THE CALCIUM QUESTION Part II©

By Adolf F. Klostermann

(Part I appeared April '91 FAMA)

If conditions are favorable for calcification, the lost mineral content, consisting essentially of calcium (Ca⁺²) and carbonate (CO₃^-2) ions, must constantly be replenished. This replenishment may be in a number of forms, each with specific advantages or drawbacks. However, before any methods arc analyzed, a brief overview of the chemical processes involved in calcification will be presented.

Calcium, although essential, is not the limiting or controlling element. Inorganic carbon, present in much smaller concentration (429mg vs. 27mg per liter), the bulk of which is in the form of bicarbonate, could be considered the limiting substance. The controlling factors are catalysts, inhibitors, pH, temperature, etc.

The bulk of the calcium is directly available as the free ion. Inorganic carbon however, present as the bicarbonate (HC03-), must constantly dissociate to form carbonate (CO₃^-2). Examining the basic chemical reaction in (l) it

Ca⁺² + 2HCO₃^- = CaCO₃(solid) + H₂CO₃ (1)

H₂CO3 = H₂O + CO₂(gas) (2)

can be noted that two ions of bicarbonate are lost from solution as one molecule of solid calcium carbonate is formed and one molecule of dissolved carbon dioxide is liberated. Or, conversely, from the reaction in (2), as one molecule of carbon dioxide (C02) is taken up via photosynthesis two ions of bicarbonate are lost and one molecule of calcium carbonate is deposited. In either case the continuous production of solid calcium carbonate must be associated with the uptake of dissolved carbon dioxide.

To place this in some form of perspective, a volume of standard seawater in equilibrium with the atmosphere (constant carbon dioxide content) could produce roughly 41mg of solid calcium carbonate per liter of water. This ionic loss results in a drop in the aragonite saturation index (ASI) from an initial value of 3.65 to a final value of 1.9. The ongoing rate at this reduced saturation index is likely to be insignificant, since calcification, even under optimum conditions, does not proceed extremely rapidly. Furthermore, the 41mg deposit of solid calcium carbonate is predicated on an uptake of 11 mg of carbon dioxide per liter of water. If the uptake is less, the 50% ASI is reached prior to the expected yield of CaC0₃. Also, since calcification and C02 assimilation arc rate dependent, the equilibrium constants used to calculate the drop in the ASI arc only a guide in a dynamic system. Last, not all the calcium carbonate formed is in the form of aragonite. Calcite, which is more stable, may account for a significant percentage as may non-crystalline and mixed deposits. Considering the 41mg a reasonable estimate and attributing 25% to the formation of aragonite produces a yield of slightly more than two grams from 200 liters (50 gallons) of water. At an aragonite density of 2.93g/cc the total skeletal volume becomes less than three quarters of a cubic centimeter. The production of aragonite is quite small, yet 96% of the initial calcium concentration is still available. The limitation is not calcium, but the saturation condition of the free calcium and carbonate ions.

There are a number of alternatives available to restore the loss of the ions. The first and most obvious method is the water change. Without much calculation it becomes readily apparent that if 100% of the water were changed on a monthly basis, the yield from 200 liters would still be just a little more than 2 grams. Even though water changing may be the most obvious, unless large quantities are processed, the method is not a means of promoting active calcification.

A second alternative is to supply the ions via the addition of fresh water due to evaporation losses. If fresh water remains in intimate contact with calcareous material, in this case aragonite2 14mg of solid CaC0₃ will dissolve per liter of water when equilibrium is reached.3 At an evaporation rate of 2% per day 20% of the ionic loss can be restored in the 200 liters of water in thirty days. Or, the ongoing aragonite production, based on the foregoing estimate of yield, is 0.4 grams per month. Hardly worth the effort. The problem lies in the fact that aragonite, even though more soluble than calcite, still is nearly insoluble in neutral fresh water. Also, of course, there is a dependence upon evaporation.

A somewhat enhanced method with increased availability of dissolved CaC0₃ is to acidify the fresh water with carbon dioxide as it contacts aragonite. The simplest means is to supply air to the water. When the water is in complete equilibrium4 with the air, due to the partial pressure of 3.4xl0-4 atm of carbon dioxide, S7mg of calcium carbonate dissolve in one liter of water. At the same evaporation rate 35 in the previous example, 83% of the ionic loss is restored or 1.7 grams of aragonite could be produced on an ongoing basis each month. This is a more than fourfold increase from the prior method, but is still quite small.

If the fresh water were actually treated with carbon dioxide (the partial pressure raised above 3.4xl0^-4 atm) the dissolution of the aragonite present would be greatly enhanced. At a C0₂ pressure of one atm ⁵ 800mg of CaC0₃ dissolve per liter of water. Not only have the lost ions been restored, there is actually an overabundance. The ongoing aragonite production, based on the same percentage of yield and evaporation would be 24 grams on a monthly basis. However, to achieve this high concentration of dissolved calcium carbonate in fresh water requires a large quantity of dissolved carbon dioxide. If four liters of this solution were added to 200 liters of standard seawater in one dose The pH would be severely depressed, somewhere slightly below seven. Hence, the doses must be incremental and the system must be able to cope with the excess carbon dioxide.

