2023, Vol. 44
2. 陕西科技大学环境科学与工程学院, 西安 710021;
3. 西安理工大学应用化学系, 西安 710048
2. School of Environmental Science and Engineering, Shaanxi University of Science & Technology, Xi'an 710021, China;
3. Department of Applied Chemistry, Xi'an University of Technology, Xi'an 710048, China
碘代X射线造影剂(iodinated X-ray constrast media, ICM)是一类由侧链带有—OH和—COOH的2, 4, 6-三碘苯甲酸衍生的有机化合物, 主要用于加强血管、腔道形态和人体器官成像.ICM本身对人体无害, 但其转化过程中通常伴随碘代消毒副产物(iodinated disinfection by-products, I-DBPs)的生成, I-DBPs具有很高的遗传毒性和细胞毒性.其中碘帕醇(iopamidol, IPM)是ICM中生成I-DBPs的主要前驱体, 且使用最广泛和用量最大[1].IPM具有较强的稳定性, 在人体内不能代谢, 极易通过生活污水进入市政污水管道, 常规污水处理工艺不能有效去除, 导致IPM在水环境中残留, 浓度甚至高达几千μg·L-1[2, 3], 因此强化IPM的去除十分必要.
目前, 水体中IPM的去除主要有高级氧化、生物降解和吸附等, 但关于IPM的吸附鲜见报道, 吸附因操作简易、吸附剂可重复使用和不产生副产物等优点受到关注[4].常见的吸附剂有活性炭[5, 6]、沸石[7]和生物炭[8, 9]等, 其中以农业废弃物为原料的生物炭成本较低、芳香结构复杂和石墨烯结构发达[10].但原状生物炭对污染物的吸附性能有限, 因此改性优化其理化性质和吸附能力尤为重要.
生物炭常用的改性方法有酸碱、金属盐、黏土矿物和球磨改性等[11], 其中酸碱改性可有效改变生物炭的理化性质, 碱在提高生物炭比表面积和孔隙结构方面具有显著优势[12].Jang等[13]和Yang等[14]利用NaOH对火炬松和柳枝进行活化, 所得生物炭的比表面积分别为959.9 m2·g-1和3 342 m2·g-1, 对四环素的吸附量达274.8 mg·g-1和1 300 mg·g-1.Chen等[15]发现KOH和NaOH混合比单独KOH或NaOH具有更好的活化作用, 活化玉米秸秆生物炭比表面积为1 993 m2·g-1, 对罗丹明B的吸附量达1 578 mg·g-1.相比较NaHCO3作为一种温和的“产气”试剂, 在热解过程中使生物炭形成分级孔隙的多孔“碳泡沫”(类似于使用酵母和小苏打制作面包)[式(1)~(5)], NaHCO3在高温活化过程中与生物炭发生一系列热化学反应, 碱性化合物(Na2CO3、Na2O和Na)进一步与生物炭反应, 腐蚀生物炭表面, 进而影响生物炭性质[16, 17], 各种气体从碳基质中逸出, 金属钠嵌入碳晶格, 导致孔隙发育, 比表面积增加[18, 19].此外, NaHCO3具有成本低、环境足迹少和腐蚀性小等优点, 基于绿色环保和可持续发展的理念, 本研究旨在进一步考察NaHCO3活化生物炭的效果及其对IPM的吸附.
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荞麦皮是荞麦加工过程中产量最大的副产品, 富含纤维素、木质素及多糖等, 本课题组前期研究了荞麦皮生物炭对硝基咪唑类抗生素的吸附效果[20].本文通过进一步考察NaHCO3对荞麦皮生物炭的性能优化, 制备环保型吸附剂材料.通过SEM、N2吸附脱附、XPS、XRD、Raman和FTIR对其进行表征, 探讨NaHCO3改性对荞麦皮生物炭理化性质的优化.选择IPM为目标污染物, 考察改性生物炭的吸附性能, 结合吸附动力学和等温线进一步探讨吸附机制, 并与其他几种碱活化生物炭的效果进行比较, 探索NaHCO3改性生物炭的实际应用和重复利用, 以期为农业废弃物的资源化及水中含碘有机物的有效去除提供一定基础资料.
