Advances in photocatalytic materials for algae inhibition

LI Dongpeng; YANG Lan; ZHAO Xiaoxiang; SONG Xinshan; CAO Xin

doi:10.7524/j.issn.0254-6108.2023051802

2023 Volume 42 Issue 11

Article Contents

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LI Dongpeng, YANG Lan, ZHAO Xiaoxiang, SONG Xinshan, CAO Xin. Advances in photocatalytic materials for algae inhibition[J]. Environmental Chemistry, 2023, 42(11): 3951-3964. doi: 10.7524/j.issn.0254-6108.2023051802

Citation:

LI Dongpeng, YANG Lan, ZHAO Xiaoxiang, SONG Xinshan, CAO Xin. Advances in photocatalytic materials for algae inhibition[J]. Environmental Chemistry, 2023, 42(11): 3951-3964. doi: 10.7524/j.issn.0254-6108.2023051802

Advances in photocatalytic materials for algae inhibition

1.
College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China
2.
Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, China

Corresponding author: CAO Xin, caoxin@dhu.edu.cn
Received Date: 18/05/2023
Accepted Date: 02/07/2023
Available Online: 27/11/2023

Fund Project: the National Natural Science Foundation of China（51909034, 52170152），the Fundamental Research Funds for the Central Universities（2232023D-14）and the Science and Technology Innovation Action Plan Social Development Key Science and Technology Project of Shanghai (21DZ1202402).

Abstract

The eutrophication induced by harmful algal blooms has seriously endangered human health and ecological environment worldwide. It is a challenge that has plagued researchers for decades in the research direction of lake ecology and environment restoration. In the context of the existing algae removal methods, photocatalytic technology, with its advantages of high efficiency, cleanliness, environmental friendliness, and simplicity of operation, has some prospects for application in controlling harmful algal blooms and eutrophication. Therefore, this study reviews the research progress of photocatalytic materials for removing harmful algae in recent years, clarifies the mechanism of algae inhibition by photocatalytic materials, and emphasizes the release and removal of toxic pollutants in the process of algae removal. Finally, we summarize the crucial limiting factors of photocatalytic materials for algae inhibition, point out the remaining shortcomings in the application and promotion of this technology. We hope this review provides an outlook on the future research of photocatalytic materials to enhance the self-purification ability of aquatic ecology.
- photocatalysis,
- nanomaterials,
- water eutrophication,
- algae removal.

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沈阳化工大学材料科学与工程学院沈阳 110142

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Advances in photocatalytic materials for algae inhibition

Corresponding author: CAO Xin, caoxin@dhu.edu.cn

Received Date: 18/05/2023

Accepted Date: 02/07/2023

Available Online: 27/11/2023

Fund Project: the National Natural Science Foundation of China（51909034, 52170152），the Fundamental Research Funds for the Central Universities（2232023D-14）and the Science and Technology Innovation Action Plan Social Development Key Science and Technology Project of Shanghai (21DZ1202402).

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Abstract: The eutrophication induced by harmful algal blooms has seriously endangered human health and ecological environment worldwide. It is a challenge that has plagued researchers for decades in the research direction of lake ecology and environment restoration. In the context of the existing algae removal methods, photocatalytic technology, with its advantages of high efficiency, cleanliness, environmental friendliness, and simplicity of operation, has some prospects for application in controlling harmful algal blooms and eutrophication. Therefore, this study reviews the research progress of photocatalytic materials for removing harmful algae in recent years, clarifies the mechanism of algae inhibition by photocatalytic materials, and emphasizes the release and removal of toxic pollutants in the process of algae removal. Finally, we summarize the crucial limiting factors of photocatalytic materials for algae inhibition, point out the remaining shortcomings in the application and promotion of this technology. We hope this review provides an outlook on the future research of photocatalytic materials to enhance the self-purification ability of aquatic ecology.

