磁性壳聚糖微球的改性研究进展及其在水处理中的应用

冯颖; 崔倩; 解玉鞠; 赵孟杰; 张建伟; 董鑫

doi:10.13801/j.cnki.fhclxb.20211105.003

磁性壳聚糖微球的改性研究进展及其在水处理中的应用

沈阳化工大学　机械与动力工程学院, 沈阳 110142

基金项目: 国家自然科学基金(21406142)；辽宁省自然基金(2020-MS-230)；辽宁省教育厅科学研究项目(LJ2020036)

详细信息

通讯作者:
董鑫，博士，讲师，硕士生导师，研究方向为环境流体多相流传递理论与技术装备　E-mail: dongxin1106@syuct.edu.cn

中图分类号: TQ424.3;
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出版历程
- 收稿日期: 2021-09-27
- 修回日期: 2021-10-21
- 录用日期: 2021-10-30
- 网络出版日期: 2021-11-07
- 刊出日期: 2022-05-31

Research progress on modification of magnetic chitosan microspheres and its application in water treatment

School of Mechanical and Power Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China

摘要

摘要: 磁性壳聚糖微球(Magnetic chitosan microsphere，MCM)是一种新型吸附材料，具有独特的磁响应特性和良好的吸附性能，以其突出的环保和可控性在生物医学、食品工程和污水处理等许多领域受到高度重视。传统方法制备的MCM存在纳米粒子易溶于酸性溶液、应用范围窄等问题，因此研究者们在其优化改性方面展开了大量工作。本文从磁性纳米粒子改性和壳聚糖改性两个方面详细综述了优化MCM的研究进展，包括磁性纳米粒子的修饰与替换，壳聚糖分子印迹改性、接枝改性、金属螯合改性、烷基化改性等方法。总结了改性后MCM对废水中重金属离子、印染废料中阴阳离子染料的吸附情况和去除效果。最后讨论了改性MCM面临的问题与挑战，展望了其未来发展趋势，提出了进一步提高改性MCM应用效率的方法和设想。
- 壳聚糖微球 /
- 磁性纳米粒子 /
- 重金属 /
- 吸附剂 /
- 改性方法 /
- 废水
Abstract: Magnetic chitosan microsphere (MCM) is a new type of adsorption material, which has unique magnetic response characteristics and good adsorption performance. With its outstanding environmental protection and controllability, it has attracted high attention in many fields such as biomedicine, food engineering, sewage treatment and so on. MCM prepared by traditional methods has some problems, such as nanoparticles are easy to dissolve in acidic solution and narrow application range. Therefore, researchers have carried out a lot of work in its optimization and modification. In this paper, the research progress of optimizing MCM was reviewed in detail from two aspects: magnetic nanoparticles modification and chitosan modification, including modification and replacement of magnetic nanoparticles, chitosan molecular imprinting modification, grafting modification, metal chelation modification, alkylation modification and so on. The adsorption and removal effects of modified MCM on heavy metal ions in wastewater and ionic dyes in printing and dyeing waste were summarized. Finally, the problems and challenges faced by modified MCM were discussed. Its future development trend was prospected, and the methods and ideas to further improve the application efficiency of modified MCM were put forward.
- chitosan microspheres /
- magnetic nanoparticles /
- heavy metals /
- adsorbents /
- modification method /
- wastewater

HTML全文

果蔬、食品、乳酸菌、血液、疫苗等对环境温度敏感的物质，采用冷链物流耗能较大，而利用材料在相变过程中储存或释放能量调节周围环境的温度，可以改进并解决其保质运输与储存的问题。相变储能材料(PCMs)储能密度大、相变温度稳定、装置简单、设备灵活，能快速解决能源在时间及空间上不匹配的矛盾^[1-2]，在军工、航天、纺织、制冷设备、食品及农产品等领域有广泛的工程应用价值^[3-9]。在冷链物流包装中，大多数果品贮藏温度为0~8℃，且具有相变温度稳定性好、相变潜热高等优点。相变储能材料按照材料类别可以分为无机相变储能材料、有机相变储能材料及共晶相变储能材料^[10-11]。无机相变材料具有较高导热性和相变潜热，主要应用于中高温材料系统^[12]，但是普遍存在严重的过冷和相分离现象^[13]。有机相变储能材料基本没有过冷度及相分离现象、化学性能稳定且成本低，但是储能密度相对较低，主要应用于中低温材料系统^[14-15]。共晶相变储能材料是由两种或两种以上的成分组成的低共熔物，在一定程度上克服了有机相变储能材料与无机相储能材料的局限性，但在导热性能、循环稳定性和储热性能方面仍需改善^[10]。

目前常用的有机相变储能材料主要包括石蜡、酯、脂肪酸、醇和烷烃等，固体成型性好、不易燃、不易发生过冷和相分离现象^[16-18]。脂肪酸的过冷度小，有可逆的融化和凝固性能，是性能良好的有机相变储能材料。月桂酸、正癸酸、棕榈酸、肉豆蔻酸及它们的共混物是应用比较多的相变材料^[19-22]。石蜡是一种固-液相变材料，具有相变潜热高、无过冷和相分离、熔点低、化学性质稳定、价格低廉等优点，通常用来改善正十二烷、正十四烷等烷烃的导热性能或熔点^[23-24]。在0℃左右的有机相变储能材料中，正癸醇结构简单、相变潜热较高、性质稳定，是一种适合的相变材料，通常将其与其他材料复配得到可调节相变温度的材料^[17]。

