Extractants and extraction methodologies play a vital role in many industrial processes, from the concentration of precious metals from ores to the separation of longlived nuclei from radioactive waste as well as the removal of heavy metals from soils and water for remediation. The vast majority of extractants rely on the use of nitrogen, oxygen, sulfur or selenium as Lewis base donor atoms to form coordination complexes with the metal ions of interest. These extractants often make use of the chelate effect and/or the macrocyclic effect in order to form stable complexes. Some of the best known types of chelate extractants include polyaminopolycarboxylic acids (N and O donors), polyamines (N donors), dithiocarbamate (S donors) and aminopolythias (N and S donors). The most extensively investigated types of macrocycles include crown ethers (O donors), thia crowns (S donors), aza crowns (N donors) and thiacrown ethers (S and O donors).
A conspicuous omission from the list of donor atoms is phosphorus. It is noted that phosphorus has been employed as a backbone atom in the development of extractants, primarily in phosphonates, phosphates and phosphine oxides. The omission of phosphorus is interesting from two points. First, many of the precious and heavy metal ions of interest (Pd2+, Ag+, Pt2+, Pb2+, Cd2+and Hg2+) can be classified as soft Lewis acids, according to Pearson’s HSAB theory. The relative softness of phosphorus as a Lewis base as compared to oxygen and nitrogen indicates that phosphorus would be a very good donor atom toward these soft metal cations. Secondly, chelating agents containing phosphorus donors form stable complexes with transition metal cations in a variety of oxidation states due to their versatile bonding capability. The !-donor characteristics of the phosphine donor coupled with the ability to " accept from filled or partially filled d orbitals of the metal cations result in strong phosphine-metal bonds.
| Identifer | oai:union.ndltd.org:WKU/oai:digitalcommons.wku.edu:theses-1111 |
| Date | 01 August 2009 |
| Creators | McDaniel, Alicia L. |
| Publisher | TopSCHOLAR® |
| Source Sets | Western Kentucky University Theses |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Masters Theses & Specialist Projects |
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