Facile Synthesis of Porous Organic Polymer/Chitosan Composites and the Removal Effect of Hg(Ⅱ)
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摘要:
特定多孔结构和杂原子掺杂的吸附剂对提高重金属离子的吸附性能具有重要意义。传统的多孔有机聚合物材料大多在溶剂中合成，且为粉末状的形式，但在合成方法优化和实际应用便捷性方面仍有一定的发展空间。本文针对水体中的汞污染，采用简单快速的机械研磨法以1,3,5-三醛基间苯三酚(Tp)和硫脲(TU)制备硫掺杂的多孔有机聚合物(TpTU)与壳聚糖(CS)复合材料TpTU@CS。采用X射线衍射光谱、N2吸附-解吸和傅里叶变换红外光谱等技术对TpTU@CS复合材料进行表征。通过扫描电镜证实了TpTU@CS复合材料的多孔结构。由于在分子网络中引入了−C=S−基团，合成的TpTU@CS在水溶液中对Hg(Ⅱ)具有更高的吸附选择性和亲和力，吸附容量高(249.21mg/g)，吸附动力学快(10min)。通过表征分析得出，TpTU@CS捕获Hg(Ⅱ)的主要机制是C=S中S与Hg的键合以及C−N与Hg(Ⅱ)的配位相互作用。与其他研究相比，TpTU@CS具有优异的吸附性能，且具有易于处理和可回收利用的优点。同时，该复合材料TpTU@CS对于Hg(Ⅱ)低浓度污染的实际水样和高浓度的加标水样，均具有较高的去除能力，去除率可达到77.0%～100.0%。
要点(1)基于席夫碱反应，采用机械研磨法能够快速便捷地合成元素掺杂多孔有机聚合物。
(2)多孔有机聚合物与壳聚糖制备的复合材料TpTU@CS捕获Hg(Ⅱ)的主要机制是C=S中S与Hg的键合以及C-N与Hg(Ⅱ)的配位相互作用。
(3)在pH=2～7范围内，溶液的pH对TpTU@CS吸附Hg(Ⅱ)的影响较小，Hg(Ⅱ)的三种形态Hg2+、Hg(OH)+和Hg(OH)2均有利于吸附。
HIGHLIGHTS(1) Based on the Schiff base reaction, element-doped porous organic polymers can be synthesized quickly and easily by the mechanical grinding method.
(2) For TpTU@CS, the main mechanism of capturing Hg(Ⅱ) is the bonding between S and Hg in C=S and the coordination interaction between C−N and Hg(Ⅱ).
(3) In the range of pH=2−7, the adsorption properties of Hg(Ⅱ) are less affected by pH, and the three forms of Hg(Ⅱ) including Hg2+, Hg(OH)+ and Hg(OH)2 are favorable for adsorption.
Abstract:Specific porous structures and heteroatom-doped adsorbents have great importance in improving the adsorption performance of heavy metal ions. Traditional porous organic polymer materials are mostly synthesized in solvents in the form of powder, so it is significant to develop a highly efficient adsorption material and apply it to the adsorption and removal of Hg(Ⅱ). In the research, S-doped porous organic polymer (TpTU) and chitosan (CS) composites TpTU@CS were prepared by using 1,3,5-trialaldehyde phloroglucinol (Tp) and thiourea (TU) by a simple and rapid mechanical grinding method. The TpTU@CS composites were characterized by X-ray diffraction spectroscopy, N2 adsorption-desorption, scanning electron microscope and Fourier transform infrared spectroscopy. Due to the introduction of the −C=S− group into the molecular network, the synthesized TpTU@CS has high adsorption selectivity and affinity for Hg(Ⅱ) in aqueous solution, with high adsorption capacity (249.21mg/g) and fast adsorption kinetics (10min). Through the characterization analysis, it is concluded that the main mechanism of trapping Hg(Ⅱ) by TpTU@CS is the bonding between S and Hg in C=S and the coordination interaction between C−N and Hg(Ⅱ). Meanwhile, the composite TpTU@CS has a high removal capacity (77.0%−100.0%) of Hg(Ⅱ) for both the actual samples and the marked samples. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202211170219.
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Keywords:
- porous organic polymer /
- composite microsphere /
- mercury /
- structural characterization /
- adsorption property /
- X-ray photoelectron spectroscopy
BRIEF REPORTSignificance: Heavy metal pollution has caused great losses to national health and economic development. Mercury(Hg) is one of the most toxic heavy metal pollutants[1]. It easily accumulates in the human body and can cause birth defects, brain damage and other diseases in humans and animals. Adsorption method is one of the more convenient and low-cost methods for Hg(Ⅱ) removal, but the adsorption capacity of traditional adsorbent materials is low, and ineffective in practical application. Therefore, it is crucial to develop new materials with high affinity and adsorption properties for Hg(Ⅱ). Porous organic polymers (POPs) are emerging materials. In recent years, POPs based adsorbents have been increasingly used to remove harmful substances[2-3]. The inclusion of functional groups (e.g. −NH4, −SH) in the synthesis of POPs adsorbents can enhance their affinity and selectivity for metal ions[4-5]. However, POPs commonly direct synthesis by a bottom-up method using monomers containing hybrid elements, which has the disadvantage that functional groups can be damaged during the reaction process. In the research, based on the Schiff base reaction theory of tautomerism of enol-ketone, a novel C=S functionalized POPs material was developed by a green preparation method of mechanical grinding, which was then integrated into a microsphere with chitosan. The rich free −NH2 and −OH functional groups in chitosan interacts with various organic groups to form a three-dimensional network with good stability. In addition, the highly ordered microporous/mesoporous structure of the novel materials is conducive to the diffusion of adsorbed substances and exposure of active sites, thus improving the mass transfer efficiency.
