Chromium speciation in natural waters using serially connected supported liquid membranes

A supported liquid membrane (SLM) method for the speciation of chromium has been developed. The method is based on selective extraction and enrichment of anionic Cr(VI) and cationic Cr(III) species in two serially connected S LM units. Methyltricaprylammonium chloride (Aliquat) and di-(2-ethylhexyl) phosphoric acid (DEHPA), respectively were used as the selective extractant s in the membrane liquid. Graphite furnace atomic absorption spectrometry ( GFAAS) was utilised for final determination. Optimised conditions for the D EHPA membrane were, sample solution at pH 3, acceptor solution 0.1 M HNO3 a nd 10% w/w carrier in kerosene. The corresponding values for the Aliquat me mbrane were pH 7, 0.75 M HNO3 and 6% w/w carrier in di-n-hexylether. This g ave extraction efficiencies for Cr(III) and Cr(VI) of 90 and 40%, respectiv ely. The method was used to measure the concentration of Cr III and Cr VI i n surface water from an abandoned tannery site. Storage experiments at diff erent pH showed that preservation at neutral pH gave almost constant values over a period of one month. At acidic pH (pH = 3.0) the concentration of C r(VI) decreased rapidly while the concentration of Cr(III) increased. The d etection limit, expressed as three times the standard deviation of enriched blank samples was 0.01 mu g l(-1). (C) 1999 Elsevier Science B.V. All righ ts reserved.