pH SWITCHES FOR THE SELECTIVE EXTRACTION OF

METAL IONS FROM BIPHASIC WATER-CO2 SYSTEMS

Kirk J. Ziegler a, John P. Hanrahan a, Jeremy D. Glennon a, David C. Steytler b,
Julian Eastoe c and Justin D. Holmes a,∗
a
Department of Chemistry and the Supercritical Fluid Centre,
University College Cork, Cork, Ireland
b
School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
c
School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
∗
To whom correspondence should be addressed: Fax: +353 21 4274097;
E-mail: j.holmes@ucc.ie

ABSTRACT
Supercritical fluids are quickly becoming a realistic alternative to conventional
organic media used in the extraction of metal ions from aqueous phases. However,
supercritical fluid extraction (SFE) is complicated by the inherent low pH (~ 3) associated
with CO2 in contact with water. However, we have recently shown that selective extraction
of the metal ions can be achieved through the control of the aqueous phase pH and the use of
a hydrocarbon surfactant. Furthermore, we demonstrate that pressure-induced pH switches of
the aqueous phase can be utilized for the selective separation of metal ions from an aqueous
metallic broth. SANS results show that this metal ion extraction occurs through the formation
of reverse micelles using the hydrocarbon surfactant Triton X-100 eliminating the use of
fluorinated surfactants and the uptake of water.

INTRODUCTION
Over the last ten years, an increased awareness of the problems associated with metal
ions in the environment has resulted in demands for more selective and effective separation
processes. Supercritical fluid extraction (SFE) [1, 2] is an increasingly popular alternative to
conventional liquid-liquid extraction from contaminated samples. Supercritical carbon
dioxide (sc-CO2) is the most utilized supercritical fluid for SFE as it is non-toxic, non-polar,
non-flammable, inexpensive, and easily recyclable [3].
SFE from an aqueous phase is made possible through the use of metal chelating
agents. Fluorinated chelating agents have been highly successful in the extraction of metal
ions from aqueous matrices by SFE [4, 5] due to high solubility in CO2 [2] and can be
engineered for the targeted extraction of a specific metal ion [6, 7]. However, fluorinated
molecules are often costly and time consuming to synthesise. Additionally, the extraction of a
specific metal ion from a broth of metallic ions typically requires the use of multiple chelating
agents limiting their usefulness in SFE.
Although hydrocarbons are soluble in CO2, [8] their larger cohesive energy densities
compared to fluorinated surfactants has made it challenging to form hydrocarbon reversed
micelles in CO2. Recently, several researchers have utilised hydrocarbon surfactants in CO2
for stabilising emulsions and microemulsions by minimising the cohesive energy density of
the hydrocarbon surfactant [9-12]. The use of hydrocarbon chelating agents in SFE, [13-17]
is clearly appealing as they are easier to synthesise and cheaper to produce than their
fluorocarbon analogues.
 Extraction of metal ions from an aqueous phase into CO2 is complicated by the
presence of carbonic acid which lowers the pH of the aqueous phase to approximately 3 [18,
19]. While some metals are extracted efficiently at pH ~ 3 [18, 20], other metal ions, cannot
be efficiently extracted at such an acidic pH. Under these conditions, a large excess of ligand
is often required to achieve complete extraction of the targeted metal limiting the practical
application and economic feasibility of large-scale extractions from contaminated soils and
wastes. Recently, Holmes et al [21, 22] have demonstrated that the pH of the aqueous phase
in a biphasic water–CO2 system can be altered from pH~3 to values approaching 7 expanding
the pH region available for the SFE of metal ions. In addition, pH ‘switches’ of 1-2 pH units
can be readily obtained [23] by taking advantage of the fact that modest changes in pressure
and temperature result in large changes in the solubility of CO2 in the aqueous phase. In the
presence of a buffer, these changes in the solubility of CO2 can then be manipulated to affect
the extent of neutralisation of carbonic acid.
Here we describe an extraction process for the selective removal of metal ions into
supercritical CO2 using reversed micelles. This extraction process overcomes the inherent
low pH problem associated with water in contact with CO2 and utilizes a cheap, commercially
available hydrocarbon surfactant as a micellar complexing agent with no uptake of water into
the micelle core. The aggregation behaviour of these surfactants as well as their metal
analogues were investigated by the direct method of small angle neutron scattering. Other
indirect spectroscopic methods are also utilized to suggest aggregation into micelles.

