Supercritical Fluid Extraction of Metal Chelate: A Review

ABSTRACT

Supercritical fluid extraction (SFE), as a new green extraction technology, has been used in extracting various metal species. The solubilities of chelating agents and corresponding metal chelates are the key factors which influence the efficiency of SFE. Other main properties of them such as stability and selectivity are also reviewed. The extraction mechanisms of mainly used chelating agents are explained by typical examples in this paper. This is the important aspect of SFE of metal ions. Moreover, the extraction efficiencies of metal species also depend on other factors such as temperature, pressure, extraction time and matrix effect. The two main complexation methods namely in-situ and on-line chelating SFE are described in detail. As an efficient chelating agent, tributyl phosphate-nitric acid (TBP-HNO₃) complex attracts much attention. The SFE of metal ions, lanthanides and actinides as well as organometallic compounds are also summarized. With the proper selection of ligands, high efficient extraction of metal species can be obtained. As an efficient sample analysis method, supercritical fluid chromatography (SFC) is introduced in this paper. Recently, the extraction method combining ionic liquids (ILs) with supercritical fluid has been becoming a novel technology for treating metal ions. The kinetics related to SFE of metal species is discussed with some specific examples.

KEYWORDS:

• Chelating agent
• metal chelate
• supercritical fluid extraction
• TBP-HNO₃ complex

Acknowledgments

The authors would like to acknowledge the following financial supports: PetroChina Innovation Foundation (2014D-5006-0401), National Natural Science Foundation of China (No. 21376186), the Ministry of Education (Doctoral Special Research Foundation No. 20110201110032) China, Fundamental Research Funds for the Central Universities (New Teacher Research Support Plan No. 08141002, International Cooperation Project No. 2011jdhz37 and Integrated Cross Project xjj2014136 in Xi'an Jiaotong University), Natural Science Basic Research Plan in Shaanxi Province of China (No.2012JM2010) and Sci. & Tech. Project for Overseas Scholars (the Ministry of Human Resources and Social Security of China, No.19900001).