Environmental constraints have forced the metal plating industry to reduce their emissions to water systems, otherwise mass usage of metals could cause severe environmental problems. Conventional waste water cleaning technology does not meet all the new regulations for releases from surface finishing plants. There is thus a need for new, more effective methods. Selective ion exchange offers a good solution for cleaning many waste streams. The aim of this work was to study and develop selective ion exchange materials for effective and economical applications in waste water treatment in the metal finishing industry.
The research was focused on waste effluents produced in water-consuming rinsing operations, and the metals of interest were chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and cadmium (Cd). The performance of 49 ion exchangers and sorbents was determined by studying their ion exchange equilibria. The batch method with rapid tracer technique was first used to evaluate large group of ion exchangers and sorbents in order to select the best sorbents for waste treatment. Then the batch method was used to determine the equilibrium data of the selected sorbents. Column mode experiments were used to test the selected sorbents with real waste solutions.
The best selective ion exchangers were iminodiacetate and aminophosphonate resins, and the fibrous polyfunctional exchanger with imidatsoline/carboxyl functionalities. Other exchangers of interest were oxine resin, zeolites A, X, L, and ferrierite and inorganic sodium titanate. Compared to conventional cleaning methods exchangers exhibited a more effective performance with low metal concentration solutions, in column tests exchangers removed 99.9% of the nickel and 99.5% of the zinc in solution. The main equilibrium parameters affecting the ion exchange were pH value, concentration of complexing ligands and concentration of competing counter ions. Commercially available exchangers can be used for purification of metal plating wastes, especially in applications where metal concentrations are low e.g. rinse waters or end of pipe polishing. With ion exchange technology it is possible to reach the most strict limits for the emissions and ion exchangers can be utilised as a supporting system for conventional purification technology