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藻类暴发作为世界范围内备受关注的水环境问题,关系到居民观感、水环境安全以及饮用水安全。近年来频发的水华和赤潮现象引发了对于藻类暴发现象析因和治理方法的研究热潮,同时,对于藻华所造成的生态及健康风险的研究也广泛开展。藻源有机质(algal organic matter, AOM)是藻类在其生命周期内代谢渗出或细胞自溶裂解而产生的一类有机物,在水华暴发期间大量存在于水体中,并在天然有机质(natural organic matter, NOM)中占有相当高的比重。对于美国的湖泊采样调查显示,有近40%的湖泊中浮游植物的生物量超过水体总碳库的10%[1],而通过13C标记追踪法发现在贫营养化的湖泊中藻类贡献的DOC可以占到水体总DOC的20%,在富营养化的湖泊中甚至占到40%[2]。AOM的特征与水体中普遍存在的陆生来源NOM之间具有一定差异,比如其亲水组分往往更多、氮含量较高、芳香族含量则较低,这使得其对于水处理工艺的影响相比于NOM中其他组分也有所差异[3-5]。此外,AOM中作为可溶性有机质(dissolved organic matter, DOM)的部分往往具有较NOM中陆源组分更强的亲水性,更难被自来水厂混凝沉淀措施除去[6],从而会在预氧化以及消毒工艺下与氧化剂或氧化生成的活性卤物质反应生成消毒副产物(disinfection by-products, DBPs)。
饮用水的氯化消毒作为公共卫生领域的重要突破,在世界范围内被广泛应用。然而水体中存在的有机质在氯化消毒条件下发生反应生成DBPs,并由于细胞和遗传毒性及致癌风险使得其自上世纪70年代以来受到广泛关注。以三卤甲烷(trihalomethanes, THMs)为例,其中氯仿、一溴二氯甲烷、二溴一氯甲烷和溴仿四种物质浓度总和在美国环保署(United States Environmental Protection Agency, US EPA)的限值为0.08 mg·L−1;而在中国,氯仿和一溴二氯甲烷的限值为0.06 mg·L−1,溴仿和二溴一氯甲烷的限值为0.1 mg·L−1[7]。当前,在水处理过程中又发现了碘代消毒副产物(iodinated disinfection by-products, I-DBPs)、溴代消毒副产物(brominated disinfection by-products, Br-DBPs)以及卤代乙酰胺乙腈为代表的氮质DBPs(nitrogenous disinfection by-products, N-DBPs),这些DBPs由于其更高的细胞和基因毒性应该得到重视[8-11]。
当前已有大量关于AOM的表征和DBPs生成潜能的研究,揭示了AOM与腐殖质和黄腐质类陆源NOM的区别[6, 12-13]。然而,由于AOM结构复杂并且系统的表征方法比较困难,当前对于其表征的研究仍不够全面,难以有效地在AOM的表征和其DBPs生成潜能间建立有效可预测的联系。
本文综述了当前对于AOM的表征及DBPs生成潜能的研究。介绍AOM的来源与表征,并在此基础上分析了以AOM为前体物质的DBPs生成潜能影响因素及机制,总结了基于AOM表征对DBPs生成潜能尝试进行预测的研究现状,最后探讨了当前研究中存在的不足与未来应着重发展的方向。
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AOM作为在藻类暴发时期大量释放在水体中的一类有机物,其具有自身独特的周期性和分布性,因而与分布在水体中的NOM相比具有不同的性质。研究AOM的来源和表征,不仅有助于研究DBPs的生成潜能,同样对于指导预氧化、混凝、过滤等饮用水处理工艺优化具有重要意义。
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AOM根据其来源在过去往往被分为胞外有机质(extracellular organic matter, EOM)和胞内有机质(intracellular organic matter, IOM),其中EOM主要成分为藻类代谢产物,伴随藻类生长过程从藻类细胞中释放。而IOM则被认为是细胞代谢过程中不会自行向外界释放的一类有机质,其往往在细胞死亡裂解之后向外释放。在藻类大量暴发的水体中,随着暴发时期推移,藻类种群逐渐衰亡,IOM逐渐由细胞破裂释放,在水体AOM中占比逐渐升高[14]。在研究中,通常直接通过离心手段提取EOM,再通过冻融、超声或研磨这类物理手段破碎细胞以提取其中的IOM组分,以便分别对其进行表征和组分研究[13, 15-16]。在过去对AOM的研究中主要针对以上两类物质进行分级分离研究,而忽略了残留的细胞碎片的作用。在实际环境中,不能排除藻类碎片中多糖和多肽及其它与细胞碎片结合紧密的组分在溶出后对AOM产生贡献的可能,这些在藻类自身生长过程中不向外溶出,只在其生命周期外溶出并对水中溶解性有机质(dissolved organic matter, DOM)产生贡献的有机质被归类为细胞结合有机质(cellular bound organic matter, COM)。
在AOM的基本形态上,提取出的EOM由于碳水化合物、氨基酸、酶这类代谢产物的存在,呈现出淡黄色;而由于藻蓝蛋白和叶绿素的存在,IOM呈现出蓝绿色[15]。在AOM各组分浓度上,EOM组分DOC浓度范围在9.28—79.12 mg·L−1之间,而IOM则为5.52—100.5 mg·L−1[15, 17-18]。在当前,由于藻种类、生长时期以及分离手段的不同,对于AOM不同来源的组分性质和浓度并不能作统一的概括,而需要继续根据这些因素所带来的影响做更具体的表征。
