环境科学  2023, Vol. 44 Issue (3): 1214-1227   PDF    
引用本文
孙红文, 方博, 陈浩, 赵茂森, 张耀之, 乔碧汀, 喻豪. 全氟和多氟烷基化合物微生物降解与转化研究进展[J]. 环境科学, 2023, 44(3): 1214-1227.
SUN Hong-wen, FANG Bo, CHEN Hao, ZHAO Mao-sen, ZHANG Yao-zhi, QIAO Bi-ting, YU Hao. Advances in Microbial Degradation and Transformation of Per- and Polyfluoroalkyl Substances(PFASs)[J]. Environmental Science, 2023, 44(3): 1214-1227.
全氟和多氟烷基化合物微生物降解与转化研究进展
孙红文, 方博, 陈浩, 赵茂森, 张耀之, 乔碧汀, 喻豪     
南开大学环境科学与工程学院, 环境污染过程与基准教育部重点实验室, 天津 300350
收稿日期: 2022-04-14; 修订日期: 2022-06-22
基金项目: 国家自然科学基金项目（22036004，22006074）；高等学校创新引智计划（T2017002）
作者简介: 孙红文(1967~), 女, 博士, 教授, 主要研究方向为新型有机污染物的环境行为, E-mail: sunhongwen@nankai.edu.cn
摘要: 全氟和多氟烷基化合物（PFASs）因其持久性、长距离迁移性、生物积累性和生物毒性而受到广泛关注.目前世界上对环境中PFASs的监测和管控主要针对全氟烷基酸（PFAAs）.而大部分多氟烷基化合物在环境中能够被微生物降解为PFAAs，也被称为前体物.因此，探究前体物在环境中的微生物转化行为有助于综合评价PFASs的环境风险，以及制定相关的管控和修复措施.虽然PFAAs一直被认为是环境中的"永久化合物"，但近年来，PFAAs的厌氧微生物还原脱氟作为一项极具潜力且充满挑战的修复技术，成为研究的一个前沿热点.系统总结了前体物（氟调化合物和全氟辛烷磺胺衍生物）、PFAAs和新型PFASs在微生物作用下的降解规律和转化路径，并讨论了PFASs微生物降解的影响因素，最后提出未来的研究方向.
关键词: 全氟和多氟烷基化合物(PFASs)      全氟烷基酸(PFAAs)      微生物降解      转化路径      氟调化合物      全氟辛烷磺胺衍生物     
Advances in Microbial Degradation and Transformation of Per- and Polyfluoroalkyl Substances(PFASs)
SUN Hong-wen , FANG Bo , CHEN Hao , ZHAO Mao-sen , ZHANG Yao-zhi , QIAO Bi-ting , YU Hao     
Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China
Abstract: Per- and polyfluoroalkyl substances (PFASs) have attracted extensive attention because of their persistence, long-distance migration ability, bioaccumulation, and biological toxicity. Currently, regulatory strategies concerning PFASs in the environment primarily focus on perfluoroalkyl acids (PFAAs). However, most polyfluoroalkyl compounds can be degraded to PFAAs by environmental microorganisms, also known as precursors. Exploring the microbial transformation behavior of precursors is fundamental to comprehensively evaluate the environmental risk of PFASs and formulate control and remediation schemes of PFAS-contaminated sites. Furthermore, anaerobic microbial reductive defluorination of PFAAs is a potential and challenging remediation technology. This review summarizes degradation rules and transformation pathways of precursors (fluorotelomer compounds and perfluorooctane sulfonamide derivatives), PFAAs, and novel PFASs by microorganisms and discusses factors affecting the microbial degradation. Finally, the future research directions are put forward.
Key words: per- and polyfluoroalkyl substances (PFASs)      perfluoroalkyl acids (PFAAs)      microbial degradation      transformation pathways      fluorotelomer compounds      perfluorooctane sulfonamide derivatives     

