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Reverse Osmosis
How Reverse Osmosis Works
Understanding
Reverse Osmosis
by
Anne Marie
Helmenstine, Ph.D.
Reverse osmosis or RO is a filtration method that is used to
remove ions and molecules from a solution by applying pressure to the solution
on one side of a semipermeable or selective membrane.
Large molecules (solute) can't cross the membrane, so they remain
on one side. Water (solvent) can cross the membrane.
The result is that solute molecules become more concentrated on
one side of the membrane, while the opposite side becomes more dilute.
How Reverse Osmosis Works
In order to understand reverse osmosis, it helps to first
understand how mass is transported via diffusion and regular osmosis.
Diffusion is the
movement of molecules from
a region of higher concentration to
a region of lower concentration.
Osmosis is
a special case of diffusion in which the molecules are water and the
concentration gradient occurs across a semipermeable membrane.
The semipermeable membrane allows the passage of water, but
notions (e.g., Na+, Ca2+,
Cl-) or larger molecules (e.g., glucose, urea,
bacteria).
Diffusion and osmosis are thermodynamically favorable and will
continue until equilibrium is reached.
Osmosis can be slowed, stopped, or even reversed if sufficient
pressure is applied to the membrane from the 'concentrated' side of the
membrane.
Reverse osmosis occurs when the water is moved across the
membrane against the concentration gradient, from lower
concentration to higher concentration.
To illustrate, imagine a semipermeable membrane with fresh water
on one side and a concentrated aqueous solution on the other side.
If normal osmosis takes place, the fresh water will cross the
membrane to dilute the concentrated solution.
In reverse osmosis, pressure is exerted on the side with the
concentrated solution to force the water molecules through the membrane to the
freshwater side.
There are different pore sizes of membranes used for reverse
osmosis.
While a small pore size does a better job of filtration, it
takes longer to move water.
It's sort of like trying to pour water through a strainer (large
holes or pores) compared to trying to pour it through a paper towel (smaller
holes).
However, reverse osmosis is different from simple membrane
filtration because it involves diffusion and is affected by flow rate and
pressure.
Uses of Reverse Osmosis
Reverse osmosis is often used in commercial and residential
water filtration. It is also one of the methods used to desalinate seawater.
Reverse osmosis not only reduces salt, but can also filter out
metals, organic contaminants, and pathogens.
Sometimes reverse osmosis is used to purify liquids in which
water is an undesirable impurity. For example, reverse osmosis can be used to
purify ethanol or grain alcohol to increase its proof.
History of Reverse Osmosis
Reverse osmosis is not a new purification technique.
The first examples of osmosis through semipermeable membranes
was described by Jean-Antoine Nollet in 1748.
While the process was known in laboratories, it wasn't used for
desalination of seawater until 1950 at the University of California in Los
Angeles.
Multiple researchers refined methods of using reverse osmosis to
purify water, but the process was so slow that it wasn't practical on a
commercial scale. New polymers allowed for the production of more efficient
membranes.
By the beginning of the 21st century, desalination plants became
capable of desalinating water at the rate of 15 million gallons per day, with
around 15,000 plants in operation or planned.
Anne
Marie Helmenstine, Ph.D.
Ph.D. in
biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge
National Laboratory.
Science
educator with experience teaching chemistry, biology, astronomy, and
physics at the high school, college, and graduate levels.
ThoughtCo
and About Education chemistry expert since 2001.
Widely-published
graphic artist, responsible for printable periodic tables and other
illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
She has worked as a research scientist and also abstracting and indexing
diverse scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor
in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
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