在当前,围绕EOM和IOM进行的研究比较丰富,但关于COM的研究还很欠缺。Liao等对于铜绿微囊藻和梅尼小环藻的AOM研究中,在超声破坏藻细胞提取出IOM后将剩余滤渣洗涤3次,并收集为细胞碎片组分重悬,以此将AOM分离为EOM、IOM以及细胞碎片三部分[19]。而Hua等在提取出IOM之后将小球藻细胞碎片在70 ℃下加热30 min,使得COM溶出后,在2700 g相对离心力下离心提取[20]。在藻类大量暴发时期的水体中,存在着大量藻细胞的繁殖和破裂,其细胞碎片可能带来的COM溶出对于DOM的贡献不可被忽视;同时,由于除藻剂在含藻水体中的加入以及预氧化过程可能对藻的破坏,COM在这些过程中的溶出在过去也未被充分考虑。因而在当前研究分离EOM和IOM的基础上,应当规范IOM的提取,并对于剩余细胞碎片的溶出因素进行系统的考察,并对于溶出的COM进行全面的表征以及DBPs生成潜能分析,以了解其对于水质安全的影响。
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溶解性有机碳(dissolved organic carbon, DOC)和溶解性有机氮(dissolved organic nitrogen, DON)作为有机质表征的基本参数指标,直接关系到DBPs生成潜能。通常在藻类生长和稳定期,EOM作为代谢产物被释放到水体中,对水环境中DOC造成贡献;处于胞内的IOM虽然理论上(根据其高占比的生物量)可以贡献出更多的DOC,但这些物质往往要在衰亡期细胞裂解后才会释放到水体。而对于溶解性有机氮而言,IOM的DOC/DON比值则相对EOM较低[13, 15, 18],这可能是由于IOM组分中有更多的肽和蛋白质组分。
波长254 nm处的紫外吸收UV254与DOC的比值被称为SUVA,常用来衡量有机质中芳香族物质的含量。就SUVA值而言,通常认为其值在4 L·(mg·m)−1以上则代表其组分总体是疏水的,而其值在2 L·(mg·m)−1以下则认为其组分以亲水组分为主[21-23]。对于AOM而言,其SUVA值往往在1.5 L·(mg·m)−1以下[24-25],以铜绿微囊藻为例,其IOM往往具有比EOM更低的SUVA值,并且随着生长时期的推移,稳定生长期藻EOM的SUVA值比指数增长期时更低[25-26]。已有的研究将SUVA值与混凝效率建立了联系,结果表明SUVA更高的成分混凝处理效果更好,而AOM则由于其低SUVA值混凝效果较差,使得SUVA值可以成为评价AOM混凝处理效率的有效指标[18, 27]。
三维荧光光谱是由激发波长和发射波长及荧光强度作为三维的激发发射矩阵(excitation-emission matrix spectra, EEM),可以通过光谱图鉴别有机质的组分类别,是当前最直观了解有机质组分及差异的手段之一,但对于组分的定性和定量则要依托于更多分析技术[28-31]。在当前,主成分分析(principal component analysis, PCA)和平行因子分析法(parallel factor analysis, PARAFAC)是相对主流和可靠的定性方法,可运用于成分识别和分类[31-33]。而结合荧光区域积分法(fluorescence regional integration, FRI)则可以对EEM表征的组分进行半定量的分析,尽管各组分浓度与荧光强度之间的关系并非线性,但这有助于在一定程度上了解并建立其与DBPs生成潜能之间的联系[34]。对于AOM而言,其EOM组分中类腐殖酸和黄腐酸的组分往往占比较高,而IOM中则以氨基酸和芳香蛋白组分为主,这也正与两者的DOC/DON比例差异相契合[35-36]。基于当前的研究,有理由认为EEM是对于AOM进行表征的有力手段,但仍需要结合更多手段对分析方法进行系统化和规范化。
在分子量方面,在早期使用超滤膜将AOM分为多个分子量等级,而随着技术的发展和表征细节的需要,高效尺寸排阻色谱法(high-performance size exclusion chromatography, HPSEC)成为了对有机质进行分子量表征的重要手段[5, 13, 37-39]。对于AOM而言,Li等[15]在早期的研究表明其各有分布特征,但IOM平均分子量略大于EOM。在不同分子量组分分布方面,EOM和IOM组分呈现双峰分布,小分子量组分(<1 kDa)和大分子量组分(>100 kDa)都在其组分中占比较高,而中间级别的分子量组分反而占比较低[6, 40]。Pivokonsky等[16]则通过对分子量进行分级表征发现IOM要比EOM含有更多高分子量的肽和蛋白质,同时随着藻类生长高分子量的有机质占比逐渐上升。Hua等[20]利用尺寸排阻色谱法研究得到的结果则表示IOM、EOM、COM之间并不存在明显差异性。AOM分子量表征对于了解其对滤膜的影响显然最为直观,小分子量的AOM组分在过滤过程中可能难以被除去,而大分子量的AOM组分则会引起膜结垢在膜上形成滤饼影响过滤效率[41-43]。
AOM的亲疏水性同样也是其重要表征元素之一。影响IOM和EOM亲水性的组分被认为是碳水化合物、亲水氨基酸、低分子量的羧酸、烷基醇、醛和酮等成分,而疏水性则主要由烃、高分子量的烷基胺、高分子量的烷基羧酸(脂肪酸)和芳香族酸、酚和腐殖质等组分贡献[6, 16, 22]。起先研究人员将肽/蛋白质组分归类为疏水性组分的贡献组分[22],然而随着研究的发展,在亲水性组分中也发现了肽/蛋白质组分[44],并且在蓝藻的肽/蛋白质组分表面上发现了一定数量的极性带电荷官能团[45],说明肽/蛋白质组分对AOM的亲水性也有所贡献。IOM通常具有和EOM相近或更高的亲水性,而较高的亲水性也会导致传统混凝沉淀手段对其处理效率较低[18, 24]。例如对于铜绿微囊藻和小球藻而言,其作为藻细胞在凝结和过滤之后的去除效率分别为94.8%和97.3%;但是其对应的AOM在同样工艺中的去除率分别只有71%和55%[46]。
