Fabrication of polyvinylidene fluoride blending membranes filled by La-TiO2-reduced graphene oxide with photocatalytic activity
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摘要: 为提高聚偏氟乙烯(PVDF)超滤膜的通量及抗污染性能,首先利用吸附相反应技术耦合乙醇热处理制备La掺杂TiO2-还原氧化石墨烯(La-TiO2-RGO),再将其与PVDF共混制备La-TiO2-RGO/PVDF抗污染超滤膜。结果表明,均匀分散于PVDF高分子中表面亲水的La-TiO2-RGO增多,La-TiO2-RGO/PVDF共混膜的水通量和抗污染性能也显著提升。当La-TiO2-RGO/PVDF共混膜中出现团聚体,则会削弱其膜通量和抗污染性。在La-TiO2-RGO填充量(与PVDF质量比)为2.0%时,La-TiO2-RGO/PVDF共混膜具有最优纯水通量。La-TiO2-RGO/PVDF共混膜最高纯水通量可达171.5 L·m−2·h−1,是PVDF膜的5倍以上,其通量衰减速率也明显低于PVDF膜。另外,由于La-TiO2-RGO具有可见光催化活性,被污染后的La-TiO2-RGO/PVDF共混膜经过光照处理后用水清洗,其膜通量恢复率较直接用水清洗后的通量恢复率大幅提高;热处理温度升高,La-TiO2-RGO弱可见光活性增强,光照后La-TiO2-RGO/PVDF共混膜通量恢复率变大。但过高热处理温度抑制了La-TiO2-RGO中Ti3+形成,且削弱其光活性,La-TiO2-RGO/PVDF共混膜通量恢复率反而下降;对于La-TiO2-RGO填充量为2.0%的La-TiO2-RGO/PVDF共混膜,被污染后分别采用直接水清洗、仅光照处理2 h、先光照处理2 h后水清洗的膜通量恢复率分别为79.28%、52.42%、90.01%。
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关键词:
- 聚偏氟乙烯(PVDF)共混膜 /
- 抗污染性 /
- 通量恢复率 /
- 弱可见光催化活性 /
- La掺杂TiO2-还原氧化石墨烯
Abstract: To improve the flux and antifouling performance of polyvinylidene fluoride (PVDF) ultrafiltration membrane, La doped TiO2-reduced graphene (La-TiO2-RGO) was first synthesized by the adsorption phase reaction coupled with alcohol solvothermal reduction processes. Followed that, La-TiO2-RGO was blended with PVDF to fabricate a La-TiO2-RGO/PVDF ultrafiltration membrane with high anti-fouling performance. The results show that the water flux and antifouling performance of the La-TiO2-RGO/PVDF blending membrane increase, when more well-distributed La-TiO2-RGO with hydrophilic groups added. The aggregations generate in the La-TiO2-RGO/PVDF blending membranes will depress their water flux and antifouling performance. When the loading content (mass ratio to PVDF) of La-TiO2-RGO is 2.0%, the La-TiO2-RGO/PVDF blending membrane has the best pure water flux. The optimum pure water flux of the La-TiO2-RGO/PVDF blending membrane reaches 171.5 L·m−2·h−1, which is 5 times as high as that of the PVDF membrane. And the flux decay