膜污染问题极大限制了膜分离技术在饮用水处理方面的应用。水体天然有机物（NOM）被认为是造成传统膜材料污染的主要物质，其中腐殖质、蛋白质和多糖是造成低压膜污染的主要成分。由于具有较好的化学稳定性、较高的导电性、较强的吸附能力，以及较低的水力学阻力，新型碳纳米管改性膜在膜法水处理领域展现出广阔的应用前景。

本研究选用三种不同性质的多壁碳纳米管，即未改性碳纳米管（MWCNT），羟基化碳纳米管（MWCNT-OH）和羧基化碳纳米管（MWCNT-COOH），采用过滤附载的方法制备了三种不同结构的碳纳米管改性膜，并系统表征了其理化性质。然后，通过不同水质条件下的过滤实验，分别考察了三种改性膜对腐殖酸（HA，腐殖质类模型污染物）、牛血清蛋白（BSA、蛋白质模型污染物）、海藻酸钠（SA，多糖模型污染物）的截留效果，并结合过滤前后NOM的分子量/颗径表征，探究了NOM的截留机制。然后，通过XDLVO理论分析，并结合膜污染动态过程分析和对污染后改性膜的红外光谱表征，探究了改性膜抗NOM污染的机理，进而筛选出具有高NOM去除率和抗污染性能的碳纳米管改性膜材料。最后，采用该材料进行商业中空纤维膜改性，并用于天然地表水的多周期过滤-反冲洗实验，初步验证了改性膜在实际水处理中的过滤及抗污染效果，以及可重复利用性。

研究的主要内容和结论如下：

（1）碳纳米管改性膜的制备和表征。选用MWCNT、MWCNT-OH和MWCNT-COOH，采用清洁无化学污染的超声分散-基底膜过滤的附载方法，分别制备了三种碳纳米管改性的平板膜和MWCNT-COOH改性中空纤维膜。表征结果显示，MWCNT膜表面呈现出非均匀分布的团聚状结构，膜孔径分布主要取决于基底膜。MWCNT-OH膜和MWCNT-COOH膜表面形成了均匀分布的网状结构，膜孔径分布主要由碳纳米管层的孔隙决定。碳纳米管层的附载降低了改性膜透水率，三种平板膜的渗透通量为153-1092 L m² h^-1 bar^-1，且随膜孔径增大而增大。

（2）改性膜截留水体中不同NOM的效果及机理。通过探究不同水质条件对碳纳米管改性膜截留三种NOM的影响，发现在溶液中不存在Ca²⁺的条件下，酸性和中性pH值有利于碳纳米管对NOM的截留，而离子强度变化（5 - 20 mM）对NOM去除影响不大。结合对膜孔径结构和NOM尺寸的表征结果，发现MWCNT膜的孔径远大于三种NOM的粒径，其对NOM的截留主要依靠吸附作用；MWCNT-OH膜和MWCNT-COOH膜对HA的截留主要依靠吸附作用，对BSA的截留依靠吸附和筛分的共同作用，而对分子量较大的SA的截留主要依靠筛分作用。当溶液中存在Ca²⁺时，Ca²⁺容易与NOM上的-COO^-络合，使之聚集为粒径大于改性膜孔径的团聚体，此时碳纳米管改性膜对NOM的截留主要依靠筛分作用。同时，Ca²⁺还可以减弱NOM和碳纳米管层之间的静电斥力作用，促进改性膜对NOM的吸附和筛分。此时，MWCNT-COOH改性膜的NOM截留率高于95%。

（3）改性膜抗NOM污染的效果与机理。采用XDLVO理论分析发现，在本研究采用的水质条件下，MWCNT膜与NOM之间表现为引力，而MWCNT-OH膜和MWCNT-COOH膜与污染物之间表现为斥力，且MWCNT-COOH膜的斥力大于MWCNT-OH膜，因此两种氧化碳纳米管改性膜的抗污染性能强于MWCNT膜，而MWCNT-COOH改性膜的抗污染性能最好。根据比通量随时间变化曲线的拟合分析发现，在溶液中不含Ca²⁺时，NOM对MWCNT膜的污染机制主要为孔缩；HA对MWCNT-OH膜和MWCNT-COOH膜的污染机制为孔缩，而BSA和SA对两种膜的污染机制为表面孔堵；当溶液中含有1.0 mM的Ca²⁺时，HA和BSA对三种碳纳米管改性膜的污染机制为表面孔堵，SA的污染机制为形成滤饼层。

（4）碳纳米管改性膜在天然地表水处理中的稳定性和抗污染性。通过碳纳米管改性平板膜过滤天然地表水实验，发现NOM去除效果和膜污染趋势与配水实验结果一致，MWCNT-COOH膜的NOM去除效果和抗膜污染性能最好。多周期过滤-反冲洗实验发现，MWCNT-COOH中空纤维膜对天然水中有机物的去除率始终高于70%，并且总膜污染和水力学不可逆膜污染程度均远低于未改性商业膜。排阻色谱分析结果进一步表明，天然水中的生物大分子有机物对膜的反冲洗效果有重要影响。

Membrane fouling has been a major obstacle for the application of membrane techonology to drinking water treatment. Aquatic natural organic matter (NOM) is considered the most important contaminant for membrane fouling. Among them, humic substances, proteins and polysaccharides are major NOM components that cause the fouling of traditional low-pressure membranes. Carbon nanotubes (CNTs) have demonstrated a broad application potential for membrane-based drinking water treatment due to their good chemical stability, high water conductivity and strong adsorption capacity.