1 材料与方法 1.1 材料与试剂无水乙醇、NaHCO3、NaOH、HCl、KOH、Na2CO3、KHCO3和Ca(HCO3)2均为分析纯, 超纯水机(四川优普超纯科技有限公司), 马弗炉(5E-MF6100, 长沙开元仪器股份有限公司), 气浴恒温摇床(HZ-8811K, 常州德欧), Aglient1200型液相色谱仪(HPLC, 美国安捷伦公司), 碘帕醇(纯度≥98%, 江苏艾康生物医药研发有限公司), 分子式C17H22I3N3O8.
1.2 生物炭的制备荞麦皮产自内蒙古地区, 首先将荞麦皮生物质清洗和粉碎, 过30目筛, 用60%乙醇超声提取色素并去除表面杂质, 然后将NaHCO3和荞麦皮生物质按一定质量比x: 1(x为0, 0.1, 0.25, 0.5, 1, 2)置于100 mL水溶液, 用玻璃棒慢速搅拌直至完全混合, 摇床上25℃, 180 r·min-1振荡12 h, 得到的混合物转移至30 mL的陶瓷坩埚, 于马弗炉中以5℃·min-1的升温速率至700℃, 保持2 h, 冷却后用去离子水清洗至滤液pH稳定在7.0~8.0之间, 105℃干燥12 h, 制备的生物炭分别标记为BC、0.1N-BC、0.25N-BC、0.5N-BC、N-BC和2N-BC.其它碱按照m(碱): m(荞麦皮)=0.25:1进行活化, 方法同上.
1.3 生物炭表征采用扫描电子显微镜(SEM, Zeiss Sigma300)测定生物炭改性前后的表面形貌特征; 全自动比表面积及孔隙分析仪(BET, V-Sorb2800TP, 金埃普)测定生物炭的比表面积与孔体积; 有机元素分析仪(Vario EL cube, Elementar, Germany)测定生物炭的C、H、N和S含量, 差减法计算O含量; X射线衍射仪(XRD, Ultima IV)进行晶相分析; 拉曼光谱仪(Raman, LabRAM HR Evol, Jobin-Yvon, France)测定生物炭的缺陷程度和石墨化程度; 傅里叶红外光谱仪(FTIR, Thermo scientific Nicolet iS20)测定生物炭所含官能团种类; X射线光电子能谱仪(XPS, Thermo scientific K-α)分析生物炭中主要元素的化学形态.
1.4 IPM分析IPM浓度通过Agilent 1200液相色谱仪分析, 色谱柱为Eclipse Plus C18(4.6 mm×150 mm, 5 μm); 流动相为10%甲醇和90%、pH=3.21的磷酸盐缓冲液, 进样量10.0 μL, 流速1 mL·min-1, 检测波长254 nm, 柱温30℃, 在此条件下IPM的保留时间为5.78 min.标准曲线方程(浓度范围0~20 mg·L-1): Y=16.036X+1.72 (R2=0.999 9).
1.5 吸附实验与数据分析 1.5.1 序批吸附实验取一定量浓度为500 mg·L-1的IPM储备液于100 mL容量瓶中, 定容, 转移至150 mL具塞三角瓶, 加入0.20 g生物炭, 密封后放入气浴恒温摇床中(25℃, 200 r·min-1)振荡180 min, 一定时间取样, 过0.22 μm滤膜, 高效液相色谱仪测定IPM浓度.所有实验重复3次, 取平均值.
吸附动力学: 准确称取0.20 g生物炭于150 mL具塞锥形瓶中, 加入100 mL浓度为10 mg·L-1的IPM溶液, 将锥形瓶置于恒温摇床中振荡, 在设定时间取样, 条件同上.
吸附等温线: 准确称取0.20 g生物炭于150 mL具塞锥形瓶中, 分别加入100 mL浓度为5、10、15、20、25、30、40和50 mg·L-1的IPM溶液, 将锥形瓶置于恒温摇床中振荡, 条件同上, 180 min后取样, 计算平衡吸附量.
