water separation

water separation

Science of the Total Environment 694 (2019) 133671 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www...

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Science of the Total Environment 694 (2019) 133671

Contents lists available at ScienceDirect

Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

A facile approach to ultralight and recyclable 3D self-assembled copolymer/graphene aerogels for efficient oil/water separation Shumei Zhang a, Guijun Liu a, Yue Gao b,⁎, Qinyan Yue a,⁎, Baoyu Gao a,⁎, Xing Xu a, Wenjia Kong a, Nan Li a, Wenqiang Jiang c a b c

Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Qingdao 266000, PR China School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China College of Environmental Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250100, Shandong Province, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• P(AM-DMDAAC))/ graphene aerogel (PGA) was prepared via a facile onestep hydrothermal method. • PGA shows excellent recyclability, outstanding hydrophobicity and oil adsorption capacity. • PGA shows the huge potential in the dynamic oil/water separation process. • P(AM-DMDAAC) has been used to enhance the hydrophobicity and oil/ water separation properties of materials. • The interfacial wettability of PGAs can be controlled by changing the pH values.

a r t i c l e

i n f o

Article history: Received 19 April 2019 Received in revised form 23 June 2019 Accepted 29 July 2019 Available online 31 July 2019 Editor: Baoliang Chen Keywords: Graphene aerogel Dimethyldiallylammonium chloride acrylamide polymer Oil/water separation Adsorption

a b s t r a c t In this paper, a facile approach was developed for highly effective oil/water separation by incorporating of the dimethyldiallylammonium chloride acrylamide polymer (P(AM-DMDAAC)) into graphene aerogels. The functionalized 3D graphene aerogel integrated a series of excellent physical properties, including low density (11.4 mg/cm3), large specific surface area (206.591 m2/g), and great hydrophobicity (contact angle of 142.7°). The modified aerogel showed excellent adsorption capacity for oils and organic solvents (up to 130 g/g). The saturation can be reached in a short time and the adsorption capacity remained nearly unchanged after repeated heating cycles. Meanwhile, we found a simple method to achieve controlled wettability transition of P(AMDMDAAC)/graphene aerogels (PGAs) by changing the pH values. The hydrophobic PGA prepared at pH 2.03 showed outstanding oil/water separation performance (130 g/g). As the pH increased, the oil adsorption capabilities of PGAs decreased slightly, but the adsorption performance for the hydrophilic organic dye was significantly improved. Therefore, as a recyclable and efficient water purification material, the sustainable and environmentfriendly polymer-modified graphene aerogel has great application potential. © 2019 Published by Elsevier B.V.

1. Introduction ⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Gao), [email protected] (Q. Yue), [email protected] (B. Gao).

https://doi.org/10.1016/j.scitotenv.2019.133671 0048-9697/© 2019 Published by Elsevier B.V.

In recent years, pollution by oil spill, chemical leaks, and toxic organic solvents has become a global concern (Shamaei et al., 2018; Bi

