New idea for calcium

[Foxy Brown]

Hi all,
I've been away for a while, life and all that. I'm starting up a small nano tank on my desk, and have been thinking of how I would like to go about calcium supplements. I travel alot so I can't be there to watch it every day, and a nano can be quite temperamental. I don't want to add just DI for a week or two and then get behind on Ca dosing. I need something bulletproof.

My normal approach for larger tanks is kalkwasser mainly because it's cheap, but it always makes me nervous about pH spikes. There's CaCO3/CO2 reactors which I've never really liked due to complexity and crashes (same as kalk but the other way around). Next comes 2-part or balling which is great and flexible but is still too complex. So I got to thinking.

I often add vinegar to kalk to tone down pH and to help w/ nitrates (like vodka dosing) but that gets one worried about bacterial blooms, and you can pre-mix much. But wait a minute, you could just as easily use vinegar in a CaCO3 reactor as the acid source instead of CO2. The great thing about this is that it doesn't wreck pH. I tested this notion w/ an air-tight container of aragonite and a bit of grocery store vinegar. Add a few ml of vinegar to DI water, pour it into the aragonite, seal it tight and wait. You always get pH~8.2 after is ages for a while (this makes sense when you think about how aragonite buffers pH).

So it seems like this is a neat little idea for a calcium reactor since I add the vinegar anyway, and it eliminates the need for a CO2 tank... just spike my top-off water w/ vinegar and pump it in nice and slow w/ a peristaltic pump through a column of aragonite and then into the tank.

Next I thought, but what if I get a bacterial bloom in the reactor? Since I'm proposing a slow flow (few ml/minute probably) it could go anoxic. Well, since I'm adding DI plus vinegar, there no sulfate present so no reason to have H2S and the bugs will just convert the acetate to CO2, which will dissolve more CaCO3, no problem there. If the reactor volume is small, the dissolved O2 in the top-off water will likely be nowhere near enough to allow all the vinegar to be consumed. Likewise, I can make sure to keep the vinegar concentration, low enough that I never precipitate calcium bicarb in the reactor, but this stuff is remarkably soluble - so it's not likely. In the end, the rate of Ca dosing vs evap make-up is controlled by vinegar concentration - so test over time and adjust the regimen as needed.

That's the simple case.

Then I got to thinking a bit more....

What if I make use of all those anoxic bacteria to remove nitrates... Say we slowly pump aquarium water into the bottom of a column of crushed aragonite, while also slowly pumping in dilute vinegar? (in this case the circulation pump runs constantly and the vinegar dosing is run from the top-off float switch). The vinegar drops the pH of the incoming tank water, and dissolves some aragonite. Next, the bacteria begin to consume O2 to convert acetate to CO2, which goes on to dissolve more aragonite. Next, NO3 gets converted to N2 to finish consuming the acetate. Of course, like any denitrator we must now be wary of sulfate going to H2S, but this can be prevented by dialing in the circulation pump speed to make sure enough oxygenated water is flowing in to stop the process at NO3 -> N2 More complicated than just dosing, but it's one little system doing a whole lot of chemistry in an elegant way, which is always satisfying.

OK everyone, shoot holes in it. I'm hoping Randy will chime in at some point.

 

[bertoni]

I would worry about the mixture encouraging anaerobic decay. There will be other nutrients in the water, vinegar, and aragonite, as contaminants. Don't you see a lot of fizzing when you add the vinegar to aragonite? Randy tried that, I think.

 

[Foxy Brown]

The fizzing is CO2, if it's slowly added to the bottom of a column, the CO2 should stay in solution and react with more aragonite, just like in a CO2 reactor. I mentioned the anerobic decay, and it might be managed to be useful.

 

[disc1]

How much calcium does the tank need every day? How much vinegar will it take to dissolve that much aragonite to get that much calcium? Is that an OK amount of vinegar to add to a tank in a day?

That would be my first calculation. We've looked before at using calcium acetate to replenish calcium, which is basically what you are doing, and it worked out to be a little heavy on the acetate dosing. At least in the case I remember. Run the figures for yours and see what it comes out to.

 

[Foxy Brown]

That's a really good question, as I am only in the planning phase I'm not sure what the demands will be. I have this vision/fantasy of stocking a crocea clam and a pygmy wrasse (Wetmorella or Pseudocheilinops ataenia) plus assorted zoanthids and ricordeas and a small halimeda (which will need regular pruning) in an 8g nano. I know most folks will probably say this is on the small side for a clam, even a crocea. These guys can use up a lot of Ca in such a small tank - which is why I think I will need more than just manual dosing and water changes.

