多酸嫁接强化硫化磁赤铁矿低温化学吸附零价汞的性能及机制研究

The existing mercury pollution control technologies in coal-fired power plants fail to achieve the centralized control of elemental mercury emission, but it converts elemental mercury into divalent mercury with a higher toxicity, which is eventually transferred into fly ash or desulfurized gypsum. However, it is easy to produce secondary pollution. Our research group has proposed a new idea of using a recyclable magnetic sorbent for recovering elemental mercury to achieve the centralized control of elemental mercury from coal-fired flue gas as a co-benefit of wet electrostatic precipitators. In our previous study, sulfurated maghemite was developed to recover elemental mercury from coal-fired flue gas as a co-benefit of wet electrostatic precipitators. However, the thermal stability of sulfurated maghemite was poor, which made it prone to phase transition and loss of magnetism during the process of recovering elemental mercury through the thermal desorption. This was not conducive to recovering the sorbent for recycling. In this project, sulfurated maghemite will be modified with polyoxometalates grafting. On the one hand, the phase transition of sulfurated maghemite will be restrained through regulating the interface effect of polyoxometalates and maghemite, which will significantly improve its thermal stability and magnetism. On the other hand, the amount of active sulfur species on sulfurated maghemite will also be notably increased by controlling the sulfuration effect on polyoxometalates grafted maghemite, which will remarkably improve its performance for elemental mercury capture at low temperatures. In summary, this project will develop high-performance and recyclable magnetic sorbents for recovering elemental mercury from coal-fired flue gas as a co-benefit of wet electrostatic precipitators, which provides an important technology support for promoting the transformation and upgrading of mercury pollution control in China’s coal-fired power plants from valence state regulation to the centralized control.

现有燃煤电厂汞污染控制技术未能实现零价汞排放的集中控制，而是将零价汞转化成毒性更高的二价汞，最终转移到飞灰或脱硫石膏中，极易产生二次污染。我们课题组提出了利用可循环使用的磁性吸附剂协同湿式电除尘器回收燃煤烟气中的零价汞加以集中控制的新思路，并试图将硫化磁赤铁矿与湿式电除尘器相结合集中控制燃煤烟气汞污染。但硫化磁赤铁矿的热稳定性较差，在热脱附回收零价汞的过程中易发生相变而失去磁性，不利于循环使用。为此，本项目拟通过多酸嫁接对硫化磁赤铁矿进行改性：一方面通过调控多酸与磁赤铁矿的界面效应抑制磁赤铁矿相变而提高其热稳定性，进而提高其磁性；另一方面通过控制多酸嫁接磁赤铁矿表面的硫化效应而提高其表面活性硫物种的含量，进而提高其零价汞吸附能力。本项目将开发出能协同湿式电除尘器回收燃煤烟气中零价汞的高效并可循环使用的磁性吸附剂，为推动我国燃煤电厂汞污染控制从形态调控向集中控制的转型升级提供重要技术支撑。

随着具有全球法律约束力的国际公约《水俣公约》的正式生效，我国作为全球最大的汞排放国，面临巨大的汞减排压力。燃煤电厂是我国人为汞排放的重要来源，加强燃煤电厂汞污染控制技术研究已迫在眉睫。但现有的燃煤电厂汞污染控制技术未能实现气态零价汞排放的集中控制，而是将气态零价汞转化成毒性更高的二价汞，最终转移到飞灰或脱硫石膏中，极易产生二次污染。.为此，我们提出了利用可循环使用的磁性吸附剂协同湿式电除尘器回收燃煤烟气中的气态零价汞加以集中控制的新思路。针对硫化磁赤铁矿热稳定性较差的问题，通过多酸嫁接对硫化磁赤铁矿进行改性：一方面通过多酸嫁接抑制磁赤铁矿相变而提高其热稳定性，进而提高其磁性；另一方面通过增加表面硫化物的形态提高其气态零价汞吸附能力。.研究发现磷钼酸和磷钨酸嫁接能够显著抑制磁赤铁矿相变而大幅提高硫化磁赤铁矿的热稳定性，进而显著增强硫化磁赤铁矿的磁性。研究还发现气态零价汞在改性硫化磁赤铁矿表面的吸附主要遵循Mars-Maessen机制，即气态零价汞首先物理吸附在吸附剂表面，然后被吸附剂表面的S22-氧化成硫化汞。依据气态零价汞在改性硫化磁赤铁矿表面的吸附机制，建立了气态零价汞吸附动力学模型。通过吸附动力学研究建立了硫化磁赤铁矿的改性、改性硫化磁赤铁矿的物理化学性质及其低温吸附气态零价汞性能三者之间的构效关系，即改性硫化磁赤铁矿吸附气态零价汞的速率与其表面S22-数量和气态零价汞物理吸附位数量的乘积成正比，吸附气态零价汞的容量等于其表面S22-数量和气态零价汞物理吸附位数量两者的较低值。依据建立的构效关系，通过磷钼酸和磷钨酸嫁接等手段进一步提高了硫化磁赤铁矿的气态零价汞吸附能力，开发出了能够协同湿式电除尘器回收燃煤烟气中气态零价汞的高效并可循环使用的磁性吸附剂。.本项目形成的科研成果将为推动我国燃煤电厂汞污染控制从形态调控向集中控制的转型升级提供重要技术支撑。