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et al., 2012; Gołub and Piekutin, 2018). The main methods to solve these problems consist of physical, chemical, biological and combined processing methods. However, most of these conventional methods have some drawbacks, such as secondary pollution, low efficiency and poor recyclability (G. Jiang et al., 2017; J. Jiang et al., 2017; Ma et al., 2017; Victoria and Keller, 2006). Physical adsorption is one of the most effective methods to deal with oil pollution. Therefore, it is necessary to develop materials with high adsorption ability, low cost, and high recyclability (Bi et al., 2014a; Wan et al., 2016; Wang et al., 2019; Zheng et al., 2014). Various absorption materials (e.g. cotton, wool fibers and straw) have been applied in oil/water separation (H. Cheng et al., 2017; Y. Cheng et al., 2017; Ibrahim et al., 2010; Paulauskiene, 2018). But most of these conventional absorbents showed weak adsorption capacity and poor recyclability, which restricted their application in field (Atta et al., 2006; Farag and Elsaeed, 2010; Li et al., 2011). Graphene is a two-dimensional material composed of carbon atoms through sp2 hybrid orbital, which has the advantages of large electronic capacity, large specific surface area and low density, and has been widely used in many fields (Papkov et al., 2013; Wang et al., 2009a, 2009b). More recently, three-dimensional (3D) network assembled by graphene oxide sheets with excellent hydrophobic property and environmental acceptability has attracted wide interests in adsorbing oil and organic solvents in water. However, the graphene sheets are easy to stack and reunite because of the Van der Waal's and π-π interactions, leading to dramatic decrease in specific surface area and low adsorption capacity for oils and organic solvents (Chen et al., 2018a, 2018b). Our previous studies showed that synthetic polymer and organic species can be used to modify the bare graphene to suppress the stack and reunite between the graphene sheets, which could significantly improve the surface hydrophobicity of materials with high adsorption capacity (Li et al., 2014; Zhu et al., 2016; Li et al., 2013; Cao et al., 2018). Dimethyldiallylammonium chloride acrylamide polymer, abbreviated as P(AM-DMDAAC), a kind of liner and binary cationic copolymer rich in quaternary ammonium ions, was synthesized by copolymerization of dimethyldiallylammonium chloride (DMDAAC) and acrylamide (AM) (Yong et al., 2017). In previous studies, P(AM-DMDAAC) has been widely used in oil extraction, papermaking, flocculation, and textile industry wastewater treatment due to its high charge density, alkali resistance and temperature resistance (G. Jiang et al., 2017; J. Jiang et al., 2017; Yang et al., 2010). However, there are few studies on the P(AMDMDAAC)-doped graphene aerogels. P(AM-DMDAAC) is positively charged, which can be used as the precursor to be combined with two or more graphene sheets by electrostatic action, thereby promoting the self-assembly of graphene hydrogel with a three-dimensional fold structure (Chun and Gaoquan, 2014; Liu et al., 2016; Xu et al., 2010). Thus, it is expected that the adsorption capacity and mechanical properties could be improved when incorporating P(AM-DMDAAC) into graphene aerogels. Herein, we reported a one-step hydrothermal method to prepare PGA (P(AM-DMDAAC)-doped graphene aerogel) for oil and organic solvents adsorption. The feasibility of situ polymerization graphene-based aerogel for adsorption of various oils and organic solvents were evaluated (Luo et al., 2018; Wang et al., 2010; Zhang et al., 2014). Meanwhile, the graphene aerogels (GAs) without incorporation of P(AM-DMDAAC) were also prepared comparison. The structures and the physicochemical properties were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunner-Emmet-Teller measurements (BET), Scanning electron microscopy (SEM), Water contact angle test (WCA), and Thermal gravimetric analysis (TGA). Finally, the oil/water separation at dynamic state as well as the reusability of the prepared PGA was tested in details.

2. Materials and methods 2.1. Materials Flake graphite was provided by Qingdao Jinrilai Carbon Reagents Co., Ltd. and Dimethyldiallylammonium chloride acrylamide polymer (m/ m, AM/DMDAAC = 7/3, with the solid content of 15% at 25 °C) was obtained from Binzhou Jiayuan environmental protection Co., Ltd. H3PO4, H2SO4, HCl, urea, H2O2, KMnO4, methylene blue, methyl orange and organic absorbate (chloroform, N, N-dimethylformamide (DMF), carbon tetrachloride, n-hexane, toluene, acetone and petroleum ether) were purchased from Sinopharm Chemical Reagent Co., Ltd. The synthesis of graphene oxide (GO) was based on an improved Hummer's method (details in S1). 2.2. Preparation of ultralight 3D GA and PGA Typically, the fabrication of ultralight 3D PGA included four steps. (i) 0.2 g of GO was dissolved in 40 mL deionized water to form a homogeneous solution. (ii) Then a colloidal solution was obtained by mixing with 0.08 g P(AM-DMDAAC), 1 g urea and GO solution under magnetic stirring. (iii) The suspension was transferred into teflon lined stainless steel autoclave and heated at 200 °C for 8 h. (iv) The as-prepared hydrogel cylinder was soaked with distilled water for 24 h, and freeze-dried under vacuum to obtain P(AM-DMDAAC)/graphene aerogel (PGA). The pure graphene aerogel (GA) was also obtained according to the similar procure without the addition of P(AM-DMDAAC). The schemes of GA and PGA were shown in Fig. 1. 2.3. Preparation process of PGAs at different pH values Firstly, the pH of the graphene solution was adjusted to 3, 5, 7, 9 and 11, and then 0.08 g P(AM-DMDAAC) and 1 g urea were added into each GO solution under magnetic stirring. Different PGA samples were then fabricated according to the procedures as mentioned above. They were named as PGA-3, PGA-5, PGA-7, PGA-9 and PGA-11 according to the pH conditions. The pH value of graphene solution was 2.03 without adjustment, and the as-prepared sample was named PGA-2.03 (PGA). 2.4. Morphology and structural characterizations The surface morphologies were investigated with a scanning electron microscopy (SEM, SIGMA 500, ZEISS). The specific surface area and pore size distribution were determined by N2 adsorption/desorption using a surface area analyzer (JW-BK122W). Thermal behaviors were investigated on a TGA-50 analyzer at a 10 °C min−1 heating rate under nitrogen atmosphere. The X-ray diffraction (XRD, mode Ultima IV) was used to identify the chemical composition and crystal structures of as-prepared materials and the scanning angle ranged from 10° to 90° with the rate of 10° min−1. The X-ray photoelectron spectroscopy (XPS) measurements results were recorded on an X-ray photoelectron spectrometer with a focused (Thermo ESCALAB 250XI). Contact angle was carried out on a HARKE-SPCA contact angle goniometer at room temperature. 2.5. Adsorption capacity tests The aerogel samples were placed in different kinds of oils and organic solvents such as engine oil, toluene, chloroform and peanut oil etc. until reaching the adsorption equilibrium (Bi et al., 2014a). After being taken from the adsorbates, the aerogel was wiped with a filter paper and weighed. The adsorption capacity (Q) of the aerogel sample was calculated with the following equation: Q¼

