Surfactant Gel Extraction of Metal Ammine Complexes using SDS and KCl at Room Temperature, and a Small-angle X-ray Diffraction Study of the Surfactant Phase Shoji TAGASHIRA, Tatsuya ICHIMARU, Kouji NOZAKI and Yoshiko MURAKAMI * Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan (Received December 10, 2012; Accepted January 29, 2013) Micellar solutions of sodium dodecylsulfate (SDS) exhibit the property of being separated into two phases as a result of a temperature change or the addition of salts. Upon addition of KCl to an SDS solution, the surfactant phase of Na + DS - changed to the potassium dodecylsulfate phase of K + DS - (KDS) at room temperature around 25°C. The ammine complexes of metal ions such as copper reacted with the dodecylsulfate anion (DS - ) to form the corresponding ion pair, and were separated from the solution as the surfactant phase. After the phase separation of the solution containing SDS, KCl, ammonia, and copper, the surfactant phase consisted of KDS and the ion pair of [Cu(NH 3 ) 4 2 + ](DS - ) 2 (abbreviated CuDS). The structures of the surfactant phases were investigated by small-angle X-ray diffraction (SAXRD) and differential scanning calorimetric measurements. The surfactant phases had lamellar structures with layer distances (d (001) ) of 3.4 nm for KDS and 2.4 nm for CuDS. This surfactant gel extraction method was applied to the mutual separation of copper and zinc in a brass alloy. The extractability of the metals was regulated by the initial metal concentration in the sample solution. The percent extraction was (98.7 ± 0.6)% and (6.8 ± 1.4)% for copper and zinc, respectively. 1. Introduction Valuable metals play a significant role in our daily lives and always coexist in environmental wastes with other metals. Thus, there is a need to find suitable techniques for their simultaneous extraction, separation, and selective recovery. Removal of heavy metals can be conducted using techniques such as precipitation, ion exchange, adsorption, and extraction [1]. The most widely employed extraction methods Solvent Extraction Research and Development, Japan, Vol. 20, 39 – 52 (2013) - 39 -
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Surfactant Gel Extraction of Metal Ammine Complexes using SDS and KCl at Room Temperature,
and a Small-angle X-ray Diffraction Study of the Surfactant Phase
Shoji TAGASHIRA, Tatsuya ICHIMARU, Kouji NOZAKI and Yoshiko MURAKAMI*
Graduate School of Science and Engineering, Yamaguchi University, Yamaguchi 753-8512, Japan
(Received December 10, 2012; Accepted January 29, 2013)
Micellar solutions of sodium dodecylsulfate (SDS) exhibit the property of being
separated into two phases as a result of a temperature change or the addition of salts.
Upon addition of KCl to an SDS solution, the surfactant phase of Na+DS
- changed to
the potassium dodecylsulfate phase of K+DS
- (KDS) at room temperature around
25°C. The ammine complexes of metal ions such as copper reacted with the
dodecylsulfate anion (DS-) to form the corresponding ion pair, and were separated
from the solution as the surfactant phase. After the phase separation of the solution
containing SDS, KCl, ammonia, and copper, the surfactant phase consisted of KDS
and the ion pair of [Cu(NH3)42+](DS
-)2 (abbreviated CuDS). The structures of the
surfactant phases were investigated by small-angle X-ray diffraction (SAXRD) and
differential scanning calorimetric measurements. The surfactant phases had lamellar
structures with layer distances (d(001)) of 3.4 nm for KDS and 2.4 nm for CuDS. This
surfactant gel extraction method was applied to the mutual separation of copper and
zinc in a brass alloy. The extractability of the metals was regulated by the initial
metal concentration in the sample solution. The percent extraction was (98.7 ± 0.6)%
and (6.8 ± 1.4)% for copper and zinc, respectively.
