Novel infinite coordination polymer (ICP) modified thin-film polyamide nanocomposite membranes for simultaneous enhancement of antifouling and chlorine-resistance performance

Highlights

• Fabricating Infinite Coordination Polymer (ICP) nanoparticle mixed PA RO membrane.

• Low contents of ICP increased membrane flux, chlorine and fouling resistances.

• NaCl rejection improved from 97.1% to 98.8% due to increased surface negative charge.

• ICP modified membranes presented smoother surfaces with reduced contact angles.

• UV irradiation improved FRR of ICP/RO membranes due to photocatalytic activity.

Abstract

In this study, a hydrophilic and photocatalytic Infinite Coordination Polymer (ICP) nanoparticle was synthesized and used to fabricate reverse osmosis (RO) membrane to reduce antifouling and improve desalination performance. Co-BDC ICP (BDC = benzene-1,4-dicarboxylic acid) was synthesized and incorporated with different contents into an m-phenylene diamine aqueous solution to create a polyamide mixed matrix layer on polysulfone support with the interfacial polymerization (IP) technique. Due to the presence of –CO and end –COOH groups in the Co-ICP, the hydrophilicity of the fabricated membranes improved, caused to improving permeability and antifouling properties. By blending 0.02 wt% Co-ICP, NaCl solution flux increased from 37.4 (for unfilled) to 59.1 L/m². h and the FRR improved from 84.0% to 95.5%. The salt rejection was also improved from 97.1% to 98.8% due to increased surface negative charge. In addition, due to the photocatalytic activity of the synthesized Co-ICP, when UV irradiation was applied during the membrane washing, the FRR of the membranes was improved. The addition of Co-ICP improved the chlorine resistance of the membranes probably because of acted as a trap for the chlorine radicals. This study confirmed that the low amounts of the applied Co-ICP could be an effective additive to improve the desalination performance of RO membranes.

Introduction

A large world population faces water shortage problems due to urbanization and industrialization, which cause freshwater pollution [1]. In addition, global warming causes melting glaciers, resulting in penetration into groundwater resources and endangers people’s health. Based on the World Health Organization (WHO), about fifty percent of the population in the world will live in the water-crisis regions by the year 2025 [2,3]. This leads us to use alternative approaches like wastewater treatment or desalted seawater for solving this issue. Desalination of seawater is a crucial technological option that helps us increase clean and drinking water which is a United Nations 6th sustainable development goal (SDG6) “to ensure availability and sustainable management of water and sanitation for all” [4]. It can supply further clean water for the global population for drinking or irrigation in water-crisis regions and decrease the pressure on freshwater supplies in these areas.

Reverse Osmosis (RO) is a membrane-based method for desalination, mainly used for seawater and brackish water purification. While the RO process has some advantages that attract us to use it, such as simple design, easy system expansion, low maintenance cost, and low energy requirement [5], it has some drawbacks that should be considered. For example, the RO process removes both harmful and desirable minerals due to its fine construction. Another problem is the fouling of the RO membranes due to organic pollutants, which reduce the membrane flux and lifetime [6]. Like for instance, Guan et al. confirmed that unmodified thin-film composite (TFC) RO membranes (SW30 XLE, Dow Chemical, US) were shown a higher tendency to foul after using it for organic solutions in comparison with modified membranes by grafting sulfonic groups [7].

For using RO membranes in large-scale desalination, it is essential to have high salt rejection and permeation rate properties, so some developments methods are used for increasing these properties [6,8,9]. On the other hand, the biofouling and chlorine resistance tests were applied to overcome the significant problem of using TFC polyamide (PA) membranes. However, owing to the sensitivity of PA membranes in free chlorine, which leads to chemical reactions of chlorine oxidant residues with aromatic PA, the membrane is failed [[10], [11], [12], [13]].

Recently, several techniques such as investigation of polymerization parameters, surface modification, and incorporation of nanomaterials into PA thin film have been studied to improve the performance of the TFC-RO membranes in terms of antifouling property, chlorine resistance, and salt rejection [[14], [15], [16]]. Different nanomaterials are used on membrane surfaces to enhance the anti-biofouling ability. For example, Jeon et al. incorporated silver nanoparticles (AgNP) on the surface of the membrane, and thus AgNP prevented the activity of the microorganisms [17]. Ansari et al. used carbon nanomaterials such as graphene oxide (GO), which has advantages like high surface area, great hydrophilicity, compatibility, and superior mechanical properties, resulted in improving anti-biofouling [18].

Coordination polymers (CPs) are a class of compounds comprising metal cation centers connected by ligands through repeating coordination units spreading in one, two, or three dimensions [19]. These materials could be not only functionalized but also applied in various areas of research [20,21]. However, there are few papers in the case of membrane fabrication application. Xu et al. [22] fabricated fluorescent mixed matrix membranes by blending two CP particles of [Cd₂(dpa)₂(cda)Cl₂]_n and [Cd(dpa)₂(cda)]_n into the polymethyl methacrylate (PMMA) matrix and the fabricated membranes presented high selectivity and sensitivity for sensing dichromate ions in water. Mirante et al. [23] impregnated the active lamellar [Gd(H₄nmp)(H₂O)₂]Cl·₂H₂O (UAV-59) CP into a polymethyl methacrylate (PMMA, acrylic glass) support membrane to prepare catalytic membrane. This catalytic membrane application as an alternative to a powder catalyst arises as a high benefit related to the capability of catalyst handling while avoiding catalyst mass loss. Li et al. [24] showed the ability of CP glasses as an inorganic porous membrane for size-exclusive gas separation. Chu et al. [25] used two crystalline CPs with uncoordinated protonated –COOH groups to increase the proton conductivity of the Nafion membrane.

Infinite coordination polymers (ICPs) as a class of CPs are amorphous coordination polymers developed by Mirkin et al. [26] in recent decades. They are a well-defined framework due to a lack of crystallinity. The ICPs can be designed for various applications and research areas with an appropriate choice of metal ions and organic linkers, such as wastewater treatment, drug delivery, ion exchange, and gas adsorption [[27], [28], [29]]. Due to several varieties of electronic transitions, tunability, and stability, ICPs can be an excellent platform for making well-organized photocatalysts with rationally combining and assembling suitable components [30].

To the best of our knowledge, there is not any study about the use of ICP nanoparticles in synthesizing RO membranes. For this purpose, we synthesized Co-ICP photocatalyst, Co-BDC (BDC = benzene-1,4-dicarboxylic acid), and embedded it into the PA layer to fabricate Co-ICP/RO membranes during the interfacial polymerization. Due to hydrophilic (-CO and end –COOH groups) and photocatalytic properties of the synthesized Co-ICP, it increased membrane permeability and antifouling characteristics. The effect of various Co-ICP concentrations on Co-ICP/RO membranes was investigated in terms of desalination, antifouling, and chlorine resistance properties.