Study on Mechanism of Synergistic Effect of pH and Dissolved Organic Matter in Water on Photochemical Transformation Kinetics of Antibiotics
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摘要:
光化学转化是水中抗生素类新污染物主要的消除途径，水体pH和溶解性有机质(DOM)是影响抗生素光化学转化的重要因素，但其协同影响机理尚不清楚。本文以诺氟沙星(NOR)和环丙沙星(CIP)作为抗生素代表，以Suwannee River天然有机质(SRNOM)和提取自松花江的DOM (S-DOM)作为DOM代表，在模拟太阳光照射下，通过光化学模拟实验和高效液相色谱检测法，研究了pH和DOM对抗生素光化学转化动力学的协同影响；通过探究不同pH值对DOM及其小分子类似物光敏化生成3DOM*、1O2和•OH量子产率的影响，从分子层面阐明了影响机制。结果表明，pH=9.0时NOR的表观光解速率常数kobs (0.39min−1)分别是pH=5.0时(kobs=0.05min−1)和pH=7.0时(kobs=0.18min−1)的7.80倍和2.17倍，pH=9.0时CIP的kobs(0.43min−1)分别是pH=5.0时(kobs=0.05min−1)和pH=7.0时(kobs=0.20min−1)的8.60倍和2.15倍，即pH影响NOR和CIP的赋存形态进而直接影响其光化学转化，两种抗生素以两性离子形态存在时(pH=9.0)光化学转化最快，与前人结果一致。pH=9.0时DOM介导的抗生素的kobs分别是pH=5.0和pH=7.0时的7.20～14.00倍和1.87～2.25倍，1O2和•OH对抗生素光化学转化的促进作用最为显著，贡献率分别为36.50%和12.58%。PPRIs的贡献随pH增大而减弱，pH=5.0时贡献率为29.17%，pH=7.0时贡献率为16.00%。DOM的E2/E3等光物理性质和酚类等官能团可影响PPRIs的量子产率，pH影响DOM可解离组分的解离程度和DOM不可解离组分的氧化还原活性官能团进而改变PPRIs的量子产率，间接影响抗生素的光化学转化。综上，抗生素的光化学转化具有pH依赖性，DOM通过光敏化生成PPRIs促进抗生素的光化学转化。
要点(1) pH影响NOR和CIP的赋存形态进而直接影响其光化学转化，两种抗生素以两性离子形态存在时(pH=9.0)光化学转化最快。
(2) 深入探讨DOM对抗生素光化学转化动力学的影响，引入了可解离和不可解离的DOM小分子类似物，揭示DOM特性对污染物光化学转化的关键作用。
(3) pH通过影响DOM光敏化PPRIs的生成进而间接影响NOR和CIP的光化学转化，1O2、•OH和3DOM*对抗生素光化学转化的促进作用依次减弱，且PPRIs的贡献随pH增大而减弱。
HIGHLIGHTS(1) pH affects the occurrence form of NOR and CIP to directly influence their photochemical transformation, which is the fastest when the two antibiotics exist in the form of zwitterion (pH=9.0).
(2) The effect of DOM on the photochemical transformation kinetics of antibiotics was deeply discussed by introducing the dissociable and non-dissociable small molecular analogues of DOM to reveal the key role of DOM characteristics in the photochemical transformation of contaminants.
(3) pH indirectly affects the photochemical transformation of NOR and CIP by affecting the formation of DOM photosensitized PPRIs, during which the promoting effect of 1O2, •OH and 3DOM* on the photochemical transformation of antibiotics decreases in turn and the contribution of PPRIs reduces with the increase of pH.
Abstract:pH and dissolved organic matter (DOM) are important for photochemical transformation of antibiotics in water, yet their synergistic mechanism remains unclear. In the study, through photochemical simulation experiments and high performance liquid chromatography (HPLC), the synergistic effect of pH and DOM on the photochemical transformation kinetics of antibiotics was studied, and the molecular mechanism was elucidated by exploring how different pH values influenced quantum yields of 3DOM*, 1O2 and •OH generated by photosensitization of DOM and its small molecule analogues. As a result, pH, which influences occurrence forms of NOR and CIP, directly affects their photochemical transformation, and that of the two antibiotics is the fastest in the form of zwitterion (pH=9.0), consistent with previous results. 1O2 and •OH play the most significant role in promoting the photochemical transformation of antibiotics, with a contribution rate of 36.50% and 12.58% respectively, while the contribution of PPRIs decreases as pH increases, with a contribution rate of 29.17% at pH=5.0 and 16.00% at pH=7.0. The photophysical properties and functional groups can affect quantum yields of PPRIs, and pH, by influencing the dissociation degree and the redox active functional groups of DOM, changes quantum yields and indirectly impacts the photochemical transformation of antibiotics.
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Keywords:
- pH /
- DOM /
- antibiotic /
- photochemical transformation /
- high performance liquid chromatography
BRIEF REPORTSignificance: Antibiotics，as a class of emerging pollutants，have drawn a great deal of attention，and photochemical transformation represents a significant environmental transformation behavior of antibiotics in water［3-4］. Photochemical transformation brings about irreversible alterations to antibiotic molecules，which governs the persistence of antibiotics within the aquatic environment. Consequently，it exerts an influence on the reduction and ultimate fate of antibiotics therein and may also lead to changes in the toxicity，bioavailability，and ecological risks associated with antibiotics. The increasing presence of antibiotics in aquatic environment poses significant risks to ecosystems and human health. pH affects the photochemical persistence of organic pollutants［5-6］，and dissolved organic matter (DOM)，which is ubiquitous in water，is an important photosensitive substance that mediates the photochemical transformation of organic pollutants［1-2］. Therefore，investigating the effect of different pH on the photochemical transformation of pollutants mediated by DOM and revealing the influencing mechanism from the DOM molecular level are of great significance for evaluating the environmental fate of organic pollutants.
Globally，research on the photochemical transformation of antibiotics has gained momentum，yet significant challenges remain. Many studies have focused on isolated factors affecting degradation rates，such as light intensity，temperature，pH and DOM and so on. 3DOM* has been proved to be the main photochemically produced reactive intermediate (PPRI) mediating the photochemical transformation of quinolones［7］，sulfonamides［8］ and tetracyclines［9］，and antibiotics can react quickly with 1O2 and •OH［10-11］. However，the synergistic effect of pH and DOM in influencing the photochemical transformation of antibiotics has not been thoroughly explored. This gap in knowledge limits the ability to predict the environmental fate of these pollutants under varying conditions.