全氟和多氟烷基化合物(per- and polyfluoroalkyl substances, PFASs)是指结构中至少含有一个全氟甲基(—CF3)或亚甲基(—CF2—), 且全氟碳上无H/Cl/Br/I原子相连的化合物[1].F原子的引入使PFASs具有良好的表面活性、疏水疏油性和稳定性, 被广泛应用于纺织、皮革、电镀、石油、采矿、航天和半导体等工业领域, 以及水性成膜泡沫(aqueous film-forming foams, AFFFs)、农药、夹克、个人护理产品、油漆、不粘炊具和食品接触材料等商业产品中[2~4].由于PFASs的持久性、长距离迁移性和生物积累性, PFASs在土壤[4]、大气[5]、地表水[6]、沉积物[7]和海洋[8]等环境介质以及植物[9]、动物[10]和人体[11]等生物组织中被普遍检出.毒理学研究表明, PFASs具肝毒性、神经毒性、生殖和发育毒性、内分泌干扰效应和致癌性等多种生物毒性, 对人类和其他生物健康构成严重威胁[12~16].

引起广泛关注的PFASs主要是全氟烷基羧酸(perfluoroalkyl carboxylic acids, PFCAs)和全氟烷基磺酸(perfluoroalkyl sulfonic acids, PFSAs)等全氟烷基酸(perfluoroalkyl acids, PFAAs).其中, 全氟辛烷磺酸(perfluorooctane sulfonic acid, PFOS)和全氟辛烷羧酸(perfluorooctanoic acid, PFOA)因其高历史产量、强稳定性、高生物富集性和毒性, 分别于2009年和2019年被列入斯德哥尔摩公约持久性有机污染物附件名单[17].不仅关于PFASs的环境调查主要关注PFAAs, 目前世界各国对PFASs的管控条约也主要针对PFAAs.例如, 我国生活饮用水卫生标准(GB 5749-2022)规定, ρ(PFOA)和ρ(PFOS)的限值分别为80 ng ·L-1和40 ng ·L-1, 美国环保署建议饮用水中ρ(PFOA)和ρ(PFOS)总和不超过70 ng ·L-1[18].

大部分多氟烷基化合物在环境中可以被微生物降解为PFAAs, 也被称为前体物.因此, 在进行PFASs污染场地的风险评估、管控和修复时, 不仅要考虑当下存在的PFAAs, 也有必要把前体物列入监测范围.探究前体物的微生物降解与转化行为能够为管理者制定相关策略提供理论依据.此外, PFAAs虽然一直被认为难以被环境微生物降解, 是环境中的“永久化合物”, 但近年来有研究成功实现了PFAAs的厌氧微生物脱氟.本文系统总结了PFASs微生物降解的研究进展, 深入讨论了：①前体物的降解规律和转化路径；②PFAAs的厌氧微生物脱氟；③其他新型PFASs的微生物转化；④影响PFASs微生物降解的因素.为叙述简洁, 下文提到的“降解”和“转化”均与微生物有关, 化合物以缩写表示, 具体信息见表 1.为方便理解, 部分化合物命名为非标准命名.

表 1 本文提及化合物的缩写及中英文全称 Table 1 Acronym and Chinese and English names of compounds mentioned in this review

1 前体物的微生物降解与转化

根据PFASs生产工艺可将前体物分为氟调化合物(产自氟调工艺)和全氟烷基磺胺(perfluoroalkyl sulfonamide, FASA)衍生物(产自电化学氟化工艺)两大类[2, 19, 20].其中, 6 ∶2氟调化合物、8 ∶2氟调化合物和全氟辛烷磺胺(perfluorooctane sulfonamide, FOSA)衍生物是常见的前体物.降解半衰期(t1/2)和产物的摩尔产率两个指标常用于评估前体物的降解和转化, 前者指母体化合物摩尔转化率达到50%所需时间, 后者是产物与母体化合物摩尔比.

