HYDROGEN SULFIDE, CARBONYL SULFIDE, AND CARBON DISULFIDE

11.10. HYDROGEN SULFIDE, CARBONYL SULFIDE, AND CARBON DISULFIDE

Hydrogen sulfide is produced by microbial processes including the decay of sulfur compounds and bacterial reduction of sulfate (see Chapter 6). Hydrogen sulfide is also released from geothermal steam, from wood pulping, and from a number of miscellaneous natural and anthropogenic sources. Because it is so readily

oxidized, most atmospheric hydrogen sulfide is rapidly converted to SO 2 . The organic homologs of hydrogen sulfide, the mercaptans, enter the atmosphere from decaying organic matter and have particularly objectionable odors.

Hydrogen sulfide pollution from artificial sources is not as much of an overall air pollution problem as sulfur dioxide pollution. However, there have been several acute incidents of hydrogen sulfide emissions resulting in damage to human health and even fatalities. The most notorious such incident occurred in Poza Rica, Mexico, in 1950. Accidental release of hydrogen sulfide from a plant used for the recovery of sulfur from natural gas caused the deaths of 22 people and the hospitalization of more than 300. The symptoms of poisoning included irritation of the respiratory tract and damage to the central nervous system. Unlike sulfur dioxide, which appears to affect older people and those with respiratory weaknesses, there was little evidence of correlation between the observed hydrogen sulfide poisoning and the ages or physical conditions of the victims.

In a tragic incident that occurred in February 1975, hydrogen sulfide gas leaking from an experimental secondary-recovery oil well near Denver City, Texas, killed In a tragic incident that occurred in February 1975, hydrogen sulfide gas leaking from an experimental secondary-recovery oil well near Denver City, Texas, killed

the hazard from hydrogen sulfide. A pocket of H 2 S was struck at 15,000 feet while drilling such a well near Athens, Texas, in 1978. Leakage of hydrogen sulfide on May 12, 1978, forced the evacuation of 50 families. As an emergency measure in such cases, the gas may be ignited, although that releases large quantities of

pollutant SO 2 to the atmosphere. Hydrogen sulfide at levels well above ambient concentrations destroys immature plant tissue. This type of plant injury is readily distinguished from that due to other phytotoxins. More sensitive species are killed by continuous exposure to around

3000 ppb H 2 S, whereas other species exhibit reduced growth, leaf lesions, and defoliation. Damage to certain kinds of materials is a very expensive effect of hydrogen sulfide pollution. Paints containing lead pigments, 2PbCO 3 • Pb(OH) 2 (no longer used), are particularly susceptible to darkening by H 2 S. Darkening results from exposure over several hours to as little as 50 ppb H 2 S. The lead sulfide originally produced by reaction of the lead pigment with hydrogen sulfide eventually may be converted to white lead sulfate by atmospheric oxygen after removal of the source of

H 2 S, thus partially reversing the damage.

A black layer of copper sulfide forms on copper metal exposed to H 2 S. Eventually, this layer is replaced by a green coating of basic copper sulfate, CuSO 4 • 3Cu(OH) 2 . The green “patina,” as it is called, is very resistant to further corrosion. Such layers of corrosion can seriously impair the function of copper contacts on electrical equipment. Hydrogen sulfide also forms a black sulfide coating on silver.

Carbonyl sulfide, COS, is now recognized as a component of the atmosphere at a tropospheric concentration of approximately 500 parts per trillion by volume, corresponding to a global burden of about 2.4 million tons. It is, therefore, a significant sulfur species in the atmosphere. It is possible that the HO • radical-

initiated oxidation of COS and carbon disulfide (CS 2 ) would yield 8-12 million tons as S in atmospheric sulfur dioxide per year. Though this is a small yield compared to pollution sources, the HO • -initiated process could account for much of the SO 2 burden in the remote troposphere.

Both COS and CS 2 are oxidized in the atmosphere by reactions initiated by the hydroxyl radical. The initial reactions are

(11.10.1) HO • + CS 2 → COS + HS •

HO • + COS → CO 2 + HS •

(11.10.2) These reactions with hydroxyl radical initiate oxidation processes that occur through

a series of atmospheric chemical reactions. The sulfur-containing products that are initially formed as shown by Reactions 11.10.1 and 11.10.2 undergo further reactions to sulfur dioxide and, eventually, to sulfate species.