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近年来，有害藻类大量繁殖导致的水体富营养化现象在世界各国频繁出现^{[1 − 3]}. 水华爆发时，以蓝藻为代表的有害藻类大量消耗水中溶解氧，造成水中其它水生植物、动物死亡，最终影响水环境质量，破坏水生态环境平衡，对人类健康和社会经济造成严重危害^[4]. 同时，藻类在死亡过程中释放出的藻毒素和有机质等物质对环境造成二次污染. 因此，控制有害藻华繁殖，防治水体富营养化成为水污染控制的重要议题. 在此背景下，国家和地方在水华频繁爆发的地区制定出台政策文件，旨在控制水体富营养的危害. 国家发展改革委于2021年发布的《关于加强长江经济带重要湖泊保护和治理的指导意见》（发改地区[2021]1617号）明确，到2025年，确保太湖、巢湖地区不发生大面积蓝藻水华导致水体黑臭现象，以确保供水水源安全.

目前，已有多种除藻以及控制水华爆发的技术手段. 传统的治理手段主要包括化学法、物理法和生物法^[5]. 化学法采用向水中投加混凝剂、絮凝剂等化学药剂，如CuSO₄、Cl₂等^[6]. 在此过程中可能生成二次污染物，对水体造成不利影响^[7]. 物理法采用超声波^{[8 − 10]}、人工捕捞、紫外辐照^{[11 − 13]}等手段，往往成本高且治标不治本，效果无法长期维持. 生物法采用种植沉水植物，改善水生态群落结构的方法，见效缓慢且作用效果不稳定，无法广泛应用^{[14 − 16]}. 鉴于目前已有的除藻手段存在各自的缺陷，寻求一种新型高效环境友好的除藻手段迫在眉睫. 光催化技术是利用自然界中存在的太阳能激发材料产生高活性的自由基等，从而达到去除水中污染物的目的. 之前的研究中，光催化技术已被广泛应用于去除重金属^[17]、活性染料^[18]、抗生素类药物^[19]等污染物，均取得了良好效果^[20]. 蓝藻作为一种特殊的水体污染物，在光催化材料的作用下可被完全灭活从而从水体中去除. 同时，对于藻类死亡过程中产生的藻毒素类物质具有良好的协同降解作用，最终达到促进水体自净、恢复水生态平衡的目的. 本文从光催化材料的概念出发，系统梳理了已有的可除藻的光催化材料种类，阐明了光催化除藻的机理，并分析影响材料发挥性能的主要因素，并探讨了溶藻过程中藻毒素等有害副产物的释放与去除，对于现有光催化材料抑藻过程中尚存的不足进行分析，以期为后续研究提供参考和支撑.

1. 光催化抑藻材料分类、性能和形态（Classification, properties and morphology of materials for photocatalytic algae inhibition）

光催化材料因其高效性，已被广泛用于杀菌消毒，降解难降解污染物等领域. 蓝藻作为一类古老的原核生物，在生理形态上与细菌有诸多相似性，使用光催化材料除藻具有丰厚的理论支撑，并展现出诱人前景^{[21 − 23]}. 该综述包含对现有光催化材料的添加量，作用时间和藻去除率的调研，并特别考察了起作用的活性基团以及材料的可回收性. 表1列出了已有报道的光催化材料种类.

1.1. 光催化抑藻材料的种类

传统的半导体光催化材料（如TiO₂）只可在紫外光下激发发挥作用，而对藻类的去除需要以阳光为能量来源. 为了满足在太阳光下发挥高效除藻率的特点，现有光催化除藻材料的研究大多是基于可见光进行. 使用贵金属沉积，如Pt、Ag等或非金属元素（如C、N、P元素）与TiO₂材料掺杂^[62,51,57]，可缩窄禁带宽度，将光能的利用扩展到可见光范围. 染料敏化^[50]或半导体复合^[35]等方式形成异质结也是增强单一材料可见光利用率常用手段. 在目前研究中应用于抑藻的光催化材料种类除TiO₂光催化剂之外^{[24,28,30,37,39,41-42,62-63]}，还包括氧化物光催化剂^[35,61,64]，石墨相氮化碳光催化剂^[65]、铋基光催化剂^[19,38,47]和石墨烯光催化剂^[28,33,38]等. 近年来，有关金属有机框架（MOF）材料的新型光催化除藻剂^{[31,32,36,45]}成为新的研究热点. 与传统纳米光催化剂先比，其具有比表面积大、热稳定性好、孔隙率高等有点. 有助于抑制电子-空穴的复合过程. 光催化材料的作用时间从数十分钟到数小时不等，大多数可实现90%以上的除藻效率.

