In a recent article,¹ Ron Shimek showed that two salt mixes and natural seawater were more conducive to the development of sea urchin embryos than were two other salt mixes and the water from two reef aquaria using one of them. In that article, Shimek suggested that elevated metal levels, such as copper, could have been responsible. The two salt mixes with poor development are reported to have higher levels of a number of potentially toxic metals than the other two salt mixes or natural seawater.

While there may be other explanations for the observed differences, including ammonia, nitrite, pH, organic compounds present intentionally or as impurities, and the elevation or depletion of many different inorganic chemicals, the hypothesis concerning toxic metals is quite reasonable. As a result of that article, as well as earlier ones that showed the elevated levels of certain metals in reef aquaria and salt mixes,^2,3 many aquarists have become interested in finding out what they can do to lower the toxic metals concentrations in their aquaria. This article starts with the premise that toxic metals are responsible for the biological effects that Shimek observed, and explores what options aquarists have to solve the problem.

Some have assumed, seemingly incorrectly, that if they use natural seawater or one of the salt mixes shown to have lower metals, that the problem will be solved. Unfortunately, some of the things that aquarists need to do in maintaining reef aquaria may well be a primary source of metals. Specifically, the foods and calcium and alkalinity supplements that aquarists use are a big source of metals to reef aquaria. The complication of these inputs may make it difficult, if not impossible, to maintain an aquarium with natural levels of these metals. Both foods and supplements have enough metals in them to take natural seawater to the levels found in the "high metals" salt mixes in a fairly short period of time. In evidence of this possibility, the one aquarium in Shimek's study of reef aquaria that used natural seawater had amongst the highest levels of many of these toxic metals.² Habib Sekha has also indicated that he has obtained similar results on two other aquaria using natural seawater (pers. comm.). That is, the copper and nickel levels were equivalent to those levels found by Shimek in aquaria using artificial salt mixes such as Instant Ocean.² The copper levels in these three aquaria using natural seawater are also comparable to the levels found in my own tank using only Instant Ocean salt mix (10-13 ppb copper).

Additionally, one would expect that the total input of these different metals would be very different between reef aquaria. And yet, all of the aquaria in the study mentioned above had copper concentrations within a factor of two of each other.² That suggests to me that the controlling factor is more likely to be export than import, and this possibility and how it impacts the actions of aquarists wanting to maintain aquaria with low metals is discussed at the end of this article.

In this article, I will focus on those potentially toxic metals that have been found to be elevated in operating reef aquaria,² including copper, nickel, zinc, molybdenum, and cobalt. I will also use the premise that aquarists want to maintain an aquarium with a lower concentration of such potentially toxic metals, and will provide some guidance and general information on how aquarists might usefully go about operating aquaria in that fashion. In some cases, these techniques will relate to many metals, and in some cases they will only apply to certain metals, such as copper.

Using methods described in this article, and other articles to follow that will attempt to further quantify how effective these suggestions are, some aquarists can potentially begin to operate aquaria with much lower levels of such metals. Whether such a reduction will have important advantages that aquarists will appreciate can only be demonstrated when reef aquaria are operated under such low metal concentrations and compared to reef aquaria operated in more traditional means. Once that has been accomplished, then aquarists will be able to look at the results and determine for themselves if this is a necessary, or at least a prudent action for them to adopt.

To begin, however, it is important to note that there are no simple solutions to this potential concern. The two obvious ways to consider lowering the metals concentrations in aquaria are to add less, and export more. There are things that aquarists can do to immediately reduce additions of toxic metals, and these will be detailed in this article. Whether the "normal" export systems (skimming, growth and harvesting of macroalgae, activated carbon, etc.) can be useful may in part depend on how much of an input load for which they are expected to compensate. Other "chemical filtration" systems have been promoted to hobbyists for this purpose, but in the end may not be successful in reducing metals levels below those already present in many aquaria, regardless of the inputs. Still other approaches to export, such as binding metals to calcium carbonate media, are more speculative and will be mentioned as suggestions that will be reported on in more detail in future articles.

In order to understand how and why certain methods of dealing with metals may work (or not), we also need to know the nature of these metals in aquaria. Many aquarists will naturally think of these metals as being like the more familiar calcium ion, floating free through the water on its own. The true situation for many of these metals is, however, much more complicated. It is expected that more than 99.9% of the copper ions present in the water column of reef aquaria are attached to something else, and what they are bound to has a big impact on how they behave. Consequently, before getting into metal removal methods, this article will describe what forms the metals take in seawater, and what forms they are expected to take in reef aquaria.

Metals in Seawater and Aquaria

Different metals take different forms in seawater. Some exist primarily as free metal ions in solution. Of those that aquarists are most familiar with, sodium (Na⁺), potassium (K⁺), calcium (Ca⁺⁺) and magnesium (Mg⁺⁺) fall into this category.⁴ All of these may, to some extent, form complexes with other ions and organic materials, but the free ion tends to predominate in seawater and in typical aquaria.

The metals that we are most concerned about in this article, however, have much more complex behavior. Copper, for example, exists in a multitude of forms.⁴ In natural seawater, it has recently become clear that copper is almost completely bound by organic materials.⁵ Many of these organics are called chelators. A chelating agent is one that can grab onto the copper from two or more directions at once.

