Natural coral rock curing has turned black and brown

[iBleedSaltwater]

I have a 55g barrel packed full of dry natural rock that has been salt curing since April 14th. This rock had been left in the backyard when I moved in and I know for a fact that the previous owners used it in a huge reef tank. I power washed all this white rock off before dumping it all in a clean barrel with 15lbs of freshly mixed Instant Ocean. About a month ago I added a small powerhead for circulation and siphoned out 20g of water (very brown- see images) and replaced it with old water from my display tank. Today, I was supposed to take it all out of the barrel to let it bleach dry in the sun, but I came to find the barrel smelling like iron and maybe sulfur, and now the rock is brown like the water with what looks like black carbon on the bottoms of each rock. The rocks all feel normal (no slime or algae), but when I dunk them in a bucket of clean saltwater small particles of the black "carbon" flake off the rocks. I had intended to get the barrel water ICP tested since I want to sell some of the rock, but just by looking at it, I don't think this is safe for any aquarium. Please help.

 

[Saltyreef]

Looks like something got into the barrel and contaminated it.....
Did you ever cover it up?
And what was in the barrel prior to the liverock and what type of barrel?

Possible someone could have dropped or poured something into it?

I would not ever use it in my reef tank after that level of contamination unless you want to spend time properly acid washing it.
And even then youll lose some mass.

 

[Jekyl]

Did you keep circulation going the entire time? Was the rock clean prior to putting it in water?

 

[iBleedSaltwater]

@Saltyreef @Jekyl
The barrel had the lid on it the entire time however, because of how full the barrel was with rock, I could not seal the lid so there was a ~1" gap all the way around between the lid and the barrel. I imagine a few bugs may have fallen in the water, but I doubted anything would want to mess with the saltwater.
The rock had been sitting in the yard sun bleaching for over a year. A few pieces had a small bit of soil and moss/algae attached from being in a flower bed next to the house, but like I said, I power washed what I could get off. The barrel had previously contained non-toxic craft/paper glue. I scrubbed off all residue, then scrubbed entirely with bleach and let soak for several days with 3 or 4 gals of bleach mixed in tap water. I then emptied and let dry. Inside the barrel it smelled of bleach and glue so I scrubbed again, soaked again with less bleach and a tiny powerhead for circulation, then let dry. Still had a faint smell of glue and bleach, but I mixed up the Instant Ocean in tap water and filled it up with the rock. As of 3/24/21, the tap water here in Arlington, TX had pH 7.8, Ammonia 1.0ppm, Nitrite/Nirate 0, Phosphate 0.25 ppm (ml/Liter).

Now I'm leaning towards power washing again and curing the rock in bleach water, per the BRS tutorial video. BRS did a comparision and I can't think of a reason to do a muriatic acid cure, plus it wouldn't be worth the cost. I'll just cancel the project and throw the rock back out in the yard if a bleach cure doesn't work.

 

[MabuyaQ]

Lack of oxigen which caused hydrogen sulfide to form, which then reacted with metals in the rock creating the black discoloration.

 

[iBleedSaltwater]

Lack of oxigen which caused hydrogen sulfide to form, which then reacted with metals in the rock creating the black discoloration.

So what you're saying is, the good news is pretty much any live organics that might have been in the rock are now dead, the bad news is... toxic hydrogen sulfide. Am I right? Will a bleach cure neutralize the hydrogen sulfide in the rock or am I just going to create a more toxic reaction? Is there anything other than a ICP test that I could try to help me figure out when this rock will be safe enough to put in a display tank?

 

[MabuyaQ]

Don't think bleach will do anything to the ironsulfide and other sulfides that formed and discolored your rock given it is alkaline. Believe only an acid can bring the sulfide back in solution. Question is do you want to do this, as it will also dissolve your rock. Under normal aquarium conditions this sulfide will remain fixed in the rock. There is plenty of large pieces life rock in aquariums that have similar sulfide deposits deep on the inside or on the undersides when burried deep enough in a sandbed. You could test with one rock to see how deep the discoloration has gone, possibly a surface wash with a strong acid is enough.