EXPERIMENTAL
Materials and Reagents. The non-ionic surfactants Triton X-100 (n = 9.5), Triton X-
114 (n = 7), and Triton X-45 (n = 2) are based on a polyethylene oxide (PEO) repeating unit
with an average molecular weight of 625, 536, and 426 (g mol-1), respectively. The metal ion
sources used for extraction were obtained from cobalt (II) nitrate, gold (III) chloride (prepared
in approx. 0.5 M hydrochloric acid), copper (II) nitrate, chromium (III) nitrate, and nickel (II)
nitrate. Sodium hydroxide was used to vary the pH of the aqueous phase. All reagents were
used as received. De-ionised water was used in all experiments and carbon dioxide was used
as received. All surfactant metal salts were prepared by mixing a known volume of metal salt
with 10 mL of Triton X-100 solution. The quantity of salt added was the maximum possible
soluble in that quantity of surfactant.
pH measurements. All experiments were carried out in a 25 mL stainless steel view
cell fitted with two UV-grade sapphire windows. Extraction temperatures were controlled by
isopad heating tape (Type ITW//SS-M) and thermostated to ± 0.2 °C using a platinum
resistance thermometer and a temperature controller. Pressure was controlled to ± 0.2 bar
using an ISCO (Lincoln, NE) syringe pump. An external magnetic stirrer drove a PTFE
coated magnetic stirrer bar. For each extraction, an aqueous sample of metal ions was
carefully loaded into the cell using a Pasteur pipette (typically 2.5 mM gold (III) chloride, 70
mM copper (II) nitrate, 80 mM cobalt (II) nitrate, 30 mM nickel (II) nitrate, and 30 mM
chromium (III) nitrate). A known volume of surfactant was introduced into the cell using a 1
mL syringe. 15 mL of CO2 was then introduced into the cell at ambient temperature.
Extractions were carried out at various temperatures between 22 and 60 °C for approx. 2 hr
and monitored in-situ using a HP 8453 UV-vis spectrometer. The pH of the aqueous layer
was controlled by the addition of NaOH and/or the temperature and pressure. The aqueous
layer pH was calculated using a Newton-Raphson method to solve a set of non-linear
equations based on a thermodynamic model. This model had been previously used to predict
the pH of the aqueous layer in a water–CO2 biphasic system. [23]
 UV-vis data for the extraction of metals from the aqueous phase was monitored by the
disappearance of the characteristic metal ion species peak. Concentrations were related to the
peak height using the Beer-Lambert law. Each Triton-metal complex had a characteristic
UV-vis peak (due to a ligand metal charge transfer band) different from that of the metal ion
species. This characteristic LMCT peak appeared in the CO2 phase and not in the aqueous
phase confirming that the surfactant-metal species resides in the CO2 phase and not the
aqueous phase. The appearance of the characteristic nitrate peak was witnessed in the CO2
phase for all extracted metals, except gold(III), suggesting that the nitrate ion is also extracted
with the metal ion. Control experiments were performed without the presence of Triton X-
100 to ensure that metal ion species were being extracted and not precipitating from solution.
The maximum error attributable to precipitation occurred at basic conditions (pH ~7.8) and
was no larger than 5%.
Phase Behaviour. Phase behaviour was studied by immersing the view cell in a water
bath and controlling the temperature to ± 0.1 °C by a water circulator. Each sample was
loaded into the cell using a 1 mL syringe. CO2 was loaded into the cell at ambient
temperature using a 260 mL ISCO syringe pump. The mixture was stirred using a PTFE
coated magnetic stirrer bar driven by an external stirrer. The pressure was raised to the point
where a clear, single phase solution was obtained (after the collapse on an initial unstable
emulsion). The phase boundary or cloud point was found by subsequently lowering the
pressure until the solution became cloudy. Such transitions, measured at constant temperature
(allowed to equilibrate for 10 min) with decreasing pressure, are clearly visible and highly
reproducible to within ± 10 bar. The rate of pressure decrease (7 bar per min) could be
controlled with the ISCO pump using a gradient program.
Small-angle neutron scattering. SANS measurements were performed on the LOQ
spectrometer using the ISIS pulsed neutron source of the CLRC Rutherford Appleton
Laboratory, UK using a stirred high-pressure optical cell (maximum pressure 500 bar, path-
length 12 mm) as described previously. [24] The magnitude of the transfer vector Q is given
by Q = 4π/λ sin (θ/2) where λ is the incident wavelength (2.2-10 Å), determined by time of
flight, and θ is the scattering angle. The intensity of the neutrons was recorded on a position
sensitive 64 × 64 pixel 2-D detector at a fixed sample to detector position (4.43m) providing
an effective Q range from 0.01 to 0.2 Å-1 in a single measurement. The data was corrected for
transmission and incoherent background scattering and normalised to absolute scattering
probabilities using standard procedures. I(Q) data were analysed using the multi-model FISH
program. [24]