1.1 6 ∶2氟调化合物 1.1.1 传统6 ∶2氟调化合物

6 ∶2 FTOH、6 ∶2 FTI和6 ∶2 FTSA(图 1)是生产6 ∶2氟调化合物的主要原料[19, 21, 22], 也直接应用于工业或商业用途[23].好氧条件下, 6 ∶2 FTOH的降解速率(t1/2＜3 d)[24~32]略快于6 ∶2 FTI(t1/2=4.5 d)[33].6 ∶2 FTSA在不同条件下的降解速率差异显著(表 2)[34~38], 在河流沉积物中t1/2＜5 d[35], 而在好氧污泥中t1/2高达2 a[37].差异的产生与体系中不同的微生物类型和硫酸盐含量有关.脱磺酸作为6 ∶2 FTSA降解的限速步骤, 硫酸盐利用细菌在该过程起到重要作用.过高的硫酸盐含量会抑制—SO3H的断裂[38], 6 ∶2 FTSA降解菌Gordonia sp. NB4-1Y[34, 39]和Rhodococcus jostii RHA1[40]均只能在无硫培养基进行脱硫和脱氟.厌氧环境中, 6 ∶2 FTOH的降解速率显著降低(t1/2=30 d)[41], 而6 ∶2 FTSA十分稳定(表 2)[35, 38, 42].理论上, 6 ∶2 FTSA在厌氧条件下具有一定脱硫潜能, 但相关研究中较高的硫酸盐的含量和碳源的缺乏限制了硫酸盐还原菌对—SO3H的利用.

改自文献[25, 26, 33~36, 41, 43~47] 图 1 6 ∶2氟调化合物微生物转化路径 Fig. 1 Microbial transformation pathways of 6 ∶2 fluorotelomer compounds

表 2 6 ∶2氟调化合物微生物降解半衰期及产物产率对比1) Table 2 Comparison of half-life and product yield of 6 ∶2 fluorotelomer compounds by microbial degradation

6 ∶2 FTUCA是6 ∶2 FTOH的重要中间产物, 其能通过两条不同的途径进行转化：①末端产物为PFBA、PFPeA和PFHxA的PFCAs途径；②末端产物为4 ∶3 Acid和5 ∶3 Acid的x ∶3 Acids途径(图 1)[25, 26, 41].6 ∶2 FTSA和6 ∶2 FTI均能转化为6 ∶2 FTOH, 后续转化路径与6 ∶2 FTOH相同[33~36].Sun等[30]报道在垃圾填埋场土壤中, 6 ∶2 FTOH也能够转化为超短链的TFA.与6 ∶2 FTOH不同的是, 6 ∶2 FTI可通过中间产物为6 ∶2 FTUI的途径生成PFHpA(图 1)[33].

多氟磷酸酯是一类典型的前体物, 6 ∶2 PAP(图 1)、8 ∶2 PAP(图 2)和SAmPAP(图 3)分别由6 ∶2 FTOH、8 ∶2 FTOH和N-EtFOSE合成, 主要用作食品包装材料涂层[48], 能通过水解反应生成对应的醇.对于磷酸二酯, 8 ∶2 diPAP(t1/2＞114 d)的好氧降解速率慢于6 ∶2 diPAP(t1/2为12~15 d)[44], di-SAmPAP在海洋沉积物中(t1/2＞380 d)[49]的降解慢于淡水沉积物(t1/2=88 d)[50].

改自文献[36, 41, 44, 59~61, 68, 69, 72, 73] 图 2 8 ∶2氟调化合物微生物转化路径 Fig. 2 Microbial transformation pathways of 8 ∶2 fluorotelomer compounds

改自文献[49, 76~79, 81~84] 图 3 全氟辛烷磺胺衍生物微生物转化路径 Fig. 3 Microbial transformation pathways of perfluorooctane sulfonamide derivatives

