Reef Chemistry Question of the Day #21

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #21

Which of the following would be expected to most readily pass through a reverse osmosis membrane?


A. Sodium and chloride
B. Magnesium and sulfate
C. Carbon dioxide and hydrogen sulfide
D. Proteins and carbohydrates
E. Bacteria and viruses

Good luck!




















.

 

[beaslbob]

c

Oh yea I need 3 characters to post.

 

[Hodge Podge]

I would guess "C" as well. Both are gasses and should pass the membrane.

 

[w.gurney77]

C is the correct answer. Both molecules are small enough to pass through the membrane.

 

[Vpham]

E is my final answer. ..

 

[Steve_Mac]

I'm going with "C" also.

 

[beaslbob]

Dern

you guys know how dangerous it is to follow the dreaded beaslbob?



:xd:

 

[swseipel]

E is my guess

E is my guess

 

[firstlight10]

C..

 

[prsnlty]

E. I believe

 

[reggaedrummin]

My guess is "c" because carbon dioxide and hydrogen sulfide are both gases and so their molecules move more freely. I would guess that their ability to move freely would give them more ability to pass through the membrane and I believe it requires a special degassing membrane to remove gas molecules from liquid.

 

[TheSaint216]

All the Above

 

[WindeyD]

C is my choice

 

[Montireef]

None of them

 

[Randy Holmes-Farley]

Nice, we have many respondents with correct answers!

And the answer is..... C. Carbon dioxide and hydrogen sulfide

The reason they more readily pass an RO membrane is because they are actually much smaller than any of the other choices, so can get through the pores more easily.

Now, that may seem weird. A sodium ion itself is smaller than carbon dioxide, but an important complication is that ions have a very tightly held set of surrounding water molecules, and the ions cannot easily shed those water molecules to pass through a little pore. So even a tiny ion is effectively larger than a small uncharged molecule.

As to bacteria and viruses, they won't pass through an RO membrane since they are too large, but bacteria can grow on the back side and downstream of an RO membrane. Consequently, if you do not sterilize the water (as with a UV), there can be bacteria in it, and those bacteria may even be pathogenic to people if you consume or inject them.

Here's a more detailed explanation:

Reverse Osmosis Membranes

Reverse osmosis membranes consist essentially of a sheet of porous organic polymer. A variety of different materials is used commercially, including cellulose acetate/triacetate blends (sometimes called CTA), thin film/thin layer composites (sometimes called TLC or TFC), and modified polysulfone (sometimes called SPSF). The relative advantages of each of these materials are detailed in other articles, but some important points are:


CTA membranes are inexpensive and resistant to oxidation by chlorine.

TFC membranes are more costly, but have high impurity rejection. They must be protected from chlorine and chloramine.

SPSF membranes are generally optimal only in special situations, such as very soft source water.


If the membrane's pore sizes are made just a bit larger than water molecules, then water can pass through them, but larger compounds cannot. Size in this case is a somewhat simplified idea. Many ions are smaller than a water molecule (Figure 3), but it turns out that charged ions (such as sodium, Na+) in solution contain several very tightly bound water molecules. Removing all of these attached water molecules requires a lot of energy, so when passing through a porous membrane, they act as if they are as large as the whole hydrated assembly (Figure 4). These larger assemblies cannot pass through an RO membrane as readily as they could without the tightly bound water molecules (Figure 5).

The more charges an ion has, the more water molecules are attached and the harder they are to remove. It has recently been suggested that the ratio of the hydrated volumes of two ions approximates the ratio of the square of the charges of the same two ions. So, for any simple inorganic X, Y, and Z, X+ is one-quarter the size of Y++, and X+ is one-ninth the size of Z+++. The same holds true for negatively charged ions.1 For these reasons, the relative order of rejection by RO membranes is typically trivalent > divalent > monovalent, as shown below.


