Inverse opal-structured all-in-one water purification filter designed for the effective removal of multi-pollutants
Graphical abstract
Introduction
Water scarcity is one of the most significant global crises as it directly impacts humans and ecosystems [1]. The wastewater from various sources has led to drastic deterioration of water quality, leading to hazardous health consequences such as kidney failure, cancer, heart attack, and brain disorders [2], [3], [4], [5]. Over the last century, centralized water treatment systems, which involve the transportation of wastewater and treated water through a broad network of pumps and pipes, have been the norm [6]. However, centralized water treatment systems demand high costs for constructing and maintaining the relevant infrastructure and transporting polluted and treated water over long distances. Moreover, the unsecured quality of treated water due to a deteriorated water supply network and huge water treatment and transportation budget makes centralized water treatment systems inappropriate for isolated areas. As a solution to these drawbacks, decentralized water treatment systems with different types of technology have been studied worldwide [7], [8], [9], [10], [11]. These decentralized treatment systems enable on-site water treatment without any transport infrastructure, lowering the cost of installation of sustaining infrastructure. Furthermore, decentralization enhances infrastructure resiliency by quickly recovering the damaged or disrupted treatment system, providing easy control of accidentally released contaminants, and ensuring high-quality water meets its intended use [7], [8]. In particular, an on-site water treatment system could contribute to ensuring a resilient and safe water supply in peri-urban areas or isolated locations.
Several efforts have been made to remove pollutants in three representation treatments, such as biological, chemical, and physical treatment. The biological treatments are suitable for biodegradable organic substances and usually economical [12]. However, this method has several disadvantages, including the need for a large land area, some chemical toxicity, and a limit of organic substances that can be removed [13]. On the other hand, the chemical and physical treatment has been widely used due to its removal efficiency and various types of contaminants that can be removed, and include various purification methods, such as adsorption, chemical precipitation, disinfection, photocatalytic process, reverse osmosis, ion exchange, and membrane [12], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Of these methods, adsorption is preferred as a potential method for water purification due to its low cost, simplicity, and potential for recycling. Real wastewaters are aqueous mixtures of diverse neutral and ionic pollutants such as heavy metals and organic compounds containing nitrogen and sulfur [2], [23], [24], [25]. However, to date, several pollutant removal studies have focused on removing a single component. Considering the realistic chemical compositions found in the wastewater matrices, the technical efforts made so far to design the treatment system for a specific single target substrate are impractical. Hence, studies focusing on the simultaneous removal of contaminants and multiple pollutants have recently gained much more attention [26], [27], [28], [29], [30], [31]. The functionalized cellulose was introduced for lead (Pb), cadmium (Cd), and nickel (Ni) removal in multi-pollutant systems, demonstrating remarkable high adsorption capacities of 295.20 mg/g Pb, 151.51 mg/g Cd, and 72.80 mg/g Ni [26]. Cao et al. observed the heavy metal adsorbing properties in co-exists of different heavy metals (e.g., Pb, Cd, copper (Cu), Ni, and zinc (Zn)) using multi-pore activated carbons and reported that their competitive adsorption was as follows: Cd > Pb > Ni > Cu > Zn [32]. The La@MgAl nanocomposites for the simultaneous treatment of co-existing phosphate and fluoride from water have also been investigated, and high adsorption capacities of 101.59 mg/g for phosphate and 51.03 mg/g for fluoride were observed [33]. However, the strategies suggested previously aimed to eliminate similar categories of pollutant species, including heavy metals, inorganic anions, and organic compounds, so the technical challenges in treating natural wastewater containing pollutants that belong to diverse categories have yet to be addressed. For more practical utilization, Afridi et al. studied the phosphate-removing properties of anodized iron oxide in a variety of simulated wastewaters having anions, heavy metals, and organic matter and confirmed the inhibitory effect of co-existing contaminants on the removal efficiency [34]. Although the study reported the effect of co-existing pollutants of different species, further research on multi-pollutants removal at once should be conducted.
