How does the molecular weight cut-off (MWCO) of a membrane filter determine its effectiveness in removing specific contaminants?

The molecular weight cut-off (MWCO) of a membrane filter determines its effectiveness in removing specific contaminants by defining the size of the smallest molecules that the membrane will effectively retain. Membrane filters, such as ultrafiltration (UF) and nanofiltration (NF) membranes, are designed to remove contaminants based on their size and shape. The MWCO is a measure of the membrane's pore size, expressed in Daltons (Da) or kilodaltons (kDa). It represents the molecular weight of the smallest molecule that is 90% or more retained by the membrane. A membrane with a lower MWCO will have smaller pores and will be able to remove smaller molecules, while a membrane with a higher MWCO will have larger pores and will allow smaller molecules to pass through. Therefore, to effectively remove a specific contaminant, the MWCO of the membrane must be smaller than the molecular weight of the contaminant. For example, if the goal is to remove a protein with a molecular weight of 50 kDa, a membrane with an MWCO of 30 kDa would be more effective than a membrane with an MWCO of 70 kDa. The 30 kDa membrane will retain most of the 50 kDa protein, while the 70 kDa membrane will allow it to pass through. However, it's important to note that the MWCO is not an absolute cutoff. Molecules with molecular weights slightly below the MWCO may still be partially retained due to factors such as molecular shape, charge, and concentration. Furthermore, membrane fouling, which is the accumulation of particles on the membrane surface, can also affect its performance and alter the effective MWCO. For example, a nanofiltration membrane used to remove natural organic matter (NOM) which has a MWCO range of 100-1000 Daltons is better than an ultrafiltration membrane which typically has a MWCO between 1,000-100,000 Daltons.