This document discusses different membrane separation techniques including reverse osmosis, dialysis, and electrodialysis. Reverse osmosis uses pressure to force purified water through a semi-permeable membrane, leaving dissolved ions behind. Dialysis relies on diffusion across a semi-permeable membrane to remove low molecular weight solutes from fluids. Electrodialysis transports ions through ion exchange membranes under an applied electric potential to purify solutions.

1. Presented by :
Arvind Singh Heer
MSc-II
(Sem-IV)
Analytical Chemistry
Paper-I
MITHIBAI COLLEGE
MEMBRANE SEPARATION

2. CONTENT
 INTRODUCTION
 REVERSE OSMOSIS
 DIALYSIS
 ELECTRODIALYSIS

3. INTRODUCTION
What is Membrane ?
 A membrane is a selective barrier that permits
the separation of certain species in a fluid by
combination of sieving and diffusion
mechanisms.
 Membranes can separate particles and
molecules and over a wide particle size range
and molecular weights.

6. REVERSE OSMOSIS
 Reverse osmosis (RO) is a water purification
technology that uses a semi permeable membrane to
remove larger particles from drinking water. In reverse
osmosis, an applied pressure is used to
overcome osmotic pressure, a colligative property, that is
driven by chemical potential,
a thermodynamic parameter. Reverse osmosis can
remove many types of molecules and ions from
solutions, including bacteria, and is used in both
industrial processes and the production of potable water.
The result is that the solute is retained on the
pressurized side of the membrane and the
pure solvent is allowed to pass to the other side. To be
"selective", this membrane should not allow large
molecules or ions through the pores (holes), but should

7.  In the normal osmosis process, the solvent
naturally moves from an area of low solute
concentration (high water potential), through a
membrane, to an area of high solute
concentration (low water potential). The
movement of a pure solvent is driven to reduce
the free energy of the system by equalizing solute
concentrations on each side of a membrane,
generating osmotic pressure. Applying an
external pressure to reverse the natural flow of
pure solvent, thus, is reverse osmosis. The
process is similar to other membrane technology
applications. However, key differences are found

8.  The predominant removal mechanism in
membrane filtration is straining, or size exclusion,
so the process can theoretically achieve perfect
efficiency regardless of parameters such as the
solution's pressure and concentration. Reverse
osmosis also involves diffusion, making the
process dependent on pressure, flow rate, and
other conditions.Reverse osmosis is most
commonly known for its use in drinking water
purification from seawater, removing the salt and
other effluent materials from the water molecules.

9. PROCESS
 Osmosis is a natural process. When two liquids with
different concentrations of a solute are separated by a
semi permeable membrane, the fluid has a tendency
to move from low to high solute concentrations for
chemical potential equilibrium.
 Formally, reverse osmosis is the process of forcing a
solvent from a region of high solute concentration
through a semi permeable membrane to a region of
low solute concentration by applying a pressure in
excess of the osmotic pressure. The largest and most
important application of reverse osmosis is the
separation of pure water from seawater and brackish
waters; seawater or brackish water is pressurized
against one surface of the membrane, causing
transport of salt-depleted water across the membrane
and emergence of potable drinking water from the

10.  The membranes used for reverse osmosis have a
dense layer in the polymer matrix—either the skin of
an asymmetric membrane or an interracially
polymerized layer within a thin-film-composite
membrane—where the separation occurs. In most
cases, the membrane is designed to allow only water
to pass through this dense layer, while preventing the
passage of solutes (such as salt ions). This process
requires that a high pressure be exerted on the high
concentration side of the membrane, usually 2–
17 bar (30–250 psi) for fresh and brackish water, and
40–82 bar (600–1200 psi) for seawater, which has
around 27 bar (390 psi) natural osmotic pressure that
must be overcome. This process is best known for its
use in desalination (removing the salt and other
minerals from sea water to get fresh water), but since
the early 1970s, it has also been used to purify fresh
water for medical, industrial, and domestic

12. DIALYSIS
 A process for selectively removing low mol. wt.
solutes from solution by allowing them to diffuse
into a region of lower concentration through thin
porous membranes. There is little or no pressure
difference across the membrane and the flux of
each solute is proportional to the concentration
difference. Solutes of high mol. wt. are mostly
retained in the feed solution, because their
diffusivity is low and because diffusion in small
pores is greatly hindered when the molecules are
almost as large as the pores.
 Uses thin porous membranes

13. PRINCIPLE
 Dialysis works on the principles of the diffusion of
solutes and ultra filtration of fluid across a semi-
permeable membrane. Diffusion is a property of
substances in water; substances in water tend to
move from an area of high concentration to an area of
low concentration. Blood flows by one side of a semi-
permeable membrane, and a dialysate, or special
dialysis fluid, flows by the opposite side. A semi
permeable membrane is a thin layer of material that
contains holes of various sizes, or pores. Smaller
solutes and fluid pass through the membrane, but the
membrane blocks the passage of larger substances
(for example, red blood cells, large proteins). This
replicates the filtering process that takes place in the
kidneys, when the blood enters the kidneys and the
larger substances are separated from the smaller

14. ELECTRODIALYSIS
 Electrodialysis (ED) is used to
transport salt ions from one solution through ion-
exchange membranes to another solution under the
influence of an applied electric potential difference.
This is done in a configuration called an
electrodialysis cell. The cell consists of a feed (dilute)
compartment and a concentrate (brine) compartment
formed by an anion exchange membrane and
a cation exchange membrane placed between
two electrodes. In almost all practical electrodialysis
processes, multiple electrodialysis cells are arranged
into a configuration called an electrodialysis stack,
with alternating anion and cation exchange
membranes forming the multiple electrodialysis cells.

15.  Electrodialysis processes are different
than distillation techniques and other membrane
based processes (such as reverse osmosis(RO)) in
that dissolved species are moved away from the feed
stream rather than the reverse. Because the quantity
of dissolved species in the feed stream is far less than
that of the fluid, electrodialysis offers the practical
advantage of much higher feed recovery in many
applications.Electrodialysis reversal(EDR) is an
advanced electrodialysis process which utilizes a flow
and polarity reversal to de-scale membrane surfaces
and enable high concentration operation. EDR is used
in higher salinity commercial applications where brine
volume and scaling is a concern.

18. At the anode small amounts of oxygen gas are
generated:
 Electrode reactions:
Small amounts of hydrogen gas are
generated at the cathode:
2 22e 2H O H (g) 2OH 
  
2 2
-
2
H O 2H 1/ 2O (g)
(also possible 2Cl Cl (g) 2e )


 
 

19. REFERENCE
Handbook of Industrial Membrane
Technology
M.C. Porter
-