Development of a multifunctional membrane for chromatic warning and enhanced adsorptive removal of heavy metal ions: Application to cadmium

In this study, a multifunctional membrane for visual warning and enhanced adsorptive removal of cadmium ions in aqueous solutions was prepared and examined. 5,10,15,20-Tetrakis (1-methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate), an optical indicator ligand for cadmium ions, was immobilized onto a porous chitosan/cellulose acetate blend base membrane through the polymer brushes grafted on the membrane surface via a surface-initiated atom transfer radical polymerization (ATRP) method. The prepared membrane was found to be capable of displaying distinctive color changes in response to the presence of cadmium ions in aqueous solutions. In addition, it also showed significantly enhanced performance for adsorptive removal of cadmium ions. Experimental results indicated that the developed membrane can exhibit quick (within 2 min) color change (from yellow to green) when it was in contact with cadmium ions in a solution even at relatively low concentrations (e.g. 5 mg/L). The interference study revealed that the functionalized membrane also possessed selectivity towards cadmium ions when other cationic species such as K+, Na+, Ca2+ and Mg2+ ions existed together in the solutions. Hence, the development in this study demonstrated the prospect to make porous membranes truly multifunctional with added advantages for adsorptive removal as well as for chromatic warning of heavy metal ions in the solutions that are treated.