傅里叶变换红外吸收光谱(fourier transform infrared, FTIR)是有机物官能团表征的重要手段。Chu等[47]通过FTIR对AOM的表征发现,细胞表面存在着可以被溶出提取的蛋白质和多糖组分,这可以为COM的溶出提供理论依据,而在1720 cm−1处与腐殖酸样联系紧密的羧酸C=O吸收带不明显,说明EOM中的腐殖质类组分并不是从细胞表面溶出释放的。在对于中肋骨条藻的高分子量EOM进行的FTIR分析中则发现了象征脂肪族基团、羧酸、类蛋白样、多糖、以及肽聚糖降解产物的官能团吸收峰带的存在[48]。而Zhou等[24]则对造成膜结垢的AOM组分进行了FTIR分析,分析发现了其中的酰胺基团峰和O—H、C—H拉伸峰以及醇的C—O拉伸峰,表明AOM造成膜结垢的主要组分是蛋白质和多糖。由此可见FTIR技术作为一种先进的官能团表征手段,在AOM成分的具体溯源和表征方面具有良好前景。
核磁共振波谱(nuclear magnetic resonance spectroscopy, NMR)在当前也开始被用于AOM的表征。对于AOM的NMR波谱显示其中有明显的碳水化合物峰以及NCH峰,同时也具有解析度良好的芳香蛋白信号[25, 49]。对比不同藻种则发现了绿藻AOM组分相比于蓝藻具有更高的脂质含量[50]。对于中肋骨条藻的高分子量EOM进行的1H和13C NMR分析则发现其在碳水化合物区域具有较脂质和蛋白质更显著的特征峰[48]。对于小球藻EOM和IOM的13C NMR光谱分析显示,EOM谱中除了一些低强度的碳水化合物信号峰以外,还存在一个羧基的特征峰;而IOM谱图则显示其为混合了脂肪族、芳香族化合物、氨基酸、碳水化合物和羰基组分的复杂物质,与EOM相比,IOM中脂肪族和芳香族的峰强要高得多[51]。同FTIR技术一样,NMR技术作为有机物组分定性分析的强力手段在未来AOM的表征研究中非常重要。
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AOM作为水华暴发时期DBPs生成的重要前体物,其复杂的组分特征使得对于其在不同氧化条件下生成不同DBPs的研究被广泛开展,力图建立起组分特征与DBPs生成特征之间的联系。
表1 汇总了先前部分以AOM为前体物的DBPs生成潜能研究。在DBPs角度上,最初的大多数研究聚焦于THMs与卤代乙酸(haloacetic acids, HAAs)这两类碳质DBPs(carbonaceous disinfection by-products, C-DBPs)上,而后的研究则开始渐渐关注起毒性更高的N-DBPs如卤代乙腈(haloacetonitriles, HANs)、卤代硝基甲烷(halonitromethanes, HNMs)、卤代乙酰胺(haloacetamides, HAMs)等。同样也出于毒性和水体中其他卤素贡献的考虑,对于Br-DBPs和I-DBPs的研究也逐渐展开。对AOM进行氯化时,由于过量的氯源输入,生成的THMs中三氯甲烷(trichloromethane, TCM)占主要成分,在所有生成的THMs中占比甚至超过90%,而当原水中溴源和碘源比例上升时,Br-和I-THMs的比例也会相应上升[52],而二氯乙酸(dichloroacetic acid, DCAA)和三氯乙酸(trichloroacetic acid, TCAA)在生成的卤乙酸中占主要组分,二氯乙腈(dichloroacetonitrile, DCAN)则占HAN主要成分[15, 25, 51, 53]。这可能是由于AOM的氯化实验中少有溴化物和碘化物的加入从而导致其他卤素元素的匮乏,生成物主要以氯代产物为主。
此外,将AOM表征及氧化条件与DBPs生成潜能建立了联系,发现了不同特征的AOM在不同氧化条件下具有不同的DBPs生成潜能。
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对于AOM的不同来源而言,当前研究普遍发现IOM具有比EOM更强的DBPs生成潜能,这与IOM与EOM的组分构成有重要关联,IOM中含有更多的蛋白质样相比于EOM中占主要组分的腐殖质和黄腐质样具有更高的DBPs生成潜能[13, 15-16]。当前对于COM的DBPs生成潜能研究尚少,研究表明,对于铜绿微囊藻而言,其EOM、IOM和细胞碎片的THMs生成量分别为30.9、14.0、8.3 μg·L−1,而对于梅尼小环藻而言,则分别为12.8、11.0、13.1 μg·L−1,这意味着细胞碎片相比于EOM和IOM具有相近的DBPs生成潜能[19]。此外,指数生长后期小球藻的EOM、IOM和COM的C-DBPs生成量分别为21.7、49.9、21.3 μg·mg−1 C,而对于衰亡阶段的小球藻则分别为24.6、38.4、17.1 μg·mg−1 C[20],充分表明了COM具有与EOM和IOM相近的DBPs生成潜能。