由于单一有机相变储能材料存在相变温度不可调、相变潜热较低等缺点，无法满足冷链物流用相变储能材料熔点为0~8℃、相变潜热高的要求，且部分有机相变储能材料价格昂贵，无法在生产生活中大量应用。因此，本文选用正癸酸、月桂酸甲酯、正癸醇、月桂酸及十四烷，通过物理共混法制备正癸酸-月桂酸甲酯、正癸酸-正癸醇、月桂酸-十四烷三种二元有机复配物，并针对二元有机复配物在相变过程中的泄漏问题利用凝胶对其进行吸附以期获得适用于果品保质包装与物流技术的有机相变储能材料。

1. 实验部分

1.1 原材料

正癸酸，99%，分析纯；正癸醇，98%；月桂酸，99%，优级纯；十四烷，98%；月桂酸甲酯，99%；N-异丙基丙烯酰胺，98%；N,N'-亚甲基双丙烯酰胺(MBA)，99%；过硫酸铵，≥98%，分析纯；四甲基乙二胺，99%；聚乙二醇1000；聚乙二醇4000，化学纯；聚乙二醇8000，分析纯；聚乙二醇10000，上海阿拉丁生化科技股份有限公司。聚乙二醇200，化学纯，北京国药集团化学试剂有限公司。

1.2 样品制备

1.2.1 凝胶的制备方法

利用在氧化还原体系下引发单体进行原位自由基聚合的方法制备凝胶。首先，选择1 mol/L的N-异丙基丙烯酰胺(NIPAM)作为单体，MBA为交联剂，为单体总质量的0.5%，聚乙二醇1000(PEG1000)为致孔剂，为单体总质量的40%，将NIPAM、MBA、PEG1000共同溶解于去离子水中。其次，在过硫酸铵(APS)和四甲基乙二胺(TEMED)氧化还原体系中保持20℃引发聚合反应24 h，形成以PEG1000作致孔剂的聚N-异丙基丙烯酰胺(PNIPAM)凝胶，凝胶的命名规则为PNIPAM-y%PEGx，x代表聚乙二醇的分子量，y代表聚乙二醇占NIPAM质量的百分比(表1)。例如，PNIPAM-40%PEG1000为以PEG1000作为致孔剂的PNIPAM凝胶，且致孔剂含量为NIPAM质量的40%。制备过程如图1所示。APS广泛应用于水相自由基聚合，TEMED作为促进剂用于加快聚合反应的进行。第三步，将制备好的凝胶在去离子水中浸泡三天，每6 h换一次去离子水，以去除多余的单体及不参与反应的PEG1000，将浸泡处理之后的PNIPAM取出并放置于−25℃的冰箱中冷冻24 h，随后将冷冻好的PNIPAM凝胶放进冷冻干燥机(LGJ-10C 型，江苏金坛市环宇科学仪器厂)冷冻干燥72 h，冷肼温度为−55~−50℃，真空度为0.5~50 Pa。

表 1 聚N-异丙基丙烯酰胺(PNIPAM)-聚乙二醇(PEG)凝胶的命名

Table 1. Naming of poly(N-isopropylacrylamide) (PNIPAM)-polyethylene glycol (PEG) gel

Sample	Mass ratio of PEG∶NIPAM/%	Molecular weight of PEG
PNIPAM-y%PEGx	y	x
Note: NIPAM—N-isopropylacrylamide.

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| 显示表格

图 1 PNIPAM-40%PEG1000凝胶制备过程

Figure 1. PNIPAM-40%PEG1000 gel preparation process

PEG1000—Polyethylene glycol 1000; APS—Ammonium peroxodisulphate; TEMED—Tetramethylethylenediamine

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1.2.2 二元有机复配物的制备方法

依据混合物凝固点下降原理，利用两种单一物质的物理共混加热法制备二元有机复配物，均匀的二元有机复配物体系的FTIR图谱如图2所示。首先，称取一定摩尔比的正癸酸、月桂酸甲酯于透明玻璃瓶中，将其置于60℃的恒温水浴锅中融化10 min，正癸酸融化完全。其次，用旋涡震荡仪(XW-80A型，佛山予华仪器科技有限公司)混合10 min，再用超声波震荡仪将混合物震荡2 min，正癸酸与月桂酸甲酯充分混合，得到正癸酸-月桂酸甲酯二元有机复配物体系，二元有机复配物体系没有分离现象，正癸酸-正癸醇及月桂酸-十四烷二元有机复配物体系采用相同的方法制备。

图 2 二元有机复配物FTIR图谱

Figure 2. FTIR spectra of binary organic compound

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1.2.3 相变材料的制备方法

NIPAM的分子链上同时具有亲水性的酰胺基团和亲油性的异丙基团。当NIPAM通过自由基聚合形成PNIPAM-40%PEG1000凝胶，具有大分子网状结构，凝胶在液体中可显著溶胀。将冷冻干燥后的PNIPAM-40%PEG1000凝胶浸泡于二元有机复配物中进行溶胀。PNIPAM-40%PEG1000凝胶在二元有机复配物中的溶胀过程实际上是两种相反趋势的平衡过程，溶剂分子试图渗透到网络内部，使得凝胶体积溶胀从而导致三维分子网络的伸展平衡，交联点之间的分子链的伸展降低了它的构象熵值，分子网络的弹性收缩力使网络收缩。当两种相反的倾向互相抵消时，达到溶胀平衡，凝胶的溶胀程度用溶胀率(Swelling ratio，SR)表示。PNIPAM-40%PEG1000凝胶在三种二元有机复配物中溶胀(水浴60℃)，得到PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯、PNIPAM-40%PEG1000/正癸酸-正癸醇和PNIPAM-40%PEG1000/月桂酸-十四烷三种相变储能材料。