THEORY
For small particles (or micelles) of volume Vp present at a number density np, the
normalized SANS intensity I(Q) (cm-1) may be written as I(Q) = np(ρp-ρm)2Vp2S(Q)P(Q)
where ρp and ρm are the mean coherent scattering length densities of the dispersed phase (e.g.
particles) and solvent medium, respectively. P(Q) is the single particle form factor describing
the angular distribution of the scattering owing to the size and shape of the particle.
Expressions for P(Q) representing various particle shapes, such as spheres, rods, disks,
ellipsoids, etc., can be used to model SANS data in order to determine particle shape and size.
S(Q) is the structure factor which arises from spatial correlations between particles. For
reversed micelles far removed from phase boundaries, i.e., in the absence of attractive
interactions between micelles, and at low micelle concentration S(Q) → 1.0. Under these
conditions, I(Q) is a direct measure of P(Q), i.e. I(Q) = np(∆ρ)2Vp2P(Q). For reversed micelle
systems at fixed volume fraction, the S(Q) contribution becomes increasingly significant as
the aggregation number (micelles size) decreases. Theoretical models are then available for
S(Q) when the micelles are spherical but as yet no simple formalism exists to account for
interactions between anisotropic rod or disk-shaped structures. However, since the
aggregation number is then appreciable, the S(Q) contribution will be less significant than for
spherical micelles formed at the same surfactant concentration. Since P(Q) = 1 when Q = 0
and the I(Q) data are fitted in absolute units, the value of the scale factor (np(∆ρ)2Vp2) is a
self-consistency check on the model since both npVp (=Φ) and ∆ρ2 are known. The fitting
program we have developed allows us to examine a wide range of models from which
physically unrealistic solutions can be eliminated using, in part, the scale factor criteria.

RESULTS AND DISCUSSION

pH effects. Despite the obvious importance, pH dependent SFE has not received much
attention and any attempts to do so has been limited to controlling the aqueous phase pH
before the addition of CO2. However, the effects of the aqueous phase pH on the extraction
efficiencies of various metals are clearly demonstrated in Figure 1a for a biphasic water–CO2
system using Triton X-100 as the extractant. Selective metal ion extraction can be achieved
by simple pressure-induced pH switches as seen in Figure 1b. These changes in the extraction
efficiency for the different metal ions as a function of pH can be attributed to changes in the
speciation of the metal complexes as the aqueous phase pH changes. However, pH and
speciation effects may also affect the formation and stability of micelles formed in CO2 [25].
(b) pH
(a) 8.0 7.5 7.0 6.5
90 100

80
Extraction Efficiency (%)

Au(III)
80
70 Cr(II)
Extraction Efficiency (%)

Co(II)
60
60
50

40
40
30 Copper
Gold
20
Chromium 20
10
Cobalt

0 0
0 100 200 300 400 500
3 4 5 6 7 8
Pressure (bar)
Aqueous phase pH

Figure 1. (a) pH effect of metal ion extraction in biphasic water-CO2 systems using TX-100 as
the extractant; and (b) pressure-induced pH switches for selective metal ion extraction.