I have planned a small 3g gravity-return fuge on reverse daylight cycle to help stabilize the pH. Maybe adding small skimmer like the one from gnome, depending on how it goes with nitrates - though they say clams appreciate a bit of nitrate as longs as it's not out of hand.

For dosing/top-off I'll probably use a 1.1ml/min pump from BRS. I have the tank up and cycled but not stocked and it so far uses about 1 gallon week for evap. At this rate it wouldn't be too tough for anaerobic bacteria to convert any acetate to CO2 before it left the reactor.

Perhaps I will set this up as an experiment and just run it for a month and collect the effluent to see what I get. I'm lucky enough to have access to the necessary lab equipment at work.

 

[disc1]

I'd run that calculation with some reasonably predicted numbers before I went any further. If it turns out to be the addition of a half gallon of vinegar to the tank or something then it is a non-starter. If it could be done with a reasonable amount of vinegar then it might be worth looking further into the experiment.

 

[Hockeynut120]

Hi I'm new to this site but figured I would chime in and ask, what's the size of the tank?
I have been running a 20 gallon for almost 3 years with lots of lps and sps and have never dosed a thing. I also have a 8 gal pico and what I have found is that there is no need for additives with a once a week water change. In the 20 I change 5 gals a week and only 1 gal in the pico. I have found this method to keep the cal and alk a very respectable levels. I try the KISS method for small tanks seems to work for me. Good luck

 

[kenny b]

I posted about this type of idea not too long ago, I think about 2 weeks or so. I built a little prototype reactor to test this out but I haven't set it up quite yet. I'm waiting to update my plumbing so I can get a more reliable drip rate into the reactor. I proposed having an adjustable flow rate of tank water through the reactor on the order of tenths of milliliters per minute (a drop about every 10 seconds) and a vinegar drip rate of probably a tenth of that. These are guesses and wont really know until I test it out. BTW I have a 10g tank that it eating up alkalinity like no ones business.

 

[Foxy Brown]

OK let's do this... say we want to raise Ca by 20ppm. Water volume is ~32L which makes for 0.64g of Ca. At 40.078 g/mol this is 16mmol of Ca2+

Now the important piece of this is that the reaction is done slowly in a high surface area reactor at low concentrations, so can be expected to be in equilibrium. That means we don't get CO2 fizz, but rather it goes into solution and so there will be 2 steps.

1) 2 H3COOH + CaCO3 -> Ca2+ + 2 H3COO- + H2CO3
2) H2CO3 + CaCO3 -> Ca2+ + 2 HCO3-

So 1 moles of acetic acid + 1 mole CaCO3 gives a mole of Calcium, 1 of bicarbonate and 1 of acetate. Molar mass of acetic acid is 60.05 and pKa is 4.76 so should be fully dissociated in equilibrium with carbonate. At 5% concentration, that's only 0.8meq/ml which means 20mL of 5% vinegar.

Not sure how much sense that's going to make, but it's probably higher than I would expect to dose. Of course much depends on the rate of Ca consumption, which was already said. If those 20 ppm of Ca last a week, that it will probably be OK. OF course, it's not hard to add more acid without adding more acetate, just bring up acidity w/ HCl.

 

[Foxy Brown]

Of course as we've already said, the acetate will go on to form CO2 either in the reactor if it becomes anoxic or in the tank later... so it makes more alkalinity in the end (acetate itself doesn't count toward alkalinity).

Interestingly, if this metabolism happens in the reactor, it should make 2 more moles of CO2 which reacts with more aragonite and makes two more moles of Ca(HCO3)2. Then the amount of vinegar drops to only ~ 6ml for the same 20ppm of Ca.

 

[disc1]

OK let's do this... say we want to raise Ca by 20ppm. Water volume is ~32L which makes for 0.64g of Ca. At 40.078 g/mol this is 16mmol of Ca2+

Now the important piece of this is that the reaction is done slowly in a high surface area reactor at low concentrations, so can be expected to be in equilibrium. That means we don't get CO2 fizz, but rather it goes into solution and so there will be 2 steps.

1) 2 H3COOH + CaCO3 -> Ca2+ + 2 H3COO- + H2CO3
2) H2CO3 + CaCO3 -> Ca2+ + 2 HCO3-

I think you mean H3CCOOH and H3CCOO-

So 1 moles of acetic acid + 1 mole CaCO3 gives a mole of Calcium, 1 of bicarbonate and 1 of acetate. Molar mass of acetic acid is 60.05 and pKa is 4.76 so should be fully dissociated in equilibrium with carbonate. At 5% concentration, that's only 0.8meq/ml which means 20mL of 5% vinegar.

Not sure how much sense that's going to make, but it's probably higher than I would expect to dose. Of course much depends on the rate of Ca consumption, which was already said. If those 20 ppm of Ca last a week, that it will probably be OK. OF course, it's not hard to add more acid without adding more acetate, just bring up acidity w/ HCl.

