Selective separation of heavy metal ions using amine-rich polyamide TFC membrane

An amine-rich thin film composite (TFC) membrane was prepared and used to selective separate of heavy metals (cadmium, copper and chromium). The rejection rate of metals in single metal system has an ascending order of Cd ²⁺ < Cu ²⁺ < Cr ³⁺ (for the range of pH investigated), indicates that the separation is governed by the hydrated radius, valence charge and the diffusion coefficient of the metal cations as well as its affinity towards the polymer layer. Highest rejection rate is obtained at pH 5, with the value of 99.8, 98.6 and 98.6% for chromium, copper and cadmium respectively. Cadmium (<0.01 mg/cm ² ) was found to be the least that adsorbed onto the membrane surface, followed by copper (0.205 mg/cm ² ) and chromium (0.325 mg/cm ² ). The adsorption behaviour of metals onto the membrane surface was explained using hard–soft acid base (HSAB) principle. In adsorptive filtration of metal mixture at pH 4, flaky-shaped crystal layer which mainly made up of copper (67%) was formed on top of membrane surface. The formation of metal crystal was due to adsorption of metal cations onto membrane surface through polymer-metal ion complexation, followed by reduction of cations that aggregated to form microparticles. The used membrane was restored to its initial condition, while the metal crystal can be potentially recovered by flushing the membrane with nitric acid solution at pH 3 for 30 min.