Methods: A S-doped porous organic polymer and chitosan composite material TpTU@CS was prepared by adopting green strategy. 34.2mg thiourea and 530.3mg p-toluenesulfonic acid were ground in agate mortar for 5min. After grinding evenly, 63.1mg 1,3,5-trialdehyde-resorcinol was added and grinding continued for 5−8min until even again. A small amount of water was added during grinding to give it a certain fluidity. The mixture was then transferred to the Teflon lining of the reactor and reacted at 140℃ for 12h. After the reaction, it was cleaned three times in turn with hot water, N,N-dimethylacetamide, hot water and acetone, and dried in vacuum for subsequent use, and the obtained material was named TpTU. The TpTU material 0.5g and chitosan 2.5g were added to 20mL 1% (V/V) acetic acid solution. After stirring for 3h, it was dropped into 1.0g/L NaOH solution, the obtained microspheres were washed with ultra-pure water until the pH becomes neutral, and then 10mL 25% glutaraldehyde solution was added. After storing for 3h, the microspheres were collected and washed with ultra-pure water several times. TpTU@CS is obtained by freeze-drying.
The specific surface area and pore size distribution were calculated using the nitrogen adsorption-desorption system based on Bruner-Emmett-Teller and Barrett-Joyner-Halenda methods. The nitrogen adsorption-desorption isotherm was collected at 77K and high purity nitrogen (>99.999%) was measured. Fourier transform infrared (FTIR) spectra of different samples were collected by infrared spectrometer with a resolution of 4cm-1 and a range of 400−4000cm-1. The crystal structure of the sample was analyzed by X-ray diffractometer. The surface morphology and elemental content of the samples were recorded by Merlin scanning electron microscopy and its energy dispersive fluorescence X-ray system (EDS). The composition and chemical state of the surface elements were studied by X-ray photoelectron spectroscopy (Al Kα radiation), with emphasis on the fine spectrum analysis of O, N, S and Hg.
Data and Results: The SEM shows that the synthetic material is a cluster of spheroidal particles with a rough surface (Fig.1). In the infrared spectra (Fig.2a), after the reaction of TU and Tp, the −NH2 tensile vibration peak (1414cm-1) of the original TU molecule almost disappears, and a new C−N bond peak is formed at 1286cm-1, indicating that Tp and TU are successfully polymerized. The BET specific surface area of TpTU@CS is 82.138m2/g, and the porous structure of TpTU@CS is conducive to improving the mass transfer rate. A large specific surface area can increase the number of adsorption sites.
The adsorption experiments show that the adsorption pattern of Hg(Ⅱ) on TpTU@CS is more consistent with the Freundlich model, indicating that Hg(Ⅱ) is more consistent with non-uniform adsorption on TpTU@CS. The adsorption of Hg(Ⅱ) is more consistent with pseudo-first-order kinetics, which indicates that the diffusion step controls the adsorption. The synthesized TpTU@CS has higher adsorption selectivity and affinity for Hg(Ⅱ) in aqueous solution, with high adsorption capacity (249.21mg/g) and fast adsorption kinetics (10min). As shown in Table 2, this composite material TpTU@CS is successfully used to remove Hg(Ⅱ) from water, showing high removal capacity (77.0%−100.0%) for both actual water samples contaminated with low concentration of Hg(Ⅱ) and high concentration marked water samples. TpTU@CS material can achieve 100% removal rate of Hg(Ⅱ) aqueous solution within 10min, which can be attributed to its regular pore structure (13nm).
Through XPS characterization in Fig.5 and Fig.6, it is found that the chemical mechanism is mainly the bonding effect between S and Hg. The formation of a covalent bond ensures the excellent adsorption capacity of TpTU@CS for Hg(Ⅱ). The strong coordination of −S−Hg− is also confirmed by exploring the influence of pH on adsorption. Under the test conditions [5mg TpTU@CS in 5mL 10mg/L Hg(Ⅱ) at different pH values], and among all the pH values tested, TpTU@CS achieves 100% removal rate of Hg(Ⅱ), which proves that the three forms of Hg(Ⅱ) including Hg2+, Hg(OH)+ and Hg(OH)2 are favorable for adsorption.
