2023, Vol. 44
, ZOU Jia-li , XU Hao , WANG Teng , SHI Yu-xiang , CHEN Yong-jian , LI Bin-yang , WANG Ya-yi , FENG Cang , WU Min
近年来, 抗生素的过度使用对生态环境和人类健康已构成严重威胁[1, 2].作为喹诺酮类抗生素的典型代表, 环丙沙星(CIP)具有结构复杂、残留时间长和污染范围广等特点, 在水体中被频繁检出, 引起广泛的社会关注[3, 4].
目前, CIP去除方法包括物理处理、生物处理和化学处理[5].常见的物理吸附法处理周期长, 实际应用较少[2, 6].尽管传统生物处理法操作简单且成本较低, 但受抗生素废水的污染物浓度限制易出现污泥膨胀[7, 8].相比之下, 基于过一硫酸盐(PMS)的高级氧化技术以氧化性强、适用范围广和环境友好等优势, 逐渐成为解决水中抗生素污染的首选方式[9, 10].然而, PMS性能稳定且自分解速率较慢, 导致单独添加PMS时污染物去除率偏低[11, 12].因此, 需外加催化剂活化PMS, 促进自由基产生, 提高污染物降解效率.
污水厂大规模建设导致污泥产量逐年增加, 污泥处理处置面临严峻挑战[13~15].采用脱水干化污泥制备生物炭, 可以活化PMS去除污染物, 并提供一种经济环保且可持续的污泥处理模式[16].然而, 生物炭对PMS活化机制尚未明确.
本文采用某污水厂脱水干化污泥制备生物炭(BC), 考察BC/PMS体系对CIP降解效果, 探究最优反应条件, 通过自由基淬灭实验和活性位点等分析对降解机制进行深入探讨, 并提出CIP可能降解路径, 以期为控制污染物排放及指导污水处理提供理论依据.
1 材料与方法 1.1 材料过一硫酸氢钾、氢氧化钠、盐酸、甲醇、甲酸、乙腈、碳酸氢钠、氯化钠、盐酸、硫酸钠、硝酸钠、无水乙醇、叔丁醇和环丙沙星来自国药集团, 若无特殊说明为分析纯.实验用水为去离子水.
1.2 生物炭制备取适量脱水干化污泥(含水率40%)置于石英舟中, 移入管式炉, 持续通入氮气形成无氧环境.以恒定速率10℃·min-1加热至450℃, 恒温热解120 min.随后, 在氮气保护下冷却20 min, 将制得BC研磨, 过200目筛, 在60℃烘箱中干燥6 h.
为排除BC对CIP吸附影响, 设置BC单独吸附CIP实验, 实验条件为: BC投加量1.0 g·L-1、CIP质量浓度20 mg·L-1、溶液初始pH 6.0以及反应时间120 min(结果见图 2, CIP去除率为12.69%).待反应完成后, 将BC从溶液中过滤并自然晾干, 回收BC用于下一步实验操作.
1.3 实验方法将装有200 mL CIP溶液的锥形瓶放入摇床中, 25℃、200 r·min-1恒温振荡.分别考察PMS投加量、BC投加量、初始pH值和无机阴离子对BC/PMS体系对CIP降解的影响.每隔一定时间用移液枪取样4 mL, 并立即采用PTFE针式过滤器(0.22 μm)过滤, 加入1 mL甲醇溶液终止反应.同一组实验重复3次, 取平均值.
在PMS投加量影响实验中, PMS投加量分别设定为0、0.25、0.5、1.0、2.0、3.0和4.0 mmol·L-1, BC投加量为1.0 g·L-1, 溶液初始pH为6.0, ρ(CIP)为20 mg·L-1; 在BC投加量影响实验中, BC投加量分别设定为0.2、0.4、0.8和1.0 g·L-1, PMS投加量为3.0 mmol·L-1, 溶液初始pH为6.0, ρ(CIP)为20 mg·L-1; 在溶液初始pH影响实验中, 使用浓度为1.0 mol·L-1 NaOH和H2SO4调节初始pH至设定值2.0、4.0、6.0、8.0和10.0, PMS投加量为3.0 mmol·L-1, BC投加量为1.0 g·L-1, ρ(CIP)为20 mg·L-1; 在无机阴离子影响实验中, 在催化反应前向CIP溶液中分别加入NaNO3、NaHCO3、NaCl和Na2SO4, 每种阴离子浓度为1、5和100 mmol·L-1, 以不添加无机阴离子实验组作为空白组.
