In the removal of pollution, the most expensive fraction of pollution to remove is the?

CO2 separation from air is expensive than co2 separation from flue gas emitted by power plant based on natural gas combustion which is expensive than co2 separation from flue gas emitted by power plant based on coal combustion which is expensive than co2 separation from bio-gas or landfill gas.

There is a diminishing return with regard to cost-effectiveness of processes such as most depollution methods. The last bit is almost always the most difficult and thus costly to remove. It is almost impossible to truly remove all of the pollutants.

Generally the last fraction because in most cases fairly simple technology will deal with higher levels of pollution but entirely new techniques and new equipment will be needed when you are treating to higher standards. True in most cases but not all. There will be some exceptions.

The most expensive is the first fraction because you have to build the machinery and hire the people to take on the job, before you can even begin to remove the pollution.