Which fits better in the following context?
"Due to the physicochemical properties of nitrogen oxides asactive molecules, the purification/filtration of this group of pollutants has always been considered as a matter of concern for specialists."
Since it calls these things "pollutants", I would assume specialists are trying to filter them out of other things which would result in the purification of the other things not the purification of the pollutants. Of the two choices, it seems to me that "filtration" is the only choice for this sentence. The sentence should probably be rewritten.
Filtration is the most common way to remove contaminants, but the contaminant must be of a size that is large enough to filter out (about 2 microns for most air filters).
There is electrostatic precipitation, that works on smaller particles. There is the use of a binding agent to allow filtration. The agent binds to the too-small-to-filter materials and creates a particle that is large enough to filter.
Reverse osmosis is used for conversion of salt water to drinking water, but it will not filter out some chemicals or viruses.
It sounds like the nitrogen oxides are molecules (probably too small to filter directly). A quick Google search for "Nitrogen dioxide removal" reveals that there are several patented methods of removing these molecules including precipitation/filtration, electrostatic precipitation, chemical scrubbing and others.
I still think "removal" is more encompassing and it is likely that filtration alone will not remove the molecules.