Aquarium Chemistry: Regeneration of Granular Ferric Oxide Media with Sodium Hydroxide
If you're reading this article beyond the title it's likely you're already familiar with the importance of phosphate in reef tanks and how granular ferric oxide (GFO) media can be used to control it, but here's a refresher anyway. Phosphate is an algal nutrient and can limit the ability of calcifying organisms to form a calcium carbonate skeleton (Farley, 2006). The general consensus is to limit reef aquaria to lower than 0.05 ppm phosphate (as PO43-). This can be achieved in a variety of ways, but using GFO to adsorb phosphate may be necessary for some aquarists and is a safe and effective tool for managing phosphate in reef aquariums (Farley, 2004). The downside to GFO is that it's rather expensive. For aquarists who keep non-photosynthetic corals that require massive feeding, have large tanks, or a dense fish population the costs of regular GFO replacement can be significant. With a few simple tools and techniques we can easily regenerate GFO so that it can be reused several times over. Below we describe a procedure that has worked for us to regenerate GFO effectively. Before going out and buying any of the materials required, please carefully read the safety instructions at the bottom of this article.
- Rinsing with a dilute acid (this step is likely optional for many people, see below): For every 500mL of GFO, add 8L of fresh water mixed with 6.4mL of full strength muriatic acid. Rinse for 4-6 hours and then drain.
- Regeneration of GFO with sodium hydroxide: For every 500mL of GFO, add 8L of fresh water mixed with 320g of sodium hydroxide. Rinse for 72-96 hours and then drain.
- Fresh water rinse with deionized water (The effectiveness of this step depends on the method you use to rinse the GFO--see the section on rinsing below for more details): For every 500mL of GFO, rinse with at least 20-25L fresh water.
Rinsing with a dilute acid
This step is intended to remove any deposits of calcium carbonate on the GFO and may be unnecessary for some people. Under certain conditions calcium carbonate will abiotically precipitate onto GFO surfaces (Farley, 2004), which will limit the amount of available surface area of the GFO particles. How do you know if you need to do this step or not? Expose your GFO to some white vinegar, or diluted muriatic acid as described above, and watch for small bubbles to form. Bubbling indicates the presence of calcium carbonate. Exposure to a dilute acid for a few hours should remove these deposits. If you don't see any bubbles forming, you can safely skip this step. We have not found it necessary to perform this step on the GFO from 3 different reef systems, but your mileage may vary. This step alone will NOT regenerate the GFO, but it may be an important step for those who have significant scale coating on the granules. Increasing the concentration of the acid or lengthening the rinse time may lead to significant dissolution of the GFO surface and may reduce the number of times you can repeat the regeneration process in the future (more on this later).
Some people have used anywhere from 1:1 to 10:1 dilutions of white vinegar for this step instead of muriatic acid. This may work fine, but we have not tested it so cannot comment on its efficacy or its effect on the GFO. For users who are regenerating a lot of GFO frequently, muriatic acid is significantly cheaper than using vinegar. A gallon of full strength (38% by volume) muriatic acid can be purchased at many home improvement, hardware, or pool supply stores for under $10, and this should last an average reefkeeper for years. It also can be used in a 1:10 dilution for cleaning scale deposits off of pump impellers and heaters. The downside to having full strength muriatic acid around is that it is very potent stuff--as you can see from the recipe it takes a little more than a teaspoon in one gallon of water to dissolve away scale. This presents safety concerns for you, your kids, and your pets. Please see the important safety information at the end of this article before using it. The diluted solution described in step 1) above is so dilute (0.03%) that it's rather harmless so long as you don't ingest it or get it in your eyes.
Regeneration of GFO with sodium hydroxide
Phosphate is believed to bind to GFO via an ionic bond that displaces two hydroxide ions into solution (Farley, 2004). The simplest way to think of regeneration is the reversal of that process. By flooding the GFO with a very high concentration of hydroxide ions we are swapping the phosphate ions out and putting them into solution, and replacing them with hydroxide ions. As a result the regeneration solution becomes flooded with phosphate ions-we have measured it as high as 270 ppm PO4- that we will later rinse away from the GFO.