采用甲醇(MeOH)作为硫酸根自由基(SO4-·)和羟基自由基(·OH)淬灭剂[17], 叔丁醇(TBA)作为·OH淬灭剂[18], L-组氨酸作为单线态氧(1O2)淬灭剂[19].上述物质投加浓度分别为500、500和10 mmol·L-1, 定时取样并测定CIP质量浓度.
1.4 分析方法采用扫描电子显微镜-X射线能谱仪(SEM-EDS, ZEISS Gemini 300, 德国)、傅里叶变换红外光谱仪(FTIR, TENSOR27, 布鲁克)、X射线衍射仪(XRD, Rigaku SmartLab SE, 日本)、X射线光电子能谱仪(XPS, Thermo fisher Scientific, 美国)和Zeta电位分析仪(Malvern, 英国)测定消除吸附影响后的BC性质.采用电子自旋共振(EPR, Bruker, 德国)检测消除吸附影响后BC中持久性自由基.采用液相色谱仪(LC-20A, SHIMADZU, 日本)测定CIP质量浓度, 分离色谱柱采用C18反相色谱柱(150 mm×4.6 mm, 5 μm), 流动相为70%纯乙腈和含有0.1%甲酸溶液的超纯水, 流速1.0 mL·min-1, 柱温40℃, 进样体积10 μL, 检测波长277 nm.采用高效液相色谱-质谱法(HPLC-MS, Agilent 1100-Thermos TSQ Quantum Ultra, 美国)测定CIP降解中间产物.对CIP降解进行动力学拟合[20], 表观速率常数kobs计算如下:
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(1) |
式中, t为反应时间(min), ρt和ρ0分别为t时刻CIP质量浓度和CIP的初始质量浓度(mg·L-1).
2 结果与讨论 2.1 BC表征由图 1(a)和图 1(b)可知, BC具有不规则片状结构, 且表面存在少量碎屑, 这是由灰分中盐类分解重组和挥发性部分流失所致[21, 22].污泥炭化时, 溢出大量气体, 导致BC出现明显孔隙结构, 拥有更多活性位点.EDS结果见图 1(c), BC含有大量O元素, 以有机和无机组分形式存在, 有机组分主要包括羧基、羟基、醌类和酯基等表面含氧官能团, 无机组分主要包括硫酸盐、碳酸盐和磷酸盐等[23, 24].此外, BC还含有Ca、Mg和Fe等金属元素, 均来自污泥脱水时添加的调理剂.
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图 1 BC的SEM-EDS、FTIR和XRD表征 Fig. 1 Characterization of BC through SEM-EDS, FTIR, and XRD |
FTIR分析结果见图 1(d), 3 428 cm-1处为较明显的—OH基团伸缩振动峰, 可能来自C—OH键和H2O[25]; 高于3600 cm-1处可能是游离—OH的伸缩振动峰[26]; 2 914 cm-1附近可能是C—H的伸缩振动峰; 1 612 cm-1处可能是羰基、羧基和芳香环中C＝O、—CONH—和C＝C的伸缩振动峰[27], 以上官能团通常作为BC催化PMS降解污染物活性位点[28].此外, 1423 cm-1附近的吸收峰与—CH2和—CH3键有关, 1 033 cm-1处和649 cm-1附近的吸收峰分别与C—O伸缩振动和金属氧化物有关[29].
XRD结果见图 1(e), 2θ位于20.86°、26.64°、50.14°和59.96°尖锐峰分别代表SiO2(100)、(101)、(112)和(211)晶面[30, 31]; 2θ位于23.02°、29.41°、35.97°、39.40°和48.51°尖锐峰分别代表CaCO3(012)、(104)、(110)、(113)和(116)晶面, 表明SiO2和CaCO3是BC主要结晶结构.
