[ Hongjie Water ] What are the ions that can pass through the RO reverse osmosis membrane ?

1. Basic principles of RO reverse osmosis membrane

1.1 Selective permeability of semipermeable membranes

As a semi-permeable membrane, the selective permeability of RO reverse osmosis membrane is mainly reflected in its ability to separate water molecules and other solutes. The pore size of RO membrane ranges from about 0.1nm to 2nm, which is much smaller than the size of most ions and molecules, so it can effectively prevent them from passing through. In practical applications, RO membrane can almost completely block dissolved salts, colloids, microorganisms, organic matter, etc., and only allow water molecules to pass through.

According to the information provided, the high selectivity of RO reverse osmosis membranes is based on sub-nanometer pores between polyamide chains, which hinder the transport of ions relative to smaller water molecules. This size-based selective permeability makes RO membranes particularly important in the field of water treatment, especially in seawater desalination and pure water preparation.

Data show that the desalination rate of RO reverse osmosis membranes can generally be stabilized at more than 90%, and the desalination rate of double-stage reverse osmosis systems can generally be stabilized at more than 98%. This high desalination performance further confirms the selective permeability of RO membranes. In addition, RO membranes can effectively remove microorganisms such as bacteria, organic matter, and inorganic matter such as metal elements, and the effluent water quality is better than other methods.

1.2 Pressure difference as driving force

The operation of RO reverse osmosis membranes relies on pressure difference as a driving force. In the natural osmosis process, water molecules move from the low concentration area to the high concentration area through the semipermeable membrane until the concentrations on both sides reach equilibrium. However, in the RO process, by applying a pressure greater than the osmotic pressure on the high concentration side, water molecules can be reversed through the membrane, thereby separating pure water from the high concentration solution.

Specifically, when pressure is applied to the liquid on one side of the membrane, and the pressure is greater than the osmotic pressure of the solution, the solvent (usually water molecules) will reverse the direction of natural osmosis. In this process, the low-pressure side of the RO membrane obtains the permeated solvent, i.e., the permeate, and the high-pressure side obtains the concentrated solution, i.e., the concentrate. This pressure-driven separation process makes RO membrane technology particularly effective in treating high-concentration solutions, such as seawater desalination and wastewater treatment.

In actual operation, the working pressure of the reverse osmosis membrane is usually about 9-12 kg, and the difference between the pressure before the membrane and the pressure after the membrane will not exceed 1 kg. If the pressure difference exceeds this range, it may mean that the membrane is blocked or needs to be cleaned. Therefore, maintaining an appropriate pressure difference is crucial to the normal operation and life of the RO membrane.

2. Ions that can pass through the RO reverse osmosis membrane

2.1 Water molecule permeability

RO reverse osmosis membranes are designed and manufactured to efficiently allow water molecules to pass through while blocking other larger solute molecules and ions. The size of a water molecule is approximately 0.324nm, while the pore size of an RO membrane is typically between 0.1nm and 2nm, which allows water molecules to pass through the membrane relatively easily. In actual applications, the permeability of the RO membrane is very high for water molecules, which is achieved by applying a pressure difference, allowing water molecules to pass through the membrane in the opposite direction of natural osmosis.

Data show that the water production rate of RO reverse osmosis membranes is related to their pore size and the hydrophilicity of the membrane material. Under standard conditions, the water production rate of RO membranes can reach several liters per square centimeter per hour, depending on the operating pressure and the specific characteristics of the membrane. For example, the 8040 model RO membrane mentioned in a study can produce 1 ton of water per hour under normal operating pressure. This efficient water molecule permeability is the key factor in the widespread application of RO membrane technology in the field of water treatment.

2.2 Permeability of some mineral ions

Although RO reverse osmosis membranes are primarily designed to block ions, not all ions are completely blocked. In some cases, some mineral ions may still pass through the membrane, although this permeability is much lower than that of water molecules. These ions that can pass through are usually those with smaller sizes and lower hydration energy.

