Regenerating GFO

[blanden.adam]

I've found that rinsing GFO in RODI to remove the regeneration hydroxide can take quite a few rinses - I calculated 9 volumes of RODI per volume of GFO, with 10 minute equilibrations for each rinse (to allow hydroxide in the pores of the solid to diffuse into the bulk liquid). The end-point I was using was phenolphthalein, which is colorless below pH 8.2.

A considerably more efficient rinse was 0.01N HCl (obviously, since the HCl will neutralize the NaOH to salt and water), which took 4 volumes/volume of GFO to reach the phenolphthalein end point.

Now that's some good info. Definitely adding that to the notebook.

 

[dkeller_nc]

I happened to have phenolphthalein on hand, but I'm thinking that since the regeneration solution is a strong base, and if one is using a strong acid like hydrochloric in the rinses, any of the indicator dyes that come in alkalinity test kits would work. The end point would be different than phenolphthalein, but I wouldn't think that would matter once the GFO is put back in use in saltwater.

One could also use a pH meter, of course, but I personally find indicator dyes to be a lot less of a PITA.

 

[elviraveloz]

I always mix GFO and carbon on reactor. Can I regenerate GFO while mixed with carbon?

 

[blanden.adam]

I always mix GFO and carbon on reactor. Can I regenerate GFO while mixed with carbon?

No, because the carbon wont regenerate in the same way and may release the bound compounds back into the water. Regeneration is one of the many reasons to run them in separate reactors if possible.

 

[elviraveloz]

Thank you.

 

[blanden.adam]

Because you regenerate GFO by treating it with sodium or potassium hydroxide. This knocks off the bound phosphate and replaces it with (you guessed it), hydroxide. This process will not regenerate carbon, so you'd essentially be putting spent carbon back into your tank.

 

[dkeller_nc]

Because you regenerate GFO by treating it with sodium or potassium hydroxide. This knocks off the bound phosphate and replaces it with (you guessed it), hydroxide. This process will not regenerate carbon, so you'd essentially be putting spent carbon back into your tank.

There's a bit of a qualification on this that might interest some; the sodium hydroxide won't regenerate carbon by the mechanism that is active in GFO, but soaking carbon in sodium hydroxide will break down absorbed proteins, amino acids, and other organic compounds that are hydrolyzable. One can then either bake the carbon at a high temperature or (better) steam it to complete the regeneration cycle.

But on the whole, even high-grade GAC is so inexpensive that regenerating it just isn't worth it.

 

[blanden.adam]

I'd be very careful regarding how you interpret the information in that paper. The journal isn't very well known and the quality of the chemical literature coming out of china (especially 5+ years ago) is kinda hit or miss. All that aside, the paper uses a very narrow test system, one that isn't representative of the compounds in a reef tank.

Reneration of activated carbon depends on a lot of things, most notable the nature of the compounds absorbed. Soaking in hydroxide for a certain period of time may or may not be sufficient to hydrolyze certain proteins and lipids, or other organics with hydrolizable bonds as dkeller_nc notes. But, many organics are quite base stable, and even if they aren't just because they oxidize doesn't mean they will come off the carbon.

It should also be noted that the organics we are trying to removed with carbon aren't amphipathic compounds like proteins or amino acids -- that's what the protein skimmer is built to remove. The carbon is generally considered to be there to remove small hydrophobic molecules such as those produced for coral warefare as an example.

The thing about regenerating the carbon in our tanks is we (or at least I) don't know the composition of the compounds that are being absorbed sufficiently well to make an informed decision about which technique to use to regenerate it. It would also then be best practice to empirically test the efficiency of the process you've chosen, and then try to optimize it. I'm just not willing to take the time to do all of that to save a dollar a month , and that's assuming the cost of regeneration is 0.

 

[bertoni]

I agree that the article covers only a very narrow use of GAC.

