关键词: 脱硫废水, 电氧化, 析氢, 硫酸铵, 资源化

Abstract:

It is preferred to simultaneously recover resource and energy from waster. Sulfur dioxide, SO₂, a common air pollutant, a potential energy resource, is a key link to sulfur nature circulation. SO₂ can be conversed to NH₄HSO₃ and (NH₄)₂SO₃ during the ammonia desulfurization process, which can be used to produce (NH₄)₂SO₄ fertilizer. For high quality (NH₄)₂SO₄ fertilizer and high heat transfer efficiency of the evaporative crystallization, HSO₃^- or SO₃^2- needs to be oxidized to form SO₄^2- before evaporative crystallization. Anodic oxidation of HSO₃^- or SO₃^2- coupled with hydrogen evolution can significantly reduce cost of hydrogen evolution due to a low reaction potential. This work uses a filter-press membrane electrode assembly electrochemical reactor to recover commercially valuable (NH₄)₂SO₄ fertilizer and produce hydrogen. It can simultaneously achieve waster recycle and energy storage, which is conformed to the domestic circulation and dual carbon goals. The electrooxidation mechanisms and dynamic parameters of (NH₄)₂SO₃ and NH₄HSO₃ on homemade PtPd_2.75/C catalyst were investigated, particularly by cyclic voltammetry and rotating disk electrode system. According to Randles-Sevĉik equation and Levich equation, the number of the electron transfer during the electro-oxidation of SO₃^2- or HSO₃^- is 1.86. The diffusion coefficients of SO₃^2- and HSO₃^- are 2.29 × 10^-6 cm²·s^-1 and 1.18 × 10^-5 cm²·s^-1, respectively. A 1 cm × 1 cm electrolyser was homemade by graphite. The desulfurization wastewater was used as the anolyte, while water as the catholyte. The anolyte and catholyte were separated by a proton exchange membrane. The homemade PtPd_2.75/C catalyst was used as both sulfite oxidation catalyst and hydrogen evolution catalyst. The catalyst was loaded to carbon clothes firstly, and then hot-pressed to the proton exchange membrane to fabricate the membrane electrode assembly. The influences of operation conditions on the electrolyser performances have been studied by potentiodynamic scans and electrochemical impedance spectroscopy. The optimal conditions were chosen as follow: pH = 7.5 of the ammonium sulfite wastewater as the anolyte, pure water as the catholyte, 50 ^oC. The electrolyser exhibited excellent SO₃^2- electro-oxidation performance and stability. Under the optimal experimental conditions, the electrolyser could achieve 294.63 mA·cm^-2 at 1.5 V. At a current density of 200 mA·cm^-2, the SO₃^2- conversion rate could reach 94% without exceeding the applied cell voltage of 2 V. It could produce 0.70 t ammonium sulfate and 2.98 kg hydrogen when 1 m³ ammonium sulfite wastewater was electrolyzed for 20 h. The electricity consumption was 137.24 kWh per m³ wastewater, which can create a profit of 1302.70 yuans. Such a strategy has shed a light on further development towards industrial application.

Key words: desulfurization wastewater, electro-oxidation, hydrogen evolution, ammonium sulfate, recycling