A sustainable filtering material for efficient removal of volatile organic compounds from their aqueous mixtures

Volatile organic compounds (VOCs) are hazardous to the environment and human health. Thus, tremendous effort has gone into developing effective/efficient techniques for separation of VOCs from their aqueous mixtures. In this work, a simple and versatile strategy was proposed to fabricate a new material for hydrophobic VOC separations, in which a self-assembling hydrogel consisting of TEMPO-oxidized cellulose nanofibers (TOCNs) and cationic guar gum (CGG) was used. Specifically, TOCNs and CGG were deposited onto a filter paper via a layer-by-layer procedure, resulting in a TOCN/CGG hydrogel-coated filtering material, in which no crosslinking agent was used. The hydrogel is spontaneously formed by electrostatic interactions and hydrogen bonding. The as-prepared filtering material exhibits oil-repellence and underwater oleophobicity resulting from the formation of hydration layer on hydrogel, thus demonstrating good VOC separation performance. One-layer of TOCN/CGG hydrogel coating (dried mass of 0.032 g·m− 2) was proved to realize the VOC separation (separation efficiency of around 99 % and water flux of as high as 905.14 L·m− 2·h− 1). Further increase of the hydrogel layers has negligible influence on the separation efficiency although it decreases the water flux. This filtering material also exhibits good stability during recycling. Consequently, this TOCN/CGG self-assembling hydrogel-coated filtering material has great potential in VOCs-contaminated wastewater treatment.


Introduction
Organic solvents and their products are widely used in industrial activity, which results in large amounts of organic solvent-contaminated wastewater that seriously threatens the global environment and even causes human health issues (Kang et al. 2019;Levchuk et al. 2014;Yang et al. 2021). Among organic solvents, volatile organic compounds (VOCs) are largely used when preparing various products, such as paints, adhesives and petroleum products (Fei et al. 2018, Zhou et al. 2013, Kujawa et al. 2015. Nonetheless, VOCs are serious precursors of fine particulate matter (such as PM 2.5) and are also the main components of photochemical smog for they are very volatile at room temperature and pressure (Farhadian et al. 2008;Hassan and Sorial 2010). VOC emissions rise continuously with the rapid industrial development, and they constitute around 7 % of the atmospheric pollutants (Delhoménie and Heitz 2005). Thus, it is urgent to eliminate or recover VOCs from wastewater. VOCscontaminated wastewater treatment can be implemented with various techniques, such as adsorption, distillation, air stripping and membrane technology (Zheng et al. 2013;Gallego et al. 2014;Ma et al. 2018). However, the separation efficiency of many existing techniques is still low, and some complex instruments also limit the practical application of these techniques (Zhu et al. 2015(Zhu et al. , 2016Tummons et al. 2016). In this context, novel materials with wettability stand out as a promising alternative, owing to their good performance in separating oil/water two-phase mixtures. Hydrogels, which consist of three-dimensional polymer networks filled with abundant water inside their interstitial space, are a kind of ideal superwetting material for oily wastewater treatment (Dai et al. 2019a, b;Lv et al. 2021;Su et al. 2019;Sun et al. 2021). Nevertheless, few works have been reported on the utilization of hydrogels for VOCs-contaminated wastewater treatment.
Hydrogels are generally prepared with crosslinked hydrophilic polymers. Cellulose (Rohrbach et al. 2014), guar gum (Dai et al. 2017) and chitosan (Tu et al. 2017) have been widely used to fabricate hydrogels due to their abundance, renewability, and functionality, which perfectly meets the requirements of ''green'' and sustainable development. In particular, nanocellulose with abundant hydroxyl groups and good water dispersity has been reported to design different hydrogels (Joseph et al. 2020). Moreover, hydrogels without chemical crosslinkers are preferred in many situations to avoid the toxicity from crosslinkers. In this sense, self-assembling hydrogels without the use of extraneous crosslinkers have attracted a lot of attention. We previously reported self-assembling hydrogels made of guar gum and nanocellulose (Dai et al. 2019a, b).
In this work, we proposed to use the hydrogel to modify conventional filter papers to endow them with wettability that may allow the resultant hydrogelmodified filter papers to separate VOCs from their water mixtures. Specifically, the TEMPO-oxidized cellulose nanofibers (TOCNs) and cationic guar gum (CGG) were adopted to coat filter papers through a layer-by-layer procedure, which leads to the formation of a hydrogel-coated filtering material since TOCNs and CGG autonomously generate a hydrogel due to the electrostatic interactions and hydrogen bonding between them. The layer-by-layer process also allows the modulation of the filtering material. The performance of TOCN/CGG self-assembling hydrogelcoated filtering material in removing VOCs from their aqueous mixtures was thoroughly studied.