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随着工业和经济的快速发展，环境污染特别是水中重金属(如汞、铅、铜、镉)污染日益严重［1-2］。中国也深受重金属污染的危害，污染调查公报显示全国重金属土壤污染率为16.1%［3-5］，重金属污染对国民健康和经济发展造成了巨大损失。汞是最具毒性的重金属污染物之一，它极易在人体内积聚，可导致出生缺陷、脑损伤以及人类和动物的其他疾病，对公共卫生和环境构成严重威胁［6-8］。联合国粮食及农业组织和世界卫生组织(粮农组织/世卫组织)建议，饮用水中Hg(Ⅱ)的最大允许浓度为30nmol/L［6］。因此，开发更有效的技术和方法从水体中去除这些污染物具有重要意义。
吸附［9］、离子交换［10］、膜过滤［11］和化学沉淀［12］等多种修复技术，在Hg(Ⅱ)的高效去除中被大量研究。在这些处理策略中，吸附处理因其高效便捷、低成本而备受关注［13-14］。吸附剂是吸附过程中最重要的部分，决定了该方法的选择性、吸附效率和应用领域［15］。传统的吸附剂材料如活性炭［16］、生物炭［17-18］、二氧化硅［19］和沸石［20］已被用于去除Hg(Ⅱ)离子。但由于其比表面积小，官能团密度不够，在实际应用中效果不佳，如生物炭材料对Hg(Ⅱ)的吸附容量约在μg/g级别［18］。因此，开发对Hg(Ⅱ)具有高亲和力和高吸附性能的新型材料至关重要。
多孔有机聚合物(POPs)是一类新兴材料，具有高比表面积、可控的结构、高化学稳定性和易后修饰的特性［21-22］。一些具有良好性能的POPs材料已被开发出来，在催化、光电、能源、环境等方面具有巨大的应用潜力［23］。近年来，基于POPs的吸附剂被越来越多地用于去除有害物质。为了增强它们对金属离子的亲和力和选择性，人们引入了多种功能基团［24］。氮原子上存在孤对电子，引入氨基会增加材料的缺陷位，提供更多的活性位点用于螯合重金属离子，显著提高吸附选择性和吸附容量［15］。大量研究表明，通过引入含硫官能团改性不同材料的表面，可以有效地提高其吸附率和选择性。如磁性硫掺杂多孔碳(MSPC)［25］材料、巯基功能化金属有机骨架材料［26，27］](MOFs)和共价有机多孔聚合物(COPs)［28-29］等。Ren等［28］通过重氮偶联反应，成功制备了一种具有分层孔结构和丰富硫醇基团的共价有机高分子材料(HP-COP-SH)。丰富的巯基位点使吸附剂与汞具有较强的亲和力，从而大大提高了材料对Hg(Ⅱ)的吸附速度和选择性。Li等［26］通过表征和DFT计算相结合，证明复合材料上的巯基对Hg(Ⅱ)的吸附起着重要作用。总体而言，相对于其他新型材料，POPs材料具有优异的结构可控性、功能可调性和化学稳定性等优点［30］，极大地满足了实际应用的要求。然而，目前常用的POPs合成方法为利用含有杂化元素的单体，采用自下而上法直接合成，这种方法存在官能团容易在合成时遭到破坏的缺点［30］。除此之外，目前所开发的POPs材料大多以粉末状态为主，在实际使用时难以收集重复使用，复合材料则能很好地解决这一问题。
基于已有研究基础，本文以硫脲(TU)作为二胺单体和功能化试剂，基于烯醇-酮的互变异构的席夫碱反应［31］，采用机械研磨的绿色制备方法，开发了一种新型的C=S功能化POPs材料［32］。然后，利用壳聚糖将其整合为微球的形态［33］，壳聚糖中丰富的游离-NH2和-OH官能团能够与各种有机基团相互作用，形成稳定性好、易于恢复的三维网络。通过扫描电镜(SEM)、傅里叶红外变换(FTIR)、X射线衍射(XRD)和氮吸附-脱附测试对合成的TpTU@CS进行表征。通过静态吸附实验探讨了吸附动力学、吸附等温线和吸附选择性。同时利用X射线光电子能谱(XPS)推测可能的吸附机理。最后，将TpTU@CS应用于实际水体中的Hg(Ⅱ)去除，揭示其在捕获环境样品中的金属离子方面的巨大潜力。
1. 实验部分
1.1 仪器及工作条件
Hg(Ⅱ)检测仪器：采用电感耦合等离子体质谱仪(ICP-MS，ICS900-iCAPQ，美国)测定目标金属离子。对操作条件进行了优化，测定程序条件如下：ICP-MS的射频功率1560W，等离子体气体流速14L/min，载气流速0.80L/min，雾化器气体流速0.99L/min。
TpTU@CS表征仪器：通过氮气吸附-解吸系统(Autosorb-iQ-MP，康塔，美国)，基于Brunauer-Emmett-Teller和Barrett-Joyner-Halenda方法计算比表面积和孔径分布。在77K下收集氮气吸附-解吸等温线，并使用高纯氮气(＞99.999%)进行测量。采用红外光谱仪(Nicolet 5700，美国)采集不同样品的傅里叶变换红外光谱(FTIR)，其分辨率为4cm−1，范围为400～4000cm−1。利用X射线衍射仪(XRD，D/MAX2500，日本)对样品的晶体结构进行分析研究。采用Merlin扫描电子显微镜(SEM，Carl Zeiss，Oberkochen，德国)及其配备的能量色散型X射线荧光系统(EDS)分别记录样品的表面形貌和元素含量。用X射线光电子能谱仪(Al Kα辐射)研究样品的表面元素组成和化学状态，重点对O、N、S、Hg进行精细谱的分析。
1.2 主要试剂
