Physical damages in membrane is a common phenomenon. The physical damage could be such as increasing salt passage, change in flux and so on. With the good experience and our study in membrane autopsy, we found physical damage and irreversible damage to the membrane caused by the crystallized minerals majorly silica where colloidal silica plays vital role. In such cases, the foulant composition contain silica near about 10% – 50%.
SILICA has the major role in most of the physical and irreversible damage of the membrane by forming crystallized minerals.
Silica is one of the major culprit which can cause the irreversible damage on membrane surface. It may affect the membrane surface by different ways such as by forming deposition/fouling on membrane or by causing impact on the flux and increasing the trans membrane pressure on membrane, etc.
The presence of reactive and colloidal silica in feed water plays an important role in pretreatment of water. While designing system, colloidal silica is rarely analysed. Most of the time, the parameter analyzed is reactive silica but not the colloidal silica. Predicting the trend of colloidal silica is a difficult task because it may not be similar to that of calcium, magnesium and other minerals and metals. Silica is a complex molecule to understand, interpret and predict because it is very difficult to obtain trend of Colloidal Silica in water.
In such cases the pretreatment of feed water is an important part. Generally raw water pretreatment includes clarification followed by ultrafiltration which plays major role in reducing the colloidal silica. The ultrafiltration process removes the colloidal particles higher than 0.01μm but the size of colloidal silica particles can be smaller up to 0.001μm. So here we understand the limitation of UF filter and how the colloidal silica get skip from ultrafiltration process. Uncertain trend of colloidal silica and limitation of UF make pretreatment more crucial. Hence, to get rid of colloidal silica in feed water, good design and healthy operation practices of pretreatment are essential.
Let’s understand the silica chemistry in detail, to understand it’s effect on membrane as per the changes in different environmental condition as well as changes in feed water chemistry.
Sources of Silica
The presence of colloidal silica is commonly found in river water, sea water, pond water and well water as well as different types of surface water. The quality of surface water changes as per the geographical condition of that particular area. Also the raw water quality changes according to the seasonal changes such as rain, flood etc. which causes both increase and decrease in the minerals, metals and nonmetal concentrations in the raw water.
There is lots of variation and deviation in colloidal silica content of the sample at same location, condition and drawn at the same time. Thus, the variation and deviation of silica forms takes place not only as per the pH and temperature but also lots of other factors.
Factors affecting on silica solubility
The presence of colloidal silica in water and solubility of silica and silicates depends on various factors. They are as follows,
01.Saturation level of silica in water.
02.pH
03.Temperature
04.Content of different minerals and metals in water
05. Source of water.
Different Forms of Silica
Silica exists in either crystalline or amorphous form. Silica is observed in different forms as follows,
01. Dissolved (or reactive silica) :
‘Soluble’ or ‘dissolved’ silica containing monomers, dimers and polymers of silicic acid.
02. Polymerized silica (slow reactive silica) :
It is the form of Silica which results from polymerisation of silicic acid forming particles. The oligomers of silicic acid are generally referred to as polysilicic acid or polymeric silica. Silica polymerization depends on temperature, pH, ionic strength and concentration of silica in water while the rate of polymerization is strongly pH-dependent. It covers a wide range of pH but comparatively rate is higher in neutral and slightly alkaline condition, and decreases to a minimum at pH above 9.5 and below 6.5, respectively. Most of the time the polymerization process is mainly catalyzed by H+ ions or OH- ions depending on the pH.
The supersaturation of water with silicic acid causes the polymerization in which silanol bond (≡ Si-OH) on the surface of two different particles forms siloxane bonds (≡ Si-O-Si ≡). The siloxane bond requires less space than two silanol group. This causes the formation of monomeric silicic acid to oligomeric silica and then gel is formed.
Thus, the hydroxide ion catalyzes the condensation of monomeric silicic acid. The deprotonated or anionic form of silicic acid bind with silanol bond of neutral silicon atom to form siloxane bond. This reaction is accelerated in presence of salts such as Calcium Chloride and Magnesium Chloride.
Silica polymerization is also affected by the presence of other ions in water. Both calcium and magnesium increase the silica polymerization rate. Magnesium hardness is more effective than calcium hardness.
The reactivity of polymeric silica is lower than that of silicic acid due to less silanol groups available. Polymeric silica with low molecular weight is considered to be unstable, and to have only a transient existence.
03. Colloidal (or unreactive silica) :
Highly polymerized silica are generally referred to as colloidal silica. It is a nonreactive form of silica. Colloidal silica is formed due to the polymerisation of silicic acid containing particles and three-dimensional gel networks of silica.
04. Suspended (or particulate silica) :
Some silica species are always in transition depending on pH and concentration of other silica species. In case of the detection process only colloidal, reactive and total silica is easily detected.
Silica precipitation in different conditions :
Dissolved silica separate out from the supersaturated solution in three forms:
01. Monomeric silicic acid deposits on any solid surface that has an −OH group on it with which it can react.
02. Colloidal particles due to polymerization of silica which grows to form a 3D polymer.
03. Biogenic amorphous silica by living organisms – Sessile micro-organisms, Diatom and algae present in the biofilm may absorb the colloidal silica. Also soluble silica have higher affinity toward the extracellular biopolymers produced by micro organisms. Amorphous silica deposits in presence of calcium carbonate and calcium sulphate.
