pH Manipulation: A Facile Method for Lowering Oxidation State and Keeping Good Yield of Poly(m-phenylenediamine) and Its Powerful Ag+ Adsorption Ability

A method of pH manipulation has been used to improve chemically oxidative polymerization of m-phenylenediamine (mPD) through concurrent addition of NaOH when adding oxidant (NH4)(2)S2O8. pH detection and open-circuit potential technique were adopted to monitor the polymerization process of mPD and to explain the oxidation state-pH and yield-pH relationships. Results from Fourier transformed infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies indicate that a low oxidation state is under control by regulating NaOH concentration. At 2.5 M NaOH, the oxidation state of poly(m-phenylenediamine) (PmPD) is 64.7 mol % (measured by molar content of quinoid imine from XPS), while the yield is 84%. The synthesized PmPD possesses better Ag+ adsorption performance when lowering its oxidation state. Moreover, the Ag+ adsorbance of PmPD can reach 1693 mg g(-1). Meanwhile, Ag+ adsorption mechanism was studied by pH tracking, X-ray diffraction (XRD) patterns, and X-ray photoelectron spectroscopy. The adsorption process includes redox reaction, chelation, and physical adsorption.