rate of the La-TiO2-RGO/PVDF blending membrane is significantly lower than that of the PVDF membrane. The flux recovery of the contaminated La-TiO2-RGO/PVDF blending membrane treated by illumination then washing is obviously higher than that just treated by washing, due to the addition of La-TiO2-RGO with photocatalytic activity. The increase in the solvothermal temperature enhances the photocatalytic activity of La-TiO2-RGO, thus improving the flux recovery rate of the La-TiO2-RGO/PVDF membranes after light irradiation. However, too high solvothermal temperature inhibites the formation of Ti3+ in La-TiO2-RGO, which weakens its photoactivity and decreases the flux recovery rate of the resulting La-TiO2-RGO/PVDF blending membranes. For the La-TiO2-RGO/PVDF blending membrane filled with La-TiO2-RGO loading of 2.0%, the flux recovery rates of contaminated membrane are 79.28%, 52.42% and 90.01%, respectively, after washing, illumination for 2 h and illumination for 2 h then washing. -
超滤膜可以有效去除水中多种污染物,如悬浮颗粒、微生物和大分子有机物等,是废水处理及回用、饮用水安全保障等领域中最具潜力的技术之一[1-3]。聚偏氟乙烯(PVDF)是目前最常用的超滤膜材料,具有热稳定性和化学稳定性好、易成膜等特点[4-6]。但PVDF是一种典型的非极性、疏水性高分子材料,过滤时水中的有机污染物、微生物及大分子有机物等极易沉积于膜平面和膜孔隙内,从而降低其分离性能,增加运行成本[6-7]。目前,引入亲水性组分(或基团)是提升PVDF膜的亲水性和增强膜的抗污染性能的共识之一[6-7]。常用的亲水性组分(或基团)包括亲水性高分子、两亲性高分子及表面亲水的无机纳米材料(无机纳米粒子、碳纳米管和氧化石墨烯)[8-10]。而表面包覆、化学接枝改性和共混是三种引入亲水组分(或基团)的主要方法,其中共混法由于易于成膜而备受关注[11-13]。近年来,通过共混引入表面亲水且具有光催化活性和抗菌功能的无机纳米粒子(如TiO2、Ag纳米粒子等),来提升PVDF膜的亲水性能和抗污染性能是构建高性能超滤膜研究的热点之一[14-16]。
围绕提升PVDF超滤膜亲水性和抗污染性能的研究虽取得了较大进展,但采取任何方法和措施都不能消除膜的污染,运行一段时间后膜仍会被微生物、大分子有机物污染,还要对污染后的膜进行清洗,膜的运行→污染→清洗→再运行→,如此循环往复。因此如何提升膜的抗污染性能、延长膜的清洗周期、减缓污染膜清洗后的通量衰减是提升超滤膜的运行效能和拓展超滤膜的应用领城的关键问题[14-16]。将具有光催化活性、表面富含羟基的TiO2纳米粒子引入PVDF高分子材料中,不仅可以显著提升PVDF共混膜的亲水性,改善共混膜的力学性能和抗污染性。且可以利用共混膜中TiO2对微生物、大分子有机物等的光催化作用而达到抗污染效果,进一步降低膜处理成本[17-18]。但构建包含TiO2纳米粒子超滤膜还需要面临两方面挑战,首先TiO2纳米粒子在高分子中的分散性。小粒径TiO2纳米粒子与PVDF的亲和力不强,难以保持纳米粒子在PVDF中的分散性[19-20]。课题组前期工作中发现,TiO2纳米粒子经氧化石墨烯(GO)负载后形成的复合物在较高填充量下仍能在PVDF中均匀分散,所制备的共混膜具备较高水通量和良好的抗污染性能[21]。另一方面,在填充光催化活性组分共混膜的研究中,往往采用几百瓦功率的紫外光或可见光灯(几百瓦的高压汞灯或氙灯),高光强的光源(光强大于1 W/cm2)长时间照射下高分子材料会被一定程度的破坏[17-18]。在课题组对弱可见光催化过程研究的基础上[22],本文提出将弱可见光响应的La掺杂TiO2-还原氧化石墨烯(La-TiO2-RGO)作为填充组分,与PVDF构建La-TiO2-RGO/PVDF共混膜。利用La-TiO2-RGO表面亲水基团提高PVDF膜亲水性和抗污染性能,同时再通过弱可见光激发La-TiO2-RGO的光催化作用来去除黏附和吸附在膜表面上及孔隙中的大分子有机物和微生物,进一步提升La-TiO2-RGO/PVDF共混膜的抗污染性能和清洗效果,拓展PVDF超滤膜的应用。
