ION EXTRACTION
Pierre Bauduin and Luc Girard, ICSM UMR5257 CEA/CNRS/UM2/ENSCM, BP 17171 CEA Marcoule,
30207 Bagnols-sur-Cèze, France
Pierre.Bauduin@cea.fr
Definition
The term ion extraction refers to the process of extracting one or several ions from a liquid phase, usually an
aqueous phase, to another phase that can be either solid or liquid. The general aim is to separate ions from a
native solution or to concentrate ions in order to handle smaller volumes. When several ions need to be
recovered from each other, i.e. performing a selective extraction of each ion, the term ion separation is used.
During the last decades the market for metal resources has been greatly stimulated by the emergence of the
new technologies and by the world population increase. On the other hand, the limited metal resources as
well as fossil fuels will constraint governments to set up restriction measures and will undoubtedly lead to
limit export and raise the price. Moreover the extraction of metal ions from ores leads to toxic wastes whose
disposal is now becoming expensive because of increasingly environmental protection regulations. Most of
these wastes are classified as hazardous and toxic mainly due to the presence of different metals such as
cadmium, chromium, lead, arsenic whose extraction will be required in the future. As a consequence there
will be a growing need for the development of new efficient and eco-friendly recycling systems based on ion
extraction process in order to avoid future mining of underground resources.
The recovery of metals from ores and recycled or residual materials is usually performed by the use of
aqueous chemistry. The overall process is covered by the field of hydrometallurgy and is typically divided in
three general parts: (1) leaching, which converts metals into soluble salts in aqueous media, (2) solution
concentration and purification, which are performed by the use of ion extraction techniques and (3) metal
recovery from the purified solution for example by electrolysis. An attempt is made here to give a brief
description of important ion extraction methods: solvent extraction, ion exchange, ion flotation, cloud point
extraction, precipitation and membrane separation. All these techniques are based on the more or less specific
recognition of ions with chemical/physical sites. Depending on the technique this recognition can originate
from (i) electrostatics including ion-pairing properties, surface effects e.g. charge density, polarizabilities of
the species… and/or (ii) physical effects i.e. size and shape recognition, steric hindrance in a porous structure
may strongly influence ion selectivity. Moreover the bulk physico-chemical properties of the ions in water
may play an important role in the ion extraction/separation process. To give an example, ion hydration,
which is characterized by hydration entropy and enthalpy, is known to be involved in many specific ion effect
in solution.
Solvent Extraction
Solvent extraction is the general term referring to the distribution of a solute between two immiscible liquid
phases in contact, usually water and oil (organic solvent) also called “diluent” in hydrometallurgy [1]. When
coexisting solutes (metal ions) have different distribution ratios (D), defined as the ratio between the solute
concentration in the organic phase and its concentration in the aqueous phase see Eq. 1, separation of these
solutes can be obtained. D depends on the thermodynamic properties of the system, like temperature,
2 – 5 pages (1,200 – 3,000 words or 8,000 – 20,000 characters with spaces)
concentration, pressure, and so on and can be related to the Gibbs free energy of the extraction process
(ΔG).
DM 
[ M ]t ,org
[ M ]t ,aq
; G   RT ln DM
(1)
with [M]t being the sum of the concentrations of the metal species in its aqueous medium or in its complexed
organic form (denoted with horizontal bar).
Turbulent stirring of the two liquid phases is usually employed to increase surface contact between oil and
water so as to reach rapid distribution of the solute. This emulsification process is then followed by the
settling of the two liquid phases which has to be fast and efficient for industrial applications. The extraction
process is performed in the presence of hydrophobic extractant molecules that aim to enable the solute
transfer from the water phase to the organic phase, see Fig.1. Hydrophobic extractants are constituted by a
polar complexing part and a hydrophobic part. Extractants are designed to achieve selectivity for ion
separation.
Fig.1: Scheme of an ion extraction system in the water/oil interface region. Ions in the aqueous phase are transferred in extractant
reverse micelles present in the organic phase. (Courtesy Philippe Guilbaud CEA Marcoule, France)
Owing to their amphiphilic structures extractant molecules show self-assembly properties in the organic
phase. Extractants form spontaneously small reverse micelles having low aggregation numbers less than ten
molecules, see Fig. 1 in the organic phase. The core of the aggregate is less than 0.5 nm, therefore complexing
agents are always in first or second coordination spheres of the extracted ions. Therefore metal ions are
usually completely dehydrated in the extractant reverse micelles.
