2022, Vol. 43
有机磷酸酯(organophosphate esters, OPEs) 在世界范围内广泛用作阻燃剂、增塑剂和消泡剂等添加剂[1~3].环境中的OPEs均来自于人类工业制品, 没有自然源.OPEs在工业应用中是以物理添加的方式进入到材料中, 由于不与给定材料发生化学键合, 很容易通过挥发、磨损和溶解排放到环境中[4~7].OPEs是由磷酸根骨架和3个取代基组成, 根据取代基种类的差别, 可分成烷基类、氯代类和芳香基类[8].芳香基类OPEs如磷酸三苯酯(triphenyl phosphate, TPHP)常被作为增塑剂用于电子设备和装修材料中[9].目前, 在大气[10~16]、灰尘[16~21]、水体[22~26]、沉积物[26~28]、土壤[28, 29]和食物[28, 30]等多种环境介质均检测到TPHP的存在, 其中水体是TPHP重要的汇.TPHP已被多项研究证实具有毒性效应、生物富集和放大效应, 对环境和人体健康产生较大威胁[31~34].然而, 目前未查到国内外水体中TPHP的相关管控标准.因此, 急需研发能够有效去除水体中OPEs尤其是TPHP的方法.
水体中OPEs常见的降解方法可分为化学法和生物法.其中, 化学法主要是通过各种反应生成具有强氧化性的自由基(·OH和SO4- ·等)将OPEs去除.由于其具有经济和高效等优势, 近年来在污染物降解方面发挥了重要作用[35~41].此外, 特定紫外光源由于具有极高能量易使电子发生跃迁产生具有极强氧化性的自由基, 因此利用紫外光耦合传统化学法的紫外驱动高级氧化技术应运而生. Ruan等[42]运用紫外光UV-O3联合的技术, 在最佳处理条件下, 60 min可以有效降解100 mg ·L-1 TCPP, 总矿化度能够达到94.3%, 在实际废水处理厂应用时60 min总矿化度可达81.3%. Cristale等[43]使用UV-H2O2方法降解废水中的OPEs, 结果显示芳香基和烷基OPEs易被降解, 而氯代OPEs(TCEP、TDCPP和TCIPP)难以被降解. Ruan等[44]研究了UV-H2O2体系对水中TCEP的降解情况, 总矿化度达到86%, 光密度、初始pH、TCEP和H2O2浓度对反应效果有影响.然而, 目前关于芳香基OPEs-TPHP的降解研究并不多见.关于OPEs降解机制的研究多集中于根据最终产物反推反应过程, 对其降解历程的在线观测甚少, 且没有明确的结论.因此, 本文选择TPHP为研究对象, 利用傅里叶红外光谱仪(FT-IR)和液相色谱-串联质谱联用仪(LC-MS/MS)对比分析TPHP在紫外-过氧化氢(UV-H2O2)、紫外-二氧化钛(UV-TiO2)和紫外-过硫酸盐(UV-PS)这3种高级氧化体系(UV-AOPs)下的降解情况, 并结合红外谱图探究TPHP的降解路径, 旨在为水体中OPEs特别是芳香基OPEs的去除提供数据.
1 材料与方法 1.1 试剂和仪器实验试剂：TPHP标准品(纯度≥99%, 美国Sigma-Aldrich公司); TiO2、K2S2O8、过氧化氢(纯度30%)、氯化钠、邻苯二甲酸氢钾和硼酸等均购自成都市科隆化学品有限公司.
实验仪器：傅里叶红外光谱仪(Nicolet is 50, 美国赛默飞世尔科技公司)搭载ATR附件(液体分析型, 美国PIKE公司), 用EZ OMNIC数据处理软件进行红外光谱分析.超高效液相色谱-三重四级杆串联质谱仪(1290 InfinityⅡ-6470, 美国安捷伦公司), 多试管搅拌式光化学反应仪(XPA-7, 南京胥江机电厂), 20W紫外灯, 氙灯光源(PLS-SXE300, 北京泊菲莱科技有限公司), 涡旋混合器(XR-C, 江苏金怡仪器科技有限公司), pH计(HQ30D, 美国HACH公司), 石英管(15 mL, 南京胥江机电厂), 搅拌子(3 mm, 成都科隆试剂), 微孔滤膜(0.45 μm, 成都科隆), 电子天平(SQP, 赛多利斯科学仪器, 北京), 超纯水机(UPT-Ⅱ-10T, 四川优普超纯科技有限公司).