1.2. 光催化除藻材料的形态

传统的光催化材料大都为粉末状，直接投加到水体中易分散，影响其发挥除藻性能. 近年来，越来越多的学者将研究的重点放在开发新型可回收的光催化材料上. 良好的可回收性能有助于提升材料的环境友好特性，使用后不产生二次污染，同时对水生生物及生态环境造成尽可能小的影响. Song等用Al₂O₃改性的膨胀珍珠岩^[47]作为可漂浮载体，将光催化剂负载于其上，在增加对可见光利用率的同时方便材料使用后的回收. 同时，膨胀珍珠岩的物理特性强化了负载材料对铜绿微囊藻的吸附性能. Tu等^[38]研究了使用原料环保易得的丝瓜瓤负载rGO/BiOBr光催化材料后的除藻效果. Fan等^[55]将AGUN光催化材料负载于自浮涂层海绵上，作为浮动载体，不仅增加了催化剂与藻细胞的有效接触，减少了水相中的能量损失，同时促进了对可见光的利用. 该研究团队还成功将Ag/AgCl@ZIF-8纳米材料负载于聚氨酯海绵^[31]上，海绵疏松多孔的特性为纳米材料的结合提供了更多的结合位点，同时良好的亲水性增加了对藻细胞的吸附性能. 海绵中吸附的藻细胞一部分能够从海绵中去除，另一部分在吸附饱和后重新返回到溶液中，最终能够去除99.9%的藻细胞. 负载材料的使用还可以增加光催化剂的循环使用次数. Sun等^[60]通过对聚氨酯海绵负载GBA催化剂的循环利用性能进行考察后认为，通过载体负载后的光催化剂至少可被重复回收使用3次. Wang等^[24]用膨胀珍珠岩为载体负载F-Ce-TiO₂纳米颗粒，借助膨胀珍珠岩优良的吸附性能，在初始1h内可吸附53.7%的铜绿微囊藻，最终除藻率可达98.1%. 此外，使用载体包覆纳米材料可防止材料中的重金属离子在使用过程中的释放. Fan等^[45]使用壳聚糖和聚乙烯醇形成的具有3D交联结构的水凝胶对Ag₂O/g-C₃N纳米材料进行包裹负载，利用材料的可漂浮性能便于使用后的回收，同时防止Ag⁺的释放对水体造成危害.

除可漂浮基材外，通过掺杂磁性材料的方法，使得制备的光催化剂在使用后可通过外部磁场回收，也是一种提升材料回收性能的方式. Fan等^[59]制备的ZnFe₂O₄/Ag₃PO₄/g-C₃N₄材料在可见光下可实现对铜绿微囊藻和MC-LR的同步去除. 除了掺杂含铁化合物外，Serrà等^[61]合成的磁性Ni基材料Ni@ZnO@ZnS，其对勃那特螺旋藻和水华鱼腥藻具有良好的去除效果.

2. 光催化材料抑藻机理（Mechanism of algae inhibition by photocatalytic materials）

3. 除藻过程中有害副产物的释放与去除（Release and removal of harmful by-products in the process of algae removal）

在光催化材料产生的自由基作用下，藻细胞裂解死亡. 此过程常伴随胞内物质的释放，包括藻毒素和有机质等. 副产物的释放加剧水体的污染状况. 因此，现有研究常需考察光催化材料对除除藻过程有害副产物的协同去除情况.

3.1. 藻毒素类物质

藻毒素是一种由藻类释放，具有高稳定性、高毒性、难降解的环状七肽化合物. 其累积可造成人类肝损伤、神经麻痹，对水生生物也有潜在危害^[82]. 藻毒素可随食物链、食物网迁移累积，最终进入人体或饮用水中，对人类健康造成威胁^[83]. 藻华爆发的过程中，随着藻类的生长繁殖及死亡，藻类释放的藻毒素不断进入水体^[84]. 使用光催化除藻手段使藻细胞裂解死亡时，也伴随着大量藻毒素的释放. 因此，对于藻类被光催化材料去除过程中藻毒素的释放与降解也是研究的重要一环，旨在达到去除有害藻类协同脱除藻毒素的目的. Jin等^[39]对了N-TiO₂材料对拟柱孢藻和柱孢藻毒素的降解的研究揭示了藻毒素释放快慢与细胞受损程度间的关系，初始时由于藻细胞形态完好，柱孢藻毒素释放降解较为缓慢. 随着藻细胞裂解程度的加深，12 h后大量藻毒素由胞内释放到胞外，胞内藻毒素浓度下降而胞外藻毒素浓度上升，藻毒素降解速度加快.

