Principles of Deionization
Deionization is the process of removing ionizable solids from
water using the principles of ion exchange. In a softener, the
ion exchange process is relatively simple, and consists essentially
of exchanging the minerals calcium and magnesium (and sometimes
iron and manganese), for the softer mineral, sodium. Deionization
is more complicated because it involves the removal of virtually
all ionizable particles from water.
All dissolved minerals in water are composed of both a metallic
part (a positively charged cation) and a non-metallic part (a
negatively charged ion). The softener requires one resin to accomplish
its job because it exchanges only cations. The deionizer requires
two resins because it exchanges both cations and anions. No single
resin can exchange both, because ion exchange depends on the tiny
electrical charges in which like particles repel one another and
unlike particles attract. A single resin can not be both positive
and negative. A cation exchange resin is chemically formulated
to attract positive ions; an anion exchange resin is formulated
to attract negative ions.
The simplest deionizer is a two-column unit in which the cation
exchange resin is held in one pressure vessel and the anion exchange
resin in another. Water first passes through the cation tank,
then the anion tank.
Cation Exchange Process
As water passes down through the cation tank, it encounters millions
of resin beads, each of which contains a large number of negatively
charged exchange sites in the pores and microscopic paths of its
structure. When the resin is in the regenerated state, each exchange
site is occupied by a positively charged hydrogen ion (H+). As
the positively charged cations in the water contact the beads,
they are attracted to the negative exchange sites. Since they
(cations in the water) are stronger in their positive charge than
the positive hydrogen ions in the resin, they drive off the hydrogen
ions and attach to the exchange sites. By so doing, they maintain
a balance between positive and negative charges. The displaced
hydrogen ions (H+) pass down through the resin bed and are discharged
from the tank.
Because the hydrogen ions are acidic, the exchange can also be
described as a displacement of acidic ions by metallic ions, and
water from the cation tank is a stream of dilute mineral acid.
At the same time, the non-metallic components, or anions, such
as sulfates, chlorides and other elements pass through the cation
tank unchanged. These anions, plus the hydrogen ions released
from the cation exchange resin beads, are piped to the anion tank.
Anion Exchange Process
The anion exchange process is similar to the cation exchange
process. There are two kinds of anion resin: strong base and weak
base. A strong base anion resin is made of beads which have positive
exchange sites, and which in the regenerated state are occupied
by negative hydroxide ions (OH-). As the negatively charged non-metallic
anions contact the beads, the same attraction-repulsion process
takes place, and negative hydroxide ions are dislodged and replaced
by the stronger negative non-metallic anions.
The hydroxide ions (OH-) pass down through the anion resin and
are discharged from the tank. At the same time, the hydrogen ions
(H+) from the cation tank have passed unchanged through the anion
resin and they join the hydroxide to form HOH or H2O (water).
A weak base resin will neutralize mineral acid but does not use
ion exchange. Strong base and weak base resins are used for different
purposes:
• Weak Base
Employed when removal of carbon dioxide or silica from water is
not required. Weak base resins are generally higher in acid-removing
capacity than strong base resins and are thermally stable. If
silica and CO2 removal are not required, weak base resin is the
logical choice.
• Strong Base, Type I
Removes mineral acids most completely, including silica as silicic
acid and CO2 as carbonic acid. Is stable at temperatures to 140"F.
• Strong Base, Type II
Removes mineral acids efficiently, but does not remove silica
as completely as Type I. Is stable at temperatures to 105?F. Is
higher in capacity than Type I.
Regeneration
Cation exchange resins are regenerated by hydrochloric or sulfuric
acid. As acid passes down through the resin bed the positively
charged hydrogen cations in the chemical force off the positively
charged cations (calcium, magnesium, sodium, etc.) that were attracted
and held during the deionizer service cycle. The positive hydrogen
ions attach to the negative exchange sites on the beads, restoring
the resin to its regenerated hydrogen form.
Anion exchange resins are regenerated by sodium hydroxide (caustic
soda). In a strong base resin, the alkaline solution passes down
through the resin bed and exchanges with the mineral acids attracted
and held by the beads during the service cycle, restoring the
resin to its original regenerated basic form. In a weak base,
resin, the alkaline solution regenerates the resin by a process
of acid neutralization, not ion exchange.
Mixed Bed Deionizers
If the cation-anion exchange process could be repeated many times,
the efficiency of ion exchange and removal would improve remarkably.
Since no exchange process is 100% efficient, successive ion exchanges
would remove even more ions, since in effect, it would be deionization
of water that had already been deionized. The result would be
an improvement of water purity with each successive ion exchange.
This is exactly what happens when the cation and anion resins
are mixed together in a mixed bed deionizer. As water passes through
the mixed bed, it has millions of chances to contact a cation
resin bead, then an anion, then another cation, another anion,
and so on. An exchange takes place, of course, only when a positive
ion contacts a negative exchange site, and vice versa. With each
exchange, purity of the water improves because more ions are removed
and held by resin beads.
Water Quality Measurement
Water quality can be measured quantitatively in milligrams per
liter (mg/1) or parts per million (ppm) of total dissolved solids
(TDS) or electrically by conductance or resistivity. Electrical
measurements are based on the fact that the electrical conductance
or resistance of water is directly related to the amount of ionizable
impurities in the water. Thus a measure of conductance or specific
resistivity is in effect a measure of the ionic content, or purity
(quality) of the water.
Mixed bed deionizers are quite superior to two-column deionizers
in terms of the water quality they produce. A two column deionizer
yields water with specific resistivity of about 250,000 ohms/cm.
Mixed bed deionizers yield water of 1,000,000 ohms/cm and up to
18,300,000 ohms/cm specific resistivity.
It should be pointed out that deionizers remove ionizable solids
only, and have little or no effect on most dissolved gases, particulate
matter, colloids, dissolved organic matter, or biological impurities.
And even though a strong base resin will remove CO2 chemically,
it may be more economical to remove it with a mechanical degasifier,
especially when large amounts of CO2 are involved. Such considerations
underscore the need for a systems engineering approach to the
problems of water treatment. Systems engineering views the total
picture in terms of the many impurities that water can contain,
identifies them, and engineers a system utilizing the proper pieces
of equipment and the appropriate processes for removing them.
Water is a simple compound, but the impurities in it, and their
removal, can be highly complex.
Electro-Deionization, (EDI)
EDI, or Electro-deionization, uses an electric field to remove
ions and polar species from an aqueous stream. EDI is used with
reverse osmosis to replace ion-exchange resin mixed beds (permanent
or exchange-basis), which require chemical regeneration either
onsite or offsite.
The primary environmental and economic benefit of EDI is the elimination
of the use of resin regeneration chemicals.
The primary quality benefit of EDI is the continuous process eliminates
spikes and upsets.
EDI removes ions from water using conventional ion-exchange resin,
but with a key benefit. In EDI an electrical current is used to
force a continuous migration of contaminant ions out of the feed
water, through the resin bed, into the concentrate stream. The
current also splits the water molecules into hydrogen and hydroxyl
ions, continuously regenerating the resin bed. EDI replaces the
primary mixed-bed in conventional water treatment systems, predictably
and consistently producing water of the highest quality.