Materials
Chemicals including N-heptane, petroleum ether, styrene, etc., were bought from Aladdin (Shanghai, China). TOCNs was purchased from Tianjin Woodelf Biotechnology Co., Ltd. (China), and CGG was sourced from a local company. Filter papers (pore size of 80-120 lm) were obtained from Hangzhou Newstar Paper (China).

Fabrication of TOCN/CGG hydrogel-coated filtering material
The filtering material was fabricated via the assembling of TOCN/CGG hydrogel onto filter papers as a coating. In detail, TOCNs and CGG with both concentrations of 0.01 wt% were sequentially deposited onto the filter paper via a layer-by-layer process. One layer of hydrogel coating (dried mass of 0.032 gÁm -2 ) consisted of one layer of TOCNs and one layer of CGG. The filtering materials of different layers of hydrogel coating were obtained by repeating the deposition procedure, and all these filtering materials were air-dried at room temperature.
Volatile organic compound (VOC) separation N-heptane, petroleum ether, styrene, etc. were used to implement the VOC separation. VOC aqueous mixtures were prepared by mixing one type of VOCs with equivalent deionized water. A piece of TOCN/ CGG hydrogel-coated filtering material was mounted onto a glass filter to separate the abovementioned VOC aqueous mixtures, during which the gravity drove the separation process. The separation efficiency was calculated as the weight ratio of collected water and initial water in the mixture. Water flux was calculated with the volume of collected water in a time interval. Filtering materials with different hydrogel layers (1 to 4 layers) were used for the separation study. Moreover, different VOCs, including petroleum ether, styrene, n-hexane, ethylbenzene, and turpentine, were adopted to demonstrate the separation performance of this TOCN/CGG hydrogel-coated filtering material for VOCs.

Characterizations
The quantitative determination of total organic carbon (TOC) in the filtrate was performed with a TOC analyser (Vario TOC). The samples were collected and sealed and were sent for analysis within 24 h to prevent the evaporation. The morphology and structure of the as-prepared filtering materials were observed with a scanning electron microscope (SEM, JEOL 6400) with the acceleration voltage of 15 kV, before which the specimens were dried at ambient conditions and sputter-coated with gold.