对于藻类的生长期而言,部分研究认为不同生长期的藻所贡献AOM的DBPs生成潜能在归一化后并无显著差异[53],尤其是对于EOM而言,其在藻类不同生长阶段的DBPs生成潜能变化很小,利用EEM观察荧光组分则发现EOM和IOM的各种荧光组分构成在不同生长期内都相当稳定[36]。然而在另一些研究中则发现AOM在衰亡期中具有比指数生长期更高的DBPs生成潜能[54],使得该问题仍然有待研究。然而,需要注意的是衰亡期的细胞破裂会导致原本处于细胞内的IOM释放从而引起AOM对水体DOM的贡献增加,因此无论如何仍需要注意衰亡期藻华构成的威胁。
对于不同藻种类而言,由于不同藻AOM组分构成的不同而呈现不同的DBPs生成潜能。碳水化合物、脂质以及蛋白质在藻细胞中的相对含量随藻种类的变化而产生变化。相比于脂质,同等DOC浓度下碳水化合物和蛋白质生成的THMs明显较低[55],说明脂质对于THMs的生成而言是更有利的有机前体物[56-57]。同时对于不同藻种亲疏水性组分的探究也表明硅藻相比绿藻和蓝藻具有更高比例的疏水组分,同时拥有着更高的DBPs生成潜能[18]。同时相比于蓝藻和绿藻,硅藻在常见藻种中的脂质占比较高[55]. 联系这些因素可以推断,主要构成细胞内疏水性组分的脂质对于DBPs的产生具有更高的潜能,硅藻暴发相对于其他藻种暴发可能具有更高的DBPs生成威胁,需要更加重视[58]。
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对于不同氧化剂而言,当前对于DBP生成潜能的研究多围绕氯消毒展开,因而传统的氯化消毒和新兴的氯胺消毒被作为重点研究。由于氯化消毒对于受管制的DBPs(THMs和HAAs等)的生成较高,而氯胺消毒则由于有效减少了这类DBPs的生成而受到青睐[59]。对于以AOM为前体物的消毒过程,该规律依然适用[13, 60]。然而,传统DBPs多为氯代产物,当溴和碘这类卤素加入后,DBPs的生成规律会变得不同。以I-DBPs的产生为例,在氯化消毒中,碘源很容易被次氯酸氧化成次碘酸,之后由次氯酸和次碘酸反应生成碘酸根离子,从而最小化其生成I-DBPs的可能,然而对于氯胺消毒而言,氯胺仅能将水体中的碘源氧化成短暂存在但可以与前体物反应的次碘酸,而不能够将其充分氧化成碘酸根离子,使得氯胺消毒具有着比氯化消毒更高的I-DBPs生成潜能[61-62]。在AOM的消毒方面,氯胺消毒对比氯化消毒所产生的Cl-和Br-THMs、HANs和卤代乙醛的生成都要更低,却在I-THMs和一些N-DBPs的控制上不如人意[13, 63]。由于I-DBPs相对于氯代和溴代产物具有更高的细胞和遗传毒性,而N-DBPs则相对于C-DBPs具有更高的细胞和遗传毒性[9, 64],因而面对存在着较高碘源浓度的水体,在进行AOM的消毒处理时尤其需要注意I-DBPs的产生以控制消毒后水体的整体毒性。
对于卤素来源而言,由于常见的氯消毒工艺和自然水体中的氯离子使得Cl-DBPs成为DBPs中最主要的一类,溴源和碘源在天然水体中μg·L−1级别的存在,也使得Br-DBPs和I-DBPs在饮用水处理的过程中产生[65-67]。尤其在海滨城市,海水的入侵可能使得水体中溴化物和碘化物的含量更高[9],使得饮用水处理及海水淡化受到影响。除了以无机物形式存在以外,随着以碘帕醇为代表的碘代X射线造影剂在医疗上的使用和排放,使得碘源还具有人为有机来源[62, 68],并且在自然水体中也有I-DBPs被检出[69-70]。对于溴离子,增加反应时初始溴离子的投放量则会同步引起氧化剂需求量的增加[71]。在保证残留氯充足的情况下,增加初始投放溴离子的量,会发现三氯甲烷的生成量降低,取而代之的是Br-THMs的增加,并且发现在初始投放氯浓度是溴的20倍时,生成的二溴一氯甲烷和二氯一溴甲烷量相近,而总卤乙酸的生成量基本稳定,总卤乙腈的生成量上升,而随着溴离子浓度的增加加速了溴代卤乙醛的分解[72]。氯化和氯胺化消毒的原理是次氯酸的强氧化性,而溴源和碘源的存在则会使得次溴酸和次碘酸生成并与AOM反应,显然次氯酸和次溴酸拥有比次碘酸更强的氧化竞争地位[73]。正如上文对于氯和氯胺消毒的差异叙述,在氯消毒中碘源多被氧化为碘酸盐[61],而在氯胺消毒过程中,I-THMs是决定了处理后总DBP细胞毒性的主要因素[63]。Br-DBPs和I-DBPs由于其更强的毒性受到更多的关注[10, 74-75],而已有研究表明藻细胞具有能够将水体中的溴浓缩在自身囊泡中的能力[76],使得对于AOM的研究需要格外注意其他卤素的介入。
对于氧化过程中水体的pH而言,在常见的pH6—9下,无论是对于氯化还是氯胺化消毒,目前普遍观察到pH的上升引起THMs生成的增加[72, 77-78]。因为在碱性条件下,碱催化水解更容易形成THMs,THMs结构简单,往往是最后的水解产物[63, 79-80]。而结构稍复杂的卤代乙腈、卤代乙酰胺、卤代乙醛等物质在pH较高时由于水解,其生成潜能会降低[80]。卤乙酸受pH影响的机制与THMs相反,其往往随着pH的升高而降低[81]。随着pH的升高,一卤代乙酸和二卤代乙酸的生成量并没有明显改变,而三卤代乙酸的生成量则会下降,当前机制研究认为,HAAs的生成前体物结构是R-CO-CX3的形式,在酸性环境下,R基团如果是容易被氧化的基团,则容易被氧化生成三卤代乙酸,否则在碱性条件下则容易水解产生THMs,而当R集团为甲基时,则不容易生成三卤代乙酸[79]。虽然在紫外光联合氯消毒下观察到较高pH下实验组的DBPs生成潜能较对照组低,但这同时可能伴随着紫外光照下游离氯/氯胺的分解[82]。
