Development of a biomimetic lithium recovery technique inspired by brown algae in seawater

DoctorWith increasing global demands for lithium resources, the separation of lithium from seawater-based resources is receiving large attention. Among various lithium separation techniques, adsorption method has been widely utilized due to its advantages of high adsorption efficiency, easy handling, and relatively low energy consumption. In this thesis, a novel biomimetic lithium separation method was proposed by fabricating alginate (Alg) composite inspired by the lithium adsorption characteristics of brown algae. Alg is a main component of brown algae. It can crosslink polyvalent cations with forming a hydrogel. By incorporating crystalline materials of phosphonate metal-organic framework (pMOF) into Alg composite (pMOF@Alg), lithium selectivity can be modulated depending on the type of crosslinking cations of Cu2+ or Al3+ ions. The growth of pMOF in Cu2+-based alginate hydrogel was found to enhance Li+ adsorption, but inhibit Mg2+ adsorption with a small difference. By contrast, the growth of pMOF homogeneously distributed in Al3+-based alginate hydrogel (pMOF@Alg(Al)) greatly enhanced Mg2+ adsorption, while rejecting Li+ adsorption. A novel lithium separation method was proposed with a peculiar adsorption behavior of pMOF@Alg(Al) hydrogel in which partial dehydration may occur depending on the dehydration energy of hydrated metal ions. The peculiar adsorption mechanism of pMOF@Alg(Al) was examined by employing advanced analytical techniques. The results of synchrotron X-ray diffraction experiment with varying the M/G ratio of Alg network exhibit the formation of versatile amorphous structure of pMOF@Alg composite. Transmission electron microscopy (TEM) results corroborate the formation of amorphous structure with the incorporation of pMOF into Alg composite, unlike the structure of crystalline MOFs. Chemical mapping by using Fourier transformed-infrared (FTIR) technique shows that Al3+−phosphonate organic ligand complexes of pMOF are simultaneously intertwined with Al3+−alginate crosslinks with the aid of partial hydrolysis. The amorphous pMOF@Alg(Al) composite is found to have tunable sieving of alkaline and alkaline earth metal ions in the experiments with varying the degree of intertwinement of amorphous structures and Al3+−phosphonate organic ligand complexes. Especially, normal Alg composite has a high lithium selectivity due to strong repulsive force of crosslinked Al3+ ions, while a certain type of amorphous structure can effectively reject Li+ ions with small hydration energy through dehydration. In general, crystalline materials have an ion selectivity depending on the size of pores. In this study, we proposed a new tunable ion sieving technique with the aids of strong repulsive force of crosslinking cations and amorphous structure of metal organic framework. Based on the strong repulsive force of crosslinked cations in Alg composite, a green recovery technique for Li+ from Li-spiked seawater was developed by incorporating thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAAm). The thermoresponsive structural behavior of Al3+ crosslinked PNIPAAm incorporated Alg (PNP/Alg(Al)) composite was thoroughly characterized by in situ TEM and in situ FTIR experiments. As a result, the polymeric networks exhibit structural rearrangement with a retrogressive phase change from hydrophobic to hydrophilic. PNP/Alg(Al) composite exhibits very rapid Li+ adsorption due to its hydrophilicity. In addition, 7.3% of Li+ ions are recovered from Li-spiked seawater. In addition, Li+ ions could be extracted from the PNP/Alg(Al) composite with a small thermal energy. The strong repulsive force of Alg composite can be utilized for simultaneous production of fresh water and electricity with the incorporation of polypyrrole (PPy) which has a high light absorption and a high electrical conductivity. As the content of PPy increases, the evaporation rate and photothermal conversion efficiency increases. Furthermore, the PPy incorporated Alg (PPy/Alg) composite exhibits crystalline antifouling property with the aid of intrinsic hydrophilicity and strong repulsive force. The strong repulsive force of crosslinked Al3+ or Zr4+ cations induces a salinity gradient between the bottom and upper surfaces inside a hydrogel by rejecting salt cations against ion transport from the bottom to the upper surfaces by solar irradiation. With the help of selective rejection of cations, salinity gradient where salinity on upper surface was lower than bottom surfaces induces a potential difference, thereby spontaneously generating electricity depending on the composition of PPy/Alg composite. Conducting polymer PPy was found to enhance the rapid production of electricity at an initial stage, while the strong repulsive force improves a stability with increasing the amount of generated electricity. Conventional selective lithium separation methods have technical limitations due to their intrinsic structural features based on the size confinement effect. The proposed strong repulsive force-based Alg composite might be utilized as a breakthrough material to have a wide range of applicability, including selective lithium separation and simultaneous production of fresh water and electricity