In natural seawater, these organics take many forms. Humic and fulvic acids, for example, are two of the most important types of materials that bind copper and other metals in seawater.⁶ They are also known to greatly reduce the toxicity of metals because in many cases it is the free copper that is the most toxic.⁵ These classes of organic materials comprise what remains when proteins, carbohydrates, and many other naturally occurring organic materials are biodegraded to a state where further degradation is very slow. Humic and fulvic acids (the distinction between the two being just that humic acids are more hydrophobic than fulvic acids) have a wide range of structures and physical properties. They typically are high molecular weight organic acids, with sizes ranging from 500 to 10,000 daltons (grams/mole). They can also be parts of larger assemblies of organic materials that would be called colloidal (very small particulates) rather than "dissolved". The humic and fulvic acids are comprised of amino acids, sugars, amino sugars, fatty acids, and other organic functional groups. Different localities and depths in the ocean have different amounts and specific types of these organic materials present. Typical values for the total dissolved organic carbon are on the order of 1 ppm carbon for tropical surface seawater.⁶ Humic substances typically account for about 10-20 % of that total, and fulvic substances can account for more than 50%.⁶

Within these structures will be sites where several carboxylic acid, phenolate, thiolate, amino, or other metal-binding groups come together. These sites are where a metal ion will be most strongly bound. Structurally, it is hard to show a "typical" humic acid binding to copper, but the structure below shows one possibility:

In this figure, the central positively-charged copper ion (Cu⁺⁺) is chelated by the larger humic acid shown in green. It is bound ionically by two negatively charged carboxylic acid groups and complexed by one neutral amino group. Together these three groups may hold the copper ion more strongly by many orders of magnitude more strongly than could any individual binding group.

In the very extensive book "Biogeochemistry of Marine Dissolved Organic Matter",⁶ it is stated:

"It is now widely accepted that the chemical speciation of most bioactive metals in seawater is regulated by strong complexation with natural organic chelators…The cycling of bioactive metals therefor is intrinsic to the behavior of this subset of organic constituents."

And also:

"The collective findings establish that a significant component of bioactive, or nutrient, metals (Mn, Fe, Co, Ni, Cu, Zn, Cd) occur in the colloidal phase along with numerous other trace metals."

Free Metal Ions

In seawater, copper is expected to be bound to organic materials. In one recent study of copper in natural seawater, more than 99.97% was bound to organic materials.⁵ Other metals, such as zinc, may not be as extensively chelated. In aquarium water, where the level of both metals and organics can be higher than in seawater, the percentage bound to organics may be even greater. Nevertheless, unchelated metals are very important. In the case of copper, for example, the unchelated copper ions may represent that portion of the total copper that is toxic to many organisms.⁵ These inorganic forms of copper and other metals are also expected to predominate in freshly mixed artificial salt water that has not been exposed to sources of organic materials.

Assuming that the organism does not take up the entire organic molecule to which the metal is attached (and many humic acids are known to be poorly taken up due to their refractory chemical nature⁶), then chelated metals are often much less toxic than unchelated metals. In seawater, for example, the speciation of copper (i.e., whether and how it is chelated) is often much more important for understanding overall toxicity than is the total copper concentration.⁷

The portions of metals in seawater that are not bound to organic materials are very complicated in their own right. Copper, for example, takes at least 7 different soluble inorganic forms in seawater.⁴ It is comprised of Cu⁺⁺ (3.9% of the inorganic copper), CuOH⁺ (4.9%), Cu(OH)2 (2.2%), CuSO4 (1%), CuCO3 (73.8%), Cu(CO3)2^-- (14.2%) and Cu(HCO3^- )⁺ (0.1%). Similar complications hold for many of the metals that we are concerned with in this article.⁴

Speciation and its Effect on Binding Metals in Aquaria

Since these metals take many different forms in aquaria, one must consider the nature of these different forms when developing methods for removing them. For example, metal ions such as Cu⁺⁺ or Ni⁺⁺ will never absorb at the air/water interface to permit selective removal by skimming. However, if the same metals were bound to an organic material that itself adsorbed to the air/water interface, the metals might well be exported by skimming. Similar concerns relate to claims about metal removal using activated carbon, polymeric ion exchange and complexation resins, and binding to inorganic materials like iron oxides and calcium carbonate. In fact, any proposed method of metal removal will be significantly impacted by the nature of the metal speciation. Depending on what is added to any particular aquarium, the speciation may actually vary from aquarium to aquarium, potentially making generalizations about them less useful.

Further, any experiment that purports to show how well something works must be carried out under conditions of the real speciation present in aquaria. Tests run in artificial salt water (or worse yet, fresh water) are not necessarily of any use in predicting efficacy of metal removal if not carried out in the presence of the typical organics present in aquaria. So when you see products make claims about their ability to remove metals, be skeptical unless you understand what conditions the claims relate to.

Input of Metals: Foods

If the goal is to reduce metals, then looking at the foods that you feed can be important. It will be impossible to eliminate all additions of metals this way, because all marine-sourced materials contain significant amounts of metals that they absorbed when growing in the ocean. Some of these metals are used by the organisms involved, and some are just incidentally accumulated. Nevertheless, there are some things that you might consider when selecting foods if you want low metals.