1.3 性能测试与结构表征

1.3.1 傅里叶红外测试

参考GB/T21186—2007^[25]“傅里叶变换红外光谱仪”的实验方法，利用傅里叶红外扫描仪(Nicolet 6700型，赛默飞世尔Thermo-fisher科技(中国)有限公司)对正癸酸、月桂酸甲酯、正癸醇、月桂酸、十四烷、正癸酸-月桂酸甲酯、正癸酸-正癸醇及月桂酸-十四烷进行红外测试，将单一有机物的图谱与二元有机复配物的图谱进行对照，考察二元有机复配物是否有新的吸收峰生成，判断两种单一有机物的化学相容性。

1.3.2 差示扫描量热分析测试

参考GB/T 19466.3—2004^[26]“塑料差示扫描量热法(DSC)”的实验方法，利用差示扫描量热仪(TGA/DSC1型，梅特勒-托利多国际贸易(上海)有限公司(METTLER TOLEDO))对正癸酸、月桂酸甲酯、正癸醇、月桂酸、十四烷、正癸酸-月桂酸甲酯、正癸酸-正癸醇及月桂酸-十四烷进行DSC测试，测定其相变初始温度、相变终止温度及相变潜热。称取10 mg左右的样品放置于铝坩埚中，精确度为0.01 mg，用压样机进行压制，实验参比侧放置了一个标准的空铝坩埚。将温度从30℃降温至−30℃(速度：20℃/min)，再升温至30℃(速度：20℃/min)，这个升降温过程重复进行两次，去除材料的热历史，将物质在30℃恒温保持5 min，再降温至−30℃得到冷冻放热曲线(速度：5℃/min)，然后再升温至30℃得到融化吸热曲线(速度：5℃/min)。月桂酸、正癸酸这两种物质的温度设置为0~80℃。DSC升降温速率是由N₂作用控制的，其中N₂作为反应气的速率为20 mL/min，N₂作为保护气的速率为150 mL/min。月桂酸甲酯、正癸醇、十四烷、正癸酸-月桂酸甲酯及正癸酸-正癸醇的测试温度为−30~30℃，而正癸酸、月桂酸及月桂酸-十四烷的测试温度为−30~50℃。

参考GB/T 14797.3—2008^[27]“浓缩天然胶乳硫化胶乳溶胀度的测定”的实验方法，对凝胶在二元有机复配物的溶胀性能进行测试。将冷冻干燥好的凝胶称重(万分之一天平，AR224CN型，奥豪斯仪器有限公司)W₁，将其浸入二元有机复配物中，在60℃的水浴环境下充分溶胀24 h，过程中每6 h测量一次凝胶的重量W₂，并计算凝胶在二元有机复配物中的溶胀率Q：

$Q=\frac{{W}_{2}-{W}_{1}}{{W}_{1}}\times 100{\%}$

(1)

其中：Q为凝胶在二元有机复配物中的溶胀率；W₁、W₂为凝胶溶胀前、后的质量。

2. 二元有机复配物热物性能估算

基于表2所示的正癸酸、月桂酸甲酯、正癸醇、月桂酸和十四烷的热物性数据，利用凝固点下降定律、热力学第二定律和相平衡关系，评估两种或多种物质共混时的相变温度。

表 2 有机物的热物性数据

Table 2. Thermophysical data of organic matter

Sample	Phase transition temperature/℃	Latent heats of phase transition/ (J·g⁻¹)
Decanoic acid	31.39	153.72
Methyl laurate	4.74	179.25
1 decanol	6.13	200.31
Lauric acid	44.54	181.14
Tetradecane	5.68	215.85

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利用Schroder Van Laar公式从理论上估算二元有机复配物体系的相变温度，即

$\left\{ \begin{aligned} & {T = \dfrac{1}{{\dfrac{1}{{{T_{\rm{A}}}}} - \ln{X_{\rm{A}}}\dfrac{R}{{\Delta {H_{\rm{A}}}}}}}}\\& {T = \dfrac{1}{{\dfrac{1}{{{T_{\rm{B}}}}} - \ln{X_{\rm{B}}}\dfrac{R}{{\Delta {H_{\rm{B}}}}}}}}\\& {{X_{\rm{A}}} + {X_{\rm{B}}} = 1} \end{aligned} \right.$

(2)

其中：T为二元有机复配物的最低共熔点温度(K)；X_A、X_B为组分A、组分B是摩尔分数(%)；T_A、T_B为组分A、组分B的理论熔点(K)；R为气体常数且取值为8.315 (J·mol⁻¹·K⁻¹))；ΔH_A、ΔH_B为组分A、组分B的相变焓(J·mol⁻¹)。

对于表2所列的单一有机物的热物性数据，由式(2)分别得到五种物质的Schroder Van Laar公式：

正癸酸-月桂酸甲酯：

$\left\{ \begin{aligned} & {\ln{X_{\rm{A}}} = 10.46 - 3184.58/T}\\& {\ln{X_{\rm{B}}} = 16.63 - 4620.83/T}\\& {{X_{\rm{A}}} + {X_{\rm{B}}} = 1} \end{aligned}\right.$

(3)

正癸酸-正癸醇：

$\left\{ \begin{aligned} & {\ln{X_{\rm{A}}} = 10.46 - 3184.58/T}\\& {\ln{X_{\rm{B}}} = 13.64 - 3809.57/T}\\& {{X_{\rm{A}}} + {X_{\rm{B}}} = 1} \end{aligned} \right.$

(4)

月桂酸-十四烷：

$\left\{ \begin{aligned} & {\ln{X_{\rm{A}}} = 13.47 - 4369.92/T}\\& {\ln{X_{\rm{B}}} = 18.47 - 5150.03/T}\\& {{X_{\rm{A}}} + {X_{\rm{B}}} = 1} \end{aligned} \right.$

(5)

通过MATLAB软件分别计算上述方程组(3)~(5)，获得正癸酸-月桂酸甲酯、正癸酸-正癸醇和月桂酸-十四烷二元复合物体系的理论共熔点及对应的摩尔比，如表3所示。显然，这三种二元有机复配物的理论最低共熔点在−1~4℃的范围内，接近于目标相变温度0~8℃，故可进一步通过实验分析来探究它们的热物性能。

