1.1 Introduction
1.1.2 Mechanism of transport of solute in LM
Three different phases are involved in the system of LM. The solute of interest is transported from feed phase to other aqueous phase,viz. receiving or strip phase, via membrane phase. The transport of solute occurs through extraction from feed phase to recovery/re-extraction in strip phase due to feeble hydrogen bonding or by strong chemical reaction between solute and carrier. When solutes are transported by simple solution-diffusion mechanism due to different solubility or chemical potential between different phases, the transport is termed as passive transport. Passive transport is direct transport and it does not require any chemical energy to accomplish the solute movement. There are two basic types of principles involved in LM operation viz.
passive transport and active transport.
Passive transport: In passive transport, ion/atom/molecule move from a zone of higher concentration to a zone of lower concentration across membrane driven by the growth of entropy of the system and does not require any chemical energy. Four different types of transport can be seen in passive transport such as solution diffusion, facilitated diffusion, filtration and osmosis. In case solution diffusion solute dissolves in membrane material and is transported through membrane due to concentration gradient. The process continues till the gradient is eliminated [32, 36]. Transport of solute A from feed phase to strip phase occurs due to concentration gradient from feed phase to strip phase as shown in Figure 1.1. In this case, the rate of diffusive transport can be low due to solubility of solute A in feed phase as well as strip phase.
Figure 1.1: Ordinary diffusive transport of component, A through LM
In order to achieve high selectivity, a substrate specific carrier must be present in the LM. The type of molecular movement across membrane is called facilitated diffusion or carrier mediated diffusion. This type of solute transport is also called passive transport as there is no energy required for transporting solute. The carrier added in the membrane phase should be soluble only in the LM phase and must have the capability to form complex reversibly with specific solute. The mechanism is repre- sented schematically in Figure 1.2. The transport mechanism is called as uniport, where only single component is transported through LM. The solute transport of feed phase component A is enhanced by the carrier C. The carrier C present in the LM forms complex AC at the feed-membrane interface. Complex AC diffuses through membrane from feed-membrane interface to strip-membrane interface due to con- centration gradient and releases solute A at the strip-membrane interface. There after the carrier C again diffuses back to the feed-membrane interface and binds with A and cycle continues. There are two types of transport mechanism involved in the carrier mediated transport, (1) some portion of A transport freely due to concentration gradient which is called solution-diffusion mechanism and (2) A forms complex with carrier which enhances the solubility of A in membrane phase to boost transport rate of solute A. There are some important basic features that should be
1.1. Introduction 7
present in the carrier mediated transport. One of the basic features is formation of solute-carrier complex should be reversible, i.e. the solute-carrier complex formed in feed-membrane interface should be easily separated at the strip-membrane interface, otherwise carrier present in the membrane phase will get saturated with solute-carrier complex which would limit the solute transport. Secondly, complex should not be very strong or very weak. Strong complex will create problem in releasing solute at strip-membrane interface whereas, very weak complex will not form solute-carrier complex very easily at feed membrane interface which eventually slows down the so- lute transport rate. There should be optimum bond energy in the reversible complex i.e. 10–50 kJ/mol [29].
Figure 1.2: Mechanism of carrier mediated or facilitated transport in LM with mobile carrier
There are two types of carrier mediated coupled transport viz. co-transport and counter transport. The transport mechanism are shown in Figure 1.3. In co-transport carrier can form complex with two different species at the same time and transport in the same direction (refer Figure 1.3a). A and B form complex with C and gets transported at same direction. It is called as co-transport. In case of counter trans- port the carrier forms complex with two different species at the same time but gets transported in opposite direction (refer Figure 1.3b). A and B form complex with carrier C and gets transported in the opposite direction.
(a) Co-transport
(b) Counter transport
Figure 1.3: Mechanism of coupled transport in LM
Active transport: Active transport, also known as uphill transport. This type transport occurs against the concentration gradienti.e. lower concentration to higher concentration. Active transport is mainly driven by oxidation-reduction, catalytic reactions, biochemical conversions on the membrane interfaces as shown in Figure 1.4.
These reactions give energy to maintain a proper balance of ions and molecules. That is why active transport works against a concentration gradient. The active transport is very solute specific or highly selective and chemical reactions are irreversible [30, 37].
1.1. Introduction 9
Figure 1.4: Mechanism of active transport in LM