Water, Water Everywhere . . .
83
of membrane per day. Attempts to improve on this by using very
thin lms failed. Some other way of increasing the ow was indi—
cated. In 1960 S. E. Loeb and F. Milstein of the University of
California described a specially cast cellulose acetate membrane
through which water owed 50 to 100 times faster. This membrane
rejected some 98- 5 per cent of the salt and, since the rejection was
geared to the pressure, it was theoretically possible to reach a point
where no salt was allowed through.
3. Chemical processes
[on exchange. Records going back to the time of Aristotle indicate
that sand lters have been used to purify salt water and make it
drinkable. Sir Francis Bacon described experiments in which he
passed salt water through pots lled with earth in order to remove
the salt—he even mentioned imaginary water treatment plant in
The New Atlantis (1623). In spite of this early start, the rst
quantitative information on ion exchange came not from the
development of water purication but from experiments With farm
manure. Farmers and scientists wanted to know, for example, why
Chemicals such as potassium and ammonium salts were held by the
soil suîciently rmly to prevent the rain from washing them away
and yet remained available to the growing crops. From their experi—
ments grew a new body of information as to how such ions behave—
information upon which purication by ion exchange ultimately
depends.
Modern desalting systems do not rely on natural substances
capable of exchanging various chemical ions; instead they use
specially developed synthetic resins. As might be expected from the
earlier mention of electrodialysis these ion exchangers have to
process both positive ions (cations) and negative ones (anions). Beds
of cation and anion exchangers are placed in series—the sea—water
erst passes through the cation exchanger, where sodium ions are
exchanged for hydrogen ions, and then ows through the anion
xchanger where chloride ions are replaced by hydroxyl groups.
The hydrogen and hydroxyl ions combine to make more water.
As long as the ion exchange resins remain active, fresh water
from the sea together with that formed by the combined ions Will
ow, but the resins gradually lose their supply of exchangeable ions
until eventually they eau no longer remove the salt. At this point,
83
of membrane per day. Attempts to improve on this by using very
thin lms failed. Some other way of increasing the ow was indi—
cated. In 1960 S. E. Loeb and F. Milstein of the University of
California described a specially cast cellulose acetate membrane
through which water owed 50 to 100 times faster. This membrane
rejected some 98- 5 per cent of the salt and, since the rejection was
geared to the pressure, it was theoretically possible to reach a point
where no salt was allowed through.
3. Chemical processes
[on exchange. Records going back to the time of Aristotle indicate
that sand lters have been used to purify salt water and make it
drinkable. Sir Francis Bacon described experiments in which he
passed salt water through pots lled with earth in order to remove
the salt—he even mentioned imaginary water treatment plant in
The New Atlantis (1623). In spite of this early start, the rst
quantitative information on ion exchange came not from the
development of water purication but from experiments With farm
manure. Farmers and scientists wanted to know, for example, why
Chemicals such as potassium and ammonium salts were held by the
soil suîciently rmly to prevent the rain from washing them away
and yet remained available to the growing crops. From their experi—
ments grew a new body of information as to how such ions behave—
information upon which purication by ion exchange ultimately
depends.
Modern desalting systems do not rely on natural substances
capable of exchanging various chemical ions; instead they use
specially developed synthetic resins. As might be expected from the
earlier mention of electrodialysis these ion exchangers have to
process both positive ions (cations) and negative ones (anions). Beds
of cation and anion exchangers are placed in series—the sea—water
erst passes through the cation exchanger, where sodium ions are
exchanged for hydrogen ions, and then ows through the anion
xchanger where chloride ions are replaced by hydroxyl groups.
The hydrogen and hydroxyl ions combine to make more water.
As long as the ion exchange resins remain active, fresh water
from the sea together with that formed by the combined ions Will
ow, but the resins gradually lose their supply of exchangeable ions
until eventually they eau no longer remove the salt. At this point,