表 3 二元有机复配物最低共熔点

Table 3. The lowest common melting point of binary organic compound

Sample	The lowest common melting point/℃	Molar ratio
Decanoic acid-methyl laurate	−0.86	29∶71
Decanoic acid-1 decanol	−0.74	29∶71
Lauric acid-tetradecane	3.58	13∶87

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| 显示表格

3. 实验结果与讨论

3.1 二元有机复配物的热物性能

图2为正癸酸、月桂酸甲酯、正癸醇、月桂酸、十四烷及二元有机复配物(正癸酸-月桂酸甲酯、正癸酸-正癸醇、月桂酸-十四烷)的FTIR图谱，由单一有机材料物理共混加热得到二元有机复配物，其FTIR图谱中并没有新的官能团生成，这说明二元有机复配物各组分之间能够共存，即共混过程不发生化学反应。

图3和图4分别描述了不同二元有机复配物的差示扫描量热曲线及热物性数据，“↑exo”表示放热方向， ${T}_{0}$ 、 ${T}_{1}$ 、ΔH分别表示起始温度、终止温度和相变焓。通过对比分析这两幅图，可获得二元有机复配物的热物性能。

图 3 二元有机复配物的差示扫描量热曲线

Figure 3. Differential scanning calorimetry curves of binary organic compound

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图 4 二元有机复配物的热物性数据

Figure 4. Thermophysical data of binary organic compound

T₀—Initial temperature; T₁—Final temperature; ΔH—Enthalpy of phase change

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对于正癸酸-月桂酸甲酯二元有机复配物，当组分摩尔比在3∶97~30∶70范围内，月桂酸甲酯能够溶解正癸酸，差示扫描量热曲线光滑且只有一个吸热峰。当组分摩尔比在33∶67~66∶34范围内，月桂酸甲酯饱和，无法溶解过量的正癸酸，在室温下有不溶物质，正癸酸-月桂酸甲酯的差示扫描量热曲线出现第二个吸热峰，如图3(a)、图3(b)和图3(c)所示。当组分摩尔比在3∶97~66∶34范围内，二元有机复配物相变初始温度在1.62~3.48℃的范围内变化，波动比较小，相变潜热处于165.35~193.40 J/g之间。当正癸酸与月桂酸甲酯的摩尔比为30∶70，相变初始温度为1.62℃、相变终止温度为8.15℃、相变焓为193.40 J/g，如图4(a)所示，此时相变潜热最高，性价比最高，相变初始温度和相变终止温度合适，适用于果品保质包装与物流技术。

对于正癸酸-正癸醇二元有机复配物，当组分摩尔比在3∶97~36∶64范围内，正癸醇能够溶解正癸酸，差示扫描量热曲线光滑且只有一个吸热峰。当组分摩尔比在39∶61~66∶64范围内，正癸醇饱和，无法溶解过量的正癸酸，在室温下有不溶物质，正癸酸-正癸醇的差示扫描量热曲线出现第二个吸热峰，如图3(d)和图3(e)所示。当组分摩尔比在3∶97~66∶34范围内，二元有机复配物的相变初始温度在−2~8℃范围内变化，相变潜热处于154.97~198.27 J/g之间。相变初始温度先从4.6℃降低到−2℃，当正癸酸摩尔比达到24%时，相变初始温度开始上升到0℃以上，最后稳定到4℃左右，波动较大。正癸酸-正癸醇二元复配物的相变初始温度范围广、相变潜热较高、规律性较明显。当正癸酸与正癸醇的摩尔比为36∶64，相变初始温度为3.80℃、相变终止温度为11.72℃、相变焓为180.94 J/g，如图4(b)所示，此时相变潜热较高，性价比最高，相变初始温度和相变终止温度合适，亦适用于果品包装。

对于月桂酸-十四烷二元有机复配物，当组分摩尔比在3∶97~21∶79范围内，十四烷能够溶解月桂酸，差示扫描量热曲线光滑且只有一个吸热峰。当组分摩尔比在24∶76~30∶70，十四烷饱和，无法溶解过量的月桂酸，在室温下有不溶物质，月桂酸-十四烷的差示扫描量热曲线出现第二个吸热峰，如图3(f)所示。二元有机复配物的相变初始温度为5~6℃且波动最小，相变潜热为205.80~217.94 J/g。十四烷成本较高，它与月桂酸的复配有利于大幅度降低成本。当月桂酸与十四烷的摩尔比为21∶79，相变初始温度为5.51℃、相变终止温度为26.74℃、相变焓为216.46 J/g，如图4(c)所示，此时相变潜热较高，性价比最高，也适用于果品包装。

3.2 相变储能材料的热物性能

3.2.1 MBA交联剂含量对溶胀程度的影响

PNIPAM凝胶在二元有机复配物的溶胀程度取决于pH值、温度以及凝胶的交联强度等，选用单体浓度分别为0.5 mol/L、1 mol/L、1.5 mol/L和2 mol/L的PNIPAM凝胶进行溶胀，以探究单体浓度对溶胀程度的影响。由于NIPAM浓度为0.5 mol/L的凝胶成胶性弱，无法得到成型性好的凝胶，故无法进行溶胀实验。如图5(a)所示，单体浓度为1 mol/L的PNIPAM凝胶具有最高的溶胀度，在其他条件不变时，单体浓度越小，越有利于凝胶对二元有机复配物的吸附。选用交联剂含量分别为NIPAM质量的0.25%、0.5%、0.75%和1%的PNIPAM凝胶进行溶胀，进一步探究MBA交联剂含量对溶胀程度的影响。图5(b)所示的实验结果表明，交联剂含量为NIPAM质量的0.25%的PNIPAM凝胶具有最高的溶胀度，在其他条件不变时，交联剂越少，越有利于凝胶对二元有机复配物的吸附，但是交联剂含量为NIPAM质量的0.25%的PNIPAM凝胶在溶胀之后过于柔软，形状稳定性不好。因此制备PNIPAM凝胶的单体浓度为1 mol/L，交联剂含量为NIPAM质量的0.5%。