In this study, pristine multi-walled CNT (MWCNT), hydroxylated MWCNT (MWCNT-OH) and carboxylated MWCNT (MWCNT-COOH) membranes with different nanoporous strucures were prepared by a filtration method. Then after, the retention mechanisms of humic acid (HA, model humic substance), bovine serum albumin (BSA, model protein) and sodium alginate (SA, model polysaccharide) by these membranes were investigated according to the filtration results obtained in different feedwater conditions, as well as the characteristics of NOM. Meanwhile, the resulting membrane fouling and its mechanism were explored according to the extended Derjaguin?Landau?Verwey?Overbeek (XDLVO) theory and the analyses of the temporal fouling profiles. Finally, the membrane with the highest NOM retention and best anti-fouling property was identified and used in multi-cyle filtration of a natural surface water to verify its performance and stability in realistic water treatment.

The main contents and findings of this study are as follows:

(1) Preparation and characterization of CNT-modified membranes. Pristine MWCNT, MWCNT-OH and MWCNT-COOH were selected to prepare three types of CNT-modified, flat-sheet membranes and one type of MWCNT-COOH modified hollow-fiber membrane, by a dispersion-filtration method without the use of chemical binders. The characterization results revealed that the surface of the pristine MWCNT membrane was dominated by granular MWCNT aggregates and the pore size distribution (PSD) of this membrane was primarily determined by the substrate membrane. The MWCNT-OH membrane and the MWCNT-COOH membrane possessed homogeneous and porous networks of CNT fibers, and their PSDs were governed by the tight interstitial spaces in the CNT layers. Meanwhile, the water permeability (153-1092 L m² h^-1 bar^-1) of the modified membranes decreased compared to the surbstrate membrane due to the CNT coating, and increased with the measured membrane pore size.

(2) Efficacy and mechanism of NOM retention by CNT-modified membranes. Based on the changes in NOM retention with feedwater quality, it was found that in the absence of Ca²⁺, acidic and neutral pH conditions enhanced NOM retention, while inonic strength (5 - 20 mM) exerted little effects. Characterization results further showed that the pore size of the MWCNT membranes were significantly greater than NOM sizes, and therefore, adsorptive filtration was the major mechanism for NOM retention. Meanwhile, HA retention by the MWCNT-OH membrane and the MWCNT-COOH membrane mainly depended on adsorption, while BSA retention and SA retention were achieved by the combined adsorption and sieving and sieving alone, respectively. In the absence of Ca²⁺, Ca²⁺ tended to form complexes with -COO^- in NOM, which promoted the formation of large NOM aggregates that were subject to membrane sieving. Besides, Ca²⁺ also weakened the electrostatic repulsion between negatively charged NOM and MWCNT, thereby increasing NOM sieving and adsorption by the modified membranes. As such, 95% of NOM was retained by the MWCNT-COOH membrane.

(3) Antifouling propensity and mechanism of CNT-modified membranes. According to the XDLVO theory, the net interaction between NOM and the pristine MWCNT membrane should be attractive, while the interaction with NOM should be more repulsive for the MWCNT-COOH membrane than the MWCNT-OH membrane. These results indicate that the two oxidized MWCNT membranes are more resistant to NOM fouling than the pristine MWCNT membrane, and the MWCNT-COOH membrane has the best anti-fouling property. Moreover, analyses of the temporal fouling profile suggest that NOM fouing of the pristine MWCNT membrane is possibly caused by pore constriction, whereas the MWCNT-OH membrane and the MWCNT-COOH membrane are fouled by HA via pore contriction, and by BSA and SA via pore blocking. At a calcium concentation of 1.0 mM, HA and BSA fouling of the CNT-modified membranes is due to pore blocking, while SA fouling is ascribed to cake layer formation.

(4) Stability and anti-fouling propensity of CNT-modified membranes in natural water treatment. The filtration of a natural surface water by the CNT-modified, flat-sheet membranes showed similar NOM retention efficiencies and anti-fouling propensities as obtained with the synthetic model waters. Also, the MWCNT-COOH membrane had the best perforemance. In the subsequent multi-cycle filtration experiment, the HF membrane modified with MWCNT-COOH consistently removed more than 70% of NOM in the natural water, and the total fouling and hydraulic irreversible fouling of the modified membrane were much lower than those observed with the virgin membrane. In addition, size exclusion chromatography analysis further revealed that biopolymer fraction of NOM had important impacts on the efficacy of hydraulic backwashing on membrane permeability recovery.