It turns out that there has been a fair amount of study of many foods for certain metals because they impact human health in various ways. For healthy people looking to ingest adequate copper levels in food, the USDA recommends shellfish, among other things, because they have a naturally high level of copper and zinc.^7,8 Some fish and shellfish may also be unusually high in many metals (including copper, zinc, cadmium, mercury, and lead) because of local pollution in the areas where they are harvested.^9,10

People with Wilson's disease have problems dealing with elevated copper levels, and they need to restrict dietary copper. At a website designed for people with this condition is a table listing the copper levels in many foods, including many that we feed to our aquaria (Table 1).

From this table it is clear that one can select lower copper foods when shopping at the grocery store. Scallops and shrimp, for example, would be much better choices than clams, crab, or lobster. Also, the viscera of squid and crabs contain much more in the way of heavy metals than does muscle tissue.¹⁰ Over the course of a year, these contributions really add up. If you add 5 grams of each of the foods in Table 4 to a 100-gallon aquarium every day, the addition over a year amounts to 178 ppb of copper using lobster and 1.3 ppb of copper using scallops. For comparison, the amount of copper in the salt mixes described in Shimek's article¹ as being high in metals are on the order of 100-200 ppb copper (but only 18 ppb copper in an earlier article²), and those low in copper were 1-40 ppb copper (for comparison, my aquarium using only Instant Ocean salt mix presently has 10-13 ppb copper). So obviously, the choice of foods can potentially make a big impact on the copper levels.

Many aquarists feed commercial foods to their aquaria, rather than fresh seafood. In a study of the amounts of different elements in certain foods,¹¹ Shimek presented the results shown in Tables 2 and 3. While none of these foods appears as high in copper as lobster, lancefish is close and the differences between the various foods are significant. In these tables I have highlighted those values that stand out as unusually high in red and unusually low in green. Bear in mind that some of these foods contain substantial water, and so are naturally more "dilute". For that reason, I included the first line in each table that shows the calories/gram for each food. In this sense, it is easy to see that the "wet" foods are about 4-5 times less concentrated than the dry foods, so in looking at metals, their concentrations need to be multiplied by 4-5 to get equivalent values in terms of actual dosing.

Based on this metals analysis alone, and no other nutritional properties, the Tahitian Blend would seem to be a good overall choice if lower metals were a significant goal (However, Eric Borneman has indicated that Tahitian Blend is a plant material suspension that is of a particle size that will be unusable by many organisms (pers. comm.)). If we knew exactly what specific metal to be most concerned with, the choice might well be different.

Input of Metals: Limewater

Limewater (the English word for kalkwasser, the solution that forms when either calcium oxide or calcium hydroxide is dissolved in water) turns out to be a very useful way to add calcium and alkalinity to a reef aquarium while adding a minimal amount of certain heavy metals. This result is not because the starting calcium hydroxide or oxide is especially low in impurities, but because it can be self-purifying if used correctly. It has been known for nearly 100 years, for example, that copper is not soluble in limewater. According to some folks from the US Department of Agriculture in 1908:

"The solutions obtained by adding excess of lime to copper sulphate solutions of various concentrations were found to be free from copper and were alkaline."¹²

They report that blue copper hydroxide precipitates from the solution. Of course, analytical techniques are much better now than they were in 1908, and the solutions that they thought to be "free" of copper still contain some dissolved copper at low levels, but much less than the initial solutions. Since that time, lime has been used for a variety of purposes relative to removal of copper from metal etching solutions,¹³ wastewater,¹⁴ and sewage.¹⁵ It has also been used to prevent the dissolution of copper from the insides of pipes.¹⁶ The use of lime to precipitate metals has also been shown for tin,¹⁷ and nickel.¹⁸

Why does this work and how can we use it?

It turns out that at high pH, many metals form insoluble hydroxide and oxide salts. In fact, high pH is a generally useful way to precipitate or limit the solubility of many metals.19-23 If the limewater solution is allowed to settle so that only clear limewater is dosed, then these insoluble hydroxide salts will precipitate from solution and collect on the bottom of the limewater reservoir. For example, copper in the starting lime (or in the water used to make it) will precipitate as copper hydroxide:

Cu⁺⁺  +  2OH^-  à  Cu(OH)2 solid

Consequently, as long as the solid precipitates are not dosed to the aquarium, then the solution may well be self-purifying for some of these metals. This has been evidenced by the fact that some aquarists find blue precipitates (perhaps copper hydroxide) remaining after they have used limewater. Exactly how effective this process is at reducing the concentration of metals depends on the metal, the pH, and whether vinegar (and how much) has been used in the limewater (vinegar alters the pH and hence the solubility of many hydroxide species, it also potentially allows for soluble metal acetates to exist in solution). Some metals can form soluble complexes under the conditions present in limewater. Copper, for example, can form the unusual species Cu(OH)3^-. It is the solubility of this species, rather than bare Cu⁺⁺ that limits how effective limewater is at such self-purification.²⁴

The calcium oxide that I use from the Mississippi Lime Company is food grade, but still has many impurities. Here's the typical analysis as well as the required limits to food grade CaO:

Notice that the typical analysis of the calcium oxide that I use (Table 4) can contain 2 ppm heavy metals, 0.1% aluminum, 12 ppm manganese, and less than 0.5 ppm lead.  If one were adding 2% of the aquarium volume in saturated limewater (0.0204 moles/L CaO) every day for a year, the following species would be added to the water (Table 5).

Worse yet, the food grade specification itself permits up to 15 times more heavy metals than the typical analysis, for a whopping 250 ppb of total heavy metals and 60 ppb of lead additions per year. Depending on the nature of those "heavy metals" those additions are potentially quite significant compared to the 10-40 ppb of copper found in many aquaria, and the much lower levels found in natural seawater.

