The Impact of Silicoaluminates on Reverse Osmosis Membranes
The Impact of Silicoaluminates on Reverse Osmosis Membranes
1. Formation Mechanism of Silicoaluminates
1.1 Reaction Conditions of Aluminum Ions and Silicate Ions
The reaction between aluminum ions (Al³⁺) and silicate ions (SiO₃²⁻) in water to form silicoaluminates is complex and influenced by various factors. Typically, aluminum ions undergo hydrolysis reactions with water molecules, forming a series of hydroxyaluminum ions, such as Al(OH)₂⁺ and Al(OH)₄⁻. Silicate ions, on the other hand, are relatively stable in water. When aluminum ions encounter silicate ions, their reaction primarily depends on the pH value, ion concentration, and temperature of the solution.
At lower pH values, aluminum ions mainly exist as Al³⁺, and silicate ions are scarce. As the pH increases, aluminum ions begin to form hydroxyaluminum ions, while the concentration of silicate ions also gradually rises. When the pH reaches a certain range, the reaction between aluminum ions and silicate ions becomes significant. For example, in the pH range of 6-8, the reaction between aluminum ions and silicate ions is quite active, leading to the easy formation of silicoaluminate precipitates. This is because, within this pH range, the activity of both aluminum ions and silicate ions is higher, and their electrostatic attraction is enhanced, promoting the formation of silicoaluminates.
Furthermore, ion concentration also significantly affects the formation of silicoaluminates. When the concentrations of aluminum and silicate ions are high, their collision probability increases, thereby accelerating the formation of silicoaluminates. For instance, in solutions containing higher concentrations of aluminum salts and silicates, the formation rate of silicoaluminates is noticeably accelerated. Temperature also has a certain impact on the formation of silicoaluminates; higher temperatures can increase the reaction rate and promote the generation of silicoaluminates. Generally, for every 10℃ increase in temperature, the reaction rate approximately doubles to quadruples.
1.2 Influence of pH on the Formation of Silicoaluminates
pH is one of the key factors affecting the formation of silicoaluminates. Under different pH conditions, the existence forms and reaction activities of aluminum ions and silicate ions change significantly, thereby influencing the formation of silicoaluminates.
In acidic environments (pH < 5), aluminum ions mainly exist as Al³⁺, and silicate ions are scarce. At this time, the reaction between aluminum ions and silicate ions is weak, and the formation of silicoaluminates is minimal. As the pH increases, aluminum ions begin to form hydroxyaluminum ions, such as Al(OH)₂⁺ and Al(OH)₄⁻, which have enhanced reaction activity with silicate ions. When the pH is between 6 and 8, the reaction between aluminum ions and silicate ions is the most active, and the formation of silicoaluminates reaches its maximum. For example, at a pH of around 7, the formation rate and yield of silicoaluminates are both at a high level.
In alkaline environments (pH > 8), aluminum ions mainly exist as Al(OH)₄⁻, and the concentration of silicate ions is also high. Although the concentrations of aluminum ions and silicate ions are high, their identical charges lead to increased electrostatic repulsion, reducing their reaction activity and causing the formation of silicoaluminates to gradually decrease. For example, at pH values of 9-10, the formation of silicoaluminates is significantly lower than at a pH of 7.
In summary, pH significantly affects the formation of silicoaluminates. The formation is most active in the pH range of 6-8, while in strongly acidic or alkaline environments, the formation of silicoaluminates is relatively less. This phenomenon is of great significance in practical applications, such as in water treatment processes, where adjusting the pH can effectively control the formation of silicoaluminates, thereby reducing membrane pollution.