In the study，norfloxacin (NOR) and ciprofloxacin (CIP) commonly detected in aquatic environment were taken as representatives of antibiotic emerging contaminants，and DOM extracted from Songhua River (S-DOM) and Suwannee River Natural Organic Matter (SRNOM) purchased from International Humic Substances Society were regarded as representatives of DOM. Under simulated sunlight irradiation，through photochemical simulation experiments and high performance liquid chromatography (HPLC) detection methods，the synergistic effect of pH and DOM on the photochemical transformation kinetics of antibiotics was studied. The molecular mechanism was elucidated by exploring effects of different pH values on quantum yields of 3DOM*，1O2 and •OH produced by photosensitization of DOM and its small molecular analogues.
The finding reveals that pH significantly affects the occurrence forms of NOR and CIP，leading to enhanced photochemical transformation rates at pH=9.0，where both of antibiotics exist in zwitterionic forms. The study quantifies the contributions of PPRIs generated by DOM，specifically 1O2 and •OH，demonstrating their pivotal roles in promoting antibiotic degradation. The result indicates that the contribution of these species diminishes as pH increases，highlighting the complex dynamics of antibiotic phototransformation.
Methods: S-DOM water sample was taken from Songhua River，and the sampling point was near the sand quarry beside the Wujintun Bridge Toll Station. S-DOM was obtained by elution and drying through the solid phase extraction method，representing DOM in Songhua River under natural conditions. The key step for extracting S-DOM by the solid phase extraction method were as follows: (1) Water sample pretreatment: Filtration was performed using a 0.45μm glass fiber membrane; (2) Determination of TOC in water sample: Each solid phase extraction cartridge (Agilent，1g) adsorbed DOC＜2mmol/L or ＜10L of water sample; (3) Sample acidification: The water sample was acidified to pH=2.0 with formic acid; (4) Column activation: The cartridge was rinsed with methanol of twice the column volume，and then the filler was rinsed with acidified water with a pH=2.0 of twice the column volume; (5) Sample loading: The water sample was extracted using a semi-automatic solid phase extraction instrument，and the flow rate was adjusted to 4.9mL/min; (6) Washing: The ionic components were removed with 20mL of formic acid water，and then the excess formic acid was removed with 20mL of ultrapure water; (7) Drying the cartridge: The cartridge was dried with nitrogen; (8) Elution: Elution was carried out with methanol，and the eluate was stored in a sample bottle for later use; (9) Drying the eluate: The eluate was dried with a nitrogen blowing instrument; (10) Preservation of DOM: It was dissolved with 10ml of ultrapure water and preserved after sonication.
The photochemical experiment was carried out on an XPA-7 rotary photochemical reactor equipped with a condenser. A 500W mercury lamp was selected，and 290nm filters were added to simulate UV-A and UV-B parts of sunlight. The condenser was used to maintain the temperature at 25±1°C. Phosphate buffer solutions with different pH values (5.0，7.0，9.0) were used to prepare the reaction solutions. In the experiment on the photochemical transformation kinetic of antibiotics，initial concentrations of NOR and CIP were 20μmol/L. Sorbic acid (SA)，sodium azide (NaN3)，and isopropanol (IPA) were selected as quenchers for 3DOM*，1O2，and •OH respectively，with initial concentrations of 0.1mmol/L，1mmol/L，and 1mmol/L respectively. In the experiment regarding the photosensitized generation of PPRIs by DOM and its small molecule analogues，2，4，6-trihydroxy phenol (TMP)，with an initial concentration of 0.1mmol/L，was chosen as a chemical probe for 3DOM* for the purpose of capturing it. Meanwhile，furfuryl alcohol (FFA)，at an initial concentration of 50μmol/L，was selected as a probe for 1O2 to trap it. Additionally，benzene，with an initial concentration of 3mmol/L，was picked as a probe for •OH to catch it. Initial concentrations of SRNOM and S-DOM were 5.0mgC/L，and that of small molecule analogues in the experiment were all 10mg/L. At the selected time intervals，1mL of sample was taken from each quartz tube and injected into a brown liquid bottle. All photochemical experiments were carried out in triplicate. Then，the concentrations of the target compounds were analyzed using HPLC. The relevant parameters for detecting compound concentrations by HPLC are shown in Table 1.
During the photochemical transformation process，apparent photolysis rate constants (kobs) of NOR and CIP were fitted using a first order kinetic model. The quantum yield coefficient of 3DOM* (fTMP)，the quantum yield of 1O2 (Φ1O2)，and the quantum yield of •OH (Φ•OH) were analyzed and processed based on formulas (1)-(9).
Data and Results: The synergistic effect of pH and DOM on the photochemical transformation kinetics of NOR and CIP was studied by using photochemical simulation experiments and HPLC. The photochemical transformation of NOR and CIP is pH-dependent，and DOM inhibits their photochemical transformation through the light screening effect，as shown in Fig.1. When pH=5.0，7.0 and 9.0，kobs of NOR in pure water are 0.05，0.18 and 0.39min−1respectively (Fig.1a); kobs of CIP in pure water are 0.05，0.20 and 0.43min−1 respectively (Fig.1b); kobs of NOR in SRNOM (S-DOM) solutions are 0.04，0.14 and 0.31min−1 (0.05，0.16 and 0.36min−1) respectively; kobs of CIP in SRNOM (S-DOM) solutions are 0.02，0.15 and 0.28min−1(0.03，0.15 and 0.30min−1) respectively. Through quenching experiments，contributions of PPRIs generated by DOM under different pH conditions to the photochemical transformation of NOR and CIP were studied，and contributions of 1O2 and •OH are the most significant，as shown in Fig.2.
Regarding the effect of pH on the photosensitized generation of 3DOM*，1O2，and •OH by DOM，results are as follows: (1) Under the same pH value，the ability of S-DOM to generate PPRIs is higher than that of SRNOM，and pH can affect the ability of DOM to generate PPRIs and the steady state concentrations of PPRIs，as shown in Fig.3. (2) DOM small molecule analogues with a higher degree of dissociation have a higher fTMP，while DOM small molecule analogues with a lower degree of dissociation have a higher Φ1O2 and Φ•OH，and pH affects the generation ability of PPRIs by controlling the dissociation forms of dissociable DOM small molecule analogues，as shown in Fig.4. (3) pH affects the generation ability of PPRIs by influencing the redox activity changes of the quinone groups of three non-dissociable DOM small molecule analogues，as shown in Fig.5.