 

[dkeller_nc]

The thing about regenerating the carbon in our tanks is we (or at least I) don't know the composition of the compounds that are being absorbed sufficiently well to make an informed decision about which technique to use to regenerate it. It would also then be best practice to empirically test the efficiency of the process you've chosen, and then try to optimize it. I'm just not willing to take the time to do all of that to save a dollar a month , and that's assuming the cost of regeneration is 0.

Absolutely - chemical engineering industrial processes that use GAC as a color-absorbing compound generally use high-pressure steam to do it. The temperature of the regeneration cycle is typically at least 300 degrees celsius (570 degrees farenheit) or higher, and precautions are taken to ensure that there's no oxygen in the regeneration gas. The reaction is exothermic, so controls are in place to prevent the reactor from getting too hot.

Not exactly a practical thing to do for a hobbyist, and potentially quite dangerous.

 

[tmz]

The original question is whether gfo and gac can be regenerated together. I don't know of anyone in the hobby who is actually regenerating gac alone much less together with gfo.

The answer as a practical matter ,imo, is no.

The processes of adsorbtion for gac and gfo are very different.

Gac does not adsorb material by ion electro static binding reliant on the relative charges of the ions involved as gfo does. Gac relies on weaker and fundamentally different intermolecular forces from quatum mechanics(Vander Walls type forces )which do not change the molecule when it's adsorbed.

Vanderwaal's type forces rely on intermolecular variations in polarity within a molecule(quantum dynamics) and work in the absence of stronger molecular binding forces like covalent binding and electro static binding as far as my limited understanding takes me.


As I read the study linked earlier , it seemed the researchers were attempting to overwhelm the Vanderwaals forces in play between the gac and adsorbate by introducing concentrations of sodium hydroxide which broke up the molecules bound to the gac or pulled them away,thus freeing binding sites on the gac for intermolecular adsorbtion( Vanderwalls) binding again once the hydroxide was removed from the solution. That paper is 5 years old,though ;if the process were superior to the usual thermal methods, I think we'd have seen more industrial applications by now. There is a lot of research on regeneration methods for gac but thermal methods are still the mainstay as far as I can tell.

Gfo regeneration relies on the oxide remaining on the gfo for binding.

 

[tmz]

I've found that rinsing GFO in RODI to remove the regeneration hydroxide can take quite a few rinses - I calculated 9 volumes of RODI per volume of GFO, with 10 minute equilibrations for each rinse (to allow hydroxide in the pores of the solid to diffuse into the bulk liquid). The end-point I was using was phenolphthalein, which is colorless below pH 8.2.

A considerably more efficient rinse was 0.01N HCl (obviously, since the HCl will neutralize the NaOH to salt and water), which took 4 volumes/volume of GFO to reach the phenolphthalein end point.

For those looking to do this that read the Advanced Aquarist article: please do not set up a reactor and a pump to circulate 1M NaOH through GFO. This is really dangerous, as you're pressurizing a caustic chemical that will burn your skin and eyes faster than you can rinse it off. All it takes is a hose barb connection to come loose to get sprayed.

And - circulating NaOH through GFO via a pump and a reactor isn't necessary. Simply soaking GFO in 3 successive 2X volumes of NaOH with 24 hours between changes is all that's required.

Thanks for the information.
I have a question. How does the acid rinse not effect the oxide binding sites on the gfo itself?

 

[blanden.adam]

Thanks for the information.
I have a question. How does the acid rinse not effect the oxide binding sites on the gfo itself?

This is my understanding of the process, I'm sure dkeller will pop in to answer the question directed at him (sorry for crashing your party man )

The bound hydroxide (and phosphate) are negatively charged. At normal water concentrations, the phosphate is able to compete off the hydroxide and bind the ferric iron. To regenerate the GFO, we expose it to a high concentration of hydroxide so that we "force" the hydroxide to compete off the phosphate by sheer number.

The positively charged iron can't be left without a counterion in solution, so the protons can't react away the hydroxide (and even if such a reaction did happen, the ferric iron would act as a lewis acid and just grab a hydroxide from the water, generating a proton. No net reaction). So the real question is can the negative ion in solution (in this case chloride) out-compete they hydroxide for binding to the ferric iron? Iron chloride is quite soluble in solution (the ions readily dissociate), so under the given conditions, no.