Unfortunately, not all metals will be removed by precipitation as hydroxide or oxide salts. Aluminum, for example, forms certain complexes in water that are soluble at high pH. In future articles, I hope to show, either through literature references or via experiment, how effective limewater is at limiting the delivery of many of the metals that we have been most interested in within this article.

Input of Metals: CaCO3/CO2 Reactors

The amounts of metals that are added to an aquarium when using a CaCO3/CO2 reactor can also be determined. Unfortunately, such a system is not "self-purifying", unlike limewater. Consequently, most impurities in the CaCO3 media make it into the aquarium water. The impurities present in such media varies with the source or brand of the media, as has been shown in different articles by Craig Bingman²⁵ and Greg Hiller²⁶. If we make the assumption that we want the same total amount of calcium and alkalinity as in the limewater case described above, then we can calculate the following amounts of metals added over a year: Those blocks colored red add more than the amount detected in Shimek's study on Instant Ocean (IO) salt mix² (many more should perhaps be colored red, but with none detected in IO salt, it is not always possible to determine if it is higher or lower than the IO salt mix).

As can be seen, the amount added over the course of a year can be quite substantial. In particular, for several of the species of most interest from a toxicity perspective (copper, zinc, nickel, aluminum, etc.) the amount added is enough to take an aquarium from zero metals to more than that present in a "high metal" salt mix, like Instant Ocean.²

Input of Metals: Two-Part Calcium and Alkalinity Supplements

When many aquarists first hear that the two-part calcium and alkalinity additive systems (e.g., B-ionic, C-balance, Kent Tech CD) contain copper and other metals, their reaction is often quite negative. The claim of B-ionic, for example, is that all ions present are in natural seawater ratios except calcium and the bicarbonate and carbonate that provide alkalinity. To fulfill that claim, copper and every other metal must be present. Moreover, if they are truly present at those levels, then they are not going to contribute to elevated metals, but will, in fact, tend to actually reduce them slightly over time.

Most aquarists realize that if a metal such as copper is elevated in an aquarium, that a water change with natural seawater will have a lowering effect on the concentration of that metal. If these two-part additives are made exactly as claimed, then the net effect of using them is the same as a small water change with natural seawater (in addition to supplementing calcium and alkalinity). For more skeptical aquarists, I have shown how this works out mathematically in a previous article.

That said, I've seen no independent analyses of any of these products. They may, in fact, not meet their claim of having all ions in natural ratios. I hope to experimentally test some of these products in the future.

Export of Metals: Macroalgae Growth and Harvesting

There are a number of potential ways to export metals from reef aquaria. One that is already in use by many aquarists, although perhaps not optimized for metal export, is the growth and harvesting of macroalgae. There have been a great many studies of the metals content of macroalgae growing in the oceans of the world, and it has been found that these algae accumulate metals to a significant degree. Table 7, for example, summarizes some of the literature data.

As can be seen from this data, the amounts of metals in these macroalgae is significant, and not very different from that reported for nori and other foods earlier in this article. They are also similar to the values reported for Caulerpa sp. harvested from aquaria (~12 ppm dry weight copper, 200 ppm dry weight zinc, 10 ppm dry weight lead, etc.).³⁰ If significant amounts of these macroalgae are removed from an operating reef aquarium, then the amount of export can be significant. Depending on the type of food being used, and the extent of macroalgae growth and export from the aquarium, the net export of metals via macroalgae might even be more than the input from foods.

Export of Metals: Activated Carbon

Activated carbon is known to bind copper from freshwater,³¹ and it is obviously known to bind organic materials in both marine and freshwater aquaria. Since much of the copper present in the water column of aquaria will be bound to organics, it stands to reason that some of these organics will contain copper. I've not seen any study of this effect, however, and so the usefulness of activated carbon at metals export remains unknown.

As a first pass, it should be relatively easy to study by determining the copper concentration in a sample of aquarium water (such as mine containing 10 ppb or so of copper), exposing it to carbon, and redetermining the copper concentration. In this experiment, copper is a good test metal because it is largely bound by organics and so should be a representative "best case" for metals export via activated carbon. That experiment should at least indicate if there is any potential use of carbon for aquarists interested in low metals, and I hope to report on it in a future article.

Export of Metals: Skimming

None of the inorganic forms of any of the metals that we are interested in will be attracted to an air/water interface. In fact, because of their extreme hydrophilicity, they are generally repelled by it. Consequently, these forms will not be appreciably removed by skimming.

Those metals that form complexes with organic materials will, however, be removed by skimming as the organics themselves will carry the metals out. It is difficult to say, a priori, how effective skimming will be in such an application. Some initial studies on skimmate show it to have an appreciable metals content (including the metals of interest to us),³² though in order to determine how effective this process is for any given aquarium, one must do careful balance studies on the skimmate and the various sources of metals to any given aquarium.

Obviously, however, the more efficient is the skimmer, the more likely is this pathway to be important. Aside from boosting the skimming with a more powerful skimmer, some folks have considered whether certain organic materials can be added to an aquarium for the specific purpose of binding metals and subsequently being skimmed out. Many commercial polymers, for example, will likely function in this context. Polyethyleneimine, for example, is shown complexing copper in Figure 2. The concern with any material added for this purpose is toxicity. Without knowing whether the material to be added is safe for EVERY organism in the aquarium, it is not possible to know whether it is worth the trade off of reduced metals for a different potential toxin.