Unlike previous studies that mostly focused on a single factor，this paper comprehensively considered the synergistic effects of pH and DOM on the photochemical transformation kinetics of the two antibiotics，NOR and CIP. Moreover，it further explored the ability of DOM，dissociable DOM small molecule analogues，and non dissociable DOM small molecule analogues to generate PPRIs and their influencing factors，providing a more comprehensive perspective for in depth understanding of the role of DOM in the photochemical transformation process.
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光化学转化是地表水中有机污染物的主要消除途径，水中普遍存在的溶解性有机质(DOM)是介导有机污染物的光化学转化的重要光敏物质。在太阳光照条件下，DOM可以吸收光子生成DOM激发三线态(3DOM*)，进而敏化水中溶解氧生成多种光活性中间体(PPRIs)，如单线态氧(1O2)和羟基自由基(•OH)等，这些PPRIs对污染物的降解具有重要作用［1-2］。DOM组分十分复杂，按照可否解离，分为可解离组分和不可解离组分。水环境的变化可影响DOM的理化性质，如水体pH的变化会导致DOM性质、粒径、官能团和分子量的变化，进而影响DOM光化学活性。尤其是DOM可解离和不可解离组分性质的显著差异，使得水体pH的变化对DOM组分中常见的可解离和不可解离组分的影响不同，进而影响DOM参与有机污染物光化学转化的过程。因此，考察不同pH环境条件下对DOM介导污染物光化学转化的影响，从DOM分子层面揭示影响机理，对于评价有机污染物的环境归趋具有重要意义。
抗生素是一类备受关注的新污染物，光化学转化是其在水中重要的环境转化行为［3-4］。光化学转化不可逆地改变了抗生素分子，决定了抗生素在水环境中的持久性，从而影响水环境中抗生素的消减和归趋，也可能改变抗生素的毒性、生物有效性和生态风险。Boreen等［5］研究了5种磺胺类抗生素的光化学转化行为，发现它们的直接光解速率与杂环取代基团类型有关，且受到pH的影响。Jin等［6］研究了初始浓度、pH和温度对土霉素的直接光解的影响，发现土霉素的4种解离形态有不同的直接光解量子产率，且土霉素直接光解速率常数受pH、初始浓度和温度的影响。3DOM*已被证实是介导喹诺酮类［7］、磺胺类［8］、四环素类［9］抗生素光化学转化的主要PPRI，且抗生素可以和1O2、•OH发生快速反应，与1O2和•OH反应的二级速率常数分别在1.4×103～6.5×107L/(mol·s)、3.7×109～2.2×1010L/(mol·s)［10-11］范围。水中DOM在光照条件下可敏化产生3DOM*、1O2和•OH，因此，DOM在抗生素类污染物光化学转化过程中可发挥重要作用。多数天然水体呈中性到弱碱性，但受水生生物活动、气候和污染物以及DOM特性等影响，水体的pH可发生明显变化［12-14］。pH的变化一方面可影响可解离抗生素的赋存形态，进而影响抗生素直接光化学转化；另一方面可影响水中DOM的存在形态和结构，影响DOM的表观分子量和荧光强度［15］，进而影响DOM光化学性质。例如，刘砚弘等［16］研究表明当pH从6.0增加到7.0时，Suwannee River腐植酸产生1O2的能力和Suwannee River天然有机质产生•OH的能力显著下降。于莉莉等［17］发现高pH值可促进DOM本身的光降解。马哲等［18］研究发现对于黄海海水DOM，其Φ3DOM*值在pH=9.0 (16.83×10−3)高于在pH=7.0 (9.25×10−3)和pH=4.0 (3.52×10−3)的相应值。DOM的光化学活性通过测定DOM在波长254nm和365nm的吸光度比值(E2/E3)来指示，pH可以影响DOM的光学性质，而Φ3DOM*与E2/E3具有正相关关系［19-20］。pH对DOM的光化学性质产生影响，但pH对DOM光敏化生成PPRIs的速率及量子产率差异的内在机制有待进一步研究。
新污染物的环境行为日益收受到人们关注，pH和DOM是影响抗生素类新污染物光化学转化的重要因素，但pH和DOM对抗生素光化学转化动力学的协同影响机理尚不清楚，pH对DOM光敏化生成PPRIs的速率及量子产率差异的内在机制有待进一步研究。因此，本文以水环境中普遍检出的诺氟沙星(NOR)和环丙沙星(CIP)为抗生素类新污染物代表，以提取自松花江流域的DOM (S-DOM)和购于国际腐殖质协会的Suwannee River天然有机质(SRNOM)为DOM代表，在模拟太阳光照射下，研究pH和DOM对抗生素光化学转化动力学的协同影响；同时以SRNOM和S-DOM为DOM代表，探究不同pH值条件对DOM光敏化生成3DOM*、1O2和•OH量子产率的影响，并选取可解离和不可解离的DOM小分子类似物进一步从分子层面探究pH对DOM生成PPRIs的影响。研究过程中，利用总有机碳(TOC)分析仪测定DOM水样的TOC以制备特定浓度SRNOM和S-DOM溶液，利用高效液相色谱法测定探针和抗生素的浓度。
1. 实验部分
1.1 实验样品与处理
S-DOM样品采集自松花江流域乌金屯大桥收费站附近采砂场旁，通过固相萃取法提取并干燥后获得溶解性有机质(S-DOM)，用以代表松花江流域自然状态下的DOM。固相萃取法的具体操作步骤如下：①预处理：将水样通过0.45μm玻璃纤维滤膜过滤；②TOC测定：确保每根固相萃取小柱(Agilent，1g)吸附的溶解性有机碳浓度低于2mmol/L或水样体积小于10L；③酸化处理：用甲酸将水样pH值调节至2.0；④活化柱子：依次用两倍柱体积的甲醇和pH=2.0的酸化水冲洗小柱；⑤上样：使用半自动固相萃取仪以4.9mL/min流速进行萃取；⑥淋洗：先后用20mL甲酸水和20mL超纯水去除离子成分及残留甲酸；⑦干燥：用氮气吹干小柱；⑧洗脱：以甲醇为洗脱剂，收集洗脱液并储存于样品瓶中；⑨干燥洗脱液：利用氮吹仪将洗脱液吹干；⑩DOM保存：用10mL超纯水溶解洗脱物，超声处理后保存备用。
