Elsevier

Applied Surface Science

Volume 396, 28 February 2017, Pages 1478-1489
Applied Surface Science

Full Length Article
Surface modification of commercial seawater reverse osmosis membranes by grafting of hydrophilic monomer blended with carboxylated multiwalled carbon nanotubes

https://doi.org/10.1016/j.apsusc.2016.11.195Get rights and content

Highlights

  • A commercial PA RO membrane was modified by grafting of hydrophilic acrylic acid.

  • COOH-MWCNTs were mixed in grafting layer to increase permeability and antifouling.

  • However, more increase of CNTs caused in reduction of flux of the membranes.

  • Effect of acrylic acid amount, contact time and curing time was optimized.

Abstract

In this study, modification of commercial seawater reverse osmosis membranes was carried out with simultaneous use of surface grafting and nanoparticle incorporation. Membrane grafting with a hydrophilic acrylic acid monomer and thermal initiator was used to increase membrane surface hydrophilicity. The used nanomaterial was carboxylated multiwalled carbon nanotubes (MWCNTs), which were dispersed in the grafting solution and deposited on membrane surface to reduce fouling by creating polymer brushes and hydrodynamic resistance. Effectiveness of the grafting process (formation of graft layer on membrane surface) was proved by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses. Increase of membrane surface hydrophilicity was approved with contact angle test. First, the grafting was performed on the membrane surfaces with different monomer concentrations, various contact times and several membrane curing times (three variables for optimization). The modified membranes were tested by a cross-flow setup using saline solution for permeability and rejection tests, and bovine serum albumin (BSA) solution for fouling test. The results showed that the modified membranes with 0.75 M of monomer, 3 min contact time and 80 min curing time in an oven at 50 °C presented the highest flux and lowest rejection decline related to the commercial reverse osmosis membrane. In the next step, the optimum grafting condition was selected and the nanotubes with different weight percentages were dispersed in the acrylic acid monomer solution. The membrane containing 0.25 wt% COOH-MWCNTs showed the highest fouling resistance.

Introduction

Nowadays, the need to fresh water is ever more serious. In recent years, the use of membranes for water purification and desalination has increased rapidly [1]. Most of these methods have used a reverse osmosis membrane [2], [3] and most of these membranes are made by interfacial polymerization process [4], [5]. The greatest important problem in these membranes is the fouling [6]. The reverse osmosis membrane surfaces are usually rough; a rougher surface has more wettability and higher surface area (and therefore more water flux), but it has less fouling resistance [7]. A surface with more hydrophilicity could provide more wettability (and therefore more water throughput) and induce more fouling resistance. However, thin film composite (TFC) reverse osmosis membranes have high cross-linked polymer network membrane surface and poor hydrophilic characteristics.

There are various ways for modification of membranes and making them more resistance against the fouling. Among them, surface modification (grafting) and nanoparticle incorporation have been identified to be the most effective ones [8]. The polyamide membrane surfaces have free amino and carboxyl groups on their surface inherently due to incomplete polymer crosslinking. It has been confirmed that large negatively charged functional groups are on the surface of TFC polyamide membranes [8], [9].

There are two problems in reverse osmosis membranes surface modification: first is membrane flux decline because of creating an additional layer for mass transfer, which this problem can be reduced using more hydrophilic surface modification materials. Second, in some cases, is salt rejection decrease because some grafting layers could reduce surface charge and thus, could decrease effect of the Donnan repulsion [8], [10].

It is confirmed that the surface charge of membrane depends on protonation and deprotonation of different functional groups such as carboxyl and amino groups on the surface of membrane. Therefore, hydrophilic grafting materials containing carboxyl and amine functional groups are ideal to improve antifouling characteristic of membranes. One of these materials is acrylic acid, which is hydrophilic and has carboxyl groups.