采用超纯水溶解HgCl2制备Hg(Ⅱ)标准溶液(1000mg/L)，然后将特定浓度的溶液在使用前进行相应稀释。
硫脲(TU)、壳聚糖(CS)、盐酸、硝酸、乙酸、氢氧化钠、戊二醛、N,N-二甲基乙酰胺(DMAc)、丙酮等均为分析纯，购自国药集团。
1,3,5-三醛基间苯三酚(Tp，分析纯，麦克林公司)，对甲苯磺酸(PTSA，分析纯，麦克林公司)。
所有试剂未作进一步净化处理。实验用水采用Milli-Q净水系统(18.25MU·cm，Millipore，Aquelix 5 & Simplicity，美国)提供。
1.3 吸附材料制备
1.3.1 TpTU材料的制备
称取34.2mg 硫脲和530.3mg 对甲苯磺酸在玛瑙研钵中研磨5min，研磨均匀后加入63.1mg的1,3,5-三醛基间苯三酚，并继续研磨5～8min至均匀。在研磨时滴加少量水使其具备一定流动性。然后将上述混合物转移至反应釜的聚四氟乙烯内衬中，在140℃下反应12h。反应结束后，依次用热水、N,N-二甲基乙酰胺、热水和丙酮清洗三次，并在真空中干燥，以备后续使用，所得到的材料命名为TpTU。
1.3.2 TpTU@CS材料的制备
在20mL 1% (V/V)乙酸溶液中加入上述合成的TpTU材料0.5g和壳聚糖2.5g。搅拌3h后，用注射器将其滴加到1.0g/L 氢氧化钠溶液中，得到的微球用超纯水洗涤至pH变为中性，然后加入10mL 25%戊二醛溶液。保存3h后，收集微球，用超纯水洗涤数次。通过冷冻干燥得到TpTU@CS。
1.4 TpTU@CS处理水中Hg(Ⅱ)的吸附研究
1.4.1 吸附等温线实验
为了探究TpTU@CS吸附Hg(Ⅱ)的最大吸附容量，首先进行了吸附等温线相关的实验。将5mg的TpTU@CS分别投至5mL不同初始浓度(C0=10、50、100、150、200、300mg/L)的Hg(Ⅱ)标准溶液中，振荡吸附8h，确保吸附已经达到平衡。吸附完成后测试剩余Hg(Ⅱ)的浓度。TpTU@CS对Hg(Ⅱ)的平衡吸附容量的计算公式为：
$$ Q_{\mathrm{e}}=\frac{\left(C_0-C_{\mathrm{e}}\right)·V}{m} $$ (1) 式中：Qe为吸附达到平衡时TpTU@CS对Hg(Ⅱ)的吸附容量(mg/g)；C0为Hg(Ⅱ)的初始浓度；Ce为吸附达到平衡时溶液中Hg(Ⅱ)的浓度(mg/L)；V为标准溶液体积(mL)，m为加入的TpTU@CS吸附剂的质量(mg)。
1.4.2 吸附动力学实验
为了探究TpTU@CS吸附Hg(Ⅱ)的吸附动力学性能，收集并测试了在时间t时(t=2、5、10、20、30、60、120min)的Hg(Ⅱ)浓度。此时，TpTU@CS对Hg(Ⅱ)的吸附容量计算公式为：
$$ {Q}_{t}=\frac{\left({C}_{0}-{C}_{t}\right)·V}{m} $$ (2) 式中：Qt为在时间t时TpTU@CS对Hg(Ⅱ)的吸附容量(mg/g)；C0为Hg(Ⅱ)的初始浓度；Ce为时间t时溶液中Hg(Ⅱ)的浓度(mg/L)。
1.4.3 吸附选择性实验
为了探索共存离子对TpTU@CS吸附Hg(Ⅱ)的影响，将10mg的TpTU@CS吸附剂添加到分别含有10mg/L Hg(Ⅱ)、Pb(Ⅱ)、Cr(Ⅲ)、Cd(Ⅱ)、Mn(Ⅱ)、Co(Ⅱ)、Ni(Ⅱ)、Cu(Ⅱ)、Zn(Ⅱ)的水溶液(10mL)中。将混合物在室温下振荡吸附6h后，过滤以去除吸附剂。最后，使用ICP-MS分析溶液中各种金属离子的浓度。
1.4.4 洗脱实验
为了验证材料的重复使用性，将5mg的TpTU@CS吸附剂添加到Hg(Ⅱ)标准溶液(5mL，500mg/L)中。在室温下振荡吸附12h后过滤，并用5mL去离子水清洗。将获得的TpTU@CS材料在5mL 2mol/L盐酸酸化的2mol/L硫脲溶液中解吸12h，然后过滤，用20mL热去离子水洗涤至中性，干燥后用于下一循环实验。
2. 结果
2.1 TpTU和TpTU@CS材料的表征
采用扫描电镜研究了TpTU@CS的特征形貌和结构，表征结果如图1所示，证实合成材料为球状颗粒聚集而成的团簇状，具有较为粗糙的表面。使用扫描电镜电子能谱仪(EDS)对材料进行了元素分布分析，结果表明C、O、N和S元素在材料中均匀分散，说明合成的材料较为均匀。
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图 1 TpTU@CS的SEM图像： (a) 200nm尺度形貌；(b) 100nm尺度形貌Figure 1. SEM images of TpTU@CS: (a) 200nm morphology; (b) 100nm morphology使用FTIR对TpTU@CS和中间体TpTU的化学结构进行了表征，如图2a所示。当TU与Tp反应后，原始TU分子的−NH2拉伸振动峰(1414cm−1)几乎消失，1286cm−1处的C−N键新峰的形成，表明Tp和TU聚合成功。TpTU聚合物的谱图中，发现对应TU的C=S拉伸振动的1085cm−1和730cm−1两个峰仍然存在，表明材料中成功引入了C=S官能团［5］，进一步证明了TpTU材料的合成成功。通过X射线衍射仪表征了材料的晶体结构，如图2b所示。该材料没有明显的衍射峰，表明制备的TpTU和TpTU@CS均具有非晶态结构。