1.2 主要试剂
抗生素光化学转化动力学实验中，NOR (纯度98%)购自上海易恩化学技术有限公司，CIP (纯度98%)、山梨酸(SA，纯度99%)购自梯希爱(上海)化成工业发展有限公司，异丙醇(IPA，纯度99.5%)购自百灵威科技有限公司。
DOM及其小分子类似物光敏化生成PPRIs实验中，2，4，6-三羟基苯酚(TMP，纯度99%)、糠醇(FFA，纯度98%)、苯(纯度98%)、苯乙酮(纯度98%)、1，4-萘醌(纯度99%)、核黄素(纯度98%)、2-氯对苯二酚(纯度85%)、7-羟基香豆素(纯度98%)购自百灵威科技有限公司，杜醌(纯度97%)购自西格玛奥德里奇贸易有限公司。
磷酸氢二钠(纯度98%)和磷酸二氢钠(纯度98%)购自百灵威科技有限公司，用于配制磷酸缓冲溶液。甲醇(色谱纯)、乙腈(色谱纯)、磷酸(分析纯)购自百灵威科技有限公司，用作高效液相色谱的流动相。SRNOM购自国际腐殖质协会。超纯水(PW，18.2MΩ·cm)由成都超纯水技术有限公司生产的净水系统制备。
1.3 实验方法
1.3.1 TOC的测定
水样的预处理步骤如下。首先通过0.45μm滤膜进行过滤，随后用浓硝酸将水样的pH值调节至2.0。S-DOM水样和SRNOM水样的TOC采用总有机碳分析仪(TOC-V CPN SHIMADZU型)进行测定，以制备5.0mgC/L的S-DOM溶液和SRNOM溶液。
1.3.2 光化学实验
光化学实验采用XPA-7型旋转光化学反应仪(南京胥江科技有限公司)进行，该设备配备冷凝器以维持温度在25±1℃。实验选用500W汞灯作为光源，并加装290nm滤光片以模拟太阳光中的UV-A和UV-B波段。汞灯的光强通过TriOS-ramses光谱辐射计(TriOS，GmbH，Germany)进行测定。样品置于石英管中。采用不同pH值(5.0、7.0、9.0)的磷酸缓冲液配制反应溶液。NOR和CIP的初始浓度为20μmol/L，选择SA、叠氮化钠(NaN3)、IPA分别用作3DOM*、1O2、•OH的淬灭剂，初始浓度分别为0.1、1、1mmol/L。选择TMP用作3DOM*的化学探针对其进行捕捉，TMP的初始浓度为0.1mmol/L；选择FFA作为1O2的探针对其进行捕捉，FFA的初始浓度为50μmol/L；选择苯作为•OH的探针对其进行捕捉，苯的初始浓度为3mmol/L。SRNOM和S-DOM的初始浓度为5.0mgC/L，实验中小分子类似物的初始浓度均为10mg/L (核黄素为5.4mgC/L；2-氯对苯二酚为4.2mgC/L；7-羟基香豆素为6.7mgC/L；苯乙酮为8.0mgC/L；杜醌为6.3mgC/L；1，4-萘醌为7.6mgC/L)。实验过程中，每隔一定时间从石英管中取出1mL样品，存放于棕色瓶内，所有实验均设置3组平行。随后，利用高效液相色谱仪(HPLC)对目标化合物的浓度进行分析。
光化学转化过程中NOR和CIP的表观光解速率常数(kobs)采用一级动力学模型拟合。DOM激发三线态量子产率系数(fTMP)基于公式(1)～(3)计算：
$$ f_{\mathrm{TMP}}=\frac{R_{\mathrm{TMP}}}{\sum_\lambda k_{{X}-{a}}(\lambda)[\mathrm{X}]} $$ (1) $$ {k_{X - a}}\left( \lambda \right) = \frac{{{I_p}{\varepsilon _X}\left( \lambda \right)\left( {1 - 1{0^{ - \left( {\alpha \left( \lambda \right) + {\varepsilon _X}\left( \lambda \right)\left[ {\mathrm{X}} \right]} \right)z}}} \right)}}{{{\varepsilon _X}\left( \lambda \right)\left[ {\mathrm{X}} \right]z}} $$ (2) $$ z = \frac{{{\text{π}} {r^2}}}{{2r}} $$ (3) 式中：RTMP为TMP的降解率[mol/(L·s)]；kX-a(λ)为光敏剂X的特征光吸收率(s−1)；[X] 为光敏剂的浓度(mol/L)；Ip为入射光的强度[Einstein/(s·cm2)]；εX(λ)为光敏剂的摩尔吸收率[L/(mol·cm)]；α(λ) 为吸光度；z为光程(cm)；r为圆柱形石英管的内径(cm)。
单线态氧量子产率(Φ1O2)基于公式(4)～(6)计算:
$$ {\Phi _{1O2}} = \frac{{{R_{1O2}}}}{{\sum\nolimits_\lambda {{k_{X - a}}\left( \lambda \right)\left[ X \right]} }} $$ (4) $$ {R_{1O2}} = {R_{{\mathrm{FFA}}}}\frac{{{k_d} + {k_{{\mathrm{FFA}}}} \cdot \left[ {{\mathrm{FFA}}} \right]}}{{{k_{FFA}} \cdot \left[ {{\mathrm{FFA}}} \right] + {k_d}}} $$ (5) $$ {\left[ {{}^1{O_2}} \right]_{ss}} = \frac{{{R_{1O2}}}}{{{k_{{\mathrm{FFA}}}} \cdot \left[ {{\mathrm{FFA}}} \right] + {k_d}}} $$ (6) 式中：R1O2为1O2的产生速率[mol/(L·s)]；RFFA为FFA的降解率[mol/(L·s)]；kd为与水分子碰撞时1O2的淬灭速率常数(2.5×105s−1)［21］；kFFA为FFA与1O2之间反应的二级速率常数[1.0×108L/(mol·s)]［21］；[FFA]为FFA的初始浓度(mol/L)；[1O2]SS为1O2稳态浓度(mol/L)。
苯可以与•OH反应生成苯酚，生成产率为0.7［22］，所以通常采用苯作为测定•OH的化学探针并计算产生速率，苯初始浓度为3mmol/L，羟基自由基量子产率(Φ•OH)基于公式(7)～(9)计算：
