Vending machines, operating at a water temperature of 95oC and atmospheric pressure usually form calcitic limescale. This is the trade off to having coffee that tastes somewhat decent and means that standard vending machines need to be cleaned every 6 months or so. Rarely though the limescale that forms is aragonitic (which isn't stable at the temperatures and pressures present in a vending machine). My question is, why is the aragonite forming in conditions for which it is not stable?
That's interesting. The stuff was determined to be aragonitic by means of XRD analysis? Two things I could think of: the particular composition of the water (high-Mg/Ca?), or microbial intervention.
Thanks. Yes, XRD analysis confirmed that it was aragonite. The morphology is pretty distinctive as well which was part of the problem. Because aragonite is orthorhombic it furs up the filters in about half the time that calcite does, meaning that cleaning was required twice as often. I was assuming it was water chemistry related as well, and it may well still be but there are vending machines affected in both hard water and soft water areas. Do you have any references for particular water chemistry controls? Was wondering whether differences in the steel of the vending machine (i.e. substrate affects) could play a part?
I attach a paper that contains references to the influence of Mg (although it isn't the main point of the paper). I am not sure whether the total hardness is important - it may just be the ration of Ca/Mg in the water. But perhaps the substrate could have something to do with it - either its chemistry or its microstructure. The test is whether the same kinds of vending machines always have the problem (substrate option preferred); or different vending machines in the same areas (water chemistry preferred); or that things are random (microbial action preferred - although that could also be assisted by water chemistry and substrate).
Nucleation of aragonite instead of calcite is a well-documented phenomenon in the living world. Practically all shell mollusks build their shells by alternating layers of the polymer chitin and CaCO3. The latter is normally orthorhombic aragonite rather than rhombohedral calcite. This is not due to the ignorance of thermodynamics on the part of the mollusks, which would actually be understandable, but it’s actually a very clever measure they undertake in order to have a shell which, although built under unstable conditions for aragonite, it turs out to be much less soluble in a water environment than calcite, hence providing a much more long lasting home even under changing pH conditions!
I’m not sure this has direct implications in the building-up of aragonite deposits in a coffee vending machine, but it may be a hint into another way in which aragonite may form under non-equilibrium conditions, not necessarily connected with a particular “intelligence” on the part of the machine!
Thanks Romano. Do you know how mollusks get over the thermodynamic barrier? I heard mention that the presence of organic ligands may help provide a suitable substrate that favours the formation of aragonite over calcite - but can't see that being a factor in a steel boiler system.
Organisms essentially get over the thermodynamic barrier by putting energy into the job of making aragonite instead of calcite. But I do hope you don't have mollusks living in the vending machines!
Yes, Christopher, Marlina's explanation is correct in that there is a biological intervention which favors the nucleation of aragonite instead of calcite in mollusks. However, the details of this biological activity are still debatable. One could invoke some templating effect of the polymer chitin or some other kind of reaction which favors the speed at which aragonite crystals are made to grow faster than calcite (i.e. a crystal growth effect). Perhaps some work has been done in this direction. I hope someone can clarify this point in this forum.
As to the growth of aragonite in a steel boiler, unless there is a surface effect also here (by some surface treatment of the steel, or by the steel surface preparation itself?).
Another explanation may be found in a variation of pressure in the steel vessel. Is it possible that an initial calcite deposit, caused by the removal of CaCO3 from the water at near boiling temperature may clog the exhaust of the vessel and build-up enough pressure to favor the growth of aragonite?
Thanks for the comments Marlina and Romano. Have just moved house and have thus been temporarily without internet to continue the discussion.
After careful investigation I can confirm that there are no molluscs within the vending machine. With regards the temporary increase in pressure. I am under the impression that pressure changes within fluid based systems, (and without large differences in vertical height), propagate through the system almost instantaneously. (This is just an impression mind you - certainly willing to be corrected). Either way the stability field for aragonite (@95 degrees celsius) kicks in at about 4 Kbar and surface pressure within a vending machine should be about 1 Kbar. If the vending machine was experiencing localised pressure forces of 4 Kbar I am sure something more fundamental would be wrong with the machine, and with the coffee!
Especially glad to hear about the lack of molluscs in the coffee! I would still try the systematic approach of: type of vending machine (substrate); area (water composition); and then if neither gives any systematic results, I'd think there may be microbes involved (somewhat more difficult to detect than the molluscs). One could probably try and get a culture of any microbes present, but then I am the last one to give advice on how to do that (petri dishes with agar-agar spring to mind).
What about the different coffees, from different kinds of coffee beans or different provenance of the coffee itself?
It is definitely a non-equilibrium growth, hence the formation of aragonite must come from a nucleation booster of that structural form, rather than that of calcite.
The different coffee types should not be relevant as the boiler system precedes the mixing of water with the coffee. I agree with Marlina that the best way forward would be to systematically rule out substrate effects, water composition effects and then possibly bacterial effects. My hunch would be for a water composition control and the need to stick an ion exchange filter upstream to mitigate against this problem.
Very interesting topic! Coincidentally, I have just examined some limscale under the SEM... here is an example, field of wiew is 60 microns.
Nice picture, Vincent. How and where was this limescale formed? It would be useful to compare with the example at hand...
Thanks Romano. Actually, this limescale comes from my domestic kettle. In my county , tap water is extremely hard, and such deposits form within weeks - something like 3-4 weeks for a 500 microns thick deposit.
OK, Vincent, the (pseudo-)hexagonal morphology of many (most?) crystals in your picture is strongly suggestive of an aragonite deposit.
All we need in order to solve Christopher's case now is an XRD pattern of the deposit in your kettle to confirm the possible formation of aragonite in simple boiling water condition in a steel vessel.
As to the reasons for the formation of aragonite instead of (or together with?) calcite, a chemical control by the water contents (in terms of soluble ions) seems the only logical explanation.
Regarding Christopher's idea on how to solve the contingent coffee problem, an ion exchange could certainly keep the machine clean. However, as the word “exchange” implies, it’s a matter of give and take. So, if the removal of Ca implies the exchange with Na (more soluble), the coffee may not only taste different (most likely worse) but it might also contain more Sodium than people are supposed to have as their average suggested daily intake! In other words, in order to save the life-time of a filter…. At this point one has to hope that the coffee would really taste awful ! :-)
Yes, I also have the XRD, this is aragonite. I have seen a few rhomboedrons (calcite?), and extremely elongated scalenohedrons.
Thanks Vincent, that is certainly highly suggestive of a water-chemical control - unless you are very unlucky in your kettle manufacturer! Romani, I think the ion exchange could still work as in theory you wouldn't seek to exchange out the Ca. The chemical control would presumably be excessive Mg or Sr or some other cation whose ratio with Ca leads to the stabilisation of aragonite. As such you would probably start by exchanging out the heavy ions first and exchanging in more Ca.