Shaping hydrotalcite-like materials into microspheres and their use for sorption applications

Abstract

Hydrotalcite-type materials, also called layered double hydroxides (LDHs), are a family of naturally occurring anionic clays. They are represented by the empirical formula [M2+1-xM3+x(OH)2]x+ (An-x/n) · mH2O, where: M2+ are divalent cations (Mg2+, Zn2+, Ca2+, etc.), M3+ are trivalent cations (Al3+, Fe3+, Mn3+, etc) and An- are interlayer anions (CO32-, NO3-, etc). These materials can be used in sorption applications, e. g. sorption of phosphates, due to their large surface areas, high anion-exchange capacity, and flexible interlayer region accommodating various anionic species. Moreover, their chemical composition can be easily tuned allowing not only a high ion exchange capacity but also high selectivity towards phosphate ions. However, a powder form of these sorbent materials are not suitable in wastewater treatment systems due to the mass transfer limitations or difficulties in handling and recovery at the end of the process. The focus is on structuring the powder into an optimal architecture, e.g. porous microspheres. This study explores the added value of structuring the sorbent powders into porous microspheres, with controlled porosity and pore structure. These shaping efforts should lead to a decrease of the mass transfer limitations, a lower pressure drop and faster sorption kinetics. Pural MG63HT commercial Hydrotalcite-type clay material used in the present study was provided by Sasol Germany GmbH. The powder was processed into suspensions by mixing with sodium alginate polymer which was further cross-linked into a coagulation bath containing Ca2+ ions. The shaped products were obtained using both the gravitational as well as the vibrational drip casting technique using a BRACE GmbH device for microsphere production. The research combines physico-chemical characterization (XRD, SEM, porosity measurements), material shaping (zeta potential and viscosity measurements of the slurries and suspensions) and phosphate sorption testing by UV-vis Spectroscopy