Removal mechanisms depend on the pollutant species, so proper adsorbents should be designed and adapted to remove each pollutant. For instance, the adsorption process by electrostatic interaction of different charged pollutants (e.g., heavy metals are positively charged and anionic substances are negatively charged) needs oppositely charged adsorbents. Meanwhile, most previously reported water treatment studies conducted batch experiments to test adsorption performance. The quantitative information from batch experiments may not be readily adapted for designing practical water purification systems due to the discriminating differences between the water flow conditions. Minimal studies have evaluated adsorption performance under flow-through conditions in multi-pollutants systems [35], [36]. Vu et al. performed column flow-through experiments in multi-pollutants systems (e.g., phosphate, zinc, and caffeine) using multifunctional Al-Mg/GO as an adsorbent, which exhibited an adsorption capacity of phosphate 1.64 mg/g, zinc 0.56 mg/g, and caffeine 0.53 mg/g [35]. However, to utilize in actual water treatment, low adsorption capacity still remains a hurdle to overcome owing to the trade-off between adsorption ability and water flux.
Inverse opal (IO) structures have been employed as water purification membranes due to their high porosity, large surface area, and good interconnectivity [37], [38], [39], [40]. Zhang et al. prepared TiO2 IO photonic crystal (PC) combined with NaYF4:Yb3+, Er3+(NYF) up-conversion nanoparticles for use in catalytic membrane. The NYF/TiO2 IO showed enhanced photocatalytic activity (high inactivation rate of 100 % for bacteria under 11 h), caused by highly ordered IO structure [40]. Na et al. employed the 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) immobilized IO for use in catalytic membrane [39]. The TEMPO-immobilized IO exhibited outstanding performance of high conversion efficiency (>97 %), maintaining a permeation flux higher than 300 L/m2·h@1 bar. In spite of the structural benefits of IO structure, their use in practical water purification system is still remained as challenges due to the difficulty in scalable process and controlling surface properties to adsorb specific pollutants. Also, researches on the IO water purification system for multi-contaminants has not been conducted yet.
In this study, we have designed an all-in-one water filtration unit using an inverse opal (IO) structure for a fit-for-purpose water treatment system. IO-based filters can encompass porous materials with a broad range of pore sizes, morphological features, and chemical compositions. It is technically possible to fabricate multifunctional IO integrating diverse inorganic nano-adsorbents that result in the miniaturization of natural wastewater treatment plants. The IO-based filter consists of three substances, Fe2O3 IO, NiOx IO, and sulfur-doped graphene (S-G), to remove heavy metals, anionic substances, and organic matters, respectively. The flow rate of water according to the pore size of IO structure was analyzed, and the water purification performance of the IO-based filter for multi-pollutants was evaluated. Also, an optimized IO-based all-in-one filter system was designed by comparing two different fixed-bed continuous water flow systems to suggest a practical water purification system.
Section snippets
Materials
Iron (III) chloride hexahydrate (FeCl3·6H2O, 99 %), urea (CH4N2O, 99.5 %), gelatin (from bovine skin, Type B), sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O, 99 %), nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, 99.9 %), sodium hydroxide (NaOH, 98 %), dimethyl sulfoxide (DMSO, (CH3)2SO, 99.9 %), hydrochloric acid (HCl, 34 wt%), poly(vinylpyrrolidone) (PVP, MW = ∼40,000), styrene, 2,2′-Azovis (2-methylpropionamidine dihydrochloride), 4,4′-Azobis (4-cyanovaleric acid), l-ascorbic acid (C6H8
Design of an all-in-one filter for multi-pollutants removal
Fig. 1 shows the IO-based all-in-one filter with three different components, S-G, Fe2O3, and NiOx IOs, which were designed to remove organic pollutants, heavy metals, and anionic substances from multi-pollutants in wastewater, respectively. First, porous S-G was prepared (Fig. S1 (a) and (b)) to remove organic pollutants through π-π electron donor-acceptor (EDA) interaction. As the S-G has π-electrons perpendicular to the surface, the primary adsorption mechanism was governed by π-π EDA
Conclusions
In summary, an IO-structured all-in-one filter composed of three different layers of absorbents (i.e. S-G, Fe2O3 IO, NiOx IO) was designed as a novel water purification system. The IO-structured filter showed an enhanced water flux of 1167 L/m2·h compared to the powder of NPs (572 L/m2·h). The adsorption property was also investigated in a continuous fixed-bed column to remove pollutants simultaneously from multi-pollutants systems, including phenol, Zn, and phosphate. The sequential
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
H.J.K. and G.S.H. contributed equally to this work. This work was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) (20016588) grant funded by the Ministry of Trade, Industry and Energy (MOTIE) of Republic of Korea, and the National Research Foundation of Korea (NRF) (2022M3J1A1064315 and 2021R1I1A1A01060152) grant funded by the Ministry of Science and ICT (MSIT) of Republic of Korea.
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These authors contributed equally to this work.