$$ {\varPhi _{ \bullet {\mathrm{OH}}}} = \frac{{{R_{ \bullet {\mathrm{OH}}}}}}{{\sum\nolimits_\lambda {{k_{X - a}}\left( \lambda \right)\left[ X \right]} }} $$ (7) $$ \frac{{d\left[ {{\mathrm{Phneol}}} \right]}}{{dt}} = 0.7{k_{ \bullet {\mathrm{OH}}}}\left[ { \bullet {\mathrm{OH}}} \right]\left[ {{\mathrm{Benzene}}} \right] = 0.7{R_{ \bullet {\mathrm{OH}}}} $$ (8) $$ {\left[ { \bullet {\mathrm{OH}}} \right]_{ss}} = \frac{{{R_{ \bullet {\mathrm{OH}}}}}}{{\sum\nolimits_i {{k_{{S_i}{\text{，}} \bullet {\mathrm{OH}}}}\left[ {Si} \right] + {k_d}} }} $$ (9) 式中：R•OH为•OH的产生速率[mol/(L·s)]；k•OH为苯与•OH反应的二级速率常数[7.8×109L/(mol·s)]［22］；[•OH]和[Benzene]分别为该体系中•OH和苯的浓度(mol/L)；R•OH为•OH的生成速率[mol/(L·s)]；[•OH]ss为•OH的稳态浓度(mol/L)；kSi，•OH为淬灭剂与•OH反应的二级速率常数[L/(mol·s)]；[Si]为淬灭剂的浓度(mol/L)；kd为与水分子碰撞时1O2的淬灭速率常数(2.5×105s−1)；•OH的主要淬灭剂为探针苯，根据苯的初始浓度和与•OH反应的二级速率常数，计算出kSi，•OH[Si]值为2.34×107s−1。
1.4 抗生素和探针浓度的分析
使用Agilent 1260Ⅱ高效液相色谱系统(Agilent，California，USA)，结合紫外可见检测器和Ultimate TM AQ-C18色谱柱(250mm×4.6mm，5μm，Welch Materials，Maryland，USA)，在30℃柱温下进行抗生素和探针的定量分析。精密度要求相对标准偏差RSD≤2.0%，准确度要求回收率应在98.0%～102.0%且RSD≤2.0%，线性范围相关系数R≥0.998，检测限要求主峰与噪声峰的信号强度比不小于3，定量限要求不小于10。具体相关参数列于表1。
表 1 NOR、CIP、TMP、FFA和苯酚的HPLC分析条件Table 1. HPLC analysis conditions for NOR, CIP, TMP, FFA and phenol检测物质 流动相比例 检测波长
(nm)流动相流速
(mL/min)NOR 乙腈-磷酸(0.1%)=20∶80 274 1.0 CIP 乙腈-磷酸(0.1%)=20∶80 274 1.0 FFA 乙腈-磷酸(0.1%)=15∶85 219 1.0 TMP 乙腈-磷酸(0.1%)=45∶55 220 1.0 苯酚 甲醇-水=45∶55 225 1.0 2. 结果与讨论
2.1 pH和DOM对NOR和CIP光化学转化动力学的影响
2.1.1 pH和DOM对NOR和CIP光化学转化动力学的协同影响
暗对照实验中NOR和CIP的浓度均没有显著降低(＜3%)。经光照后，NOR和CIP均有不同程度地降解，且存在pH依赖性(图1)。在pH=5.0、7.0、9.0条件下，NOR在纯水中的kobs分别为0.05、0.18和0.39min−1(图1a)，CIP的kobs分别为0.05、0.20和0.43min−1(图1b)。pH=9.0时NOR的kobs分别是pH=5.0时和pH=7.0时的7.80倍和2.17倍，pH=9.0时CIP的kobs分别是pH=5.0时和pH=7.0时的8.60倍和2.15倍。因此，在pH 5.0～9.0范围内，两种抗生素的kobs随pH增大而显著增大(p＜0.05)，说明pH对抗生素类污染物光化学转化的突出作用。从分子结构分析，氟喹诺酮类抗生素包含1个羧基和3个碱性氮，使其在水溶液中呈现5种不同的解离形态。在pH＞8.0时NOR和CIP的羧基发生去质子化，而哌嗪环上的氨基发生质子化，两种抗生素光化学转化最快，与前人报道结果一致［23-24］。由此可见，NOR和CIP以两性离子形态存在时光化学转化最快。
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图 1 不同pH条件下(a)诺氟沙星(NOR)和(b)环丙沙星(CIP)的表观光解速率常数(kobs)Figure 1. Apparent photolysis rate constants (kobs) of norfloxacin (NOR) and ciprofloxacin (CIP) under different pH values: (a) NOR; (b) CIP在pH 5.0、7.0、9.0时，NOR在SRNOM溶液中的kobs (0.04、0.14和0.31min−1)低于在纯水中的kobs，约为纯水中的0.80倍，在S-DOM溶液中的kobs (0.05、0.16和0.36min−1)则为纯水中的0.89～1.00倍(图1a)，CIP在SRNOM (0.02、0.15和0.28min−1)和S-DOM (0.03、0.15和0.30min−1)中的kobs则分别为纯水中的0.40～0.75倍和0.60～0.75倍(图1b)，pH=9.0时两种抗生素的kobs是pH=5.0时的7.20～14.00倍，pH=7.0时的1.87～2.25倍。同一pH条件下，NOR和CIP在SRNOM溶液中的kobs皆低于在S-DOM溶液中的kobs。NOR和CIP在DOM溶液中的光化学转化明显慢于纯水体系，SRNOM对两种抗生素光化学转化的抑制作用更强。因此，不同pH值溶液中，DOM可通过光屏蔽效应抑制NOR和CIP的光化学转化。前人也指出，DOM抑制抗生素光化学转化可归因于DOM的光保护能力，它会产生屏蔽效应、PPRIs淬灭和调节抗生素分子固有的激发态特性，从而抑制抗生素的光化学转化［25］。
2.1.2 PPRIs对NOR和CIP光化学转化动力学的影响