1.2 实验方法 1.2.1 在线降解实验光源发生装置为20 W紫外灯管和氙灯光源, 取TPHP溶液10 mL于石英管中, 加入不同浓度的H2O2, 充分振荡后用移液管取石英管中样品溶液2.0 mL于ATR晶面上, 打开UV灯电源, 设置红外光谱扫描范围为500~4 000 cm-1, 分辨率为32 cm-1.实验装置如图 1.
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图 1 在线光催化装置示意 Fig. 1 Schematic diagram of photocatalytic device |
光化学反应装置为XPA-7型多试管搅拌式光化学反应仪.取500 μg ·mL-1的TPHP溶液10 mL加入不同浓度的反应液, 充分摇匀后转移至石英管中, 放入光化学反应仪中进行光催化降解.
采用傅里叶红外光谱仪在线观测TPHP基团变化并进行半定量分析.样品采集条件为扫描次数32, 分辨率32, 吸光度, 矫正为ATR; 光学台条件为检测器MCT-A, 扫描范围为500~4 000 cm-1, 增益为自动增益, 光栅120, 衰减轮为不衰减.
1.3 分析方法 1.3.1 红外光谱图解析方法根据朗伯-比尔定律, 红外光谱中峰强度与样品的浓度有关.获得OPEs与水的混合物和水的红外吸收光谱图后采用差减技术获得OPEs的红外光谱图.根据Mangolini[45]对降解指数(DI)的定义, 本文采用其对DI值的计算方法, 但对参数的定义进行调整.DI值越低, 表明该基团降解得越完全, 去除效果越好.
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(1) |
式中, η为降解指数(DI); At为降解反应进行t(min)后, TPHP的红外特征峰的强度; A0为降解反应开始时TPHP的红外谱图特征峰的强度.
1.3.2 LC-MS/MS分析方法采用安捷伦超高效液相色谱-三重四级杆串联质谱仪(1290-6470), C18色谱柱(Zorbax SB-C18, 2.1 mm×50 mm, 1.8 μm), 柱温为30℃.仪器进样量为5.0 μL.定量分析时水相为1%的甲酸铵水溶液, 有机相为甲醇, 流速为0.2 mL ·min-1, 流动相起始为水(100%), 0.5 min后为90%水+10%甲醇, 5 min后为10%水+90%甲醇.质谱条件为：负离子模式(ESI), 多反应离子监测(MRM)模式分析, 雾化气压力0.24 MPa, 电喷雾电压3 000 V, 干燥气温度250℃.
1.3.3 反应动力学环境中有机物污染物的降解一般符合一级或准一级动力学.本研究假定OPEs的降解也符合一级反应动力学进行计算.一级反应动力学的微分速率方程为：
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(2) |
经变化得：
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(3) |
式中, c0为光照前污染物的浓度, ct为光照时间为t时的污染物浓度, K为速率常数, t为反应时间, dct/dt为反应速率.
降解半衰期(t1/2)的表达式为：
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(4) |
进行影响因素分析之前, 首先对TPHP的红外光谱图进行分析(图 2).TPHP的4个特征峰的波数分别为：958(归属于P—O的反对称伸缩振动频率)、1 183(归属于C—O的反对称伸缩振动频率)、1 303(归属于P＝O的伸缩振动频率)和2 870 cm-1(归属于C—H的对称伸缩振动频率).其中最强峰为P—O特征峰.苯环结构中的C＝C伸缩振动频率位于1 529 cm-1, H2 O的变角振动吸收峰波数为1 558 cm-1.由于两吸收峰波数相近, 水峰基本掩盖了苯环结构的特征峰, 故本文未对苯环结构的C＝C特征峰进行讨论.
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图 2 差减后TPHP的红外光谱图 Fig. 2 Subtraction infrared spectrum of TPHP |
高级氧化反应中, 自由基活性物种自身能发生淬灭现象[46], 故要合理控制试剂的浓度以获得最合理的反应条件.在UV-H2O2体系下, 当H2O2浓度在0~0.146 mol ·L-1范围内变化时, TPHP在降解过程中P—O特征峰的DI值随时间变化见图 3.随着H2O2浓度增加, TPHP特征峰的强度下降速度加快; 当H2O2浓度高于0.019 mol ·L-1时, 反应30 min后TPHP的P—O特征峰完全消失.
