chapter 7 
The Use of Genetically Engineered 
Micro-Organisms in the Environment 
Although most genetically engineered micro- 
organisms are being designed for contained fa- 
cilities like fermenters, some are being exam- 
ined for their usefulness in the open en\ iron- 
ment for such purposes as mineral leaching and 
reco\ erv, oil reco\ erv, and pollution control. 
■All three applications are characterized by: 
• the use of large \olumes of micro-orga- 
nisms: 
• less control o\er the behavior and fate of 
the micro-organisms; 
• a possibility of ecological disruption; and 
• less basic research and development (R&.D) 
—and a higher degree of speculation— than 
the industries previously discussed. 
Mineral leaching and recovery 
.All micro-organisms interact with metals. 
Two interactions that are of potential economic 
and industrial interest are leaching metals from 
their ores, and concentrating metals from 
wastes or dilute mixtures. The first would allow 
the extraction of metals from large quantities of 
low-grade ores: the second would provide meth- 
ods for recycling precious metals and control- 
ling pollution caused by toxic metals. 
Microbial leaching 
In microbial or bacterial leaching, metals in 
ores are made soluble by bacterial action. Even 
before bacterial leaching systems became ac- 
cepted industrial practice, it was known that 
dissolved metals could be recovered from mine 
and coal wastes. Active mining operations cur- 
rently based on this process (such as those in 
Rio Tinto, Spain) date back to the 18th century. 
Presently, large-scale operations in the United 
States use bacterial leaching to recover copper 
from waste material. Estimates for the contri- 
bution of copper leaching to the total annual 
U.S. production range from 11.5 to 15 percent. 
Leaching begins with the circulation of water 
through large quantities— often hundreds of 
tons— of ore. Bacteria, which are naturally asso- 
ciated with the rocks, then cause the metals to 
be leached by one of two general mechanisms: 
either the bacteria act directly on the ore to ex- 
tract the metal or they produce substances, 
such as ferric iron and sulfuric acid, which then 
extract the metal. It appears that simply adding 
acid is not as efficient as using live bacteria. 
Although acid certainly plays a role in metal ex- 
traction, it is possible that direct bacterial attack 
on some ores is also involved. In fact, some of 
the bacteria that are known to be involved in 
mineral leaching have been shown to bind tena- 
ciously to those minerals. 
The application of the leaching process to 
uranium mining is of particular interest be- 
cause of the possibility of in situ mining. Instead 
of using conventional techniques to haul urani- 
um ore to the surface, microbial suspensions 
can extract the metal from its geological setting. 
Water is percolated through underground 
shafts where the bacteria dissolve the metals. 
The solution is then pumped to the surface 
where the metal is recovered. This approach, 
also called "underground solution mining,” is 
already used in Canadian uranium mines, 
where it began almost by chance. In 1960, after 
only 2 years of operation, researchers at the 
Stanrock Uranium Mine found that the natural 
underground water contained large amounts of 
leached uranium. In 1962, over 13,000 kilo- 
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