Review of the Chemical Separation

The chemical separation is used to reduce the quantity of potentially toxic or hazardous materials discharged to the environment. In addition, separations that lead to recycle, recovery, or reuse of materials also prevent discharge. It is can assists in resolving some of the environmental challenges, so it became more important for researchers, in particular, environmental specialists. Hence, this article aimed to collection information about the chemical separation to help the researchers to resolve the environmental problems. Overall, it can concluded that the extraction is the first step to separate the desired substance from the raw materials and there are various methods for extraction such as sublimation, distillation method and solvent extraction. Correspondently, there are many types of separation processes, the most widely used method is the chromatography method. Keywords— chemical separation, gas chromatography, separation methods, solvent extraction, environment.


I. INTRODUCTION
Separation processes, or processes that use physical, chemical, or electrical forces to isolate or concentrate selected constituents of a mixture, are essential to the chemical, petroleum refining, and materials processing industries (NRC, 1998). Chemical separations are of central importance in many areas of environmental science, whether it is the clean-up of polluted water or soil, the treatment of discharge streams from chemical processes, or modification of a specific process to decrease its environmental impact (Noble and Terry, 2004). Hence, this report will supply the reader with principle information about separation processes; functions, and methods of separation, more detail about extraction and chromatography methods, in particular, gas chromatography; classification, fundamentals and advantages and disadvantages of gas chromatography.

II. THE FUNCTIONS OF SEPARATION PROCESSES
Separation processes are used for three primary functions: purification, concentration, and fractionation. Purification is the removal of undesired components in a feed mixture from the desired species. For example, acid gases, such as sulfur dioxide and nitrogen oxides, must be removed from power plant combustion gas effluents before being discharged into the atmosphere. Concentration is performed to obtain a higher proportion of desired components that are initially dilute in a feed stream. An example is the concentration of metals present in an electroplating process by removal of water. This separation allows one to recycle the metals back to the electroplating process rather than discharge them to the environment. Lastly, in fractionation, a feed stream of two or more components is segregated into product streams of different components, typically relatively pure streams of each component. The separation of radioactive waste with short half-lives from that having much longer half-lives facilitates proper handling and storage (Noble and Terry, 2004).

III. SEPARATION METHODS
There are many different separation techniques which may be broadly classified into processes of mechanical separation and separation by diffusion or others, include various types of separation processes (Taulbee and Maroto-Valer, 2000; Moskvin, 2016 and Ibrahim, 2018), for example: extraction, chromatography, crystallization, filtration, Decantation and Sublimation. We will discuss extraction and chromatography, because they are widely used. Extraction is the first step to separate the desired substance from the raw materials. There are many methods for extraction such as solvent extraction, distillation method, pressing and sublimation. However, the most widely used method is solvent extraction (Zhang et al., 2018). These methods were summarized in (Table 1.). Soxhlet extraction and distillation method will be discussed as sample for extraction methods. The extraction of organic compounds, including pesticides, polycyclic aromatic hydrocarbons and phenols from matrices (soils, sewage sludges, vegetables, plants), has historically been carried out by using Soxhlet extraction.
The mode of operation of the extraction system is that organic solvent under the influence of heat (and pressure) will desorb, solvate and diffuse the organic compounds from the sample matrix allowing them to transfer into the bulk (organic) solvent ( Dean, 2009).
The Soxhlet apparatus consists of a solvent reservoir, extractor body, an electric heat source and a water-cooled reflux condenser.
In the practice, the solid sample is loaded on the Soxhlet thimble and placed in the inner tube of the extractor body. The extractor body is then fitted to a round bottomed flask containing the chosen organic solvent and to a reflux condenser, then turn on heat source. The samples take few hours to extract, the process is repeated many times with new samples to get the required quantity (Dean, 2009;Elamin and Satti, 2012). Soxhlet extraction uses a range of organic solvents to remove organic compounds from samples, but solvent properties are very important to the extraction process (Prado et al., 2015 andDean, 2009).