1. 实验材料和方法
1.1 原材料
聚乙二醇2000(PEG2000)、N,N-二甲基乙酰胺(DMAc)、牛血清白蛋白(BSA)、钛酸四丁酯(C16H36O4Ti)、无水乙醇、乙二醇、硝酸镧(La(NO3)3),均为分析纯,上海晶纯化学试剂有限公司;聚偏氟乙烯(PVDF,FR904) ,上海三爱富公司。
1.2 La-TiO2-还原氧化石墨烯(RGO)的制备
氧化石墨烯(GO) 制备、吸附相反应制备TiO2-GO及La3+掺杂过程详见文献[22]。取0.5 g GO、200 mL无水乙醇和1.5 mL去离子水置于三口烧瓶中,30℃下搅拌,直至达到吸附平衡。另取2.15 g钛酸四丁酯和0.77 mg 硝酸镧溶解于50 mL无水乙醇中,然后把该乙醇溶液滴加到已达吸附平衡的GO体系中,钛酸四丁酯通过扩散作用到达GO表面,在吸附层中水解反应形成TiO2,反应5 h后,离心得到La掺杂TiO2-氧化石墨烯(La-TiO2-GO),其中La的掺杂量为0.05%(与TiO2的原子比)。
超声作用下将La-TiO2-GO均匀分散于120 mL无水乙醇中形成悬浮液,然后将该悬浮液放入200 mL聚四氟乙烯烧杯中,再将聚四氟乙烯烧杯加盖后转移至反应釜中,反应釜密封后放入鼓风干燥箱中,分别在160℃、170℃、180℃下热还原处理24 h。热处理后停止加热,自然冷却至室温,打开反应釜取出聚四氟乙烯烧杯,将杯中的悬浮液离心、干燥得到不同温度热还原处理的La-TiO2-RGO粉末,分别标记为La-TiO2-RGO(160℃)、La-TiO2-RGO(170℃)、La-TiO2-RGO(180℃)。
1.3 La-TiO2-RGO/PVDF共混膜的制备
共混膜的制备过程参考前期TiO2-GO/PVDF共混膜的工作[21]。分别量取二份等体积的DMAc放入圆底烧瓶中。称取适量的La-TiO2-RGO放入其中一圆底烧瓶中,超声分散。称取适量的PVDF和PEG2000放入另一圆底烧瓶中,搅拌溶解。将两烧瓶中的溶液混合,60℃的油浴中搅拌混合8 h,再自然冷却至室温,静置8 h后得铸膜液。选用0.22 μm聚丙烯平板膜作支撑层,支撑层先用50wt%乙醇溶液浸润再晾干待用。室温下将铸膜液倾倒在支撑层上,再用刮刀刮出150 μm厚度的初生液膜,液膜在空气中静置约40 s后,浸入30℃、20wt%的DMAc水溶液中凝胶成膜,然后将膜放入纯水中浸泡24 h,使用前再用纯水反复冲洗。La-TiO2-RGO/PVDF共混膜中所填充的La-TiO2-RGO及其铸膜液组成如表1所示。表1中数据还列出了通过单轴拉升实验获得的各个膜最大拉伸强度Emax,发现添加La-TiO2-RGO后的PVDF膜力学性能得到提高。
表 1 La-TiO2-还原氧化石墨烯(RGO)/聚偏氟乙烯(PVDF)共混膜各组分含量Table 1. Contents of components of La-TiO2-reduced graphene oxide(RGO)/ polyvinylidene fluoride(PVDF) blending membranesMembrane La-TiO2-RGO VDMAc/
mLMass of
PVDF/gMass of
PEG2000/gMass ratio of La-TiO2-
RGO to PVDF/%Emax/
MPaPVDF — 50 7.5 2.5 0 75.3 La-TiO2-RGO(160)/PVDF-1 La-TiO2-RGO(160) 50 7.5 2.5 1.0 86.7 La-TiO2-RGO(160)/PVDF-2 La-TiO2-RGO(160) 50 7.5 2.5 2.0 98.9 La-TiO2-RGO(160)/PVDF-3 La-TiO2-RGO(160) 50 7.5 2.5 3.0 88.1 La-TiO2-RGO(170)/PVDF-1 La-TiO2-RGO(170) 50 7.5 2.5 1.0 87.2 La-TiO2-RGO(170)/PVDF-2 La-TiO2-RGO(170) 50 7.5 2.5 2.0 101.1 