光激发产生的ROS常进攻Adda侧链上的甲氧基、芳香环结构或碳碳双键，使其发生断裂或脱羧、脱酰、羧基化等反应^[33]. 值得注意的是，MC-LR在降解过程中由于氨基酸支链的结构稳定，ROS的进攻很难将其完全矿化为小分子有机物. 但该过程中藻毒素的毒性也会降低，从而减小其环境危害性^[85]. Wang等^[30]研究了使用PDDA改性后的TiO₂材料除藻的同时对藻毒素的降解路径. 他们发现材料在可见光激发下产生的活性基团可攻击藻毒素分子中的Adda侧链，并脱去谷氨酸、丙氨酸等基团，揭示了藻毒素质荷比由995.6下降到835.4的可能途径. 最终，使用0.2PDDA@NPT-EGC的光催化材料可在6 h后降解几乎100%的藻毒素. 这充分说明光催化技术具有灭藻细胞的同时降解产生的藻毒素的功能. 他们的研究也证实，藻细胞存在对藻毒素的降解效率也存在影响，这可能是由于MC-LR与藻细胞之间对光催化活性位点存在竞争关系. Fan等^[34]研制的AGON光催化材料在120 min内可降解0.5 mg·L⁻¹的MC-LR，而当体系中同时存在藻细胞与MC-LR时，8 h后的降解率只有91.2%. 同时，MC-LR的存在对于铜绿微囊藻的去除也存在抑制作用. 此外，Pelaez等^[86]发现，同种材料对微囊藻毒素的不同亚型之间的降解率也有显著差异，相同时间内MC-LR的降解程度高于MC-RR.

3.2. 溶解性有机质

溶解性有机质（dissolved organic matter, DOM）是一种广泛存在于水体环境中的复杂有机质，由脂肪族和芳香族以及多种含有羟基、羧基和氨基等官能团组成的有机物连续体或混合体. 水体中的DOM来源包括动植物残体腐败、藻类代谢产生的排泄物^[87]. DOM能够与水体中重金属相互作用，影响营养盐释放，且可影响水体中微量金属离子的形态、毒性和迁移转化过程^[88]，在水域生态系统和生物地球化学循环过程中起重要作用^[89]. DOM可分为胞外溶解性有机质（EDOM）和胞内溶解性有机质（IDOM）. 在光催化除藻的过程中，藻类膜结构破坏，IDOM释出，EDOM含量提升. 目前研究多针对EDOM和IDOM的转化和藻死亡后EDOM的去除. Wang等^[24]细致研究了F-Ce-TiO₂/EP450除藻过程中胞外溶解性有机质（EDOM）与胞内溶解性有机质（IDOM）的含量变化情况. 结果显示EDOM包含的两个主峰中其中一个强度显著降低，说明EDOM得到部分去除；而另一个峰强度增加，说明IDOM随细胞破裂释出转化为EDOM. IDOM的峰强则未发生显著变化.

藻细胞表面带负电荷，其zeta电位呈现负值，而释放出的EDOM会附着在藻细胞表面，这会抑制藻细胞的灭活过程^[90]. Zhou等^[37]发现，藻类死亡初期释放的DOM包括蛋白质类物质、富里酸和腐殖酸等. 初期藻细胞破裂时DOM释出，随后在大量活性基团的作用下得到一定程度的降解. 仍有一部分DOM留存于水体中无法被彻底降解，后续产生的环境影响以及与其它污染物质相互作用机制尚待研究. Fan等^[31.55,59]指出，IDOM的成分主要为芳香族蛋白质（如酪氨酸和色氨酸等），EDOM中的成分为腐殖酸和富里酸. IDOM的释放导致蛋白质类物质转化为富里酸和腐殖酸类物质. 相比于蛋白质类物质，腐殖酸和富里酸类物质更易被降解. 经过光催化材料作用后，其主要特征峰强度均减弱，而最终并未完全去除. 这说明只依靠ROS的作用，很难将EDOM彻底去除. 但在ROS的作用下，EDOM含量通常比藻华爆发时更低. 使用氧化剂（如臭氧^[91]和高铁酸钾氧化^[92]）或电渗析法^[93]可进一步去除水体中残留的EDOM.