Different types of ion extractant are available and they could be classified according to their extraction
mechanism as follows [2]:
1- extraction by cation exchange
e.g. with acidic function like in HDEHP (bis (ethylhexyl) phosphoric acid)
2- extraction by solvation
e.g. with a neutral function like in TBP (tri-n-butyl phosphate), TOPO (trioctyl
phosphine oxide)
3- extraction by formation of ion pairs
2 – 5 pages (1,200 – 3,000 words or 8,000 – 20,000 characters with spaces)
e.g. amine salts like TOA (tri-n-octylamine)
4- extraction by complex synergistic effects
e.g. enhanced U(IV) extraction by coupling of an acidic extractant with a neutral one
(HDEHP-TOPO)
The chemical behaviour of these four classes of extractant molecules in solution gives the mechanism of
transfer of the ion into the organic phase: 1 and 3 are based more on the ionisability of the molecule and
though roughly on the electrostatic interaction with the metal ion whereas 2 is based on the competition of
the extractant with the first solvation shell of the metal ion which once replaced facilitates the transfer of the
so hydrophobic ion complex into an organic phase; 4 combines both mechanisms.
Ion exchange
Ion exchange is, with solvent extraction, the most used process for ion extraction. It is based on the
competitive adsorption process between two ions at a charged solid surface. This process is reversible, the ion
exchanger can be regenerated or loaded by washing with the appropriate ions [3, 4].
The materials used as ion exchangers can have different chemical nature: functionalized porous or gel
polymer known as ion exchange resins, zeolites, clays (montmorillonite), and soil humus. Ion exchangers are
anionic, cationic or amphoteric respectively when cations, anions or both have to be separated from the
solution. However the amphoteric exchange can be more efficiently performed in mixed beds containing a
mixture of anion and cation exchanger, or passing the treated solution through several different ion exchange
materials. Zeolites are all-inorganic microporous materials and are widely used in ion extraction [3]. They are
aluminosilicate minerals with a porous structure that can accommodate a wide variety of cations and shows
ion selectivity according to the cation and pore sizes. Ion exchange resins are porous support structures,
usually small beads (1-2mm), made of an insoluble organic polymer substrate (most of them based on
crosslinked polystyrene). The pores on the surface are used to easily trap and release ions. The trapping of
ions takes place only with the simultaneous release of other ions.
The four main types of ion exchange resins differ in their functional groups: strongly acidic (typically, sulfonic
acid groups, e.g. sodium polystyrene sulfonate or polyAMPS), strongly basic, (quaternary amino groups, for
example, trimethylammonium groups, e.g. polyAPTAC), weakly acidic (mostly, carboxylic acid groups) and
weakly basic (primary, secondary, and/or ternary amino groups, e.g. polyethylene amine). There are also
specialised resin types containing chelating moieties (iminodiacetic acid, thiourea, and many others).
Depending on the nature of the functional group, ion exchangers can be non ion-specific or can have binding
specificity for certain ions or classes of ions [4]. The active groups can be introduced after polymerization, or
substituted monomers can be added during the polymerisation process. Ion exchange resins are not only
designed as bead-shaped materials but are also produced as membranes (see the section Membrane
separation).
Ion flotation
Ion flotation (or foam fractionation) is an extraction/separation process used to concentrate ionic species
present in a dilute aqueous solution [5]. This is a simple and cost effective technique that can be used for the
concentration of valuable materials or removal of toxic materials from very large volumes of very dilute
solutions, an ideal proposition for waste-water treatment [6]. Ion flotation is a sub-category of the more
general flotation process also called froth flotation. Froth flotation is a process for separating solid minerals
from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between
2 – 5 pages (1,200 – 3,000 words or 8,000 – 20,000 characters with spaces)
valuable minerals and waste gangue are increased through the use of surfactants that adsorb on the mineral
surface. The selective separation of the minerals makes processing complex ores economically feasible.
Ion flotation involves the addition of an ionic surfactant (collector)
to a solution containing ions of opposite charge (colligend). Gas is
then bubbling into the solution. As the bubbles rise, the surfactant
molecule, which consists of a hydrophobic tail and a charged
hydrophilic head group, adsorb at the surface of the bubble, and the
bubble surface becomes charged. Ions in the solution adsorb then at
the bubble surface by an ion exchange process with the surfactant
counter ion:



Y aqu.  X int
Y int erface  X aqu. 
erface
Fig.2: Schematic representation of
an ion flotation lab setup. (Courtesy
Caroline Bauer MedesisPharma, France)
Owing to the foam’s large surface area over liquid volume ratio, the
liquid that results upon collapse of the foam is manifold enriched in
the ion compared to the initial solution. The ion extraction and
separation largely depends on the selectivity of the charged
surfactant interface for the ion in the presence of competing
counterions.