对于氧化工艺而言,预氧化是当前水处理中常见的一项工艺,该过程一方面会对藻细胞造成破坏,释放其中较难被混凝工艺去除的IOM组分,使其进入消毒流程并作为前体物生成DBPs;另一方面由于氧化剂、原水中卤素以及有机前体物的存在,此过程在消毒之前就可以生成DBPs[83]。因而当前出现了两种不同的研究方向,一种致力于在保证本身预氧化效率的情况下不破坏藻细胞,不让IOM释放;另一种则致力于在已经破坏了藻细胞使得内容物释放的情况下将IOM彻底氧化处理,从而防止其对混凝效率造成影响。高锰酸盐预氧化凭借其对藻细胞完整性破坏小,释放的IOM少,成为相比于预臭氧化、预氯化更优的工艺,然而其缺点在于对于水色造成的影响较大[84-87]。UV/H2O2联合的高级氧化工艺在引起藻细胞破裂后,对于释放出的有机质的降解则高度依赖于光催化下羟基自由基产生的量[88-89]。基于紫外光照直接对AOM照射的研究发现,紫外光照降低了AOM的SUVA值,EOM中的腐殖酸和黄腐酸样有机质和IOM中的蛋白样有机质最容易受紫外光照影响减少,然而在后续的氯化和氯胺化实验中DBPs生成的变化随着DBPs种类而异[90-91]。将紫外光照射联合氯化/氯胺化的预氧化过程拥有着强氧化作用,其不仅可以有效破坏藻类细胞,还能够继续氧化AOM中的蛋白质和氨基酸,被认为具有发展前景[65, 83, 92],但需注意其生成的Br-DBPs毒性,在紫外/氯化条件下含AOM和溴化物的原水中生成的Br-HANs是DBPs毒性的主要贡献因素[65]。对于含藻水的预氧化工艺在当前逐渐向高级氧化方向发展,然而在当前并不存在完美的处理工艺,对于富藻水的处理仍需要混凝、过滤以及消毒工艺的联合优化从而达到最佳处理效果。
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在当前,对于AOM的DBPs生成潜能主要基于表征展开,力图通过对AOM的表征揭示其DBPs生成潜能。
在光学表征方面,AOM各级组分的DOC/DON值可以最直观地反映其组分中碳质有机前体物和氮质有机前体物的含量,从而与C-和N-DBPs生成潜能相关联。而IOM相较EOM更低的DOC/DON比意味着相对于EOM其更容易生成N-DBPs,而相比于C-DBPs,N-DBPs往往具有更高的细胞毒性[8]。SUVA值作为芳香族化合物的有效指标在当前并没有与DBPs生成潜能建立稳定有效的联系[53, 72],例如其与THMs形成的联系在不同的研究中呈现出不同的结果[35, 93],然而其与混凝效率建立的有效联系直接影响到后续进入消毒流程中的AOM多少,因而虽然并不能够直接将SUVA用于DBPs生成潜能的预测,但仍可以将其作为参考指标。
在亲疏水性方面,AOM总体都具有着比NOM更高的亲水性,对于EOM和IOM而言,EOM的疏水组分>亲水组分,而IOM的亲水组分>疏水组分[6, 94]。EOM和IOM中的疏水性组分体现出了比亲水组分和中性组分更高的DBPs生成潜能[24, 37]。疏水组分较容易被混凝去除,而占比较高的亲水性组分虽然单位生成潜能不如疏水组分,却更难被混凝去除从而进入消毒环节,因而依据亲疏水性预测AOM的DBPs生成潜能需要结合混凝效率综合判断。
在分子量大小方面,AOM的中间级别分子量组分(1—100 kDa)的DBPs生成潜能最低,而小分子量(<1 kDa)和大分子量(>100 kDa)的DBPs生成潜能则较高[37, 40, 95]。显然,大分子量的组分更容易被混凝过滤去除,因而小分子量的AOM组分更值得注意。
当前利用模型预测DBPs生成潜能的研究已有开展[81, 96],尤其是利用常规的水质参数(pH、DOC、卤素离子浓度等)来进行模型预测。Chowdhury等[81]针对各项依据水质以及反应参数建立的DBPs生成预测模型进行了综述,其中大多数研究基于原水中前体物条件(如DOC)、水质参数(如pH、温度)、反应条件(如消毒剂剂量、反应时间)作为变量构建模型,并指出尽量考虑更多影响因子的实验对于模型建立具有重要意义,未来需要用数十年时间建立起DBPs生成的系统数据库。Sohn等[97]以DOC、紫外吸光度、Br−浓度、温度、pH、Cl2投加量、反应时间建立起对原水和混凝水的DBPs生成预测模型,多个模型的R2值在0.70—0.94之间。Beauchamp等[98]利用UV272在反应前后的变化与DBPs生成量建立模型,得到了良好的线性关系,R2值从0.62—0.99,并且受季节水文特征影响有所不同。然而,这些研究建立模型时所参考的有机物表征参数多为DOC浓度和紫外吸光度等,重条件而轻表征,受制于有机物组分的复杂性,考虑MW、亲疏水性、EEM等有机物细化表征建立的模型仍较为少见。Hua等[36]利用HPSEC联合有机碳检测器(organic carbon detector, OCD)结合峰拟合技术选取分子量特征成分为因子构建DBPs生成潜能预测模型[20],并利用EEM各组分的平均荧光强度(average fluorescent intensity, AFI)将AOM各成分与DBPs生成潜能进行联系,其结果表明各区域AFI值与THMs的生成量相关性较差,与卤乙酸和C-DBPs之间则有一定的相关性,其中芳香蛋白组分相关性最高,然而R2值最高仍未超过0.9,其余如可溶性微生物、类腐殖酸、类黄腐酸成分的R2值甚至未超过0.6。包含COM的小球藻AOM的研究则表明,其EOM主要组成为类腐殖酸(43%)和类黄腐酸(35%)的成分;IOM主要组成则为芳香蛋白样(52%)和类可溶性微生物质成分(23%),而COM以芳香蛋白样(45%)和黄腐酸类成分(25%)为主,将芳香蛋白组分和可溶性微生物质组分作为因子,并搭配尺寸排阻色谱的峰进行拟合,则将对于C-DBPs的预测R2值提高到了0.916,对于卤乙酸的预测R2值提高到了0.891,展现出了两种方法搭配对于DBPs生成潜能进行预测的前景[20]。通过EEM的平行因子法,Ma等[99]发现AOM中的色氨酸样物质C1与THMs和HANs的生成潜能具有强相关性,而氨基酸组分C2则与TCNM的生成相关,这些研究可能对于DBPs的生成潜能预测具有参考意义。基于当前的研究,有理由认为多表征结合建立模型是对于AOM的DBPs生成潜能进行预测的有力手段,但限于AOM表征以及DBPs生成条件的多样性,目前仍未有相对标准的规范方法,模型建立的数据样本量难以得到保证。未来仍需要结合更多信息进行模型拟合才能确认其对于DBPs生成潜能的预测效果。
