5.3.1 Liquid-Liquid Extraction
Liquid-liquid extraction (LLE) also known as ‘solvent extraction’ or ‘partitioning’ is
a method to separate compounds or metal complexes based on their relative
solubilities in two different liquids that differ in their density, polar (usually water)
and non-polar (organic solvent) (Berk 2013). Most organic compounds will partition
into the organic phase and salts and ionic forms of solutes will remain in water
(Tissue 2013). In the simplest case, the system consists of three components: solute
(s), carrier liquid and solvent. The initial mixture, containing one or more solutes
dissolved in the carrier liquid, is mixed with the solvent. Upon mixing, the solute is
transferred from the carrier liquid to the solvent. The denser solution settles to the
bottom. The location of the solute will depend on the properties of both liquids and
the solute (JoVE 2019). Hexane and cyclohexane are solvents from the alkane
family widely used for extracting non-polar compounds (e.g. organochlorine and
some organophosphorus pesticides), whereas dichloromethane and chloroform are
typical solvents used for the extraction of non-polar to medium polarity organic
pollutants (Kassim et al. 2005; Chirila and Drăghici 2013).
Although LLE is probably the oldest separation technique in analytical chemistry,
it still remains one of the most powerful and versatile sample clean-up and
preconcentration methods (Jeannot 2007; Saha et al. 2017). It is frequently used in
the chemical and petrochemical (for the separation of aromatic components and the
extraction of metals from aqueous solution), mining (as a second step out of three in
oxide ore processing) and pharmaceutical industry to separate products from each
other (Kiezyk and Mackay 1971; Berk 2013; Stanley et al. 2015). In wastewater
treatment, it is primarily used for the removal of phenols, creosols and other phenolic
acids. These wastewater streams arise principally in petroleum refineries, coke-oven
plants in steel industry and plastics industry (Kiezyk and Mackay 1971).
Even if LLE offers many advantages, it is gradually being set aside due to an
extensive use of SPE (Saha et al. 2017). Many recent developments in LLE have
focused on the reduction of the amounts of solvent used, on environmental factors as
well as on the automation of the process in conjunction with chromatographic
analysis. To minimize the disadvantages of LLE while preserving the advantages
it offers, various modifications of the technique have been introduced recently, such
as direct extraction from aqueous solution into a small volume of organic solvent,
headspace techniques and microextraction followed by back-extraction into a small
volume of aqueous solution. These methods are referred to as liquid-phase
microextraction (LPME) or solvent microextraction techniques (Jeannot 2007;
Saha et al. 2017).
There are three main variants of LPME, single-drop liquid-phase microextraction
(SD-LPME or SDME), hollow fibre liquid-phase microextraction (HF-LPME) and
dispersive liquid-liquid microextraction (DLLME); the latter represents the latest
development in LPME. All of them are simple, rapid, cost-effective and environmentally friendly since the volume of organic solvent used is typically in the
microliter range. These methods also do not have sample carryover problems
associated with SPME (Quigley et al. 2016).
5 The Recent Strategies Employed in Chemical Analysis of Contaminated Waters,. . .
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Liquid-liquid extraction (LLE) also known as ‘solvent extraction’ or ‘partitioning’ is
a method to separate compounds or metal complexes based on their relative
solubilities in two different liquids that differ in their density, polar (usually water)
and non-polar (organic solvent) (Berk 2013). Most organic compounds will partition
into the organic phase and salts and ionic forms of solutes will remain in water
(Tissue 2013). In the simplest case, the system consists of three components: solute
(s), carrier liquid and solvent. The initial mixture, containing one or more solutes
dissolved in the carrier liquid, is mixed with the solvent. Upon mixing, the solute is
transferred from the carrier liquid to the solvent. The denser solution settles to the
bottom. The location of the solute will depend on the properties of both liquids and
the solute (JoVE 2019). Hexane and cyclohexane are solvents from the alkane
family widely used for extracting non-polar compounds (e.g. organochlorine and
some organophosphorus pesticides), whereas dichloromethane and chloroform are
typical solvents used for the extraction of non-polar to medium polarity organic
pollutants (Kassim et al. 2005; Chirila and Drăghici 2013).
Although LLE is probably the oldest separation technique in analytical chemistry,
it still remains one of the most powerful and versatile sample clean-up and
preconcentration methods (Jeannot 2007; Saha et al. 2017). It is frequently used in
the chemical and petrochemical (for the separation of aromatic components and the
extraction of metals from aqueous solution), mining (as a second step out of three in
oxide ore processing) and pharmaceutical industry to separate products from each
other (Kiezyk and Mackay 1971; Berk 2013; Stanley et al. 2015). In wastewater
treatment, it is primarily used for the removal of phenols, creosols and other phenolic
acids. These wastewater streams arise principally in petroleum refineries, coke-oven
plants in steel industry and plastics industry (Kiezyk and Mackay 1971).
Even if LLE offers many advantages, it is gradually being set aside due to an
extensive use of SPE (Saha et al. 2017). Many recent developments in LLE have
focused on the reduction of the amounts of solvent used, on environmental factors as
well as on the automation of the process in conjunction with chromatographic
analysis. To minimize the disadvantages of LLE while preserving the advantages
it offers, various modifications of the technique have been introduced recently, such
as direct extraction from aqueous solution into a small volume of organic solvent,
headspace techniques and microextraction followed by back-extraction into a small
volume of aqueous solution. These methods are referred to as liquid-phase
microextraction (LPME) or solvent microextraction techniques (Jeannot 2007;
Saha et al. 2017).
There are three main variants of LPME, single-drop liquid-phase microextraction
(SD-LPME or SDME), hollow fibre liquid-phase microextraction (HF-LPME) and
dispersive liquid-liquid microextraction (DLLME); the latter represents the latest
development in LPME. All of them are simple, rapid, cost-effective and environmentally friendly since the volume of organic solvent used is typically in the
microliter range. These methods also do not have sample carryover problems
associated with SPME (Quigley et al. 2016).
5 The Recent Strategies Employed in Chemical Analysis of Contaminated Waters,. . .
141