Silica:colloidal or reactive?

[shahyar]

Usually, when people report silica content of their waters, they do not say if it is "Total silica", "Reactive silica", or "Colloidal silica".
How do you interpret such numbers?

Thanks

 

[Muggle]

Normally, the lab will provide a analysis designation number/abbreviation - which describes the procedure and what it is measuring - total soluble and collodial silica, or only one - the lab should be able to provide the description for you, but you can normally find the procedure outlined by a web search - normally, the EPA site also will have the description if it conforms to EPA standard testing procedures.

 

[bimr]

The measurement of total silica is generally considered an off-line procedure; in most cases, a water sample is transported to a lab for analysis.

Colloidal silica is typically found in surface waters and has created problems for water treatment because of its stability as an un-ionized compound, making it difficult to remove using ion exchange processes.

Silica is at the lower end of selectivity for anion resins, creating a scenario where silica breakthrough is one of the first to occur. As a result, silica can be effectively removed only if the ion exchange resins are completely and properly regenerated.

With the introduction of RO as a pretreatment process, the colloidal silica can be removed very effectively simply by virtue of the filtration capabilities of a typical RO membrane. In fact, if one uses a conventional definition of colloidal silica as that which measures greater than 50 angstroms, even ultrafiltration with a molecular-weight cutoff (MWCO) of 100,000 daltons is effective at removing colloidal silica. The RO also helps the ion-exchange process by alleviating the overall silica loading on the resin.

Pretreatment is prudent on a demineralizer makeup water supply. You are trying to make ultrapure water and there is typically a lot of crap in the normal municipal water supply.

 

[shahyar]

Muggle,
we do not have access to the lab which provided the analysis,always. I read bunch of articles in Chemical engineering magazine, CEP, and so on and I have seen some papers in recent "international water conference", But none of them addressed differentitation between TSiO2 and RSiO2.
and my question still is: What do those numbers mean?

Regards

 

[bimr]

Silica is generally found in water supplies in three different forms: reactive, colloidal and suspended particles (e.g., sand), with the reactive being that portion of the total dissolved silica that is readily reacted in the standard molybdate colorimetric test, and the colloidal being that which is not.

Total silica should be the sum of the reactive silica and colloidal silica. But total silica may not be expressed that way and colloidal silica may not show up on such tests.

Colloidal silica is insoluble and is something that can be filtered out with a whatman filter. Reactive silica is dissolved

Colloidal silica is difficult to detect in low (ppb) concentrations, and not many labs are capable of providing analyses.

 

[cub3bead]

There are two methods for doing this analysis: Atomic Absorption or Ion Couple Plasma (AA/ICP) or the molybdate blue method.

With the AA/ICP method, a portion of the sample is injected into the machine and the silica content is measured. This is the total silica (both reactive and colloidal). Another portion of the sample is filtered through a 0.45 um filter (colloidal silica is retained on the filter). The filtered sample is injected into the machine and the silica content measured. This result is the reactive silica. The difference between the two silica readings is colloidal silica.

A similar proceedure for the molybdate blue method is followed. A portion of the sample is analyzed according to the ASTM method, resulting in the reactive silica content. To convert the collodial silica to reactive form requires a hot, sulfuric acid digestion first. Then the molybdate blue proceedure is run. This results in a total silica reading. The difference between the two is colloidal silica.

The AA/ICP method is more accurate and faster but requires much more in analytical equipment investment and a much higher level of training for the personnel operating the AA/ICP.

 

[shahyar]

Thanks bimr and Cub3bead for your excellent explanations.
So, appearently, there is no way to understand or even guess if a "silica" concentration is total or reactive or colloidal, if originator of article did not say anything.

Thanks again

 

[cub3bead]

Unless specified, it would be a guess as you said. That said, I have guessed "reactive" in the past but still ordered collection of new samples and proper testing.

The colloidal silica is the most troubling, especially if the treating water for steam generator makeup since the temperature and pressure in unit will convert the colloidal form to the reactive form.