I try and think about GFO as an affinity resin for anions, with the solubility of the resultant compounds as the indicator for affinity. Less soluble, higher affinity. It works at least to a first approximation .

 

[tmz]

Thankyou, Nice and clear.

 

[dkeller_nc]

This is my understanding of the process, I'm sure dkeller will pop in to answer the question directed at him (sorry for crashing your party man )

The bound hydroxide (and phosphate) are negatively charged. At normal water concentrations, the phosphate is able to compete off the hydroxide and bind the ferric iron. To regenerate the GFO, we expose it to a high concentration of hydroxide so that we "force" the hydroxide to compete off the phosphate by sheer number.

The positively charged iron can't be left without a counterion in solution, so the protons can't react away the hydroxide (and even if such a reaction did happen, the ferric iron would act as a lewis acid and just grab a hydroxide from the water, generating a proton. No net reaction). So the real question is can the negative ion in solution (in this case chloride) out-compete they hydroxide for binding to the ferric iron? Iron chloride is quite soluble in solution (the ions readily dissociate), so under the given conditions, no.

I try and think about GFO as an affinity resin for anions, with the solubility of the resultant compounds as the indicator for affinity. Less soluble, higher affinity. It works at least to a first approximation .

Yep, that's exactly the way I would've explained it. One addition is worth mentioning - ferric oxide is incredibly insoluble in aqueous alkaline or neutral conditions. However, it's somewhat soluble in aqueous acidic conditions. So, in theory adding a bit of dilute acid will "etch" the surface of the particles. And for our purposes, that's probably a good thing, since in use the particles will develop a coating of all sorts of stuff from the tank water (proteins, lipids, inorganic metal precipitates, bacteria, etc...).

In theory, the sodium hydroxide will take care of the proteinaceous materials by hydrolysis, and it will saponify fats and lipids, rendering them soluble and therefore removing them from the surface. But it won't take care of inorganic metal precipitates (calcium carbonate being just one possibility), so slightly etching the surface of the particles with a dilute acid doesn't seem like a bad idea.

In the Advanced Aquarist article on regeneration of GFO, they first soak the GFO in dilute HCl to remove any precipitated calcium carbonate. In my case, I don't bother - I feed heavily, so my GFO is only in use for one week before regeneration, and I've never observed the kind of caking that other hobbyists have noticed, particularly in tanks with high calcium and alkalinity concentrations.

 

[elviraveloz]

I followed the guidelines for regenerating GFO. Today I place 200g regenerating GFO on reactor took a sample of water coming out of reactor 3 hrs after, and read 0.94. Is this normal. I'm using new Hanna reagents so I don't know if I should trust because my tank reading this morning was 0.75 using new GFO. I change every week.

 

[Tomoko Schum]

A considerably more efficient rinse was 0.01N HCl (obviously, since the HCl will neutralize the NaOH to salt and water), which took 4 volumes/volume of GFO to reach the phenolphthalein end point.

I did this with approximately 2 cups of GFO which was placed in a 200 micron bag. Since I added more 0.01N HCl than necessary and the pH of the rinse solution turned out acidic (below 7.4) afterward, I decided to rinse the recharged GFO with RODI water to wash off the acidic solution. Then, I measured the pH of the RODI water with GFO, it read well over 8.8. I am wondering if I should have left the GFO longer in the acid solution to neutralize longer. I am using a high range API pH test kit to measure the pH.

 

[bertoni]

More time exposure might have been useful. Did you measure the TDS of the water along with the pH?

 

[Tomoko Schum]

No I did not. The water is rather reddish. I think TDS reading would be high with rust in the water.

 

[bertoni]

Well, I'd probably try another rinse. You could try testing the alkalinity of the rinse water, I guess, although it's probably very small, which would mean that the media should be safe enough to use.