Ion Separation of Binary Metallic Aqueous Solutions at Acidic Langmuir Monolayer Surfaces

We focused on analyzing the capability of the acidic monolayer surfaces for separation of toxic metal ions out of house-prepared binary inorganic ionic solutions such as calcium-lead, calcium-chromium, calcium-copper, and calcium-zinc aqueous systems. The affinities of the films to toxic metal ions were analyzed by using Fourier transform infrared spectroscopy. A model considering both the electrochemical and thermodynamic aspects was also applied to quantify the surface ion affinities. It is noted that surface ion binding capability for binary ionic solutions can be much different from that for pure ionic solutions. As a result, surface binding constants were found to be 4.5×10⁶,⁺⁺ for lead ions, 1.5×10⁶ for chromium ions, 5.5×10⁵ for copper ions, and 6×10⁴ for zinc ions, respectively, at pH=5.5. For the separation experiments done at pH=5.5, lead, copper, zinc ions were separated more efficiently from the mixed ionic solutions by the factors of ca. 30,000, 10,000, 3,700, and 400, respectively, compared to calcium ions of which binding constant is 1.5×10². Interestingly, when compared to corresponding pure ionic systems, copper and lead ions were separated as much, while chromium and zinc ions were less by the factor of 500 and 50, respectively.