1.5.2 数据分析生物炭对IPM的去除率和吸附量由式(6)~(7)计算:
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式中, c0和ct分别为0和t时刻IPM的浓度, mg·L-1; V为反应液体积, L; m为生物炭的添加量, g.
动力学实验数据采用准一级、准二级和粒子内扩散模型[式(8)~(10)].
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式中, qt为t时刻IPM的吸附量, mg·g-1; qe为平衡吸附量, mg·g-1; t为吸附时间, min; k1为准一级吸附动力学速率常数, min-1; k2为准二级吸附动力学速率常数, g·(mg·min)-1; kd为颗粒内扩散系数, mg·(g·min0.5)-1; D为与边界层厚度相关的常数.
吸附等温线采用Langmuir和Freundlich模型拟合[式(11)~(12)].
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式中, ce为平衡时溶液中IPM的浓度, mg·L-1; qe为平衡时IPM的吸附量, mg·g-1; qm为最大吸附量, mg·g-1; KL为Langmuir方程的特征常数, L·g-1; KF和n为Freundlich方程常数.
1.6 吸附剂再生和重复使用再生实验条件与1.5.1动力学研究相同. 0.1 mol·L-1 NaOH作为解吸剂再生0.25N-BC, 将0.25N-BC和50 mL 0.1 mol·L-1 NaOH的混合物置于摇床中, 在25℃, 200 r·min-1条件下振荡24 h进行解吸, 连续进行3个再生循环.
1.7 实际水体应用实际水体取自西安市第四污水处理厂二沉池出水(SW)和西安湖湖水(LW), 水体水质参数见表 1, 0.25N-BC添加量2 g·L-1, IPM模拟浓度为10 mg·L-1.
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表 1 两种实际水体的水质参数/mg·L-1 Table 1 Water quality parameter of two water samples/mg·L-1 |
2 结果与讨论 2.1 NaHCO3活化荞麦皮生物炭的理化性质 2.1.1 BET和SEM
图 1(a)为BC热重分析图, TG曲线主要有3个失重过程: 200℃以下的水分蒸发; 250~400℃挥发性物质释放; 400~700℃衍生碳前驱体结晶区域的气化[21].700℃基本炭化, 热稳定性极佳, 后续实验选取热解温度为700℃. xN-BC的N2吸附-脱附等温线如图 1(b)所示, 根据IUPAC分类, 该曲线属于Ⅰ型和Ⅳ型[22], 材料主要为微孔和介孔, 孔结构更加丰富.图 1(c)孔径分布可以看出, xN-BC的孔径主要集中在1~10 nm, 分布强度大小为: 0.25N-BC>0.5N-BC>N-BC>2N-BC>BC, 0.25N-BC的孔隙结构最为显著[12], Mestre等[23]报道IPM分子的单体、二聚体和三聚体尺寸均小于2 nm, 由表 2可知, xN-BC的Davg在2~3 nm之间, 有利于目标污染物的吸附.随m(NaHCO3): m(荞麦皮)增加, 由0增加到0.25:1和2:1, 改性生物炭的SBET由480.40 m2·g-1先增至572.82 m2·g-1, 随后降低为422.84 m2·g-1; Vtot由0.294 cm3·g-1先增加到0.402 cm3·g-1, 而后降至0.309 cm3·g-1; Smic和Vmic也呈现先增加后减少的趋势, 说明适量NaHCO3活化能够改善生物炭孔隙结构, 过量则可能导致孔壁坍塌[24].结合图 6(a), 0.25N-BC在30 min对IPM的吸附率达到100%, 实验确定m(NaHCO3): m(荞麦皮)为0.25:1, 记为0.25N-BC.
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图 1 BC的热重分析和xN-BC的N2吸附-脱附等温线和孔径分布 Fig. 1 Thermogravimetric analysis of BC and N2 adsorption-desorption isotherms and pore size distribution of xN-BC |
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表 2 xN-BC的比表面积和孔隙结构参数 Table 2 Specific surface area and pore structure parameters of xN-BC |
BC和0.25N-BC的SEM如图 2所示, BC表面平整光滑, 0.25N-BC表面出现许多凹凸不平的絮状结构; 低分辨率图也可清楚看到BC孔隙结构有限, 0.25N-BC有块状的堆叠颗粒, 呈现密集的不规则孔隙和裂纹, 这是由于NaHCO3作为膨松剂活化产生气体造成的[25].
