Reef Chemistry Question of the Day #288: Deionizing Resins for RO/DI

[Randy Holmes-Farley]

Reef Chemistry Question of the Day 288

Deionizing resins used in an RO/DI have ionic charges that are permanently attached to the resin, and other ionic charges that are free to move around and potentially come out of the resin.

Which of the following ions comprise the free to move around ions in new DI resin?

A. Sodium (Na+) and chloride (Cl-)
B. Calcium (Ca++) and sulfate (SO4--)
C. Hydrogen ion (H+) and chloride (Cl-)
D. Hydrogen ion (H+) and hydroxide (OH-)
E. Sodium (Na+) and hydroxide (OH-).

Good luck!

Previous Reef Chemistry Question of the Day:

Reef Chemistry Question of the Day #287: Sources of Color

Reef Chemistry Question of the Day #287 There are many sources of color in reef aquaria, and many questions I could ask about them. Let's start with a hard one. Which of the following is not true? 1. The spines of some sea urchins are purple due to the presence of iodine in them. 2. The...

www.reef2reef.com

 

[Saltyreef]

D!

 

[BrokenReefer]

C (?) ... don't fail me prof!

 

[Derrick0580]

Answer F….doesn’t matter to me as long as it works!

 

[gbroadbridge]

Reef Chemistry Question of the Day 288

Deionizing resins used in an RO/DI have ionic charges that are permanently attached to the resin, and other ionic charges that are free to move around and potentially come out of the resin.

Which of the following ions comprise the free to move around ions in new DI resin?

A. Sodium (Na+) and chloride (Cl-)
B. Calcium (Ca++) and sulfate (SO4--)
C. Hydrogen ion (H+) and chloride (Cl-)
D. Hydrogen ion (H+) and hydroxide (OH-)
E. Sodium (Na+) and hydroxide (OH-).

Good luck!

Previous Reef Chemistry Question of the Day:

Reef Chemistry Question of the Day #287: Sources of Color

Reef Chemistry Question of the Day #287 There are many sources of color in reef aquaria, and many questions I could ask about them. Let's start with a hard one. Which of the following is not true? 1. The spines of some sea urchins are purple due to the presence of iodine in them. 2. The...

www.reef2reef.com

D

 

[Mark Gray]

I believe it is D

 

[Ucyibd1]

It is D

 

[Sean Clark]

I am going with D because reasons.

 

[Dburr1014]

D

 

[Chuk]

D but it is possible to go past the ammonia break point on the cation resin and get ammonium to come off instead of h+

 

[Fish Styx]

D.

 

[Randy Holmes-Farley]

And the answer is (as most got correct ), D.

Which of the following ions comprise the free to move around ions in new DI resin?

A. Sodium (Na+) and chloride (Cl-)
B. Calcium (Ca++) and sulfate (SO4--)
C. Hydrogen ion (H+) and chloride (Cl-)
D. Hydrogen ion (H+) and hydroxide (OH-)
E. Sodium (Na+) and hydroxide (OH-).

Deionizing Resins

The final filter in an RO/DI system is the deionizing resin. A DI resin traps all charged molecules passing through it, and leaves uncharged (neutral) molecules free to pass through. Water, for example, passes through it, as would other uncharged inorganic molecules such as oxygen (O2), nitrogen (N2) and chloramine (NH2Cl, if any remained from the previous filters). Uncharged organic molecules also pass through a DI resin, including ethanol (CH3CH2OH), methanol (CH3OH), methane (CH4), propane (CH3CH2CH3), carbon tetrachloride (CCl4), and methylene chloride (CH2Cl2). Ions such as sodium (Na+), copper (Cu++ or Cu+), ammonium (NH4+), phosphate (PO4---), silicate (Si(OH)3O-), and acetate (CH3CO2-) all get caught.

All atoms or molecules that are in rapid and significant equilibrium with their charged forms will be caught and removed just as their charged forms are. These include ammonia (NH3) caught as ammonium, silicic acid (Si(OH)4) caught as silicate, carbon dioxide caught at least partially as bicarbonate (HCO3-) or carbonate (CO3--), and acetic acid (CH3CO2H) caught as acetate, etc.

To catch these ions, the resin consists of porous beads that have fixed charges attached to them. The counterions to these fixed charges start off as H+ and OH- in a fresh resin. Normally, there are different beads intended to bind cations and anions. In a mixed bed DI resin, the beads are mixed together in a single filter. In a separate bed system, each bead type will be in a different filter, thereby potentially allowing the DI to be recharged (a process that is discussed later in this article).

Figure 6 shows a cation-binding resin bead ready to bind sodium (the fixed charges on the resin are not shown, only the replaceable H+ ions). As the sodium ions enter the bead, they bind to the fixed negative charges, and H+ is released as they swap places. The chloride ions pass through this bead unchanged since they are not attracted to the negatively charged sites in the bead. After all of the sodium ions that entered the bead are bound, the product is a hydrogen chloride solution that passes on to the next bead in the bed (Figure 7).

Figure 8 shows an anion-binding resin bead ready to bind chloride (the fixed charges on the resin are not shown, only the replaceable OH- ions). As the chloride ions enter the bead, they become bound to the fixed positive charges, and OH- is released as they swap places. As the chloride ions that entered the bead are bound and OH- is released, the H+ and OH- ions combine to form water molecules (H2O). In this way, none of the original sodium and chloride ions remain in solution, and only pure water passes out of the DI resin chamber (Figure 9).

