[Key Point] The ability of reverse osmosis membranes to remove various ions

Reverse osmosis membrane technology plays an important role in wastewater treatment. How does the ability of reverse osmosis membranes to remove various ions rank? Reverse osmosis has different separation characteristics for ions and organic substances in water, mainly including the following points:

1. Organic substances are easier to separate than inorganic ones.

2. Electrolytes are easier to separate than non-electrolytes. Electrolytes with higher charges are easier to separate, and the removal rates usually follow this order:

Al3 > Fe3 > Ca2 > Na  PO43- > SO42- > Cl-

For non-electrolytes, the larger the molecule, the easier it is to remove.

The removal rate of inorganic ions is related to hydrates and hydrated ion radii. 

3. The larger the radius of the hydrated ion, the easier it is to remove. The removal rates are as follows: 

Mg²⁺, Ca²⁺ > Li⁺ > Na⁺ > K⁺　　 F⁻ > Cl⁻ > Br⁻ > NO₃⁻

4. Separation rules for polar organic compounds: 

Aldehydes > Alcohols > Amines > Acids, Tertiary amines > Secondary amines > Primary amines, Citric acid > Tartaric acid > Malic acid > Lactic acid > Acetic acid

5. For isomers (compounds with the same chemical formula but different structures): Tertiary (tert-) > Isomeric (iso-) > Secondary (sec-) > Primary (pri-)

6. Organic sodium salts have good separation performance, while phenols and their derivatives show negative separation. For aqueous solutions of polar or non-polar, dissociated or non-dissociated organic solutes, when they are separated by membranes, the interactions between the solute, solvent, and membrane determine the membrane's selective permeability. These effects include electrostatic forces, hydrogen bonding, hydrophobicity, and electron transfer. 

7. Generally, solutes have little impact on the physical or transport properties of the membrane. In aqueous solutions, only phenols or some low-molecular-weight organic compounds cause cellulose acetate to swell. The presence of these components usually reduces the membrane's water flux, sometimes significantly.

8. Nitrates, perchlorates, cyanides, and thiocyanates are not removed as effectively as chlorides, and ammonium salts are not removed as well as sodium salts. 

9. Most components with a relative molecular weight greater than 150 can be removed effectively, whether in electrolytes or non-electrolytes. Besides, reverse osmosis membranes separate aromatics, cycloalkanes, alkanes, and sodium chloride in different orders. 

In the design and operation of a reverse osmosis system, there are many interdependent factors. So, under theoretical guidance, it’s really important to verify things experimentally, understand the properties or patterns of substances, and use reverse osmosis correctly.