I'm still trying to figure out the site.. but since I've done a similar project I figured right off the bat I'd contribute with some background. So here are a few diagrams that demonstrate how this process actually functions throughout and inside the plant..
I'm hoping this can give us some inspiration and possibly fuel (hahah pun intended) our design process...
This diagram illustrates a current system for water filtration. Basically, toxins and pollutants are pulled out of the water in 3 stages (phases, whatever). The water is strained through a ceramic filter, followed by high density activated carbon (pretty much filters gas) and lastly a heavy metal reducing media. I think our plants will likely have to filter water in stages much like this system.. of course they will be more efficient and sufficient.
This is pretty much just how a plant pulls the water up FROM its roots into the leaves (the roots get the water through osmosis). The water is transported through the xylem (think veins but plant cells), it does so by using a CAPILLARY action ~ this is where it gets a bit tricky. Capillary action is another word for opposites attract; water moves up into the roots and through the capillaries because its molecules are attracted to those in the inner walls of the plants cells (causing suction pressure).
If you're still unsure, Wikipedia gives the best description: "Capillary action is primarily responsible for water transport in plants. Water is drawn up into the plant via the roots. A network of fine tubes, collectively called the xylem, attracts water up the stem or trunk due to the adhesive forces between the water molecules and the cellulose molecules in the xylem walls. The effect of capillary action is limited by the pull of gravity"