Finely divided ferrous carbonate absorbent, siderite granules or absorbent particles made by mixing, agglomerating and shaping finely powdered ferrous carbonate, preferably siderite, in combination with minor effective amounts of water or an optional binder, followed by drying, are used to treat and significantly reduce concentrations of hydrogen sulfide, carbonyl sulfide, organic disulfides, mercaptans and other sulfurous compounds and contaminants in gaseous and liquid fluid streams such as natural gas, light hydrocarbon streams, crude oil, acid gas mixtures, carbon dioxide gas and liquid streams, anaerobic gas, landfill gas, geothermal gases and liquids, and the like.