Citric Acid dosing as a carbon source?

[arking_mark]

@Randy Holmes-Farley

I'm sure there is a good reason why it's not used and Vodka and Vinegar are preferred. Just curious...

 

[Courtney Aldrich]

I have no experience, but found citric acid has been explored as a carbon source in wastewater treatment for reducing phosphate (see attached paper). Citric acid stimulates production of polyphosphate in certain bacteria by enhancing ATP levels through the citric acid cycle in respiration. It might be worth experimenting.

 

[EuphylliaAddict]

It would be nice to see brstv do a deep dive on this one... We already use citric acid for cleaning pumps and power heads, why not a drop or 2 a day in the tank to lower po4 naturally... It would be cool to see it be a topic in the future.

 

[arking_mark]

My bet is that certain carbon sources work better with the desired bacteria.

 

[Courtney Aldrich]

Interestingly, citric acid raises the alkalinity, which is counterintuitive. However, consumption of protons would occur if the bacteria make polyphosphate,

 

[arking_mark]

Interestingly, citric acid raises the alkalinity, which is counterintuitive. However, consumption of protons would occur if the bacteria make polyphosphate,

I have no experience, but found citric acid has been explored as a carbon source in wastewater treatment for reducing phosphate (see attached paper). Citric acid stimulates production of polyphosphate in certain bacteria by enhancing ATP levels through the citric acid cycle in respiration. It might be worth experimenting.

So I recall a Dr. Tims lecture on the different heterotrophs and that the bacteria used in wastewater were not the right ones for saltwater. So these wastewater studies can't always be applied to saltwater aquariums.

 

[Courtney Aldrich]

So I recall a Dr. Tims lecture on the different heterotrophs and that the bacteria used in wastewater were not the right ones for saltwater. So these wastewater studies can't always be applied to saltwater aquariums.

Yes, I doubt the same bacteria are present in aquaria, but there are so many bacteria. I'm sure there are marine bacteria capable of metabolizing citric acid.

 

[Randy Holmes-Farley]

I elected vinegar/acetate due to the very wide range of organisms that are known to readily use it from the water. That is seemingly less true of ethanol or other organics, but it may not ultimately be important how many different species can use the organic, as long as one type of bacteria can use it and metabolize it. Overall, I do not see any reason to prefer citrate over acetate, but it may work fine.

I would say that organic carbon dosing in general is not a great way to reduce phosphate. That is because of denitrification as a process that reduces nitrate and not phosphate, and that requires consumption of organics.

 

[Randy Holmes-Farley]

Interestingly, citric acid raises the alkalinity, which is counterintuitive. However, consumption of protons would occur if the bacteria make polyphosphate,

Why would you say that?

Adding citric acid cannot raise alkalinity, either in the short term (before metabolism) or long term (after metabolism).

Adding citric acid will reduce the total alkalinity in the short term because it will release 3 protons on addition, and when doing an alk test, 1-2 on the carboxylates will not be titrated since their pKa in seawater is likely below the pH endpoint of an alkalinity test (about pH 4.3).

Adding citric acid will have no effect on total alkalinity in the long term, once it is all effectively metabolized to CO2 (which cannot impact alkalinity up or down).

Adding any citrate salt will have a more complicated effect, with the short term effect depending on the form and the long term effect to be to add alkalinity.

 

[DxMarinefish]

An old reefer I used to know (has sadly passed away a while back) used to mix citric acid (organic) in his fresh water top off, feed this into the bottom of a cylinder full of calcium reactor media, which in turn fed into he's Sump as the top up water.

He explained it to me a while back but I was a young reefer then and had no idea what he was talking about.

For some reason he said it kept his Ca/Alk & Ph stable and fed other nutrients too from the dissolved media.

Not sure about the logic or how successful it was for him but i loved he's tank.

 

[Randy Holmes-Farley]

An old reefer I used to know (has sadly passed away a while back) used to mix citric acid (organic) in he's fresh water top off, feed this into the bottom of a cylinder full of calcium reactor media, which in turn fed into he's Sump as the top up water.

He explained it to me a while back but I was a young reefer then and had no idea what he was talking about.

For some reason he said it kept his Ca/Alk & Ph stable and fed other nutrients too from the dissolved media.

Not sure about the logic or how successful it was for him but i loved he's tank.

Yes, that would work sort of like a CaCO3/CO2 reactor plus organic carbon dosing.

 

[Courtney Aldrich]

Why would you say that?

Adding citric acid cannot raise alkalinity, either in the short term (before metabolism) or long term (after metabolism).

Adding citric acid will reduce the total alkalinity in the short term because it will release 3 protons on addition, and when doing an alk test, 1-2 on the carboxylates will not be titrated since their pKa in seawater is likely below the pH endpoint of an alkalinity test (about pH 4.3).

Adding citric acid will have no effect on total alkalinity in the long term, once it is all effectively metabolized to CO2 (which cannot impact alkalinity up or down).

