Carrier-mediated extraction of bipyridilium herbicides across the hydrophobic liquid membrane

Supported liquid membrane (SLM) method for preconcentration and enrichment of the two bipyridilium herbicides, namely diquat and paraquat, from environmental water samples has been developed. The permanently charged cationic herbicides were extracted from a flowing aqueous solution to a stagnant acidic acceptor solution across a liquid membrane containing 40% (v/v) di-(2-ethylhexyl) phosphoric acid dissolved in di-n-hexyl ether. The mass transfer of analytes is driven by the counter-coupled transport of hydrogen ions from the acceptor to the donor phase. The efficiency of the extraction process depends on the donor solution pH, the amount of the mobile carrier added to the liquid membrane and the concentration of the counter ion in the acceptor solution. The applicability of the method for extraction of these quaternary ammonium herbicides from environmental waters was also investigated by spiking analyte sample solutions in river water. With 24 h sample enrichment concentrations of diquat and paraquat down to ca. 10 ng/L could be detected in environmental waters.