La-TiO2-RGO(170)/PVDF-3 La-TiO2-RGO(170) 50 7.5 2.5 3.0 89.5 Notes: Emax—Maximum tensile strength of membrane; La-TiO2-RGO(160)/PVDF-(1/2/3)—Blending membrane in which mass ratio of La-TiO2-RGO(160) to PVDF is 1%, 2%, 3%, respectively, La-TiO2-RGO(170)/PVDF-(1/2/3) is also understood like this; VDMAc—Volume of N,N-dimethyl acetamide. 1.4 共混膜的表征
采用日本Rigaku公司的D/max-rA转靶X 射线多晶衍射仪对La-TiO2-GO进行XRD 分析,采用CuKα (λ=0.15406 nm),功率为1 600 W (40 kV×40 mA)。采用美国FEI公司的FEI-Tecnai-G20型高分辨透射电子显微镜(HRTEM)对La-TiO2-GO进行分析。采用岛津的T.hermo Nicolet IR200型FTIR表征La-TiO2-GO的化学结构。采用美国Thermo Fisher Scientific公司的Thermo ESCALAB 250型XPS对La-TiO2-GO进行分析,X射线激发源为单色AlKa (hν=1 486.6 eV),功率为150 W,X射线束斑500 μm能量分析器固定透过能为30 eV。采用辰华CHI 660E电化学工作站进行光电性能测试,瞬态光电流测试在0.5 V偏压下进行,使用Pt电极为对电极,Ag/AgCl电极为参比电极。实验使用的光源为300 W氙灯,并配有AM 1.5 G滤光片。采用SEM (荷兰,Phenom G2 pro)对共混膜的表面和断面形貌进行表征,表征前首先将共混膜在液氮中脆断用镀金处理。采用美国KONO公司的SL200B静态接触角测量仪对共混膜表面亲水角进行测试。
共混膜的孔隙率(ε)利用文献和前期工作中的干湿重法来进行测定[21,23]。共混膜的平均孔径(rm)利用过滤测速法测量[24]。
1.5 共混膜通量及其通量的恢复率测试和计算
共混膜通量测试采用msc-300超滤杯(超滤杯的容积为0.1 m3),以死端过滤方式进行。测试膜的面积(A)为30 cm2。
膜纯水通量测试:在0.10 MPa (N2)下先用纯水对样品膜预压30 min,保持压力为0.10 MPa不变,每隔一段时间测量膜的纯水透过体积,重复测量3次后取平均值。纯水通量Jw计算如下:
Jw=VwAt 式中:Vw—纯水透过体积;A—膜面积;t—测试时间。
膜牛血清蛋白(BSA)溶液通量测试:在0.10 MPa压力(N2氛围)下先用纯水对膜预压30 min,将料液换成1 mg·mL−1的BSA溶液,保持压力为0.1 MPa,分别在没有光照和20 W 的LED灯光照情况下,每隔一段时间测量膜的BSA溶液透过体积,重复测量3次后取平均值,膜的溶液通量Jp计算如下:
Jp=VpAt 式中,Vp—BSA溶液透过体积。
被污染膜清洗后的纯水通量测试:实验中1 mg·mL−1的BSA溶液作为进料液,进行30 min超滤分离过程的膜作为被污染的膜。被污染膜分别采用以下三种方式进行清洗:(1)纯水清洗3次;(2)直接用20 W的LED灯光照2 h;(3)先用20 W的LED灯光照2 h后纯水清洗3次。被污染膜分别按3种清洗方式清洗后,再按纯水通量测试方法测试膜纯水通量,计算如下:
Jr=VrAtJE=VEAtJEr=VErAt 式中:Vr—清洗方式(1)下纯水透过体积;VE—清洗方式(2)下纯水透过体积;VEr—清洗方式(3)下纯水透过体积;Jr—清洗方式(1)下纯水通量;JE—清洗方式(2)下纯水通量;JEr—清洗方式(3)下纯水通量。
被污染膜清洗后通量恢复率(R)计算如下:
R=JrJw×100%RE=JEJw×100%REr=JErJw×100% 式中:R—清洗方式(1)下通量恢复率;RE—清洗方式(2)下通量恢复率;REr—清洗方式(3)下通量恢复率。
2. 结果与讨论
2.1 La-TiO2-RGO的微观形貌及结构