Export of Metals: Poly-Filters

Many aquarists have heard that Poly-Filters (made by Poly-Bio-Marine) absorb copper and other heavy metals. Poly-Filters are essentially comprised of an organic polymer that is designed to bind to a wide variety of chemical compounds in aquaria. As with all materials that bind metals, the higher the concentration of metal in solution, the more metal will be bound. Unfortunately, this fact has lead many aquarists to misunderstand whether Poly-Filters might actually help them reduce metal levels below that present in typical reef aquaria (10-40 ppb). In this section I am not discussing whether one can make a polymer that will bind copper and other heavy metals from aquarium water (that's a different discussion for elsewhere in this paper and others, but it is possible that modified Poly-Filters might work in that context). What we are interested here is in whether there is any information to suggest that currently available Poly-Filters are effective at reducing the copper concentrations below the 10-40 ppb copper reported for all of the marine aquaria in Shimek's study, and in my own aquarium (10-13 ppb copper).

It turns out that, unlike most manufacturers, Poly-Bio-Marine provides some nice data and makes the results especially clear for us. Unfortunately, what they say is that it won't work for many metals in artificial seawater. In fact, they have specifically designed these filters to not take out copper below 30 ppb. Here's a series of quotes from their website:

“ASTM Standard D 1141 lists only six (6) trace elements which are : Barium (99.4 mg/L), Manganese (34.0 mg/L), Copper (30.8 mg/L), Zinc (9.6 mg/L), Lead (6.6 mg/L) and Silver (0.49 mg/L).”

Note:  mg/L is the same as ppb (parts per billion)

Then they note:

"Our next section will go into details of how Poly-Bio-Marine, Inc.'s special manufacturing process prevents Poly-Filter from sorbing those trace elements and other major or minor synthetic seasalt components."

"In order to make a Poly-Filter not capable of sorbing trace elements we must first saturate each Poly-Filter with the trace elements found in synthetic seawater."

"Upon completion Poly-Filter will not sorb trace elements nor calcium, magnesium, strontium or fluoride."

So they add the metals listed above to the Poly-Filters during manufacturing in order to prevent them from bringing down these metal concentrations when used in aquaria. In reality, I don't know whether their statements are accurate or not in relation to real aquaria, because all of the tests were in freshwater and synthetic seawater, not in aquaria where some of these metals (especially copper) will be largely bound to organics. Nevertheless, taking their claims at face value, one is forced to conclude that Poly-Filters will not be generally useful in reducing metal concentrations below the levels shown in Table 8 when used in raw artificial seawater. In this case, only zinc appears to be at a level such that Poly-Filters will remove substantial amounts from aquaria.

It is entirely possible that these filters will be more effective than described below at removing metals when the metals are bound to organics in real aquaria. After all, these filters claim to remove organics as well. However, it is also possible that they won't be effective, and testing them under actual reef aquarium conditions is something that I hope to provide in future articles.

Export of Metals: Other Systems

There are a variety of other systems that one might imagine for metals export from reef aquaria. Some of these are briefly described below.

1. Binding of metals to inorganic media, like calcium carbonate, iron oxide or hydroxide, alumina, and clay (e.g., kaolin). In this application, these materials would be added to some sort of filter, like a fluidized sand bed or maybe even something like a CaCO3/CO2 reactor with no CO2. It is well known that metals bind to many of these materials.^33-44 Iron oxide, for example, has been used to remove copper and lead from wastewater.^33,34 The binding of copper has even been studied on many media in seawater in the presence and absence of organic materials.³⁵ Nevertheless, how effective these systems will be in a real aquarium can only be determined through experimental means, and no such results are presently available.

Interestingly, absorption of metals onto growing calcium carbonate surfaces, such as coral skeletons, may well be the primary export mechanism for metals in reef aquaria. Besides the fact that it is known that such surfaces absorb metals, it is one thing that is common to all reef aquaria. The inputs and other export mechanisms vary substantially between reef aquaria, but if this export mechanism is the one that ultimately limits the concentrations of certain metals, such as copper, then that might explain why so many disparate aquaria have similar copper levels.² Likewise, if this export mechanism is so important and widespread, then it may also be one that can be manipulated to lower the metals content. Hence the suggestion of using calcium carbonate media to bind and remove metals.

2. Polymeric resins specifically designed to bind metals. I've patented a number of such materials for human pharmaceutical use (to bind iron, copper, etc), and there are vast numbers of other similar applications that have been documented. Suitable materials would be those that strongly complex metals, and could contain any of the many organic functional groups know to interact strongly with the metals of interest, including amines, hydroxamic acids, hydroxylamines, carboxylic acids, phenols, thiols, and disulfides. These materials would look and act rather like a Poly-Filter. In fact, Poly-Filters provided without any metals content might even fit the need. How effective these would be in a real aquarium remains to be established. Some are commercially available and I hope to test some in aquarium water.

The nature of the metal speciation will greatly impact how well these materials work, and they might work much more effectively in the environment of raw artificial seawater than they will in a real aquarium environment, where chelation by organics is important. Materials can be devised that will bind the metals very strongly and can theoretically take them away from the organic material to which they are initially bound, but how fast that will happen, how much competition there might be from ions that we do not want to bind (calcium, magnesium, etc) and how much it might cost to do so leaves this option uncertain.

