Great idea! This is an original idea for a science project. You will need to identify a question to ask that you can answer with a controlled experiment. And, ideally you will need results that you can measure quantitatively.
What problem do you want to solve? Plants can remove carbon dioxide and pollutants, and they can produce oxygen, so there are a variety of subjects that you could investigate.
I recommend that you do some background reading on this topic and look for an interesting problem to solve. Here are some possibilities
Here is a project from this website, which explores the cooling effect of plants on rooftops.http://www.sciencebuddies.org/science-f ... 26.shtml''
Here's a project idea on pesticide breakdown.http://www.sciencebuddies.org/science-f ... p019.shtml
You could measure the effect of the plants on common pollutants such as ozone, carbon monoxide, or particulates, but you would need a procedure for measuring the analyte that you are interested in.
You can get more ideas by looking for literature references. I recommend that you go to Google Scholar and search of terms that might be of interest to you. You will find mostly abstracts, but you should be able to find a few articles that you have access to that have the detailed material and methods section.
Can you confirm when you project is due? Plant projects usually take some time, so the deadline may be an important consideration for this project.
Please post again in this topic if you have any questions. Once you have identified a purpose for your project, you will need to design the experiment.