2. Impact of Silicoaluminates on Reverse Osmosis Membrane Performance
2.1 Flux Decline
The deposition of silicoaluminates on the surface of reverse osmosis membranes is one of the main reasons for the decline in membrane flux. Research indicates that when the influent contains a higher concentration of aluminum salts, especially at a neutral pH, aluminum ions and silicate ions are prone to form silicoaluminate precipitates. These precipitates accumulate rapidly on the membrane surface, forming a dense inorganic pollution layer. This layer increases the membrane's resistance, hindering the passage of water molecules and leading to a significant decrease in membrane flux. For example, in an experiment, when the influent aluminum salt concentration was 20 mg/L and the pH was 6.7, the flux of the reverse osmosis membrane decreased from an initial 63.7 L/(m²·h) to 44.0 L/(m²·h) after 30 hours of operation, a decline of about 31%. Moreover, the deposition of silicoaluminates also exacerbates the phenomenon of concentration polarization, further reducing the membrane's flux. Concentration polarization increases the solute concentration at the membrane surface, raising the osmotic pressure and thereby offsetting part of the operating pressure, reducing the effective driving force for mass transfer and leading to a decrease in flux.
2.2 Changes in Desalination Rate
Silicoaluminates also have a significant impact on the desalination rate of reverse osmosis membranes. Under different pH conditions, the trend of desalination rate changes varies. When the influent pH is neutral, the formation of silicoaluminates is the most active, and the pollution layer on the membrane surface is the most severe, leading to the highest desalination rate. This is because under neutral conditions, the silicoaluminate precipitates not only block the membrane pores but also change the electrical properties of the membrane surface, enhancing the membrane's ability to retain solutes. However, this high desalination rate comes at the cost of sacrificing membrane flux, and the degree of membrane pollution is also the most severe. Under acidic or alkaline conditions, although the formation of silicoaluminates is relatively less, the desalination rate is still affected. For example, when the influent pH is 4.5, the RO membrane's desalination rate is the lowest; whereas when the influent pH rises to 11, the desalination rate increases but remains lower than under neutral conditions. This is mainly because in acidic or alkaline environments, the existence forms of aluminum ions and silicate ions change, and their interactions with the membrane surface weaken, leading to a decrease in the membrane's retention performance. In addition, the deposition of silicoaluminates can also cause physical damage to the membrane, such as scratches, further affecting the membrane's desalination performance.
3. Characteristics of Silicoaluminate-Contaminated Membranes
3.1 Physical Characteristics of the Pollution Layer
The pollution layer formed by silicoaluminates on the surface of reverse osmosis membranes has significant physical characteristics that directly affect the performance and service life of the membrane. First, the thickness of the pollution layer is an important indicator of its impact on the membrane. Research shows that as the concentration of aluminum salts in the influent increases, the thickness of the pollution layer also correspondingly increases. For example, when the influent aluminum salt concentration is 10 mg/L, the thickness of the pollution layer is about 0.5 μm; whereas when the concentration increases to 30 mg/L, the thickness of the pollution layer can reach 1.5 μm. This increase in thickness significantly increases the membrane's resistance, leading to a decrease in flux.
Secondly, the roughness of the pollution layer is also a key factor affecting membrane performance. The pollution layer formed by silicoaluminate deposition has a rough surface, increasing the resistance to water flow. Through scanning electron microscope (SEM) analysis of the contaminated membrane surface, it was found that the surface roughness of the pollution layer is the highest under neutral pH conditions, with a roughness value reaching 0.2 μm, while under acidic or alkaline conditions, the roughness values are reduced to 0.1 μm and 0.15 μm, respectively. This increase in roughness not only affects the passage of water but also easily leads to the attachment and growth of microorganisms, further exacerbating membrane pollution.
Moreover, the density of the pollution layer also has an important impact on membrane performance. The pollution layer formed by silicoaluminates is relatively dense and can effectively block the passage of water molecules. Experiments have shown that the density of the pollution layer is the highest at a pH of 6-8, at which time the decline in membrane flux is the most significant. This dense pollution layer not only reduces the effective filtration area but also increases the difficulty of membrane cleaning, shortening the service life of the membrane.