图 5 PNIPAM凝胶在二元有机复配物中的溶胀度

Figure 5. Swelling ratios of PNIPAM gels in binary organic compounds

C—Concentration

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3.2.2 PEG作致孔剂对PNIPAM-y%PEGx凝胶物化性能的影响

在PNIPAM凝胶的制备过程中加入致孔剂PEG，得到PNIPAM-y%PEGx凝胶，可以有效增加凝胶中胞元的尺寸。致孔剂聚乙二醇在PNIPAM-y%PEGx凝胶形成的过程中并不参与反应，只是占据一定的空间，在凝胶形成之后，可以通过去离子水浸泡法去除致孔剂聚乙二醇。图6为PNIPAM、PNIPAM-40%PEG1000两种凝胶的FTIR图谱，显然两种凝胶的FTIR图谱相同。其中3300 cm⁻¹为NH的伸缩振动，2945 cm⁻¹为聚合物主链上饱和CH₃的振动吸收，1650 cm⁻¹属于C=O的双键伸缩振动(酰胺I带)，1540 cm⁻¹属于CNH的摇摆振动(酰胺II带)，1387 cm⁻¹和1459 cm⁻¹为CH₃的吸收峰，在PN₁₀₀₀40凝胶的FTIR图谱中1150~1050 cm⁻¹处没有观察到典型的COC吸收，说明PNIPAM-40%PEG1000凝胶中没有PEG1000，PEG1000分子仅在凝胶聚合过程中充当致孔剂，没有参与PNIPAM-40%PEG1000凝胶的形成反应，在反应完成后通过在去离子水中浸泡除去。

图 6 PNIPAM和PNIPAM-40%PEG1000凝胶的FTIR图谱

Figure 6. FTIR spectra of the gel PNIPAM and PNIPAM-40%PEG1000

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3.2.3 PEG分子量及含量对溶胀程度的影响

凝胶PNIPAM与凝胶PNIPAM-y%PEGx内部含有丰富的微胞结构，如图7所示，从凝胶PNIPAM及PNIPAM-y%PEGx的SEM图像(S4800场发射扫描电镜冷场，S4800型，日立Hitachi公司)可以看出，PEG分子量越大，形成胞元的尺寸越大，但是溶胀度不会随着其一直增大，溶胀结果见图8所示。实验结果表明，PNIPAM-40%PEG1000凝胶在二元有机复配物中具有最高的溶胀率：在正癸酸-月桂酸甲酯中，PNIPAM-40%PEG1000凝胶的溶胀度可以达到53.31%；在正癸酸-正癸醇中，PNIPAM-40%PEG1000凝胶的溶胀度可以达到53.70%；在月桂酸-十四烷中，PNIPAM-40%PEG1000凝胶的溶胀度可以达到52.47%。这主要是由于当PEG的分子量为4000、8000和10000时，由于微胞尺寸较大且毛细管力较弱，造成微胞之间导热性能较低且凝胶吸附能力差；当不含PEG或PEG分子量为200时，凝胶为微胞单元较小，吸附二元有机复配物的能力较差，相应所得的有机相变储能材料的相变潜热也较小。凝胶完全浸没在二元有机复配物中成为一个整体，整个微胞呈现出有些透明凝胶状态，这表明此时二元有机复配物的质量超过了凝胶的饱和吸附量。用自封袋分别装入100 g的PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯、PNIPAM-40%PEG1000/正癸酸-正癸醇及PNIPAM-40%PEG1000/月桂酸-十四烷，并进行冷冻处理，随后将其放置于30℃的恒温恒湿箱中8 h(模拟夏季高温情况)，最后从自封袋中取出相变储能材料并进行称重，质量损失率分别为2.8%、5.7%和5.1%，说明PNIPAM-40%PEG1000凝胶能够有效减少二元有机复配物的泄露量。

图 7 PNIPAM-y%PEGx凝胶的SEM图像

Figure 7. SEM images of the PNIPAM-y%PEGx gels

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图 8 PNIPAM-y%PEGx凝胶在二元有机复配物中的溶胀性能

Figure 8. Swelling performance of PNIPAM-y%PEGx gel in binary organic compounds

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3.2.4 相变温度和相变潜热

相变温度和相变潜热是决定相变储能材料能否应适用于冷链物流的重要条件。通过PNIPAM-y%PEGx凝胶对二元有机复配物进行吸附得到相变储能材料，添加致孔剂PEG可以提高凝胶内部微胞的尺寸，提高凝胶的溶胀率，但是胞元越大，凝胶的毛细管作用力就越小，因此凝胶的溶胀率不会随着致孔剂分子量及含量的增加而一直增加，如图8所示。PNIPAM-40%PEG1000凝胶在三种二元有机复配物中的溶胀效果最好，相变潜热最高，如图9所示。这三种相变储能材料的热物性能为：(1) 正癸酸-月桂酸甲酯(摩尔比30∶70)对应的相变储能材料为PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯，其相变初始温度为3.2℃、相变焓为188.10 J/g；(2) 正癸酸-正癸醇(摩尔比36∶64)对应的相变储能材料为PNIPAM-40%PEG1000/正癸酸-正癸醇相变储能材料，其相变初始温度为1.2℃、相变焓为177.74 J/g；(3) 月桂酸-十四烷(摩尔比21∶79)对应的相变储能材料为PNIPAM-40%PEG1000/月桂酸-十四烷相变储能材料，其相变初始温度为4.2℃、相变焓为206.17 J/g。