Adding any citrate salt will have a more complicated effect, with the short term effect depending on the form and the long term effect to be to add alkalinity.

The paper I attached above showed citric acid ultimately raised the pH of the system. Initially, the pH was lowered as you would expect, but then the pH increased dramatically as the citrate was metabolized. As you explained to me yesterday in another post, metabolism of organics will decrease alkalinity due to the production of protons. So, I was confused why the experimental conditions showed pH actually increases. I hypothesize that the formation of polyphosphate is likely responsible as this consumes monoprotonated phosphate to add to the growing polyphosphate chain with concomitant formation of one molecule of water.

 

[DxMarinefish]

Yes, that would work sort of like a CaCO3/CO2 reactor plus organic carbon dosing.

So the citric acid in the fresh water would create a lower ph enough to dissolve the media.
I am guessing one would have to be careful how much citric acid was mixed with the fresh water.

 

[Randy Holmes-Farley]

The paper I attached above showed citric acid ultimately raised the pH of the system. Initially, the pH was lowered as you would expect, but then the pH increased dramatically as the citrate was metabolized. As you explained to me yesterday in another post, metabolism of organics will decrease alkalinity due to the production of protons. So, I was confused why the experimental conditions showed pH actually increases. I hypothesize that the formation of polyphosphate is likely responsible as this consumes monoprotonated phosphate to add to the growing polyphosphate chain with concomitant formation of one molecule of water.

That paper is very complicated to interpret, but I'm not sure it says anything unexpected.

I'll note first that it is not seawater, and uses nutrients far from reef tank conditions (>20 ppm phosphate, for example).

Most importantly, it does not mention alkalinity, and pH is not a useful surrogate measure for alkalinity if CO2 values are changing. One can lower the pH of seawater to pH 5 with CO2, and that has no impact on alkalinity. Similarly, removing CO2 from seawater will raise the pH but have no impact on alkalinity (at least until something like magnesium hydroxide precipitates).

I discuss that (The Principle of Conservation of Alkalinity) here:

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

In the paper you posted, they note that

"The introduction of a citric acid solution to the system contributed to a decrease in wastewaters pH value from pH 7.60 to pH 5.5 (Table 3). The consumption of citric acid by microorganisms and CO2expelling as a result of aeration was accompanied by an increasing pH value, which in the treated wastewaters accounted for pH 8.24–8.37 in Variant 1, for pH 8.11–8.36 in Variant 2, and for pH 8.25–8.32 in Variant 3 (Table 3)."

Thus, I do not see clear evidence for an alkalinity "change".

But I think there is likely an alkalinity increase due to the potential for denitrification, which they say they are trying to minimize, but give no data on the extent of it happening. Denitrification raises alkalinity.

 

[Randy Holmes-Farley]

So the citric acid in the fresh water would create a lower ph enough to dissolve the media.
I am guessing one would have to be careful how much citric acid was mixed with the fresh water.

Certainly, yes, just like one has to control the CO2 in a reactor.

One can also do this with vinegar and calcium carbonate, or one can buy calcium acetate (Salifert All in One) or calcium formate (Tropic Marin carbocalcium) where they have already "done that reaction" for you and you just dose the final product.

 

[Courtney Aldrich]

That paper is very complicated to interpret, but I'm not sure it says anything unexpected.

I'll note first that it is not seawater, and uses nutrients far from reef tank conditions (>20 ppm phosphate, for example).

Most importantly, it does not mention alkalinity, and pH is not a useful surrogate measure for alkalinity if CO2 values are changing. One can lower the pH of seawater to pH 5 with CO2, and that has no impact on alkalinity. Similarly, removing CO2 from seawater will raise the pH but have no impact on alkalinity (at least until something like magnesium hydroxide precipitates).

I discuss that (The Principle of Conservation of Alkalinity) here:

Chemistry and the Aquarium: What is Alkalinity?

Randy provides an overview of alkalinity as to why it's important, how it's measured, and how can it be tested.

reefs.com

In the paper you posted, they note that

"The introduction of a citric acid solution to the system contributed to a decrease in wastewaters pH value from pH 7.60 to pH 5.5 (Table 3). The consumption of citric acid by microorganisms and CO2expelling as a result of aeration was accompanied by an increasing pH value, which in the treated wastewaters accounted for pH 8.24–8.37 in Variant 1, for pH 8.11–8.36 in Variant 2, and for pH 8.25–8.32 in Variant 3 (Table 3)."

Thus, I do not see clear evidence for an alkalinity "change".

But I think there is likely an alkalinity increase due to the potential for denitrification, which they say they are trying to minimize, but give no data on the extent of it happening. Denitrification raises alkalinity.

