Regenerating GFO

[Randy Holmes-Farley]

You do not "need" to replace anything, but how you treat the surface of GFO will determine what is there, and if what is there changes when added to tank water, you can find that there may be a net addition or subtraction of alkalinity and a consequent one time initial effect on pH.

Many mineral surface will have properties that change with pH. A hydroxide, in particular, will have a variety of different surface forms. Specifically, the surface will not be just hydroxide ions, so the relative amounts of OH- will change with pH. In pure water the surface-charge determining ions are OH- and H3O+. At lower pH, there are more of the latter and the surface charge is more positive.

In other solutions, like vinegar or seawater, some of the OH- sites may be taken up by chloride, sulfate, carbonate, and in the case using a vinegar wash, acetate.

The point where the surface of iron oxide/hydroxide has no net charge is usually described as being in the pH 7-10 range.

 

[ScooterTDI]

So, I did some quick research on GFO regeneration. Two studies had some interesting info.

First, as far as fluoride desorption goes, a minimum desorption occurs under acidic conditions. I haven't found any phosphate specific acid regeration studies yet though. So, maybe acidic regeration is not the way to go. I'll see if I can find any phosphate specific information.

Second, under basic regeneration, a 15-40% loss in capacity is seen after the first regeneration cycle. It was posutulated by the authors that a phase transformation to geothite or hematite is occuring. It is interesting that tatu noted a color change upon regeneration. Just guessing, but this could possibly be the result of phase transformation to hematite.

One study found that 0.1M NaOH was sufficient for regeneration and higher concentrations yielded no additional benefit. They also found that a volume of regeneration solution 9 times the bed volume (volume of GFO) was sufficient with no additional improvement in regeneration efficiency when using 60 time the bed volume.

Scott

 

[jptenklooster]

the idea is that you replace the phosphate with OH- in order to obtain FeOH/O again. This regeneration has nothing to do with scraping of the surface, at least according to manufacturers. Personally I did not notice any colour changes, but this might depend on the brand GFO that you are using.

 

[ScooterTDI]

I don't think there is anything necessarily special about the surface hydroxide groups. They are just being displaced by phosphate.

Is there any reason that surface bound acetate would not simply be displaced by phosphate? Would acetate (with its relatively lower basicity compared to OH-) actually be a better to have on the surface because it would be more readily displaced? I'd guess that the acetate is mostly displaced from the surface by OH- fairly quickly once placed back in the tank anyway.

The phase transformation info was referring to akaganeite (sp?) being converted to hematite or geothite. Although akaganeite is reported to have the greatest binding capacity, I think some manufacturers use different forms, so a phase transformation may not occur in all brands. Again, it was just a guess.

Scott

 

[chaderic26]

I gave my old gfo I was gonna throw away a dip in lye solution and then a dip in acid then rinsed well.

Should this stuff be ready to go for next time I switch it out?

 

[Randy Holmes-Farley]

I'd guess that the acetate is mostly displaced from the surface by OH- fairly quickly once placed back in the tank anyway.

I agree that acetate will not stay bound in tank water.

 

[ScooterTDI]

I gave my old gfo I was gonna throw away a dip in lye solution and then a dip in acid then rinsed well.

Should this stuff be ready to go for next time I switch it out?

First, I'd do the acid first if I was going to use both vinegar and NaOH.

Also, I'd immerse it longer than just a dip. I don't have any definitive time period necessary to regenerate, but it certainly wouldn't hurt too much to at least immerse it for a couple of hours. That is what I've been doing with vinegar. I can't find the paper I was looking at earlier, but if I remember correctly they were just eluting a column of GFO with a few bed volumes of NaOH. Filling a column with GFO and slowly eluting with either vinegar or NaOH would probably be the preferred method of regeneration.

Scott

 

[redfishsc]

So on the practical level, we're looking at regenerating GFO by soaking it for a few hours in vinegar or sodium hydroxide?

 

[chaderic26]

I soaked it in muriatic acid then a lye solution for about an hour each with r/o rinsing in between.

 

[Fish Ace]

An interesting subject, but what is the bottom line and how can we do it cheaply?

 

[ScooterTDI]

The bottom line is that you can use acids like vinegar or hydrochloric acid. You can also use bases like sodium hydroxide or potassium hydroxide. You can also use both. It very inexpensive to do this. A gallon of vinegar is a few dollars and would be enough for many many regeneration cycles.

Scott

 

[redfishsc]

Nice.

Muriatic is cheap too but dangerous if you have kids around or are a bit of a klutz yourself.
Muriatic (hydrochloric) can be diluted down as necessary.


If one tried vinegar, how much vinegar to regenerate, say, a cup of GFO? And how long would you guys suggest?

 

[ScooterTDI]

I haven't tried muriatic acid (same as hydrochloric acid), but as you said I'd imagine that dilution is probably necessary to prevent excessive dissolution of the GFO.

I haven't experimented much yet, but I've just been soaking in a volume about 2X the GFO volume. The more solution volume you use, the more PO4 and FeOH will be released into solution before equilibrium will be established, but there is a point of diminishing returns. I'm not sure where that point is yet. Next time, I will try a few succesive soaks like this and see if I get an appreciable PO4 concentration in the 2nd and 3rd soaks.

Next time I go to Lowes, I'll pick up some trisodium phosphate and run some more controlled experiments.

Scott

 

[Fish Ace]

I'd be interested in knowing how much vinegar to GFO.

 

[reefrad]

I checked so called 'distilled vinegar' with a hannah meter and the hosphates came back at 2.5! aparently distilled referrs to the dilution of the vinegar and not the water itself...

 

[Randy Holmes-Farley]

There is some phosphorus in vinegar (as there also may be in sodium hydroxide and muriatic acid), but at the pH of vinegar it will not likely bind much to GFO.

 

[James79]

Could someone estimate how much of a 50% sodium hydroxide solution would be needed to get 1 liter at 1M?

 

[Randy Holmes-Farley]

Could someone estimate how much of a 50% sodium hydroxide solution would be needed to get 1 liter at 1M?

Sure. You need 40 grams of solid NaOH, or 80 grams of 50%. The density is probably around 1.5 g/mL, so maybe 50-60 mL.

 

[msman825]

would this work with just reg phosban

 

[James79]

Thanks Randy, this is a great way to stretch a dollar.