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为有效防治农业杂草,实现农作物保护和农产品产量的提高,近年来除草剂的使用量逐渐增加,成为全球用量最大的一类农药[1-3]。莠去津作为三嗪类除草剂的代表被广泛用于小麦、甘蔗、玉米、高粱等多种作物田间阔叶杂草的防治[4-6]。莠去津水溶性好且半衰期长(约30—100 d),通过地表径流和雨水冲刷等方式很容易进入水体,造成其在环境中的残留问题[7]。残留在环境中的莠去津对非靶标生物和人类均有致癌致畸风险[8-9]。此外,由于残效期长、生物可降解性较低,残留在环境中的莠去津还会对后茬作物产生药害[10-11]。因此,亟需开发经济高效、二次污染少的莠去津去除技术。
目前,莠去津的去除方法主要有物理吸附、生物降解和化学降解等。Yan等[12]利用碳纳米管实现对莠去津的吸附,结果表明,莠去津的吸附量不仅与碳纳米管的比表面积大小有关,材料的表面性质也会对吸附能力产生影响。Boruah等[13]设计合成了Fe3O4/石墨烯复合材料,并用于水溶液中莠去津的吸附,磁性材料的加入,使得分离过程更易实现。郑妍婕[14]通过制备多种生物炭材料,实现土壤中莠去津的吸附。生物降解主要通过筛选各种降解菌实现,耗时较长,且微生物易受到环境因素的影响[15-16]。化学降解法具有较高稳定性,其中光催化降解技术近年来发展迅速[17-19]。TiO2、ZnO、Ag3PO4、CdS等金属纳米材料及其复合材料[20-24]等被应用于莠去津的光催化降解。由于金属催化剂自身对环境具有污染性,紫外光响应的特点又限制了光能的充分利用。因此从环境保护和能源利用角度出发,可见光响应催化剂具有更大的应用潜力。
石墨型氮化碳(g-C3N4)材料是一种新型非金属纳米催化剂,可见光响应特性使g-C3N4纳米材料对自然光的利用率大大提升[25]。g-C3N4可被用于多种污染物的光催化降解。但基本型g-C3N4材料受到比表面积较低,光致空穴和电子对重组率高等的限制,光催化活性较低。因此在实现比表面积增大的基础上,调整材料的能带结构,提高光能利用率[26-27],有助于提高材料的光催化性能。从环境保护角度出发,非金属元素掺杂在提高g-C3N4催化活性的同时还能避免金属材料的二次污染问题。
基于此,开发一种高效、环保、经济的莠去津降解技术具有重要意义。本文制备了多孔氮化碳纳米材料,并将其应用于莠去津的可见光催化降解。比较水热处理和浸泡处理两种方式对材料性能的影响,并对材料进行了系统表征分析,明确材料属性与降解性能的相互关系。同时研究莠去津浓度、pH值变化及催化剂用量对降解过程的影响。结合活性物种捕获实验,对降解机理进行了阐述。以期为高残留除草剂的污染治理提供理论和技术支持。
多孔氮化碳纳米材料光催化降解莠去津的性能及机理研究
Fabrication porous carbon nitride for photocatalytic degradation of atrazine: Influencing parameters and mechanism
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摘要: 本文利用磷酸水热法制备了多孔氮化碳(PCN-H)纳米材料,通过紫外-可见吸光光谱、扫描电子显微镜、X射线衍射、X射线光电子能谱和比表面积分析等多种手段表征催化剂的形貌、光学属性及结构特点等。在可见光照射下光催化降解除草剂莠去津,评价材料对莠去津的催化降解活性。分别测试最优化氮化碳材料在不同pH条件下对莠去津降解效率的变化,并分析催化剂用量和除草剂浓度对降解率的影响。结合活性物种捕获实验,阐述莠去津可见光降解的机理。通过材料表征结果分析,PCN-H表现为独特的多孔结构,比表面积分别是基础石墨型氮化碳(MCN)和磷酸浸泡氮化碳(PCN-S)的4.3倍和3.0倍。磷酸水热处理成功实现磷元素的掺杂,可见光利用率明显提高,1 h内即可将莠去津降解率从18.4%提升至45.7%。酸性条件有助于PCN-H对莠去津催化降解。在PCN-H可见光催化降解莠去津的过程中,光致空穴和超氧自由基发挥主要作用。该方法制备的材料光能利用率高,避免了金属催化剂自身对环境的潜在污染,酸性条件下降解更为高效,有助于减轻农药污染对农业生态环境及非靶标生物造成的负面影响。Abstract: To improve the photocatalytic efficiency, bulk carbon nitride (MCN) was treated by hydrothermal process with phosphoric acid in the present study. The resulting sample was denoted as PCN-H. The morphology, optical properties, and structure of prepared CN samples were characterized by UV-Vis spectrophotometer, scanning electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, and BET analysis. The effect of the degradation of atrazine by PCN-H was investigated. The reaction mechanism was identified via active species trapping experiments. PCN-H with porous structure was successfully fabricated. The Brunauer-Emmett-Teller (BET) surface area of PCN-H is about 4.3 times and 3.0 times than that of MCN and PCN-S, respectively. Compared to PCN-S and MCN, the enhanced light absorption ability was obtained via P doping. The degradation percentage increased from 18.4% to 45.7% just in an hour. Acidic condition is conducive to high degradation efficiency. Light induced hole and superoxide radical are the main active species of PCN-H during the photocatalytic degradation of atrazine. The as-prepared PCN-H exhibits considerably high photocatalytic activity under visible light irradiation, which can also overcome contamination problem caused by metal catalyst. This work proved that PCN-H has great potential to be applied in alleviating negative effects of pesticide on agricultural ecological environment and non-target organisms.
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Key words:
- carbon nitride /
- doping /
- atrazine /
- photocatalysis /
- mechanism
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表 1 催化剂CN材料的比表面积,孔体积和平均孔径
Table 1. The SBET, pore volume and average pore diameter of the prepared CN samples
材料 Sample 比表面积 SBET/(m2·g−1) 孔体积 /(m3 ·g−1) Pore volumes 平均孔径/nm Average pore diameters MCN 14.10 0.11 32.17 PCN-S 20.36 0.21 28.19 PCN-H 60.25 0.48 23.73 -
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