Consumption of protons raises alkalinity. Thus, if bacteria are consuming protons to form polyphosphate, I think it has the potential to raise alkalinity. While pH is a poor proxy for alkalinity as you note, increasing pH is consistent with an increase in alkalinity. The bacteria used in the reactor were polyphosphate-accumulating (POA) bacteria, which are used to remove phosphate from wastewater. Many bacteria will produce high-energy polyphosphate as an energy store, but POA bacteria appear to do it even better. The general concept of removing phosphate while raising alkalinity seems plausible. Of course, the bacteria must be removed from the system through efficient protein skimming otherwise they will simply use the stored energy from polyphosphate and liberate protons when the carbon source is removed, ultimately decreasing alkalinity (I agree that you cannot escape the fact that metabolizing a carbon source generates CO2, reducing equivalents and lots of protons). I furthermore agree with you that the POA bacteria in this freshwater system are unlikely to exist in marine environments, but there are a lot of different bacteria and I have no doubt POA-marine bacteria exist. The exciting thing I extrapolated from the study was that citric acid, as key intermediate of the citric acid cycle, may perturb the normal metabolism favoring formation of polyphosphate as an energy store. All carbon sources are not the same and can have different consequences on metabolism. This is all highly speculative, and I think potentially interesting and worthwhile to explore.

 

[Randy Holmes-Farley]

Consumption of protons raises alkalinity. Thus, if bacteria are consuming protons to form polyphosphate, I think it has the potential to raise alkalinity.

But that process does not consume protons, at least in seawater. I worked through this before answering earlier, but it is not easy to get a complete handle on.

The orthophosphate is not starting as phosphoric acid.

In seawater at pH 8.1, orthophosphate consists of 0.5% H2PO4–, 79% HPO4- - and 20% PO4- - -. Only the HPO4-- and PO4--- contribute to alkalinity. H2PO4- is the form present at the end of an alk test in seawater (pH ~4.3; the first pKa of phosphoric acid in seawater is below 2).

Polyphosphate at pH 8 will be fully ionized to -1 on each P along the chain, and will remain so all through an alk test (just like H2PO4-- remains unchanged in an orthophosphate alk titration). Only the ends will be titrated, and in a long chain, the ends are minor/insignificant.

On the face of it, phosphate contributes a lot more to alkalinity than does polyphosphate.

But the conversion of orthophosphate to polyphosphate also releases alkalinity, and I think it releases the same amount as the difference in titration of orthophosphate compared to polyphosphate in an alk test.

Let's take an example of HPO4-- converted into a 100-mer (ignoring the ends for now):

100 HPO4- - + 100 H2O --> [PO(O-)]100 + 100 OH-

Thus, we see that an amount of alkalinity is released that is exactly the difference in the alk titration difference between H2PO4-- alk titration (1 unit of alk per P, or 100) and polyphosphate (0 units of alk per P).

 

[Courtney Aldrich]

Thank you for your patience and taking the time to explain. I would have thought OH- would contribute more to alkalinity than HPO4(-2) given the large difference in their basicities. OH- will quantitatively consume one proton, but I thought HPO4(2-) would not consume protons given the pKa of H2PO4- is 4.3.

 

[arking_mark]

But that process does not consume protons, at least in seawater. I worked through this before answering earlier, but it is not easy to get a complete handle on.

The orthophosphate is not starting as phosphoric acid.

In seawater at pH 8.1, orthophosphate consists of 0.5% H2PO4–, 79% HPO4- - and 20% PO4- - -. Only the HPO4-- and PO4--- contribute to alkalinity. H2PO4- is the form present at the end of an alk test in seawater (pH ~4.3; the first pKa of phosphoric acid in seawater is below 2).

Polyphosphate at pH 8 will be fully ionized to -1 on each P along the chain, and will remain so all through an alk test (just like H2PO4-- remains unchanged in an orthophosphate alk titration). Only the ends will be titrated, and in a long chain, the ends are minor/insignificant.

On the face of it, phosphate contributes a lot more to alkalinity than does polyphosphate.

But the conversion of orthophosphate to polyphosphate also releases alkalinity, and I think it releases the same amount as the difference in titration of orthophosphate compared to polyphosphate in an alk test.

Let's take an example of HPO4-- converted into a 100-mer (ignoring the ends for now):

100 HPO4- - + 100 H2O --> [PO(O-)]100 + 100 OH-

Thus, we see that an amount of alkalinity is released that is exactly the difference in the alk titration difference between H2PO4-- alk titration (1 unit of alk per P, or 100) and polyphosphate (0 units of alk per P).

Reading this makes me want to dust off my high-school chemistry books and really get into the subject.

 

[Randy Holmes-Farley]

Thank you for your patience and taking the time to explain. I would have thought OH- would contribute more to alkalinity than HPO4(-2) given the large difference in their basicities. OH- will quantitatively consume one proton, but I thought HPO4(2-) would not consume protons given the pKa of H2PO4- is 4.3.

That would be the case, but in seawater the pKa shifts down. It is much easier to stabilize ions in seawater than in low ionic strength solutions. So all acid/base equilibria generally shift toward the higher charge form. I'll post the published pKa a bit later when I find it.