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图 2 (a) TpTU和TpTU@CS的傅里叶变换红外光谱；(b) TpTU和TpTU@CS的X射线衍射分析结果Figure 2. (a) Fourier transform infrared spectra of TpTU and TpTU@CS; (b) X-ray diffraction spectra of TpTU and TpTU@CS通过分析TpTU@CS的氮吸附-解吸曲线(图3a)，发现其曲线特征符合Ⅳ型吸附等温线，表明该材料具有介孔结构。TpTU@CS的BET比表面积为82.138m2/g，TpTU@CS的多孔结构有利于提高传质速率和增加吸附位点的暴露量，使该材料在用于吸附去除重金属时，能够加快吸附速率并具有较强的富集能力。
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图 3 (a) TpTU@CS的N2吸附-解吸分析曲线；(b) TpTU@CS的孔径分布结果Figure 3. (a) N2 adsorption-desorption analysis curves of TpTU@CS; (b) Aperture distribution of TpTU@CS2.2 TpTU@CS对Hg(Ⅱ)的吸附等温线研究
为了考察材料的吸附容量，配制一系列不同浓度的Hg(Ⅱ)溶液，考察不同Hg(Ⅱ)浓度下材料的吸附等温线，结果如图4a所示。可以看出，当Hg(Ⅱ)浓度较低时，随着溶液中Hg(Ⅱ)浓度的增加，材料对Hg(Ⅱ)的吸附量也成比例増加；当Hg(Ⅱ)浓度升高到300mg/L，材料对Hg(Ⅱ)的吸附量基本到达平台，这主要是由于材料对Hg(Ⅱ)的吸附已逐渐趋于饱和。最终实验表明，材料对Hg(Ⅱ)的最大吸附容量为249.21mg/g。为进一步了解Hg(Ⅱ)在TpTU@CS材料表面的吸附模式，分别使用Langmuir和Freundlich对实验数据进行拟合。Freundlich模型能更好地拟合TpTU@CS等温线数据，说明Hg(Ⅱ)在TpTU@CS上更符合非均匀吸附。
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图 4 (a)Langmuir和Freundlich模型拟合结果；(b) 准一级动力学和准二级动力学模型拟合结果Figure 4. (a) Fitting results of Langmuir model and Freundlich model; (b) Fitting results of pseudo-first-order kinetics model and pseudo-second-order kinetics modelLangmuir模型的表达式为：
$$ Q_{\mathrm{e}}=\frac{K_{\mathrm{L}}C_{\mathrm{e}}Q_{\mathrm{m}}}{1+K_{\mathrm{L}}C_{\mathrm{e}}} $$ (3) 式中：Qe 表示平衡时吸附剂对Hg(Ⅱ)的吸附容量（mg/g）；Qm表示吸附剂对Hg(Ⅱ)的最大吸附量(mg/g)；Ce表示达到吸附平衡时溶液中剩余的Hg(Ⅱ)浓度(m/L)；KL代表Langmuir常数(L/mg)。
Freundlich模型的表达式为：
$$ {Q}_{\mathrm{e}}={K}_{\mathrm{F}}{{C}_{\mathrm{e}}}^{\tfrac{1}{n}} $$ (4) 式中：Qe 表示平衡时吸附剂对Hg(Ⅱ)的吸附容量（mg/g）；Ce表示达到吸附平衡时溶液中剩余的Hg(Ⅱ)浓度(mg/L)；KF代表Freundlich常数(mg/g)；n是Freundlich经验参数。
2.3 TpTU@CS对Hg(Ⅱ)的吸附动力学研究
除了吸附容量较大具有优势外，较快的吸附动力学在实际应用中也具有重要意义。因此，用5mg的TpTU@CS材料吸附5mL pH为3的20mg/L Hg(Ⅱ)标准溶液，以进行吸附动力学实验，结果如图4b所示。可以看出，当浓度较低时，随着溶液中Hg(Ⅱ)浓度的增加，材料对Hg(Ⅱ)的吸附增加并趋于平衡。TpTU@CS去除Hg(Ⅱ)只需5min即可达到平衡，显示了该材料吸附Hg(Ⅱ)快速的吸附速率。
为了进一步了解Hg(Ⅱ)在TpTU@CS材料表面吸附的速率控制步骤，分别采用准一级动力学模型和准二级动力学模型对实验数据进行拟合。
准一级动力学模型的表达式为：
$$Q_t=Q_{\mathrm{e}}\left[1-\exp \left(-k_1 \cdot t\right)\right] $$ (5) 式中：Qt表示TpTU在时间t时对Hg(Ⅱ)的吸附容量(mg/g)；Qe表示TpTU在该浓度下的平衡吸附容量(mg/g)；k1表示准一级动力学吸附速率常数(min−1)。
准二级动力学模型的表达式为：
$$Q_t=\frac{k_2 \cdot Q_{\mathrm{e}}{ }^2 t}{1+k_2 Q_{\mathrm{e}} t} $$ (6) 式中：Qt表示TpTU在时间t时对Hg(Ⅱ)的吸附容量(mg/g)；Qe表示TpTU在该浓度下的平衡吸附容量(mg/g)；k2表示准二级动力学吸附速率常数[g/(mg•min)]。
