Hierarchically structured metal oxides have two or more levels of structure. Their nanometer-sized building blocks provide a high surface area, a high surface-to-bulk ratio, and surface functional groups that can interact with, e.g., heavy metal ions. Their overall micrometer-sized structure provides desirable mechanical properties, such as robustness, facile species transportation, easy recovery, and regeneration. In combination these features are suitable for the removal of heavy metal ions from water. Several general synthesis routes for the fabrication of metal oxides with various morphologies and hierarchical structures are discussed including soft and hard template-assisted routes. These routes are general, reliable, and environmentally friendly methods to prepare transition and rare earth metal oxides, including cobalt oxide, iron oxide, and ceria. As-prepared hierarchically structured metal oxides show excellent adsorption capacities for AsV and CrVI ions in water.
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