The sodium hydroxide solution described in step 2) above is 16 bed volumes (the volume of regeneration solution compared to the volume of GFO being recharged) and 1.0M in concentration. We have rinsed GFO in lower concentrations (0.1M and 0.5M) at 16 bed volumes and this requires repeated regeneration cycles to remove all the phosphate from the GFO. Is the solution described above the only version that will work? Probably not, but we haven't tested other versions so can't comment on their effectiveness. Would allowing the solution to rinse longer than 96 hours pull out more phosphate? Probably not. All the testing we've done so far indicate that regeneration solutions from 0.1 - 1.0M pull out all the phosphate they can after about 72 hours. Longer rinse times may actually be problematic, because the sodium hydroxide solution will start to dissolve away some of the GFO surface. Can you double the concentration (2.0M) and lower the volume (8 volumes) or vice versa (0.5M, 32 bed volumes) and achieve the same results? We wouldn't be surprised if alternate versions of the regeneration solution would work fine as well, but further testing would be required to determine that. In our experience the combination above has worked to remove all bound phosphate from several batches of GFO used on different reef systems. If you have a good phosphate test kit at home you can play around with the concentration and volume of the regeneration solution to determine when you have "bottomed out" and removed all the bound phosphate. Most phosphate test kits work within a specific pH range, so the regeneration solution would need to be neutralized with an acid before testing. The concentration of phosphate in the solution will be much higher than most phosphate test kits can measure, so you will need to carefully dilute the sample by 100 or 200 times to get a reading that's not "off the charts".
Sources of sodium hydroxide
So where does one easily obtain sodium hydroxide? Many chemical supply houses will sell high grades of sodium hydroxide or potassium hydroxide but these are generally quite expensive. Sufficiently pure sodium hydroxide for GFO regeneration is a white powdery crystal sold by the common names "lye" or "caustic soda", and is used in soap making, biodiesel production, drain clearing, curing certain food products like olives, and even used in the manufacture of certain illegal synthetic drugs that might rhyme with the word "bethamphetamine". Rumor has it that it's used by mafia hitmen for disposing of certain "problems" as well. One of the quickest and easiest places to find small amounts of lye is in drain opening products at hardware stores. Most OSH, Lowe's, Home Depot, and True Value and Ace Hardware stores currently carry a product called "100% Lye Drain Opener" made by Rooto or a product called "Crystal Drain Opener" made by Roebic. Both of these products are suitable for regeneration of GFO. Will other products work? If the label says that they are made of 100% lye it's likely they will work fine, but we have never tried any drain opening product besides these two brands. There are other drain opening products that will certainly NOT work including some that may "contain lye" but may also contain gelling agents, strong acids, or bleach that may dissolve away your GFO or worse, wreak havoc in your aquarium if they are not thoroughly diluted during the rinsing process. As with muriatic acid, the use of sodium hydroxide presents important safety concerns for you, your kids, and your pets. Please see the safety disclaimers at the end of this article for more information.
For finding more than a few pounds of lye at considerably cheaper prices you may want to to do a search of local chemical suppliers and ask for caustic soda beads or lye in bulk. While large amounts of lye can be shipped to your door, there are substantial shipping charges added to bulk amounts of lye due to the hazardous nature of it. It is much cheaper to pick it up in person, plus you avoid the whole awkward conversation with your neighbors about what's the deal with the orange hazmat bucket that's been sitting on your porch all afternoon. In California, Gallade Chemical supplies lye as "caustic soda beads" in 50 pound buckets and bags that can be picked up in person or delivered by their own truck to certain locations for free. For $37 you can get 50 pounds of lye that should last the average reefkeeper for several years. If you're looking to cut costs, this is $0.74 per pound compared to $3-6 per pound of the Rooto or Roebic lye. In other states you should be able to find chemical supply houses by asking local soapmaking shops or internet forums where to find local bulk sources of lye. In case you're starting to do the math in your head, the cost of the sodium hydroxide used to regenerate GFO can be up to 20 times cheaper than replacing the GFO.
A quick note--Potassium hydroxide, commonly called caustic potash, can theoretically be used as a substitute for the sodium hydroxide in this step. However, at least in our area it was harder to find and more expensive where we could find it. The amounts used should be about the same for sodium hydroxide, although we haven't used it to regenerate GFO so can't comment on its effectiveness.