Now if you use HCl you're essentially consuming alkalinity to make calcium. The whole purpose of using aragonite in a reactor is to add the two in a balanced ratio. Putting HCl in there would completely defeat the purpose of what you are doing.

Of course as we've already said, the acetate will go on to form CO2 either in the reactor if it becomes anoxic or in the tank later... so it makes more alkalinity in the end (acetate itself doesn't count toward alkalinity).

Acetate does too count towards alkalinity. It's basic and can be titrated can't it?

But the extra CO2 you make does NOT count. You are creating one unit of alkalinity, but you are consuming two units (the acetate and the proton you let go of when the CO2 goes to bicarb) so that's actually going to be a net loss of alkalinity, not a gain. And it won't happen in anoxic conditions. That's going to be aerobic respiration that takes in acetate and gives you back CO2.

Interestingly, if this metabolism happens in the reactor, it should make 2 more moles of CO2 which reacts with more aragonite and makes two more moles of Ca(HCO3)2. Then the amount of vinegar drops to only ~ 6ml for the same 20ppm of Ca.

You don't get a clean 2 moles for 2 moles of CO2. Think about the equilibrium there. A good bit of your CO2 is going to go out as gas.

 

[bertoni]

How do you intend to contain the carbon dioxide released by the combination of vinegar and aragonite?

I'm still skeptical that the anaerobic process is going to be helpful.

 

[Foxy Brown]

OK, I think the whole reactor thing is not quite clear. This is the essential point, so let me explain a little more...

We will be dosing in a small amount of acetate into a column of packed aragonite. This is not the 5% stuff but rather a few ml of it added to DI, and it will be added over a long period, not all at once. We will be adding something like 20mmol of H+ per gallon of water, so something like 40x more dilute than the store-bought vinegar, and added over a week... now you can see what I mean by slow.

So, the concentration of acid is never high. Try it yourself, add a very dilute solution of acetic acid to crushed coral, you will not get fizz, but this doesn't mean nothing is happening. To form a bubble requires alot of energy. When you make a bubble you are creating a surface (gas/water interface) and surface tension is going to be fighting you, you also have to overcome the 14 psi of atmospheric pressure + the weight of the water column to form the bubble (this is why water boils at different temps depending on elevation). That means you must generate enough CO2 not only to exceed the solubility of H2CO3, but then even a bit more to overcome these effects. So a slow reaction does not "fizz".

Instead of fizzing, the CO2 generated by adding acid goes into solution where it can go on to react with something else, or will eventually leave through the water's surface (a very slow process in a packed column). Now, say we are filling at the bottom of a column, the H+ will react with CaCO3 to form HCO3- and Ca2+ or possibly H2CO3. This H2CO3 can either go on to react with more CaCO3 or form a bubble, which on its way up can re-dissolve and again has a chance to react...

The surface area available for the first reaction is large, and the second requires a huge number of H2CO3 molecule to group-up and form a bubble... the second dissolution step is far more likely when everything is happening slowly, as is the case here. This is very different from CO2 injection, which happens at high flow rates, and the water is always super-saturated w/ H2CO3.

Any acid with a pKa which is higher than that of carbonic acid (6.367) cannot count toward alkalinity (or rather its conjugate base cannot), because it will not react before reaching the end-point of the titration. So neither Cl- nor H3CCOO- counts toward alkalinity on their own. However, because of the design features that prevent us from loosing HCO3- from the system, we can still add alkalinity to the tank.

This is like saying that adding CO2 to a CO2/CaCO3 reactor cannot add alkalinity. Dosing CO2 into the tank wouldn't add alkalinity but the CO2 reactor most certainly does add it by first consuming the CaCO3 in the reactor... same thing applies here. I prefer to use acetate over chloride because Cl just increases salinity, while acetate will go on to become CO2 sooner or later. But that's not to say it would be wrong to use another acid (except nitric or phosphoric which would obviously be a terrible idea).

I need Randy to chime in and let me know if I'm missing something.

 

[disc1]

Any acid with a pKa which is higher than that of carbonic acid (6.367) cannot count toward alkalinity (or rather its conjugate base cannot), because it will not react before reaching the end-point of the titration. So neither Cl- nor H3CCOO- counts toward alkalinity on their own. However, because of the design features that prevent us from loosing HCO3- from the system, we can still add alkalinity to the tank.

This is incorrect. The endpoint in your alkalinity titration is 4.2. It's not based only on carbonic acid. Acetate most definitely counts as alkalinity.

 

[Foxy Brown]

I think you mean H3CCOOH and H3CCOO-

True

Now if you use HCl you're essentially consuming alkalinity to make calcium. The whole purpose of using aragonite in a reactor is to add the two in a balanced ratio. Putting HCl in there would completely defeat the purpose of what you are doing.