3.2 Chemical Composition of the Pollution Layer
The chemical composition of the silicoaluminate pollution layer is complex, mainly consisting of aluminum and silicon compounds, as well as other impurities. Through energy-dispersive X-ray spectroscopy (EDS) analysis of the contaminated membrane surface, it was found that aluminum (Al) and silicon (Si) are the main elements
in the pollution layer, with contents reaching 39.74% and 49.06%, respectively. In addition, the pollution layer also contains small amounts of carbon (C), sodium (Na), magnesium (Mg), and calcium (Ca), etc., the presence of which further affects the performance of the membrane.
Aluminum and silicon compounds exist in various forms in the pollution layer. Under neutral pH conditions, aluminum mainly exists in the form of polymeric hydroxy compounds and colloidal Al(OH)₃, while silicon mainly exists in the form of silicate ions (SiO₃²⁻) and silicoaluminates. These compounds form a complex network structure on the membrane surface, increasing the membrane's resistance. For example, the solubility of colloidal Al(OH)₃ is extremely low, theoretically less than 0.2 mg/L, and once it is deposited on the membrane surface, it quickly forms a dense pollution layer.
In addition, other impurities in the pollution layer also have an important impact on the performance of the membrane. For example, the presence of carbon may originate from the decomposition of organic matter, which combines with silicoaluminates to form organic-inorganic composite pollution layers, further exacerbating membrane pollution. Sodium, magnesium, and calcium, etc., may originate from the hardness components in water, reacting with silicoaluminates to form more complex compounds, increasing the difficulty of membrane cleaning.
In summary, the physical characteristics and chemical composition of the silicoaluminate pollution layer have a significant impact on the performance of reverse osmosis membranes. The thickness, roughness, and density of the pollution layer directly affect the flux and desalination rate of the membrane, while the chemical composition of the pollution layer determines the cleaning difficulty and service life of the membrane. Therefore, in practical applications, it is necessary to optimize operating conditions and pretreatment methods to reduce the formation of silicoaluminates, thereby reducing the pollution of reverse osmosis membranes.

4. Factors Affecting Silicoaluminate Pollution
4.1 Impact of Influent Water Quality
Influent water quality is one of the key factors affecting silicoaluminate pollution. Research shows that the concentration of aluminum salts, silicon content, pH value, temperature, turbidity, organic matter content, residual chlorine content, iron content, and the concentration of ions that easily form insoluble salts in the influent all have a significant impact on silicoaluminate pollution.
- Aluminum Salt Concentration: An increase in the concentration of aluminum salts in the influent significantly exacerbates the formation of silicoaluminates. For example, when the influent aluminum salt concentration increases from 10 mg/L to 30 mg/L, the thickness of the silicoaluminate pollution layer increases from 0.5 μm to 1.5 μm, leading to a significant decrease in membrane flux.
- Silicon Content: An increase in silicon content also promotes the formation of silicoaluminates. In solutions with a higher concentration of silicates, the formation rate of silicoaluminates is significantly accelerated.
- pH Value: pH is a key factor affecting the formation of silicoaluminates. In the pH range of 6-8, the formation of silicoaluminates is the most active, while in strongly acidic or alkaline environments, the formation of silicoaluminates is relatively less.
- Temperature: Temperature also has a certain impact on the formation of silicoaluminates. Higher temperatures can increase the reaction rate and promote the generation of silicoaluminates. Generally, for every 10℃ increase in temperature, the reaction rate approximately doubles to quadruples.
- Turbidity and Organic Matter Content: An increase in turbidity and organic matter content exacerbates membrane pollution. Organic matter can combine with silicoaluminates to form organic-inorganic composite pollution layers, further exacerbating membrane pollution.
- Residual Chlorine Content: Excessive residual chlorine content can cause oxidative damage to reverse osmosis membranes, affecting their performance. At the same time, residual chlorine can react with aluminum ions to form compounds such as chlorinated aluminum, further exacerbating membrane pollution.
- Iron Content: An increase in iron content promotes the formation of silicoaluminates. Iron ions can react with aluminum ions and silicate ions to form complex compounds, increasing the degree of membrane pollution.
- Concentration of Ions That Easily Form Insoluble Salts: An increase in the concentration of ions such as calcium and magnesium, which easily form insoluble silicate precipitates, exacerbates membrane pollution.
