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餐饮、食品加工等行业排放的废水具有成分复杂、有机物及悬浮物浓度高、环境污染等特点[1-2]。以豆类为原料的食品行业,含有较多蛋白质,极大提高了废水的含氮浓度,其属于可生化性较好的高浓度有机废水,比一般食品废水更难实现脱氮处理[3-4],排放不当将导致水体富营养化或黑臭现象,间接威胁人体健康[5]。
超滤是压力差推动下的孔径筛分过程,可以有效去除蛋白质、胶体等大分子物质[6-7],目前已广泛运用于食品废水处理[8-9]。然而膜污染成为了阻碍膜发展的重要因素,污染物对膜表面及孔道的堵塞使得通量下降,能耗增加,膜的使用寿命缩短[10]。学者们通过膜污染模型拟合滤出水的通量变化,从微观角度阐明污染机理,发现膜孔堵塞及滤饼层是膜污染形成不可忽视的因素[11-12]。
表面改性因操作简单,可明显提高膜的亲水性和抗污染性而被广泛研究。贺等[13]在聚偏氟乙烯(PVDF)膜表面通过多巴胺/聚乙烯亚胺纳米颗粒进行亲水改性,改性后膜的抗污染性提高,对农村含油生活污水通量的恢复率达99.4%;REN等[14]采用2-n-丙基磺化壳聚糖对PVDF膜进行表面改性以提高膜的抗污染性,改性后膜的接触角下降至39°,不可逆污染最小可达8.2%;SHEN等[15]将金属有机框架UiO-66-NH2沉积在PVDF膜表面并与全氟烷基聚氧乙酸接枝,发现膜对多种油水混合物的去除率均大于95%,在死端过滤条件下,通量恢复率达93%。然而用于表面改性的材料或纳米颗粒的制备步骤较为繁琐。天然矿物质蛭石方便易得、价格低廉,且由于自身独特的结构具有大量羟基化表面,可有效提高膜的亲水性和抗污染性。目前蛭石用于膜改性的研究报道较少,污染机制等问题尚不明晰。本实验采用蛭石纳米颗粒对PVDF膜进行抗污染改性,以减轻膜污染,延长膜使用寿命。通过5种膜污染模型分析单一牛血清蛋白及含蛋白质废水对膜的污染类别及污染行为,并对膜性能进行评估,为实际废水的膜法处理提供理论基础和参考依据。
蛭石改性PVDF超滤膜的污染行为模型解析
Exploration on fouling behavior of vermiculite modified PVDF ultrafiltration membrane by membrane fouling models
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摘要: 采用蛭石纳米颗粒(Verm NPs)、盐酸多巴胺(DA)、硅烷偶联剂KH550及三(羟甲基)氨基甲烷盐酸盐对聚偏氟乙烯(PVDF)超滤膜进行表面改性,通过5种膜污染模型对1 g·L−1的牛血清蛋白溶液(BSA)及含蛋白质的实际废水进行抗污染研究。结果表明:改性体系在pH=7.2~7.3的条件下,多巴胺与KH550的聚合交联及蛭石羟基化表面的配位作用可共同将蛭石纳米颗粒稳定黏附在膜表面,蛭石在膜表面的冲刷恰好可以与聚多巴胺网状物形成最优配合,形成克服渗透性和选择性的Trade-off效应的高渗透性高截留的表面改性层。改性膜的BSA污染减轻,5种膜污染模型拟合曲线的R2均从改性前的0.90~0.94降至0.75~0.79,且膜污染的形成主要发生在过滤初期。采用实际含蛋白质废水对膜进行抗污染测试,发现改性后5种膜污染模型拟合曲线的R2均较改性前降低,膜污染得到缓解。由于实际废水的复杂性,污染物在孔内部的不断吸附、在膜表面的沉积及最终形成滤饼层仍然是膜污染形成的重要因素。Abstract: In this investigation, commercial polyvinylidene fluoride ultrafiltration membrane was modified by vermiculite nanoparticles, dopamine and KH550. The anti-fouling performance had been further explored according to 5 kinds of membrane fouling models by using 1 g·L−1 bovine serum albumin (BSA) solution and actual protein-containing wastewater. At pH 7.2~7.3, the polymerization and cross-linking of dopamine and KH550 and the coordination of vermiculite hydroxylated surface jointly stabilized the adhesion of Verm NPs to the membrane surface. Under this condition, the scouring effect of vermiculite on membrane surface exactly coordinated with the formation of polydopamine network, which led to an excellent modified layer with high permeability and rejection rate without trade-off effect. For fouling model fitting, the corresponding R2 of pristine and modified membrane for BSA was decreased from 0.90~0.94 to 0.75~0.79, indicating the obvious fouling alleviation after modification, and membrane fouling mainly occurred at the early stage of filtration. Additionally, the R2 of actual wastewater all decreased to a certain extent after modification, showing the significant effect on fouling mitigation. Notably, the unavoidable adsorption and blockage on membrane pores and inner channels by foulants as well as the formation of cake layer became the main factor of membrane fouling due to the complexity of actual wastewater.
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Key words:
- vermiculite /
- ultrafiltration membrane /
- surface modification /
- membrane fouling /
- fouling models
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表 1 常压死端过滤下的膜污染模型
Table 1. Membrane fouling models under constant dead-end filtration
膜污染类型 模型公式 n 完全堵塞 J0-J = AV (4) 2 经典标准堵塞 =$ \sqrt{{{J}}_{\text{0}}}{-}\sqrt{{J}} $ (5)$ \text{}\dfrac{{B}{V}\sqrt{{{J}}_{\text{0}}}}{\text{2}} $ 1.5 延伸标准堵塞 -$ {{J}}_{\text{0}}^{\frac{\text{3}}{\text{2}}} $ = CV (6)$ {{J}}^{\frac{\text{3}}{\text{2}}} $ 2.5 中间堵塞 lnJ0-lnJ = DV (7) 1 滤饼层 1/J-1/J0 = EV (8) 0 注:J和J0为瞬时及初始通量,cm·s−1;A、B、C、D、E为常数。 -
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