Ions that Can Pass Through RO Reverse Osmosis Membranes
1. Basic Principle of RO Reverse Osmosis Membranes
1.1 Selective Permeability of Semi-Permeable Membranes
As a semi-permeable membrane, the primary function of an RO reverse osmosis membrane is its ability to selectively separate water molecules from other solutes. The pore size of an RO membrane typically ranges from 0.1 nm to 2 nm, which is significantly smaller than the size of most ions and molecules, thus effectively preventing their passage. In practice, RO membranes can almost completely block dissolved salts, colloids, microorganisms, and organic substances, allowing only water molecules to pass through.
This high selectivity is attributed to the sub-nanometer pores within the polyamide chains of the RO membrane, which hinder ion transport relative to the much smaller water molecules. This size-based selective permeability is crucial in water treatment applications, particularly in seawater desalination and pure water production.
Data show that the salt rejection rate of RO membranes is generally stable at over 90%, while dual-stage RO systems can achieve up to 98% or higher. This high desalination performance further confirms the selective permeability of RO membranes. Additionally, RO membranes effectively remove bacteria, microorganisms, organic substances, and inorganic metals, producing water quality superior to other treatment methods.
1.2 Pressure Difference as the Driving Force
The operation of RO membranes relies on a pressure difference as the driving force. In natural osmosis, water molecules naturally pass through a semi-permeable membrane from a low-concentration region to a high-concentration region until equilibrium is reached. In reverse osmosis, by applying pressure greater than the osmotic pressure on the high-concentration side, water molecules are forced to move in the opposite direction through the membrane, separating pure water from concentrated solutes.
Specifically, when pressure is applied to one side of the membrane that exceeds the osmotic pressure of the solution, the solvent (usually water) moves in reverse osmosis. The low-pressure side collects the permeated solvent (permeate), while the high-pressure side retains the concentrated solution (concentrate). This pressure-driven process makes RO technology highly effective for treating high-salinity solutions, such as in seawater desalination and wastewater treatment.
In practice, RO membranes typically operate at pressures around 9–12 kg/cm², and the pressure difference before and after the membrane usually does not exceed 1 kg/cm². A greater difference may indicate fouling or the need for cleaning. Maintaining an appropriate pressure difference is essential for proper RO operation and membrane longevity.
2. Ions that Can Pass Through RO Membranes
2.1 Water Molecule Permeability
RO membranes are specifically designed to allow water molecules to pass efficiently while blocking larger solute molecules and ions. A water molecule measures approximately 0.324 nm, while RO membrane pores range from 0.1 nm to 2 nm, allowing water to pass relatively easily. The permeability for water is significantly high, driven by the applied pressure difference, enabling water molecules to flow in the reverse direction of natural osmosis.
Data indicate that the water production rate of an RO membrane depends on its pore size and hydrophilic properties. Under standard operating conditions, an RO membrane can produce several liters of water per square centimeter per hour, depending on operating pressure and specific membrane characteristics. For example, an 8040 RO membrane model can produce up to 1 ton/hour under normal pressure conditions. This high water permeability is key to the widespread application of RO technology in water treatment.
2.2 Partial Permeability of Certain Mineral Ions
Although RO membranes are designed to block ions, not all ions are completely removed. In some cases, a small fraction of certain mineral ions may pass through, albeit at much lower rates than water molecules. Ions that can pass through more easily are typically smaller and have lower hydration energy.
Studies have shown that RO membranes tend to have a slightly higher transmission rate for monovalent ions (such as sodium Na⁺) compared to divalent ions (such as calcium Ca²⁺). For example, sodium ions have been observed to pass through more readily due to their smaller hydration shell and higher mobility. While RO effectively reduces total dissolved solids (TDS), specific applications may require further treatment to fully remove certain ions.
In practical applications, the permeability of mineral ions depends on factors such as membrane material, operating pressure, and feed water quality. To enhance specific ion removal, advanced membrane technologies or additional pre/post-treatment steps can be employed. For example, softening systems can be added upstream of RO to remove calcium and magnesium ions. Regular chemical cleaning of RO membranes is also crucial for maintaining performance and improving ion rejection rates.
3. RO Membrane Ion Selectivity
3.1 Effect of Ion Size and Charge
RO membranes’ selectivity is strongly influenced by ion size and charge. The pores of an RO membrane are significantly smaller than most ions, effectively preventing their passage. Hydration also increases the effective size of ions, further hindering transport.
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Ion size: Monovalent ions like sodium (Na⁺) and chloride (Cl⁻), being smaller and less hydrated compared to divalent ions like calcium (Ca²⁺) and magnesium (Mg²⁺), are slightly more likely to pass through the membrane.
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Ion charge: Divalent ions (Ca²⁺, Mg²⁺) have stronger hydration shells and larger effective sizes, making it harder for them to permeate the membrane. Experimental data show that RO membranes generally remove divalent ions more effectively than monovalent ions due to these properties.
3.2 Ion Removal Rate Differences
RO membranes exhibit varying removal rates for different ions due to differences in size, charge, and hydration effects:
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Monovalent vs. divalent ions: RO membranes generally achieve >90% removal of monovalent ions like sodium but have slightly lower efficiency for divalent ions such as calcium.
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Membrane material: Common polyamide membranes typically achieve over 90% desalination, while advanced composite membranes can reach 98% or higher.
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Operating conditions: Higher pressure increases water permeability but may slightly increase ion passage. Higher ion concentration in feedwater can lead to scaling and fouling, lowering rejection rates.
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Pre- and post-treatment: Pre-softening removes calcium and magnesium before RO, while ion exchange polishing can further improve final water quality.
4. Factors Affecting Ion Rejection
4.1 Membrane Material and Structure
The material and structure of the RO membrane significantly affect ion selectivity:
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Material: Polyamide membranes are most common due to their chemical stability and high water flux, typically achieving >90% desalination. Advanced composite membranes combine multiple materials to reach up to 98% desalination.
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Structure: Typical RO membranes have a pore size of 0.1–2 nm, much smaller than most ions. They are structured in three layers: base layer (non-woven fabric), support layer (polysulfone), and desalination layer (polyamide), each optimized during fabrication to enhance ion rejection.
4.2 Operating Conditions (Pressure and Concentration)
Operating pressure and feedwater concentration directly impact RO performance:
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Pressure: Higher pressure linearly increases water flux but only up to a certain point improves salt rejection. Beyond this limit, further pressure increase does not enhance rejection.
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Concentration: Higher salinity increases osmotic pressure, requiring higher applied pressure. If operating pressure remains constant, flux decreases, and salt passage increases, reducing rejection efficiency.
5. Summary
In this section, we explored the ion selectivity of RO membranes, including their principles, water and ion permeability, and factors affecting performance:
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Ion selectivity: RO membranes effectively block most ions, allowing mainly water molecules to pass. Typical salt rejection rates are >90%, and dual-stage systems can reach 98% or more.
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Water vs. ions: Water permeates efficiently due to pressure-driven transport, while only trace amounts of certain small monovalent ions may pass through.
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Factors influencing rejection: Membrane material and structure, along with operating conditions (pressure, concentration), determine ion selectivity. Optimizing these factors enhances performance for specific applications.
Overall, the ion rejection capability of RO membranes is a key factor in their widespread use in water treatment. By improving materials, structure, and operational settings, RO membranes can achieve even higher ion selectivity to meet diverse application needs.