Table 7. Typical Rejection Rates of Ions From RO Membranes

[SIZE=-1]Table 7. Typical Rejection Rates of Ions From RO Membranes[/SIZE]​

[SIZE=-1]Ion:[/SIZE]

[SIZE=-1]Percent Rejection:[/SIZE]​

[SIZE=-1]Typical monovalent ions (Na[SIZE=-2]⁺[/SIZE], K[SIZE=-2]⁺[/SIZE], Cl[SIZE=-2]^-[/SIZE], F[SIZE=-2]^-[/SIZE], I[SIZE=-2]^-[/SIZE], NO[SIZE=-2]3^-[/SIZE])[/SIZE]

[SIZE=-1]94-96[/SIZE]​

[SIZE=-1]Typical divalent ions (Ca[SIZE=-2]⁺⁺[/SIZE], Mg[SIZE=-2]⁺⁺[/SIZE], Cu[SIZE=-2]⁺⁺[/SIZE], SO[SIZE=-2]4[/SIZE][SIZE=-2]^--[/SIZE], CO[SIZE=-2]3[/SIZE][SIZE=-2]^--[/SIZE])[/SIZE]

[SIZE=-1]96-98[/SIZE]​

[SIZE=-1]Typical trivalent ions (Fe[SIZE=-2]⁺⁺⁺[/SIZE], Al[SIZE=-2]⁺⁺⁺[/SIZE])[/SIZE]

[SIZE=-1]98-99[/SIZE]​

[SIZE=-1]
[/SIZE]

Since RO membranes purify based on size, they are subject to some obvious limitations. Certainly, anything that is very large cannot pass though them. In this category would be bacteria (although they may colonize both sides of the filter, they cannot pass through it), viruses, large organic molecules such as proteins, and inorganic mineral particulates that were small enough to pass through the sediment and carbon filters (often called colloids).


Also, in order to get a sufficiently fast flow of water through the membrane, membrane pores are actually significantly larger than a water molecule. For this reason, some of the molecules of compounds that are somewhat larger than a water molecule can still get through (sodium ion, for example, is not perfectly rejected).


However, at the small end of the spectrum a number of compounds can pass through a reverse osmosis membrane to some extent and are, therefore, of concern to reef aquarists. These include carbon dioxide (CO2), ammonia (NH3), hydrogen sulfide (H2S, especially a concern with well water) and silicic acid (Si(OH)4, which is the uncharged and predominate form of silicate at pH values below 9.5). All of these should be trapped by a functioning DI resin (discussed below), but can still be a concern.


In the case of CO2, for example, there can be a lot of it in certain well waters, and DI resins may become rapidly depleted because the CO2 so readily passes through RO membranes (how to deal with this is discussed later in this article). As another example, ammonia that comes from chloramine in the water can be significant, and is one reason that RO/DI is greatly preferred to RO alone in those situations where chloramine is added to the tap water.


In the case of silicic acid, some types of RO membranes can be better than others at excluding it, even before it gets to the DI resins. For example, a thin-film polyamide membrane might let only 0.3% of the silicic acid pass, while a similar cellulose acetate membrane might let 12.7% of it pass.


In order to function properly, the RO membrane is coupled with a flow restrictor that allows pressure to build on the upstream side of the membrane, rather than letting the water simply run out of the unit and down the drain. This pressure helps force the water molecules (and other small molecules) through the membrane. After passing through the pores, the water then continues on to the DI resin.


Many systems will include a pressure gauge that measures the line pressure ahead of the RO membrane. It is the pressure across the RO membrane that forces water through. At low pressure, the water may simply run past the restrictor and down the drain. Most membranes need at least 40 PSI or so to get reasonable flow and purification. In my system, the pressure drops over time as sediment clogs the sediment and carbon filters. I use this gauge as an indicator that the filters before the membrane need to be replaced. Some RO/DI manufacturers (e.g., Spectrapure and Kent) sell kits that allow the membrane to be flushed with water, permitting loose sediments and calcium/magnesium carbonate that can clog it to be washed away.


Various factors, such as temperature and pressure, impact not only the flow rate through the membrane but also the purity of the resulting water. Lower temperatures make the water more viscous and less likely to flow through the small pores, reducing the production of purified water. The effect of temperature on purity is much smaller, with purity decreasing slightly at higher temperatures. Higher line pressure across the RO membrane results in higher rates of production and quality, although a pressure that is too high can damage the membrane. Any backpressure on the effluent will degrade performance. Very high TDS (total dissolved solids) in the source water also leads to higher osmotic backpressure, reducing the membrane's effectiveness. As a rough guide, every 100 ppm of TDS produces 1 psi of osmotic backpressure.




This has more:


Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com