通过淬灭实验，研究了不同pH条件下DOM生成的PPRIs对NOR和CIP光化学转化的贡献，其中，SA、NaN3、IPA分别是3DOM*、1O2和•OH的淬灭剂。从图2a可以看出，在pH=5.0、7.0、9.0条件下，在NOR+SRNOM体系中加入SA，NOR的kobs分别为0.06、0.20和0.37min−1，分别是NOR+SRNOM体系的1.50倍、1.43倍和1.19倍，kobs明显增加，说明3DOM*对NOR的光化学转化没有贡献。DOM主要通过动态淬灭作用抑制抗生素的光化学转化，且NOR在水中主要以自敏化光解为主，3DOM*的淬灭使3DOM*降低到基态，降低了对NOR的动态淬灭作用，促进了NOR的光降解。在NOR+SRNOM+NaN3体系中，NOR的kobs分别为0.05、0.15和0.25min−1，分别是NOR+SRNOM体系的1.25倍、1.07倍和0.81倍。在NOR+SRNOM+IPA体系中，NOR的kobs分别为0.04、0.23和0.25min−1，分别是NOR+SRNOM体系的1.00倍、1.64倍和0.81倍。因此，SRNOM主要通过1O2和•OH促进NOR的光化学转化。
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图 2 淬灭条件下(a，b)诺氟沙星(NOR)和(c，d)环丙沙星(CIP)的表观光解速率常数(kobs)Figure 2. Apparent photolysis rate constants (kobs) of norfloxacin (NOR) and ciprofloxacin (CIP) under quenching values: (a) NOR; (b) NOR; (c) CIP; (d) CIP从图2b可以看出，在pH 5.0、7.0、9.0条件下，在NOR+S-DOM体系中加入SA，NOR的kobs分别为0.04、0.24和0.47min−1，分别是NOR+S-DOM体系的0.80倍、1.50和1.31倍。在NOR+S-DOM+NaN3体系中，NOR的kobs分别为0.01、0.19和0.29min−1，分别是NOR+S-DOM体系的0.20倍、0.56倍和0.81倍。在NOR+S-DOM+IPA体系中，NOR的kobs分别为0.03、0.15和0.37min−1，分别是NOR+S-DOM体系的0.60倍、0.94倍和1.03倍。因此，S-DOM主要通过1O2和•OH促进NOR的光化学转化，1O2的促进能力最强，而3DOM*对NOR的光化学转化只有略微的促进作用，且S-DOM对NOR的促进程度随着pH增大而减小。
从图2c可以看出，在pH 5.0、7.0、9.0条件下，在CIP+SRNOM体系中加入SA，CIP的kobs分别为0.01、0.12和0.31min−1，分别是CIP+SRNOM体系的0.50倍、0.80倍和1.11倍。在CIP+SRNOM+NaN3体系中，CIP的kobs分别为0.003、0.05和0.17min−1，分别是CIP+SRNOM体系的0.15倍、0.33倍和0.61倍。在CIP+SRNOM+IPA体系中，CIP的kobs分别为0.01、0.07和0.25min−1，分别是CIP+SRNOM体系的0.50倍、0.47倍和0.89倍。因此，SRNOM主要通过1O2、•OH和3DOM*促进CIP的光化学转化，1O2促进作用最强，•OH次之，3DOM*最弱，且SRNOM对CIP的促进程度随着pH增大而减小。
从图2d可以看出，在pH=5.0、7.0、9.0条件下，在CIP+S-DOM体系中加入SA，CIP的kobs分别为0.03、0.09和0.52min−1，分别是CIP+S-DOM体系的1.00倍、0.60倍和1.73倍。在CIP+S-DOM+NaN3体系中，CIP的kobs分别为0.01、0.04和0.37min−1，分别是CIP+S-DOM体系的0.33倍、0.27倍和1.23倍。在CIP+S-DOM+IPA体系中，CIP的kobs分别为0.02、0.07和0.44min−1，分别是CIP+S-DOM体系的0.67倍、0.47倍和1.47倍。因此，S-DOM主要通过1O2和•OH促进CIP的光化学转化，1O2的促进能力最强，而3DOM*对CIP的光化学转化只有略微的促进作用。
基于以上结果可以得出，1O2、•OH和3DOM*对NOR和CIP光化学转化的促进作用依次减弱，前两者贡献率分别约为36.50%、12.58%，且PPRIs的贡献率随着pH增大而减弱，pH=5.0时贡献率约为29.17%，pH=7.0时贡献率约为16.00%，pH=9.0时贡献率可忽略不计，由此推测，pH可影响DOM生成PPRIs的能力，进而影响抗生素光化学转化。
2.2 pH对DOM光敏化生成3DOM*、1O2和•OH的影响
2.2.1 SRNOM和S-DOM光敏化生成PPRIs的量子产率
以SRNOM和S-DOM为DOM代表，探究了SRNOM和S-DOM光敏化生成PPRIs的量子产率，结果如图3所示。可以看出，在pH 5.0、7.0、9.0条件下，S-DOM的fTMP (4.52、2.63、2.71)是SRNOM (2.45、1.46、1.11)的1.84倍、1.80倍、2.44倍(图3a)，S-DOM的Φ1O2 (3.23×10−2、2.21×10−2、2.00×10−2)是SRNOM (1.60×10−2、1.94×10−2、7.65×10−3)的2.02倍、1.14倍、2.61倍(图3b)，S-DOM的Φ•OH (1.15×10−4、1.41×10−5、2.39×10−5)是SRNOM (8.89×10−5、1.25×10−5、9.35×10−6)的1.29倍、1.13倍、2.56倍(图3c)。因此，同一pH条件下，S-DOM生成PPRIs的能力高于SRNOM。研究发现，测定DOM在波长254nm和365nm的吸光度比值(E2/E3)可以用来指示DOM的光化学活性，比值越高，光化学活性越低［26］。在pH=5.0、7.0、9.0条件下，SRNOM的E2/E3 (9.28，6.37，6.11)分别是S-DOM的E2/E3 (6.08，3.92，3.55)的1.53倍、1.63倍、1.72倍，表明S-DOM的光化学活性明显高于SRNOM。因此，在同一pH条件下，S-DOM的fTMP、Φ1O2、Φ•OH较高，其介导NOR和CIP的间接光解强于SRNOM。
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图 3 不同pH条件下DOM的3DOM*、1O2和•OH量子产率Figure 3. Quantum yields of 3DOM*，1O2 and •OH of DOM under different pH values: (a) fTMP; (b) Φ1O2; (c) Φ•OH2.2.2 pH对SRNOM和S-DOM光敏化生成PPRIs的影响
