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全氟化合物(perfluorinated compounds, PFCs)是消防泡沫、石油勘探、食品包装、服装和纺织品等生产活动的常见排放物。近年来,由于我国氟化工产业迅速发展造成其在多种水体中频繁检出[1-3]。SUN等[4]调查了上海市39个地表水样品中17种PFCs的污染状况,发现有10种PFCs检出。QI等[5]在天津市地下水中检测到13种全氟辛烷磺酸,其中全氟辛酸(perfluorooctanoic acid, PFOA)和全氟丁烷磺酸(perfluorobutane sulfonic acid, PFBS)含量最高。LI等[6]调研了中国79个城市饮用水中17种PFCs的含量,总浓度为4.49~174.93 ng·L−1,平均值为35.13 ng·L−1。PFCs通过呼吸、饮水和饮食等途径进入人体后,对肝脏功能、脂肪代谢和遗传发育均有不良影响,可能导致哮喘,肝肿瘤和慢性肾病等[7]。因此,亟需研发水中PFCs的高效去除技术。
PFCs分子具有高能C—F键,其结构性质稳定且极难被氧化或微生物降解。吸附是目前去除水PFCs最为经济可行的技术,常用的吸附剂主要有活性炭、树脂等。其中活性炭作为一种具有发达孔隙结构及高比表面积的广谱吸附剂,因其相对低廉的成本而被广泛应用。大量研究表明活性炭在去除PFCs方面,尤其是长链PFCs,具有较好的应用潜能。例如,孙博等[8]发现采用椰壳粉末炭实现了超纯水中90%以上长链PFOA的去除,但对于短链全氟丁酸(perfluorobutyric acid, PFBA)去除效果不佳(低于10%)。SON等[9]研究也发现煤质和椰壳粉末活性炭对长链全氟辛烷磺酸(perfluorooctanesulfonic acid, PFOS)和PFOA去除率分别可达50%~95%和30%~90%,但对短链PFBS去除率仅为20%~40%。因此,活性炭吸附是目前去除水中PFCs最为可行处理技术工艺,然而如何将活性炭吸附与其他工艺组合提升PFCs,尤其是短链PFCs的处理效能,仍需进一步深入研究。
在目前的净水工艺中,尤其在农村净水工艺中,普遍采用高级氧化与吸附技术相结合。基于UV的高级氧化技术操作简单、氧化能力强、且二次污染较小,得到了广泛应用。该过程主要通过光化学反应激发氧化剂产生高反应活性的自由基实现污染物的高效去除,其中UV与H2O2的组合是最常见的高级氧化过程。JAVED等[10]的研究表明在UV体系中H2O2对水中PFOA去除率的提高没有明显作用,UV的直接光降解是PFOA的主要降解机制。然而TANG等[11]的研究得出了不同的结果,报道显示UV-Fenton高级氧化在1h内可实现87.9%的PFOA去除率,且UV照射时长和氧化剂质量浓度对PFOA的去除有一定影响,氧化剂质量浓度增加对PFOA的去除有一定增益,但超过20 mmol·L−1时,PFOA的去除率会降低。此外,PFCs的初始质量浓度对高级氧化对其的降解效率有一定影响,质量浓度较高时PFCs降解速率受到抑制,因此研究人员建议PFCs的高级氧化降解探究应采用μg·L−1质量浓度水平[12]。目前关于高级氧化降解PFCs的研究主要集中在PFOA和PFOS上,PFOS可以经自由基反应转化为PFOA,最后转化为氟离子、甲酸和二氧化碳等[13-14]。虽然对于高级氧化技术去除PFCs有了一定的研究进展,然而高价氧化处理污染水体,会存在矿化不完全的问题,易形成毒性较高的中间产物,因此需要与活性炭吸附技术联用,消除中间产物的潜在环境风险。此外,目前对于高级氧化与活性炭吸附联合处理PFCs的报道较少,且已有研究存在一定的分歧,因此针对水中PFCs去除的UV高级氧化与吸附联用处理技术的进一步探究是有必要的。
因此,本研究利用快速小柱实验法(rapid small-scale column tests, RSSCT),对比分析了UV/GAC、H2O2/GAC和UV/H2O2/GAC 3种组合工艺对水中不同链长的PFCs包括PFBA、PFBS、PFOA和PFOS的去除效果和影响机制,以期为水中PFCs去除的工艺选择提供科学依据。
UV/H2O2/GAC工艺去除水中全氟化合物
Removal of PFCs from water by UV/ H2O2/GAC process
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摘要: 全氟化合物(PFCs)持久存在于水环境中,难以通过传统的水处理工艺去除,提高水中PFCs的去除性能具有重要意义。本研究采用快速小柱实验探究了UV、H2O2和颗粒活性炭(GAC)组合工艺对水中4种PFCs的去除效果,并探究了UV照射时长和H2O2质量浓度对PFCs的直接去除效果及其对后续GAC吸附的影响。结果表明:不同工艺条件下,长链全氟辛酸(PFOA)和全氟辛烷磺酸(PFOS)始终表现出较短链全氟丁酸(PFBA)和全氟丁烷磺酸(PFBS)更高的去除率。单独GAC吸附对长链全氟PFOA和PFOS去除率为59.6%和64.3%,但对短链PFBA和PFBS去除率仅为11.7%和13.1%。单独UV或单独H2O2分别与GAC联用时,随着UV照射时长和H2O2质量浓度的增加,4种PFCs的去除率略有增加,且UV照射的增益效果较H2O2更优。UV/H2O2/GAC联用工艺对水中4种PFCs的去除效果最佳,30 min UV/H2O2处理后经GAC吸附可去除90%以上的PFOA和PFOS,短链PFBA和PFBS去除率也达到50%以上。UV/H2O2/GAC联工艺的良好效果是由于UV/H2O2高级氧化过程产生的羟基自由基氧化反应降低了水中大分子有机物的含量,从而减弱了有机物对PFCs的竞争吸附作用。本研究可为水中PFCs的高效处理提供技术参考。