Research paperPreparation and characterization of polysulfone/organoclay adsorptive nanocomposite membrane for arsenic removal from contaminated water
Graphical abstract
Introduction
Arsenic is one of the most toxic elements in the environment and has negative impact on human health when presented at elevated levels in surface water. It is usually found in an inorganic form like oxyanions of arsenite (As(III)) and arsenate (As(V)) (Camacho et al., 2011). Arsenate is the dominant species in surface water due to high redox potential while arsenite is found in anaerobic groundwater conditions (Sun et al., 2014). Arsenic exposure in drinking water has been proven to leave serious negative impacts on human health, causing skin, lung, liver, bladder, kidney and lymphatic cancer (Giles et al., 2011, Wu et al., 2012). Due to these health hazards associated with arsenic contaminated water, the World Health Organization (WHO) has decreased the maximum contaminant level (MCL) of arsenic in drinking water from 50 ppb to 10 ppb (Chandra et al., 2010, Smedley and Kinniburgh, 2002). Therefore, researchers have focused on the development of effective, low cost and robust technologies for arsenic removal from water resources. Various treatment technologies including precipitation (Harper and Kingham, 1992), coagulation (Balasubramanian and Madhavan, 2001, Baskan and Pala, 2010, Parga et al., 2005), ion exchange and adsorption (Banerjee et al., 2008, Chutia et al., 2009, Guan et al., 2009, Mandal et al., 2011, Manna and Ghosh, 2007) have been extensively studied for arsenic removal. Nevertheless, all these methods have drawbacks, especially regarding efficiencies and costs. Generation of toxic sludge in coagulation and precipitation method; separation of adsorbents from filtrate in batch adsorption, severe pressure drop in column adsorption process and high cost operation in ion exchange are difficult to be eliminated (Chatterjee and De, 2014, Ladhe et al., 2009, Lin et al., 2013, Sogaard, 2014, Zheng et al., 2011). Fortunately membrane technology, which has attracted considerable attention in recent years owing to its efficiency, ease of scale up and cost effectiveness, provides a powerful tool for solving some of the major problems pertaining to pollutant separation (Ahsani and Yegani, 2015, Jafarzadeh et al., 2015, Romero-Dondiz et al., 2015, Zheng et al., 2011). Among the pressure driven membranes, nanofiltration (NF) and reverse osmosis (RO) membranes are capable of removing arsenic; however, they require high operational pressure and costly membranes in comparison with low pressure processes such as microfiltration (MF) and ultrafiltration (UF) membranes. MF and UF membranes could not remove arsenic, mainly due to their larger pore sizes (Fu and Wang, 2011). In this regard, developing the MF/UF membranes to remove small size pollutants such as arsenate would be a challenging step in overcoming the disadvantageous of NF and RO membranes. For this purpose, the concept of adsorptive membranes was introduced. Adsorptive membranes are a kind of MF/UF membranes developed by incorporation of inorganic adsorptive materials into polymeric matrix (Niedergall et al., 2013, Yin and Deng, 2015). This separation technique combines the specificity of small adsorbents of particles with the convenience of filtration membranes. The inorganic dispersed phase provides selectivity due to its specific chemical structure and polymer matrix grants mechanical support. During the past years, several adsorptive membranes have been developed for an efficient removal of pollutants such as arsenic, fluoride, lead, sulphur and phenolic compounds from water. Zheng et al. (2011) embedded zirconia as inorganic adsorbent in porous PVDF membrane for arsenic removal and found that these membranes can adsorb arsenic and remove other contaminants like microorganisms. Gohari et al. (2013b) demonstrated that PES/Fe-Mn membrane is capable of As(III) removal from contaminated groundwater by producing permeate containing As(III) less than MCL. They also successfully removed the Pb(II) from water by PES/HMO ultrafiltration membrane (Gohari et al., 2013a). (Lin et al. (2013) used adsorptive membrane with metal organic frameworks for sulphur removal. Chatterjee and De (2014) added activated alumina to cellulose acetate phthalate matrix and investigated the removal of fluoride from water. It is noticeable that important issues associated with adsorptive membranes are using proper polymer as supporting matrix and inorganic materials with high adsorption capacity, high selectivity, low cost, high binding capacity and easy regeneration (Ladhea et al., 2009). A suitable polymer for the fabrication of adsorptive membranes should have high permeability, proper hydrophilicity and excellent chemical resistance. As for the polymer matrix, polysulfone (PSf) has been widely used in ultrafiltration membranes due to its outstanding properties such as low cost, availability, high mechanical strength, thermal and chemical stabilities, resistance over wide range of pH, as well as easy processability and variety of active functional groups (Anadão et al., 2010, Choi et al., 2006, Fan et al., 2008, Ganesh et al., 2013, Yang et al., 2007). Among different types of adsorbents, montmorillonite (Mt), due to its availability, low cost, variety of surface and structural properties and expanding property has been selected as inorganic material in preparation of nanocomposite membranes (Musso et al., 2014, Ozdes et al., 2011, Xi et al., 2004). However, the presence of negative charges on the surface of Mt. has made it unsuitable adsorbent for anionic pollutants such as arsenate. In order to remove negatively charged pollutants, organically modified clays, known as organoclays, have been developed. The most common surfactants used for Mt modification are quaternary ammonium salts (Li and Bowman, 2001). The final adsorption properties of organoclays depend on the molecular length and functional groups of surfactants. It has been proved that a long chain of organic surfactants not only improves the anion adsorption capacity of Mt., but also greatly facilitates its dispersion in polymer matrix by increasing the interlayer space of clay minerals (Shah et al., 2013). Although there are some papers regarding the effects of organoclays on the structure and fouling property of PSf/organoclay membranes (Ma et al., 2012a, Ma et al., 2012b, Monticelli et al., 2007), there is no desired outcome discussing the capability of the adsorptive removal of contaminants from water by PSf/organoclay membranes. By taking this point into consideration, commercial sodium Mt modified with quaternary ammonium salt; Cloisite 30B (C-30B), was embedded in PSf membranes. The main goals of this study include (a) preparation of PSf/organoclay adsorptive nanocomposite membranes to investigate its ability in arsenate removal from surface water, (b) structural and operational characterizations of the prepared membranes using FE-SEM, XRD, AFM, contact angle measurement, pure water flux and mechanical strength, (c) investigation the batch adsorption characteristics and the mechanisms of arsenate adsorption using adsorptive nanocomposite membranes by fitting the experimental data to Langmuir and Freundlich isotherms, as well as pseudo-first and second order kinetics models and finally (d) demonstration of the applicability of membranes for As(V) removal from water resources in dead-end filtration experiments.
Section snippets
Materials
Udel P-1700 polysulfone was purchased from Solvay Advanced Polymer LLC and used as polymer. The organoclay used in this study was the commercially available Cloisite 30B nanoclay (Cation exchange capacity = 90 meq/100 g, 90% of the dry particles having size < 13 μm), a natural montmorillonite modified with a quaternary ammonium salt with the hydroxyl groups and long straight alkyl chain supplied by Southern Clay (USA) Products. The organic modifier is a methyl, tallow, bis-2-hydroxyethyl, quaternary
Membrane morphology
In order to study the effect of organoclay on the microstructure of the membranes, FE-SEM images of both cross-sections and top surfaces of neat and nanocomposite membranes have been shown in Fig. 1. Formation of asymmetric structure at the cross-section of all the samples can be easily observed which is the typical result of NIPS method (Crock et al., 2013). The cross sectional images of membranes reveal the presence of macrovoids in neat PSf membrane and extending the macrovoids over the
Conclusions
The performance of PSf/organoclay nanocomposite membrane fabricated via non-solvent induced phase separation (NIPS) method for arsenic removal from surface water was studied. In comparison with neat PSf membrane, nanocomposite membranes exhibited higher pure water flux, surface hydrophilicity and roughness and better mechanical strength. XRD analysis revealed that the organoclays were completely exfoliated when the organoclay content increased from 0 to 1.5 wt%. However, further increase in
Acknowledgements
The authors would like to express their appreciation to Water and Wastewater Company's Laboratory of East Azarbaijan for the partial support of this study.
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