What is molecular weight?
Before going any further, it is important to understand what molecular weight is. Also known as "molecular mass", this term refers to the mass of a molecule or substance. This measurement is based on the atomic weight of carbon-12. In other words, molecular weight represents the mass/weight of any substance or molecule.
To identify the molecular weight of a molecule, simply add up the weights of the atoms in the molecule. For example, the water molecule (H2O) has a molecular weight of 18.
There are two hydrogen atoms and one oxygen atom. The hydrogen atom has an atomic mass of 1.008, while the oxygen atom has an atomic mass of 15.999.
The two hydrogen atoms (1.008 x 2) added to the oxygen atom (15.999) give a molecular weight of 18.
What is the molecular weight cut-off of a membrane?
Now that we know that the molecular weight represents the weight of any molecule, what is the molecular weight cut-off of a membrane?
It is a very useful data in the water treatment field that allows the categorization of membranes according to their ability to treat certain contaminants. In other words, the molecular weight cut-off of a membrane refers to the capacity of a membrane to retain at least 90% of a solute of known molecular weight. The smaller the molecular weight of a solute, the finer the filtration the membrane must provide in order to achieve the 90%+ retention rates.
To illustrate, a reverse osmosis membrane with a molecular weight cut-off of 200 Daltons should have a 90% rejection rate of dissolved materials with a molecular weight greater than 200 Daltons. So, let's take the example of PFAS, which is problematic contaminants primarily caused by humans. With an approximate mass of 414.07 Daltons, RO membranes should be able to remove them. On the other hand, a nanofiltration membrane, generally offering cutoffs ranging between 200 and 1000 Daltons, could encounter problems in removing this type of contaminant.
- Without going into details, the Dalton is a unit of measurement with the symbol "Da" and represents one 12th of the mass of a carbon-12 atom. For instance, one Dalton represents approximately 1.660 x 10-27kg.
For the selection of a membrane, the standard is that a membrane with a molecular weight cutoff of three to six times smaller than that of the chosen solution should be chosen. This is because the environment (temperature, pH, flow rate, etc.) affects the membranes and the solubility of water and contaminants. In addition, the varying shapes of molecules can affect filtration. For example, linear molecules such as some polymers can pass more easily than other types of molecules.
Types of membranes and their typical molecular weight limits
Molecular weight cut-off
> 100 kDa
Suspended solids, fat, bacteria, etc.
10 – 500 kDa
Bacteria, viruses, colloidal silica, etc.
0.2 – 10 kDa
Multivalent ions, organic matter, sugars, etc.
Reverse osmosis (RO)
< 200 Da
salt ions, PFAS, monovalents ions
- kDa represents one kilodalton, or 1000 Daltons.