Basics of Environmental Science

102 / Basics of Environmental Science

Osmosis

If two solutions of different strengths are separated by a membrane that allows

molecules of the solvent to pass, but not those of the solute (the dissolved

substance), solvent molecules will cross the membrane from the weaker to

the stronger solution until the two are of equal strength. The membrane

separating them is called ‘differentially permeable’ if it allows water molecules

to pass but slows the passage of larger molecules or prevents some of them,

or ‘semi-permeable’ if it is completely permeable to molecules of solvent and

completely impermeable to those of the solute. Cell membranes are differentially

permeable. Membranes that allow the passage of some but not all molecules

are now often described as ‘partially permeable’.

The passage of water through a membrane requires energy. Pure water is

considered to possess zero energy and a solution to have a negative energy

value. Osmosis occurs when there is an energy difference between two

solutions and the energy involved, known as the ‘osmotic pressure’ or ‘water

potential’, can be measured.

In reverse osmosis sufficient pressure is applied to a solution to overcome the

water potential and force water molecules to cross a semi-permeable membrane

from the higher to lower concentration. The pressure required is about 25×10^5

Pa (25 times ordinary sea-level atmospheric pressure).

Electrolytic desalination involves pumping sea water into a chamber containing electrodes. Some
ions are attracted to the positive electrode (anode), others to the negative electrode (cathode) and
partly purified water is extracted from the middle.

As its name suggests, reverse osmosis is based on a natural process. A partially permeable membrane
separates fresh from sea water and the pressure of the sea water is increased. The high pressure

Figure 3.6 Multistage flash evaporation