How do I pretreat H2S in Reverse Osmosis feed water?

H2S acts as a weak acid, much like carbonic acid:

$${H_2S \Leftrightarrow HS^- + H^+}$$$${HS^- \Leftrightarrow S{^2}{^-} + H^+}$$

If your pH is above 5.5 and oxygen is allowed to contact with air, then sulfide ions will oxidize to element sulfur. We have recently come accross a case where a plant in South America made the mistake of using peroxide to oxidize the H₂S and then attempting to remove the resulting elemental sulfur by ultrafiltration prior to the RO. We just performed an RO membrane autopsy and their membrane was completely coated with elemental sulfur.

The fact that you have H₂S implies that your source is anaerobic. You should maintain the feed water under pressure from the well directly into the RO. If you operate at pH 5.5, your H₂S will be 100% gas and pass to the permeate side. If you operate at a higher pH, then more of your H₂S will be in sulfide ion form and be rejected, ending up in the RO concentrate (as long as your water is free of oxygen or other oxidizers).  If you have iron in your water, the sulfide ion can form a black ferrous sulfide scale on your membranes (again assuming anaerobic conditions).  This can be removed with low pH membrane cleaners.

In all cases, some H₂S and/or sulfide ions will pass through to the permeate side.  Acid should be added to the permeate prior to a degasifier to maintain all sulfides as H₂S, because air stripping does not remove ions, only gases. Any ions will react with the oxygen and precipitate as elemental sulfur in your degasifier.

Some plants with  low  concentrations of sulfides use strong oxidizers such as chlorine or ozone to oxidize hydrogen sulfide (H₂S) to sulfate (SO₄).  Such a treatment is too expensive with water containing high H₂S levels.