How can lead be removed from the air, especially related to
exhaust gasses from internal combustion engines? Is there a chemical
process that can accomplish this instead of filtration?
We can distinguish several different aspects of this problem.
Alkylated lead compounds (e.g., tetraethyl-lead) is added to gasoline as an
antiknock compound. This is prohibited in the U.S. for most on-road vehicles,
but, is still used in some off-road vehicles, some older on-road vehicles, and
small planes. In addition, leaded gasoline is still used in many other
countries.
First, a certain amount of alkylated lead is emitted from vehicles using
leaded gasoline. Some of this occurs while the vehicle is at rest, i.e., not
even turned on, as alkylated lead is somewhat volatile and can evaporate
slowly from the gas tank. And, some of this occurs while the engine is
operating. Alkylated lead will exist primarily in the gas phase at the
conditions of the engine exhaust, and also under normal ambient atmospheric
conditions. Alkylated lead is fairly reactive in the atmosphere, and the
compound is transformed to a variety of other chemicals. These compounds are
probably less volatile, and, eventually most of the lead (originally emitted
as alkylated lead) winds up on small particles in the atmosphere.
Second, most of the lead in leaded gasoline is emitted from engines in another
way. An additive -- usually ethylene dibromide (EDB) and/or ethylene
dichloride (EDC) -- is added to the gasoline. In the combustion process, the
lead combines with the chlorine or the bromine and forms lead dichloride or
lead dibromide. At the temperatures of the exhaust gases, these compounds are
both vapor-phase compounds. Thus, they are exhausted with the other gases.
The main reason why EDB/EDC is added to leaded gasoline is so the lead will
leave the engine and be exhausted, rather than stay in the engine and "gunk"
it up. Once the exhaust gases cool, these lead compounds are relatively water
soluble and relative non-volatile, and so, they are either absorbed into
atmospheric water droplets, e.g., clouds, or condense out onto small
atmospheric particles.
So, in both cases, lead leaves the engine in a chemical form that is
relatively volatile (at least at the temperature of the exhaust), and then
eventually ends up on particles in the general atmosphere. The particles will
eventually be deposited to the earth's surface through dry and wet deposition
processes (e.g., after a week or two). However, since lead is being
continuously emitted, there is plenty of lead to make up for that which is
"lost" to deposition. So, there is lead in the atmosphere.
Obviously, the very best way to remove lead from internal combustion exhaust
is to not put lead into the gasoline in the first place. This cannot be said
too strongly.
Once the lead is added to the gasoline, you are left with a variety of
difficult choices. I can perhaps imagine a device put onto the exhaust system
that would remove the lead. It would probably cool the exhaust gases down and
then condense the lead onto some sort of filter material. Alkylated lead
and/or lead dichloride/dibromide might be reacted first to form a less
volatile compound. However, such a system would likely be very expensive and
fraught with technical difficulty. Also, it would probably not work at 100%
efficiency. That is, some lead would no doubt still be emitted.
In the general atmosphere, most lead exists on small particles. The best way
-- and perhaps the only way -- to remove this lead from the atmosphere is
filter the air with filter that can remove these small particles. I don't
know of any chemical process that will do this.
Dr. Mark Cohen