图1为La-TiO2-RGO(160)、La-TiO2-RGO(170)、La-TiO2-RGO(180)的TEM和HRTEM图像。图2为La-TiO2-RGO(160)、La-TiO2-RGO(170)、La-TiO2-RGO(180)的XRD图谱。由图1并结合前期工作可知[22],不同热还原处理温度下得到的La-TiO2-RGO的TEM图像中深色物是TiO2粒子,显纱状、浅色物是GO或RGO,La掺杂及热还原处理均不影响TiO2粒子和GO (或RGO)的分散性及形貌。从HRTEM图像还可以看出,经乙醇热还原处理后,La-TiO2-RGO中GO或RGO表面上负载有显晶格条纹结晶态的TiO2纳米粒子。由图2可知,经乙醇热还原处理后,La-TiO2-RGO中GO或RGO表面上负载有显晶格条纹结晶态的TiO2纳米粒子,结果与HRTEM分析结果一致。与经乙醇热还原处理的TiO2-RGO的XRD图谱对比发现,La-TiO2-RGO(160)、La-TiO2-RGO(170)、La-TiO2-RGO(180)含锐钛矿TiO2和金红石TiO2的混晶结构,证明经La掺杂和乙醇热还原处理的La-TiO2-RGO具有弱可见光响应和催化活性[22]。
图3为La-TiO2-RGO(160)、La-TiO2-RGO(170)和La-TiO2-RGO(180)的FTIR图谱。可以看出,不同温度热处理的La-TiO2-RGO表面均仍保留一定羟基及含氧亲水基团,但较经乙醇热还原处理的TiO2-RGO[22]有所减少。羟基及含氧亲水基团的减少有利于改善La-TiO2-RGO与PVDF之间的相容性,且留存下的羟基及含氧亲水基团又可以提高共混膜亲水性能。
图4为La-TiO2-RGO (160)、La-TiO2-RGO (170)、La-TiO2-RGO (180)的XPS图谱。可以看到,160℃和170℃下乙醇热还原处理得到的La-TiO2-RGO(160)和La-TiO2-RGO(170)中存在明显的Ti3+峰,但180℃下得到的La-TiO2-RGO(180)中Ti3+峰的强度非常微弱。Ti3+是热处理过程中乙醇对TiO2纳米粒子表面还原形成的,热还原处理温度从160℃升至170℃,Ti3+峰的强度增强,表明La-TiO2-RGO(170)中形成的Ti3+较多。但过高的热还原处理温度使TiO2更倾向于形成完美的结晶结构,没有利用稳定性较差的Ti3+形成,因此La-TiO2-RGO(180)中Ti3+峰的强度非常微弱。结合前期工作中有关TiO2弱光催化研究可知[22,25],Ti3+的存在是TiO2具有弱可见光响应的前提,且Ti3+/Ti4+越多,其弱可见光响应越强。
图5为La-TiO2-RGO(160)、La-TiO2-RGO(170)、La-TiO2-RGO(180)的可见光激发光电流响应曲线。可以看出,La-TiO2-RGO(160)、La-TiO2-RGO(170)、La-TiO2-RGO(180)的可见光激发光电流相应结果与其Ti3+/Ti4+的变化规律一致,La-TiO2-RGO(170)具有最强的光电流响应值,La-TiO2-RGO(180)的可见光响应值非常微弱。因此,在后续的研究中,选择具有较好弱可见光催化活性的La-TiO2-RGO(160)和La-TiO2-RGO(170)作为填充物制备La-TiO2-RGO/PVDF共混膜进行结构分析和性能考评。
2.2 La-TiO2-RGO/PVDF共混膜的微观形貌
图6为PVDF、La-TiO2-RGO(160)/PVDF、La-TiO2-RGO(170)/PVDF和未经处理的La-TiO2-RGO/PVDF共混膜表面的SEM图像。可以看到,La-TiO2-RGO(160)和La-TiO2-RGO(170)的加入可以改善La-TiO2-RGO/PVDF共混膜的孔隙结构。由图6(c)可以看到,La-TiO2-RGO(170)/PVDF-2共混膜表面出现明显孔结构增多、孔径增大的现象。由图6(f)可以看到,乙醇热还原后改善了La-TiO2-RGO与高分子的亲和力,在同样填充量下,La-TiO2-RGO(160)和La-TiO2-RGO(170)可以更均匀地分散于PVDF高分子中。但当La-TiO2-RGO(160)和La-TiO2-RGO(170)掺杂量为3.0%时,La-TiO2-RGO(160)/PVDF和La-TiO2-RGO(170)/PVDF共混膜表面形貌中均出现团聚物体,如白色圆圈所示。
根据文献和前期工作[21,26]可知,La-TiO2-RGO(160)和La-TiO2-RGO(170)的填充对La-TiO2-RGO/PVDF共混膜孔结构的改善是由于两种粉末表面有一定数量的羟基等含氧亲水基团,其填充的PVDF共混膜在浸没凝胶相转化成膜过程中,由于La-TiO2-RGO(160)和La-TiO2-RGO(170)表面的亲水基团加快了相转化凝胶成膜过程中非溶剂和溶剂之间的质量传递,从而在膜中形成更多孔隙和更大的孔[21,26]。
