Well, if you are going to go all geeky, air absorbent is the wrong term. What the onion absorbed was not the air, but the VOCs out of the air. What you mean is something like air-filtering, air-purifying.
I tried to look, but apparently hadn't figured out how to find an answer to your question, but I'm thinking there isn't one answer. This is from a little article about shipping produce, what not to ship together:
Today, letâ€™s take a look at the odor absorption tendencies. Below are the most crucial notes for odor absorption as per the industry recognized authority on the subject, the University of California â€“ Davis. The notes below advise on what products should not be mixed in order to avoid odor absorption.
1. Odors from apples and pears are absorbed by cabbage, carrots, celery, figs, onions, and potatoes
2. Avocado odor is absorbed by pineapple.
3. Celery absorbs odor from apple, carrot, and onion.
4. Citrus absorbs odor from strongly scented fruits and vegetables.
5. Ginger odor is absorbed by eggplant.
6. Sulfur dioxide released from the pads that are used to ship table grapes will be absorbed and damage other produce.
7. Green onion odor is absorbed by fig, grape, corn, mushroom, and rhubarb.
8. Leek odor is absorbed by apple, pear, citrus, and celery.
9. Onion odor is absorbed by apple, pear, citrus, and celery.
10. Pepper odor is absorbed by avocado, pineapple, and beans.
https://uwctrucking.blogspot.com/2011/08 ... -load.html
I know this isn't directly an answer to your question, but the point is different odors (i.e. different aromatic molecules in the surrounding atmosphere) are absorbed differently by different veggies. Therefore the answer to the question of what veggie would be best to filter X molecules out of the air is very dependent on what X is.
In general houseplants are known for air purification. Specifically re your VOC's:
While most leafy plants are adept at purifying indoor air, some of the plants that scientists have found most useful in removing VOCs include Japanese royal ferns, spider plants, Boston ferns, purple waffle plants, English ivy, areca palms, golden pothos, aloe vera, snake plants and peace lilies.
https://www.livescience.com/38445-indoor ... n-air.html
In the late '80s, NASA and the Associated Landscape Contractors of America studied houseplants as a way to purify the air in space facilities. They found several plants that filter out common volatile organic compounds (VOCs). Lucky for us the plants can also help clean indoor air on Earth, which is typically far more polluted than outdoor air. Other studies have since been published in the Journal of American Society of Horticultural Science further proving the science. Want to see the best flowers? Just click through the buttons above to see all 15 plants.
https://www.mnn.com/health/healthy-space ... -fresh-air
Using living plants like this to filter/purify air, water, soil, is called phytoremediation.
People also use baking soda to absorb odors out of the air.