Some materials of this nature are available now, including the "Toxic Metal Sponge" by Kent Marine and Cuprisorb by Seachem. How well these will work at lowering the metals present in typical operating aquaria to levels approaching natural seawater remains to be established.

3. Earlier in this article I mentioned that many aquaria kept under different conditions with different types of salt water (both natural seawater and artificial saltwater) often seem to end up with similar levels of metals. Copper, for example, seems to range between 5 and 40 ppb for many aquaria. Why would that be? Certainly, binding to calcium carbonate surfaces, as described above, might be the controlling factor in driving all of these different aquaria to similar copper levels.

Another possibility, however, is biological control. Copper is known to bind to biological surfaces, ranging from bacteria to corals. Millero shows a graph of copper binding to bacteria, for example, and it demonstrates that the copper binding is a steep function of the copper concentration in the 5-50 ppb range.⁴ At 50 ppb there is about four times as much copper bound as at 5 ppb. Above 50 ppb (to about 200 ppb where the data ends) there is no additional binding. Because of this copper binding, as the dissolved copper concentration begins to rise within the range of 5-50 ppb, copper will bind to bacteria and blunt the observed rise. Likewise, as the copper concentration falls, copper may be released and blunt the observed drop. Consequently, the copper concentration may be "buffered" by binding to bacteria and other biological surfaces in the range that is typically found in aquaria. This binding may explain why so many aquaria have similar dissolved copper levels.

While it is not easy to imagine using this fact to lower concentrations below about 5 ppb (where little binding occurs), it is clear that this pool of biologically bound copper may make lowering the copper concentration in aquaria substantially more difficult than it would be in a tank that only contained water.

Summary

In this article I have provided background information on the nature of metals in reef aquaria in order to assist aquarists in their goals of understanding their aquaria, and potentially learning to operate them with low metals content. Whether this second goal ends up being worthwhile will only be known when the fruits (if any) of such efforts become generally recognized.

There are many sources of metals in aquaria, perhaps many that are not mentioned here. Reducing this input is one obvious way to try to reduce the metals content of aquaria. In particular, selecting foods and calcium and alkalinity supplements with low metals contents will be key to achieving this goal.

Increasing the export of metals is the other obvious way to achieve this goal. Some combination of activated carbon, skimming, specific metal-binding media of various types, and export of biological tissues (macroalgae, corals, etc) may all end up being part of the plan.

Both the reduction of imports and the increase of exports will likely be necessary to achieve metals levels approaching natural seawater. Along the way, however, aquarists may have significant difficulty in attaining their goal. If the metals in the water column of aquaria are "buffered" by binding of the metals to biological and inorganic surfaces, then there may have to be substantial changes in the total burden of metals in the tank before significant changes are observed in the water column.

In future articles I hope to provide additional information on how well some of the more unusual methods described in this article actually succeed.

In the meantime… Happy Reefing!

References:

1. The Toxicity of Some Freshly Mixed Artificial Seawater; A Bad Beginning for a Reef Aquarium by Ronald L. Shimek. Reefkeeping.com. Volume 2. Number 2. March 2003

2. It's (in) the Water by Ronald L. Shimek. Reefkeeping.com. Volume 1. Number 1. February 2002

3. The Composition Of Several Synthetic Seawater Mixes by Marlin Atkinson and Craig Bingman Aquarium Frontiers March 1999.

4. Chemical Oceanography, Second Edition. Millero, Frank J.; Editor. (1996), 496 pp.

5. Intercomparison of voltammetric techniques to determine the chemical speciation of dissolved copper in a coastal seawater sample. Bruland, Kenneth W.; Rue, Eden L.; Donat, John R.; Skrabal, Stephen A.; Moffett, James W. Institute of Marine Sciences, University of California at Santa Cruz, Santa Cruz, CA, USA. Analytica Chimica Acta (2000), 405(1-2), 99-113.

See also:

Chemical speciation of copper and zinc in surface waters of the western Black Sea. Muller, Francois L. L.; Gulin, Sergei B.; Kalvoy, Ashild. Department of Chemistry, University of Bergen, Bergen, Norway. Marine Chemistry (2001), 76(4), 233-251.

6. Biogeochemistry of Marine Dissolved Organic Matter. Hansen, Dennis A.; Carlson, Craig A.; Editors. USA. (2002), 774 pp. Publisher: (Academic Press, San Diego, Calif.)

7. The measurement and evaluation of zinc and copper in foods by the methods of microwave oven-based wet digestion technique and atomic absorption spectrophotometry. Andoh, Kaeko; Saitoh, Yasuko; Takatani, Akemi; Takahashi, Fumie; Tazuya, Yoko; Tsunajima, Kumiko; Motoki, Chiyomi; Yasuoka, Kiyoko; Yamaji, Yoshiko; Matsuoka, Choh. Tokushima Bunri Univ., Tokushima, Japan. Tokushima Bunri Daigaku Kenkyu Kiyo (1982), 25 113-25

8. Content of trace elements in seafoods from the Gulf of La Spezia. Baston, Walter; Cozzani, Ermanno; Nana, Alda; Tesei, Luciano. Lab. Chim. Prov., La Spezia, Italy. Bollettino dei Chimici dell'Unione Italiana dei Laboratori Provinciali, Parte Scientifica (1981), 32(S1), 35-40.