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

 

[taricha]

So is a scenario like @Chuk mentioned plausible in hobby situations: using the resin well past the point of its capacity could kick out previously bound undesirable stuff?

 

[Randy Holmes-Farley]

So is a scenario like @Chuk mentioned plausible in hobby situations: using the resin well past the point of its capacity could kick out previously bound undesirable stuff?

Definitely.

Ammonia and silicate would be especially problematic.

Deionizing Resin Depletion Issues

Eventually, all of the H+ and OH- originally installed in a DI resin will become depleted, and ions will pass through unchanged (Figure 10). When charged ions begin to pass through the DI resin, the effluent's conductivity rises. Many RO/DI systems use an inline conductivity meter to alert users when ions are starting to appear, indicating that the resin needs to be replaced. Without such an inline meter, users need to periodically monitor the effluent’s conductivity (in mS/cm or ppm TDS; details are given in the tips section on what conductivity to target for resin replacement).

Some DI resins incorporate color changes to indicate when the DI is depleted. Such indicators are typically pH indicating dyes that change color when the pH in the interior of the beads shifts from the very high or low pH values when OH- or H+ are the dominant counterions, to more neutral values when other ions dominate (such as Na+ or Cl-). Such color changes may be less effective than measuring the effluent’s conductivity for indicating early breakthrough of ions. The color change may not indicate that some beads or parts of beads may become depleted before others due to channeling of the ion flow. Consequently, I would not rely exclusively on such color changes unless they have proven to accurately predict the rise in conductivity of the effluent for a given brand of DI filter and bead.

Several issues arise relating to the depletion of the DI resins that aquarists need to be aware of. Primary among these is that when a DI resin becomes depleted, that does not simply mean that the water passes through just as it came from the RO effluent. It may actually be much worse from an aquarist’s perspective. The reason for this is that while the DI resin is functioning properly, all ions will be caught. But when it is depleted, not only the new ions are coming through and might show up in the product water, but so are all the ions that ever got into the DI resin in the first place. The total concentration of ions coming out of the exhausted DI resin will not be raised as compared to the RO's effluent, but which ions are released may be very different.

In the DI descriptions above, I did not address the fact that some ions will show a greater preference for attachment to the resin than will others. When the resins are not depleted, it does not matter what the ions’ affinity is, as all are bound. But in a depleted scenario, when there are more ions than ion binding sites, those with a higher affinity for the resin will be retained, and those with a lower affinity will be released. It turns out that silicate is found at the lower end of affinity for anion resins. Consequently, if the DI resin has been collecting silicate for a long period and is then depleted, a large burst of silicate may be released.

Perhaps even more of a concern is ammonia. In a system with chloramine in the tap water, the DI resin will serve the important function of removing much of the ammonia produced by the chloramine breakdown. Ammonia has a poorer affinity for many cation-binding resins than do many other cations (e.g., calcium or magnesium). Consequently, when the DI resin first becomes depleted, a big release of ammonia from and through the DI resin is likely. I recently had a DI resin become depleted, and the effluent contained so much ammonia that I could easily smell it.

Other complications can also impact resin depletion. One potentially important issue is that the anion and cation-binding sites may not become depleted at the same time. Figure 10 shows this scenario when both types become depleted together, with sodium and chloride in the effluent. But, it is possible for one to become depleted first, and in that case, the pH of the effluent can swing far from neutral. Figures 11 and 12 show what happens when a lot of carbon dioxide is present, as is the case with some well waters. Initially, it is mostly bound as bicarbonate, and the effluent is essentially pure water. Note, however, that as the bicarbonate is removed, the anion binding resin is being taken up with bicarbonate, while the cation-binding resin is unchanged and is therefore not being depleted.

Eventually, the anion-binding sites become fully occupied (Figure 13). At that point, additional ions coming through (such as sodium and chloride) are no longer equally swapped out to produce pure water. The sodium is swapped for H+, but the chloride does nothing, potentially leaving the effluent water with a very low pH.

A similar effect can be hypothesized for silicic acid in the RO permeate:

Si(OH)4 --> H+ + Si(OH)3O-

The effect on pH of the DI resin’s initial depletion would be similar here to the effect of carbon dioxide in the tap water.

The same can happen in the opposite sense with ammonia. If a lot of ammonia gets through the RO membrane (as is the case when chloramine is present), the ammonia will be bound in the DI resin as ammonium:

NH3 + H2O --> NH4+ + OH-

The ammonium depletes the cation-binding resin, while the OH- does not impact the anion-binding resin. Eventually, then, the cation-binding capacity can become depleted before the anion binding is depleted, and Na+/Cl- passing through is converted into Na+ and OH-, with a potentially high pH.

 

[taricha]

Good to know. So not just plausible, but pretty likely as there are other things common in water that would be more tightly bound to the resins than ammonia and silicate.

 

[Randy Holmes-Farley]

Good to know. So not just plausible, but pretty likely as there are other things common in water that would be more tightly bound to the resins than ammonia and silicate.

Yes. I once made a batch of limewater before I realized the DI had depleted, and it stank of ammonia. I don't think it would have smelled nearly as much if it was just the ammonia in the tap water (from chloramine), but I never tried it.

 

[taricha]

Yes. I once made a batch of limewater before I realized the DI had depleted, and it stank of ammonia. I don't think it would have smelled nearly as much if it was just the ammonia in the tap water (from chloramine), but I never tried it.

Agreed. I've done a batch of limewater with tap water before and I thought I could barely detect a slight ammonia smell. I needed to use a NH3 film over the limewater to be sure I wasn't imagining it.