1. Introduction
Valuable metals play a significant role in our daily lives and always coexist in environmental wastes
with other metals. Thus, there is a need to find suitable techniques for their simultaneous extraction,
separation, and selective recovery. Removal of heavy metals can be conducted using techniques such as
precipitation, ion exchange, adsorption, and extraction [1]. The most widely employed extraction methods
Solvent Extraction Research and Development, Japan, Vol. 20, 39 – 52 (2013)
- 39 -
use aqueous and organic phases whose stratification occurs as a result of the limited mutual solubility of
water and the organic solvent. Despite their advantages, extraction methods have a number of
shortcomings, the greatest of which is the need for organic solvents, which tend to be flammable and/or
toxic substances. The problem of making extraction safer can be solved by finding less toxic extracting
agents and interest has been shown in the separation of metal ions via extraction methods using ionic
liquids[2] and surfactants[3,4] as the extraction medium.
One of the developed extraction methods is based on the cloud-point phenomenon, where an
aqueous solution of a surfactant becomes turbid and separates into two isotropic phases if a condition such
as temperature is increased or an appropriate substance is added to the solution. Nonionic micelles have
been used extensively for cloud-point extraction (CPE) of metal ions [5-7]. Some CPE procedures have
been developed using mixtures of nonionic and anionic surfactants. However, the use of anionic surfactants
is still rare for the preconcentration of metals, but there are a few methods for the determination of metal
ions based on the use of anionic surfactants [8].
Another effective method is micellar extraction with micellar-enhanced ultrafiltration (MEUF)
[9-12]. When an anionic surfactant such as sodium dodecylsulfate (SDS) is added to water above the
critical micelle concentration (CMC), surfactant monomers aggregate to form negatively charged micelles.
The heavy metal cations can be mostly trapped by the micelle owing to electrostatic interactions. In this
method, separation can be achieved because the micellar aggregates have a size that prevents them from
passing through ultrafiltration membranes. Numerous investigations of the concentration of metals and
metal chelates have been conducted; however, difficulty still lies in the manipulation of highly viscous
concentrated micellar solutions.
Aqueous solutions of anionic surfactants exhibit the phase separation phenomenon when the
solutions are cooled to certain temperatures. The metal ions that were bound to the surface of the
oppositely charged micelles can be efficiently extracted into the surfactant phase, accompanying the phase
separation. Most of the separated surfactant phase exists as a gel-like solid that has the ability to bind a
solute. According to traditional extraction theory, an electrostatic attractive force is the operative
phenomenon for the formation of ion pairs of cationic metal complexes and anionic surfactants, and the
hydrophobic domain of the surfactant is the extraction medium. In order to efficiently remove metal ions
from water, the formation of stable ionic complexes is necessary. The separation of metals is also based on
the stability of the ion pair. Therefore, the extractability of metals is influenced by the concentration of the
ligand that forms the complex and the ionic surfactant that makes the ion pair. This mechanism is the same
- 40 -
as that for ion-pair extraction systems using organic solvents.
The extraction methods based on temperature-induced phase separation of aqueous ionic micellar
solutions may resemble the CPE method using nonionic surfactants [13]. However, we previously reported
that an unexpected decrease in extractability was observed at low metal concentrations for the ion-pair
extraction of dodecylsulfate anions (DS-) and cationic metal ammine complexes. This decrease has been
explained by the solubility of metal ion pairs (MDSs) at low temperature around 0°C [14]. In the present
study, the phase separation and extraction of the metal ammine complexes at room temperature were
examined by the addition of salts such as KCl to anionic surfactant systems of SDS. The solubility of the
MDS was estimated from the dependence of the metal concentration on the percent extraction and the
structure of the surfactant phase was investigated by small-angle X-ray diffraction (SAXRD). In addition,
the developed surfactant gel extraction method was applied to the separation of major base metal
components of a brass alloy. Finally, the solubility of the surfactant gel and related energetic relationships
are discussed.
2. Experimental
2.1 Reagents
All chemicals were analytical grade and were obtained from Kanto Chemical Co., Inc. (Tokyo),
Tokyo Kasei Kogyo Co., Ltd. (Tokyo) or Wako Pure Chemical Industries, Ltd. (Tokyo). The stock
solutions of divalent metal ions were separately prepared by dissolving the appropriate amounts of