Abstract: Perfluorinated compounds (PFCs) are persistently found in aquatic environment, which are difficult to be removed by the conventional water treatment process. Improving the removal effect of PFCs in water is of great significance. The removal efficacies of four kinds of PFCs in water by the combined process of UV, H2O2 and GAC were studied using the rapid small-scale column tests (RSSCT), and the effects of UV irradiation duration and H2O2 concentration on the direct oxidation of PFCs and the subsequent GAC adsorption were also explored. The results showed that long-chain perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) consistently exhibited higher removal rates than short-chain perfluorobutyric acid (PFBA) and perfluorobutane sulfonic acid (PFBS) under different process conditions. The removal rates of long chain perfluorinated PFOA and PFOS by GAC adsorption alone were 59.6% and 64.3%, respectively, but the removal rates of short chain perfluorinated PFBA and PFBS were only 11.7% and 13.1%, respectively. When UV or H2O2 were combined with GAC, the removal rates of the four PFCs slightly increased with the increase of UV irradiation time and H2O2 concentration, and the gain effect of UV irradiation was better than that of H2O2.The combined process of UV/H2O2/GAC presented the best removal effect of the four PFCs in water. After 30 minutes of UV/H2O2 treatment, GAC adsorption could remove higher than 90% of PFOA and PFOS, and the removal rates of short chain perfluorinated PFBA and PFBS were over 50%. After 30 min UV/H2O2/GAC treatment, more than 50% PFBA and PFBS were removed, and more than 90% of PFOA and PFOS can be removed. The excellent effect of the UV/H2O2/GAC combined process was due to the hydroxyl radical oxidation reaction generated by the advanced oxidation process of UV/H2O2, which weakened the competitive adsorption of organic matter on PFCs. This study can provide a technical guidance for the treatment of PFCs in water.
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表 1 三维荧光光谱5个分区
Table 1. Five regions of three-dimensional fluorescence spectra
荧光区域 有机物名称 激发波长(Ex)/nm 发射波长(Em)/nm Ⅰ 酪氨酸类 Ex<250 Em<330 Ⅱ 色氨酸类 Ex<250 330<Em<380 Ⅲ 富里酸类 Ex<250 Em>380 Ⅳ 微生物产物类 250<Ex<280 Em<380 Ⅴ 腐殖酸类 Ex>280 Em>380 表 2 目标PFCs的质谱分析条件
Table 2. Target analytes and the MS/MS parameters used in this study
目标物 母离子 子离子 破碎电压/V 碰撞能/V PFBA 213 169.1 57 10 PFBS 299 80 135 32 PFOA 413 368.9 82 4 PFOS 498.85 79.97 80 40 -
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