摘要
本研究在氧化淀粉(OS)和聚乙烯醇(PVA)的存在下,通过甲基丙烯酰氧基乙基三甲基氯化铵(DMC)和丙烯酰胺(AM)间简单的自由基聚合,制备了力学性能良好、可重复循环使用的双网络水凝胶。当OS溶液质量分数9%时,制备的水凝胶性能最佳,其压缩模量可达2.01 MPa,润胀度仅11.16 g/g,在25℃下对pH值8,浓度200 mg/L的亚甲基蓝溶液吸附480 min时吸附量最高,达602.6 mg/g。经过5次吸附-脱附过程后,双网络水凝胶对亚甲基蓝的去除率仍高达94.8%,脱附率74.2%。
染料是废水中一种常见的污染物,广泛用于食品、纺织、化妆品、印刷和造纸等行
水凝胶是一类具有三维网状结构的聚合物,通过交联形成的聚合物网络往往包括大量亲水基团,如羟基、胺基、酰胺基和羧基等,这些基团能够吸收水而不易分解,在水净化方面具有较大优
由于氧化淀粉(OS)具有丰富的含氧官能团如羰基、羧基等,使其易于交联并且不易分解,但鲜有学者将其用于废水吸附处
氧化淀粉(OS)购自东莞东美食品有限公司;1799H型聚乙烯醇(PVA)购自山东优索化工科技有限公司;亚甲基蓝(MB)、氢氧化钠(NaOH)、丙烯酰胺(AM)、甲基丙烯酰氧基乙基三甲基氯化铵(DMC)、过硫酸铵、N,N-亚甲基双丙烯酰胺(MBA)、盐酸、氨水均购自天津大茂化学试剂厂。
将不同质量OS溶解于质量分数5% NaOH溶液中,得到质量分数为3%、5%、7%、9%、10%和11%的OS溶液;将PVA置于水中,80℃加热至完全溶解,得到质量分数5%的PVA溶液。随后将10 mL PVA溶液和10 mL OS溶液搅拌混合均匀后加入3 g DMC和9 g AM,通入氮气以去除气泡。最后加入0.15 g MBA作为交联剂和0.06 g过硫酸铵作为引发剂,60℃下加热12 h。随后反复洗涤以去除游离的单体和聚合物,获得OS-PVA/P(DMC-AM)双网络水凝胶。根据OS溶液质量分数,制备的双网络水凝胶分别命名为OS3、OS5、OS7、OS9、OS10、OS11。
将10 mL质量分数9%的OS溶液和10 mL质量分数5%的PVA溶液混合,加入9 g AM,通入氮气以去除气泡。最后加入0.15 g MBA作为交联剂和0.06 g过硫酸铵作为引发剂,60℃下加热12 h。随后反复洗涤以去除游离的单体和聚合物,制得OS-PVA/PAM水凝胶。
在20 mL 5%NaOH溶液中加入3 g DMC和9 g AM,充分溶解,通入氮气以去除气泡。最后加入0.15 g MBA作为交联剂和0.06 g过硫酸铵作为引发剂,60℃下加热12 h。随后反复洗涤以去除游离的单体和聚合物,制得P(DMC-AM)水凝胶。
将10 mL质量分数9%的OS溶液和10 mL质量分数5%的PVA溶液混合均匀后置于-20℃冰箱冷冻2 h,然后在常温下解冻2 h,反复洗涤以去除游离的单体和聚合物,即可形成OS-PVA水凝胶。
将OS-PVA/P(DMC-AM)双网络水凝胶浸入25℃的水中,待润胀平衡后取出,用湿滤纸除去水凝胶表面多余水分,称取水凝胶质量。润胀度(SR)由
(1) |
式中,wt表示润胀后水凝胶质量,w0表示润胀前水凝胶质量。
制备1 g/L MB溶液,稀释至实验所需浓度。吸附实验在锥形瓶中进行,并置于25℃恒温摇床(HNY,荣耀公司)中以120 r/min速度振荡。对吸附完成的溶液通过紫外可见分光光度计(Cary5000,安捷伦)在λmax=664 nm处测量其吸光度,计算其浓度。实验分别研究了水凝胶质量、pH值、温度和MB溶液初始浓度对吸附的影响。吸附量和去除率的计算如
(2) |
(3) |
式中,qe为吸附量,mg/g;q为去除率,%;C0为MB溶液的初始浓度,mg/L;Ce为吸附平衡时MB溶液的浓度,mg/L;V为MB溶液的体积,L;M为水凝胶质量,g。
25℃下,将10 mg不同浓度OS溶液制备的水凝胶(OS3、OS5、OS7、OS9、OS10、OS11)放入50 mL、浓度50 mg/L、pH值为8的MB溶液中吸附480 min,可以得到吸附平衡后不同OS溶液质量分数水凝胶的MB去除率。
25℃下,称取10 mg OS9水凝胶,加入到50 mL、浓度50 mg/L、pH值分别为2、4、6、8、10的MB溶液中进行480 min的吸附实验(通过氨水和盐酸来调节MB溶液的pH值),研究MB溶液pH值对吸附的影响。
25℃下,分别称取25、50、75、100、150和200 mg OS9水凝胶,加入到50 mL、浓度300 mg/L、pH值为8的MB溶液中进行480 min的吸附实验,研究水凝胶的质量对吸附的影响。
15、25和35℃下称取10 mg OS9水凝胶,分别加入到50 mL、pH值为8、浓度分别为20、50、100、150、200 mg/L的MB溶液中吸附480 min,研究温度和MB溶液初始浓度对吸附的影响。
15、25和35℃下称取10 mg OS9水凝胶,加入到50 mL、300 mg/L、pH值为8的MB溶液中,分别进行0~480 min的吸附实验,以完成吸附动力学研究。
水凝胶良好的压缩性能有助于循环吸附以及吸附后水凝胶的回收。

