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在美丽中国建设和双碳国家战略背景下,对现有城镇污水处理厂出水水质的进一步提升将导致能源消耗、药剂使用等迅速增加,不利于城镇污水处理厂碳中和目标的实现. 因此,新型污水处理技术的开发将是我国下阶段城镇污水处理厂技术升级的关键. 厌氧膜生物反应器是近年来得到迅速发展的新型污水处理工艺[1],采用一段式处理工艺可实现有机污染物的高效去除,如COD浓度可降到30 mg·L−1 左右,因不需曝气、产泥量低等优点可有效降低能耗及运行费用,极大地提升污水处理的碳中和潜力;但对氮磷的去除效率较低而不能满足排放要求[2-3],限制其实际应用. 水体中硝酸盐氮既可对人体造成蓝婴综合症甚至致癌作用,也可引起水体富营养化[4]. 吸附、电絮凝和生物方法等可用于去除水中硝酸盐氮,但由于投资高等导致应用难度大[5]. 光催化因具有适应能力强、费用低、占地少以及无二次污染等优势成为关注的热点[6-7],已成为水体环境污染治理的热点技术之一[8-10]. 硝酸盐氮作为污水中总氮主要组成也成为光催化还原的主要处理对象[11-13],有望成为碳中和背景下城镇污水处理可行工艺.
TiO2作为一种常见催化剂被应用于硝酸盐氮的光催化还原[14-15]. 为了提高光催化转化效率,采用Cu[16]、Cr[17]、Co[18]、Pb[19]、Fe[14]等金属掺杂制作新型TiO2催化材料. 但由于TiO2带隙较宽,不能有效利用太阳光,限制其实际应用,g-C3N4光催化剂因带隙较窄被应用到硝酸盐氮的还原,在还原效率及氮气选择性方面均取得了较好的效果[12,20]. 同样具有可见光响应的BiOX(X=Cl、Br、I)催化剂可用于光催化还原,但主要集中在CO2[21]、H2[22]、Cr6+[23]等还原,而对硝酸盐氮还原的研究较少. BiOX是具有依靠范德华力连接的[Bi2O2]2+和X−特殊结构的P型半导体材料[24],可减少电子-空穴的复合[25]. 相对于BiOCl相对较宽的带隙(3.2—3.4 eV),BiOBr和BiOI带隙相对较窄,分别为2.6—2.9 eV和1.6—1.8 eV,具有更宽的可见光吸收范围[26]. 为了促进催化剂的光催化效果,不同的复合催化剂被开发,如Ag/Ag2O/AgBr/BiOBr[27]、Ag/AgBr/BiOBr[28]、pg-C3N4/BiOBr/Ag[29]等. 可见Ag被广泛的用于提升Bi系光催化剂的光催化性能,这主要是由于银具有高导热和高电导率,以及表面等离子体共振. 如Yu等[30]发现在可见光下1%—2%Ag沉积率可促进BiOBr催化活性的迅速增加. Cushing等[31]发现纳米银颗粒可促进催化剂的红移以及促进光催化现象. Zhu等[32]采用纳米银掺杂BiOCl,可使光吸收边界从350 nm扩展到600 nm.
本研究针对碳中和背景下城镇污水技术发展趋势,选择低浓度硝酸盐氮(25 mg·L−1以氮计)为处理对象,采用反应合成法制备BiOBr,并用光还原方法沉积Ag. 利用SEM、XRD、XPS、UV-vis等现代材料分析技术研究复合材料的物相、形貌及表面物理化学性能. 探索光催化材料对硝酸盐氮的去除效果及其氮气选择性,同时研究了pH值对光催化反应的影响及催化剂的稳定性.
Ag/BiOBr材料的制备及其光催化还原硝酸盐氮
Preparation of Ag/BiOBr and photocatalytic reduction of nitrate
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摘要: 采用反应合成法及光还原沉淀法制备Ag/BiOBr复合光催化材料,通过SEM、XRD、FT-IR、XPS、UV-vis等对其进行表征,并研究了该复合材料在金卤灯照射下对低浓度硝酸盐氮的还原效果. 结果表明,采用Ag/BiOBr光催化剂时,硝酸盐氮去除率为73.5%,相比BiOBr光催剂去除率增加31.1%;pH值为4时,硝酸盐氮去除率和产物中氮气占比最高,在180 min时硝酸盐氮去除率达82.8%,产物中氮气占比为76.5%,氮气选择性为92.4%. Ag沉积可提升BiOBr催化性能,主要是由于其费米能级相对于光催化剂价带处于更低的能级,光激发电子被Ag颗粒捕捉,减少了与空穴的复合,促进硝酸盐氮的还原. 同时空穴清除剂(甲酸)氧化过程生成过氧化物自由基(COO∙-),也可促进硝酸盐氮的还原. 经4次重复实验,硝酸盐氮去除率在82.9%以上,可见该催化剂具备良好的稳定性,具有较好的应用前景.Abstract: Ag/BiOBr composite photocatalyst was prepared by the reactive synthesis and photoreduction precipitation method, and it was characterized by scanning electron microscope (SEM), x-ray diffraction (XRD), fourier transform infrared spectra (FT-IR), x-ray photoelectron spectroscopy (XPS) and ultraviolet visible diffuse reflectance spectroscopy (UV-vis). The degradation effect of the composite material on the nitrate with low concentration under the irradiation of gold halide lamp was studied. The results showed that the removal rate of nitrate was 73.5% when Ag/ BiOBr photocatalyst was used. Compared with BiOBr photocatalyst, the removal rate increased by 31.1%. When pH value was 4, the nitrate reduction rate and nitrogen in the product were the highest. When the treating time is 180min, the nitrate removal rate, nitrogen in the production and nitrogen selectivity were 82.8%, 76.5% and 92.4%, respectively. The deposition of Ag can improve the catalytic performance of BiOBr and promotes the reduction of nitrate because it reduces the recombination of electrons with holes. This is mainly because the Fermi level of Ag is at a lower level than the valence band of photocatalyst and the photoexcited electrons are captured by Ag particles. Meanwhile, the formation of peroxide radical (COO∙-) during the oxidation of hole scavenger (formic acid) can also promote the reduction of nitrate. The results of 4 times repeated experiments indicate that the nitrate removal rate is above 82.9%. It suggests that the catalyst has good stability and good application prospect.
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Key words:
- Ag/BiOBr /
- nitrate /
- photocatalysis /
- nitrogen selectivity.
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