Investigation of the removal and recovery of metal cations and anions from dilute aqueous solutions using polymer – surfactant aggregates

Dilute metallic ion treatment (<10 mg/L) remains a challenge in water purification and resource recovery. A novel and inexpensive treatment process that employs polymer–surfactant aggregates (PSAs) has been developed and applied to remove and recover dilute metallic ions, such as Cr3+, Rh3+, Cd2+, Fe(CN)6 3- and CrO42-, from industrial process and effluent. At the heart of this process is a material that comprises a colloidal structure of polymers and surfactants, named a polymer–surfactant aggregate (PSA), that trap metallic ions. The ion loaded PSAs then coalesce and settle out. The flocs are then treated separately by acid-base wash to recover the ions in a concentrated salt and regenerate the polymer and surfactant. The regenerated polymer and surfactant can then be recycled without a deterioration of removal ability in the next cycle. This process is simple, uses low energy, and generates little material loss or discharge. The thesis is divided into three main parts: fundamentals, cation treatment and anion treatment. First, the mechanism of formation of PSAs and their interactions with metallic ions are investigated using surface tension and electrical conductivity measurements. Both measurements reveal that the PSA is formed by surfactant monomers binding to the oppositely charged polymer chains and forming micelle–like aggregates via hydrophobic and electrostatic forces. These aggregates, like micelles, can bind to the oppositely charged metallic ions, but the surfactant concentration required is a few orders of magnitude lower than that required for micelle formation. The resulting nano-size PSA has a large surface area to volume ratio, and can effectively treat dilute aqueous streams. Each PSA consists of positive and negative charges. Within a near charge neutralisation range, they can quickly self-flocculate to simultaneously remove metallic ions and settle the flocs out of aqueous solutions. Correlating the removal efficiency of ions with surface tension and electrical conductivity measurements, the results suggest that the PSA is indeed responsible for removing the ions from the streams. Based on the fundamentals, a PSA process consisting of three stages (removal, recovery and recycle) is developed to treat metal cations in dilute streams. At the removal stage, polymer and surfactant (i.e. removal agent) are used to form PSAs and trap 99% of 0.1 mM metal ions into flocs. At the recovery stage, a small amount of acid solution is added to leach out 95% of the trapped metal ions into a concentrated salt, and then using a base solution to completely dissolve and regenerate the removal agent. After that, the removal agent are recycled in the next cycle without the need for any make-up, and little deterioration of removal ability is found. The same three-stage process is also applied to recover dilute metallic anions. As the targeted ions are negatively charged, the charge of polymer and surfactant used and the order of acid-base wash are reversed as compared with the cation treatment process. The PSA process is robust under different conditions, e.g. pH, temperature, salinity and organic contaminants. Such a sustainable process thus has potential applications for the efficient removal and recovery of dilute metallic ions during process effluent water treatment.