1.1.2 新型6 ∶2氟调化合物

新型6 ∶2氟调化合物主要出现在AFFFs中, 如6 ∶2 FtTAoS、6 ∶2 FTAA和6 ∶2 FTAB(图 1)[51~53].AFFFs由碳氢表面活性剂、PFASs、溶剂和助溶剂构成[54], 能有效熄灭碳氢燃料引起的火灾, 被用作消防部门、军事基地和机场紧急情况和训练用途的灭火剂[55].Ruyle等[56]结合总氧化前体物(total oxidizable precursor, TOP)分析和贝叶斯推理, 发现目前市场上AFFFs中90%的PFASs是6 ∶2氟调化合物.6 ∶2 FtTAoS在好氧土壤(t1/2＜10 d)[45]中能快速降解, 在厌氧土壤中降解缓慢(t1/2＞100 d)(表 2)[46]. 6 ∶2 FTAB虽能被Gordonia sp. NB4-1Y高效降解(t1/2＜7 d)[34], 但在实际环境中的半衰期相对较长, 在石油烃污染土壤中t1/2=31 d[57].

好氧环境中, 6 ∶2 FtTAoS的—S—依次氧化生成亚砜[—S(O)—]和砜[—S(O)2—], 然后断裂C—S生成6 ∶2 FTSA[45].厌氧条件下—S—比较稳定, 此时6 ∶2 FtTAoS的—C(O)NH—断裂生成6 ∶2 FtTP, 该产物不再发生进一步的转化(图 1)[46].6 ∶2 FTAA和6 ∶2 FTAB的转化主要发生在—S(O2)NH—. D'Agostino等[47]发现6 ∶2 FTAA和6 ∶2 FTAB能通过非生物降解生成6 ∶2 FTOH、6 ∶2 FTSA和6 ∶2 FTSAm, 但也有研究指出6 ∶2 FTAB不发生非生物转化[34].氟调磺胺类PFASs的厌氧降解还未见报道, 但对于具有相似结构的磺胺类化合物, 如抗生素磺胺甲噁唑, 在硫酸盐还原条件下被报道能发生—S(O2)NH—的断裂[58].

1.2 8 ∶2氟调化合物 1.2.1 8 ∶2 FTOH

由于全氟碳链的增长, 8 ∶2 FTOH(t1/2＜7 d)的好氧降解速率慢于6 ∶2 FTOH(t1/2＜3 d)[59, 60].在垃圾填埋场沉积物中观察到8 ∶2 FTOH在好氧条件下t1/2＞365 d(表 3), 可能是该环境下存在高浓度的有机物阻碍了8 ∶2 FTOH的共代谢[36].厌氧环境中降解速率有所降低, 在产甲烷污泥中8 ∶2 FTOH的t1/2=145 d[41].也有报道称8 ∶2 FTOH在厌氧污泥中快速降解(t1/2=4 d), 降解速率的显著提升与体系中高浓度的污泥、充足的电子供体以及VB12的添加有关[61].其中维生素B12具有含钴原子的咕啉环结构, 是一种强亲核试剂, 能够加速碳卤键的断裂, 已被报道可以促进氯代化合物的微生物还原脱氯[62, 63], 并且能显著提高8 ∶2 FTOH在厌氧污泥中的降解和脱氟效率[64].8 ∶2 FTOH的转化路径与6 ∶2 FTOH相似(图 2), 不同的是, 在x ∶3 Acid途径中8 ∶2 FTOH只能生成7 ∶3 Acid一种x ∶3 Acid.

表 3 8 ∶2氟调化合物微生物降解半衰期及产物产率对比1) Table 3 Comparison of half-life and product yield of 8 ∶2 fluorotelomer compounds by microbial degradation