Studies have shown that RO membranes generally have higher removal rates for monovalent ions (such as sodium ions Na+) than for divalent ions (such as calcium ions Ca2+). For example, in one study, the transmittance of Ca2+ and Na+ was calculated and it was found that the transmittance of Na+ was relatively high, which may be due to its smaller hydration and larger mobility. This difference means in practical applications that although RO membranes can effectively reduce total dissolved solids (TDS) in water, their removal rate for certain specific mineral ions may not be sufficient to meet the requirements of certain specific applications.

In practical applications, the mineral ion permeability of RO membranes is affected by many factors, including membrane materials, operating pressure, inlet water quality, etc. In order to improve the removal rate of specific ions, more advanced membrane technology or additional pretreatment and post-treatment steps may be required. For example, for applications that require further reduction of hardness, a softening step can be added before the RO system to remove calcium and magnesium ions from the water. In addition, the maintenance and cleaning of RO membranes are also essential to maintain their ion removal performance. Regular chemical cleaning can remove contamination and scaling on the membrane surface, thereby restoring the performance of the membrane and improving its removal rate of mineral ions.

3. RO reverse osmosis membrane's ability to screen different ions

3.1 Effect of ion size and charge

The screening capacity of RO reverse osmosis membranes is significantly affected by the size and charge of the ions. The pore size of the membrane is usually between 0.1nm and 2nm, which is much smaller than the size of most ions and can therefore effectively block them. The hydration of ions also affects their ability to pass through RO membranes because hydration increases the effective size of the ions, making it more difficult for them to pass through the membrane pores.

- Effect of ion size: Monovalent ions such as sodium (Na+) and chloride (Cl-) are weakly hydrated with water molecules due to their smaller size, so they can pass through RO membranes more easily than divalent ions such as calcium (Ca2+) and magnesium (Mg2+). A study showed that the transmittance of Na+ was relatively high, which may be due to its smaller hydration and greater mobility.

- Effect of ion charge: Divalent ions such as Ca2+ and Mg2+ have a higher charge and are more hydrated, resulting in an increase in effective size, making it more difficult for them to pass through the RO membrane. Experimental data show that the removal rate of Ca2+ by RO membrane is usually lower than that of Na+, because Ca2+ is more hydrated, resulting in a lower mobility in the membrane pores.

3.2 Difference in ion removal rate

RO reverse osmosis membranes have different removal rates for different ions, which is mainly due to the differences in ion size, charge and hydration.

- Difference in removal efficiency between monovalent and divalent ions: RO membranes generally have higher removal efficiency for monovalent ions than for divalent ions. For example, the removal efficiency for Na+ may be above 90%, while the removal efficiency for Ca2+ may be slightly lower than this value. This difference is caused by the stronger hydration and larger effective size of divalent ions.

- Differences in removal rates of different membrane materials: Different membrane materials have different removal rates for ions. Polyamide membrane is the most common RO membrane material, and its desalination rate for inorganic salts is generally stable at more than 90%, while some improved membrane materials such as composite membranes can achieve a desalination rate of more than 98%.

- Effect of operating conditions on removal rate: Operating pressure and influent water quality will also affect the ion removal rate of RO membrane. Higher operating pressure can increase the permeability of water molecules, but it may also increase the permeability of certain ions. In addition, the concentration and type of ions in the influent water will also affect the removal rate. High concentrations of ions may cause faster contamination and scaling of the membrane surface, thereby reducing the removal rate.

- Effect of pre-treatment and post-treatment on removal efficiency: In order to improve the removal efficiency of specific ions, it may be necessary to add a pre-treatment step before the RO system, such as softening, to remove calcium and magnesium ions. Post-treatment steps, such as ion exchange, can also further improve water quality and remove ions that are not completely removed during the RO process.

4. Factors affecting RO reverse osmosis membrane ion screening

4.1 Membrane materials and structures

The ion screening ability of RO reverse osmosis membranes is significantly affected by their materials and structure. The chemical properties and physical structure of the membrane material determine its selectivity for different ions.