图 9 PNIPAM-y%PEGx/二元有机复配物的相变焓

Figure 9. Enthalpy of phase transition of PNIPAM-y%PEGx/binary organic compounds

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4. 结论

(1) 通过物理共混法得到三种相变温度适当、相变潜热较高的二元有机复配物：正癸酸-月桂酸甲酯、正癸酸-正癸醇和月桂酸-十四烷，这三种二元有机复配物无过冷和相分离现象、相变温度范围是0~5℃，且具有可逆的熔化和凝固性能，满足果品保鲜包装的要求。

(2) 由于二元有机复配物在相变过程中会有液相生成，不可避免地会发生泄漏，利用聚N-异丙基丙烯酰胺(PNIPAM)凝胶的吸油性可以对三种二元有机复配物进行吸附，可以减少其在应用过程中的泄漏量。

(3) 当在PNIPAM凝胶的制备过程中加入致孔剂聚乙二醇1000(PEG1000)，凝胶在二元有机复配物的溶胀度提高，制备出三种适用于果品保质包装与物流技术的有机相变储能材料：PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯、PNIPAM-40%PEG1000/正癸酸-正癸醇及PNIPAM-40%PEG1000/月桂酸-十四烷相变储能材料。

图 1 磁性壳聚糖微球(MCM)的制备过程

Figure 1. Preparation of magnetic chitosan microspheres (MCM)

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图 2 分子印迹聚合物制备过程(a, b, c, d, e代表模板分子上与功能单体的作用点)

Figure 2. Preparation process of molecularly imprinted polymer (a, b, c, d and d represent the interaction point between the template molecule and the functional monomer)

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图 3 不同接枝方式

Figure 3. Different grafting methods

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图 4 壳聚糖分子间氢键

Figure 4. Intermolecular hydrogen bond of chitosan

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表 1 常用交联剂的结构及特点

Table 1 Structures and characteristics of common crosslinking agents

Crosslinker	Structure	Advantage	Shortcoming	Ref.
Glutaraldehyde		Aldehyde group combines with the amino group to form a new nitrogen-carbon double bond by Schiff base reaction and cross-linking, resulting in a stable structure and increased hydrophobic strength	Two carbonyl groups are far away from each other, the crosslinking site is bound and the spatial site resistance is large, and the product is not highly crystalline	[14-16]
Sodium tripolyphosphate		Non-toxic, amino protonation cross-linked with phosphate ions to improve the chemical stability of the substance while introducing phosphate	At higher pH, the product surface is negatively charged and the phosphate dissociates, reducing the adsorption capacity	[17-19]
Glyoxal		Amino group at the C2 position reacts with the carbonyl group in the Schiff reaction, and the hydroxyl group at the C6 position reacts with the carbonyl group in the acetal reaction, which is conducive to improving the fiber strength of the product	—NH² generates —NH₃⁺, which is not suitable for strong acid environment because it is not conducive to Schiff reaction.	[20-21]
Epichlorohydrin		Carbon and chlorine bonds are broken and the hydroxyl group becomes ether after epoxy ring opening, so the cross-linking efficiency is high and the product is more stable; and the amino group is not occupied, so more sites are combined with pollutants	Suitable for alkaline environment, the preparation process should be cross-linked before neutralizing with acid	[22-23]

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表 2 不同材料对磁性Fe₃O₄纳米粒子的修饰效果

Table 2 Modification effects of different materials on magnetic Fe₃O₄ nanoparticles

Modifying substance	Modification effect	Experimental result	Advantage	Ref.
Oleic acid	It is spherical and becomes larger in size and evenly dispersed	When the reaction temperature was 31℃, 400 nm microspheres were prepared, showing high sensitivity	Reduce surface energy without changing crystal structure and improve magnetic nanoparticle dispersion	[24]
SiO₂	SiO₂uniformly coated magnetic nanoparticles	When pH=2.0, the maximum adsorption capacity of Cr(Ⅵ) is 236.4 mg·g⁻¹	Improve the acid resistance of adsorbent, reduce the content of magnetic particles, and still ensure the smooth separation	[25]
Attapulgite	Nanoparticles were successfully attached to the rod	When pH is adjusted to 0.97%, the maximum adsorption rate of Pd⁴⁺ and Pd²⁺ is 0.98%	High density adsorption of iron ions on the rocks, while retaining a large number of adsorption sites for polyelectrolytes	[26]
Phase change material	Phase change material and magnetic iron oxide were encapsulated in chitosan microspheres at the same time	Enthalpy of crystallization at 30 and 32°C and the enthalpy of melting at 43℃ dropped to 75.0 and 73.0 J·g⁻¹, respectively	Combination of the two substances as a core effectively enriches the functionality and applicability of chitosan, while increasing the magnetic	[27]

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表 3 部分新型粒子替换Fe₃O₄ 粒子的实例

Table 3 Examples of some new particles replacing Fe₃O₄

Magnetic particle	Product characteristic	Experimental result	Ref.
CoFe₂O₄	Uniform dispersion of magnetic particles, excellent magnetic response, good adsorption characteristics	At the end of adsorption, the solution was nearly colorless and the maximum desorption efficiency was about 92.85%	[28]
MnFe₂O₄	Does not damage the structure of the position itself, and at the same time has a synergistic effect on the adsorption performance of chitosan and MnFe₂O₄	Maximum removal efficiency of MB at pH=6.0 and initial concentration of 671.4 mol·L⁻¹ was 96.4%, and the maximum adsorption capacity was about 85.3 mg·g⁻¹	[29]
Co_0.5NiFe₂O₄	Excellent core-shell structure and magnetic response performance	Maximum decolorization rate of cationic dyes reached over 95.0% at a dose of 20 mg·g⁻¹	[30]
Magnetic graphene oxide	Preparation of composite products of graphene oxide and iron oxide for solid-liquid separation by external magnetic field and high adsorption of pollutants	Average adsorption capacity of the product was 80.8 mg·g⁻¹; the maximum desorption efficiency was 86.3 mg·g⁻¹ at 40℃ and pH=6.8	[31]

