Fabrication of amino acid functionalized CNT/polyamide thin film nanocomposite desalination membranes

Water sustainability has become a worldwide concern due to limited natural resources and population growth. Considering that saline water constitutes nearly 97% of the total resources, water demand of agricultural, industrial and energy sectors leads a growing interest in desalination of seawater and brackish water. Reverse osmosis (RO) which constitutes approximately 65% of the worldwide desalination capacity has emerged as the leading technology for potable water production. Polyamide based thin film composite (TFC) membranes are extensively used in RO due to their superior separation performance; however, there are still important limitations such as permeability-selectivity trade-off, rejection of small and neutral species and susceptibility to fouling. Thin film nanocomposite (TFN) membranes has become a widely used approach to overcome limitations of conventional TFC membranes. These mixed matrix membranes are prepared by incorporation of various nanomaterials into the thin selective layer. Carbon nanotubes (CNTs) stand out as a potential TFN material due to their unique water transport properties. Experimental and computational studies suggest that functionalization of CNT tips may improve ion rejection as well. In this study, effect of amino acid functionalized carbon nanotubes on the structure, morphology and separation performance of TFN membranes was investigated. Inspried by structure of biological aquaporin channels, asparagine (Asn) amino acid was used as functional group in order to improve water permeability and salt rejection. The second amino acid, 8-aminocaprylic acid (ACA) was used due to its bulky structure in order to create steric hindrance for ions by narrowing the entrance of CNTs. Amino acid functionalization of carboxylated CNTs (COOH-CNTs) having average inner diameter of 1 nm was carried out via carbodiimide mediated reaction and confirmed by TGA, XPS and Raman spectrometry analyses. TFN membranes were prepared in two steps: First, functional CNTs dispersed in water by means of surfactant and centrifugated to remove large CNT bundles and impurities. Then CNT dispersion was vacuum-filtrated through the pores of support layer in order to provide partial aligment. Second, selective polyamide layer was synthesized via interfacial polymerization between MPD and TMC monomers. TFC membranes were also synthesized under identical conditions for comparison. Resulting TFC and TFN membranes were characterized by FTIR, XPS, SEM, AFM, polarized Raman spectrometry, water contact angle measurements and performance tests. TFN membranes were prepared with COOH-CNT loadings of 2.4, 1.2, 0.6 and 0.3x1013 CNTs/cm^2 in order to determine optimum CNT loading. XPS, SEM, AFM and polarized Raman analyses showed that concentration of CNT significantly affects dispersion and alignment of CNTs inside the polymeric matrix as well as homogeneity, thickness and crosslinking ratio of polyamide layer. In addition, COOH-CNT incorporation decreased hydrophilicity of surface indicated by high water contact angles for all loadings and surface roughness slightly increased as well. Based on salt rejection and water flux as well as structural and morphological properties, optimum CNT loading was determined as 0.6x10^13 CNTs/cm^2. Then, TFN membranes containing Asn-CNTs and ACA-CNTs were prepared at this loading and compared with TFC and TFN membranes contaning COOH-CNTs. Asn amino acid was thought to increase compatibility of CNTs with polyamide, leading formation of a thin, homogeneous polyamide layer having a similar surface with TFC. Asn also significantly increased surface hydrophicility compared to COOH due to its high hydrogen bonding capacity. Use of asparagine as functional group improved average water flux from 21.1 to 26.5 LMH and average salt rejection from 97.8% to 98.3% compared to TFC membranes. On the other hand, ACA amino acid resulted in a morphologically different polyamide layer, possibly having pin-hole defects and reduced crosslinking ratio. Bulk structure of the molecule did not provide expected steric hindrance and caused a slight decrease in both water flux and salt rejection.