- Membrane materials: RO membranes are usually made of materials such as polyamide, the chemical structure of which gives the membrane specific ion screening properties. Polyamide membranes are widely used due to their good chemical stability and high water flux. Studies have shown that the salt rejection rate of polyamide membranes for inorganic salts can generally be stabilized at more than 90%. In addition, new membrane materials such as composite membranes can achieve higher salt rejection rates of more than 98% by combining the advantages of different materials.

- Membrane structure: The structure of the RO membrane also has an important influence on its screening ability. The pore size of the membrane ranges from about 0.1nm to 2nm, which is much smaller than the size of most ions and molecules, so it can effectively prevent them from passing through. The RO reverse osmosis membrane of the interfacial polymerization method is divided into three layers: base layer (non-woven fabric) + support layer (polysulfone) + separation layer desalination layer (polyamide), which can be optimized separately during the preparation process, thereby further improving the performance of the RO reverse osmosis membrane.

4.2 Operating conditions (pressure, concentration)

Operating conditions, especially pressure and influent concentration, have a significant effect on the ion screening ability of RO reverse osmosis membranes.

- Pressure: The inlet pressure has a direct impact on the water flux and desalination rate of the RO membrane . Higher operating pressure can increase the permeability of water molecules, but it may also increase the permeability of certain ions. Specifically, when the inlet pressure increases, the water flux increases almost linearly, which is mainly due to the decrease in viscosity and increase in diffusion capacity of water molecules passing through the membrane [4]. However, there is an upper limit to increasing the salt removal rate by increasing the inlet pressure. Beyond a certain pressure value, the desalination rate no longer increases.

- Concentration: The salt content in the influent water also affects the ion screening ability of the RO membrane. The osmotic pressure is a function of the concentration and type of salt or organic matter in the water. As the salt concentration increases, the osmotic pressure also increases. Therefore, the size of the influent driving pressure required to reverse the direction of natural osmotic flow depends mainly on the salt content in the influent water. If the pressure remains constant, the higher the salt content, the lower the flux. The increase in osmotic pressure offsets the driving force of the influent water and increases the salt flux through the membrane (reducing the desalination rate).

5. Conclusion

In this section, we have explored in depth the ion screening ability of RO reverse osmosis membranes, including its basic principles, the permeability of water molecules and some mineral ions, and the factors that affect the screening ability of RO membranes.

5.1 Ion screening capability of RO reverse osmosis membrane

The design of RO reverse osmosis membrane enables it to effectively block most ions and only allow water molecules to pass through. This selective permeability is based on the sub-nanopore structure of the membrane. These pores, relative to the size of water molecules, hinder the transmission of ions. The desalination rate of RO membranes can generally be stabilized at more than 90%, and the desalination rate of double-stage reverse osmosis systems can generally be stabilized at more than 98%. This efficient desalination performance further confirms the selective permeability of RO membranes.

5.2 Comparison of the permeability of water molecules and ions

RO reverse osmosis membranes have extremely high permeability to water molecules, which is achieved by applying a pressure difference , allowing water molecules to pass through the membrane in the opposite direction of natural osmosis. In contrast, although some mineral ions may pass through the membrane, their permeability is much lower than that of water molecules. Studies have shown that the removal rate of RO membranes for monovalent ions (such as sodium ions Na+) is generally higher than that of divalent ions (such as calcium ions Ca2+), which may be due to the smaller hydration and greater mobility of monovalent ions.

5.3 Factors Affecting RO Membrane Ion Screening

The ion screening ability of RO reverse osmosis membrane is affected by many factors, including membrane material, structure, and operating conditions (such as pressure and concentration). Polyamide membranes are widely used due to their good chemical stability and high water flux, while new membrane materials such as composite membranes can achieve higher desalination rates. Changes in operating pressure and influent concentration will also affect the ion screening ability of RO membranes. Higher operating pressure can increase the permeability of water molecules, but it may also increase the permeability of certain ions. As the salt content in the influent increases, the osmotic pressure also increases, thus affecting the ion screening ability of the RO membrane.

In summary, the ion screening ability of RO reverse osmosis membrane is the key factor for its wide application in the field of water treatment. By optimizing membrane materials and structures, as well as adjusting operating conditions, the ion screening performance of RO membrane can be further improved to meet the needs of different application scenarioses.

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