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图 2 BC和0.25N-BC的SEM表征 Fig. 2 SEM characterization of BC and 0.25N-BC |
BC和0.25N-BC的XRD和Raman见图 3, 图 3(a)中(002)和(101)晶面衍射峰分别对应无定形碳和石墨碳结构[26].与BC相比, 0.25N-BC的无定形碳特征峰明显变宽, 强度减弱, 表明改性后的生物炭结晶度降低.图 3(b)中1 335 cm-1和1 580 cm-1分别为碳缺陷诱导Raman峰(D峰)和有序石墨结构峰(G峰)[27], 0.25N-BC的ID/IG值为3.15, 是BC的1.22倍, 与徐晋等[28]报道KOH活化小麦秸秆生物炭前后的1.50倍相接近.结合表 2, 0.25N-BC的比表面积和总孔体积分别为BC的1.19倍和1.37倍, 表明其存在高度无序结构, 石墨碳被金属插入或碱腐蚀破坏[29], 与XRD分析一致.
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图 3 BC和0.25N-BC吸附IPM前后的XRD和Raman表征 Fig. 3 XRD and Raman characterization of BC and 0.25N-BC before and after adsorption of IPM |
图 4为BC和0.25N-BC的FTIR图, 3 600~3 200 cm-1为醇或酚—OH引起的伸缩振动[30], 0.25N-BC的吸收峰强度略小于BC, 这可能是NaHCO3活化加速生物炭中纤维素和半纤维素的分解. 2 933.20 cm-1是—CH3的不对称伸缩振动峰, 1 666.67 cm-1的强吸收峰为羧基和内酯基中C＝O或芳香环上C＝C伸缩振动[31], 0.25N-BC在该区域的振动强烈, 说明其具有丰富的芳香结构[32]. 1 332.20 cm-1为碳水化合物的C—O特征吸收峰, 893.12 cm-1可能是由苯环中C—H平面振动所引起[16].
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图 4 BC和0.25N-BC吸附IPM前后的FTIR表征 Fig. 4 FTIR characterization of BC and 0.25N-BC before and after adsorption of IPM |
生物炭改性前后的元素组成见表 3, 与BC相比, 0.25N-BC中C和H减少, O增多, NaHCO3与生物炭发生热化学反应促进H释放及炭化程度加强, 含氧官能团增加[12].BC和0.25N-BC的H/C值接近, 说明均高度芳香化, 0.25N-BC的O/C和(N+O)/C值大于BC, 亲水性和极性增加[30].
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表 3 BC和0.25N-BC的元素组成 Table 3 Elemental analysis of BC and 0.25N-BC |
2.1.4 XPS
图 5(a)XPS全扫描光谱显示, 与BC相比, 0.25N-BC的C降低(84.19%→76.36%), O(13.52%→17.65%)含量增加, 与表 3结果一致, 且检测到Na[25].0.25N-BC中C 1s分解为284.80、286.40、288.00和289.70 eV, 对应官能团C—C/C＝C、C—O、C＝O和O—C＝O[12, 33, 34]; O 1s分解为531.67 eV和533.39 eV的C＝O和C—O—C/C—OH; N 1s分解为398.61 eV和399.94 eV, 对应吡啶氮和吡咯氮.
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图 5 BC和0.25N-BC吸附IPM前后的XPS谱图 Fig. 5 XPS characterization of BC and 0.25N-BC before and after adsorption of IPM |
图 5(b)显示, 与BC相比, 0.25N-BC中C 1s出现新的官能团O—C＝O, C—C/C＝C含量由53.82%增至72.06%, 这与0.25N-BC的高度芳香化有[35, 36]关; 图 5(c)中O 1s的C＝O含量由54.55%增至72.46%; 图 5(d)中N 1s的吡啶氮随吡咯氮含量的增加而降低[18], 可以看出改性后生物炭的官能团种类更加丰富.
