this invention relates to a process for stabilizing heavy metals in ash . the steps of the process include : preparing an aqueous slurry of ash and water ; decreasing the ph of the aqueous slurry to less than about 5 . 0 in the presence of an iron salt ; treating the aqueous slurry with an organo sulfur containing compound , an alkaline extract of peat moss , and an alkaline earth metal containing compound . heavy metals in ash treated by this process will not leach into the environment .