Cloud point extraction
Cloud point extraction (CPE) is an analytical tool sometimes presented as a solvent-free alternative to liquidliquid extraction. CPE is based on the property of many non-ionic surfactants, mainly poly-ethoxylated
surfactants, in aqueous solutions to form micelles and to undergo liquid-liquid phase separation when heated
to a certain temperature called “cloud point” [7]. For analytical purpose CPE is useful as pre-concentration
method that has many advantages, such as low cost, safety, simple procedure, rapid and high capacity to
concentrate a wide variety of analytes.
Any species that interacts with the micellar system may be extracted from the initial solution. In order to
extract ions the addition of a suitable hydrophobic ligand or extractant, that is solubilised in the non-ionic
surfactant micelles, is needed. An approach based on the use of a surfactant functionalized by a complexing
part has been recently proposed [8]. The use of such a surfactant enables to increase the extraction
effectiveness and the ion separation factor compared to the system when the surfactant is simply mixed with
the ligand.
The process of CPE is divided in three steps: (1) solubilisation of the analytes in the micellar aggregates, (2)
clouding by increasing temperature and (3) phase separation. Above the cloud point of the surfactant, the
solution separates into two distinct phases: one water-rich phase containing the surfactant at a concentration
below, or equal to, its critical micelle concentration and the other one a surfactant-rich phase containing the
ion-extractant complex.
Precipitation
Chemical precipitation is widely used for heavy metal removal from inorganic effluent [9]. The main principle
is based on adjusting pH to basic conditions (pH >9), in order to change the dissolved metal ion speciation
and to convert the metal ions into an insoluble solid phase via a chemical reaction with a precipitant agent
such as lime. Typically, the metal precipitates from the solution in its hydroxide form with a general reaction
described by the following equation:

n
 M (OH )
M aqu.  nOH aqu. 
n
(2)
2 – 5 pages (1,200 – 3,000 words or 8,000 – 20,000 characters with spaces)
where Mn+ and OH− represent the dissolved metal ions and the precipitant, respectively, while M(OH)n is the
insoluble metal hydroxide.
Selective metal ion precipitation from acidic aqueous waste solutions (liquid-solid extraction) was also
proposed for example as a simple way to separate actinides by using cationic surfactant as precipitant [10].
This approach has some interesting advantages over solvent extraction, because several steps are omitted like
stripping of the extracted species or solvent washing. Moreover, the amount of waste is decreased
considerably since no contaminated organic solvent is produced.
Membrane
Membrane filtration is capable of removing not only suspended solid or organic compounds, but also
inorganic compounds such as heavy metal ions. Depending on the nature of the inorganic compound three
different processes can be employed: ultrafiltration (UF) nanofiltration (NF) and reverse osmosys (RO) [11].
UF uses permeable membranes with membrane weight of separating compounds of 1000 to 100,000Da and
a pore size of 5 to 20nm allowing the passage of water and low molecular weight solids, while retaining the
macromolecules which have a size larger than the pore size. The membranes are made of cellulose acetate,
polyamide or silica and alumina.
The ion separation concern several ions such as Co(II), Ni(II), Zn(II), Cr(III) and Cd(II). Depending on the
membrane characteristics it can achieves more than 90% of removal efficiency with metal concentration from
10 to 112mg/L. The separation could be enhanced by using a surfactant such as sodium dodecyl sulphate
(SDS) to form micelles or a water born polymer such as chitosan to complex ions. The membrane fouling
problems have hindered this technique from a wider industrial waste water treatment.
NF involves steric and electrical (Donnan) effect. The interest of this membrane lies in its small pore (~1nm)
and surface charge, which allows charged solutes smaller than the membrane pores to be rejected along with
the bigger neutral solutes and salts. The membranes can be organic or inorganic, e.g. polyvinyl alcohol or
TiO2. NF can be assisted by chelating agents such as DTPA, EDTA, HEDTA for example in the
actinides(III)/lanthanides(III) separation [12]. Although NF is able to treat inorganic effluents with a metal
concentration of 2g/L in a rather wide pH range (3-8) at pressure of 3-4 bars, it is less intensively investigated
than UF and RO for heavy metal removal.