图7为PVDF、La-TiO2-RGO (160)/PVDF、La-TiO2-RGO (170)/PVDF和未经处理的La-TiO2-RGO/PVDF共混膜断面的SEM图像。可以看到,La-TiO2-RGO (160)/PVDF和La-TiO2-RGO (170)/PVDF共混膜由于La-TiO2-RGO (160)和La-TiO2-RGO (170)具有一定的亲水性,在浸没凝胶相转化成膜过程中,液固相分离速率较纯PVDF膜显著加快,易于形成指状孔[27],La-TiO2-RGO(160)/PVDF和La-TiO2-RGO (170)/PVDF共混膜的指状孔结构得到发展,海绵状孔结构减少[27]。当La-TiO2-RGO(160)和La-TiO2-RGO (170)的填充量达到3.0%时,La-TiO2-RGO (160)/PVDF和La-TiO2-RGO (170)/PVDF共混膜中均出现少量团聚颗粒(如图6(d)和图6(e)中亮白色颗粒物所示),导致共混膜中指状孔道明显变短。而未经乙醇热还原处理La-TiO2-GO/PVDF共混膜在La-TiO2-GO填充量为2.0%时就出现了大量团聚物,堵塞了膜的部分孔隙。
2.3 La-TiO2-RGO/PVDF共混膜的性能
2.3.1 共混膜的水通量及其变化规律
表2为PVDF膜和La-TiO2-RGO/PVDF共混膜的孔隙结构、表面亲水角和膜通量。从宏观角度,膜的孔隙结构和表面亲水性是影响膜通量的关键因素。可以看出,La-TiO2-RGO/PVDF共混膜的纯水通量和BSA溶液水通量均远大于PVDF膜;随La-TiO2-RGO (160)或La-TiO2-RGO (170)填充量的增加,La-TiO2-RGO/PVDF共混膜的纯水通量和BSA溶液水通量呈先增大后减小的趋势,最大纯水通量和BSA溶液水通量可分别达171.5 L·m−2·h−1和42.8 L·m−2·h−1;La-TiO2-RGO (160)和La-TiO2-RGO (170)的填充量相同时,La-TiO2-RGO (170)/PVDF共混膜的纯水通量和BSA溶液水通量高于La-TiO2-RGO (160)/PVDF共混膜,且共混膜通量的变化规律与膜的孔隙率、平均孔径和亲水性的变化趋势和结果一致。La-TiO2-RGO (160)和La-TiO2-RGO (170)的微观结构和La-TiO2-RGO/PVDF共混膜微观结构分析也证明了膜水通量变化规律。
表 2 PVDF膜和La-TiO2-RGO/PVDF共混膜的孔隙结构、表面亲水角和膜通量Table 2. Porosities, mean pore sizes, water contact angles and flux of PVDF membrane and La-TiO2-RGO/PVDF blending membranesMembrane Porosity/% Mean pore size/nm Water contact angle/(°) Jw/(L·m−2·h−1) Jp/(L·m−2·h−1) PVDF 30.2 35.1 91.3 37.9 7.4 La-TiO2-RGO(160)/PVDF-1 61.3 51.5 66.7 145.3 31.5 La-TiO2-RGO(160)/PVDF-2 71.1 62.3 65.3 169.2 39.6 La-TiO2-RGO(160)/PVDF-3 68.8 49.7 61.2 119.1 22.4 La-TiO2-RGO(170)/PVDF-1 65.3 57.2 56.4 149.2 33.4 La-TiO2-RGO(170)/PVDF-2 70.7 65.3 55.8 171.5 42.8 La-TiO2-RGO(170)/PVDF-3 69.7 59.7 58.9 155.1 38.7 Notes: Jw—Water flux; Jp—Bull serum albumin (BSA) flux. 2.3.2 光照运行对共混膜通量衰减的影响
图8为光照对PVDF膜和La-TiO2-RGO (170)/PVDF-2共混膜通量衰减过程的影响。可以看出,La-TiO2-RGO (170)/PVDF-2混合膜由于填充了具有可见光催化活性和亲水性的La-TiO2-GO (170),在光照作用下其通量衰减速率明显低于无光照作用。无论是在光照或无光照条件下,La-TiO2-RGO (170)/PVDF-2混合膜的膜通量衰减速率均明显低于PVDF膜,而PVDF膜通量衰减情况不受光照影响。说明通过填充La-TiO2-GO,La-TiO2-RGO/PVDF共混膜的通量和抗污染性得到提高;在外加光源条件下运行,La-TiO2-RGO/PVDF共混膜的通量和抗污染性可以进一步提高。
2.3.3 清洗方式对被污染的共混膜通量恢复效果的影响