图1 不同质量分数OS溶液对OS-PVA/P(DMC-AM)双网络水凝胶的压缩模量、润胀度和吸附性能的影响
Fig. 1 Effect of concentration of OS solution on compression modulus, swelling and adsorption properties of OS-PVA/P(DMC-AM) double network hydrogel
吸附剂材料若是润胀度过高,会导致其在吸附染料的同时吸附大量水,不利于吸附的进行。从
在25℃下,不同质量分数OS溶液制备的水凝胶对MB的去除率如


图2 OS9水凝胶的SEM图
Fig. 2 SEM images of OS9 hydrogel

图3 OS-PVA、P(DMC-AM)、OS-PVA/PAM和OS9水凝胶的FT-IR图
Fig. 3 FT-IR spectra of OS-PVA、P(DMC-AM)、OS-PVA/PAM and OS9 hydrogel

图4 pH值对吸附量的影响
Fig. 4 Effect of pH value on the adsorption capacity

图5 水凝胶质量对MB的吸附量和去除率的影响
Fig. 5 Effect on the adsorption capacity and removal rate of OS9 hydrogel dosage towards MB

图6 MB初始浓度和温度对吸附量的影响
Fig. 6 Effect of initial MB concentration and temperature on the adsorption capacity

图7 不同温度下水凝胶吸附性能随时间的变化
Fig. 7 Effect on the adsorption capacity of adsorption time of OS9 hydrogel towards MB at different temperature
吸附动力学可以用来描述吸附量随时间的变化情况、吸附速率和吸附机理。伪一级动力学模型一般对应物理吸附,吸附速率主要由扩散决定;伪二级动力学模型则对应化学吸附,吸附之间的相互作用(如静电作用)是决定吸附速率的重要因素。本研究通过伪一级动力学模型和伪二级动力学模型来确定吸附机
(5) |
(6) |
式中,qt是t时刻的吸附量;kl是伪一级吸附速率常数;k2是伪二级吸附速率常数。通过对OS9水凝胶对MB的吸附过程进行伪一级动力学和伪二级动力学模型线性拟合,得到
Langmuir等温吸附模型认为同一吸附剂表面上饱和的吸附质是单层覆盖且均匀吸附,Freundlich等温吸附模型则假设在不均匀的吸附位点上存在多层覆盖。Langmuir和Freundlich等温吸附模型如
(7) |
(8) |
式中,qmax是理论最大吸附量;KL和KF分别为Langmuir和Freundlich吸附常数。此外,作为一个无量纲的均衡参数,RL的定义如
(9) |
根据Langmuir和Freundlich等温吸附模型线性拟合结果可得到
本研究通过两步一锅法制备了一种氧化淀粉基双网络水凝胶,对亚甲基蓝(MB)具有良好吸附性能且可循环重复使用。在OS溶液质量分数为9%时,制备的双网络水凝胶性能最佳。水凝胶润胀度仅为11.16 g/g,压缩模量可达2.01 MPa。水凝胶在25℃、pH值为8、MB溶液浓度200 mg/L、吸附时间480 min时,对MB的最大吸附量为602.6 mg/g,吸附遵从伪二级动力学模型和Langmiur等温吸附模型。此外,水凝胶在经过5次循环吸附-解附实验后对MB去除率仍可高达94.8%,脱附率74.2%。
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