1.2.2 8 ∶2氟调聚合物及其单体

氟调聚合物(fluorotelomer-based polymers, FTPs)被广泛用于地毯、纺织品、室内装饰和造纸工业的表面保护剂, 占全球所生产氟调化合物的80%以上[65].由于8个及以上的全氟碳能提供足够的表面保护特性, FTPs通常由8 ∶2氟调化合物单体聚合而成[66, 67].FTPs单体通常由8 ∶2 FTOH通过酯键(如8 ∶2 TBC、8 ∶2 FTS、8 ∶2 FTAC和8 ∶2 FTMAC)或氨基甲酸酯键(如8 ∶2 HMU和8 ∶2 FTU)与碳氢骨架相连(图 2), 能以杂质的形式存在于FTPs中, 也会在含FTPs材料的使用和处置过程中从聚合物中释放出来.单体的不同结构导致了降解速率的巨大差异, 比如8 ∶2 FTAC的t1/2=5 d[68], 而8 ∶2 FTU和8 ∶2 HMU的t1/2长达几百天[69].在转化路径上, 单体都是通过酯键的水解生成8 ∶2 FTOH(图 2).FTPs的降解难以直接进行定量测定, 只能通过测定转化产物的产率间接评估FTPs的降解速率, 其降解半衰期长达几十至几千年[65, 70, 71].考虑到FTPs的高产量, FTPs是环境中PFAAs不可忽视的重要来源.

1.3 FOSA衍生物 1.3.1 N-EtFOSA衍生物

N-EtFOSA衍生物是以FOSA为基础, 在磺胺N原子上引入一个乙基和一个含有不同官能团的烷基链, 如N-EtFOSE和N-EtFOSA(图 3). N-EtFOSE作为合成聚合物的原材料, 曾经是3M公司销售量最高的PFOS前体物[49].在不同环境介质中, N-EtFOSE的t1/2从小于1 d到44 d不等(表 4)[49, 76~79]. N-EtFOSE能从3条不同的路径开始转化(图 3), 但其转化产物主要是N-EtFOSAA(产率为25%~75.7%, 表 4), 表明—CH2 OH氧化为—COOH是主要途径, 氧化产物N-EtFOSAA在环境中较稳定, t1/2高达几百天[78, 79]. N-EtFOSA是一种杀虫剂(氟虫胺), 在家庭和农业上被用于消灭白蚁、蚂蚁和蟑螂[80], 同时它也是N-EtFOSE的降解中间产物. N-EtFOSA通过N原子上的—CH2CH3直接脱去或先氧化再脱去形成FOSA, 最终转化为PFOS(图 3).在缺氧或厌氧条件下N-EtFOSA不发生降解[81]. N-EtFOSA虽然与氟调磺胺类化合物同样含有—S(O2)NH—, 但与磺胺直接相连的全氟碳链导致其稳定性增强, 其在厌氧条件下难以发生降解.

表 4 全氟辛烷磺胺衍生物微生物降解半衰期及产物产率对比1) Table 4 Comparison of half-life and product yield of perfluorooctane sulfonamide derivatives by microbial degradation

1.3.2 FOSA-胺类化合物

FOSA-胺类化合物是末端为含氮官能团的FOSA-衍生物, 如PFOSAmS、PFOSNO、PFOSB和PFOSAm, 末端官能团分别为季胺、氧化胺、甜菜碱和叔胺, 这些化合物在2002年以前是AFFFs配方的重要组成成分[82].即使结构相似, 但它们在环境中的稳定性差异较大, 好氧土壤中t1/2排序为：甜菜碱≈季胺≫叔胺＞氧化胺(表 4)[82].FOSA-胺类化合物的好氧转化首先发生在末端含氮官能团, 涉及到的反应主要脱烷基化和氧化, 生成FOSAA后进一步降解为FOSA和PFOS, 也能直接通过磺胺官能团的水解形成PFOS(图 3)[82~84].酰胺类衍生物也是电化学氟化的产物, 但磺胺类比酰胺类稳定性更强.一方面, 磺胺比酰胺类衍生物多一个全氟碳, 且磺胺官能团体积更大, 会促进磺胺类在环境中的不可逆吸附, 降低生物可利用性；另一方面, 磺胺官能团的S—N比酰胺官能团的C—N更难断裂[82].

