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ISSE Seed Project: Investigation of Crown Ether-Modified Diatoms for the Removal of Alkali Metals from Aqueous Solutions

Team: Angelica Palomino and Kimberly Carter; Civil and Environmental Engineering; Brian Long, Department of Chemistry

2015-2016 Funded Proposal

Providing access to clean water is one of the National Academy of Engineering’s Grand Challenges. However, the criteria for meeting this challenge are that the proposed water treatment technology should be environmentally sound, non-energy intensive, and economical for the removal of bacteria, suspended solids, and metal species. Metals such as arsenic and mercury are known to be toxic to human health, harmful environmental pollutants, and difficult to remove from solution, especially at low concentrations. While reverse osmosis membranes and ion exchange resins have been used to remove salts and metal ions from solution, these technologies are generally expensive and energy intensive, especially in remote areas where little infrastructure for water treatment facilities are in place.

The purpose of this proof-of-concept study is to investigate the use of crown-ether modified diatoms for water treatment applications, in particular the removal of alkali metal ions (K+ and Na+), from aqueous solutions. If successful, the techniques and materials developed here will be further modified to address the removal of heavy metal species such as arsenic or mercury. The scope of this project is multidisciplinary, falling at the interface of water purification, geo-materials engineering, environmental engineering, chemistry, and material science that can impact society as a whole.

diagram of crown-ether attachment
Example of crown-ether attachment to the surface of a diatom

The two main components of the proposed system are crown ethers and diatoms. Crown ethers are cyclic polyethers that have found tremendous utility due to their remarkable ability to bind cationic metal salts. Previous studies have investigated their use when immobilized on silica surfaces to aid in the removal of various metals from solution. Diatoms are the skeletal remains of algae and plankton that are typically found in lakes, rivers, and other large bodies of natural waters. The skeletons are microscopic in size (~10-100 μm) and primarily composed of biogenic silica.

The advantages of using diatom surface immobilized crown ethers in environmental applications are many. First, the cost of the diatomaceous material is low. Second, silica-based materials, such as diatoms, are relatively chemically inert. Lastly, diatoms have a much higher surface area than traditional silica-based materials, which is extremely attractive as to maximize crown ether attachment and concentration. In sum, though previous studies have investigated attachment of crown ethers to silica-based surfaces for the removal of metal ions, the proposed work aims to provide proof-of-principle results that a simpler, more environmentally-friendly approach can be achieved by the use of surface immobilized crown-ethers on a less costly silica material, i.e. diatoms.