Nanohybrid Graphene-Based Materials for Advanced Wastewater Treatment: Adsorption and Membrane Technology

INTRODUCTION

Graphene is considered to be one of the most hoi materials of recent years because of its numerous unique properties (mechanical, optical, environmental, etc). In 1986, Boehm et al. [1] described in detail the structure of graphite having a single atomic sheet [1]. The major turn in graphene history occurred from 2000-2010, when it was proved that 2-D crystals (such as graphene) had no thermodynamic stability, suggesting their nonexistence at room temperature [2]. In particular, graphene sheet is thermodynamically unstable if its size is less than about 20 nm (graphene is the least stable structure if lower than about 6,000 atoms); it becomes the most stable fullerene (as within graphite) only for molecules larger than 24,000 atoms [3].

The leader of graphene science is Konstantin Novoselov who successfully isolated and characterized an exfoliated graphene monolayer with various techniques [4]; A.K. Geim and K.S. Novoselov were awarded the Nobel Prize in 2010 for their impact on graphene science. But what is graphene? The reply was clear and was given by IUPAC: graphene is a carbon layer (single) of graphite, having a structure/nature similar or analogous to an aromatic hydrocarbon (polycyclic) of quasi-infinite size [5]. This means that graphene is a flat monolayer of hybridized sp² atoms of carbon, which are densely packed together into an ordered two-dimensional honeycomb network [6]. An hexagonal unit cell of graphene comprises two equivalent sublattices of carbon atoms, joined together by sigma (a) bonds with a carbon - carbon bond length of 0.142 nm [7]. Each carbon atom in the lattice has a н-orbital that contributes to a delocalized network of electrons, making graphene sufficiently stable, compared to other nanosystems [8]. The applicability of graphene is based on an advantageous network provided by this material: combination of high three-dimensional aspect ratio and large specific surface area, superior mechanical stiffness and flexibility, remarkable optical transmittance, exceptionally high electronic and thermal conductivities, and impermeability to gases, as well as many other supreme properties. Due to all of these characteristics, Novoselov characterized graphene as a miracle material [9].

In some of the most important applications of graphene in wastewater treatment, the oxidized form of graphene (graphene oxide) is used. The large scale production of functionalized graphene at low cost should result in good adsorbents for water purification [10], due to the two-dimensional layer structure, large surface area, pore volume, and presence of surface functional groups in these materials; the inorganic nanoparticles also prevent the adsorbent aggregation. Water, as it is known (i.e., from several good handbooks), can be treated and purified by multiple techniques such as desalination, filtration, membranes, flotation, adsorption, disinfection, and sedimentation. Certainly, adsorption holds advantages over other methods (various methods are discussed in the following sections), such as ease of operation and comparatively low cost. Adsorption is the surface phenomenon in which pollutants are adsorbed on the surface of a material (adsorbent) via physical and/or chemical forces. It depends on many factors such as temperature, solution pH, concentration of pollutants, contact time, particle size, temperature, and nature of the adsorbate. In addition, apart from its use in adsorption, graphene oxide can be used as a supplement in membrane technology, especially in nanofiltration. Therefore, in this chapter the use of graphene oxide is analyzed for wastewater treatment and many examples for membranes (antifouling properties) and adsorbents are given.

SYNTHESIS PROCEDURES

Before further analysis, it is mandatory to give a definition about graphene composites. Graphene composites are considered to include all graphene- based materials that have been modified (e.g., grafting with reactive groups, functionalizations with polymers, complexes with other sources).

Graphene oxide (GO), which is considered to be the most known graphene composite material, results from the chemical exfoliation of graphite. It is a highly oxidized form of graphene, consisting of numerous and varying types of oxygen functionalities. Many theories have been developed in the past for the determination of the exact chemical structure of GO [11. 12]. This is mainly because of the complexity of the material (including sample-to-sample variability), and, of course, its amorphous, berthollide character i.e. nonstoi- chiometric atomic composition [13]. The Lerf-Klinowski model describes a theory according to which, the carbon plane in GO is decorated with hydroxyl and epoxy( 1,2-ether) functional groups [14]. The consideration for the existence of some carbonyl groups is correct, most likely as carboxylic acids along the sheet edges, but also as organic carbonyl defects within the sheet [15, 16]. The synthesis of GO is based on three preparation methods: (i) Brodie’s method [17], (ii) Staudenmaier’s method [18], or Hummers' method [19].The major part of all methods is the chemical exfoliation of graphite using oxidizing agent in the presence of mineral acid. Two methods (Brodie’s method and Staudenmaier’s method) apply a combination of K.CIO4 with HNO3 to oxidize graphite. Hummer’s method adds graphite to potassium permanganate and H2SO4. The oxidation of graphite breaks up the л-conjugation of the stacked graphene sheets into nanoscale graphitic sp2 domains surrounded by highly disordered oxidized domains (sp³ CC) as well as defects of carbon vacancies