Sb −Sa Sa

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where Sa (g) was the initial weight and Sb (g) was the terminal weight after reaching adsorption equilibrium of aerogel samples, respectively. The heat treatment was used for the regenerating of the aerogel. After the oil adsorption reaching equilibrium, PGA was sufficiently burned in the air and collected for further recycle experiment. The dye adsorption experiments of PGAs prepared under different pH value conditions were carried out. Cationic MB and anion MO were selected as test dyes. In adsorption kinetic experiments, 0.15 g of the adsorbent (PGA-11, PGA-9, PGA-7, PGA-5, PGA-3, PGA-2.03) was added into 1000 mL of MB or MO solution with an initial concentration of 50 mg/L. 1 mL of the solution was taken at various time intervals and tested for concentration by TU-1810.

3. Results and discussion 3.1. Characterizations of GA and PGA As the concentration of the polymer increases, the density of the materials dramatically decreased from 18.9 to 11.4 mg/cm3 (Figs. S1 and S2). Therefore, the PGA with diameter of 22.4 mm and height of 21.1 mm could stand on a single leaf stably (Fig. S4). Interestingly, the PGA had an ultra-light weight, but its load carrying capacity was remarkable. Fig. S5 described that GA sample could not support mass loading of 100 g, whereas PGA was able to support a 100 g weight without collapsing, suggesting that PGA obtained excellent mechanical property due to the incorporation of P(AM-DMDAAC). Its mechanical properties can be evaluated by balancing weights. In Fig. S6, 94 mg of PGA (11.4 mg/cm3, 21.1 mm in height and 22.4 mm in diameter) can support a weight of 200 g, which was equivalent to about 2128 times its own weight. The calculated compressive strength of PGA was 4.79 kPa (calculated by F = mg/S), which was higher than GA (Fig. S7, 2.35 kPa). According to the previous studies, the mechanism of P(AMDMDAAC) doping into graphene sheets can be summarized as follows: (i) P(AM-DMDAAC) is a positively charged cationic copolymer, and the graphene sheet is negatively charged due to the functional group (hydroxyl and carboxyl group) on the graphene sheet layer. Therefore, P (AM-DMDAAC) can be combined with the graphene sheet layer by electrostatic action (Liu et al., 2016); (ii) The hydrophilic edges of GO sheets could bound with P(AM-DMDAAC) branches via hydrogen bonding and Van der Waal's force (Li et al., 2016). Therefore, P(AM-DMDAAC) can bound with different graphene sheets forming more stable 3D aerogel