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图 3 H2O2浓度对TPHP光降解的影响 Fig. 3 Effect of H2O2 concentration on photodegradation of TPHP |
当降解反应时间为5 min, 比较分析H2O2浓度对各特征峰的影响(图 4).随着H2O2浓度升高, 4个特征基团的DI值呈下降趋势; 当H2O2浓度达到0.097 mol ·L-1时, C—H和P＝O峰消失.故增大H2O2浓度可有效提高TPHP的降解速率.
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图 4 反应5 min时不同H2O2浓度下TPHP特征峰的DI值 Fig. 4 DI value of TPHP characteristic peaks at different H2O2 concentrations at 5 min of reaction |
在UV-TiO2体系下, 不同TiO2浓度时TPHP的P—O特征峰的DI值随时间变化见图 5.当TiO2浓度为0时, 无降解效果; 当TiO2浓度逐渐增加至0.013 mol ·L-1, TPHP的P—O特征峰的DI值逐渐降低, 降解效果明显, 其中TiO2浓度为0.013 mol ·L-1时P—O特征峰的DI值下降速率最快, 但当反应进行至15 min, P—O特征峰的DI值便不再下降.这与H2O2反应时不同.这是由于H2O2浓度的增加, 提高了TPHP和·OH碰撞的可能性从而促进了TPHP的降解.TiO2是固体催化剂, 需要紫外光照射在表面产生·OH, 当TiO2浓度过量时会导致TPHP的降解效率降低[47].
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图 5 TiO2浓度对TPHP光降解的影响 Fig. 5 Effect of TiO2 concentration on photodegradation of TPHP |
真实的环境水体的成分复杂, 多种OPEs组分共同存在, 干扰组分多, 影响因素复杂, 故本文结合文献考察了几种主要影响因素对TPHP降解过程的影响.
首先, 光源的选择是TPHP降解反应顺利进行的关键环节.本文对3种光照条件(紫外灯、氙灯和黑暗)下TPHP的降解情况进行了分析, 结果表明汞灯发出的紫外光对TPHP的光催化降解效果最优, TPHP在氙灯模拟的日光和黑暗条件下基本不降解.
其次, 自然水体中存在大量阴阳离子, 可能会通过与TPHP竞争相同的自由基活性物从而影响光催化降解效果.本文选取自然水体中常见的Cl-探究阴离子对TPHP降解的影响.结果表明, 添加少量Cl-的水样相较未添加Cl-的水样更有利于降解反应, 这是由于Cl-与部分·OH发生反应产生少量的无机自由基, 可与·OH共同降解TPHP.但是, 较高浓度的Cl-会把·OH反应完, 导致只剩氧化能力较弱的无机自由基降解TPHP.
另外, pH被认为是光催化反应过程中重要的影响因素.本文探讨了不同pH对TPHP降解的影响.结果表明酸性和中性环境下TPHP的降解效果较好, 但碱性环境会抑制TPHP的降解.主要原因是体系中H+不足时会导致降解反应中·OH浓度下降, 不能有效降解TPHP.故后续实验的水溶液不添加pH缓冲溶液, 其pH值为7左右.
2.4 TPHP降解过程的红外光谱分析TPHP在UV-H2O2、UV-TiO2和UV-PS这3个高级光降解体系下降解过程中不同基团DI值的变化见图 6.
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图 6 3种体系中TPHP降解过程中不同基团DI值的变化 Fig. 6 Changes in the DI value of different groups during the degradation of TPHP in three systems |
在TPHP的降解过程中, 500~4 000 cm-1波数范围内没有其他基团产生, 说明TPHP的降解过程为长链断裂为短链, 最终矿化为无机物质.该结果与Antonopoulou等[48]报道的TCPP的光催化降解过程相似, 主要为羟基化、氧化、脱氯和脱烷基过程.