4.2 Impact of Operating Conditions
Operating conditions also have a significant impact on silicoaluminate pollution. These mainly include operating pressure, temperature, recovery rate, and flow rate.
- Operating Pressure: An increase in operating pressure can increase the permeation rate of water, but at the same time, it also exacerbates the phenomenon of concentration polarization, leading to an increase in the solute concentration at the membrane surface and an increase in the deposition of silicoaluminates. For example, when the operating pressure increases from 10 bar to 40 bar, the permeation flux increases by 188%, but the deposition of silicoaluminates also significantly increases.
- Temperature: An increase in temperature increases the permeation rate of water, but at the same time, it also promotes the formation of silicoaluminates. Generally, for every 1℃ increase in temperature, the water permeation rate increases by about 3%, but the deposition of silicoaluminates also correspondingly increases.
- Recovery Rate: An increase in recovery rate leads to an increase in the solute concentration at the membrane surface, increasing the deposition of silicoaluminates. For example, when the recovery rate increases from 50% to 75%, the deposition of silicoaluminates on the membrane surface significantly increases.
- Flow Rate: An increase in flow rate can reduce the phenomenon of concentration polarization, lower the solute concentration at the membrane surface, and thereby reduce the deposition of silicoaluminates. For example, when the flow rate increases from 2 L/min to 4.4 L/min, the deposition of silicoaluminates decreases.
In summary, influent water quality and operating conditions are important factors affecting silicoaluminate pollution. By optimizing these factors, the formation of silicoaluminates can be effectively reduced, the pollution of reverse osmosis membranes can be lowered, and the service life of the membranes can be extended.
5. Control and Mitigation Measures for Silicoaluminate Pollution
5.1 Pretreatment Methods
To effectively control the pollution of reverse osmosis membranes by silicoaluminates, it is crucial to adopt appropriate pretreatment methods. The following are some common pretreatment methods:
- pH Adjustment: Based on the formation mechanism of silicoaluminates, adjusting the pH of the influent can significantly affect their formation. Controlling the pH within a range where silicoaluminates form less, such as below 5 or above 8, can effectively reduce the generation of silicoaluminates. For example, by adding an appropriate amount of acid (such as hydrochloric acid) or alkali (such as sodium hydroxide) to the influent, the existence forms of aluminum ions and silicate ions can be changed, reducing their reaction activity and thereby reducing the precipitation of silicoaluminates.
- Reducing Aluminum Salt Concentration: Reducing the concentration of aluminum salts in the influent is a key measure to control silicoaluminate pollution. This can be achieved by optimizing the coagulation process, reducing the dosage of aluminum coagulants, or replacing part of the aluminum coagulants with other non-aluminum coagulants. Additionally, increasing the retention time of the sedimentation tank or using more efficient sedimentation equipment can improve the removal efficiency of aluminum salts and reduce the concentration of aluminum salts entering the reverse osmosis system. For example, reducing the influent aluminum salt concentration from 20 mg/L to 10 mg/L can significantly reduce the thickness and roughness of the silicoaluminate pollution layer, improving the flux and desalination rate of the membrane.
- Removing Silicon Content: Reducing the silicon content in the influent is also an effective method to control silicoaluminate pollution. This can be achieved by using methods such as lime softening and ion exchange to remove silicates from the water. For example, lime softening can react with silicate ions in the water to form insoluble calcium silicate precipitates, thereby reducing the silicon content. Ion exchange can use specific ion exchange resins to adsorb silicate ions in the water, achieving the purpose of silicon removal. Reducing the influent silicon content from 10 mg/L to 5 mg/L can effectively reduce the formation of silicoaluminates and alleviate the degree of membrane pollution.