通常，准一级动力学模型假设扩散步骤控制吸附，而准二级动力学模型对应于受化学吸附机制影响的吸附过程。表1列出TpTU@CS吸附Hg(Ⅱ)时两个模型的拟合结果，表明TpTU@CS对Hg(Ⅱ)的吸附更符合准一级动力学。
表 1 TpTU@CS对Hg(Ⅱ)的吸附等温线和动力学拟合结果(25℃)Table 1. Adsorption isotherms and kinetic fitting results of TpTU@CS adsorption on Hg(Ⅱ) at 25℃吸附等温线 Freundlich吸附模型 Langmuir吸附模型 n KF (mg/g) R2 Qm (mg/g) KL (L/mg) R2 3.704 249.21 0.9659 227.52 0.79 0.9033 吸附动力学 拟一级动力学模型 拟二级动力学模型 k1 (min−1) Qe (mg/g) R2 k2 [g/(mg·min)] Qe (mg/g) R2 0.39 189.72 0.9938 0.003 202.53 0.9868 2.4 TpTU@CS对Hg(Ⅱ)吸附的重复使用性和选择性
使用盐酸酸化的硫脲溶液，考察了TpTU@CS的重复使用性。在重复吸附-解吸5次之内，吸附Hg(Ⅱ)去除率均维持在90%以上，说明吸附过程是可逆的，且所制备的材料具有良好的稳定性和可重复使用性。因此，TpTU@CS是一种可回收和有竞争力的吸附剂。同时，在初始浓度为10mg/L含Mg(Ⅱ)、Cr(Ⅲ)、Mn(Ⅱ)、Co(Ⅱ)、Ni(Ⅱ)、Cu(Ⅱ)、Zn(Ⅱ)、Cd(Ⅱ)、Pb(Ⅱ)和Hg(Ⅱ)的水溶液中，研究了TpTU对Hg(Ⅱ)吸附去除的选择性。如图5所示，TpTU@CS对Hg(Ⅱ)的吸附性能要明显高于其他离子，但由于共存离子的存在，与Hg(Ⅱ)产生竞争吸附，因此吸附效率有所下降。
2.5 TpTU@CS处理实际水样中Hg(Ⅱ)去除效果
将TpTU@CS用于处理实际水样。考虑到Hg(Ⅱ)可能存在于自然界不同水体中，且不同水体的基质也都有所不同。因此，为了考察不同水体中竞争离子和天然有机物对吸附效率是否存在影响，分别采集了溪水、湖水及池塘水三种样品。所采集的样品均含有一定初始浓度的Hg(Ⅱ)，对应的浓度分别为0.6、6.6和9.6μg/L。在该水平基础上，目标污染物可以被有效地去除且去除率达到100%。同时，为了进一步验证在高浓度污染情况下材料的应用效果，在相应样品中进行了不同浓度的加标实验。表2结果表明，对于1、2、5mg/L的Hg(Ⅱ)污染样品，Hg(Ⅱ)去除率可保持在77.0%～83.7%之间。证实了制备的TpTU@CS具有从实际水样中去除Hg(Ⅱ)的优越潜力。
表 2 TpTU@CS处理实际水样中Hg(Ⅱ)的吸附效果Table 2. Adsorption effect of Hg(Ⅱ) in real water samples pretreated by TpTU@CS实际水样 Hg(Ⅱ)浓度
(μg/L)Hg(Ⅱ)去除率
(%)Hg(Ⅱ)加标浓度
(1mg/L)Hg(Ⅱ)加标浓度
(2mg/L)Hg(Ⅱ)加标浓度
(5mg/L)去除率
(%)RSD
(n=3)去除率
(%)RSD
(n=3)去除率
(%)RSD
(n=3)溪水 0.64 100.0 83.4 0.51 83.7 1.56 78.7 1.19 湖水 6.61 100.0 77.4 0.83 81.5 1.18 77.0 1.33 池塘水 9.72 100.0 78.6 1.19 79.8 1.29 78.9 0.62 与其他材料相比，TpTU@CS对于Hg(Ⅱ)的吸附容量(249.21mg/g)处于中上水平，如富硫醚介孔聚合物S-CX4P的吸附容量为278mg/g［34］，巯基修饰的锆基MOF (Zr-DMBD) 的吸附容量为171.5mg/g［27］，双硫腙进行磁化功能化活性炭材料的吸附容量为255mg/g［16］。
3. 讨论
3.1 TpTU@CS对Hg(Ⅱ)的吸附机理分析
为了进一步研究TpTU@CS与Hg(Ⅱ)之间可能的吸附机理，测定了TpTU@CS吸附前后的高分辨率XPS谱，如图6和图7所示。根据设计的预期材料结构分析，Hg(Ⅱ)最有可能被TpTU@CS中的S元素捕获，XPS中S元素的精细光谱分析证实了这一猜想。在图7a中，通过比较吸附前后S2p的精细光谱峰拟合结果，发现分别代表C=S键S2p3/2化学状态的163.24eV处的结合能表现出0.06eV的红移，分别降低到163.18eV。在以往的报道中，由于金属离子的吸附，S2p轨道的结合能降低，范围为0.20eV至1.20eV。原因是由于S和Hg产生的共享电子对转移到了S，导致S在金属吸附后的结合能降低［24，35］。上述结果表明，Hg(Ⅱ)在TpTU@CS上的吸附主要发生在S原子上，其化学机制主要是S和Hg的键合效应，共价键的形成确保了TpTU@CS对Hg(Ⅱ)的优异吸附能力［36］。
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图 6 TpTU-CS和TpTU-CS@Hg的XPS全扫描光谱Figure 6. XPS full-scan spectra of TpTU-CS and TpTU-CS@Hg![]()
