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地下水中硝酸盐污染严重,不仅对生态环境稳定[1]、动植物生存造成潜在威胁[2],甚至其直接或间接进入人体后,直接危害人类健康[3-5]。因此,世界卫生组织和我国最新实施的生活饮用水卫生标准(GB 5749-2022)规定饮用水中硝酸盐氮(NO3−-N以N计)含量都为不超过10 mg·L−1。基于对人类健康和环境保护的角度,去除地下水中的硝酸盐意义重大。
常见的处理方法有生物处理法[6]、物理处理法[7]和化学还原法[8]。电催化还原法易于控制、反应高效、催化剂稳定且能二次利用[9],在废水脱氮领域优势独特。有研究表明利用三维粒子电极催化还原NO3−-N较二维催化效果提升显著[10],然而三维电催化还原技术中对催化剂载体的精准化设计和调控技术瓶颈突出,其差异直接影响催化剂催化活性组分的分散性和电子效应,从而制约催化反应活性[11]。有研究表明,生物炭[12]、金属氧化物[13]和金属氢氧化物[14]等可作为还原NO3−-N的催化剂载体,生物炭因其高比表面积、强稳定性结构和可再生等特点倍受关注。目前,关于以生物炭为基材的负载型催化剂颗粒尺寸往往只在纳米或亚纳米级别[15-16],这种传统催化剂的原子利用率和去除效果不足以满足现有高质量浓度NO3−-N废水处理的需求[17],而将颗粒尺寸从纳米或亚纳米级别减小到单一原子可大幅度提高催化剂的原子利用效率、催化活性和结构稳定性[18-20]。
因此,本研究拟通过杂原子氮掺杂的方式,利用氮掺杂+煅烧还原法将毛竹炭载体与目标金属钯铜原子的配位环境及颗粒尺寸进行有效控制,制备出单原子钯铜改性毛竹炭,并应用于电催化高效还原水中的NO3−-N,同时对粒子电极的循环利用性以及NO3−-N电催化还原过程机理进行研究,结果以期为地下水脱氮提供技术方法支持。
单原子钯铜改性毛竹炭电催化高效还原水中硝酸盐
Electrocatalytic efficient reduction of nitrate in water by single-atom palladium-copper modified bamboo biochar
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摘要: 针对地下水中硝酸盐氮去除难的问题,通过氮掺杂+煅烧还原法制备获得了单原子钯铜改性毛竹炭(palladium-copper single-atom catalysts supported on nitrogen-doped bamboo biochar, SAC-Pd/Cu@NBC),以其为三维粒子电极,考察了初始硝氮质量浓度、粒子电极投加量、电流强度对SAC-Pd/Cu@NBC电催化高效还原硝酸盐氮的机理。结果表明:在电催化反应90 min时,SAC-Pd/Cu@NBC显著提高了硝酸盐氮的去除率,较纳米钯铜改性毛竹炭、二维电催化还原体系分别提升了2.52倍和17.16倍;在粒子电极投加量为0.100 g、初始硝氮质量浓度为100 mg·L−1、电流强度为220 mA和反应时间为180 min的条件下,SAC-Pd/Cu@NBC对硝酸盐氮去除率可达99.62%,质量催化活性为0.689 4 mg·(g·min)−1;粒子电极经过3次循环使用后,对硝酸盐氮的去除率仍达89.72%;HAADF-STEM等表征结果表明单原子Pd、Cu的成功负载,其可高效还原硝酸盐氮的机理主要为单原子Pd位点与Cu位点的协同催化。因此,SAC-Pd/Cu@NBC具备良好的电催化还原水中硝酸盐应用前景。Abstract: Nitrate removal from groundwater is a difficult problem, palladium-copper single-atom catalysts supported on nitrogen-doped bamboo biochar (SAC-Pd/Cu@NBC) were prepared through the nitrogen doping- calcination reduction method. Taking SAC-Pd/Cu@NBC as a three-dimensional particle electrode, the effects of initial nitrate concentration, particle electrode dose and current intensity on the performance and mechanism of electrocatalytic efficient reduction of nitrate nitrogen by SAC-Pd/Cu@NBC were investigated. The results showed that SAC-Pd/Cu@NBC significantly enhanced the removal rate of nitrate after 90 min electrocatalytic reaction, which was 2.52 times and 17.16 times higher than that of nano-palladium-copper modified bamboo biochar and two-dimensional electrocatalytic reduction system, respectively. At particle electrode dose of 0.100 g, initial nitrate concentration of 100 mg·L−1, current intensity of 220 mA and reaction time of 180 min, the removal rate of nitrate nitrogen by SAC-Pd/Cu@NBC could reach about 99.62%, and the mass catalytic activity was 0.689 4 mg·(g·min)−1. After three cycles of the particle electrode, the removal rate of nitrate nitrogen was still 89.72%. HAADF-STEM and other characterization results showed the single-atom Pd and Cu were successfully loaded on the surface of biochar, and the mechanism of highly efficient reduction of nitrate was dominated by the synergistic catalytic effect of single-atom Pd and Cu sites. Therefore, these results suggested that the Pd/Cu single-atom catalytics had a good prospect in the electrocatalytic reduction of nitrate.
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表 1 不同粒子电极投加量下催化剂的质量催化活性及NO3−-N去除率
Table 1. Mass catalytic activity and NO3−-N removal rate of different particle electrodes
投加量/g 质量催化活性/(mg·(g·min)−1) NO3−-N去除率/% 0.075 0.751 0 82.31 0.100 0.672 1 96.21 0.125 0.509 4 95.99 0.150 0.452 8 94.63 表 2 粒子电极的质量催化活性及一级反应动力学参数
Table 2. Mass catalytic activity and first-order reaction kinetic parameters of the particle electrode
初始NO3−-N质量
浓度/(mg·L−1)质量催化活性/
(mg·(g·min)−1)一级动力学参数 k/min−1 R2 50 0.327 5 0.015 8 0.959 8 75 0.485 1 0.016 5 0.997 0 100 0.672 1 0.018 3 0.996 9 125 0.789 6 0.014 3 0.997 8 -
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