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networks, thereby improving the mechanical properties of the graphene aerogel. The surface microstructure and composition of the materials were characterized by SEM (Fig. 2a–d) and EDS analyses (Fig. S8). Based on the Fig. 2a and b, it was obvious that the structure of GA was unitary. The PGA layers were loosely stacked with obvious wrinkles, and the degree of continuous cross-linking between the sheets was enhanced. Therefore, P(AM-DMDAAC) effectively overcame the stack of graphene sheets and cross-linked well with the graphene sheet in the aerogel structure, which was beneficial for improving the oil absorption performance of PGA. For further analyzing the hydrophobic properties of PGA and GA, water or oil contact angels of aerogels were further detected and the results were shown in Figs. 3 and S3. The contact angle of PGA was 142.7°, which was much higher than that of GA (80.2°). As shown in Fig. 3b, oil can be absorbed instantly when it was dropped onto the surface of PGA, suggesting the lipophilic characteristic of PGA. However, when water droplets are dropped onto the PGA surface, they can remain on the surface of the PGA (Fig. 3d). This indicated the excellent hydrophobic property of PGA after incorporating of P(AM-DMDAAC). According to the Cassie-Baxter equation, the more surface layer numbers lead to superior hydrophobic property (Kota et al., 2013). Therefore, this phenomenon can be explained by the pleat structure of the PGA. The XPS spectra further investigated the surface composition and chemical characteristics of PGA. XPS spectra of GA and PGA indicated that the main constituents in GA and PGA were based on the C, N, and O elements (Fig. 4a and b). The increased N element in PGA was due to the introduction of nitrogen groups from P(AM-DMDAAC). In the O 1s XPS spectra of GA and PGA (Fig. 4c and d), four types of oxygencontaining functional group were located at 530.2 (O=C), 530.9 (O\\C), 531.7 (C-O-C), and 533 (Chemisorbed water molecules) (Ganguly et al., 2011; Lei et al., 2017; Periasamy et al., 2017). However, O_C peak in PGA was significantly decreased and chemisorbed water molecules was increased as compared with those in GA, which suggested the less O_C bonds and more chemisorbed water molecules in PGA after incorporating P(AM-DMDAAC) into graphene aerogels. In the XRD patterns of GA and PGA (Fig. 5a), no peak was observed at about 12°, which indicated that most oxygen-containing functional groups were removed after hydrothermal synthesis (Bi et al., 2014b; Donghui et al., 2010; Hwang et al., 2007; Papkov et al., 2013; Zu and Han, 2009). The PGA exhibited a strong diffraction peak at 2θ = 25.66°, and the layer spacing was calculated to be 3.5 Å, which was

Fig. 1. Preparation scheme of GA and PGA.

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Fig. 2. SEM of GA (a, b) and PGA (c, d).

similar to that of GA. This indicated that the incorporating of P(AMDMDAAC) has little effect on the ordered structure of graphene aerogel. Fig. 5b showed the Raman spectra of GA and PGA with two remarkable absorption bands at around 1340 and 1580 cm−1, which were

attributed to the typical D band and G band of carbon (Yuan et al., 2018; Li et al., 2018; Chen et al., 2019; Shang et al., 2019). The D band was related to the structural disorder of the material, and the G band was associated with the first order scattering vibration of sp2 hybridized

Fig. 3. Contact angle test for PGA (a) and GA (c). Oil drops on the PGA 2s (b) and water drops on the surface of PGA (d).

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Fig. 4. XPS survey spectrum of GA (a) and PGA (b). XPS survey spectrum of O 1s for GA (c) and PGA (d).

Fig. 5. XRD patterns of GA and PGA (a). Raman spectra of GA and PGA (b). N2 adsorption isotherms for GA and PGA (c). Pore size distributions for GA and PGA (d).

