Reagents and Materials. Di(2-ethylhexyl) phosphoric
acid (D2EHPA), purchased from Shanghai Laiyashi Chemical
Co., Ltd., was used as the extractant without further purification.
Chloroform, toluene, n-hexane, cyclohexane, nonane, kerosene,
dodecane, and trialkylphosphine oxide (TRPO) were used as
diluents. Poly(ethylene glycol) with average molecular weight
2000 (PEG) stock solution of 50% (w/v) was prepared. A series
of (NH4)2SO4 stock solutions with different initial pH values
(pHini) were prepared. Two stock solutions of mixed Cr(III) and
Cr(VI) were prepared with CrCl3 3 6H2O and K2CrO4. One (Cr
stock 1) contained 100 mmol 3 L1 Cr(III) and 50 mmol 3 L1
Cr(VI). The other (Cr stock 2) contained 50 mmol 3 L1 Cr(III)
and 50 mmol 3 L1 Cr(VI). Solutions of 1 mol 3 L1H2SO4 or 1.5
mol 3 L1 NaOH were used for pH adjustment as appropriate. All
aqueous solutions were prepared using analytical reagent grade
chemicals (Sinopharm Chemical Reagent Co., Ltd.) and deionized
water.