Regular ArticleDeveloping new adsorptive membrane by modification of support layer with iron oxide microspheres for arsenic removal
Graphical abstract
Introduction
Arsenic (As) is a common contaminant widely existing in surface water and groundwater and has become a critical environmental problem due to its tremendous toxicity and carcinogenicity [1], [2]. Arsenic-contaminated water has significant adverse impacts on human health and ecosystems [3], [4]. The World Health Organization (WHO), US Environmental Protection Agency (US-EPA) and other agencies have established a strict maximum contaminant level (MCL) of permissible arsenic in drinking water, with the concentration reduction from 50 to 10 μg/L [5]. Thus, removal of such toxic arsenic from natural water is of great importance [6].
Many methods have been used for removing arsenic from drinking water, including ion exchange [7], precipitation-coagulation [8], [9], adsorption [10], membrane processes (e.g. reverse osmosis and nanofiltration) [11], [12], [13] and so on. Precipitation-coagulation can remove multifarious pollutants effectively, but the sludge as a byproduct causes environmental concerns. The ion exchange process is complicated and time-consuming when recycling the exchange resin, and produces excess waste.
Adsorption is a very popular method to remove arsenic due to its low material costs, easy preparation and operation [14]. Metal (hydro) oxides have been intensively investigated for arsenic removal, particularly iron (hydro) oxides [1]. For example, Fe3O4 particles have excellent properties, such as high adsorption capacity and ease of separation and reuse, and thus have been widely used in the removal of arsenic from drinking water [15], [16]. However, the release of particles to the ultimate water may cause considerable risks [2].
The past 20 years have witnessed the rapid development of membrane technology in water treatment because of its small footprint, high efficiency and stability, ease of operation and relatively low energy consumption [17], [18]. Operating pressures of microfiltration (MF) and ultrafiltration (UF) is lower than those of nanofiltration (NF) and reverse osmosis (RO). However, MF and UF can only remove some viruses and suspended solids, while NF and RO can effectively remove heavy metal ions and fluorides [19], [20]. Although NF and RO could decrease arsenic concentration from 1000 to below 10 μg/L, high fouling potential and high operating pressure may hinder their full-scale applications [21].
Recently, some studies combined adsorbents and membrane technology [22], namely, mixed matrix membranes (MMMs), to treat wastewater containing arsenic, integrating the advantages of both adsorption and membrane separation [23]. MF and UF membranes are selected due to the high water permeability, low operating pressure and relatively low preparation/operating costs [24], [25], [26]. Some researchers reported that the performance of the membrane will be enhanced with the increase of functional adsorbent in a certain range [19], [24], [27]. For example, the addition of hydrophilic nanoparticles will increase the pure water flux of the treated membrane without sacrificing the BSA rejection [28], [29], [30], [31]. However, blending excessive nanoparticles into membranes may damage the structure of the membrane, and thus deteriorate filtration performance. It is suggested that the content of adsorbent in the membrane matrix should usually be less than 6 wt% [32], [33]. Blending minor amounts of adsorbents leads to a low adsorption capacity of the membrane. Also, the blended nanoparticles in the membrane matrix may suffer from the particle leaching problem, causing secondary contamination.
Apart from bulk blending, surface modification is another very common membrane modification strategy to improve separation performance of the membrane [34]. However, surface modification may increase mass transfer resistance of the membrane if an extra layer is coated or the surface pores are blocked. Additionally, the long-term stability of surface modification is a practical concern [35]. Compared with bulk blending and surface modification of membranes, modification of the support layer is much less studied [32].
In this work, we developed a new adsorptive membrane by modifying the membrane support layer with Fe3O4 microspheres. Fe3O4 microspheres were loaded in the support layer by reverse filtration in a self-made apparatus (Fig. 1A) and then sealed by self-polymerization of dopamine. The prepared adsorptive membrane was compared with a pristine membrane and a blend membrane in terms of the membrane microstructure, pure water permeability, selectivity to organic matters and As removal efficiency. The membrane regenerability and As removal efficiency with the presence of organic matters were also investigated.
Section snippets
Material
Polyvinylpyrrolidone (PVP) (K-30) was purchased from Shanghai Titan Scientific Co., Ltd. Polyethersulfone (PES) was obtained from BASF Company (Germany). Ethylene imine polymer (PEI) (Mw = 10,000) was bought from Aladdin company. N, N-dimethylformamide (DMF) and Trisodium citrate dihydrate were all purchased in Shanghai Chemical Reagent Company. Bovine serum albumin (BSA) (Mw = 67,000) was the product of Sinopharm Chemical Reagent Co, Ltd. Ferrosoferric oxide (Fe3O4) was prepared via
Characterization of Fe3O4 microspheres
FESEM and TEM images show the morphology, structure and size of the as-synthesized microspheres (Fig. 2). As displayed in Fig. 2A and B, the microspheres had uniformly dispersed spherical structures with average diameters were about 300 nm. Actually, these microspheres were composed of many smaller particles of 20 nm (Insert Fig. 2A), which is similar to the reported value [36], [37]. The component of the as-prepared microspheres was indicated by XRD analysis (Fig. 2C). Six obvious diffraction
Conclusions
The new adsorptive membrane was fabricated by modifying the support layer of the membrane with loading Fe3O4 microspheres. The introduction of Fe3O4 microspheres endows the polymeric membranes with excellent adsorption ability while conserves its UF performance. The adsorptive membrane had comparable water flux (250.67 L/m2/h) and BSA rejection (94.5%) performance with the pristine membrane without Fe3O4, but higher dynamic adsorption capacity (277.9 mg/g) than the blending membrane. Specially,
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant no. 51678307). Funding from the Faculty of Science and Engineering Visiting Researcher Scheme at Macquarie University is gratefully acknowledged.
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