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carbon atoms (Duan et al., 2019; Cançado et al., 2006; Kong et al., 2019). The intensity ratio of D to G band (ID/IG) was usually used to determine the extent of structural defects. The value of ID/IG decreased slightly from 1.09 to 1.08 when the GA was modified with PGA, suggesting that the polymer was introduced into the graphene aerogel without introducing more defects. The porous properties of PGA and GA were demonstrated by nitrogen adsorption and desorption experiments. As shown in Fig. 5c, the adsorption-desorption curve of PGA exhibited type II isotherm with an obvious H3-type hysteresis loop, indicating the existence of mesopore for oil/water separation. Besides, BET surface area of PGA apparently increased to 206.591 m2/g after the incorporation of P(AM-DMDAAC) as compared with that of pristine GA (101.405 m2/g). Two aspects contributed to this phenomenon: (i) a large number of pleated structures in the PGA resulted in the significant improvement of specific surface area; (ii) the 3D pleated structure of the PGA forms an interconnected network structure, effectively preventing the accumulation of graphene sheets (Chen et al., 2018a, 2018b; H. Cheng et al., 2017; Y. Cheng et al., 2017; Li et al., 2013, 2014). The pore size distributions of PGA and GA calculated by BET (Fig. 5d) showed that GA and PGA had a sharp peak at 3.6 and 4.2 nm. Both the average pore size and the total pore volume of PGA were larger than GA (Table S1), ascribed to the loosely porous structure of PGA, which were beneficial to enhance the adsorption capacity of PGA (Priyanka and Saravanakumar, 2018). FT-IR spectra of GO, GA and PGA samples were shown in Fig. 6a. The GO powder showed strong adsorption bands at 1046, 1625, 1734 and 3340 cm−1, which could be attributed to the stretching vibration of C\\O, C_C, C_O, and O\\H (Peng et al., 2018; Li et al., 2014; Wang et al., 2009b). For GA and PGA, the significant decrease in stretching

vibration of O\\H, C_O was mainly due to the removal of unstable oxygen-containing functional groups after hydrothermal reaction (Li et al., 2013). By comparing the infrared spectra of GA and PGA, the intensity of the stretching vibration of C_C and C\\O was obviously enhanced and the position was shifted after adding P(AM-DMDAAC), this result indicated that P(AM-DMDAAC) was successfully doped into the graphene sheets in a hydrogen bonding. PGA had excellent fire resistance as shown in Fig. S9, which can be used to heat-treatment after oil absorption to achieve the recycling. The thermal stability of GA and PGA was measured by TGA and the resulting curves were shown in Fig. 6b. Because PGA had better hydrophobic properties and absorbs less moisture than GA, the weight loss caused by evaporation of absorbed water of PGA (8%) was less than that of GA (10%) when temperature increased to 100 °C. The reduction of sample weight from 210 °C to 350 °C was mainly due to the loss of oxygen-containing groups (Chengzhou et al., 2010; Li et al., 2014). As a result, the content of unstable PGA-containing oxygen groups was less than GA. 3.2. Adsorption capacity of GA and PGA In order to examine the adsorption capacity of GA and PGA for various oils and organic solvents, adsorption kinetics experiment, reusability and cycling experiment as well as dynamic oil/water separation experiments were carried out. As shown in Fig. 7a, PGA had higher adsorption capacity for various oils and organic solvents as compared with GA. The adsorption capacities of PGA ranged from 40 to 130 times of its original weight, which was higher than those of sorbent materials previous reported (Table 1). The adsorption capacity for the engine oil was greater than other oils and organic pollutants, owing to the higher viscosity and density of the liquids. As the viscosity and density of the oil increased, the adhesion of the oil to the surface of the absorbent also increased, resulting in a higher oil absorption capacity (Singh et al., 2017). 3D structure polymer modified aerogels with ultra-low density and high surface hydrophobicity were also considered to be the ideal materials for separation of oil/water mixtures. PGA was used to separate toluene and chloroform dyed by Sudan (I) from water. Chloroform was quickly absorbed within 2 s when PGA was submerged (Fig. S10a and Movie S1), which indicated its selective adsorption capacity. In addition, PGA could quickly adsorb light oil (toluene) in water and still float on the top of water as the adsorption was completed (Fig. S10b). The rapid and thorough oils adsorption indicated excellent adsorption performance of PGA. In order to further prove the rapid adsorption capacity of PGA, the adsorption kinetics of organic pollutants onto PGA was explored. When the PGA was placed in an oil or organic solvent, the adsorption increased rapidly and reached equilibrium within 30 s (Fig. 7b). It was much faster than other adsorption materials like cotton, wool, and expanded perlite (Bastani et al., 2006; Bo et al., 2014). Moreover, it was worth noting that the adsorption rate of peanut oil was lower than those of other organic solvents (n-hexane, carbon tetrachloride and petroleum ether), indicating that the adsorption rate was related to the viscosity of the oils. In order to further study the mechanism and characteristics of adsorption kinetics, the experimental data were linearly fitted with a pseudo-first-order model. The relationship between ln (Q-Qt) and t was shown in Fig. 7c, and the regression coefficients were listed in Table S2. The experimental data fit well with the pseudo-first-order kinetic model, and the K values (equilibrium rate constant) were inversely proportional to the viscosities of organic pollutants. The high adsorption rate of PGA was more conducive to its practical application. 3.3. Reusability and cycling experiments of PGA

Fig. 6. FT-IR spectra of GA and PGA (a). TGA curves of GA and PGA (b).