图9是被污染的PVDF膜及La-TiO2-RGO/PVDF共混膜经水清洗、光照处理、光照处理后再水清洗三种方式清洗后的纯水通量。表3为被污染的PVDF膜及 La-TiO2-RGO/PVDF共混膜经水清洗、光照处理、光照处理后再水清洗三种方式清洗后的通量恢复率。可以看出,由于La-TiO2-RGO/PVDF共混膜填充具有亲水和弱可见光催化活性组分,其亲水和抗污染性能明显优于PVDF膜。被污染的La-TiO2-RGO/PVDF共混膜只采用水清洗其水通量就能得到较好的恢复,通量恢复率为60%~80%。污染的La-TiO2-RGO/PVDF共混若仅用弱可见光光照处理也具有一定的清洁效果,通量恢复率为30%~55%。污染的La-TiO2-RGO/PVDF共混经弱可见光光照处理再用水清洗的水通量恢复效果非常显著,可达70%~90%。
表 3 PVDF膜和La-TiO2-RGO/PVDF共混膜的抗污染性能Table 3. Anti-fouling performance of PVDF membrane and La-TiO2-RGO/PVDF blending membranesMembrane R/% RE/% REr/% PVDF 32.65 14.19 34.86 La-TiO2-RGO(160)/PVDF-1 77.40 34.70 82.22 La-TiO2-RGO(160)/PVDF-2 79.79 48.47 84.51 La-TiO2-RGO(160)/PVDF-3 63.69 33.55 72.09 La-TiO2-RGO(170)/PVDF-1 78.59 43.78 86.00 La-TiO2-RGO(170)/PVDF-2 79.28 52.42 90.01 La-TiO2-RGO(170)/PVDF-3 66.12 40.87 72.81 Notes: R—Flux recovery rate of contaminated membrane after washing; RE—Flux recovery rate after illumination for 2 h; REr—Flux recovery rate after illumination for 2 h then washing. 由于La-TiO2-RGO(170)的可见光响应性能更优,在同样填充量下的La-TiO2-RGO(170)/PVDF共混膜被污染后直接用弱可见光光照处理、先光照处理再水清洗的
纯水通量和通量恢复率均优于La-TiO2-RGO(160)/PVDF共混膜。对于同一La-TiO2-RGO/PVDF共混膜,如La-TiO2-RGO(170)/PVDF-2共混膜被污染采用先光照处理再水清洗的清洗方式的通量恢复率可达90.01%,明显高于仅采用光照处理的通量恢复率(52.42%)和直接水清洗的 通量恢复率(79.28%),表明吸附在被污染的La-TiO2-RGO/PVDF共混膜表面的BSA经光照处理后有一定的降解或去除,再用水清洗,使膜的清洗效果更明显,通量恢复率也更高。若被污染的La-TiO2-RGO/PVDF共混膜经光照处理后没有进一步的水清洗,虽然吸附在膜表面的BSA有一定的降解或去除,但还有一部分BSA及其部分降解产物留存在膜表面,膜通量的恢复率低于直接水的恢复率。 3. 结 论
(1) 采用吸附相反应技术耦合乙醇热还原处理可以获得具有弱可见光催化活性和亲水性的La-TiO2-还原氧化石墨烯(RGO),热还原温度为160℃时获得的La-TiO2-RGO中Ti3+较少,弱可见光响应特性较弱,亲水性较好,但过高的热处理温度(180℃)也抑制了La-TiO2-RGO中Ti3+的形成,弱可见光响应特性较弱,且亲水性变差。
(2) La-TiO2-RGO/聚偏氟乙烯(PVDF)共混超滤膜呈指状孔结构,其平均孔径和孔隙率得到显著提高,因此La-TiO2-RGO/PVDF共混膜也具有较大的水通量及较低的通量衰减速率,最高纯水通量可达171.5 L·m−2·h−1;La-TiO2-RGO/PVDF共混膜被污染后若采用先光照处理再水清洗的方式进行清洗,膜通量的恢复率较高,可达90%以下。
(3) 通过热还原处理温度可以调控La-TiO2-RGO的亲水性和弱可见光响应特性,进而可以调节La-TiO2-RGO/PVDF共混膜的微观结构和性能。
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表 1 La-TiO2-还原氧化石墨烯(RGO)/聚偏氟乙烯(PVDF)共混膜各组分含量
Table 1 Contents of components of La-TiO2-reduced graphene oxide(RGO)/ polyvinylidene fluoride(PVDF) blending membranes
Membrane La-TiO2-RGO VDMAc/
mLMass of
PVDF/gMass of
