Regenerating GFO

[Anonymous]

First regen cycle: 96 hours of soaking the GFO (1L of granules) in 7.6L (2 USG) of a 0.1M NaOH solution yielded 243 mg of PO4 released. For a sense of scale, this would be about 0.25 ppm PO4 in a 300g tank. In the regen bucket, it represents 32.0 ppm PO4!

We'll be testing for PO4 again after 120 hours and 144 hours to see if it is rising or steady. After that I plan on immediately adding a fresh batch of NaOH solution and testing for PO4 again on the same timeline. My goal here is to discover exactly how much NaOH and time it takes to regenerate X volume of GFO, assuming I'm starting with fully exhausted GFO.

 

[Anonymous]

Well we skipped the 120 hour test, but we did test again today at 144 hours. The PO4 level in the bucket was 28 ppm. Appears to have dropped, but this is easily within the range of experimental error.

To elaborate, we are not taking a direct measure of the PO4 level in the bucket because our PO4 test is "off the chart" above 3.5 ppm. So in order to measure such a high level, we remove 1.0mL from the bucket and dilute it into 99mL of DI water, then test that solution and multiply the result by 100. The difference between 0.032 ppm and 0.028ppm is obviously a small one (finer than hobbyist kits can even measure), and could be due simply to small errors in the dilution or the test itself.

So it appears that the 0.1M solution removed all the PO4 it could after 4 days (and maybe even sooner?), but I don't know if it removed all the PO4 from the GFO. I will add a fresh batch of NaOH solution tomorrow and start testing again.

 

[Anonymous]

why not dilute it by 9mL of water? because the difference between 0.35ppm and 0.28ppm would be quite a bit more substantial unless of course that's still "off the charts" for your testing equipment.

 

[Anonymous]

why not dilute it by 9mL of water? because the difference between 0.35ppm and 0.28ppm would be quite a bit more substantial unless of course that's still "off the charts" for your testing equipment.

That would work perfectly, except we didn't know when we tested it that we were going to be almost exactly 10 times the concentration of the test's upper limit. I thought it might be much higher. As to why we didn't just re-test it at 10x dilution, lack of time, mostly.

 

[Anonymous]

Drained the bucket today, and then flushed with DI. I set it up so that rinse water left the reactor to drain, not back into the bucket. It takes about 10 turnovers of the reactor volume (measured at the output) to get it to what we deemed an "acceptable" level (it took about one drop of 10% HCl to acidify a whole liter of the rinse water). The apparent pH as measured by a probe remains high even after multiple flushings.

Then I started the whole process back up again. I'll update with new PO4 readings on Tuesday.

 

[Anonymous]

Same process this time yielded 31 ppm PO4 in the water. Seems pretty clear that I am limited by the amount of NaOH, not time. I'm going to bump the solution up to 2USG of 0.5M NaOH and repeat again...

 

[Len]

Thanks for the update, but I gotta wonder: Is NaOH that much cheaper than replacing the GFO? Just asking ....

 

[Anonymous]

Thanks for the update, but I gotta wonder: Is NaOH that much cheaper than replacing the GFO? Just asking ....

In our case? Abso-frickin-lutely. For the average hobbyist? Probably not even worth the hassle and time unless you have a big or NPS tank.

I am being super thorough about the testing of this process because we're setting up a GFO reactor on our 200,000 gallon reef tank. In that case, not only will it save us a lot of money to regenerate the GFO, it will actually save us a ton of time/labor. Imagine removing 40 gallons of wet GFO out of large acrylic cylinders and disposing of it every month, then reloading 40 gallons of dusty new stuff. Not fun! It will be much easier to turn some valves, add some powder, walk away, neutralize it, then send it to drain. The GFO can stay inside the units this way.

The total cost of the NaOH to do this monthly will be something like $10-20, total cost if we had to replace all the GFO even at bulk prices is something like $600 if I remember right. That's money we can spend on Vortechs/skimmers/LEDs instead...

 

[Anonymous]

Does your facility also regen DI resin too?

 

[Anonymous]

Does your facility also regen DI resin too?

Nope. Siemens services them. Not sure why we don't do it ourselves.

 

[Anonymous]

If are are working with strong acid and bases, and have the safety equip, it is not much work once you set it up. For the scale of thing there, I am sure you guys can save a bundle to a point that you can even skip the RO stage.

 

[Anonymous]

If are are working with strong acid and bases, and have the safety equip, it is not much work once you set it up. For the scale of thing there, I am sure you guys can save a bundle to a point that you can even skip the RO stage.

We don't use an RO stage already. We do have mixed bed canisters, not separate cation/anion resins...any idea how or if those can be recharged w/o separation?

 

[Anonymous]

I forget which is which, but being as a base recharges one, and an acid recharges the other... nope, need them separate. Now you can separate them, recharge, then remix together but it'd probably be a hassle.

 

[Anonymous]

You need to have two separate canisters for this to work efficiently. Basically, you dump the used mixed resin in acid (HCl), and they will separate. Once you separated them, you keep them separated, and don't mix them up again. Then you regenerate the other resin with NaOH (lye), and you will have the cation and anion resins. The detailed instruction is online somewhere. It will be your homework tonight.

Alternatively, you can buy them separately (same price), but it is a waste since you already have the mixed resin already.

 

[liquid]

We've got instructions for recharging mixed bed DI resins:

http://www.reefs.org/library/article/g_ ... truct.html

http://www.reefs.org/library/article/twp_recharge.html

I've done it 1x so far using the second link above. Essentially the instructions explain how you separate the two resins starting with an appxox 25-35% NaOH (lye) solution. The lye solution will "exhaust" the cation resin which sinks to the bottom of your container. The anionic resin will float and you can use a turkey baster to suck out the floating anionic resin into a separate container. After that I'd probably run each in its own canister to ease recharging. Read over the instructions for complete details.

As always, respect the chemicals. Have plenty of ventilation and wear proper PPE (goggles, gloves, cover arms with a lab coat or at minimum a long sleeved shirt, wear jeans and no open-toed shoes). The last thing you want to do is to splash a strong acid or base on you. If you do, IMMEDIATELY wash it off and get medical attention if necessary. I've seen my fair share of chemical burns (30% peroxide + sulfuric acid is NOT pretty) in the lab and always practice safety first.

Shane