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早在20世纪,硝酸盐污染就已成为国际上普遍关注的问题,硝酸盐污染主要来源于大气沉降、土壤中化肥农药的大量使用、生产生活中排放的污水及生物的代谢废物等,其特点主要有稳定性高、溶解性高和易迁移[1]。特别是对于以地下水为主要饮用水源的国家和地区来说,该问题还突出表现为对人体健康的潜在威胁,根据国际流行病学研究,饮用水中高水平的硝酸盐除了具有婴幼儿高铁血红蛋白症的急性风险,还可能会增加人体内致癌化学物质的形成。新西兰的一项关于饮用水中硝酸盐污染程度的研究认为,3.26%的结直肠癌可归因于硝酸盐,导致了100例确诊病例和41例死亡病例[2]。为此,世界卫生组织(WHO)以及各国均制定了水中硝酸盐(以N计,下同)的浓度标准,如欧盟规定了饮用水中硝酸盐的质量浓度应少于11.3 mg·L−1,WHO、美国、日本、加拿大规定了硝酸盐的最高限值为10 mg·L−1,我国最新修订的《生活饮用水卫生标准》(GB 5749-2022) 中也规定了饮用水中硝酸盐的限值为10 mg·L−1。
当前,我国北方的地下水中硝酸盐超标情况不容乐观。一项关于北京市潮白河中上游地区地下水中硝酸盐含量的研究[3]表明,267个水样中硝酸盐氮超标率达到28.84%,平均质量浓度为15.89 mg·L−1,最高值为65.5 mg·L−1。有学者对青岛市农区地下水的检测发现约28.57%的地下水样品严重超标,85.71%超过了WHO规定的标准[4]。内蒙古的一项研究[5]表明,该研究区地下水中硝酸盐质量浓度在0.8~109.57 mg·L−1,平均为23.57 mg·L−1,且该地区地下水的非致癌性慢性毒性影响中约有10%的儿童超出其能够接受的健康风险。
目前去除水中的硝酸盐的方法包括物理法如离子交换法[6]、反渗透法[7]、生物反硝化[8]和化学法[9]。近年来,高级还原技术表现出了良好的发展潜力和应用前景,其原理是通过催化剂或紫外光等活化手段,使得还原剂产生具有强还原能力的自由基,能够将水中具有氧化性的污染物还原去除。目前已有多种可以去除硝酸盐的高级还原体系被提出,如产生水合电子(eaq−)的UV/亚硫酸盐体系[10]、产生二氧化碳自由基(·CO2−)的UV/甲酸体系[11-13]和UV/过氧化氢/甲酸盐体系[11]等。
·CO2−是一种强还原性物质,氧化还原电位为E0=-1.9 V[14],其来源大多为含有羧基基团的简单有机物或有机盐,通过加热或紫外照射等方式活化产生。常见的产生方法是将氧化剂过硫酸盐与甲酸耦合[15],但该方法会引入硫元素,存在二次污染的风险,因此,在选择·CO2−的活化底物时应考虑其清洁性和安全性,同时有广泛的获取途径。
乙酸钠作为一种含有羧基基团的小分子有机酸盐,无毒无害,价格低廉,是污水处理厂最常用的碳源之一。为此,本研究提出一种利用乙酸钠作为产生·CO2−的前体物,在紫外光的照射下还原水中硝态氮的方法,考察了乙酸钠投加浓度、初始pH、Cl−浓度、SO42−浓度、HCO3−浓度、溶解性有机物浓度、光照强度对该体系还原效能的影响,并通过ESR对反应中所产生的自由基种类进行了鉴定,且进一步推测反应机理,以期为水体中硝酸盐高效快速去除提供一种方法参考。
紫外光/乙酸钠体系去除水体中硝态氮
Nitrate reduction by UV/ sodium acetate system
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摘要: 通过构建紫外光(UV)/乙酸钠高级还原体系(ARP),产生强还原性的二氧化碳自由基(·CO2−),从而将水中硝酸盐(NO3−)转化为气体溢出。采用电子自旋共振波谱仪(ESR)对反应体系中·CO2−进行了鉴定,并考察了乙酸钠初始投加量、初始pH、水中常见的离子及溶解性有机物对反应体系还原效能的影响。结果表明:在硝酸盐的初始质量浓度(以N计)为16 mg·L−1,乙酸钠投加量为4 mmol·L−1,反应溶液的pH为7.45,采用125 W、365 nm的高压汞灯作为光源时,紫外光/乙酸钠体系对水中硝酸盐和总氮(TN)的去除率分别为98.4%和86.4%。水体的酸性条件和紫外灯高功率有利于提高反应效能,而水中常见的阴离子如氯离子(Cl−)、硫酸根离子(SO42−)、碳酸氢根离子(HCO3−)及溶解性有机物如腐殖酸(HA)会对NO3−的去除效率也有一定影响。Abstract: The ultraviolet light (UV)/sodium acetate advanced reduction process (ARP) system was constructed to produce the carbon dioxide anion radical (·CO2−) with a strong reducing power, which could converted nitrate in water into gas overflow. The production of ·CO2− in the reaction system was confirmed by electron spin resonance (ESR), and the effects of the initial dosage of sodium acetate, initial pH, common ions and dissolved organic matter on the efficiency of reaction were investigated. The results showed that when the initial concentration of nitrate was 16 mg·L−1 and the dosage of sodium acetate was 4 mmol·L−1, the pH of the reaction solution was 7.45, and a 125 W, 365 nm high-pressure mercury lamp was used as the light source, the degradation rates of nitrate and total nitrogen (TN) in the UV/sodium acetate system reached 98.4% and 86.4%, respectively. Acidic conditions and high power of UV lamp were conducive to improving reaction efficiency. Common anions in water such as chloride ion, sulfate ion, bicarbonate ion and dissolved organic matter such as humic acid (HA) had a certain impact on the removal efficiency of nitrate.
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