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表 4 磁性壳聚糖微球(MCM)分子印迹改性

Table 4 Molecular imprinting modification of magnetic chitosan microspheres (MCM)

Template molecule	Crosslinking agent	Advantage	Results of control experiments	Results of control experiments	Ref.
Cu²⁺	Epichlorohydrin	Cu²⁺ cross-linked with I-CM, breaking the coordination of —NH₂ and —OH, decreasing crystallization ability and acid solubility	Surface leveling of the blank control was completely soluble in acetic acid, and the adsorption rate was about 36.7%	At 80℃ and pH=5.0, the initial mass concentration of Cu²⁺ was 338.7 mg·L⁻¹ and the product adsorption amount was 72.8 mg·g⁻¹	[33]
2,4,6 Trichlorophenol	Ethylene glycol dimethacrylate	A large number of binding sites to improve product stability and recognition	Maximum adsorption of the product was about 58.0 mg·g⁻¹ at 25℃ and pH=2.0-6.0	Maximum adsorption of the product was about 75.0 mg·g⁻¹ at 25℃ and pH=2.0-6.0	[34]
Uranyl ion	N. N- Methylene bisacrylamide	C=C bond interacts with uranyl ion and the binding energy increases, electrostatic attraction between phosphoric acid and lanthanum, effective adsorption of uranium by phosphoric acid and N-isopropylacrylamide	Maximum adsorption capacity was 105.0 mg·g⁻¹ at 25℃ and pH=6.0	At 25℃ and pH=6.0, the maximum adsorption amount was 232.0 mg·g-1 and U(VI) was reduced to U(IV)	[35]
Piroxicam (PIX)	Ethylene glycol dimethyl acetamide	Cavity fits to the target template, PIX has high affinity to the active site and forms stable hydrogen bonds with the acrylic group	pH=4.0, magnetization saturation value of 20.0 emu·g⁻¹, low adsorption efficiency	pH=4.0, maximum adsorption efficiency of 42.3 mg·g⁻¹, and magnetization saturation value of 38.0 emu·g⁻¹	[36]
Deep eutectic solvent	Glutaraldehyde	Increased specific surface area for efficient selective identification and binding to cavities	20-60℃, the average adsorption capacity is about 15.0 mg·g⁻¹, which is about 1.9 times that of MgO	20-60℃, the average adsorption capacity is 80.8 mg·g⁻¹, which is about 11.0 times that of MgO	[31]

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表 5 MCM的接枝改性

Table 5 Graft modification of MCM

Grafting material	Product preparation	Removal of ions	Control experiment	Experimental result	Ref.
Glutamine	Glutamine-modified magnetic chitosan microspheres	Acid green 25 (AG25), Mercury ions	Maximum adsorption of G25 at pH=2.0 was 460.0 mg·g⁻¹ and the maximum adsorption of Hg²⁺ at pH=6.0 was 80.0 mg·g⁻¹	Maximum adsorption of G25 was 900.0 mg·g⁻¹ at pH=2.0 and 140.0 mg·g⁻¹ for Hg²⁺ at pH=6.0	[42]
Ammonium persulfate (NH4)	Aminated magnetic chitosan microspheres	Methylene blue (MB), Brilliant red (RBR)	Adsorption amount for MB was 105.0 mg·g⁻¹ at pH=12.0 and 450.0 mg·g⁻¹ for RBR at pH=10.0	Maximum adsorption amounts for MB and RBR were 210.9 and 638.7 mg·g⁻¹ at pH=12.0, respectively	[43]
1,6-hexanediamine	1, 1,6-hexanediamine-functionalized magnetic chitosan microspheres (AF-MCTS)	Cr(Ⅵ)	Maximum adsorption capacity was 110.0 mg·g⁻¹	Maximum adsorption capacity was 208.3 mg·g⁻¹	[44]

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表 6 壳聚糖烷基化改性的不同位置优缺点

Table 6 Advantages and disadvantages of different positions of chitosan alkylation modification

Route	Modification position	Advantage	Disadvantage
O-site alkylation	Hydroxyl groups at C3 and C6 positions	Amino group has a polycationic character, decreases crystallinity, weakens intermolecular hydrogen bonds, and interacts with negative charges to inhibit bacteria and sterilization	Amino group is more active than the hydroxyl group, and the reaction starts with the amino group at the C2 position, which is not easy to prepare
N-position alkylation	Amino in the C2 position	Intermolecular hydrogen bonds are broken, regularity is reduced, crystallinity of the molecule is decreased, new functional groups are introduced, and the complex has new properties	Special nature of arsenate, solution alkaline good, the removal of metal ions when the solution pH plays an important role, the two need to coordinate
O, N-position alkylation	Amino group at C2 position, hydroxyl group at C3 and C6 positions	Adding substances with different biocompatibility differences, hydrophobicity changes, and simpler preparation process	Introduction of large substituents reduces intermolecular hydrogen bonds and changes hydrolysis ability