2 PFAAs厌氧微生物脱氟

由于C—F较高的键能和F原子相对于H原子较大体积产生的屏蔽作用, 使PFAAs具有较强的稳定性[85].PFAAs被一直被认为是“永久化合物”, 在环境中难以被微生物降解.2010年, Liou等[86]探究了PFOA在污泥、沉积物和土壤等5种环境介质中的微生物降解性能, 发现PFOA具有较强的顽固性.Kwon等[87]和Beškoski等[88]虽然在微生物培养体系中观察到PFOA和PFOS含量降低, 但是未检测到氟离子的释放, 认为体系中部分PFOA和PFOS被微生物吸附.Yang等[89]和Ji等[90]利用X射线光电子能谱和傅里叶红外光谱进一步证实污泥通过吸附去除PFOA和PFOS.

与好氧条件相比, PFAAs的厌氧还原脱氟在热力学上是可行的, 但对体系中的氧化还原电位要求较高[85].PFAAs的化学降解也表明高级还原技术比氧化技术更有利于C—F的断裂[91].可以推测, 环境中具有脱氟功能的微生物极大可能存在于厌氧生境中.近几年来, 有学者成功筛选出具有还原脱氟功能的微生物, 实现了PFAAs的降解与脱氟.普林斯顿大学Jaffe教授团队在2019年报道, 1株具有铁氨氧化功能的Acidimicrobium sp.A6能够以Fe3+为电子受体, 以NH4+/H2作为电子供体, 通过共代谢实现PFOA和PFOS的脱氟, 培养100 d后PFOA和PFOS的去除率高达60%[92].之后他们将Acidimicrobium sp.A6分别应用于厌氧消化污泥[93]和微生物电解池[94]中, 对PFOA均取得了良好的降解效果.但是, 目前对于Acidimicrobium sp.A6厌氧降解PFOA/PFOS的机制和相关基因的表达还不明确.

3 其他新型PFASs的微生物降解

随着PFOA和PFOS等传统长链PFASs的逐渐淘汰, 越来越多的新型PFASs涌入市场, 它们通常以下两个特征：①全氟碳数目较少；②结构中含有C=C、—O—和—S—等官能团.新型PFASs的微生物降解潜能近年来也逐渐成为研究热点.

6 ∶2 Cl-PFESA(商品F-53B的主要成分)自20世纪70年代以来在中国被生产用作镀铬工业中的抑雾剂[95].其在好氧土壤中十分稳定[96], 在厌氧污泥或脱氯微生物作用下, 其头部碳原子上的Cl会被H取代生成6 ∶2 H-PFESA(图 4), 但难以发生进一步的降解转化[97].这两项研究同时也表明, 与全氟(亚)甲基相连的—O—具有较强的抗生物降解性.一项关于土壤中氯代全氟聚醚羧酸及其转化产物筛查的研究也报道未在土壤中发现醚键断裂的产物[98], 进一步证实了其稳定性.

改自文献[97, 99, 100] 图 4 其他新型PFASs微生物转化路径 Fig. 4 Microbial biotransformation pathway of other novel PFASs

diFESOS是2018年进入欧洲市场的一种新型表面活性剂, 其结构中含有—O—、—S—和—COO—等官能团(图 4)[99].在好氧污泥中, diFESOS首先通过—COO—水解生成醇, 再依次断裂—S—和—O—[99].虽然该物质与6 ∶2 FtTAoS都含有—S—(图 1), 但并未观察到—S—的氧化产物.有趣的是, 与全氟碳相连的─OH, 能够脱去HF形成酮, 这种转化方式是首次报道.此外, 由2H-3 ∶2 PFECA转化为PFPrA可知(图 4), 只要降低PFECA中—O—临位碳上的氟化程度, 就有可能改变其生物可降解性.

PFMeUPA和FTMeUPA分别为含有不饱和双键的全氟羧酸和氟调羧酸(图 4).在厌氧脱氯微生物的作用下, PFMeUPA双键上的C—F能发生断裂, FTMeUPA其sp3杂化叔碳上的F会被H取代, 但对于它们的饱和酸来说, 对应位置上的F原子十分稳定, 说明不饱和双键对C—F的断裂具有重要影响[100].

