Osmosis is defined as a spontaneous phenomenon involving liquid solutions separated by a semipermeable membrane, capable of allowing the passage (or rather the diffusion) only of the solvent and prevent it from the solute, from an area with a higher concentration of solvent to one with a lower concentration. The process depends only on the number of particles in solution and not on the nature of the particles themselves.

The osmosis process continues until an equilibrium situation is reached, in which both solutions gain and maintain the same concentration. Each solution has an osmotic pressure that is directly proportional to its concentration. As mentioned above, osmosis is a spontaneous physical process, meaning that it occurs without external input of energy, which tends to reduce the difference in concentration of a solution.

In biology, osmosis is involved in some passive transport processes (of sugars, proteins and mineral salts and other substances) through biological cell membranes; it is of vital importance for animals and plants, since all cells of living organisms are enclosed by semi-permeable cell membranes, which allow the passage of water molecules but not to certain solutes (of higher molecular weight) such as dissolved salts, amino acids, sugars and proteins. Osmotic balances in the body fluids are essential to provide cells with an optimal environment in which to live.

Due to osmosis, a cell placed in distilled water (hypotonic environment compared to the cell) will tend to swell with water until it bursts, while a cell placed in a solution of sucrose (molecule that can not cross the cell membrane) will lose water, because in this case it is immersed in a hypertonic environment.

Osmotic pressure

Osmotic pressure is a colligative property associated with solutions. When two solutions with the same solvent, but different concentrations of solute, are separated by a semi-permeable membrane (i.e. which allows the solvent molecules to pass but not the solute molecules), the solvent molecules move from the solution with lower concentration of solute (therefore higher concentration of solvent) to the solution with higher concentration of solute (therefore lower concentration of solvent), so as to equalize (or better, make close) the concentrations of the two solutions; the pressure that must be applied to the solution so that the passage of the solvent does not occur is called “osmotic pressure”.

The osmotic pressure of cellular and intercellular fluids plays a very important role for living beings, and its value, like that of other chemical and physical constants related to the internal environment of organisms, can not vary beyond certain limits without compromising the functionality and survival of the cells (see homeostasis): osmotic pressure determines for example the concentration of plasma proteins.

The mechanism of osmotic pressure can be interpreted considering that the particles of a solute tend to disperse uniformly in the solvent, even against gravity, as well as those of a gas tend to occupy all the space available to them: the particles of solute exert a pressure similar to gaseous pressure.

If we consider a system consisting of a solution and its pure solvent separated by a semi-permeable membrane, the number of solvent molecules that in the unit of time cross the membrane towards the solution is higher than the number of solvent molecules that cross it in the opposite direction, because in the first case the molecules that come into contact with the membrane are all solvent molecules, in the second case there is a certain percentage of solute particles, which do not pass through but exert a pressure on the membrane with their impact, the osmotic pressure. The mechanism responsible for the passage of the solvent to the solution with higher concentration is currently believed to be related to the interactions of solute molecules with the membrane and subsequent transfer of momentum away from the membrane to the solvent molecules.

The pressure generated by the collisions of the solute molecules is not directly measurable, whereas the hydrostatic pressure that results in the solution due to more solvent molecules entering it than leaving it is measurable.

Reverse osmosis

Reverse Osmosis (abbreviation: RO), also called hyperfiltration (abbreviation: IF), is the process in which the passage of solvent molecules from the most concentrated solution to the less concentrated solution is forced by applying to the more concentrated solution a pressure greater than the osmotic pressure. So the flow of the solvent occurs from the more concentrated solution to the more diluted one, thus further increasing the difference in concentration between the two compartments. In practice reverse osmosis is realized with a membrane that holds the solute on one side preventing it from passing and allowing the pure solvent to be obtained on the other side. This phenomenon is not spontaneous and requires a mechanical work equal to that necessary to cancel the effect of osmotic pressure.

This process represents the finest water filtration technique as it does not simply consist of a physical obstacle, determined by the size of the pores, to the passage of molecules, but exploits the different chemical affinity of the species with the membrane, allowing in fact the passage of hydrophilic (or water-like) molecules, that is chemically similar to water, such as short-chain alcohols. From the plant point of view, the method uses the principle of tangential filtration, as well as other separative techniques through membranes such as microfiltration, ultrafiltration and nanofiltration. Reverse osmosis is used in water treatment both for desalination and for the removal of traces of phosphates, calcium and heavy metals, pesticides, radioactive materials and almost all polluting molecules.

In recent years, “zero liquid discharge” plants are being built in which the reverse osmosis section increases the concentration of chemical species present in the wastewater to values close to or above their solubility (over-saturated solutions).

Thin Film Composite Membranes (TFC or TFM) are used in the reverse osmosis process. These membranes are semipermeable and manufactured primarily for use in water purification or desalination systems. They also have uses in chemical applications such as batteries and fuel cells. In essence, a TFC material is a molecular sieve constructed in the form of a film of two or more layered materials.

Membranes used in osmosis are generally made of polyamide, a substance chosen primarily for its water permeability and relative impermeability to various dissolved impurities, including salt ions and other small molecules that cannot be filtered out. Another example of a semi-permeable membrane is one used in dialysis.

Fields of application of reverse osmosis

This process typically finds application in:

  • reverse osmosis plants for boiler feedwater production, steam generators;
  • desalination plants for medium salinity and brackish water for the production of water for drinking, irrigation and industrial use; and seawater desalination plants;
  • reverse osmosis plants for the production of water for washing solar panels;
  • industrial reverse osmosis plants for car washes;
  • production of demineralized water for dust abatement systems;
  • reverse osmosis concentrators for the production of fruit juices, wine, breweries, dairy industry;
  • production of osmosis water for pharmaceutical, chemical and cosmetic industry;
  • advanced industrial reverse osmosis applications for wastewater treatment and recovery;
  • reverse osmosis plants for water cutting machinery (hydro-cutting, waterjet).
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