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表 7 MCM做吸附剂处理印染废水

Table 7 Treatment of printing and dyeing wastewater with MCM as adsorbent

Adsorbent	Dye	Principle of action	Experimental result	Repeat regeneration experiment	Ref.
β-cyclodextrin modified magnetic chitosan microspheres	Methylene blue dye	Electrostatic adsorption between -OH and methylene blue	Maximum adsorption capacity was 123.7 mg·g⁻¹ and the maximum adsorption capacity was positively correlated with pH and negatively correlated with temperature	Adsorption amounts of the first and third experiments were 123.7 mg·g⁻¹ and 115.9 mg·g⁻¹, and the decolorization rates were 93.7% and 92.7%, respectively	[54]
Silver particle-modified magnetic chitosan microspheres	Composite dye system	The —NH₂ and —OH of the shell layer are protonated and deprotonated significantly at different acid and alkaline levels, and are easily bound to anionic and cationic dyes	At 35℃ and pH=4.0, the maximum dye adsorption was 271.2 mg·g⁻¹; the maximum removal rate of dye was 99.5%.	Adsorption rates of dyes were 99.0%, 95.0% and 91.0% for the three repeated regeneration experiments, respectively	[55]
Polyacryloyloxyethyl trimethyl ammonium chloride grafted magnetic chitosan microspheres	Sunset yellow dye	Quaternary ammonium group has increased hydrophilicity, and the quaternary ammonium group can be electrostatically attracted to sunset yellow with or without -NH2 protonation	Maximum adsorption capacity at 25℃ and pH=2.0 was 787.1 mg·g⁻¹	Adsorption rate was 96.2% after five adsorption elution cycles	[56]
Sr_3.8Fe_25.7O_70.4 Chitosan magnetic particles	Crystalline Violet (CV) Alkaline Red (BR9)	Initial concentration difference provides resistance to two-phase mass transfer and promotes dye adsorption through hydrogen bonding, electrostatic adsorption	Maximum removal rates of CV and BR9 at 30-40℃ and pH>7 are 94.5% and 97.5%, respectively, and are highly efficient and environmentally friendly.	Dye removal rate remains above 90.0% after five cycles	[57]

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表 8 MCM作吸附剂处理重金属离子

Table 8 Treatment of heavy metal ions with MCM as adsorbent

Adsorbent	Heavy Metal	Principle of action	Experimental result	Repeat Regeneration Experiment	Ref.
Magnetic chitosan microspheres	P	Adsorption of porous carbon films depends mainly on electrostatic effect	Maximum adsorption amount was 4.8 mg·g⁻¹ at pH=7.0	After treatment and reuse, the adsorption effect is almost unaffected	[62]
Quaternary ammonium magnetic chitosan microspheres	Cr，P	Adsorption mechanism of chromium and phosphorus is mainly electrostatic interactions	At 25°C and pH=6.0, the saturation adsorption amounts of P and Cr were 416.0 and 419.0 mg·g⁻¹, respectively	After performing four adsorption-desorption cycles, the adsorption effect was not affected by	[63]
Novel cross-linked chitosan magnetic beads modified with cysteinyl urea Schiff base	Cu、Cr	Acidic environment -NH₂ and -C=N- have high nitrogen content to facilitate chelation, and both protonate and electrostatically adsorb with CRO₄^2- and HCrO₄⁻	The maximum adsorption amounts of most Cr at pH=2.0 and pH=5.0 were 156.5 and 138.5 mg·g⁻¹, respectively	After three adsorption-desorption cycles, the adsorption efficiency of metal ions was still higher than 91.0%	[64]
Magnetic carboxymethyl chitosan composite microspheres	Mn²⁺	pH=6.5-7.5 to prevent the generation of manganese hydroxide precipitation; electrostatic adsorption of surface adsorption sites with Mn²⁺	At 25°C and pH=7.0, the adsorption capacity of Mn²⁺ was 75.7 mg·g⁻¹ and the adsorption rate was higher than 90.0%	After five elution tests, the adsorption efficiency for Mn²⁺ was still 77.9%	[65]
Nanoporous magnetic cellulose-chitosan composite microspheres	Cu	Hydrogen bonding and miscibility between the two effectively suppress the crystal structure; copper and nitrogen atoms are chemically bonded	Adsorption amount was 65.8 mg·g⁻¹ at pH=5.0	Five cycles of experiments, the adsorption capacity of Cu, no significant decline	[47]
Magnetic Fe₃O₄ chitosan microspheres	Ag	Electrostatic interaction causes surface cations to form complexes with electron-rich organic ligands	Extraction efficiency of lakes and wastewater ranged from about 84.9% to 98.8%	Average extraction efficiency was still 77.2% after multiple repetitions of the experiment	[66]

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表 9 在不同领域作吸附剂的应用实例

Table 9 Application examples of adsorbent in different fields

Adsorbent	Adsorbed material	Mechanism of action	Experimental result	Ref.
PEI-modified magnetic chitosan microspheres	Ibuprofen	Regular spherical shape, narrow particle size distribution, successful introduction of a large number of amino	Maximum adsorption capacities of the three composites with different ratios were 89.3, 100.0 and 138.6 mg·g⁻¹, respectively	[67]
Magnetic chitosan microspheres	Apple juice organic acid	Acid ions form ionic bonds with protonated amino groups	At 25℃, the average adsorption capacity was 112.4 mg·g⁻¹ and the saturation capacity was 188.7 mg·g⁻¹	[68]
Molecularly imprinted polymer modified magnetic group microspheres	Chloramphenicol	Hydrogen bonds, ionic bonds make strong interactions between molecules	30-40℃, the maximum adsorption capacity is 32.5 mg·g⁻¹	[69]
Quaternary ammonium-functionalized magnetic chitosan microspheres	Beet juice coloring agent	Electrostatic adsorption of quaternary ammonium groups and impurities, ion exchange reaction between Cl⁻ and quaternary ammonium cations	Maximum adsorption efficiency was 99.4% and the maximum adsorption capacity was 127.2 mg·g⁻¹	[70]
Novel magnetic chitosan microspheres	Lysozyme	Cavity-specific recognition of lysozyme	25℃, the maximum adsorption capacity is 130.0 mg·g⁻¹	[71]

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