以SRNOM和S-DOM为DOM代表，探究了pH值对DOM光敏化生成PPRIs的影响。在pH 5.0、7.0、9.0条件下，SRNOM的[1O2]SS分别为1.53×10−13、1.62×10−13和8.18×10−14mol/L，SRNOM的[•OH]ss分别为9.15×10−18、1.12×10−18和1.08×10−18mol/L，S-DOM的[1O2]SS分别为2.55×10−13、2.10×10−13和1.81×10−13mol/L，S-DOM的[•OH]ss分别为9.80×10−18、1.45×10−18和2.33×10−18mol/L。由此可知，SRNOM和S-DOM在pH=9.0时的[1O2]SS和[•OH]ss皆低于pH=5.0时，为pH=5.0时的0.12～0.71倍。同时，从图3可以看出，SRNOM和S-DOM在pH=9.0时的fTMP、Φ1O2、Φ•OH皆低于pH=5.0时，为pH=5.0时的0.11～0.62倍，且NOR和CIP在pH=9.0时kobs最大，说明pH=9.0能够抑制DOM光致生成PPRIs，pH=9.0时[PPRI]ss值较低，pH=9.0时DOM介导NOR和CIP的光化学转化最弱，两者光化学转化最快。因此，pH可影响DOM生成PPRIs的能力和PPRIs的稳态浓度，进而影响抗生素光化学转化。
SRNOM与S-DOM是两种不同来源的DOM，二者的酚类基团所占比重不同，氧化还原性质有差异，DOM中抗氧化酚部分对pH的依赖性抑制作用可能会对DOM光致生成PPRIs的过程产生影响，导致量子产率的差异［27］。此外，SRNOM和S-DOM在DOM的分子量、亲疏水性、结构与官能团、芳香性等方面存在差异，影响PPRIs的产生，对pH变化的响应有差异，对抗生素光化学转化的影响也存在差异。
2.3 pH对DOM光敏化生成3DOM*、1O2和•OH影响的分子机制
2.3.1 pH对可解离DOM小分子类似物光敏化生成PPRIs的影响
以可解离的核黄素、2-氯对苯二酚和7-羟基香豆素为DOM小分子类似物代表，研究了pH值对其生成PPRIs的影响。从图4a可以看出，核黄素在pH 5.0、7.0、9.0条件下fTMP分别是5.13、4.93和5.44，明显高于2-氯对苯二酚(1.24、2.08、0.31)和7-羟基香豆素(1.73、1.26、1.71)。从图4b可以看出，在pH 5.0、7.0、9.0条件下，2-氯对苯二酚的Φ1O2分别是5.57×10−2、4.08×10−2和2.53×10−2，核黄素的Φ1O2分别是3.66×10−2、3.07×10−2和2.75×10−2，7-羟基香豆素的Φ1O2分别是1.48×10−2、8.55×10−3和2.35×10−2，2-氯对苯二酚在pH=5.0、7.0条件下的Φ1O2明显高于核黄素和7-羟基香豆素的相应值，pH=9.0时三种可解离DOM小分子类似物Φ1O2值相近。从图4c可以看出，在pH 5.0、7.0、9.0条件下，2-氯对苯二酚Φ•OH分别是2.88×10−4、3.43×10−4和6.10×10−5，核黄素的Φ•OH分别是1.07×10−4、1.80×10−5和5.45×10−5，7-羟基香豆素的Φ•OH分别是1.38×10−4、7.30×10−5和7.32×10−5，2-氯对苯二酚在pH=5.0、7.0条件下的Φ•OH明显高于核黄素和7-羟基香豆素的相应值，pH=9.0时三种可解离DOM小分子类似物Φ•OH值相近。杜超等［28］发现DOM结构中官能团的可解离程度对PPRIs生成具有一定影响。核黄素的解离常数pKa值(1.7)最小，7-羟基香豆素的pKa值(7.11)较大，2-氯对苯二酚的pKa值最大(9.21)，解离出氢离子的能力依次减弱。故在相同pH条件下，可解离程度较高的DOM小分子类似物的fTMP更高，可解离程度较低的DOM小分子类似物的Φ1O2和Φ•OH更高，可解离程度较低的DOM小分子类似物可能在能量传递过程中更稳定，不易因解离而损失能量，由此推测，可解离程度低的DOM小分子类似物对抗生素光化学转化的促进作用更为明显。但也有研究表明，具有不同pKa值的模型化合物在不同的pH条件下产生活性氧物种的能力与其解离性整体上呈正相关［29］，与本研究结果相反。
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图 4 不同pH条件下三种可解离DOM小分子类似物的3DOM*、1O2和•OH量子产率Figure 4. Quantum yields of 3DOM*，1O2 and •OH for three dissociable DOM small molecule analogues under different pH values: (a) fTMP; (b) Φ1O2; (c) Φ•OH在三种pH条件下，核黄素在三种pH条件下的解离形态基本相同，核黄素的fTMP范围为4.93～5.44，Φ1O2范围为2.75×10−2～3.66×10−2，Φ•OH范围为1.80×10−5～1.07×10−4，PPRIs的量子产率(系数)变化趋势较小。2-氯对苯二酚在三种pH条件下解离形态不同，fTMP范围为0.31～2.08，Φ1O2范围为2.53×10−2～5.57×10−2，Φ•OH范围为6.10×10−5～3.43×10−4，PPRIs的量子产率(系数)变化趋势较大。核黄素分子中的核糖醇部分的羟基和异咯嗪环上的氮原子可能参与解离反应，但在pH=5.0到pH=9.0范围内，这些基团的解离程度有限，因此核黄素在三种pH条件下的解离形态基本相同。而2-氯对苯二酚分子中含有酚羟基，在酸性条件下2-氯对苯二酚主要以分子形式存在，随着pH值的升高2-氯对苯二酚会逐渐形成酚氧负离子，在碱性条件下2-氯对苯二酚主要以酚氧负离子的形式存在，因此2-氯对苯二酚在三种pH条件下解离形态不同。因此，pH通过控制可解离DOM小分子类似物的解离形态，影响PPRIs的生成能力，进而影响抗生素的光化学转化。
2.3.2 pH对不可解离DOM小分子类似物光敏化生成PPRIs的影响
以不可解离的苯乙酮、杜醌和1，4-萘醌为DOM小分子类似物代表，研究了pH值对其生成PPRIs的影响。在pH=5.0、7.0、9.0条件下，苯乙酮的fTMP分别是3.06、7.62和7.33(图5a)，是杜醌(9.44、4.92、6.22)的0.32倍、1.55倍和1.18倍，Φ1O2 (1.33×10−2、3.44×10−2、3.56×10−2)是杜醌(4.59×10−2、2.13×10−2、1.74×10−2)的0.29倍、1.62倍和2.05倍(图5b)，Φ•OH (1.87×10−4、4.62×10−5、3.82×10−5)是杜醌(4.15×10−5、3.79×10−5、2.14×10−5)的4.51倍、1.22倍和1.79倍(图5c)。在pH=5.0、7.0、9.0条件下，苯乙酮的fTMP是1，4-萘醌(1.52、1.26、2.67)的2.01倍、6.05倍和2.33倍，Φ1O2是1，4-萘醌(1.20×10−2、4.21×10−2、1.16×10−2)的1.11倍、0.82倍和3.07倍，Φ•OH是1，4-萘醌(1.13×10−5、1.84×10−5、1.58×10−5)的16.55倍、2.51倍和2.42倍。因此，同一pH条件下，苯乙酮的PPRIs的量子产率(系数)明显高于杜醌和1，4-萘醌，杜醌的PPRIs的量子产率(系数)高于1，4-萘醌。