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AOM作为藻类暴发时期水体中的重要有机质,其对于饮用水安全的影响及重要性在当前已经得到了充分重视。本文通过对于AOM基本表征的描述,并综合当前已有对于AOM的DBPs生成潜能的研究总结了影响其DBPs生成潜能的相关机制,最后探讨了利用AOM表征对其DBPs生成潜能进行预测的现状和发展方向。对于AOM的DBPs生成潜能研究,未来需要着重于:
(1) 当前已有大量针对AOM的表征研究,然而由于AOM的复杂性,仍然需要利用更多手段以完善不同藻种类和状态下的AOM表征。需要注意的是,COM对于饮用水安全的影响在过去并没有得到充分考虑。应当将COM放在和EOM及IOM同等地位考虑其对于饮用水处理的影响,全面地对COM进行表征以及DBPs生成潜能研究。
(2) 以AOM作为有机前体物研究DBPs生成潜能的研究已经初步展开,然而难以统一的AOM提取条件和氧化条件使得数据难以进行横向对比,需要规范和细化各项操作,如藻种类的选取标准和培养条件、AOM的提取步骤和氧化条件等以增加数据可对比性。
(3) 基于AOM表征及其DBPs生成潜能建立模型,并依据模型开展的DBPs生成潜能预测研究仍处于起步阶段,应当结合多方面表征因素(如光学表征、分子量分级、亲疏水性等)尝试对DBPs生成潜能进行拟合,从而由AOM表征预测其对于饮用水消毒可能造成的威胁,为藻类暴发时期饮用水厂消毒对策提供有效参考。
藻源有机质表征及消毒副产物生成潜能研究进展
Characterization and formation potential of disinfection by-products of algal organic matter: The critical review
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摘要: 随着当前水华和赤潮现象在世界范围自然水体中的广泛发生,藻源有机质(algal organic matter, AOM)对饮用水安全的影响受到广泛关注。AOM作为有机前体物生成高毒性的消毒副产物(disinfection by-products, DBPs)直接影响饮用水健康。因此,阐明AOM结构特征与DBPs的生成潜能具有十分重要的意义。本文综述了AOM表征方法,并结合其DBPs生成潜能和影响因素,探讨了通过AOM结构表征来预测DBPs生成潜能的可行性;提出未来需结合多种AOM表征方法和多因素分析来尝试预测其DBPs生成潜能。本文对于AOM提取和表征方法的规范化以及水华时期饮用水安全的保障具有参考意义。Abstract: With the current occurrence of water blooms and red tides in natural water bodies around the world, the impact of algal organic matter (AOM) on drinking water safety has received widespread attention. As an organic precursor, AOM generates highly toxic disinfection by-products (DBPs), which directly affect the health risks of drinking water. Therefore, it is of great significance to clarify the structure characterization of AOM and the formation potential of DBPs. This study reviews the AOM characterization methods, combined with its DBPs formation potential and influencing factors, and explores the feasibility of predicting the formation potential of DBPs through AOM structure characterization. In the future, it is necessary to combine a variety of AOM characterization methods and multi-factor analysis to try to predict its DBPs formation potential. It is of reference significance for the standardization of AOM extraction and characterization methods and the guarantee of drinking water safety during the water bloom.
