Simple and scalable synthesis of earth-alkaline hydroxides nanoparticles in aqueous suspension

Introduction: Ca-hydroxide nanoparticles are emerging as effective conservation materials for all carbonatic-based substrates and mortars of Cultural Heritage, guaranteeing compatibility, durability, reduced environmental impact and human risk. Parallely, the development of nanoscale Mg-hydroxide, revealed considerable interest in the de-acidification of paper and wood, but also as antibacterial, flame retardant agent, and as MgO nanoparticles precursors. Ca- or Mg- hydroxide nanoparticles are obtained by several methods, generally characterized by some critical drawbacks: high temperature, processes are time and energy consuming, and low specific yield in the production, so determining a limit to their application to particular or restricted fields. In this work we present a facile and scalable method, recently patented (RM2011A000370, WO2014020515A1), to synthesize Ca- and Mg- hydroxide nanoparticles in aqueous suspension. This eco-friendly and time-energy saving route is based on an ion exchange process, operates at room temperature, starts from cheap or renewable reactants, with no intermediate steps (washings/purifications), drastically reduces times of synthesis and has the ability to scale up the nanoparticles production. The produced nanoparticles are pure and crystalline, with dimension generally less than 80 nm. Materials and methods: To synthesized Ca(OH)2 (or Mg(OH)2) nanoparticles, a CaCl2·H2O (MgCl2) aqueous solution and an ion-exchange resin are mixed together, at room temperature and under moderated stirring. The substitution of hydroxyl groups on the resin substrate with the chlorides ions in solution leads, in supersaturation conditions, to the formation of a white precipitated phase inside the preparation, already after few minutes; maintaining the stirring, the preparation is rapidly separated from the resin by a sieving procedure. Resin can be regenerated to be reused for a scale-up production, according a cyclic procedure. The produced nanoparticles are characterized from structural and morphological point of view, by means of X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermal analyses (TG-DTA), surface area measurements (BET) and transmission electron microscopy (TEM). Carbonatation efficiency of the produced Ca-hydroxides nanoparticles suspension is investigated too. Results and discussion: From XRD measurements, only pure and crystalline Ca(OH)2 (Mg(OH)2) phases are produced after few minutes, as evidenced by the absence of peaks belonged to other substances in the experimental XRD pattern. The purity of the obtained phases is confirmed by FTIR and TG-DTA measurements. From BET measurements, we have obtained that the synthesized Mg(OH)2 sample reaches values of surface area up to 155 m2/gr, generally higher than those reported in literature. TEM images referred to Ca(OH)2 and Mg(OH)2) samples, respectively, show nanoparticles characterized by a plate-like morphology, with dimensions generally ranging from 20 to 80 nm. Moreover, observations reveal that nanoparticles are formed by an oriented aggregation of Mg(OH)2 primary hexagonal particles (singlets), crystalline, homodispersed, and with dimensions of about 3 nm. The analysis of Ca(OH)2 nanoparticles carbonatation process, investigated when the suspension is exposed to air (T = 20°C, RH = 70%), shows a complete transformation of Ca(OH)2 into CaCO3 (calcite), already after 3 h of air exposure time. This result is very interesting for the fast reactivity but also for the formation of pure calcite, that guarantees the perfect compatibility with all the carbonatic substrates.