These methods, excluding water change, have added the lost calcium directly as the free Ca⁺² ion and the lost carbon in the form of the three carbon species. Since the ratio of the three species is pH dependent, the total carbon added via each method contains the following percentage of the three species: (See Table l)

Of the total carbon added only 66, 50 and 16% respectively for each method is converted to carbonate. The balance of the carbon must be taken up via photosynthesis or be equilibriated with the atmospheric carbon dioxide. The first method actually has the best utilizable carbon percentage. Unfortunately, the overall quantity is very small. The second method provides essentially all bicarbonates, therefore 50% can be converted to carbonates. The third method that provided the highest yield of ions also has the lowest utilizable carbon. Nearly 70% of the carbon added is in the form of carbon dioxide that must be eliminated from the system.

A fourth method deals with the addition of calcium ions without the associated carbon species. A solution of calcium hydroxide sometimes called by the German "Kalkwasser" can provide large quantities of calcium. A saturated solution contains roughly 1.6 grams of solids, dissolved, per liter. The calcium content is over 800mg and the alkalinity (caustic) is 43 mequ. per liter. If the same measure of yield and evaporation is applied as in the prior methods the potential aragonite production is 64 grams in 200 liters on a monthly basis. Since this solution contains no carbon species the

carbon must be provided as dissolved atmospheric C0₂ and respiratory C0₂ that continuously enter the system. The reaction with the carbon species available in the water, as the calcium hydroxide dose is increased, may be illustrated as follows:

Equation (3) shows a small dose of calcium hydroxide converting dissolved carbon dioxide to bicarbonate. Equation (4) shows an increase in the carbonates as the calcium hydroxide dose is increased. Equation (5) shows the loss of all bicarbonates and equation (6) shows the existence of caustic alkalinity. The calcium hydroxide dose must be limited to the reactions shown in equations (3) and (4) to avoid extremes in pH and spontaneous precipitation of calcium carbonate. As with method three, which contained excess carbon dioxide, this method calls for incremental additions in most cases. If four liters were added to 200 liters of standard seawater in one dose the pH would rise to 8.6. A modification to this method, that gains independence from evaporation, is to feed a slurry. This method must be under positive pH control since the concentration of the calcium hydroxide added can easily raise the pH above nine.

In summary, the potential aragonite production, for the methods described, in 200 liters of water on a monthly basis is as follows:

All methods, discounting the water change, deal with adding the lost ions via evaporated fresh water. Obviously only the latter two provide significant results. If these methods are combined larger increments may be added at one time without the rapid changes in pH. Up to this point the added ions consisted of Ca⁺² (calcium), HCO₃^-(bicarbonate), CO₃^-2 (carbonate) and OH^- (hydroxide) along with the molecule H₂C0₃ (dissolved carbon dioxide). None of these substances are cumulative. They are all utilized or expelled or deposited, and the overall water chemistry does not change.

There are other methods of supplying calcium and carbon that deal with the addition of chemicals that leave cumulative ions behind. Using the method that produced the highest potential aragonite production as a reference, the quantity of chemicals that must be added to produce that equivalent amount of calcium carbonate can be determined. Since the aragonite produced was only 25% of the total calcium carbonate, the total calcium carbonate must have been 256 grams. If calcium chloride and sodium bicarbonate are used to provide the calcium and carbon respectively, the following equations apply:

From equation (8) it can be seen that for each molecule of solid CaCO₃ formed one molecule of Ca(HCO₃)₂ is consumed, and from equation (7) for each molecule of Ca(HCO₃)₂ formed two molecules of NaCl (sodium chloride) are also formed. From the law of equal proportions 256 grams of CaCO₃(s) is produced from 415 grams of Ca(HCO₃)₂ which in turn is produced from 430 and 284 grams of sodium bicarbonate and calcium chloride respectively. The byproduct of unwanted cumulative molecules is 300 grams of sodium chloride. Using this technique eliminates the necessity of small incremental feeding of chemicals, since wide variations of pH are not experienced. However, there is a definite side affect. There

is an accumulation of both sodium and chloride ions. If the initial sodium and chloride concentrations were 11 and 19.8 grams per liter, the addition of 300 grams of sodium chloride to 200 liters of standard seawater in one month would raise the concentrations to 11.59 grams of sodium and 20.7 grams of chloride per liter. In six months the sodium concentration is 32% higher and chloride concentration is 28% higher. This increase in major ion content would reflect an increase in salinity. If the rise in salinity is corrected by the addition of more freshwater, the balance of the constituents are effectively reduced. Small monthly water changes do not negate this effect sufficiently.

In conclusion, there is no assurance that aragonite producing corals will grow by leaps and bounds if any of these methods are used for calcium and carbon enrichment. However, there definitely will be no growth if nothing is done. It depends on the individual aquarist to choose the method or combination of methods that fit into the scheme of his system and the potential results desired.

ABOUT THE AUTHOR…

The author was born in Germany in 1942 and immigrated to America in 1955. After finishing his education he joined the United States Air Force and, during his travels had the opportunity to see and experience his first coral reef off the island of Okinawa in the Pacific Ocean. The spark of interest in the mysterious creatures of the sea, that had always been there, was finally ignited. A hobbyist was born in the early sixties. After leaving the Air Force the author pursued a career in electronics. He picked a company in Ft. Lauderdale to be near the only reefs in the United States. In 1986, and just prior to his retirement as Vice President of Sunair Electronics, he founded Coral Reef Research. The company is dedicated, through continuing and from prior years of research, to the development of a saltwater processing system that maintains the original, essential, water chemistry of closed systems indefinitely.

. . . At this date, indefinitely is 14 years.