Organic Arsenic Compounds in Petroleum and Natural Gas

Abstract

Arsenic, a monoisotopic element with mass number 75, and several of its compounds have been known since ancient times. The average concentration of arsenic is estimated to be 2.5 mg/kg in the earth’s crust and 1.5 μg/kg in ocean water. Thus, arsenic is about as abundant - or better - scarce as the elements uranium, tungsten, molybdenum, cesium, and most of the rare-earth elements (Cox, 1989). Arsenic is ubiquitous. Arsenic concentrations of several thousand mg/kg can be found in soils - even in those not influenced by human activities that dispersed arsenic compounds into the environment (U.S. National Academy of Sciences, 1977; Tanaka, 1988). Ocean water with 1.5 μg arsenic per kg has one of the lowest arsenic concentrations among environmental compartments. In contrast, marine organisms have arsenic concentrations several thousand-fold higher than their aqueous environment (Hanaoka et al., 1988). Because petroleum and natural gas were formed during geological time periods from arsenic-rich marine organisms, these hydrocarbon materials should be blessed with significant concentrations of arsenic compounds. Very little information is available in the literature about the concentrations of total arsenic in crude petroleum, in liquid hydrocarbons derived from petroleum, and in natural gas.