The regenerability and economic viability of PGA were also evaluated and the heat treatment method was used in the recycle

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Fig. 7. Absorption capacities of GA and PGA for various oil and organic solvents (a). Adsorption of different oils and organic solvents by PGA in terms of time (b). The pseudo-first-order adsorption kinetics of PGA for four kinds of oils (c). Recyclability of the PGA for n-hexane and N,N-dimethylformamide (d).

experiments due to its fire-resistant property (Fig. S11). As being illustrated in Fig. 7d, the high recoverability (more than 99%) of n-hexane and N, N-dimethylformamide (DMF) saturated PGA after 10 cycles was observed. According to the difference of adsorbing organic solvents (n-hexane and N,N-dimethylformamid), the PGA samples after 10 times of regeneration were named as PGA-10H and PGA-10D, respectively. For further verify the microstructure of the reused PGA after the recycle experiments, N2 adsorption-desorption curves and SEM images were characterized. As shown in Fig. S13, there were no obvious change in the morphological structures of PGA after ten times of regeneration, which could be attributed to the well maintenance of interconnected porous structure (Chen et al., 2018a, 2018b). It can be seen from the N2 adsorption-desorption curves that after 10 cycles of experiments, the H3-type hysteresis loop of PGAs were more obvious, indicating that the materials produce more mesopores after combustion (Li et al., 2014).

3.4. The oil/water separation at dynamic state of PGA The oil collecting device using PGA acted as a filter and peristaltic pump as driving force was applied and the schematic diagram was given in Fig. 8a. The dynamic oil collecting operation was shown in Fig. 8b–e. As shown in Fig. 8b and Movie S2, when the peristaltic pump was turned on, no water flowed out after a period of time, which exhibited the excellent water resistance of PGA. Fig. 8c–e and Movie S3 showed that the PGA acts as a filter could quickly collect the organic solvent (toluene dyed by Sudan (I)) from the oils/ water mixture and then it was pumped through a rubber tube into the collection container. Owe to its excellent hydrophobic properties and promising structure, the 100 mL dyed toluene was quickly collected through a filter into a collection container after 3 min. This showed the huge potential of PGA in the dynamic oil/water separation process.

Table 1 Comparison of adsorption properties of various oil-absorbing materials. Adsorbents

Adsorption capacity (g/g)

Adsorbate

Reference

Peanut shell modified graphene Expanded perlite Carbon aerogel Silylated nanocellulose Sponges Cotton-cellulose aerogels Polydimethylsiloxane sponge Biomass-derived aerogel 3D graphene/polypyrrole foams PGA

38.2–79.3 12.6 12.8–26.75 49–102 40–100 4–11 20–40 37.01–108.76 40–130

Oils, organic solvents Medium Asian oil Oils, organic solvents Oils, organic solvents Oils, organic solvents Oils, organic solvents Oils, organic solvents Organic solvents Oils, organic solvents

(Li et al., 2019) (Bastani et al., 2006) (E et al., 2018) (Zhang et al., 2014) (H. Cheng et al., 2017; Y. Cheng et al., 2017) (Choi et al., 2011) (G. Jiang et al., 2017; J. Jiang et al., 2017) (Li et al., 2013) This work

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Fig. 8. Schematic diagram of an oil collection device consisting of PGA and peristaltic pump (a). Collecting water continuously with a collecting device (b). Collecting toluene (dyed by Sudan (I)) continuously with a collecting device: 10 s (c) 1 min (d) 3 min (e).