2.2 影响因素分析 2.2.1 PMS投加量由图 2可见, 仅投加3.0 mmol·L-1 PMS时, CIP质量浓度基本不变, 说明未经催化的PMS无法直接氧化CIP.投加PMS和1.0 g·L-1 BC时, CIP质量浓度逐渐降低.PMS投加量由0.1 mmol·L-1增至3.0 mmol·L-1, CIP降解率与kobs由25.73%和0.010 1 min-1升至49.09%和0.027 2 min-1, 说明PMS经BC催化后加速降解CIP.当PMS投加量增至4.0 mmol·L-1时, CIP降解率基本不变, kobs降至0.026 4 min-1, 可能是过量PMS导致SO4-·自淬灭, 生成不具有强氧化性的SO42-[式(2)][32]; 过量PMS与SO4-·和·OH反应生成氧化性更低的SO5-·[式(3)~(4)], 影响降解效果.因此, 最佳PMS投加量为3.0 mmol·L-1.
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(2) |
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(3) |
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(4) |
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图 2 PMS投加量对BC/PMS体系降解CIP的影响 Fig. 2 Effects of PMS dosage on CIP removal in BC/PMS |
由图 3可见, 当BC投加量为0.2 g·L-1时, 催化剂活性位点不足导致PMS无法被有效活化, CIP降解率仅为11.95%, kobs仅为9.79×10-4 min-1.当BC投加量逐渐增至0.4、0.6、0.8和1.0 g·L-1时, CIP降解率分别升至36.41%、41.96%、46.34%和49.09%, kobs分别升至0.003 9、0.011 2、0.017 8和0.027 2 min-1.这是因为高含量催化剂可提供更多活化PMS的表面活性位点, 提高污染物降解率[33, 34].因此, 最佳BC投加量为1.0 g·L-1.
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图 3 BC投加量对BC/PMS体系降解CIP的影响 Fig. 3 Effects of BC dosage on CIP removal in BC/PMS |
如图 4(a)和图 4(b)所示, 当pH值由2.0增至10.0时, CIP降解率由59.05%降至32.33%, kobs由0.056 5 min-1降至0.011 4 min-1, 这是由于初始pH直接影响CIP(pKa为6.2和8.8)[8]、BC和PMS的状态.由图 4(c)可见, pH>8.8时CIP以CIP-为主, pH < 6.2时以CIP+为主, 6.2 < pH < 8.8时为CIP±.由图 4(d)可见, BC零电荷点pHpzc为3.26, Zeta电位随pH增加由正变负.在酸性条件下, 尽管BC与CIP间存在静电斥力, 但溶液中SO4-·浓度较高, 促进CIP降解; 在碱性条件下, PMS自我分解生成氧化能力较弱的SO5-·, 同时SO4-·与OH-生成氧化能力较弱的·OH[35], 减缓CIP降解.实际污废水pH值通常为6.0~8.0, 考虑实用性, BC/PMS体系溶液初始pH值调至6.0.
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图 4 溶液初始pH对BC/PMS体系降解CIP的影响 Fig. 4 Effects of the initial pH on CIP removal in BC/PMS |
由图 5(a)和图 5(b)可见, 投加1、5和100 mmol·L-1 SO42-或NO3-, CIP降解率基本不变, 说明它们对BC/PMS体系的影响可以忽略.投加HCO3-后, CIP降解率降低, 由图 5(c)可见, HCO3-浓度由1 mmol·L-1增至100 mmol·L-1时, CIP降解率由48.78%降至32.43%.这可能是因为过量HCO3-与SO4-·和·OH反应生成氧化能力较弱的CO3-·[36, 37], 影响CIP降解[式(5)和式(6)].