9. Trace elements in seafood organisms around southern California municipal waste water outfalls. Young, David R.; Moore, Michael D. California State Water Resour. Control Board, Sacramento, CA, USA. Publication - California State Water Resources Control Board (1978), 60 104 pp.

10. Survey on the heavy metal contents of food. Part 3. Ohtsuki, Kazuko; Momokawa, Akira; Sato, Nobutoshi; Hosoya, Yoshitaka; Sakai, Keiichi; Hayasaka, Kunio; Onodera, Tsuneyuki; Matsuzaki, Sakuo. Eisei Kenkyujo, Japan. Miyagi-ken Eisei Kenkyusho Nenpo (1976), 84-8.

11 Necessary Nutrition, Foods and Supplements, A Preliminary Investigation by Ronald L. Shimek. Aquarium Fish Magazine. 13: 42-53.

12. The Action of Lime in Excess on Copper Sulphate Solutions. Bell, J. M.; Taber, W. C. Bur. Soils, U. S. Dept. Agr, J. Physic. Chem. (1908), 11 632-36.

13. Recovery of copper from spent etching solution containing copper chloride. Tatsumi, Kenji; Wada, Shinji; Yukawa, Yasuhiro. (National Institute of Advanced Industrial Science and Technology, Japan; Mitsubishi Corporation). Jpn. Kokai Tokkyo Koho (2002), 5 pp.

14. Heavy metals in wastewater: modeling the hydroxide precipitation of copper(II) from wastewater using lime as the precipitant. Baltpurvins, K. A.; Burns, R. C.; Lawrance, G. A. The Department of Chemistry, The University of Newcastle, Newcastle, Australia. Waste Management (Oxford) (1997), Volume Date 1996, 16(8), 717-725.

15. Effects of lime treatment on fractionation and extractabilities of heavy metals in sewage sludge. Hsiau, Ping-Chin; Lo, Shang-Lien. Graduate Inst. Environmental Eng., Natl. Taiwan Univ., Taipei, Taiwan. Journal of Environmental Science and Health, Part A: Environmental Science and Engineering & Toxic and Hazardous Substance Control (1997), A32(9 & 10), 2521-2536.

16 Prevention of copper dissolution from city water pipes by pH control. Tateishi, Keiichiro; Inoue, Michio; Hirose, Kenichi. Japan. Osaka-shi Suidokyoku Komubu Suishitsu Shikensho Chosa Hokoku narabini Shiken Seiseki (1975), Volume Date 1972, 24 38-42.

17. Precipitation of tin(IV) from hydrochloric acid solutions by calcium hydroxide. Toptygina, G. M.; Evdokimov, V. I.; Eliseeva, N. A.; Badanin, V. S. Inst. Obshch. Neorg. Khim. im. Kurnakova, Moscow, USSR. Zhurnal Neorganicheskoi Khimii (1978), 23(6), 1471-6.

18. Redox precipitation of nickel from aqueous solution using neutralization and hydrogen. Fugleberg, Sigmund; Haemaelaeinen, Matti; Knuutila, Kari. (Outokumpu Oyj, Finland). PCT Int. Appl. (2001), 17 pp.

19. Precipitation of hydroxides and hydroxocarbonates of iron, nickel, and copper from wastewaters and process liquors. Maksin, V. I.; Valuiskaya, E. A. Inst. Kolloidn. Khim. Khim. Vody im. Dumanskogo, Kiev, USSR. Khimiya i Tekhnologiya Vody (1989), 11(1), 12-25.

20. Purification of wastewaters containing heavy metals ions. Galitskii, N. V.; Sukhareva, N. I.; Lyakhovskaya, T. G. Institute of Technology, Mogilev, Belarus. Gal'vanotekhnika i Obrabotka Poverkhnosti (1993), 2(6), 52-5.

21. Solubility of cobalt(III) hydroxide and stability constant of the hydroxy complex Co(OH)30 in aqueous solutions. Savenko, V. S.; Savenko, A. V. Geograf. Fak., Mosk. Gos. Univ. im. M. V. Lomonosov, Moscow, Russia. Geokhimiya (1999), (4), 443-445.

22. The pH of formation of cobalt hydroxide and carbonate. Chernobrov, S. M.; Kolonina, N. P. Zhur. Priklad. Khim. (1956), 29 704-8.

23. Manufacture of cobalt hydroxide. Nabeshima, Joji; Kawamata, Hiroshi. (Sumitomo Metal Mining Co., Ltd., Japan). Jpn. Kokai Tokkyo Koho (1985), 3 pp.

24. Electron spin resonance investigation of the soluble blue copper(II) hydroxide complex. Chao, Yen-Yau H.; Kearns, David R. Dep. Chem., Univ. California, La Jolla, CA, USA. Journal of Physical Chemistry (1977), 81(7), 666-8.

25. Calcium Carbonate for CaCO3/CO2 Reactors: More Than Meets the Eye by Craig Bingman Aquarium Frontiers, August 1997.

26. Alternative Calcium Reactor Substrates by Greg Hiller Aquarium Frontiers.

27. Bioactivity and elemental composition of certain seaweeds from Karachi coast. Rizvi, Muhammad Afzal; Farooqui, Shazia; Shameel, Mustafa. Bait-al-Hikmah Research Institute, Hamdard University, Karachi, Pak. Pakistan Journal of Marine Biology (2000), 6(2), 207-218.