4. 光催化材料性能的限制性因素（Limiting factors for the performance of photocatalytic materials）

4.1. 材料特性

光催化材料自身性质是影响光催化效果的首要因素. 以目前研究广泛的TiO₂光催化材料为例，锐钛矿和金红石是常见的两种TiO₂. 一般认为，金红石具有相对稳定的热力学稳定性，而锐钛矿作为亚稳定形态，具有最佳的光催化活性^[94]. 因此研究中多围绕锐钛矿TiO₂进行. Zhou等^[37]在N₂气氛中煅烧制得黑色或蓝色TiO₂. 相比于一般的黄色TiO₂而言，黑色TiO₂具有不同的纳米粒子的结构、化学、电子以及光学性质，同时具有结构和化学缺陷、更强的导电性和更窄的禁带宽度. 目前的研究中常采用掺杂的方式对单一光催化材料性能进行改良，将光能的利用范围从紫外光区扩展到可见光区，同时提升光生电子和空穴的分离效率，减少复合率，提高除藻效率的同时也极大增加了实际应用的可行性. Song等^[41]合成的g-C₃N₄/TiO₂可漂浮型光催化材料对铜绿微囊藻和MC-LR表现出极高的吸附和降解效率.

4.2. 藻细胞密度

水体中藻细胞密度对于光催化材料的去除效率也有重要影响. 已有研究表明，不同的初始藻密度对于金属或纳米颗粒的毒性效应有影响^[95,96]. Sun等^[60]的研究发现，在高浓度和低浓度的藻密度条件下，光催化材料均不能发挥最佳除藻性能. 当藻密度过低时，藻细胞与纳米颗粒间的有效接触减少，这延缓了藻细胞的灭活速率. 而当藻密度过高时，材料产生的ROS对应过多的藻细胞而减弱. 且当藻密度过高时，大量藻细胞将材料颗粒包裹覆盖，降低了材料对光能的利用效率. 同时，过多的藻细胞也了光在水中的传播，不利于材料发挥最佳性能. Wang等^[28]的研究表明，当高浓度的藻细胞抑制了材料除藻的性能，可通过延长材料作用时间以达到相同的除藻率. 藻细胞浓度为2.7×10⁶ cell·mL⁻¹时需3 h达到去除效果，而当藻细胞浓度为5.2×10⁶ cell·mL⁻¹时则需9 h.

4.3. 天然有机物（NOM）浓度

水体中的天然有机物（NOM）等腐殖质类物质对光催化效率也有影响. NOM的存在一方面增加了水体浊度，减小透光度^[97]，且易吸附在纳米材料表面，影响其光催化特性. 另一方面，Ren等^[98]研究表明，腐殖质类物质和藻细胞之间竞争光催化材料表面的活性位点，影响藻类去除. Fan等^[34]考察腐殖质中两类主要成分HA（腐殖酸）和FA（富里酸）对光催化效率的影响，在投加了10 mg·L⁻¹的腐殖酸和富里酸后，Ag₂CO₃-N:GO的除藻率有所下降，然而6 h后藻去除率均达到100%. 当剂量为5 mg·L⁻¹时，实验组中加入HA和FA的藻类细胞的去除率在5 h结束时分别为89%和97%. 随着NOM剂量的增加，藻细胞去除效率进一步降低，同时加入HA和FA的实验组藻类细胞的去除率降至68%和90%. 其中，HA对材料的抑制率比FA更为显著，这可能是由于HA比FA拥有更小的分子量和更高的O/C比，因此FA更容易被氧化. 而HA需要更长的时间和更多的ROS降解^[99]. Jeong等^[100]的研究表明，FA和HA的存在对于MC-LR的降解具有抑制作用. Tu等^[38]发现，DOM的降解过程伴随着总有机碳（TOC）含量的下降，这与藻细胞的死亡过程生物质的消减相呼应.

