Reverse osmosis separation mechanism

Basic principles of reverse osmosis equipment

When pure water and brine are separated by an ideal semi-permeable membrane, the ideal semi-permeable membrane only allows water to pass through but prevents the passage of salt. At this time, the water on the pure water side of the membrane will spontaneously flow into the salt water side through the semi-permeable membrane. This phenomenon is called For osmosis, if pressure is applied to the brine side of the membrane, the spontaneous flow of water will be inhibited and slowed down. When the applied pressure reaches a certain value, the net flow of water through the membrane is equal to zero. This pressure is called osmotic pressure. When the pressure exerted on the brine side of the membrane is greater than the osmotic pressure, the flow of water will be reversed. At this time, the water in the brine will flow into the pure water side. The above phenomenon is the basic principle of water reverse osmosis (RO) treatment.

The academic circles mainly explain the following three theories about the mechanism of reverse osmosis separation:

1. Dissolution-diffusion model

Lonsdale et al. proposed a dissolution-diffusion model to explain the reverse osmosis phenomenon. He regarded the active surface skin layer of reverse osmosis as a dense and non-porous membrane, and assumed that both solutes and solvents can be dissolved in a homogeneous non-porous membrane surface layer, and each diffused through the membrane under the chemical potential caused by concentration or pressure. The difference in solubility and the difference in the diffusibility of solutes and solvents in the membrane phase affect their energy through the membrane. The specific process is divided into: the first step, the solute and the solvent are adsorbed and dissolved on the surface of the liquid side of the membrane; the second step, there is no interaction between the solute and the solvent, they are molecularly diffused under the promotion of their respective chemical potentials Through the active layer of the reverse osmosis membrane; in the third step, the solute and solvent are desorbed on the permeate side surface of the membrane.

In the above process of solute and solvent permeating the membrane, it is generally assumed that the first and third steps are carried out very quickly. At this time, the permeation rate depends on the second step, that is, the solute and solvent are driven by the molecular Diffusion means through the membrane. due to
The selectivity of “membrane” enables the separation of gas mixture or liquid mixture. The permeability of a substance depends not only on the diffusion coefficient, but also on its solubility in the membrane.

2. Priority adsorption—capillary flow theory

When different kinds of substances are dissolved in the liquid, the surface tension will change differently. For example, organic substances such as alcohols, acids, aldehydes, and fats dissolved in water can reduce the surface tension, but the dissolution of certain inorganic salts will increase the surface tension slightly. This is because the solute dispersion is uneven That is, the concentration of the solute in the surface layer of the solution is different from the concentration inside the solution. This is the surface adsorption phenomenon of the solution. When the aqueous solution is in contact with the polymer porous membrane, if the membrane’s chemical properties make the membrane negatively adsorb solutes and preferentially adsorb water, a layer of pure water with a certain thickness will be formed on the interface between the membrane and the solution. . Under the action of external pressure, it will pass through the capillary pores on the membrane surface to obtain pure water.

3. Hydrogen bond theory

In the cellulose acetate, due to the action of hydrogen bonds and van der Waals forces, there are two parts in the film, the crystalline phase region and the amorphous phase region. Macromolecules are firmly bonded and arranged in parallel are crystalline phase regions, while macromolecules are completely disordered are amorphous phase regions, and water and solutes cannot enter the crystalline phase region. Near the cellulose acetate molecule, water and the oxygen atoms on the cellulose acetate carbonyl group will form hydrogen bonds and form so-called bound water. When cellulose acetate adsorbs the first layer of water molecules, it will cause a great drop in the entropy of the water molecules, forming a structure similar to ice. In the large pore space of the amorphous phase region, the occupancy rate of bound water is very low. There is water with a common structure in the center of the pore. Ions or molecules that cannot form hydrogen bonds with the cellulose acetate membrane enter the bound water and become Migrate in an orderly diffusion mode, passing through the membrane by continuously changing the position of hydrogen bonds with cellulose acetate.

Under pressure, the water molecules in the solution and the activation point of the cellulose acetate-the oxygen atom on the carbonyl group form a hydrogen bond, and the hydrogen bond formed by the original water molecule is broken, and the water molecule dissociates and moves to it. The next activation point forms a new hydrogen bond, and then a series of hydrogen bonds are formed and broken, so that water molecules leave the dense active layer on the membrane surface and enter the porous layer of the membrane. Since the porous layer contains a large amount of capillary water, water molecules can flow out of the membrane smoothly.

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