HYDRODISTILLATION:
Hydrodistillation is a commonly used method of extracting essential oils from plant samples. This method is divided into the subcategories of steam distillation, water distillation, and a combination of water and steam distillation (Dilworth et al., 2017). The distillation apparatus consists of a vessel for plant material and water together (water distillation) or two vessels, one for plant material and other for water (water and steam distillation) or a vessel for plant material with steam inlet (steam distillation), a condenser to cool and condense the vapour produced and a method of collection, or 'receiver'. The sample for extraction is placed in the distillation vessel. This is then heated to boiling point and the steam (water vapour) carries out the volatile oils (Clarke, 2008).

CHROMATOGRAPHY:
Chromatography is one of the most important analytical techniques. It allows the separation and subsequently the qualitative and quantitative analysis of complex mixtures, as long as the samples are volatile or soluble in a suitable solvent (Meyer, 2013). Chromatography has been defined by Irving et al. (1978) as "a method used primarily for separation of the components of a sample in which the components are distributed between two phases, one of which is stationary while the other moves. The stationary phase may be a solid, or a liquid supported on a solid, or a gel. The stationary phase may be packed in a column, spread as a layer, or distributed as a film, etc.; in these definitions Chromatographic bed is used as a general term to denote any of the different forms in which the stationary phase may be used. The mobile phase may be gaseous or liquid".

CLASSIFICATION OF CHROMATOGRAPHY:
There are many forms of chromatography based on the different mobile phases, stationary phases, and supports that can be used in this method, which has led to a wide range of applications for this technique, they are summarized in the figure, then are explained as follow (Irving et al. 1978 andMoskvin, 2016).

CLASSIFICATION ACCORDING TO PHASES USED:
In this classification the first word specifies the mobile phase and the second the stationary phase. A liquid stationary phase is supported on a solid.

Thin-layer chromatography
Chromatography carried out in a layer of adsorbent spread on a support, e.g. a glass plate.

GAS CHROMATOGRAPHY:
Gas chromatography comprises all chromatographic methods in which the moving phase is a gas.
The word chromatography itself implies that a stationary phase is present in addition to the moving phase, Includes: A. Gas-liquid chromatography comprises all gaschromatographic methods in which the stationary phase is a liquid distributed on a solid support. Separation is achieved by partition of the components of a sample between the phases. B. Gas-solid chromatography comprises all gas chromatographic methods in which the stationary phase is an active solid (e.g. charcoal, molecular sieves). Separation is achieved by adsorption of the components of a sample.  A typical gas chromatograph consists of an injection port, a column, carrier gas flow control equipment, ovens and heaters for maintaining temperatures of the injection port and the column, an integrator chart recorder and a detector (Halord and Miller, 1998 The detector is the device located at the end of the column which provides a quantitative measurement of the components of the mixture as they elute in combination with the carrier gas. These detection properties fall into two categories: bulk properties and specific properties. Bulk properties, which are also known as general properties, are properties that both the carrier gas and analytic possess but to different degrees. Specific properties, such as detectors that measure nitrogen-phosphorous content, have limited applications but compensate for this by their increased sensitivity. There are many detectors, but Mass Spectrometry Detectors detector are most powerful of all gas chromatography detectors (Table 2.).  (Skoog, et al., 2007).

Type of Detector Applicable Samples Detection Limit
Mass

IV. CONCLUSION
Solvent extraction is widely used among the extraction methods. Chromatography is one of the most important analytical techniques. It is classified based on three types classification according to phases used includes gas chromatography (GC) and liquid chromatography (LC); according to mechanisms includes adsorption chromatography, partition chromatography, ion-exchange chromatography (IEC), size-exclusion chromatography (SEC), and affinity chromatography and according to techniques used includes column chromatography (CC), open-tube chromatography, paper chromatography (PC) and thin-layer chromatography.