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New Effects of Fe Based Phosphate Binders

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NaH2O

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Boomer gave me this from Randy Holmes-Farley.

I found this very interesting, and am curious as to if it will make a difference in our systems:

Boomer said:
Randy has recently found out something interesting.

I ran across this reference today that suggests that the iron oxide/hydroxide phosphate binders (Rowaphos, Phosban, Phosphate Killer and others) may have other interesting applications in aquaria.

It claims that these materials may act as a sink for sulfide in reef aquaria. Consequently, one might hypothesize that aquaria using such materials might have a lower concentration of free sulfide than those that do not.

I have no idea whether free sulfide (hydrogen sulfide and its ionized forms) is a concern in typical reef aquaria but it is highly toxic, and can be generated in some systems (for example, those using sand bed beds or cabon denitrators).

So if the constant low level of sulfide is detrimental to anything int eh aquarium, then perahsp running such media might be beneficial.

It also suggests taht soem of the iron may become more soluble in this fashion, and thereby may become bioavailable (a potentially good thing, IMO).

The reference:

A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide. Poulton, Simon W.; Krom, Michael D.; Raiswell, Robert. Danish Cent. Earth Syst. Sci., Inst. Biol., Univ. South. Denmark, Odense, Den. Geochimica et Cosmochimica Acta (2004), 68(18), 3703-3715.

Abstract

The reaction between dissolved sulfide and synthetic iron (oxyhydr)oxide minerals was studied in artificial seawater and 0.1 M NaCl at pH 7.5 and 25°C. Electron transfer between surface-complexed sulfide and solid-phase Fe(III) results in the oxidn. of dissolved sulfide to elemental sulfur, and the subsequent dissoln. of the surface-reduced Fe. Sulfide oxidn. and Fe(II) dissoln. kinetics were evaluated for freshly pptd. hydrous ferric oxide (HFO), lepidocrocite, goethite, magnetite, hematite, and Al-substituted lepidocrocite. Reaction kinetics were expressed in terms of an empirical rate equation of the form: Ri = ki(H2S)0.5t=0A where Ri is the rate of Fe(II) dissoln. (RFe) or the rate of sulfide oxidn. (RS), ki is the appropriate rate const. (kFe or kS), (H2S)t=0 is the initial dissolved sulfide concn., and A is the initial mineral surface area. The rate consts. derived from the above equation suggest that the reactivity of Fe (oxyhydr)oxide minerals varies over two orders of magnitude, with increasing reactivity in the order, goethite < hematite < magnetite .mchlt. lepidocrocite » HFO. Competitive adsorption of major seawater solutes has little effect on reaction kinetics for the most reactive minerals, but results in rates which are reduced by 65-80% for goethite, magnetite, and hematite. This decrease in reaction rates likely arises from the blocking of surface sites for sulfide complexation by the adsorption of seawater solutes during the later, slower stages of adsorption (possibly attributable to diffusion into micropores or aggregates). The derivation of half-lives for the sulfide-promoted reductive dissoln. of Fe (oxyhydr)oxides in seawater suggests that mineral reactivity can broadly be considered in terms of two mineral groups. Minerals with a lower degree of crystal order (hydrous ferric oxides and lepidocrocite) are reactive on a time-scale of minutes to hours. The more ordered minerals (goethite, magnetite, and hematite) are reactive on a time-scale of tens of days.
Substitution of impurities within the mineral structure (as is likely in nature) has an effect on mineral reactivity. However, these effects are unlikely to have a significant impact on the relative reactivities of the two mineral groups.


This reference further shows that organic materials (the siderophores) can release iron from these binders:

Iron oxide dissolution and solubility in the presence of siderophores. Kraemer, Stephan M. Institute of Terrestrial Ecology, Swiss Federal Institute of Technology, Schlieren, Switz. Aquatic Sciences (2004), 66(1), 3-18.

Abstract

Iron is an essential trace nutrient for most known organisms. The iron availability is limited by the soly. and the slow dissoln. kinetics of iron-bearing mineral phases, particularly in pH neutral or alk. environments such as carbonatic soils and ocean water. Bacteria, fungi, and plants have evolved iron acquisition systems to increase the bioavailability of iron in such environments. A particularly efficient iron acquisition system involves the solubilization of iron by siderophores. Siderophores are biogenic chelators with high affinity and specificity for iron complexation. This review focuses on the geochem. aspects of biol. iron acquisition. The significance of iron-bearing minerals as nutrient source for siderophore-promoted iron acquisition has been confirmed in microbial culture studies. Due to the extraordinary thermodn. stability of sol. siderophore-iron complexes, siderophores have a pronounced effect on the soly. of iron oxides over a wide pH range. Very small concns. of free siderophores in soln. have a large effect on the soln. satn. state of iron oxides. This siderophore induced disequil. can drive dissoln. mechanisms such as proton-promoted or ligand-promoted iron oxide dissoln. The adsorption of siderophores to oxide surfaces also induces a direct siderophore-promoted surface-controlled dissoln. mechanism. The efficiency of siderophores for increasing the soly. and dissoln. kinetics of iron oxides are compared to other natural and anthropogenic ligands.
 

