Introduction and Literature Review
1. Introduction
1.1 Liquid membrane
1.1.1 Mechanism of separation through LM
Three different phases are involved in the system of LM. Target solute is transported from feed phase to the stripping phase via the membrane phase. Extraction and/or re-extraction may occur due to feeble hydrogen bonding or by strong chemical reaction between solute and carrier. When the solute is transported by the simple solution-diffusion mechanism by virtue of difference in solubility (chemical potential) of the solute among the phases, the transportation is termed as the passive transport. It is driven by the growth of entropy of the system and does not require any chemical energy.The transport mechanisms in LM based separation can be categorized into two major types, viz. passive transport and active transport.
1.1.1.1 Passive transport
Passive transport is a movement of ions/atoms/molecules across membranes that is driven by the growth of entropy of the system and does not require any chemical energy. The four main kinds of passive transport are solution diffusion, facilitated diffusion, filtration and osmosis. Solution diffusion is the phenomenon by which components from a high concentration zone moves towards lower concentration zone due to concentration gradient. Diffusion continues until this
Chapter-I gradient is eliminated [11]. Transport of solute A from feed phase to membrane phase occurs by higher solubility or diffusivity of solute A in the membrane phase as shown in Fig. 1.1. The rate of mass transfer in this case is low and depends on the solubility of solute in the LM as well as strip phase.
Figure 1.1: Ordinary diffusive transport of component, A through LM
Facilitated diffusion (or carrier-mediated diffusion) is the movement of molecules across the cell membrane via special carrier agents embedded within the LM. Solute molecules bind with its specific carrier agents, and the complex move through the membrane. Facilitated diffusion is also a passive process as the solutes move down the concentration gradient without using energy.
To increase the rate of mass transfer or efficiency of the LM separation, a carrier agent is added to the membrane phase. The carrier should be soluble only in LM and should have the ability to form complex reversibly with a specific solute. The mechanism is represented schematically through Fig. 1.2. This is called the uniport mechanism because a single component is transferred through the LM. Here the transport of component A is enhanced by the presence of the carrier molecule C. The carrier C forms a complex AC at the feed/membrane interface. Complex AC then diffuses through the membrane due to concentration gradient across the membrane and
Feed phase A
Stripping phase
A LM phase
Introduction and Literature Review releases the solute A at the membrane/strip interface. The free carrier C then diffuses back to the feed/membrane interface due to concentration gradient and the cycle continues. In this case two processes occur simultaneously. Part of component A is transported by free diffusion (i.e.
solution diffusion mechanism) whilst other part is transported due to the formation of solute- carrier complex that enhances the solubility of the solute A in the membrane phase. Hence the transport rate is increased. One basic feature of carrier mediated transport is that the complexation reaction must be reversible. Otherwise solute transport would stop when all the carrier molecules would have formed complex with the solute. Secondly, the affinity between the carrier and solute should not be very strong or very weak. A strong complex, i.e. one exhibiting high affinity between the carrier and solute may result in slow release at the membrane/strip interface while a weak complex, i.e. one exhibiting low affinity between the carrier and solute would yield limited facilitation. Therefore, there should be optimum bond energies of this reversible complex. This bond energy is recommended to be in the range of 1 − 5 × 104 kJ/kmol [9].
Figure 1.2: Mechanism of carrier mediated or facilitated transport in LM with mobile carrier
AC A
C
A C
Feed phase LM phase Stripping phase
Chapter-I 1.1.1.2 Active transport
In active transport, the movement of a substance across a cell membrane occurs against its concentration gradient (from low to high concentration). This is known as uphill transport.
Kedem et al. [12, 13] has proposed a more general definition. According to him, active transport is accomplished only by the cross-coupling of the flux of species, i with that of other species or the chemical reaction, and the driving force is supplied by the free energy change of the coupled processes [14]. There are two types of active transport: primary and secondary. Primary active transport, also called direct active transport, directly uses chemical energy (such as from adenosine triphosphate or ATP) to transport all species of solutes across a membrane against their concentration gradient. Uptake of glucose in the human intestines is an example of primary active transport. Secondary active transport, on the other hand, allows one solute to move downhill (along its electrochemical potential gradient) in order to yield enough entropic energy to drive the transport of the other solute uphill (from a low concentration region to a high one).
This is also known as coupled transport, as opposed to non-coupled or uniport transport where transport of a single component is facilitated. This research work basically deals with non- coupled transport and hence the coupled transport is not described here.