PEG2000/gMass ratio of La-TiO2-
RGO to PVDF/%Emax/
MPaPVDF — 50 7.5 2.5 0 75.3 La-TiO2-RGO(160)/PVDF-1 La-TiO2-RGO(160) 50 7.5 2.5 1.0 86.7 La-TiO2-RGO(160)/PVDF-2 La-TiO2-RGO(160) 50 7.5 2.5 2.0 98.9 La-TiO2-RGO(160)/PVDF-3 La-TiO2-RGO(160) 50 7.5 2.5 3.0 88.1 La-TiO2-RGO(170)/PVDF-1 La-TiO2-RGO(170) 50 7.5 2.5 1.0 87.2 La-TiO2-RGO(170)/PVDF-2 La-TiO2-RGO(170) 50 7.5 2.5 2.0 101.1 La-TiO2-RGO(170)/PVDF-3 La-TiO2-RGO(170) 50 7.5 2.5 3.0 89.5 Notes: Emax—Maximum tensile strength of membrane; La-TiO2-RGO(160)/PVDF-(1/2/3)—Blending membrane in which mass ratio of La-TiO2-RGO(160) to PVDF is 1%, 2%, 3%, respectively, La-TiO2-RGO(170)/PVDF-(1/2/3) is also understood like this; VDMAc—Volume of N,N-dimethyl acetamide. 表 2 PVDF膜和La-TiO2-RGO/PVDF共混膜的孔隙结构、表面亲水角和膜通量
Table 2 Porosities, mean pore sizes, water contact angles and flux of PVDF membrane and La-TiO2-RGO/PVDF blending membranes
Membrane Porosity/% Mean pore size/nm Water contact angle/(°) Jw/(L·m−2·h−1) Jp/(L·m−2·h−1) PVDF 30.2 35.1 91.3 37.9 7.4 La-TiO2-RGO(160)/PVDF-1 61.3 51.5 66.7 145.3 31.5 La-TiO2-RGO(160)/PVDF-2 71.1 62.3 65.3 169.2 39.6 La-TiO2-RGO(160)/PVDF-3 68.8 49.7 61.2 119.1 22.4 La-TiO2-RGO(170)/PVDF-1 65.3 57.2 56.4 149.2 33.4 La-TiO2-RGO(170)/PVDF-2 70.7 65.3 55.8 171.5 42.8 La-TiO2-RGO(170)/PVDF-3 69.7 59.7 58.9 155.1 38.7 Notes: Jw—Water flux; Jp—Bull serum albumin (BSA) flux. 表 3 PVDF膜和La-TiO2-RGO/PVDF共混膜的抗污染性能
Table 3 Anti-fouling performance of PVDF membrane and La-TiO2-RGO/PVDF blending membranes
Membrane R/% RE/% REr/% PVDF 32.65 14.19 34.86 La-TiO2-RGO(160)/PVDF-1 77.40 34.70 82.22 La-TiO2-RGO(160)/PVDF-2 79.79 48.47 84.51 La-TiO2-RGO(160)/PVDF-3 63.69 33.55 72.09 La-TiO2-RGO(170)/PVDF-1 78.59 43.78 86.00 La-TiO2-RGO(170)/PVDF-2 79.28 52.42 90.01 La-TiO2-RGO(170)/PVDF-3 66.12 40.87 72.81 Notes: R—Flux recovery rate of contaminated membrane after washing; RE—Flux recovery rate after illumination for 2 h; REr—Flux recovery rate after illumination for 2 h then washing. -
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