extractants

Extractants are chemical reagents used to selectively transfer a solute from one phase to another in solvent extraction or liquid-liquid separation processes. They are typically organic compounds that form soluble complexes with the target species, often a metal ion, and thereby concentrate the solute in an organic phase away from an aqueous phase. The efficiency of extraction is described by distribution ratios, which depend on the extractant’s structure, the solute, the diluent, pH, and temperature.

Extractants are broadly categorized as neutral, acidic, or chelating. Neutral extractants, such as certain organophosphorus compounds, act by solvating the metal ion in the organic phase. Acidic extractants, including organophosphorus and carboxylic acids, transfer metal ions by releasing protons into the aqueous phase and forming metal complexes in the organic phase. Chelating extractants bind the metal through multiple donor atoms to form stable complexes that migrate into the organic solvent. Common examples include tributyl phosphate (TBP), di(2-ethylhexyl) phosphoric acid (D2EHPA), and tri-n-butylphosphate-type reagents, as well as neutral agents like trioctylphosphine oxide (TOPO).

In industry, extractants are central to hydrometallurgy and recycling, notably in uranium and plutonium separations in nuclear fuel reprocessing (the PUREX process uses TBP in kerosene). They are also employed to purify and separate rare earth elements, enable metal finishing recycling, and remediate contaminated streams. Selectivity, phase behavior, and environmental and safety considerations guide extractant choice, including compatibility with diluents, resistance to hydrolysis, and ease of separation from the raffinate or product streams.