Results and discussion
TOCNs and CGG were coated on the filter paper through a layer-by-layer deposition process. Thanks to the non-covalent interactions (electrostatic interactions and H-bonding) between TOCNs and CGG, a self-assembled hydrogel can be easily and instantly formed on the filter paper without any cross-linking agents (Fig. 1a). Furthermore, no adhesive agents are required to attach the hydrogel onto the filter paper, for both components inherently have good affinity with the cellulosic paper. This proposed technique shows advantages over some reported counterparts. For instance, the adhesive agent was needed to bind the precursor with the substrate (i.e. steel mesh) in the preparation of polyacrylamide (PAM) hydrogelcoated steel mesh, and 90 to 120 min was needed for the hydrogel formation (Xue et al. 2011). The oil repellence performance of the hydrogel-coated filtering material was characterized with dichloromethane (dyed with oil red O) and the results are shown in Fig. 1b. The dichloromethane droplet nimbly flows across the filtering material indicating the low adhesion force between oil droplet and filtering material. Thus, the TOCN/CGG hydrogel-coated filtering material has good oil repellence property. The underwater oleophobic performance was also studied with the dichloromethane (Fig. 1c) and the underwater oil contact angle was around 153°. The oil repellence and oleophobic properties of the resultant TOCN/CGG hydrogel-coated filtering material prove its potential in separating VOCs from their aqueous mixtures.
The TOCN/CGG hydrogel-coated filtering material was analyzed with different VOCs to show its separation performance. The setup is illustrated in Fig. 2. As shown, the TOCN/CGG hydrogel-coated filtering material was mounted on the glass filter into which the petroleum ether/water mixture was poured to start the separation under the gravity force. Obviously, the water passed through the filtering material while the petroleum ether was prevented from passing through. Thus, the VOC separation from the aqueous solution was realized.
Total organic carbon (TOC), representing the total organic substances in water, is an important indicator for evaluating the degree of pollution of water bodies by organic substances. The filtered water was analyzed in terms of TOC content to further confirm the blockage of petroleum ether from passing through the hydrogel-coated filtering material. The water sample was taken out of the water phase in the oil/ water mixture as the control. As listed in Table 1, the TOC content is as low as 1.16 mgÁL -1 when one-layer hydrogel-coated filtering material is used, which indicates that the hydrogel modification is very effective in blocking VOCs. The one-layer hydrogelcoated filtering material exhibits a separation efficiency of ca. 99.06 %, indicating that one-layer hydrogel is enough to uniformly cover the filter paper surface. The high separation efficiency is possibly due to the superhydrophilicity and ultralow oil adhesion force endowed by the TOCN/CGG hydrogel. The separation efficiency negligibly changes with the increase of hydrogel layer. Nevertheless, the water flux decreases from 863.77 to 306.85 LÁmÁ -2 Áh -1 as the hydrogel layer increases from 1 to 4, which might be caused by the increased hydrogel thickness, slowing down the water penetrating speed.
Based on the aforementioned separation efficiency results, 1-layer hydrogel modification is preferred for the filtering material design. However, the thinner the hydrogel layer, the easier of the filtering material to be worn-out. In this context, the recyclability of one-layer hydrogel-coated filtering material was studied, in which petroleum ether was used as a VOC model (Fig. 3). The results show that the separation efficiency maintains around 99 % during the recycles.
To further determine the VOC separation performance of TOCN/CGG hydrogel-coated filtering material, six typical VOCs (i.e. n-heptane, petroleum ether, styrene, n-hexane, ethylbenzene, and turpentine) were used to perform the separation analysis. Figure 4a indicates that the one-layer hydrogel-coated filtering material shows high separation efficiency for different VOCs (98.32 % (n-heptane), 99.06 % (petroleum ether), 99.02 % (styrene), 98.88 % (n-hexane), 98.76 % (ethylbenzene) and 98.72 % (turpentine oil)). Furthermore, the as-prepared hydrogel-coated filtering materials with different numbers of hydrogel layer were studied in this work, and the results are  shown in Fig. 4b and d. These filtering materials all show high oil/water separation efficiency and their separation efficiency is not significantly affected by the numbers of hydrogel layer, indicating that the TOCN/CGG hydrogel-coated filtering materials possess oleophobic properties once a uniform hydrogel layer is generated. The effect of the number of hydrogel layer on water flux in the separation of VOC/water mixtures was also analyzed. Figure 5 demonstrates that the layer number of hydrogel coating significantly affects the water flux in the separation process. For instance, the water flux significantly decreases as the layer number of hydrogel coating increases in the process of separating n-heptane/water mixtures (Fig. 5a). It decreases from 848.53 to 271.27 LÁm -2 Áh -1 as the hydrogel coating increases from 1 to 4 layers. The same trend appears in separating other VOC/water mixtures (Fig. 5b-f). The decreased water reflux can be attributed to the decreased pores and voids, as well as the increased tortuosity with the increased thickness of hydrogel layer.
To further clarify this, SEM was used to observe the morphology and structure of TOCN/CGG hydrogelcoated filtering material (Fig. 6). As shown, the original filter paper exhibits typical porous structures (Fig. 6a) while the hydrogel coating covers a lot of pores and voids, thus remarkably decreasing the size  of water passage (Fig. 6b). This change is in accordance with the nanofibrillated cellulose hydrogelcoated filter papers reported by Rohrbach et al. (Rohrbach et al. 2014). It is easy to imagine that the hydrogel thickness increases as the number of hydrogel layer increases, which decreases the water flux because the water penetration speed is slowed down as a result of the decreased pores and voids, as well as the increased tortuosity.

Conclusions
In this study, a sustainable TOCN/CGG hydrogelcoated filtering material is demonstrated to efficiently separate VOCs from their water mixtures. The asprepared one-layer hydrogel-coated filtering material showed high VOC/water separation efficiency of around 99 %, with the hydrogel coating amount of only 0.032 gÁm -2 (dried mass). Meanwhile, the maximum water flux during the separation reaches 905.14 LÁm -2 Áh -1 , driven by the gravity force. The increase of the number of hydrogel layer decreases the water flux since the water penetration speed is slowed down as the hydrogel thickness increases. This filtering material also exhibits good stability for it maintains the separation efficiency of around 99.0 % after 20 times of recycling. The outstanding performance demonstrates the potential of this filtering material in treating VOC-containing wastewater in the future.