Concentration Polarization Condition in RO membrane System :
In case of RO systems, while separation of salts concentration polarization results in an increase in silica concentration or accumulation of silica at the membrane surface to form a thin boundary. This concentration polarization decreases the permeate flux as well as it affects on the rejection of solute through membrane surface. The cake layer formed on the membrane surface is responsible for hydraulic resistance and create hurdle for diffusion back of the solute into the concentrate water, resulting in a decreased permeability and salt rejection.
Concentration Polarization condition in case of RO membrane
Effect of Silica on Membrane:
The accumulation of higher concentration of colloids near the membrane surface affects the performance of the RO system in several ways:
- Increase in pressure drop is responsible for decrease in water flux.
- The decrease in water flux and increase in solute flux through the membrane affect the separation efficiency of the membrane.
- Increase in ion concentration on membrane surface causes the solutes to exceed their solubility limits and then finally responsible for precipitation and scaling.
Thus, higher concentration of silica along with changes in pH may increase fouling possibilities on the membrane surface.
Membrane fouling phenomenon depends on the changes in silica solubility limit due to changes in concentration polarization layer on membrane surface.
Solubility of different salts of silica:
- In high salinity waters sodium could prevent silica deposition on the membrane surface.
- The presence of carbonic acid (H2CO3) impart the silica both acid as well as base. characteristics. This causes changes in the interaction of silica with membrane surfaces.
- Silica forms a complex with hydrated form of calcium, aluminum, magnesium, and iron elements. This complex form polymerizes and creates colloids.
- Silicates of potassium and sodium are soluble but the silicates of iron, aluminum, and crystalline silica are having very low solubility and are nonreactive form of silica.
The reactive silica consist of low ionized forms (such as monomeric silicic acid) at pH of 6–9 and forms silica gel or cake like silica structure on membrane surface.
The concentration of silica or silicate compounds increases in the bulk solution of the RO feed and scale begins forming on the membrane surface. Thus it becomes less permeable, resulting in the decrease in permeate flux.
Fouling of Silica on Membrane Surface :
Fouling by polymerized silica, colloidal silica or silica gel, because of the polymerization of supersaturated silicic acid, in feed water occurs when the supersaturation level of silica takes place. This process of fouling is more rapid at higher temperatures, but comparatively slower at lower temperatures. Presence of multivalent metal ions such as Fe3+, Al3+, Ca2+, and Mg2+ in feed water act as catalysts during the polymerization process. These ions form complex with silica, which causes the polymerization and fouling of silica on membrane surface.
The reaction between Mg(OH)2 and silicate ions leads to the formation of magnesium silicate precipitates. These precipitates have been shown to be a major foulant because of their insolubility according to the changes in temperature and pH . In presence of calcium carbonate and calcium sulphate, amorphous deposits of silica are formed.
The colloid of silica which is formed by the complex of hydrated minerals and metals with silica grows through polymerization and bridge between organic and inorganic matter to form a gel like layer on membrane surface.
Therefore, the scaling potential of feed water is dependent on the pH and SiO2 content in the concentrate.
Effect of organic compound and silica on membrane :
Apart from above, silica also reacts with organic components including different types of proteins, humic acids and polysaccharides. The presence of silica along with organic compound in feed water causes comparatively more impact on flux than individual organic or inorganic fouling. The deposition of silica on membrane causes formation of thin film on membrane which also causes impact on polyamide layer. The hydrolysis of trimesoly chloride (which is the part of polyamide membrane) causes the formation of carboxyl group. The silica may form a bond with carbon in the carboxyl group by substituting the oxygen. The decrease in carboxyl group may cause impact on flux. These changes in carboxyl group causes irreversible damage.
Removal of Silica fouling on membrane :
The removal of scaling is difficult and expensive.
The presence of silica foulant leads to panic and harsh cleaning which are responsible for different types of physical damages. The acid and alkali cleaning also causes impact on silica fouling processes.
- The possibility of metal silica fouling increases due to frequent acidification of feed water.
- During the alkali cleaning processes, as pH increases above neutral, silicic acid dissociates into the silicate anion (SiO3 2-)n. This can react with different minerals such as calcium, magnesium as well as multivalent metals like iron, manganese or aluminum to form insoluble silicates. Among all these aluminium is one of the most powerful element which causes precipitation of silica. The presence of both Al3+ and Fe3+ in the pretreated feedwater causes the precipitation of silica even at below the saturation concentration level of silica. Hence, to maintain the concentration of both Al3+ and Fe3+ in feed water it is very important as both Al3+ and Fe3+ salts are used as coagulant in water treatment processes. So, post coagulation concentration of Al3+ and Fe3+ should be below 0.05mg/l or less.
- Hydrofluoric acid can remove the silica but in such cases the possibility of the oxidation of polyamide membrane may increase.
Control of fouling of Silica:
For preventing membrane fouling by surface waters with high silica content, the effective control of silica is essential,
The fouling of Silica can be controlled by different ways,
- Operating systems at low silica concentration levels.
- Controlling the feed water silica concentration (e.g., by lime softening or other processes).
- Selecting the proper Silica inhibitor or dispersant which will inhibit silica polymerization or cause the dispersion of the silica precipitate and form the soluble metal silicates.
- Effectively controlling the scales such as calcium carbonate, calcium phosphate as well as metal and mineral concentration in feed water
- Maintaining the pH of system less than 8.3 to avoid the precipitation of metal silicate.
To get the rid of reactive and colloidal silica problem understanding the effective pretreatment is essential and to design effective pretreatment, adequate data of reactive, colloidal and total silica is required.
So, negligence to the reactive silica and colloidal silica will cost a lot.