Fig. 1.

a Illustration of synthesis process of XFO–BFO membrane and its microscope picture (5 × 5 mm²). During the synthesis process, the first step is that Shewanella oneidensis MR-1 possesses a periplasmic [Fe]-hydrogenase that drives the microbial reduction of Fe³⁺ [Fe(OH)₃: cuboid] and the release of Fe²⁺ ions to the aqueous solution. The second step is that the =Fe²⁺(OH)₂ surfaces can absorb and immobilize X²⁺. The third step is that the functional =Fe²⁺(OH)₂ groups provide high surface area and functional sites which allow the chemical complexation of X²⁺ [21, 22]. b SEM images (200 nm) of XFO–BFO membranes