图 7 TpTU-CS@Hg的XPS S2p(a)、N1s(b)、O1s(c)和Hg4f(d)精细光谱Figure 7. XPS S2p(a), N1s(b), O1s(c) and Hg4f(d) fine spectra of TpTU-CS@Hg为了确定氮和氧是否对汞的捕获有所贡献，对N1s和O1s的XPS精细光谱进行了分析，如图7中b和c所示。通过对N1s精细光谱的分峰拟合，可以发现TpTU@CS中有两个结合能为400.64eV和397.61eV的峰，分别代表C−N和C=N的化学状态。吸附Hg(Ⅱ)后，C=N和C−N的峰均发生红移，移动到结合能400.39eV和397.73eV处。这表明碳氮中N原子与Hg(Ⅱ)的络合作用也是TpTU-CS吸附Hg的相互作用之一。O1s的精细光谱峰拟合结果表明，位于531.68eV和531.03eV处分别代表C−O和C=O的峰位置变化不大，表明O可能不参与Hg(Ⅱ)的吸附过程。据报道，在XPS表征中，HgCl2的Hg4f7/2轨道的结合能为101.4eV。然而在图7d中，当HgCl2被TpTU@CS吸附时，Hg4f7/2的结合能降低至103.25eV，这表明吸附过程中可能发生了电子转移。
总的来说，TpTU@CS捕获Hg(Ⅱ)的主要机制是C=S中S与Hg的键合以及C−N与Hg(Ⅱ)的配位相互作用。根据Lewis酸碱理论，只有作为Lewis软碱的S与Hg(Ⅱ)有较强的相互作用，而Hg(Ⅱ)正是典型的Lewis软酸。
值得注意的是，溶液的pH对材料吸附行为至关重要。适宜的pH不仅能提高吸附效率，而且可在一定程度上减少基质的干扰。Hg2+在碱性条件下易发生水解，当溶液pH＜3时，主要以Hg2+的形式存在；而当pH＞6时，主要以Hg(OH)2的形式存在；当pH介于3～6时，同时存在Hg2+、Hg(OH)+和Hg(OH)2三种形式。当溶液pH＜7时，金属离子主要通过吸附去除；pH＞7时，金属离子通过沉淀形成去除，因此研究了pH在2～7时TpTU@CS对Hg(Ⅱ)的吸附效果。在本实验测试条件下，即5mg TpTU@CS吸附剂吸附不同pH条件下5mL的10mg/L Hg(Ⅱ)，在所有测试的pH值中，TpTU@CS对Hg(Ⅱ)的去除率均达到了100%，证明Hg(Ⅱ)的吸附对pH的敏感性很低，这是由于−S−Hg的强配位性所致，该结果与其他报道的工作一致，展现出TpTU@CS对Hg(Ⅱ)优异的吸附能力。
3.2 TpTU@CS吸附Hg(Ⅱ)性能与其他材料对比
不同材料如多孔碳、水凝胶、金属有机骨架和共价有机骨架，对汞的吸附性能结果见表3。可以看出金属有机骨架和本文的复合材料具有更加优异的去除效率。这是因为它们所具有的高度有序的微孔/介孔结构有利于吸附质的扩散和活性位点的暴露，从而提高传质效率；结合吸附动力学的探究，TpTU@CS材料在10min内即可实现对Hg(Ⅱ)水溶液约100%的去除率，可归结为其规则的孔道结构(13nm)。在低浓度Hg(Ⅱ)下，汞离子与TpTU@CS材料表面足够的活性位点之间的强相互作用，大大降低了吸附剂和溶液中存在的传质阻力的影响。然而，大多数材料是粉末状态，这些粉末状的材料从水体中分离时较为困难，这将导致吸附剂损失和潜在的二次污染问题。本研究所制备复合材料的微球形态具有便于从水体中分离的优势。并且，TpTU@CS的吸附能力比文献中许多吸附剂要高得多，功能化改性的孔壁具备更丰富的活性基团，利用其与目标污染物之间的强亲合力进行吸附结合。另一方面，该材料精确可调的孔径尺寸可以增加对于污染物离子的吸附选择性，这一点在2.4节的研究中也有所体现。因此，本工作制备的TpTU@CS使用绿色便捷的制备方法，具有较高的吸附能力，更具有实际应用的吸引力。
表 3 不同吸附剂对Hg(Ⅱ)去除效果的比较Table 3. Comparison of removal effects of Hg(Ⅱ) pretreated with different adsorbents4. 结论
本工作成功地设计并制备了一种新型的、绿色环保的多孔有机聚合物/壳聚糖复合材料吸附剂，证明了基于烯醇-酮的互变异构的席夫碱反应制备具有不定性结构的多孔有机聚合物的可行性。通过表征对材料中的硫元素进行了确认，证明该合成方法能够使得S元素在材料中得到较大程度的保留。合成的材料具备介孔结构，材料表面为多孔的壳聚糖网络，能够提供较多的活性位点。壳聚糖的纤维状结构为TpTU粉末提供了稳定的支撑，使其在Hg(Ⅱ)吸附中能够更加简便地回收。吸附实验表明，材料对Hg(Ⅱ)的最大吸附容量为249.2mg/g，在10min内对Hg(Ⅱ)水溶液的去除率最高可达100%。TpTU@CS通过C=S中S与Hg的键合以及C−N与Hg(Ⅱ)的配位相互作用对水体中Hg(Ⅱ)进行快速捕获。通过吸附-解吸循环实验，证明该材料至少可重复使用5次。即使对复杂基质的实际水体样品，该材料对Hg(Ⅱ)离子的去除率可维持在80%。
本工作不仅为杂原子掺杂多孔有机聚合物的构建探索了一种创新合成方法，而且为选择性去除水中汞离子提供了一种有应用前景的吸附剂。基于该合成方法，未来可进一步探索设计引入其他掺杂元素或功能性官能团。但对于基质复杂且含有高浓度污染物的样品，该材料的应用仍存在一些限制。因此，制备更具选择性且抗干扰的吸附材料是未来研究的重点。
