Recovery of Molybdate from Dilute Aqueous Solutions by Complexation with Cationic Surfactants and Extraction with Isobutanol

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.