2.2 荞麦皮生物炭对IPM的吸附行为 2.2.1 IPM的吸附效果IPM初始浓度为10 mg·L-1, xN-BC投加量为2 g·L-1, 摇床转速为200 r·min-1, 25℃, xN-BC对IPM的吸附结果如图 6(a)所示.BC在180 min对IPM的吸附率为83.61%, N-BC和2N-BC在120 min对IPM的去除率分别为95.40%和94.53%, 0.25N-BC和0.5N-BC在30 min达到吸附平衡, 综合考虑经济成本和吸附效果, 本文选择0.25N-BC进行研究, 比Yang等[25]报道最佳比例低[m(NaHCO3): m(玉米秸秆)=1:1], 结合2.1节NaHCO3活化荞麦皮生物炭理化性质的分析, 0.25N-BC的比表面积更大, 官能团更丰富, NaHCO3活化可有效增强荞麦皮生物炭对污染物的吸附效果[11].图 6(b)为0.25N-BC吸附IPM过程中的UV-vis变化, IPM在λ242 nm处的吸收峰随时间延长逐渐降低直至消失, IPM浓度减小.
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图 6 xN-BC对IPM的吸附效果和0.25N-BC吸附IPM的紫外可见吸收光谱变化 Fig. 6 Adsorption effect of xN-BC on IPM and the UV-visible absorption spectrum of 0.25N-BC on IPM |
图 7(a)所示, BC和0.25N-BC吸附IPM的趋势大致相同, 吸附量qt随时间延长而增加, 直至饱和.采用准一级[式(8)]、准二级吸附动力学模型[式(9)]和颗粒内扩散模型[式(10)]对吸附过程进行拟合, 由表 4可知, BC吸附IPM符合准一级动力学模型(R12=0.960 2), 0.25N-BC吸附IPM更符合准二级动力学模型(R22=0.999 3), qe值与实验测得值基本相当, 0.25N-BC通过交换或共享电子与IPM发生化学吸附[22], k2＜1, 吸附过程快速, 表明π—π结合和氢键克服生物炭与IPM之间的转移阻力提高了其吸附率[37].
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图 7 BC和0.25N-BC对IPM的吸附动力学和颗粒内扩散模型 Fig. 7 Adsorption kinetics and intracellular diffusion model of IPM by BC and 0.25N-BC |
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表 4 BC和0.25N-BC对IPM的吸附动力学拟合参数 Table 4 Fitting parameters of adsorption kinetics of BC and 0.25N-BC for IPM |
粒子内扩散模型拟合结果见图 7(b)和表 5, BC和0.25N-BC对IPM的吸附过程可分为3个阶段, 第一阶段为颗粒外部扩散阶段, IPM浓度差是吸附的驱动力, kd1较大(0.358 5和0.589 0), IPM快速扩散到生物炭表面, 意味着外表面扩散是吸附过程中的速率限制阶段; 第二阶段为颗粒内部扩散阶段, 传质阻力增大[38], 吸附速率下降, kd2有所减小(由0.358 5和0.589 0减至0.104 4和0.108 0), 直至吸附位点接近饱和状态; 第三阶段为吸附平衡阶段, D值明显大于前两阶段, 边界层厚度增加[39].拟合线未通过原点, 说明吸附过程包含两个或多个步骤, 颗粒内扩散并不是唯一的限速步骤.
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表 5 BC和0.25N-BC对IPM的颗粒内扩散模型拟合参数 Table 5 Fitting parameters of the IPM intracellular diffusion model for BC and 0.25N-BC |
2.2.3 吸附等温线
Langmuir和Freundlich等温模型的拟合结果见图 8, 相关参数见表 6.Langmuir等温模型中BC和0.25N-BC吸附IPM的RL2分别为0.970 4和0.998 3, Freundlich模型的RF2分别为0.972 5和0.995 0, 两种模型都能很好地描述BC和0.25N-BC对IPM的吸附过程, 表明生物炭对IPM的吸附以单层和非均质多层吸附为主[40].IPM浓度增加, 吸附量增加, 0.25N-BC的最大吸附量(74.94 mg·g-1)是BC(7.88 mg·g-1)的9.51倍, Yang等[25]发现NaHCO3活化玉米秸秆生物炭前后对阿特拉津(35 mg·L-1)的最大吸附量增加了1.65倍, 目前未检索到IPM吸附的文献, 此处暂不作比较.Freundlich模型的1/n值均小于1, 表明生物炭吸附性能良好.