4.4. H₂O₂的添加

在体系中加入一定浓度的H₂O₂也有利于增加材料产生ROS的效率和强度. Zhang等^[51]研究发现，通过添加H₂O₂促进了b-N-TiO₂/C纳米材料对藻细胞叶绿素a的降解，同时避免了除藻过程中的反应滞后阶段. Wang等^[53]的研究发现，以40 μL作为最适剂量添加H₂O₂，S-CN材料对将铜绿微囊藻的灭活时间从3.5 h缩短到1.5 h. 这可能是由于H₂O₂可与自由电子反应，从而阻止电子-空穴的复合，增强·OH的生成和作用效果.

4.5. 藻种和水质条件

除此之外，不同藻种和藻毒素种类也会影响光催化材料除藻效率. Jin等^[39]发现拟柱孢藻及其产生的柱孢藻毒素相比于铜绿微囊藻及其产生的微囊藻毒素更难降解. 这可能是由于拟柱孢藻在降解过程中先在ROS进攻下分解成更短的细丝，随后长度不断缩短最终破碎. 由于拟柱孢藻相对难降解的特性，在治理由此类藻爆发的水华灾害时将会面临更大的挑战.

Regulska等^[101]认为，在工程项目中应用光催化降解污染物的难题主要来自于水体中复杂组分对光能捕获的干扰和对ROS的竞争. 实际水体中所含有机污染物成分和较高的盐离子浓度会抑制光催化材料的有效性^[102]，一些高盐度水体中所含的卤素离子（如Br^-、Cl^-）可与ROS反应以此竞争活性位点^[103]，从而削弱对藻类的去除作用. 水体的pH也是制约光催化剂除藻的重要条件. Fan等^[50]的研究发现，酸性和碱性条件下Ag/AgCl@g-C₃N₄@UIO-66对铜绿微囊藻的抑制率均高于中性时. 这可能由于藻细胞表面带负电荷，当pH呈酸性时，静电相互作用更强，催化剂与藻细胞更好地接触，有利于增强光催化去除率. 当pH呈碱性时，OH^-和h⁺间反应生成·OH. 这均有利于藻细胞的灭活.

5. 总结与展望（Summaries and perspectives）

本文在系统梳理已有光催化材料抑藻研究的基础上，阐明了材料发挥抑藻效能的机理. 现有的光催化除藻技术在抑藻率和对有害副产物的协同去除上展出了良好的应用前景. 凭借高效的除藻率，广泛的适用范围和低能耗的特点，将成为未来极具潜力的新型除藻方式. 这有助于解决我国长期存在的河湖富营养化问题. 但在目前实际工程的运用中，还存在以下问题待解决：（1）传统的光催化剂多为粉末状，这有利于投加到水体当中可均匀分散从而提高光催化效率，同时这也给材料的回收造成不便. 分散在水体中的材料可能会带来二次污染等问题. （2）部分研究中贵金属作为掺杂剂（如珀、银等），这些贵重金属离子在使用过程中存在释放问题，造成二次污染. 此外，贵金属（如珀、银）等的掺杂会增加光催化材料的成本，不利于其在富营养化水体中应用. 实际的富营养化水体往往面积较大，项目施工时需全面覆盖. 使用二氧化钛、石墨烯氮化碳等为基材的光催化材料则可大幅降低材料的合成成本，在大范围富营养化水体的施工应用中具有显著优势. （3）富营养化水体中常存在的重金属离子及其它有机污染物、营养盐等组分，对于光催化材料发挥效率的影响尚未完全清晰. （4）对于除藻过程中释放的有害副产物（如NOM等）的协同去除仍待探究.

综上，今后的研究可重点侧重于：（1）依托环保易回收的载体，提升材料抑藻效率的同时避免对环境产生次生危害. （2）避免贵金属掺杂剂的使用，在降低光催化抑藻材料成本的同时，可避免重金属离子的释放. 寻找抑藻效率与材料成本的最佳平衡点，为实际水体中大范围应用提供可能. （3）关注材料除藻过程中多种典型污染物以及有害副产物的协同净化效果，为净化水环境，恢复水生态平衡提供良好基础.

Figure (2) Table (1) Reference (103)

Advances in photocatalytic materials for algae inhibition