A nanocomposite material for use as an electromagnetic wave or radio frequency absorber or as a filter to trap or remove heavy metals. The nanocomposite material may be made with a one-pot synthesis, or thermodecomposition process, of magnetic graphene nanocomposites decorated with core-double-shell nanoparticles, wherein the double shell iron nanoparticles may comprise a crystalline iron core, an inner iron oxide layer around the crystalline iron core, and an outermost amorphous Si—S—O compound shell around the iron oxide layer.