SEPARATIONS Recovery of Molybdate from Dilute Aqueous Solutions by Complexation with Cationic Surfactants and Extraction with Isobutanol Nilay Sameer, Marilyn Markwei, Bandaru V. Ramarao,* and Raymond C. Francis Department of Paper and Bioprocess Engineering/Empire State Paper Research Institute, College of EnVironmental Science and Forestry, State UniVersity of New York, Syracuse, New York 13210 Although the molybdate anion in aqueous solution is known to be an effective acid catalyst for bleaching pulp with hydrogen peroxide, and other similar agents, its use commercially has been hampered by the lack of a viable recovery and recycle process for the catalyst. We developed the elements of a recovery process based on sequestering the molybdate anion using cationic surfactants from low pH solutions where molybdate exists as a polyanion. Two different surfactants: dodecylamine (DDA) and cetyl trimethyl ammonium bromide (CTAB) were both found to be effective in complexing with molybdate and could be separated out by filtering the resulting particulates. The complexes were redissolved in dilute NaOH to give concentrated solutions from which the surfactant was extracted with isobutanol (IBA or 2-methyl-1-propanol), leaving the molybdate in concentrated form in the aqueous phase. Our experiments show that nearly complete recovery of molybdate could be obtained from aqueous molybdate solutions typical of those expected in pulp bleaching process effluents, pointing to effective recovery of the molybdate using this process. IBA can be evaporated and recycled to the start of the extraction process, while the CTAB surfactant can be dissolved in warm water and recycled to the start of the molybdate complexation process. The alkaline molybdate (pH ∼10) was returned to the bleach plant. The surfactant complexes with molybdate consisted of small particles that were retained by 0.1 µm filters. Phase diagrams for complexation and particle formation were determined as a function of reactant concentrations (surfactant and molybdate) and solution conditions: pH, temperature, and electrolyte (NaCl) concentration. Particulate complexes were formed within a pH range of 3-4.5, which also depended on electrolyte concentration and temperature. Scanning electron micrographs of the CTAB-molybdate precipitate particles showed a cubical morphology, and those of DDA-molybdate showed star patterned agglomerates of needle-shaped primary particles. Introduction Molybdate can be used as an effective catalyst for bleaching wood pulps with hydrogen peroxide, oxygen, ozone, and chlorine dioxide. 1-3 In the manufacture of bleached kraft pulps, almost all of the lignin is removed by pulping, oxygen delignification, and bleaching. The kraft process uses nucleo- philic reagents, HO - and HS - /S ) , at 150-170 °C, and their addition to the lignin in wood chips leads to interunitary ether bond cleavage and depolymerization. The hydrolyzed lignin is soluble in the hot alkali, and 88-93% of the lignin is extracted. The individual fibers in the delignified chips separate with a minimal application of mechanical energy, and the washed kraft pulp (or fibers) are then bleached. The most common deligni- fication and bleaching sequence in a modern kraft mill is D 0 E O D 1 ED 2 or O-D 0 E O D 1 ED 2 where O is an alkaline O 2 delignification stage at elevated temperatures and pressures, D 0 is chlorine dioxide (ClO 2 ) delignification at pH 2-3, E is alkaline extraction, E O is O 2 addition to an E stage, and D 1 and D 2 are ClO 2 brightening stages at an ending pH of 3.5-5.5. The sequence can be divided into delignification stages D 0 E O or OD 0 E O , which lowers the lignin content of the pulp from 2-5 wt % to ∼0.5 wt %, and brightening stages (DED) that transform the pulp color from brownish-yellow to white. Hardwoods are usually cooked to a kappa number of about 15 (wt % lignin ≈ 0.15 × kappa number) in commercial practice. However, many laboratory and pilot plant studies have indicated the potential for a 2-3% yield increase (on chips) if pulping is terminated at kappa numbers close to 20. 4-7 Most of this yield increase is retained after bleaching. 5-7 The capital cost associated with upgrading the bleach plant to handle higher kappa numbers has prevented mills from realizing these yield benefits. One approach being investigated involves delignifi- cation with hydrogen peroxide (a P M stage) catalyzed by sodium molybdate. 1,2,8,9 The two chemicals can be added along with the ClO 2 solution to one of the mixers before either the D 0 or the D 1 stage, thus converting it to a D/P M stage. 3,10 Incremental H 2 O 2 delignification can be achieved without a major capital expenditure using this approach. This technology would require a recovery and recycle scheme for the molybdate anion. It is known that cationic surfactants can sequester the molybdate and that the resultant complex can be removed by flotation. 11-13 When air is bubbled through the surfactant containing solution, the surfactant-ion complex seeks the bubble interfaces and is preferentially removed in the foamate. Earlier work in our laboratory 3 showed that dodecy- lamine (DDA) can complex with molybdate and can be removed by ion flotation. Preliminary testing showed that 83% of the initial molybdate could be recovered easily by flotation of DDA-molybdate complexes. We now report our further investigation of DDA as well as the highly water soluble cetyl * To whom correspondence should be addressed. E-mail: bvramara@esf.edu. 428 Ind. Eng. Chem. Res. 2008, 47, 428-433 10.1021/ie070053q CCC: $40.75 © 2008 American Chemical Society Published on Web 12/12/2007