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表 1 先前研究中AOM各组分不同氧化条件下生成DBPs潜能
Table 1. The potential of AOM components to generate disinfection by-products under different oxidation conditions in previous studies
藻种类
Algae species氧化条件
Oxidation conditions附注
NoteDBPs生成潜能/(μg·mg−1 C)
Disinfection by-product formation potential参考文献
References实际水体 太湖 [NaClO]0=Cl2∶DOC=5∶1; pH=7.0±0.2; 反应时间=7 d; 反应温度=(22±1) ℃ AOM: 59.32 (THM+HAA) [12] Microcystis aeruginosa [DOC]=5 mg·L−1; [NaClO]0=25 mg·L−1; pH=7.0; 反应时间=3 d; 反应温度=(22±1) ℃ EOM: 16 (TCM); 11 (DCAA) [13] Microcystis aeruginosa [DOC]=5 mg·L−1; [NaClO]0=15 mg·L−1; pH=7.0; 反应时间=3 d; 反应温度=(22±1) ℃ IOM: 28 (TCM); 15 (DCAA) [13] Microcystic aeruginosa [NaClO]0=Cl2∶DOC=5∶1; 反应时间=7 d; 反应温度=20 ℃ EOM: 32.44 (THM); 54.58 (HAA)
IOM: 21.46 (THM); 68.29 (HAA)[15] Chlorella vulgaris [NaClO]0=Cl2∶DOC=1.8∶1; pH=8.0±0.2; 反应时间=24 h; 反应温度=(20±1) ℃ EOM: 12.66 (THM); 14.83 (HAA)
IOM: 17.68 (THM); 22.20 (HAA)[18] Scenedesmus quadricauda [NaClO]0=Cl2∶DOC=1.8∶1; pH=8.0±0.2; 反应时间=24 h; 反应温度=(20±1) ℃ EOM: 14.17 (THM); 23.36 (HAA)
IOM: 22.67 (THM); 25.77 (HAA)[18] Phaeodactylum tricornutum [NaClO]0=Cl2∶DOC=1.8∶1; pH=8.0±0.2; 反应时间=24 h; 反应温度=(20±1) ℃ EOM: 124.01 (THM); 146.26 (HAA)
IOM: 75.91 (THM); 91.80 (HAA)[18] Aulacoseira granulata f. curvata. [NaClO]0=Cl2∶DOC=1.8∶1; pH=8.0±0.2; 反应时间=24 h; 反应温度=(20±1) ℃ EOM: 72.91 (THM); 56.80 (HAA)
IOM: 56.92 (THM); 66.30 (HAA)[18] Microcystis aeruginosa [NaClO]0=Cl2∶DOC=1.8∶1; pH=8.0±0.2; 反应时间=24 h; 反应温度=(20±1) ℃ EOM: 21.34 (THM); 28.46 (HAA)
IOM: 24.44 (THM); 30.52 (HAA)[18] Merismopedia sp. [NaClO]0=Cl2∶DOC=1.8∶1; pH=8.0±0.2; 反应时间=24 h; 反应温度=(20±1) ℃ EOM: 54.66 (THM); 62.98 (HAA)
IOM: 62.61 (THM); 65.30 (HAA)[18] Microcystis aeruginosa [DOC]=1.8 mg·L−1; [NaClO]0=12.8 mg·L−1; pH=7; 反应时间=3 d; 反应温度=24 ℃ AOM为藻悬浮液COM为细胞碎片 AOM: 30.5a (TCM); 1.2a (TCNM)
EOM: 14.0a (TCM); 1.5a (TCNM)
IOM: 30.9a (TCM); 1.0a (TCNM)
COM: 8.3a (TCM); 0.5a (TCNM)[19] Cyclotella meneghiniana [DOC]=1.2 mg·L−1; [NaClO]0=12.8 mg·L−1; pH=7; 反应时间=3 d; 反应温度=24 ℃ AOM为藻悬浮液COM为细胞碎片 AOM: 31.8a (TCM); 1.0a (TCNM)
EOM: 11.0a (TCM); 0.4a (TCNM)
IOM: 12.8a (TCM); 0.5a (TCNM)
COM: 13.1a (TCM); 0.5a (TCNM)[19] Chlorella sp. [DOC]=5 mg·L−1; [NaClO]0= Cl2∶DOC=5∶1; pH=(7±0.1); 反应时间=7 d; 反应温度=(25±1) ℃ 指数生长后期 EOM: 5.8 (THM); 15.9 (HAA)
IOM: 9.0 (THM); 40.9 (HAA)
COM: 5.9 (THM); 15.4 (HAA)[20] Chlorella sp. [DOC]=5 mg·L−1; [NaClO]0= Cl2∶DOC=5∶1; pH=(7±0.1); 反应时间=7 d; 反应温度=(25±1) ℃ 衰亡期 EOM: 6.7 (THM); 17.9 (HAA)
IOM: 5.1 (THM); 33.3 (HAA)
COM: 5.7 (THM); 11.4 (HAA)[20] Chlorella sp. [DOC]=5 mg·L−1; [NaClO]0= Cl2∶DOC=5∶1; pH=(7±0.1); 反应时间=7 d; 反应温度=(25±1) ℃ 指数生长后期&
低硝酸盐培养基EOM: 7.0 (THM); 16.8 (HAA)
IOM: 29.4 (THM); 45.1 (HAA)
COM: 12.7 (THM); 13.4 (HAA)[20] Chlorella sp. [DOC]=5 mg·L−1; [NaClO]0= Cl2∶DOC=5∶1; pH=(7±0.1); 反应时间=7 d; 反应温度=(25±1) ℃ 衰亡期&
低硝酸盐培养基EOM: 1.7 (THM); 11.3 (HAA)
IOM: 10.8 (THM); 49.7 (HAA)
COM: 11.9 (THM); 22.9 (HAA)[20] Chaetoceros muelleri [NaClO]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ EOM: 29 (TCM) [25] Oscillatoria prolifera [NaClO]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ EOM: 30 (TCM) [25] Scenedesmus quadricauda [NaClO]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ EOM: 48 (TCM) [25] Chlorella sp. [DOC]=5 mg·L−1; [NaClO]0= Cl2∶DOC=5∶1; pH=7±0.1; 反应时间=7 d; 反应温度=(25±1) ℃ EOM: 6.4 (THM); 17.7 (HAA)
