Coextraction

Coextraction refers to the simultaneous transfer of two or more solutes from one liquid phase into another during solvent extraction. It is observed in liquid–liquid extraction and can involve metal ions, organic molecules, or ions such as acids or anions. Coextraction occurs when multiple solutes have appreciable affinity for the extractant, or when they form complexes with the extractant that are carried into the receiving phase.

The extent of coextraction depends on factors such as pH, ligand concentration, counterions (such as nitrates), temperature, extractant loading, and the phase ratio. Interactions between solutes can be synergistic or antagonistic: the presence of one solute can increase or decrease the extraction of another by altering the extractant's speciation or by competition for binding sites.

Coextraction can simplify processing by combining multiple separations into a single step, reducing solvent use and equipment needs. However, it can also hinder separation selectivity, require more complex downstream steps, and promote phenomena such as emulsification or third-phase formation.

Quantification uses distribution ratios and selectivity coefficients for each solute; multisolute equilibrium models can predict coextraction behavior. In practice, coextraction is relevant to hydrometallurgy, pharmaceutical separations, and environmental remediation, where understanding and controlling the interactions among solutes is essential.