In RO process, water can pass through the membrane, while heavy metal is retained. By applying a greater
hydrostatic pressure than the osmotic pressure of the feeding solution, cationic compounds can be removed
from water. RO is the most effective membrane separation technique for metal ions removal from inorganic
solution with 97% of rejection percentage with a metal concentration ranging from 21 to 200mg/L. RO
works (depending on the characteristic of the membrane) in a wide pH range (3-11) at 4.5-15bar of pressure.
The membranes are made of polyamide or sulfonated polysulfone. There are many advantages of the RO: the
metal removal efficiency is tuned by the pressure, the high flux rate, the chemical, thermal, biological stability
of the membrane. The main limitations of RO are the fouling of the small pores which might be irreversible
and the high energy consumption.
Apart from these techniques electrodialysis (ED) is also considered as a membrane or an electrochemical
technique [13]. It is used to transport ions from one solution through ion-exchange membrane to another
solution under the influence of an applied electric potential. The membranes are made of thin sheets of
polymer materials with cationic or anionic properties. ED is performed in an electrodialysis cell consisting of
a feed and a concentrate compartment formed by an anion and a cation exchange membrane placed between
two electrodes. For electrodialysis processes, multiple electrodialysis cells are arranged together with
alternating anion and cation exchange membranes to form an ED stack. The anions/cations pass in the
diluate stream and migrate toward the anode/cathode through the positively/negatively charged anion/cation
2 – 5 pages (1,200 – 3,000 words or 8,000 – 20,000 characters with spaces)
exchange membranes, but are prevented from further migration toward the anode/cathode by the
negatively/positively charged exchange membrane and therefore stay in the concentrate stream. The major
applications of ED are the desalination of brackish water or seawater and the production of pure and
ultrapure water by electro-deionization.
Cross-References
[Please enter your text here …]
References
[1] Rydberg J, Musikas C, Choppin GR (1992) Principles and Practices of Solvent Extraction. Marcel Dekker
Inc, New York.
[2] Danesi PR, Chiariza R, Coleman CF (1980) The kinetics of metal solvent extraction. CRC Cr Rev Anal
Chem. 10(1): 1-126.
[3] Auerbach SM, Carrado KA, Dutta PK (2003) Handbook of Zeolite Science and Technology. Marcel
Dekker Inc, New York.
[4] Korkisch J (1989) Handbook of ion exchange resins: their application to inorganic analytical chemistry.
CRC Press, Florida.
[5] Sebba F (1962) Ion flotation. Elsevier Publishing Company, Amsterdam.
[6] Zabel T, (1984) Flotation in water treatment, in: K.J. Ives (Ed.), The Scientific Basis of Flotation, Springer,
The Hague, pp. 349–378.
[7] Almeida Bezerra M, Zezze Arruda M, Costa Ferreira SL (2005) Cloud Point Extraction as a Procedure of
Separation and Pre-Concentration for Metal Determination Using Spectroanalytical Techniques: A Review.
Appl Spectrosc Rev 40(4): 269-299. Silva MF, Cerutti ES, Martinez LD (2006) Microchim Acta 155: 349-364.
[8] Larpent C, Prevost S, Berthon L., Zemb Th, Testard, F (2007) Nonionic metal-chelating surfactants
mediated solvent-free thermo-induced separation of uranyl. New J. Chem. 31(8): 1424-1428.
[9] Benefield LD, Morgan JM (1999) Chemical precipitation, in: R.D. Letterman (Ed.), Water Quality and
Treatment, McGraw-Hill Inc., New York.
[10] Strnad J, Heckmann, K (1992) Separation of actinides with alkylpyridinium salts. J Radioanal Nucl Chem.
163: 47–57.
[11] Kurniawan TA, Chan GYS, Lo WH, Babel S. (2006) Physico-chemical treatment techniques for
wastewater laden with heavy metals. Chem Eng J. 118: 83-98.
[12] Borrini J, Bernier G, Pellet-Rostaing S, Favre-Reguillon A, Lemaire M (2010) Separation of
lanthanides(III) by inorganic nanofiltration membranes using a water soluble complexing agent. J Membrane
Sci. 348: 41-46.
[13] Davis TA (1990) Electrodialysis, in Handbook of Industrial Membrane Technology, M.C. Porter, ed.,
Noyes Publications, New Jersey.
2 – 5 pages (1,200 – 3,000 words or 8,000 – 20,000 characters with spaces)