Magnesium Hydroxy Carbonate Hydrates—Nature-Inspired Water Filters for Heavy Metal Sequestration

The climate change, growing world population, together with a widespread use of chemicals in industry and our everyday life, are projected to reduce water availability in sufficient quantity and quality. Heavy metal contaminations have a seriously damaging impact on the quality of water bodies, imposing a great burden on downstream users and ecosystems. Although, natural materials, such as charcoal, sand and diatomaceous earth, have been already incorporated into modern water filtration systems (1), there is still a great need for highly efficient and environmentally sustainable sorbents. Motivated by this challenge, we focus our research on investigation of nature-inspired sustainable functional materials and their application in water filter technologies.<br/>Magnesium hydroxy carbonate hydrates are a relatively small group of naturally formed minerals that have been widely investigated for large-scale CO₂ sequestration. However, our recent study has shown that these compounds can also effectively remove heavy metals (e.g., Cr, Cd, Cu, Pb, Ni (2)) from water and act as water filters (3,4). Two members of this mineral family: <b><i>1)</i></b> dypingite (named in ’70s by Gunnar Raade, after the Dypingdal serpentine-magnesite deposit in Norway), and yet <b><i>2)</i></b> unnamed compound (hereafter referred to as UPh, a new crystalline phase identified by us while studying dypingite samples), deserve particular attention due to their unique desert rose morphology. The term “desert rose” designates self-assembled microstructures with clusters organized into fanning platelets. These 3D configurations exhibit hierarchical features that span across multiple length scales and present high specific surface area, which are crucial for many applications including sorption. Despite the remarkable sorption performance, information about the dypingite and UPh structural properties is surprisingly limited. Even though, the naturally occurring dypingite was a subject of several investigations (5,6), the available data on the material chemical composition, in particular<br/>water content, and structural features are inconsistent. The synthetic dypingite minerals has also been obtained but none of the applied approaches resulted in the formation of desert rose-like microstructures, occurring exclusively in naturally formed compounds. For better understanding of phenomena behind the physical & chemical performance of both minerals, the systematic and detailed material analysis is indispensable.<br/>To partly fill this gap, we have been performing comprehensive structural, microstructural and functional studies on natural and synthetic samples of both minerals that allow us: <b><i>i)</i></b> to define optimum synthesis conditions for the formation of materials with desert rose morphologies, <b><i>ii)</i></b> to determine relationship between the material structural/microstructural characteristics and its heavy metal sorption capacity, <b><i>iii)</i></b> to define the compound stability range, and <b><i>iv)</i></b> to identify unique powder X-ray diffraction patterns of both phases, in order to solve their crystal structures.<br/><br/>References<br/>1. McAllister, S. (2005). Analysis and comparison of sustainable water filters. EPD 397 Technical report, University of<br/>Wisconsin, US.<br/><br/>2. Uzair, M. N. (2014). Feasibility study on heavy metal adsorption by weathered dunites with emphasis on the minerals<br/>pyroaurite and dypingite. Master thesis, University of Oslo, Norway.<br/><br/>3. Hamilton, J. L., et al. (2016). Nesquehonite sequesters transition metals and CO₂ during accelerated carbon<br/>mineralisation. <i>Int. J. Greenh. Gas Control</i>, <i>55</i>, 73.<br/><br/>4. Hooper, J. C. (2019). Tracking the fate of nickel during transformation of nesquehonite to dypingite at 35°C. Bachelor<br/>thesis, Queen’s University, Canada.<br/><br/>5. Raade, G. (1970). Dypingite, A new hydrous basic carbonate of magnesium, from Norway. <i>Am. Mineral.</i>, <i>55</i>, 1457.<br/><br/>6. Suzuki, J., & Ito, M. (1973) A new magnesium carbonate hydrate mineral Mg₅(CO₃)₄(OH)₂8H₂O from Yoshikawa, Aichi<br/>prefecture, Japan. <i>J. Japan. Assoc. Min.</i>, <i>68</i>, 353.