4 PFASs降解的影响因素

分子结构、微生物类型、环境条件、底物和共存污染物等因素显著影响PFASs的微生物降解行为, 通过优化这些条件能有效降低新型PFASs的环境风险和提高PFASs的微生物修复效率.

4.1 分子结构

影响PFASs微生物降解的结构因素包括空间位阻、C—F键数量和分布和官能团的类型.PFASs作为人工合成的化合物, 被认为是通过微生物的共代谢途径进行降解和脱氟[101].F原子相对H原子具有更大的体积能有效保护碳链, 增强其稳定性.与氟调化合物不同, FOSA-衍生物由于其与全氟碳链相连的磺胺官能团具有较大的空间位阻, 阻止了电子进攻—S(O2)NH—相邻的C—F, 因此无法发生脱氟, 其末端产物只有PFOS一种PFAA[101].Che等[102]探究了C—F的数量和分布对好氧污泥降解短链氟化羧酸的影响, 发现α位上两个H都被F取代的羧酸不能被降解.含有—CF3结构的氟调羧酸, 奇数链长的羧酸具有脱氟活性, 而偶数链长的羧酸无法降解[102].再如, —O—邻位碳上F被H取代的2H-3 ∶2 PFECA能够被好氧污泥降解为PFPrA(图 4)[99].在好氧土壤中, EtFOSAA和FOSA支链异构体比直链转化更快, 而EtFOSA的直链化合物比支链优先转化[79].此外, 官能团的类型对PFAS的降解速率也有重要影响.电化学氟化法生产的PFASs, 酰胺类降解速率大于磺胺类, 末端为不同含氮官能团的PFASs之间降解速率存在显著差异[82]. 含有C=C的不饱和全氟和氟调羧酸特定位置上的C—F具有厌氧脱氟活性[100].Yu等[103]进一步探究了C=C对PFASs厌氧转化的影响, 发现在α和β位上具有C=C的含氟羧酸能通过脱氟或加氢反应进行转化, 并且随着C=C上氢取代程度的提高, 加氢反应变得更易发生.

4.2 微生物类型

自然环境中, 不同生境中的微生物类型不同, 导致PFASs降解速率的差异.海洋作为全球PFASs最终的汇[20], N-EtFOSE和SAmPAP二酯等PFASs在海洋沉积物中的降解速率显著低于陆地沉积物(表 4)[49].在活性污泥中, 氨氧化细菌分泌的氨单加氧酶能够促进FTOH降解以及PFCAs的生成[104, 105], 6 ∶2 FTSA在填埋场沉积物中的降解也得到相同的结论[106].有学者比较了Pseudomonas oleovorans、Pseudomonas butanovora和Pseudomonas fluorescens这3种微生物对6 ∶2 FTOH和6 ∶2 PAPs的降解, 它们对6 ∶2 FTOH的降解速率差别不大, 但Pseudomonas fluorescens降解6 ∶2 PAPs的产物产率明显高于另外两株微生物[31, 107].能降解PFOA和PFOS的Acidimicrobium sp.A6是具有铁铵氧化功能的厌氧化能自养菌[92], 因此在特殊的厌氧生境中更有可能存在PFAAs降解微生物.

4.3 环境条件

影响PFASs降解的环境条件主要是氧化还原电位和温度.氧化还原条件不仅影响降解速率, 也会改变转化路径, 好氧条件下产生的中间产物在厌氧环境中可能以终端产物存在[20].氟调类化合物在好氧条件下的降解速率快于厌氧条件(表 2和表 3). 6 ∶2 FTSA和N-EtFOSA能发生好氧降解, 但在厌氧环境中十分稳定[35, 42, 81].路径改变的典型例子是6 ∶2 FtTAoS, 好氧条件下—S—被氧化为—SO3H, 但厌氧条件下不会断键(图 1)[45, 46].温度通过改变微生物活性影响降解速率, 如EtFOSE在海洋沉积物中的降解, 4℃时[t1/2 =(160±17) d]的降解速率远低于25℃[t1/2=(44±3.4) d][49].

