That is a good option in my opinion, the nanocomposite can be separated with a magnet easilly after removal, so the surface area is lower than activated carbons or another substrates.
bentonite's well-known and worldwide studied adsorbent of a set of heavy metals. See artciles attached. Magnetite won't make the clay properties get depressed for sure. Your welcome
You can use bentonite/Fe3O4 nanocomposite for the removal of heavy metal somewhat extent only due to less porosity when compared to other adsorbents like CNTs, activated carbon and carbon nanofibers. If you can try with CNTs, activated carbon and carbon nanofibers removal of heavy metals in more extent...