Bionanocomposites are a novel group of organic-inorganic hybrid materials based on the assembly of natural polymers and inorganic solids, most commonly clay minerals, through nanoscale interactions between both components [1]. Similarly to conventional nanocomposites, bionanocomposites exhibít improved structural and functional properties, while offer non-toxicity, biocompatibility and biodegradability character associated with the biopolymer [1]. Amongst them, sacran is an interesting biopolymer compound for developing novel c1ay-based bionanocomposites. This extracellular polysaccharide extracted from the cyanobacteria Aphanothece sacrum is an anionic megamacromolecule of extremely high molecular weight (about 1.6 x 10 7 g/mol), provided with both carboxylate and sulfate groups, and its chains can self-orientate forming double helixes, gels or even huge domains of liquid crystals at given concentrations [2]. The aim of this work was the development of novel bionanocomposite materials based on the assembly of sacran to sepiolite, a hydrated magnesium silicate with microfibrous morphology, and montmorillonite layered c1ay, investigating the main interactions between both components [3]. Interestingly, this megamacromolecule is able not only to become associated with the external surface of sepiolite via its silanol groups but also to intercalate layered c1ays, as revealed by FTIR and DRX techniques. These bionanocomposites can be rocessed as self-supporting films owing to the film­forming ability of sacran, showing higher tensile moduli than films of pristine sacran. Their resistance and integrity in aqueous solutions are also considerably increased in comparison to neat sacran films, due to presence of the sepiolite and montmorillonite clays. Considering the interesting colloidal and metal complexing properties of sacran [4], these novel bionanocomposites result very promising for a wide range of applications, from medical purposes to environmental remediation. For instance, neodymium, europium and gadolinium ions were selected to carry out adsorption tests with a set of sacran-sepiolite materials. The bionanocomposites showed a c1ear preference for Nd3 + ions, suggesting their potential application in the selective recovery of this lanthanide.