acknowledge the funding support of the
University of Vienna (Austria). We would furthermore like tothank Dr. Kei Morisato at Kyoto University as well as Ms. Julia
Zolles and Prof. Dr. Gunda Koellensperger (Department ofAnalytical Chemistry of the University of Vienna) for theirgenerous assistance throughout the project.

2. The material, even without surface modification,achieved a high Sc adsorption capacity, ∼24.9 and∼142.7 mg/g at pH 4 and 6, respectively. Temperature
increases above 25 °C had minimal effect on Sc
adsorption.
3. The unmodified silica surfaces had a high selectivity for
Sc adsorption compared to the other REEs.
4. The DPS powder did function as an extractant for Sc ina semicontinuous flow column system, though optimal
performance was reduced by the slow Sc adsorption
reaction kinetics, with a potential minor contribution
from intraparticle diffusion within the mesopores.
5. The high Sc adsorption capacity and promising
selectivity, especially given the lack of surface
modification, combined with the excellent mass transfer
properties demonstrate that the DPS powder has great
potential for the SPE of Critical Materials.

■■

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DOI: 10.3389/fceng.2021.789416.

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acssuschemeng.3c04672.
Tracer ion (Br−) breakthrough behavior in the fixed-bed
powdered monolith column (PDF)■

Research Article

AUTHOR INFORMATION

Corresponding Authors

Aaron Brewer − Department of Functional Materials andCatalysis, Faculty of Chemistry, University of Vienna, 1090
Vienna, Austria; orcid.org/0000-0002-7562-6660;
Email: aaronbrewer5@gmail.com
Freddy Kleitz − Department of Functional Materials andCatalysis, Faculty of Chemistry, University of Vienna, 1090
Vienna, Austria; orcid.org/0000-0001-6769-4180;
Email: freddy.kleitz@univie.ac.at
Authors

Chloé Reicher − Department of Functional Materials andCatalysis, Faculty of Chemistry, University of Vienna, 1090
Vienna, Austria; Present Address: National College ofChemical Engineering, Université de Haute-Alsace, 68100
Mulhouse, France
Olivia Manatschal − Department of Functional Materials andCatalysis, Faculty of Chemistry, University of Vienna, 1090
Vienna, Austria; Present Address: Department ofChemistry, Université Paris Cité, 75013 Paris, France
Hongzhi Bai − DPS Inc., 615-8530 Kyoto, Japan
Kazuki Nakanishi − Institute of Materials and Systems for
Sustainability, Nagoya University, 464-8601 Nagoya, Japan;
Institute for Integrated Cell-Material Sciences, Kyoto
University, 606-8501 Kyoto, Japan
Complete contact information is available at:https://pubs.acs.org/10.1021/acssuschemeng.3c04672
Funding

Open Access is funded by the Austrian Science Fund (FWF).
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
Funding for this project was provided by an FWF Lise Meitner
Programme Grant (M 2750-N) to Dr. Aaron Brewer and Prof.
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