28. Manganese, zinc, copper, nickel and cobalt contents in seawater and seaweeds from Saurashtra coast. Kesava Rao, Ch.; Indusekhar, V. K. Cent. Salt Mar. Chem. Res. Inst., Bhavnagar, India. Mahasagar (1986), 19(2), 129-36.

29. Biodeposited trace metals and mineral content studies of some tropical marine algae. Sivalingam, P. M. Sch. Biol. Sci., Univ. Sains Malaysia, Pulau Pinang, Malay. Botanica Marina (1978), 21(5), 327-30.

30. Down the Drain, Exports from Reef Aquaria by Ronald L. Shimek. Reefkeeping.com. Volume 1. Number 12. December 2002

31. Removal of Cu(II), Pb(II), and Ni(II) by adsorption onto activated carbon cloths. Kadirvelu, K.; Faur-Brasquet, C.; Le Cloirec, P. Departement Systemes Energetiques et Environnement, Ecole des Mines de Nantes, Nantes, Fr. Langmuir (2000), 16(22), 8404-8409.

32. Down the Drain, Exports from Reef Aquaria by Ronald L. Shimek. Reefkeeping.com. Volume 1. Number 12. December 2002

33. Competitive adsorption of copper and lead ions on an iron-coated sand from water. Lai, C. H.; Chen, C. Y.; Shih, P. H.; Hsia, T. H. Department of Environmental Engineering and Sanitation, Fooyin Institute of Technology, Hsien, Taiwan. Water Science and Technology (2000), 42(3-4, Water Quality Management in Asia), 149-154.

34. Adsorption of Cu2+ and Ni2+ on iron oxide and kaolin and its importance on Ni2+ transport in porous media. Sen, Tushar Kanti; Mahajan, S. P.; Khilar, Kartic C. Department of Chemical Engineering, Indian Institute of Technology, Mumbai, India. Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2002), 211(1), 91-102.

35. Critical evaluation of treatment strategies involving adsorption and chelation for wastewater containing copper, zinc and cyanide. Bose, Purnendu; Aparna Bose, M.; Kumar, Sunil. Department of Civil Engineering, Environmental Engineering and Management Programme, Indian Institute of Technology, Kanpur, India. Advances in Environmental Research (2002), 7(1), 179-195.

36. Voltammetric study of adsorption of copper(II) species on solid particles added to seawater. Plavsic, Marta; Bilinski, Halka; Branica, Marko. Cent. Mar. Res., "Rudjer Boskovic" Inst., Zagreb, Yugoslavia. Marine Chemistry (1987), 21(2), 151-60.

37. The use of straw for removal of heavy metals from waste water. Larsen, Vagn Juhl; Schierup, Hans Henrik. Bot. Inst., Univ. Aarhus, Aarhus, Den. Journal of Environmental Quality (1981), 10(2), 188-93.

38. Uptake of Cu2+ by the calcium carbonates vaterite and calcite as studied by continuous wave (CW) and pulse electron paramagnetic resonance. Schosseler, P. M.; Wehrli, B.; Schweiger, A. Laboratory for Physical Chemistry, Swiss Federal Institute of Technology, Zurich, Switz. Geochimica et Cosmochimica Acta (1999), 63(13/14), 1955-1967.

39. Adsorption of copper at aqueous illite surfaces. Du, Qing; Sun, Zhongxi; Forsling, Willis; Tang, Hongxiao. Division Inorganic Chemistry, Luleaa Univ. Technology, Luleaa, Swed. Journal of Colloid and Interface Science (1997), 187(1), 232-242.

40. Role of surface precipitation in copper sorption by the hydrous oxides of iron and aluminum. Karthikeyan, K. G.; Elliott, Herschel A.; Chorover, Jon. Agricultural and Biological Engineering Dep. and Agronomy Dep., Pennsylvania State University, University Park, PA, USA. Journal of Colloid and Interface Science (1999), 209(1), 72-78.

41. Extended adsorption series of metals on pure calcium carbonate and on natural limestone. Gorlich, E.; Gorlich, Z.; Szwaja, A. Akad. Gorn.-Hutnicza, Krakow, Pol. Bull. acad. polon. sci., Ser. sci., Chim., geol., et. geograph. (1960), 8 75-8.

42. Heavy metal element solid-liquid interfacial processes at the mouth of Changjiang River [China]. II. Thermodynamic model of lead, copper, and cadmium adsorption on hydrated ferric oxide. Cheng, Song; Liao, Wenzhuo; Ai, Hongtao; Pan, Jiezai; Xu, Aiyu; Zhuang, Guoshun; Xu, Jianzhong. No. 3 Inst. Oceanogr., Natl. Bur. Oceanogr., Xiamen, Peop. Rep. China. Haiyang Xuebao (Zhongwenban) (1984), 6(3), 324-33.

43. Cobalt, copper, and manganese adsorption by aluminum and iron oxides and humic acid. Bibak, Allan. Chemistry Department, Royal Veterinary Agricultural University, Frederiksberg, Den. Communications in Soil Science and Plant Analysis (1994), 25(19 & 20), 3229-39.

44. Adsorption series of some cations on pure calcium carbonate and on natural limestone and dolomite. Gorlich, E.; Gorlich, Z. Akad. Gorniczo-Hutnicza, Krakow, Pol. Bull. acad. polon. sci., Ser. sci., Chim. geol. et geogr. (1958), 6 669-74.