To get a better filtration effect you need to force air into the root zone through an active carbon layer in the soil media. That is the method used in the original nasa paper and subsequent public labs designs when trying to remove VOC's such as those emitted by printers. Active carbon adsorbs the VOC's so that the root microbes can metabolize it. Since it is a microbial filter, it becomes more effective at filtering the contaminants that it's most exposed to. Active carbon can also remove CO2, but must be recharged with lower CO2 air periodically.

The public labs design uses a aquarium bubbler to push the air, but the flows are pretty low - with retention time in my 10x12 office at 1 month. Since the VOC's are usually heavier than air, if you place the pump inlet on the floor it performs better though.