Sulfur smell from one side of double sink
For several months now, when using one of the two sides of the double sink in our master bathroom, you pick up a bit of a sulfur / rotten egg smell. The other sink has been fine to this point. The two sinks have separate traps and enter the wall separately and I can only assume that their drains are connected within the wall. No other sinks or fixtures have any kind of smell from them.
Since I am not getting any smell from anywhere else, I have eliminated the water supply itself as a possible source. and since it isn't coming back through the second sink's drain, it seemed reasonable that the entire problem was limited to the trap of the smelly drain.
Today I finally got around to pulling the trap and was greeted by the smell. What I also found was a fairly considerable buildup of a black granular substance that I found in the bottom of the Ptrap as well as on the horizontal floor of the drain stub-out coming out of the wall. With the water out of the way, the smell from the substance had more of a burned gunpowder and sulfur smell. (My fiancee almost threw up when she caught a whiff of it)
Do you guys have any ideas what this substance might have been? Or how it might have formed?
I am going to simply replace the trap but also need to clear the buildup from the stub-out, which should at least handle the current situation.
What could be done to keep this from returning?