Ion Flotation of Chromium(VI) Species: PH, Ionic Strength, Mixing Time, and Temperature

Batch foam separation experiments of Cr(VI) anions with the cationic surfactant, ethylhexadecyldimethylammonium bromide, show a sharp increase in the flotation stoichiometry from 1.0 to 2.0+ over pH 6-8, corresponding to the conversion of Cr2072(“HCr04.”) to Cr042 with pH. In the acidic region for approximately 1.0 x 10∼3 M Cr(VI) solutions, the maximum increase in the flotation stoichiometry and decrease in the fractional removal of Cr2072” is 12 % over a fortyfold increase in ionic strength, varied with four different mono-and divalent salts ; the effect is produced by a small increase in the solubility of the (EHDA)2Cr207 precipitate. In the basic region, a twentyfold increase in ionic strength with NaCl produces greater than 100% changes in the same flotation parameters, indicating a foam fractionation mechanism and competition between Br∼, Cl∼ and Cr042∼ for surfactant exchange sites. A temperature increase from 23 to 33 C in the acidic region has no effect on the flotation, and the lack of the effect of the mixing time between the Cr(VI) and surfactant solutions over the full range of variables permits all reported data points to be the average of four replicates, within 3%.