3.5. Adsorption of dyes The hydrophobicity of the PGA surface could also be improved in different pH conditions. As shown in Fig. 9a, PGA-2.03, PGA-3, PGA-7, PGA9 were hydrophobic (the water contact angle greater than 90°), whereas PGA-5 and PGA-11 were amphiphilic. Hydrophobicity was generally related to the roughness of the surface of material (Yue et al., 2007). The engine oil was employed to evaluate the oil adsorption performance of PGAs prepared at different pH values. The hydrophobic PGA obtained at pH value of 2.03 demonstrated the outstanding adsorption capacity in engine oil (130 g/g), whereas the amphiphilic PGA obtained at pH value of 11 can also reach 100 g/g. The adsorption capacity of PGAs prepared under different pH conditions can reach more than 70 g/g, which indicated that the oil absorption performance of PGAs was stable. For further exploring the influencing factors of oil adsorption, the density, specific surface area and pore size distribution of PGAs were detected and the results were shown in Table S3. The zeta potential curves of seven PGA samples at different pH values were shown in the Fig. 9b. The isoelectric point values for GA, PGA-9, PGA-7, PGA-5, PGA-3 and PGA-2.03 were located at 4.63, 4.37, 4.47, 2.88, 3.16 and 5.41 mV, which were higher than that of PGA-11 (2.09). This result also indicated that in the neutral environment, PGA-

2.03 has more positive charge, while PGA-11 has more negative charge. The change in pH of the zeta potential was mainly due to the change of the protonation ability of surface group (Yan et al., 2018). In reality, oily wastewater or waste oils tend to have a certain chromaticity color, while materials with a chromaticity color are generally water-soluble. In order to investigate the ability of PGA to remove chromaticity color, the adsorption capacity of PGAs (PGA-11, PGA-9, PGA-7, PGA-5, PGA-3, PGA-2.03) and GA for MO and MB was also determined. Dyes have high solubility and require high hydrophilicity on the surface of materials, so the adsorption of dyes is more challenging than that of oils and organic pollutants. As shown in Fig. 9c, PGA-11 showed the highest adsorption capacity for MB (215 mg/g). According to the equilibrium adsorption capacity, the adsorption abilities of different materials for MB decrease in the following order: PGA-11 (215 mg/g) N PGA-7 (165 mg/g) N PGA-5 (118 mg/g) N PGA-9 (116 mg/g) N PGA3 (67 mg/g) N GA (47 mg/g) N PGA-2.03 (40 mg/g), while the adsorption capacities of the seven graphene aerogels for anionic MO increase from PGA-2.03(18 mg/g) to GA(70 mg/g), which were related to the hydrophobic properties and surface charge of the surface. The PGA-2.03 exhibited a larger water contact angle (142.7°), therefore, it cannot be fully contacted with contaminants for better adsorption. Adsorption of dyes was a very complex process and all factors including molecular

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Fig. 9. Absorption capacities of engine oil and contact angel for GA and PGAs (a). Zeta potential curves of GA and PGAs at different equilibrium pH values (b). Adsorption kinetic curves of MB (c) and MO (d) on GA and PGAs.

properties as well as electrostatic interactions should be considered. From Fig. S15, the molecular structure of MB was more complicated than that of MO, so the steric hindrance of binding to materials was greater than that of MO. But the adsorption capacity of PGAs for MB was higher than that of MO. Therefore, the electrostatic interaction between materials and the hydrophilicity of the PGAs both contributed to the adsorption of dyes on the material surface. The adsorption mechanism of MB by PGA was shown in Fig. 10. Thus, PGA-2.03 can remove oil slick and dispersed oil from water in practical application, while

PGA-11 can be used in the presence of both oil slick and water-soluble organic matters. 4. Conclusion In this study, a PGA with high adsorption capacity and high hydrophobicity was produced by one-step hydrothermal treatment of doping P(AM-DMDAAC) into three-dimensional graphene. The experimental results showed that PGA had excellent adsorption capacities for oils

Fig. 10. Adsorption mechanism of MB by PGA.

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and organic solvents, with the adsorption capacities ranged from 40 to 130 g/g. Moreover, the adsorption capacity of the PGA had no significant loss after ten times of recycling. Meanwhile, PGA exhibited a great adsorption capacity for hydrophilic dyes such as MB (215 mg/g) and MO (65 mg/g). Excellent adsorption capacity, recyclability, economical as well as mechanical strength make PGA a durable water purification material. Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.133671.

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