Freger et al. [11] have examined modifying commercial RO and NF polyamide membranes with grafting of acrylic acid hydrophilic monomer. They prepared solutions containing 0.1 to 1 molar concentration of acrylic acid, 0.01 M ethylene glycol dimethacrylate (as a cross-linker) and redox system of 0.01 M K2S2O5/0.01 M K2S2O8 as initiators. They illustrated that attaching of the grafted layer on the membrane surface did not cause any important change in the surface morphology, while a certain decline of roughness was observed. However, if more decrease of the surface roughness is necessary for more enhance antifouling properties, the explained grafting technique will need further modification.

Yu et al. [12] modified a commercial TFC aromatic polyamide RO membrane by depositing N-isopropylacrylamide-co-acrylic acid copolymers (P(NIPAm-co-AAc)) on the membrane surface. The modified membrane showed enhanced membrane property.

Because of the similarity of fluid transport properties of carbon nanotubes (CNTs) with water channels in biological membranes, the CNTs have attracted much attention in preparation of filtration membranes [13]. Special transport properties of the carbon nanotubes are result of uniform surface potential energy inside the nanotubes. The observed enhancements in nanofluidic flows and gas diffusivities have been primarily attributed to nanoscale inner diameter, molecular smoothness of graphitic walls, along with hydrophobic character of the nanosized CNT pores, which force polar water molecules to take linear arrangement. Thus, in usages such as water purification, large volumes of water could pass through the CNT membranes, while blocking the passing of bulky hydrated ions [14]. It is reported that addition of charges using functional groups in the entrance of the tubes can help to prevent the passage of ions, however functionalizing partly could increase water flux. In addition, the antimicrobial and protein anti-adhesion properties have been listed for the carbon nanotubes modified reverse osmosis membranes in literatures [15]. Therefore, carboxylated-MWCNTs can be used as an effective nanomaterial for modification of membranes.

The purpose of this research is modification of a commercial polyamide seawater reverse osmosis membrane with simultaneously grafting of a hydrophilic acrylic acid monomer and carboxylated multiwalled carbon nanotubes to reach the membranes with improved hydrophilicity and fouling resistance. This layer should be enhanced flux, rejection and performance of the original membrane and make the membranes useable for longer periods. The COOH-MWCNTs in combination with grafting method could add new properties to the grafting layer. The MWCNTs could increase the grafting layer porosity causes to increase in water flux.

Section snippets

Materials

Commercial polyamide seawater reverse osmosis membranes were purchased from Woongjin Company (CSM, South Korea), which known by commercial name of RE4040-SHF. Potassium persulfate (KPS, initiator), acrylic acid (AA, monomer) and ethylene glycol dimethacrylate (EGDMA, cross-linker) were obtained from Merck, Germany. Carboxylated multiwalled carbon nanotubes (COOH-MWCNTs) with 5–15 nm OD were provided from US Research Nanomaterials, Inc., USA. Bovine serum albumin (BSA) and sodium chloride used to

FTIR analyses

FTIR spectroscopy was used to investigate carboxylated carbon nanotubes structure, the RO membrane structure and effect of graft on it. In Fig. 2, FTIR spectrum of the carboxylated carbon nanotubes was shown. In this spectrum, absorption bands in the range of 1400 cm−1 to 1600 cm−1 approve Cdouble bondC double bond. A strong band at 1628 cm−1 is related to Cdouble bondO stretching vibration of single bondCOOH functional group [18]. Strong peak at 3439 cm−1 is assigned to carboxylic acid Osingle bondH stretching vibration [19].

ATR-FTIR

Conclusion

The purpose of this research was to modify a commercial polyamide reverse osmosis membranes with grafting of the hydrophilic acrylic acid monomer and simultaneously by embedding carboxylated multiwalled carbon nanotubes to reach the membranes with more fouling resistance, meanwhile flux and rejection maintains constant. This project seeks to optimize modification of the membrane to reach the best membrane performance.

FTIR analysis proved the effectiveness of the grafting procedure and creating

Acknowledgments

The authors are thankful to the support from Kharazmi University (Iran) and gratefully acknowledge the financial support by the Iran National Science Foundation (INSF) (Grant no. 95826985).

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