Additive manufacturing of geopolymers for brine filtration

The term brine refers to a highly concentrated solution of salt in water, with different types based on the source. Brine filtration has received more attention for the possibility of recovery of valuable trace elements and pure water.With the development of separation technologies, the primary sources of some met- als have gradually changed from ore to brine, which has now become the main raw material. Resources of metal-bearing brine are scattered throughout the globe, both in a natural state and as waste from industrial plants. Most brine recovery plants work using evaporation, precipitation, ion-exchange, solvent extraction, adsorption or membrane separation; the recovery or removal of trace components remains a challenge, with new methods being applied for the selective and more efficient extraction of trace elements. With this premise we decided to investigate both a new possible material class (i.e. geopolymers) for the production of selective adsorption devices, exploiting their use as inks for direct ink writing (DIW) in the production of structured monoliths, as well as new and improved design solutions. Geopolymers are inorganic ’polymers’ with ceramic-like characteristics, synthesized at relatively low temperatures (typically below 100°C). They are composed by a network (-polymer) of min- eral molecules (-geo), Si-Al or others, linked by covalent bonds. Geopolymers can be chemically defined through x SiO2–Al2O3–M2O–y H2O, where x and y are variables related to the amount of silica and water present; these two are the main factors modified during synthesis to achieve different material prop- erties. The letter M represents the species of the precursor necessary for the dissolution of the network. Raw materials such as metakaolin and industrial wastes (coal fly ashes and blast furnace slag) contain alumino silicate sources used in the geopolymerization reaction. These materials undergo geopolymer- ization via two distinct synthesis routes: in an alkaline solution containing (Na, K, Li. . . ) hydroxides and alkalisilicates, they yield poly(silicates), poly(siloxo), poly(silico-aluminates), poly(sialate) chains; in an acidic medium, such as with phosphoric acid, they instead yield poly(aluminophospho) networks. In this framework, the research focused on the selective adsorption of Lithium ions before examining a different solution for oil removal. The project’s in- dustrial partner, ENI S.p.a., produces Li-containing brines as a byproduct and wishes to upcycle them by retrieving the precious metal and removing organic pollutants (mainly oils from wastewater) to reduce their environmen- tal impact. Our main objective was defining a pathway to synthesize and characterize different geopolymers with high cationic exchange capacities and high oil affinity. We synthesized various compositions of geopolymers, employing diverse combinations of precursors and alkali ions, producing materials with controlled adsorption properties and capacities. To achieve the specific removal of lithium, we utilized lithium hydroxide as a key component alkali source) during the synthesis of the geopolymer. In the framework of oil removal we produced different sodium based composites, utilizing various carbonous fillers and employing both liquid and solid dispersions, aiming to achieve high oil removal; for each composition we defined an optimized ink for DIW to produce a filter design that could fit in the industrial partner’s lab-scale filtration system set-up.