IOM: 7.5 (THM); 31.7 (HAA)[36] Chlorella sp. [DOC]=5 mg·L−1; [NaClO]0=Cl2∶DOC=5∶1; pH=7.0±0.1; 反应时间=7 d; 反应温度=(25±1) ℃ EOM: 10.0 (THM); 20.5 (HAA)
IOM: 12.1 (THM); 25.7 (HAA)[51] Microcystis aeruginosa [游离氯]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ AOM: 42.6 (TCM); 28.7 (HAA); 1.32 (DCAN) [53] Aphanizomenon flos-aquae [游离氯]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ AOM: 56.6 (TCM); 24 (HAA); 0.12 (DCAN) [53] Scenedesmus subspicatus [游离氯]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ AOM: 19.9 (TCM); 35.8 (HAA); 1.10 (DCAN) [53] Asterionella formosa [游离氯]0=Cl2∶DOC=5∶1; pH=7; 反应时间=7 d; 反应温度=20 ℃ AOM: 18.7 (TCM); 25 (HAA); 0.53 (DCAN) [53] Microcystis aeruginosa [DOC]=45.7 mg·L−1; [NaClO]0=20 mg·L−1; 残留氯浓度=3—4 mg·L−1; pH=7; 反应时间=24 h; 反应温度=(25±1) ℃ 指数生长期 AOM: 25.5 (THM); 38.8 (HAA); 7.1 (HAN) [54] Microcystis aeruginosa [DOC]=44.7 mg·L−1; [NaClO]0=20 mg·L−1; 残留氯浓度=3—4 mg·L−1; pH=7; 反应时间=24 h; 反应温度=(25±1) ℃ 衰亡期 AOM: 55.5 (THM); 97.2 (HAA); 25.5 (HAN) [54] Oscillatoria sp. [NaClO]0=Cl2:DOC=10:1; 反应时间=4 d; 反应温度=20 ℃ 藻细胞(AOM)=
藻悬浮液-EOMAOM: 26.1 (TCM); 33.5 (DCAA); 38.5 (TCAA) [55] Chlamydomonas sp. [NaClO]0=Cl2:DOC=10:1; 反应时间=4 d; 反应温度=20 ℃ 藻细胞(AOM)=
藻悬浮液-EOMAOM: 33.9 (TCM); 28.9 (DCAA); 32.9 (TCAA) [55] Nitzschia sp. [NaClO]0=Cl2:DOC=10:1; 反应时间=4 d; 反应温度=20 ℃ 藻细胞(AOM)=
藻悬浮液-EOMAOM: 47.8 (TCM); 24.5 (DCAA); 18.5 (TCAA) [55] Cyclotella sp. 细胞浓度=20000 cells·mL−1; [NaClO]0=Cl2:DOC=14:1=15 mg·L−1; pH=7.0; 反应时间=7 d; 反应温度=20 ℃ AOM由藻细胞
反应AOM: 43a (THM); 85a (HAA)
EOM: 16a (THM); 29a (HAA)[86] Cyclotella sp. 细胞浓度=20000 cells·mL−1; [NaClO]0=Cl2:DOC=14:1=15 mg·L−1; pH=7.0; 反应时间=7 d; 反应温度=20 ℃ 藻细胞经过
1 mg/L预臭氧化处理AOM由藻细胞反应AOM: 51a (TCM); 104a (HAA)
EOM: 24a (TCM); 29a (HAA)[86] Microcystis aeruginosa [DOC]=1.30 mg·L−1; [游离氯]0=5.5 mg·L−1; pH=8; 反应时间:24 h AOM由藻细胞
反应AOM: 12.87 (THM) [87] Microcystis aeruginosa 残留[游离氯]=4.1—13.4 mg·L−1; pH=7.5; 反应时间=7 d; 反应温度=22—24 ℃ IOM: 64 (TCM); 117 (HAA); 64 (TCNM); 1.2 (DCAN) [100] Oscillatoria sp. 残留[游离氯] = 4.1—13.4 mg·L−1; pH=7.5; 反应时间=7 d; 反应温度=22—24 ℃ IOM: 47 (TCM); 121 (HAA); 41 (TCNM); 0.7 (DCAN) [100] Lyngbya sp. 残留[游离氯] = 4.1—13.4 mg·L−1; pH=7.5; 反应时间=7 d; 反应温度=22—24 ℃ IOM: 38 (TCM); 101 (HAA); 40 (TCNM); 1.0 (DCAN) [100] Microcystis aeruginosa [DOC]=5 mg·L−1; [NH2Cl]0=200 μmol·L−1; pH=7; 反应时间=72 h; [I−]0=10 μmol·L−1; 反应温度=(25±1) ℃ AOM: 19.9 (I-THM) [101] Microcystis aeruginosa [DOC]=5 mg·L−1; [NH2Cl]0=200 μmol·L−1; pH=7; 反应时间=72 h; [碘帕醇]0=
10 μmol·L−1; 反应温度=(25±1) ℃AOM: 36.4 (I-THM) [101] Microcystis aeruginosa [DOC]=5 mg·L−1; [NH2Cl]0=200 μmol·L−1; pH=7; 反应时间=72 h; [I−]0=10 μmol/L; [Br−]=5 μmol·L−1; 反应温度=(25±1) ℃ AOM: 33.4 (I-THM) [101] Microcystis aeruginosa [DOC]=5 mg·L−1; [NH2Cl]0=200 μmol·L−1; pH=7; 反应时间=72 h; [碘帕醇]0=
10 μmol·L−1; [Br−]0=5 μmol/L; 反应温度=(25±1) ℃AOM: 107.6 (I-THM) [101] Microcystis aeruginosa [DOC]=5 mg·L−1; [NH2Cl]0=200 μmol·L−1; pH=6; 反应时间=72 h; [碘帕醇]0=
10 μmol·L−1; [Br−]0=5 μmol/L; 反应温度=(25±1) ℃AOM: 125.9 (I-THM) [101] Microcystis aeruginosa [DOC]=5 mg·L−1; [NH2Cl]0=200 μmol·L−1; 反应时间=72 h; [碘帕醇]0=10 μmol·L−1; [Br−]0=5 μmol·L−1; 反应温度=(25±1) ℃ AOM: 8.7 (I-THM) [101] Anabaena flos-aquae 残留[游离氯]>0.5 mg·L−1; pH=7; 反应时间=7 d; 反应温度=21 ℃ AOM由藻细胞
反应AOM: 50 (THM); 78 (HAA)
EOM: 26 (THM); 48 (HAA)[102] Microcystis aeruginosa 残留[游离氯]>0.5 mg·L−1; pH=7; 反应时间=7 d; 反应温度=21 ℃ AOM由藻细胞
反应AOM: 61 (THM); 164 (HAA)
EOM: 28 (THM); 66 (HAA)[102] a:单位为μg·L−1 -
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