UV-H2O2光降解体系：由图 6(a)知, 降解反应发生120 min时, 表示TPHP碳链结构的P—O与C—O键仍未被完全降解, 说明TPHP的支链结构较难被破坏, 导致TPHP的降解速率较慢.C—H特征峰是TPHP降解最快的特征峰, 说明UV-H2O2体系中OPEs的降解主要是C—H键被·OH攻击的过程.
UV-TiO2光降解体系：与UV-H2O2体系相比, TPHP的特征峰位置并未发生变化, 说明体系的改变并未影响OPEs特征峰的波数.在TPHP降解的过程中, 500~4 000 cm-1范围内没有其他基团产生.由图 6(b)知, 3个主要特征峰的波数在30 min内迅速降至20%以下, 但C—H在30 min内的DI保持在95%以上.故UV-TiO2体系下TPHP的C—H键比较稳定, 较难降解.
UV-PS光降解体系：没有检测出TPHP的C—H特征峰, 见图 6(c).TPHP的C—O键特征峰的DI值下降较多.当反应结束时P—O特征峰的DI降低了73%, C—O特征峰的DI值降低了68%, P＝O特征峰的DI值降低了40%.表明UV-PS体系下, P＝O键不易断裂.
故在不同降解体系下, TPHP的降解路径不一致.若要将其应用, 需深入了解其降解历程和机制.
2.5 降解动力学 2.5.1 UV-H2O2光降解体系本实验选用优化后的实验条件, 选用波长为245 nm高压汞灯作为光源, TPHP的初始浓度为20.0 ng ·mL-1, H2O2投加量为0.049 mol ·L-1, 使用LC-MS检测TPHP在不同时间点的浓度, 结果见图 7.
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图 7 TPHP在UV-H2O2体系下的降解和动力学拟合 Fig. 7 Degradation of TPHP in UV-H2O2 system and its kinetic fitting results |
结果表明, 降解反应开始后, TPHP的浓度迅速降低, 且反应符合一级动力学方程.由图 7可知, 反应进行到120 min时, TPHP的去除率为68%.将图 7中的数据代入公式(3), 可得TPHP降解速率常数为0.009 4 min-1.根据公式(4)计算得到TPHP的半衰期为74 min.
2.5.2 UV-TiO2光降解体系本实验选用波长为245 nm高压汞灯作为光源, TiO2作为催化剂投加量0.049 mol ·L-1, TPHP的浓度为20.0 ng ·mL-1, 使用LC-MS/MS检测TPHP在降解过程中不同时间点的浓度.结果见图 8.
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图 8 TPHP在UV-TiO2体系下的降解和动力学拟合 Fig. 8 Degradation of TPHP in UV-TiO2 system and its kinetic fitting results |
在UV-TiO2体系下, TPHP的浓度迅速降低, 反应符合一级动力学方程.图 8是TPHP的降解效果和动力学拟合.反应结束时, TPHP的去除率为43%, 说明UV-TiO2体系相较于UV-H2O2体系降解效果较差.这可能是因为H2O2产生·OH的速度比TiO2更快, 每一个分子H2O2可以产生两分子的·OH[49].将图 8中的数据代入公式(3)得到TPHP的降解速率常数为0.004 6 min-1.根据公式(4)计算得TPHP的半衰期为150 min.
2.5.3 UV-PS光降解体系选用波长为245 nm高压汞灯作为光源, K2S2O8投加量为0.049 mol ·L-1, TPHP的浓度为20.0 ng ·mL-1, 用LC-MS检测TBEP和TPHP在不同时间点的浓度.图 9为TPHP在UV-PS体系下的降解效果和动力学拟合图.在UV-PS体系下, TPHP的浓度迅速降低, 反应符合一级动力学方程.反应结束时, TPHP的去除率为61%.由图 9得到TPHP速率常数为0.007 8 min-1, TPHP半衰期为89 min.
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图 9 TPHP在UV-PS体系下的降解和动力学拟合 Fig. 9 Degradation efficiency of TPHP in UV-PS system and its kinetic fitting results |
故3种降解体系中, TPHP均有一定的降解, 反应均符合一级动力学方程, 其中UV-H2O2光降解体系的效果最好.