4.4 底物

PFASs是通过微生物的共代谢发生降解, 因此底物中电子供体的种类和含量具有重要影响.向污泥中加入葡萄糖或丙酸盐, 能显著提高三氟丙酸(CF3CH2COOH)的脱氟效率[102].在向培养基中加入葡萄糖后, 木腐真菌Phanerochaete chrysosporium对6 ∶2 FTOH的降解加快[32].在乳酸盐存在的情况下, Pseudomonas butanovora降解6 ∶2 PAPs生成的产物更多样化[107].添加垃圾渗滤液能够促进填埋场沉积物对6 ∶2 FTSA的转化[23].以上结果均表明, 适当添加碳源/电子供体能够促进前体物的降解.植物根系分泌物也能增强PFASs的微生物转化.在溶液中添加大豆根系分泌物后能加速8 ∶2 FTOH降解[108].FOSA能在种植胡萝卜和莴苣的土壤中发生转化, 但在无作物土壤中较稳定[109].对于含—SO3H的PFASs, 培养体系中的硫酸盐会抑制其脱磺酸和脱氟, Gordonia sp. NB4-1Y[34, 39]和Rhodococcus jostii RHA1[40]均只能在无硫培养基中降解6 ∶2 FTSA.此外, 维生素B12被证实能够促进氟调醇的厌氧脱氟[64].

4.5 共存污染物

共存污染物对PFASs降解的影响较复杂.对于易降解的污染物, 在贫营养的条件下能作为微生物的潜在碳源, 有利于PFASs的降解.而对于结构比较稳定的污染物, 尤其是只能通过共代谢途径降解的物质, 会与PFASs竞争来自电子供体的电子.比如, 石油烃作为碳源能够促进土壤中PFASs的降解[57], 而垃圾渗滤液中存在的大量有机污染物却会抑制8 ∶2 FTOH的降解[36].Olivares等[110]探究了BTEX化合物(苯、甲苯、乙苯和邻-二甲苯)对6 ∶2 FtTAoS降解的影响, 对降解速率的影响不大, 但产物中PFCAs的产率提高了4~5倍.不同类型PFASs之间可能也存在电子竞争, 但相关研究还鲜见报道.

5 结论与展望

(1) 目前对传统PFAAs前体物微生物降解的研究趋于完善, 但对于含有不饱和双键、醚键、硫醚键、环氧醚、磺胺、季胺和甜菜碱等官能团的新型PFASs的转化研究还有所欠缺.尤其是对于氟化官能团和与氟化碳直接相连的官能团结构, 由于C—F的稳定性以及F较大的体积, 导致它们的稳定性增强.用于预测有机污染物微生物降解性能和转化路径的数据库和商用软件, 对这些结构并不适用.考虑到PFASs细微的结构差异就能导致降解性能的显著不同, 因此有必要从PFASs的结构出发, 考究结构对其环境稳定性的影响, 建立并完善相关的数据库.PFASs数据库的建立对PFAS的生产具有重要指导意义, 生产者能根据数据库预测的结果, 在不影响PFASs功能的前提下对其结构进行微调, 增强其生物可降解性, 缓解其环境风险和危害.

(2) PFAAs一直被认为无法被微生物降解, 但最新的报道指引了新的前沿.即使多氟烷基化合物在好氧环境中的降解速率更快, 但厌氧环境中更有可能存在具有还原脱氟功能的微生物, 它们极大可能作为非优势微生物而存在, 对于PFAAs的降解具有重大潜力.进一步筛选和识别具有还原脱氟功能的微生物是核心内容.利用宏基因和转录组等手段识别脱氟功能基因和了解微生物脱氟机制, 有利于脱氟微生物的筛选.脱氟微生物的实际应用是相关研究的最终目的, 微生物电化学技术能通过强化电子传递能促进微生物脱氟, 植物-微生物、活性材料-微生物和化学催化-微生物等微生物与其他技术联合可能显著提高PFASs的修复效率, 基因工程等生物学手段能扩大脱氟微生物的应用, 相关研究有待拓展和深入.

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