Triton reversed micelles in CO2. Although Triton X-100 is widely believed to form
reversed micelles in CO2, [17, 26, 27] direct evidence of micelle formation has not been
presented. Phase behaviour measurements for pure Triton X-100 in CO2 over a wide range of
concentrations show that Triton solubility is favoured at higher pressures and lower
temperatures due to the increased solvent strength of CO2. SANS data of pure Triton X-100
in CO2 in the one phase region is shown in Figure 2. The scattering intensity is very weak but
clearly indicates the presence of small aggregates. Reasonable fits were given using a
polydisperse spherical form factor model with a Schultz polydispersity function index
(σ/Rmean) of 0.2. [28] The mean radii, Rmean, representing the overall micelle dimensions were
obtained as 11 Å for Triton X-45 and 13.5 Å for Triton X-100. The absolute intensity of the
scattering is at least a factor of four lower than would be expected from a solution of Triton
X-100 at this composition assuming complete aggregation into unsolvated reversed micelles.
This attenuation in absolute intensity and small size suggest loosely packed reversed micelles
of low aggregation number with significant CO2 solvation of the aggregate structure. It is also
likely that the cµc for the surfactants in CO2 is significant at the concentration levels
employed leading to a relatively high monomer:micelle ratio. Tests using a more
parameterised core-shell model did not alter the above conclusions but the statistical
resolution of the data does not merit such detailed analysis at this stage.
0.07
0.40

0.35
0.06 copper + TX100 + BTB

Absorbance (AU)
0.30
Acetone
I(Q) (cm )

0.25
-1

TX-100 + BTB
0.05 0.20

0.15 BTB in CO2

no surfactant
0.10
0.04 Water
0.05

0.00
0.03 360 380 400 420 440 460 480 500
0.00 0.05 0.10 0.15 0.20 0.25 Wavelength (nm)
-1
Q( Å )
Figure 2. Small angle neutron scattering data Figure 3. UV-visible spectra of thymol blue
for TX-100 in pure CO2. in various solvent environments.

UV-vis dye solubilization measurements were also carried out to confirm the presence
of reversed micelles as seen in Figure 3. No thymol blue absorbance was detected in pure
CO2. However, with the addition of Triton X-100, a spectrum characteristic of thymol blue is
obtained. The shift in λmax for thymol blue in pure water compared to that in Triton and CO2
seen in Table 1 is indicative of a change in the polarity of the environment surrounding the
dye suggesting the incorporation of the thymol blue into Triton X-100 aggregates.

TX-100 and copper

0.28
Table 1. The wavelength shift of thymol
blue in various solvent environments. 0.24
I(Q) (cm )
-1

Substance λ max 0.20

Water 438nm 0.16

Acetone 396nm
Triton X100 414nm 0.12

Triton X100 + Copper 427nm

0.05 -1
0.10 0.15
Q(Å )

Figure 4. Small angle neutron scattering data

for TX-100-copper complex in pure CO2.

Triton-complex reversed micelles in CO2. SANS data of Triton X-100-copper

complex in pure CO2 is shown in Figure 4. Once again, the scattering intensity is very weak
but clearly indicates the presence of small spherical aggregates. The mean radius representing
the overall micelle dimensions was 6.5 Å. UV-visible dye experiments appear to confirm that
Triton-copper complexes form dry micelle aggregates as characterized by the shift in λmax.
This data suggest that the mechanism of metal ion extraction in aqueous matrices using Triton
X-100 proceeds via the formation of reversed micelles. The formation of reversed micelles
offers several advantages over w/c microemulsion-mediated metal ion extraction since there is
no uptake of water into the micelle core eliminating the need for further treatment of the
contaminated water.
CONCLUSIONS
Here we describe a simple and environmentally benign route for the extraction of
heavy metal ions from aqueous matrices using the cheap, commercially available hydrocarbon
surfactant Triton X-100 as an extractant in lieu of expensive, fluorinated chelating agents.
Triton and Triton-metal complexes are shown by both SANS and dye solubilization to form
reversed micelles in pure CO2 and is believed to be the mechanism for metal ion extraction in
biphasic water–CO2 matrices. The formation of reversed micelles in lieu of microemulsions
offers the additional advantage of expulsion of water from the core eliminating the need for
further processing of the water after extraction.

ACKNOWLEDGEMENTS
We acknowledge financial support from Enterprise Ireland for a postgraduate studentship for JPH and a
postdoctoral research fellowship for KJZ. Financial support from EPSRC (GR/L05532 and GR/L25653) is also
acknowledged.

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