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图 5 常见阴离子(SO42-、NO3-、HCO3-和Cl-)对BC/PMS体系降解CIP的影响 Fig. 5 Effects of anions(SO42-, NO3-, HCO3-, and Cl-) on CIP removal in BC/PMS |
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(5) |
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(6) |
由图 5(d)可见, 当Cl-浓度为1 mmol·L-1时, CIP降解率降至46.34%; 当Cl-浓度增至5 mmol·L-1和100 mmol·L-1时, CIP降解率降至39.37%和35.47%.这是因为Cl-与SO4-·和·OH反应, 生成氧化能力较弱的Cl·和ClOH-·[19], 见式(7)和式(8).当Cl-浓度继续增加时, CIP降解率下降幅度较小.这可能是因为Cl-直接与溶液中PMS反应生成HClO, 形成氧化性较强的Cl2, 氧化降解CIP[38], 见式(9)和式(10).
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(7) |
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(8) |
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(9) |
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(10) |
如图 6所示, 投加500 mmol·L-1 TBA时, CIP降解率由49.09%降至38.77%, kobs由0.027 2 min-1降至0.016 1 min-1; 投加500 mmol·L-1 MeOH时, CIP降解率降至32.23%, kobs降至0.013 1 min-1, 说明BC/PMS体系同时生成·OH和SO4-·, 贡献率分别为40.6%和11.1%, 见式(11)和式(12).因此, ·OH是主要活性物质.
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(11) |
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(12) |
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图 6 BC/PMS体系自由基淬灭实验 Fig. 6 Radical quenching experiment in BC/PMS |
在以生物炭为催化剂活化PMS降解污染物体系中, 还存在以1O2为主的非自由基催化途径[39, 40].由图 6可见, 投加10 mmol·L-1 L-组氨酸后, CIP降解受到明显抑制, 120 min内降解率仅为18.37%, 说明1O2在BC/PMS体系降解CIP中发挥作用.
2.3.2 表面活性位点及持久性自由基分析采用XPS分析反应前后BC表面化学组成, 结果见图 7.由图 7(a)可见, 在284.8 eV处存在C 1s峰, 在530.1 eV处存在O 1s峰, 在709.6 eV处存在Fe 2p峰, 在400 eV附近出现N 1s峰.
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图 7 BC反应前后XPS图谱和EPR图 Fig. 7 XPS spectra of the BC before and after the reaction and EPR spectra of BC |
C 1s细分谱图见图 7(b).反应前BC存在3个特征峰, 分别对应石墨碳的C—C或C＝C(284.8 eV)、酚醇的C—O(286.3 eV)和羧基或酯基的O＝C—O(289.3 eV).反应后C—O含量由38.33%降至32.62%, O＝C—O含量由24.47%降至11.09%, 证实酚、醇和羧基等官能团可能是BC中活性位点[41].
N 1s细分谱图见图 7(c).反应前BC存在2个特征峰, 分别对应吡啶氮(398.9 eV)和吡咯氮(400.4 eV); 反应后BC存在3个特征峰, 分别对应吡啶氮(398.9 eV)、吡咯氮(400.5 eV)和硝酸盐氮(405.2 eV).同时, 吡啶氮含量由42.37%降至23.38%, 吡咯氮含量由57.62%升至68.58%, 说明吡啶氮是BC活化PMS降解CIP主要活性位点[42, 43].
O 1s细分谱图见图 7(d).反应前BC存在3个特征峰, 分别对应金属氧化物(530.3 eV)、C—O(531.5 eV)和C＝O(533.0 eV), 含量分别为18.43%、64.40%和17.16%; 反应后酮类C＝O和酚醇类C—O含量降低.这可能是因为羰基C＝O作为具有孤对电子的Lewis碱性位点, 有效增加相邻碳原子的电子密度, 进而活化PMS[44].
Fe 2p细分谱图见图 7(e).反应前BC的Fe 2p谱图在709.7 eV、711.3 eV、723.3 eV和724.9 eV处出现4个特征峰, 分别表示Fe2+ 2p3/2、Fe3+ 2p3/2、Fe2+ 2p1/2和Fe3+ 2p1/2, 反应后4个特征峰位置在710.3 eV、711.9 eV、723.9 eV和725.5 eV.反应后BC中Fe2+的含量从50.38%下降至34.79%, 表明Fe3+/Fe2+氧化还原共轭对参与了PMS活化过程[45].