Again think about a reactor. CO2 is an acid yet a CO2 reactor add alkalinity. This is because you are consuming CaCO3 as well as adding acid.

Acetate does too count towards alkalinity. It's basic and can be titrated can't it?

No, its pKa is too low.

But the extra CO2 you make does NOT count. You are creating one unit of alkalinity, but you are consuming two units (the acetate and the proton you let go of when the CO2 goes to bicarb) so that's actually going to be a net loss of alkalinity, not a gain. And it won't happen in anoxic conditions. That's going to be aerobic respiration that takes in acetate and gives you back CO2.

Remember the CO2 is not going into the tank, but going on to consume more CaCO3 in the reactor. Acetate is not alk and a proton is anti-alk, as it consumes OH- which IS alk. Also remember that it's not as important what we put into the reactor, as it is what come out of it.

Acetate-CO2 reaction can still happen anaerobically, that just meas O2 is not the terminal electron acceptor... generally S or N becomes the terminal electron acceptor, but there is none present in DI, so in this case, I think the dissolved O2 present will be consumed and any remaining acetate (most of it as very little O2 dissolves in water) will go on un-touched. I'm OK with that.

You don't get a clean 2 moles for 2 moles of CO2. Think about the equilibrium there. A good bit of your CO2 is going to go out as gas.

Again see my description of slow kinetics in a reactor. It's all about rates an concentrations, but CO2 does not always make bubbles and leave. If that were so CO2/CaCO3 reactor could not function. Dissolved CO2 reacts rapidly with CaCO3 to form Ca(HCO3)2. Very little if any CO2 ever leaves the reactor.

 

[Foxy Brown]

This is incorrect. The endpoint in your alkalinity titration is 4.2. It's not based only on carbonic acid. Acetate most definitely counts as alkalinity.

I must disagree here. though much depends on the indicator used in the test. In any event, I would argue that the real alkalinity we are concerned about is that contributed from carbonates, as the concern is really about calcification and buffering, and acetate adds no buffering capacity in the pH range we are worried about (8.0-8.3)

 

[dkeller_nc]

One of the things I've learned over the years is that sometimes, a prototype is about the only way to truly gauge whether a concept will work or not, particularly if biology is involved. That doesn't mean that I think constructing a prototype of a perpetual-motion machine is worthwhile, but barring something that the laws of physics absolutely prohibit, it's worth messing with.

 

[kenny b]

My thoughts in the prototype reactor I built is this:

2CH3COOH+CaCO3->Ca(CH3COOH)2+CO2+H2O
and then
CO2+H2O+CaCO3->Ca(HCO3)2

and due to metabolism,
O2+CH3COOH->H2O+2CO2+OH with
2H2O+2CO2+2CaC03->2Ca(HCO3)2 if aerobic
or
5CH3COOH+8NO3->10CO2+4N2+13OH+H2O
10H2O+10CO2+10CaC03->10Ca(HCO3)2 if anaerobic

I'm an engineer and not a chemist so chime in if I have errors.

 

[Foxy Brown]

kenny b,
I just read your old post and I think you and I are on the same track, more or less. I will look over the reactions you posted later, but I suspect that they aren't balanced properly... no OH should show up for instance. I actually am a chemist with a PhD, though this is not the typical thing I work with so I'm going back about 10 years in my education. I'll want to run over it first so I don't come out looking like an idiot.

I think the thing that everyone gets stuck on is misunderstanding the difference between chemistry happening in the reactor and chemistry happening in in the tank. And of course the idea that the CO2 evolved must fizz away. I doubt that anyone has ever tried a 0.1% solution pumped slowly over a large amount of high surface area CaCO3. But we've all seen the baking soda and vinegar volcano, and that's what most folks envision here.

As you did, I think the answer is - make one!

 

[kenny b]

My first thoughts were that the metabolic component would happen inside the tank however, it will probably occur inside the reactor like your proposing which I think would be way better assuming there is enough nutrients in the tank water entering the reactor. If the reactor is designed such that just about all the acetate is consumed before it leaves the reactor, then I don't see why there would be any problems with bacteria blooms or oxygen depletion harming the tank inhabitants. The potential issues I can think of is algae consumption of CO2 in the reactor causing calcium-alkalinity imbalance and/or eventual imbalance from CO2 and acetate that manages to escape the reactor through the effluent.

In a standard Ca reactor, I don't think there are any imbalances from CO2 loss since more CO2 is just directly added to compensate.

I'm sure another carbonate mineral like Iron carbonate can be added to the reactor media to compensate for CO2 loss in the Ca acetate reactor.