Hydrogen Sulfide and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2005-12/rhf/index.php
Stability of Hydrogen Sulfide in Water: Iron Hydroxide Oxidation
It has been shown that particles of iron oxide/hydroxide can react with hydrogen sulfide fairly rapidly to produce elemental sulfur and reduced metals. For example, Fe+++ (ferric iron) is reduced to Fe++(ferrous iron) as the sulfide is oxidized to elemental sulfur. The process involves sulfide coming to the surface and giving up electrons to the metal ions that take them up.3 The rate of this process is decreased by sulfate, and less so by magnesium and calcium ions. It is strongly decreased by phosphate and silicate that bind to the particles' surfaces. Certain organic chelators (EDTA and TRIS, for example) accelerate this process by speeding the cycling between Fe++ and Fe+++.4 It has been claimed that this process is rapid enough to purify seawater polluted with hydrogen sulfide, as in mariculture systems.5
Since many reef aquarists have sufficient amounts of iron oxide/hydroxides in their systems to bind iron, this may be an important mechanism for hydrogen sulfide detoxification in some reef aquaria.
Preventing and Dealing with Hydrogen Sulfide
Aquarists sometimes have to deal with situations where hydrogen sulfide exposure is a possibility. The suggestions below may be useful in preventing or correcting such problems.
1. Avoid burying organic materials under sand or rocks. This organic material decays, and that decay can drive the formation of hydrogen sulfide. Burying live rock under sand, for example, will likely kill many organisms and when they decay, H2S may be formed.
2. When using a denitrator, be careful not to set its flow too slow so that nitrate is eliminated and sulfate reduction takes over. The
ORP in the denitrator may or may not be a useful guide here, depending on the setup.
3. Do not store live sand for extended periods without placing it in circulating aerated water.
4. Do not add organisms (such as certain fish) that dig up sand beds if there is significant potential for hydrogen sulfide release.
5. Be careful that equipment failures (such as a powerhead falling off the side of an aquarium onto the sand) will not disturb sand beds if sulfide deposits are suspected.
6. Do not stop the flow of aquarium water inside piping or other closed systems for more than an hour or two. If it is necessary to stop the flow for a longer time, collect the water rather than sending it back into the tank.
7. Do not store tank water or natural seawater without stirring and aeration for more than a few hours. Raw artificial seawater made with pure fresh water does not have this concern, as there are no organics in it to decay. It can be stored unstirred for as long as desired.
8. If an anoxic sand bed needs to be removed from a reef aquarium, and there are organisms that cannot be relocated out of harms way, the following precautions may be useful based on the principles detailed in previous sections, although I've not tested any to see how effective they are:
A. Remove delicate organisms from the tank system, if possible.
B. Perform the change when the lights are as bright as possible, preferably near the end of the light cycle. The lights drive the O2 concentration higher, speeding the oxidative removal of hydrogen sulfide, and the light itself will catalyze the oxidation of H2S.
C. Maximize aeration. A high oxygen level drives hydrogen sulfide oxidation, and high aeration will drive some off as volatile H2S gas.
D. Add an iron supplement to help catalyze oxidation of hydrogen sulfide and the precipitation of ferrous and/or ferric sulfide. Use one chelated to an organic; either ferrous or ferric iron will work.
E. Pass the water over iron oxide/hydroxide (GFO) to convert hydrogen sulfide to elemental sulfur.
F. Pass the water over activated carbon, which may bind some sulfide, and may also catalyze the oxidation. If forced to choose between carbon and GFO, I'd pick the GFO media.