此外, BC含具有氧化还原能力的持久性自由基, 可转移电子活化PMS[25, 28, 46].由图 7(f)可见, 持久性自由基浓度为15.91×1015 spins·g-1, 是以氧原子为中心的半醌类自由基(g因子>2.004 0).
2.3.3 BC/PMS体系催化机制分析BC活化PMS降解CIP机制见图 8.BC/PMS体系同时存在自由基途径和非自由基途径.前者包括: BC中C＝O、—OH和—COOH等含氧官能团及缺陷导致PMS中O—O断裂; BC中Fe3+/Fe2+氧化还原共轭对参与PMS活化; 吡啶氮具有孤对电子, 促进自由流动的π电子从sp2碳中转移到PMS, 从而活化PMS; 此外BC以氧原子为中心的半醌类自由基可转移电子活化PMS.后者包括: PMS形成SO5-·的自反应[式(13)和式(14)]; 以氧原子为中心的半醌类自由基活化氧分子生成超氧自由基及其进一步转化生成1O2[20]; 同时, BC具有一定的吸附能力, 可以促进PMS和CIP接触, 加速电子从CIP转移到PMS, 从而促进1O2的生成.
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(13) |
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(14) |
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图 8 BC/PMS体系催化机制 Fig. 8 Possible catalytic mechanism of BC/PMS system |
有研究表明, 在亲电反应中, 因电子易于在高2FEDHOMO2值处获得, 故在有较高2FEDHOMO2+FEDLUMO2值的位置容易受到自由基攻击[47].基于HPLC-MS结果, 推测CIP降解路径主要包括哌嗪环开环和羟基化反应, 见图 9.
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图 9 BC/PMS体系中CIP降解路径 Fig. 9 Possible degradation pathways of CIP in the BC/PMS system |
途径1为CIP哌嗪环开环, 有研究表明, CIP中哌嗪环是最容易受到自由基攻击的部位, 其侧环被SO4-·、·OH及1O2等氧化活性物质攻击后氧化分解[4].CIP哌嗪侧环开环后产生P1(m/z=334), 通过甲醛损失生成P2(m/z=306), 并通过CH5N基团损失以及氧化和脱碳产生P3(m/z=291).P3进一步氧化生成P4(m/z=263), 从而使得CIP中哌嗪环被完全打开.P4经过脱氟作用和氧化作用, 生成中间产物P5(m/z=208)、P6(m/z=100)和P7(m/z=88).途径2为羟基化反应, CIP经羟基化作用转化为P8(m/z=330), P8进一步被SO4-·、·OH及1O2等攻击生成P7(m/z=88).最后, BC/PMS体系生成的中间产物可能被矿化为CO2和H2O, 或是进一步降解转化为其他小分子有机物.有研究表明, CIP毒性主要与结构中哌嗪环和氟有关, 当哌嗪环开环及脱氟后, 其毒性明显降低[2].
3 结论(1) BC具有形状不规则的片状结构, 表面存在少量碎屑, 含有大量氧元素及少量金属元素、丰富的含氧官能团及脂肪结构官能团, SiO2和CaCO3为主要结晶结构.
(2) BC/PMS体系中CIP降解率随BC投加量、PMS投加量增大而升高, 随溶液初始pH增大而降低.当BC投加量1.0 g·L-1、PMS投加量3.0 mmol·L-1和初始pH为6.0时, BC/PMS体系降解效果最佳, 在120 min内20 mg·L-1 CIP降解率达到49.09%, kobs为0.027 2 min-1.SO42-和NO3-对BC/PMS体系降解效果无显著影响, 而HCO3-和Cl-具有明显抑制作用.
(3) BC/PMS体系中自由基(·OH与SO4-·)和非自由基(1O2)共同参与CIP降解, 其中·OH与SO4-·的贡献率分别为40.6%和11.1%; CIP降解路径主要包括哌嗪环开环和羟基化反应.
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