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图 8 BC和0.25N-BC对IPM的吸附等温线 Fig. 8 Adsorption isotherms of IPM by BC and 0.25N-BC |
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表 6 BC和0.25N-BC对IPM吸附等温线拟合参数 Table 6 Fitting parameters of IPM adsorption isotherms for BC and 0.25N-BC |
2.3 pH对0.25N-BC去除IPM的影响
不同pH范围IPM在BC和0.25N-BC的吸附如图 9(a)所示, 在2.0~10.0范围内, BC和0.25N-BC对IPM均具有稳定的吸附效果, pH=12.0时, BC和0.25N-BC对IPM的去除率分别为48.35%和93.78%.IPM的pKa=10.7, pH＜10.7时IPM在溶液中以分子形式存在, pH>10.7时以阴离子形式存在.
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图 9 溶液pH对IPM吸附效果的影响及BC和0.25N-BC的Zeta电位 Fig. 9 Effect of solution pH on adsorption of IPM and Zeta potential of BC and 0.25N-BC |
图 9(b)表示BC和0.25N-BC的Zeta电位, 其随pH的增加而降低, 0.25N-BC增加的含氧基团(—OH、—COOH和内酯基等)逐渐电离使生物炭表面带负电[41], pH>10.7时, IPM与表面带负电的0.25N-BC存在一定静电斥力, 但表面负电荷的增加并没有显著降低IPM的去除(93.78%), 说明静电作用不是0.25N-BC吸附IPM的主导机制, 孔隙扩散、氢键和π—π等发挥重要作用[42].
2.4 不同类型碱活化生物炭对IPM吸附效果的影响不同类型碱[m(碱): m(荞麦皮)=0.25:1]活化荞麦皮生物炭对IPM的吸附结果见图 10(吸附条件同图 6).BC对IPM的吸附率在180 min为83.61%; Ca(HCO3)2和KHCO3活化有效缩短了吸附平衡时间, 120 min对IPM的去除率分别为93.18%和96.02%; 而KOH和NaHCO3活化生物炭分别在10 min和30 min达到100%吸附; 碱活化可有效增强荞麦皮生物炭对污染物的吸附效果[12].考虑到KOH的腐蚀性和环境毒性, 相对温和且环境友好的NaHCO3作为活化剂具有明显优势.
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图 10 不同类型碱活化生物炭对IPM的吸附效果 Fig. 10 Adsorption effect of different types of alkali-activated biochar on IPM |
0.25 N-BC对IPM吸附机制包括物理和化学过程, BET和SEM表明, 0.25N-BC表面粗糙, 具有发达的孔隙结构和大的比表面积, 有利于IPM扩散到吸附剂的内孔系统中.由表 2可知, 0.25N-BC吸附IPM后SBET和Vtot分别由572.82 m2·g-1和0.402 cm3·g-1降至476.57 m2·g-1和0.324 cm3·g-1, Smic和Vmic相应由360.10 m2·g-1和0.171 cm3·g-1降至313.40 m2·g-1和0.148 cm3·g-1, 这是由于IPM吸附在0.25N-BC的网状结构上, 堵塞部分孔隙, 类似结果也有报道[43].表明孔隙填充是荞麦皮生物炭吸附IPM的主要机制之一.