- Increasing Pretreatment Processes: Adding extra treatment processes to the existing pretreatment workflow, such as ultrafiltration (UF) or nanofiltration (NF) pretreatment, can further remove suspended solids, colloids, and some dissolved substances from the water, reducing the formation of silicoaluminates. Ultrafiltration can effectively remove suspended solids and colloidal particles from the water, reducing the turbidity of the influent and reducing the deposition of silicoaluminates on the membrane surface. Nanofiltration can remove some dissolved silicates and aluminum salts, reducing the possibility of silicoaluminate precipitation on the reverse os
mosis membrane surface. For example, after ultrafiltration pretreatment, the turbidity of the influent can be reduced from 5 NTU to 0.5 NTU, significantly reducing the risk of silicoaluminate pollution.
5.2 Chemical Cleaning and Maintenance
Even with effective pretreatment measures, reverse osmosis membranes may still be subject to a certain degree of silicoaluminate pollution. Therefore, regular chemical cleaning and maintenance are important means to maintain membrane performance and extend membrane service life.
- Selection of Chemical Cleaning Agents: For silicoaluminate pollution, it is necessary to choose suitable chemical cleaning agents. Common cleaning agents include hydrochloric acid, citric acid, and hydrofluoric acid. Hydrochloric acid and citric acid are mainly used to remove inorganic scales, such as silicoaluminate precipitates. Hydrofluoric acid has a good dissolution effect on silicon scales, but its strong corrosiveness should be noted when using it. For example, for reverse osmosis membranes mainly polluted by silicoaluminates, a mixed solution of 0.1% hydrofluoric acid and 0.4% hydrochloric acid can be used for cleaning, effectively removing the silicoaluminate pollution layer on the membrane surface and restoring the flux and desalination rate of the membrane.
- Cleaning Methods and Steps: Chemical cleaning usually includes the following steps:
1. Low-Pressure Water Flushing: Before chemical cleaning, the membrane components should be flushed with low-pressure water (0.1-0.2 MPa) to remove loose pollutants and impurities on the membrane surface. The flushing time is generally 0.5-1.0 hour.
2. Preparation of Cleaning Solution: According to the degree of membrane pollution and the concentration requirements of the cleaning agent, an appropriate amount of chemical cleaning solution should be prepared. The water used for preparing the cleaning solution should be softened water or product water, free of heavy metals, residual chlorine, or other oxidants.
3. Circulation Cleaning: Inject the prepared cleaning solution into the membrane components and perform low-pressure circulation cleaning. The circulation pressure is generally controlled at 0.1-0.3 MPa, and the flow rate is determined according to the specifications of the membrane components. For example, for 8-inch membrane components, the cleaning flow rate is 6-9 m³/hr; for 4-inch membrane components, the cleaning flow rate is 1.8-2.3 m³/hr. The circulation cleaning time is generally 1-2 hours, and the specific time is determined according to the degree of pollution.
4. Soaking: After circulation cleaning, soak the membrane components in the cleaning solution to allow the cleaning solution to fully penetrate into the interior of the membrane, dissolving and loosening the pollutants. The soaking time is generally 2-24 hours, and the specific time is determined according to the degree of pollution and the characteristics of the cleaning agent.
5. Discharge and Rinsing: After soaking, discharge the cleaning solution and thoroughly rinse the membrane components with pre-treated raw water or product water to remove residual cleaning solution and pollutants. The rinsing time is generally 0.5-1.0 hour, until the pH value and conductivity of the rinsing water meet the requirements.
6. Restore Operation: After rinsing, reinstall the membrane components into the system, gradually restore the operating pressure and flow rate, and bring the system back to normal operation.
- Cleaning Cycle and Maintenance: According to the operating conditions of the reverse osmosis system and the degree of membrane pollution, a reasonable chemical cleaning cycle should be determined. Generally, when the standardized flux of the membrane decreases by 10%-15% or the standardized pressure difference increases by 15%-20%, chemical cleaning is needed. At the same time, regularly maintain and inspect the reverse osmosis system, including checking the sealing of the membrane components, the on-off state of the valves, the connection of the pipelines, etc., to timely discover and solve potential problems and ensure the normal operation of the system. For example, conducting a comprehensive inspection and maintenance of the reverse osmosis system once a month can effectively extend the service life of the membrane and maintain the efficient operation of the system.