Curtswearing

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That is awesome information. Thanks Boomer.

I recently removed my sandbed and I had a sulphide zone underneath my largest rock. This possibility is exciting to me in theory....here's my thoughts.

The table on this link Chemical Composition of ASW Mixes shows that our artificial saltwater mixes include sulpher/sulphate. This is normal because NSW also has tons of it. The only reason that our coral reefs aren't dieing from Hydrogen Sulphide is because of the lack of nutrients. I read a paper where they believed the pollution from land turned an area eutrophic instead of oligotrophic, and they believed that the Hydrogen Sulphide produced by the sulfate-reducing bacteria was responsible for a lot of deaths. Since our tanks are often much more polluted than a reef, this should possibly be more of a concern to us.

If what they are saying is true, the sulphur/sulfate that is present in the water column should end up in your fluidized bed reactor. Because of the constant input of oxygen-laden water in the reactor, you could possibly severely limit the population growth of sulphate-reducing bacteria (they live in anaerobic zones). Get rid of them and you also substantially lower the Sulphide-oxidizing bacteria (they are aerobic but need reduced sulfates for a food source). Basically, our chance for developing a sulfide zone in our sandbed would be substantially lowered and the possibility of producing Hydrogen Sulfide due to our nutrient-laden tanks is also substantially lowered.


Disclaimer....the above is entirely theoretical.
 

NaH2O

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Could this be beneficial for DSBs that have the potential of releasing any sulphorous end-products (hitting a pocket accidentally during maintenance, as an example)? What I'm not sure about, and I may need to think about it some, is whether the sulfide would even make it to the iron/oxide hyrdoxide phosphate binder before being broken down or sunk in other areas of the system. Does the sand have a stronger sinking ability compared to the phosphate remover (perhaps stronger is not the right term)? Also, we wouldn't be able to measure when the phosphate binder is saturated with sulfide, however, the frequency of replacing it (for phosphate removal) would probably be sufficient.
 

mojoreef

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Thats a tough question Nikki, I think it would be tought for the FE in the filter canister to realy effect the sulfide in the dsb. I would imagine that it could absorb the soluable Sulfide in the water column, but not the stuff in the deep part of the bed. the other thing it would do is to make the sulfide zone more visable. Mosts folks think that the Sulfide zone is black or grey in thier sand, but in most cases it is not. It only turns black if exposed to iron.


Mike
 

Curtswearing

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mojoreef said:
I would imagine that it could absorb the soluable Sulfide in the water column, but not the stuff in the deep part of the bed. Mike
I agree. I was thinking more of a new setup for someone who wanted a DSB. Lets assume they did everything right. For example:

  • They cured their LR in the tank fully with no sand in the tank.
  • Once the rock was cured, they built a racking system of some sort so the rock was neither buried in the sand nor laying on the top of the sand....rather it was slightly raised above the sand.
  • The fluidized bed reactor was put on the tank from day one of the curing process and so was an efficient protein skimmer.
  • The rock is turkey basted on a regular basis.

Do you think that would prevent the formation of a sulphide zone in the sand or at least slow the formation of one?
 

mojoreef

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Do you think that would prevent the formation of a sulphide zone in the sand or at least slow the formation of one?
No. Curt the bed is in to much of a state of flux, for that to have a real impact, and to be honest If I ran a dsb I would want a population of Sulfide reducing bacteria. SRB's are a great little bacteria. They are the ones that bind up and take out most of the toxins we have in our tanks, especially the metals. They are fine down thier in the bed, just make sure that is where they stay. When the sulfide zone raises up into a bed its not the SRB's that are the problem, the problem is that the whole zone is converting to anaerobic and the aerobic zone is loosing its oxygen content. It would be more benefical for us to keep the top inch oxygenated and not clogged then it would be to worry about SRB's.
When thinking about DSB filtration most of the conversation deals with nitrogen reducing bacteria, but since we put a hell of alot more then just nitrogen products into the bed we need bacteria such as SRB's to deal with the other elements entering the bed. They take up the other nasties and put them into cycle.

Mike
 
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