一般比表面积对目标污染物吸附起积极作用[31], N-BC和2N-BC的比表面积分别为462.56 m2·g-1和422.84 m2·g-1(表 2), 均小于BC(480.40 m2·g-1), 但对IPM的吸附效果明显优于BC[图 6(a)], 说明生物炭中的官能团起着重要作用.0.25N-BC和BC吸附IPM前后的FTIR显示(图 4), 在3 600~3 200 cm-1的吸收峰均变弱, 生物炭表面的—OH(H供体)与IPM(H受体)的含氧和含氮基团产生了偶极—偶极氢键作用(O—H—O/N)[44, 45], 同时生物炭表面的—OH可以通过Yoshida氢键与IPM的芳香环相互作用[36, 37]. 1 666.67 cm-1的吸收峰移至1 663.50 cm-1, 表明羧基和内酯基中C＝O或芳香环上C＝C参与了吸附, 吸附剂的苯环或C＝O与IPM苯环存在π—π或n—π相互作用[17].
对比吸附前后XPS谱图中C、O和N元素含量以及官能团类型和峰面积的变化, 吸附IPM后, C—C/C＝C含量由72.06%降至56.01%[图 5(b)], 进一步证明了生物炭的芳香性与IPM吸附有关.C＝O含量由72.46%降至49.71%[图 5(c)], O—C＝O官能团消失, 生物炭表面的羰基氧离子充当电子供体, 而IPM的芳香环充当电子受体, 与Tran等[36, 46]报道球型生物炭吸附对乙酰氨基酚一致[47], 表明n—π相互作用也是吸附IPM的主要机制之一.
IPM芳香环的π—电子与石墨烯层的π—电子之间存在π—π作用, XRD显示, 吸附IPM后, (002)和(101)晶面衍射峰强度减弱[图 3(a)].对比吸附前后的Raman表征[图 3(b)], ID/IG值由3.15降至1.01, 表明吸附过程中发生了从IPM分子到生物炭的强电荷转移.Ramesha等[48]也应用Raman技术来识别有机物吸附到碳基材料上的电荷转移.
综合分析, NaHCO3活化生物炭吸附IPM的主要机制有孔隙填充、氢键、π—π和n—π相互作用, 静电作用影响不大, 具体见图 11.
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图 11 0.25N-BC吸附IPM的机制 Fig. 11 Mechanism of adsorption of IPM by 0.25N-BC |
湖水(LW)取自西安湖, 二沉池出水(SW)取自西安市第四污水处理厂, 在上述相同实验条件下, 考察两种实际水体中0.25N-BC对IPM的吸附效果, 图 12显示了吸附前后各项指标的浓度, NH4+-N、NO3--N、COD和TP浓度基本不变, IPM在LW和SW中的去除效率分别达97.60%和97.30%, 0.25N-BC对IPM的吸附不受外界环境影响, 可作为实际水环境中去除IPM的良好吸附剂.
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图 12 0.25N-BC在实际水体中的应用 Fig. 12 Application of 0.25N-BC in actual water |
0.25N-BC的再生性能如图 13所示, 再生3次(R3)后对IPM的去除率仍达到74.91%, 与R0相比, IPM的去除率降低了23.36%, 这可能是由于IPM在0.25N-BC表面的不完全解吸导致活性位点减少.总体来看0.25N-BC是一种高效且经济的吸附剂.
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图 13 0.25N-BC的重复使用性能 Fig. 13 Reuse performance of 0.25N-BC |
(1) 以荞麦皮为原料, m(NaHCO3): m(荞麦皮)=0.25:1时, 生物炭0.25N-BC材料孔隙结构改善显著, 比表面积和孔体积显著增大, 对IPM的吸附量显著增加.
(2) 0.25N-BC对IPM的吸附动力学过程符合准二级动力学模型, 吸附动力学常数k2＜1, 吸附过程快速, 主要以化学吸附为主.颗粒内扩散模型拟合结果表明吸附过程可分为3个阶段, 颗粒内扩散并不是唯一的控速步骤.
(3) 0.25N-BC对IPM吸附等温线同时符合Langmuir和Freundlich模型, 吸附以单层和非均质多层吸附为主, 且qm为74.94 mg·g-1, 是BC的9.51倍.
(4) 0.25N-BC与IPM之间存在物理化学吸附, 吸附机制主要有孔隙填充、氢键、π—π和n—π相互作用, 静电作用次之, 吸附机制较为复杂.
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