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图 1 TpTU@CS的SEM图像： (a) 200nm尺度形貌；(b) 100nm尺度形貌
Figure 1. SEM images of TpTU@CS: (a) 200nm morphology; (b) 100nm morphology
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图 2 (a) TpTU和TpTU@CS的傅里叶变换红外光谱；(b) TpTU和TpTU@CS的X射线衍射分析结果
Figure 2. (a) Fourier transform infrared spectra of TpTU and TpTU@CS; (b) X-ray diffraction spectra of TpTU and TpTU@CS
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图 3 (a) TpTU@CS的N2吸附-解吸分析曲线；(b) TpTU@CS的孔径分布结果
Figure 3. (a) N2 adsorption-desorption analysis curves of TpTU@CS; (b) Aperture distribution of TpTU@CS
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图 4 (a)Langmuir和Freundlich模型拟合结果；(b) 准一级动力学和准二级动力学模型拟合结果
Figure 4. (a) Fitting results of Langmuir model and Freundlich model; (b) Fitting results of pseudo-first-order kinetics model and pseudo-second-order kinetics model
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图 6 TpTU-CS和TpTU-CS@Hg的XPS全扫描光谱
Figure 6. XPS full-scan spectra of TpTU-CS and TpTU-CS@Hg
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图 7 TpTU-CS@Hg的XPS S2p(a)、N1s(b)、O1s(c)和Hg4f(d)精细光谱
Figure 7. XPS S2p(a), N1s(b), O1s(c) and Hg4f(d) fine spectra of TpTU-CS@Hg
表 1 TpTU@CS对Hg(Ⅱ)的吸附等温线和动力学拟合结果(25℃)
Table 1 Adsorption isotherms and kinetic fitting results of TpTU@CS adsorption on Hg(Ⅱ) at 25℃
吸附等温线 Freundlich吸附模型 Langmuir吸附模型 n KF (mg/g) R2 Qm (mg/g) KL (L/mg) R2 3.704 249.21 0.9659 227.52 0.79 0.9033 吸附动力学 拟一级动力学模型 拟二级动力学模型 k1 (min−1) Qe (mg/g) R2 k2 [g/(mg·min)] Qe (mg/g) R2 0.39 189.72 0.9938 0.003 202.53 0.9868 
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表 2 TpTU@CS处理实际水样中Hg(Ⅱ)的吸附效果
Table 2 Adsorption effect of Hg(Ⅱ) in real water samples pretreated by TpTU@CS
实际水样 Hg(Ⅱ)浓度
(μg/L)Hg(Ⅱ)去除率
(%)Hg(Ⅱ)加标浓度
(1mg/L)Hg(Ⅱ)加标浓度
(2mg/L)Hg(Ⅱ)加标浓度
(5mg/L)去除率
(%)RSD
(n=3)去除率
(%)RSD
(n=3)去除率
(%)RSD
(n=3)溪水 0.64 100.0 83.4 0.51 83.7 1.56 78.7 1.19 湖水 6.61 100.0 77.4 0.83 81.5 1.18 77.0 1.33 池塘水 9.72 100.0 78.6 1.19 79.8 1.29 78.9 0.62
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表 3 不同吸附剂对Hg(Ⅱ)去除效果的比较
Table 3 Comparison of removal effects of Hg(Ⅱ) pretreated with different adsorbents
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