Fresh water rinse
Now that we've separated the phosphate into solution, we need to dilute and separate the solution from the GFO media. One of the downsides of this process is that the regeneration solution is rather "sticky"--it tends to require a lot of rinsing to get it all off. One way to rinse it is to put the media in a 5 gallon bucket, fill with water, and then decant; this is rather inefficient if you have a lot of media. The particles at the bottom of the pile have very little contact with the fresh water, and if you stir or agitate the media it will start grinding the GFO into dust. Plus the whole ordeal is just messy and a pain to deal with. We have found the easiest way to rinse off the regeneration solution is with a dedicated media reactor from Bulk Reef Supply and a small Maxijet 400 powerhead. Fresh water goes in one side and the diluted solution comes out the other to go straight to the drain. The biggest advantage is that all the GFO media is rinsed at once without any dead spots and without any stirring that can grind it into dust. For this reason, the BRS reactor is also ideal for all 3 steps of the regeneration process. It assures that the acid rinse and sodium hydroxide bath contacts all the media particles without grinding. If you also use a BRS reactor on your reef tank, you can easily swap out the internal plastic cartridge containing GFO from "reef mode" to "regeneration mode" without having to deal with transferring wet and messy GFO media from container to container. We can easily regenerate a GFO canister with just a few minutes worth of work every month, and with very little mess. You may find that regeneration using this method is easier than replacing the GFO! Regardless of which way you choose to rinse your media after regeneration, it is important to have a phosphate test to insure you have removed nearly all of the sodium hydroxide and phosphate before replacing the GFO back into your aquarium. We've found that rinsing the GFO in the reactor still requires about 40-50 bed volumes to be assured that all the phosphate has been rinsed out. Less efficient methods may require even more rinsing, but exactly how much would need to be determined experimentally.
So is there a limit to how many times you can regenerate a given batch of GFO? It doesn't appear that there is any limit on how many times you can exchange the phosphate for hydroxide and back again, but each time you do a small amount of iron oxide hydroxide from the GFO surface is dissolved into solution. This means that the granules of media will slowly start to shrink a little bit each time you regenerate them, so there is a theoretical limit on how many times regeneration can be repeated. Some hobbyists have reported no problems with regenerating the same batch of GFO more than 10 times. There may be an advantage to purchasing pelleted versions of GFO media if you intend to regenerate it. There is less surface area to bind phosphate, but some hobbyists have reported that the pellets hold up better without dissolving as much, so you may get more regeneration cycles out of the media before it needs to be replaced entirely. The GFO we use is the small grained version from Bulk Reef Supply.
This article describes procedures that involve dangerous chemicals. Sodium hydroxide/potassium hydroxide/lye/caustic soda is potentially very harmful to you, your kids, and your pets. Before using it, please look up the safety information in the links below. If those links don't work in the future, here's the gist of it: Sodium hydroxide can permanently blind you and cause really nasty chemical burns if it comes in contact with your skin. When sodium hydroxide comes in contact with water it can produce enough heat to melt through plastic and burn you. For this reason it should always be very slowly added to cold water, and not vice versa. It can also react violently with some metals such as aluminum to produce hydrogen gas, so for our purposes it should be mixed, stirred, and stored with plastic materials. Safety equipment including latex gloves and eye protection should be considered an absolute necessity any time you are handling it, even when it's diluted in water. The pH of the sodium hydroxide solution described above should be right around 14 when it's freshly mixed. Vinegar will safely neutralize sodium hydroxide so it's a good idea to have a jug of vinegar around for spills. When disposing of the sodium hydroxide solution down the drain it should be flushed with plenty of cold water and/or neutralized with vinegar.
The same goes for muriatic acid, if you choose to use it instead of vinegar. Muriatic/Hydrochloric acid is potentially very harmful to you, your kids, and your pets. Before using it, please look up the safety information in the links below. If those links don't work in the future, here's the gist of it: Muriatic acid can permanently blind you and cause really nasty chemical burns if it comes in contact with your skin. Muriatic acid should be opened only in an area of sufficient ventilation and special care should be taken to avoid exposing yourself to the fumes. All acids should be very slowly added to cold water, and not vice versa. Like sodium hydroxide it can also react violently with some metals such as aluminum and/or produce hydrogen gas, so for our purposes it should be mixed, stirred, and stored with plastic materials. Safety equipment including latex gloves and eye protection should be considered an absolute necessity any time you are handling it, even when it's diluted in water. Baking soda will safely neutralize muriatic acid so it's a good idea to have a box of it around for any spills. When disposing of the muriatic acid solution down the drain it should be flushed with plenty of cold water and/or neutralized with baking soda.
Working with these chemicals and knowing the risks is your responsibility. Please proceed carefully, and happy regenerating!
- Holmes-Farley, R. Phosphate and the Reef Aquarium. Reefkeeping Online Magazine, Sept. 2006.
- Holmes-Farley, R. Iron Oxide Hydroxide (GFO) Phosphate Binders. Reefkeeping Online Magazine, Nov. 2004.
- Pers. comm., William Wing.
- Pers. comm., The Reef Chemistry Forum, www.reefcentral.com