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图 5 不同pH条件下三种不可解离DOM小分子类似物的3DOM*、1O2和•OH量子产率Figure 5. Quantum yields of 3DOM*，1O2 and •OH for three non-dissociable DOM small molecule analogues under different pH values: (a) fTMP; (b) Φ1O2; (c) Φ•OH已有研究表明，具有芳香酮类、醌类结构的DOM对3DOM*的产生具有关键作用［30］。苯乙酮属于芳香酮类，酮类3DOM*易于失去电子生成PPRIs，且酮类失电子的反应相比于醌类3DOM*的反应较为占优势。3DOM*是1O2和•OH的前体，因此当fTMP提高，Φ1O2和Φ•OH也会随之升高，因此酮类DOM的PPRIs量子产率(系数)较高，对抗生素光化学转化的促进作用较强［31］。醌基是负责DOM氧化还原性质的主要部分，但是pH的变化会导致氧化还原活性官能团的可及性变化，使其PPRIs量子产率(系数)发生变化［30］。例如，杜醌的Φ1O2和Φ•OH随着pH的增大而降低。pH通过影响三种不可解离DOM小分子类似物的醌基的氧化还原活性变化，进而影响抗生素的光化学转化。因此，pH可通过影响DOM的官能团结构进而影响抗生素的光化学转化。
3. 结论
与以往研究多聚焦单一因素不同，本文综合考虑了pH和DOM对NOR和CIP这两种抗生素光化学转化动力学的协同影响，并进一步探讨了DOM、可解离DOM小分子类似物和不可解离DOM小分子类似物生成PPRIs的能力及其影响因素，为深入理解DOM在光化学转化过程中的作用提供了更全面的视角。
本文结合模拟光照实验和高效液相色谱法开展研究，并引入了小分子类似物的模拟研究法深入阐释了pH和DOM对水中抗生素类污染物光化学转化动力学影响的分子机制。两种抗生素以两性离子形态存在时(pH=9.0)光化学转化最快，DOM可以通过光屏蔽效应抑制两种抗生素的光化学转化。pH可影响DOM敏化生成PPRIs的能力，1O2、•OH和3DOM*对两种抗生素光化学转化的促进作用依次减弱，且其贡献随着pH增大而减弱。DOM的E2/E3等光物理性质和酚类基团等官能团结构影响PPRIs的量子产率。可解离DOM小分子类似物生成PPRIs的能力受解离常数pKa和解离形态影响，不可解离DOM小分子类似物生成PPRIs的能力受酮类、醌类基团等官能团影响。
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图 1 不同pH条件下(a)诺氟沙星(NOR)和(b)环丙沙星(CIP)的表观光解速率常数(kobs)
Figure 1. Apparent photolysis rate constants (kobs) of norfloxacin (NOR) and ciprofloxacin (CIP) under different pH values: (a) NOR; (b) CIP
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图 2 淬灭条件下(a，b)诺氟沙星(NOR)和(c，d)环丙沙星(CIP)的表观光解速率常数(kobs)
Figure 2. Apparent photolysis rate constants (kobs) of norfloxacin (NOR) and ciprofloxacin (CIP) under quenching values: (a) NOR; (b) NOR; (c) CIP; (d) CIP
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图 3 不同pH条件下DOM的3DOM*、1O2和•OH量子产率
Figure 3. Quantum yields of 3DOM*，1O2 and •OH of DOM under different pH values: (a) fTMP; (b) Φ1O2; (c) Φ•OH
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图 4 不同pH条件下三种可解离DOM小分子类似物的3DOM*、1O2和•OH量子产率
Figure 4. Quantum yields of 3DOM*，1O2 and •OH for three dissociable DOM small molecule analogues under different pH values: (a) fTMP; (b) Φ1O2; (c) Φ•OH
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图 5 不同pH条件下三种不可解离DOM小分子类似物的3DOM*、1O2和•OH量子产率
Figure 5. Quantum yields of 3DOM*，1O2 and •OH for three non-dissociable DOM small molecule analogues under different pH values: (a) fTMP; (b) Φ1O2; (c) Φ•OH
表 1 NOR、CIP、TMP、FFA和苯酚的HPLC分析条件
Table 1 HPLC analysis conditions for NOR, CIP, TMP, FFA and phenol
检测物质 流动相比例 检测波长
(nm)流动相流速
(mL/min)NOR 乙腈-磷酸(0.1%)=20∶80 274 1.0 CIP 乙腈-磷酸(0.1%)=20∶80 274 1.0 FFA 乙腈-磷酸(0.1%)=15∶85 219 1.0 TMP 乙腈-磷酸(0.1%)=45∶55 220 1.0 苯酚 甲醇-水=45∶55 225 1.0 
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