Unusual - Need to dissolve/oxidise H2S(g) into H2O(aq)

[Tridex]

I need to extract H2S(g) from polluted air source into drinking water oxidising the H2S in process. Contaminated air at 20-100ppm H2S. Resulting surfacing air must be void of odour.


Suggestions for aerater-filter-catalyst?

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[Tridex]

Activated carbon filter would do the trick BUT I can't have any maintenence rewuirments. The body of water thats to be impregnated can be replenished periodically such that the filter/catalyst/aerator will be momentarily exposed to atmosphere. I'm hoping that simply this periodic fresh replenishment will suffice to cleanse/rejuvinate the filter/catalyst/aerator.

 

[hydrae]

We in the water industry try to do the opposite, one way to get rid of the H2S in water is to aerate the water.
Other ways to get rid of H2S in water include injecting an oxidizer such as chlorine or postasium permagenate, inject a little and filter the water, or inject enough oxidant such as chlorine and the H2S converts to Na2S4O6.
But you are looking for a way to do this without maintenance... No help there.
Hydrae

 

[Tridex]

Yes I'm trying to do the exact reverse - put the H2S back in the water. I'm hoping that since the water has zero H2S to start and since the H2S conc. in the air is so low, that I may accomplish this. But while bubbling the air through the water will disolve SOME of the H2S the fugacity will still be a problem. Thus I'm hoping to catalytically convert the H2S during the aerating process. Perhaps an iron-honeycomb filter? End up with Iron Sulphates?

 

[SanEnr1]

Packed tower wet scrubbers are very common in the wastewater industry for removing H2S from contaminated air. A water shower is introduced into a chamber with packed media to increase surface area while contaminated air is sent in an upflow through the reaction chamber. Chemical addition is usually required to maintain optimal pH for soluability. I think that more maintenance (and space) is required for this design over the activated carbon system.

Do you have a place to treat the contaminated water?

I have also used the activated carbon for this application with good results and minimal maintenance requirements. I don't know of any process that is completely maintenance free.

 

[Tridex]

The contamination levels of the air are variable between 5-100000 ppb. The body of water may be considered as infinite as far as water contamination issues matter. The problem presents such that the H2S-air may only make one pass through the water and this feature is the limiting concern. Thus increasing surface area to exposure is paramount.

What I'm probably looking for then is an equilibria model that describes the activity of H2S/air vapour phase dissolution into water. I'm not sure. And I'm struggling to find it. But having found it I can then refine the parameters and construct the prototype. Again, the aerator must have maintenece issues as low as concrete - i.e. survive years of degradation. But this depends on traffic. Thats why I'm hoping something as simple as a porous iron honeycomb that rusts etc. Or a limestone aerator might do the trick.

If the pH is to be considered then we can assume for that case a finite body of water such that limestone aerator might be sufficient to increase the pH and accomadate greater solubility of H2S. But my problem is modeling the process. I have to model it.

Thanks thus far.

 

[SanEnr1]

I just did a quick Google search and found this interesting site

http://www.telusplanet.net/public/jcarroll/ION.HTM

Hope it or the references at the end help. I don't think there is much more I can offer. Good luck.

 

[Tridex]

Thanks for efforts thus far. You would have seen theres not much on google for it. I've already spoken to John Caroll at AQualibrium. He simply advised using NaOH to control the solubility - which sought of disregards what I'm trying to do. I'll write to him again and invite him to join this thread.

 

[Tridex]

I'VE SOLVED IT! Excuse the pun

Thanks for your help people.

I found out the pH of the water supply is HARD. It should be sufficient to dissolve the H2S simply with aeration. Thus zero maintenence

 

[kenvlach]

Hard water is perhaps a solution; at least it moves the problem from the air to the aqueous phase, creating suspended solids such as hydrated CaSO₄. But, is the cation concentration high enough to handle 100 ppm H₂S with finite exposure times? And note that the reaction will lower the pH, reducing H₂S solubility.

I think you are on the right track. But, I suggest using dissolved iron, which is very effective at removing H₂S. It forms a fine dispersion of fairly dense black FeS precipitate, not messy sulfates (unless aerated) at all normal pH’s for drinking water.
However, a “porous iron honeycomb” periodically exposed to air will end up a clogged mess of rust, sulfide and sulfate. Also, limestone will become coated with hydrated CaSO₄ and then become ineffective.

As a dissolved iron source, I suggest an electrocoagulation (EC) unit with mild steel plates, followed by either an automatically draining sediment trap or an auto-backflushing sand filter. The EC unit can be controlled to create the correct dosage of Fe ions. Since the H₂S range is highly variable, a feedback sensor will be needed to control the voltage and hence current to the EC unit in order to not add excess iron when not required. Current will range from zero or a trickle charge up that at the decomposition voltage for H₂O.

Controlled injection of potassium permanganate, as suggested by hydrae, followed by an auto draining trap or backflushing filter should also work. But, IMHO it will require more maintenance and space, plus you will find KMnO₄ solutions to be pretty messy. I don’t know your system size, but with ¼” thick plates in an EC unit, perhaps only once yearly maintenance will be needed.

Details of your water analysis – major cation and anion concentrations, pH – would be helpful. Also, is this water going to a settling pond or into potable water lines?

 

[bimr]

"I found out the pH of the water supply is HARD."

What are you talking about? Hardness is an indicator of calcium and magnesium not pH.

If you want to dissolve H2S in water, then you will need to raise the pH.

If you want the resulting solution to be odorless, then you are going to have to raise the pH to a high level since H2S is odorous at minute quantities. H2S is noticeable even in the cold when present in a water to the extent of 0.5 ppm or more. At a high pH value, the odor may be slight, for in such a case much of the sulfur may be present as an alkaline sulfide instead of as hydrogen sulfide.

Aesthetics are a motivation for drinking water treatment. H2S is a contaminant that affects the aethetics of drinking water. Many parts of the world have requirements that the hydrogen sulfide not be detectable by consumers.

As far as the ferrous sulfide concept, it only takes a small quantity to blacken the water.

Considering the fact that hydrogen sulfide is considered to be a contaminant, you might consider abandoning this idea and focusing on the next big thing.