Cr(Ⅵ) Removal By Electrodes Modified With Sulfuric Acid-glycine Co-doped Polyaniline

Electroreduction, a process merely depending on the electron gain and loss of electrodes to reduce, has gradually been on track to become one of the water treatment technologies with the strongest competitiveness and the widest application prospect due to its less impact on environment, owing to no reagents addition and the cleaner process with less sludge production. This technology also could be applied for the removal of wastewater containing Cr（Ⅵ）. However, suffering from slow kinetics, low current efficiency and large energy consumption, its actual applications are limited. Therefore, improving the reduction kinetics through effective means to alleviate these problemsis is crucial in the electroreduction process of Cr（Ⅵ）.In this study, we developed a novel adsorption-electroreduction system for accelerated removal of Cr（Ⅵ） by employing reticulated vitreous carbon electrode modified with sulfuric acid-glycine co-doped polyaniline（RVC/PANI-SA-GLY）. Firstly, response surface methodology（RSM） was selected for the optimum polymerization condition of co-doped PANI synthesis since uncommonly used glycine served as one of dopants. Next, studies were further focused on Cr（Ⅵ） electroreduction process, during which the influences of potential, initial concentration of Cr（Ⅵ）, initial p H and current density on the reduction of Cr（Ⅵ） by RVC/PANI-SA-GLY were investigated, based on a comparison with RVC electrode modified with sulfuric acid doped PANI（RVC/PANI-SA） and bare RVC electrode. Following these investigations, a potential adsorption-reduction mechanism of RVC/PANI-SA-GLY to Cr（Ⅵ） was verified and elucidated by Fourier transform infrared（FTIR） and Energy dispersive spectrometer（EDS） analysis technologies. Finally, the stability and recyclable performance of RVC/PANI-SA-GLY electrode was assessed.The experimental results were shown as follows. The optimal adding of glycine not only made the synthesis of co-doped PANI stay at high-point, but also maximized the adsorption of glycine theoretically and practically leading to the improvement of Cr（Ⅵ） removal. Response surface methodology confirmed the optimum polymerization condition of co-doped polyaniline for modifying ele ctrodes（Aniline, sulfuric acid and glycine respectivel y of 0.2 mol/L, 0.85 mol/L, 0.93 mol/L）. Subsequently, RVC/PANI-SA-GLY showed higher Cr（Ⅵ） removal percentages in electroreduction experiments over RVC/PANI-SA and bare RVC. In contrast to RVC/PANI-SA, the improvement by RVC/PANI-SA-GLY was more significant and especially obvious at more negative potential, lower initial Cr（Ⅵ） concentration, relatively less acidic solution and higher current densities, best achieving 7.84% higher removal efficiency with entire Cr（Ⅵ） eliminated after 900 s. Current efficiencies were likewise enhanced by RVC/PANI-SA-GLY under quite negative potentials（-0.6V、-0.8V）. FTIR and EDS analysis revealed a possible adsorption-reduction mechanism of RVC/PANI-SA-GLY, which greatly contributed to the faster reduction kinetics and was probably relative to the absorption between protonated amine groups of glycine and HCrO₄^-. Eventually, the stability of RVC/PANI-SA-GLY was proven relatively satisfactory and RVC/PANI-SA-GLY was quite feasible for cyclic utilization.RVC/PANI-SA-GLY could be a promising alternative for the electroreduction of low concentration Cr（Ⅵ） with favorable advantages of less processing time and higher current efficiency.