2.6 TPHP的降解产物和机制用LC-MS/MS鉴定TPHP在UV-H2O2体系下的光催化降解产物, 讨论其降解途径和机制.在光降解TPHP过程中, 检测到6种中间产物, 具体结果见表 1.
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表 1 TPHP的LC-MS/MS碎片信息 Table 1 LC-MS/MS data for TPHP |
TPHP由1个磷酸中心和3个苯环侧链组成, 其质荷比m/z为327.苯环结构上的C—H键, 连接P原子中心的P—O—C键是自由基攻击的主要位点.图 10是TPHP可能的降解路径.如降解路径1所示, UV-H2O2和UV-TiO2产生的·OH首先使C—O断裂, ·OH通过加成反应形成中间产物1.中间产物1为磷酸二酯产物, 它再脱去1分子的苯甲酯结构生成中间产物3, 脱除的苯甲酯结构加成·OH后形成中间产物6.TPHP经光照后, 苯环结构上的C—H键发生断裂, 生成碳中心自由基和·H, ·OH断裂P—O后与·H反应, 碳中心自由基又与UV-H2O2产生的·OH发生反应最后生成中间产物5, 与Ruan等[44]提出过TCEP在UV-H2O2体系下能够在P—O键断裂后进一步结合氢生成产物一致.
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图 10 水溶液中TPHP可能的降解路径 Fig. 10 Possible degradation path of TPHP in aqueous solution |
UV-PS体系下TPHP的降解路径如图 10中降解路径2所示.TPHP的支链上C—O被SO4- ·氧化, 随后经过H2 O分子加成和一系列电子转移过程形成了中间产物2, 它是水解产生的磷酸二酯产物; 该中间产物的另一条支链也很快被SO4- ·攻击, 断开一个苯环结构并经过加成反应生成产物4.
2.7 环境水样中TPHP的光催化降解 2.7.1 环境水样的采集本研究选择连续晴朗的第3 d采集水样, 共采集水样3个, 分别是江安河过成都城区段自然水体, 人造湿地公园——天府芙蓉园的景观水体和某大学封闭景观池塘水体的表层水样, 水样的基本指标见表 2.
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表 2 环境水体样品的相关指标 Table 2 Relevant indicators of environmental water samples |
2.7.2 环境水样中TPHP的光催化降解效果
采用UV-H2O2体系对TPHP进行降解, H2O2浓度为0.049 mol ·L-1, 结果见图 11.
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图 11 TPHP浓度随反应时间的变化 Fig. 11 Change in TPHP concentration with reaction time |
由图 11可知, 公园景观水体的TPHP浓度最高, 为30.7 ng ·mL-1.在UV-H2O2体系下降解60 min后, 公园景观水体中的TPHP去除率达到66%, 而江安河和学校池塘水中TPHP的去除率为31%和41%.说明UV-H2O2体系对多种实际水体中的TPHP均有一定的降解效果, 但由于环境水体成分复杂, 各种OPEs组分和与OPEs性质相近的污染物均可能存在, 所以在UV-H2O2体系下, ·OH也会与易发生类似反应的其他有机污染物反应产生竞争.同时, 实际环境水体的pH值和其他共存物质等均可能促进或抑制TPHP的降解反应, 需要进一步研究.
3 结论(1) 降解实验的主要影响因素：当H2O2浓度为0~0.097 mol ·L-1时, H2O2浓度升高会促进TPHP的降解; 当TiO2浓度为0~0.013 mol ·L-1时, TiO2浓度升高会促进TPHP的降解.
(2) UV-H2O2、UV-TiO2和UV-PS体系下TPHP的降解效率分别为68%、43%和64%, 反应速率常数为0.009 4、0.004 6和0.007 8 min-1.
(3) UV-H2O2体系下TPHP的降解历程中各特征峰的DI值下降速率顺序为：C—H>P＝O>C—O>P—O. UV-TiO2体系下TPHP的C—H特征峰强度几乎没有变化.UV-PS体系下TPHP未检测出C—H特征峰, 且DI值下降速率顺序为：P＝O>C—O>P—O.
(4) 红外光谱变化和液-质联用仪的检测结果表明, TPHP的光降解路径主要为P—O—C键断裂、苯环结构的C—H键断裂和水解反应.
(5) UV-H2O2体系对多种实际水体中的TPHP均有一定的降解效果, 但差异较大, 与环境因素有关.
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