[20]. The GO sheets produced consist of phenol, hydroxyl and epoxy groups, mainly at the basal plane, and carboxylic acid groups at the edges [21]; they can thus readily exfoliate to form a stable, light- brown - colored, single-layer suspension in water [20].

Next, some major synthesis procedures are analyzed in detail to describe the nanohybrid graphene oxide preparation.

Synthesis of GO

A very crucial step in preparing an efficient graphene-based material, either for modification of membranes or for simple use as adsorbent, is to select the correct synthesis route. Some slightly different ways of modified graphenes are described next.

Nanohybrid GO

A classic synthesis way was described by Suresh Kumar et al. [22]. This team used graphite for the synthesis of GO. Specifically, 3 g of graphite flakes were dissolved in a mixture of H2SO4/H3PO4 (360/40 mL), and then 18 g of КМПО4 were added slowly under continuous stirring for 12 h at 50°C. After this process, in order to exfoliate GO into single layers, the mixture was cooled down to room temperature and it was mixed with a solution ~400 mL ice water with concentration 3 mL of 30% hydrogen peroxide followed by sonic- ation for 0.5 h. In addition, the diluted mixture was centrifuged at 10,000 rpm for 15 min. Multiple washings (30% HCI) were made with water to remove the residues from the solid particles. Then, the experimental process was followed by a vacuum drying process at room temperature for 12 h [22].

The next step was the preparation of Mn Ре₂0₄ nanoparticles. Briefly, 100 mL of deionized water were used to dilute 0.845 g of MnS0₄ H₂0 and

2.7 g of FeCl.v6H20 (the molar ratio of Mn:Fe in the mixture was 1:2). The mixture was then continuously stirred, and heated at temperature 80°C.

To slightly modify the pH of the mixture to 10.5, drops of 8 M NaOH were inserted slowly to the same temperature for 5 min and then the mixture was cooled down to room temperature. To separate the blackish precipitates, a magnetic process was used. The obtained blackish precipitates were then washed with excess of water to remove the unreacted quantity, and were then washed with propanol. Finally, the obtained blackish precipitates were dried at room temperature for 24 h [22].

The final step was the synthesis of Go-MnFe₂0₄ nanohybrids. Therefore, in the final experimental process, 0.5 g of GO were inserted to 400 mL of water and were then dispersed by ultrasonication for 5 min. 0.845 g of M11SO4 H₂0 and

2.7 g FeCl.y6H₂0 were added to the colloidal GO mixture and after this, this solution was stirred for 30 min (until the increase of the mixture temperature to 80°C). Similarly, to increase the pH of the solution, 8 M NaOH was added dropwise and heated up to the same temperature with the mixture. The experimental reaction process was then continued for 5 min and then the yielded mixture cooled down to room temperature. A magnetic process was then used to separate the nanohybrid particles. The nanohybrid particles were washed with excess of H₂0 and propanol and dried for 24 h at room temperature [22].

Preparation of GO and Reduced GO (rGO) Mixture

Lee et al. [23] synthesized neat GO from natural graphite according to modified Hummers method. Potassium permanganate, nitric acid, sulfuric acid, and hydrogen peroxide were used as reagents to oxidize graphite to GO; 0.12 g of graphite flakes were added to a mixture of concentrated H₂S0₄/HN0₃ (6 mL:0.132 mL) and 0.72 g potassium permanganate was then inserted gradually to the mixture under stirring for 2 h to temperature 35°C-45°C. After this step, the mixture was heated up to 100°C and stirred for 30 min, and 42 mL of water and 1.2 mL hydrogen peroxide were added. The mixture was cooled down to room temperature to remove the acidic supernatant from the mixture, and was then centrifuged at 13,000 rpm for 15 min. After the removal of the acidic supernatant by centrifugation process, distilled water was inserted to the mixture to dilute the acidic remnant from GO. To redisperse the pellets, the mixture of GO pellets and distilled water was vortex-stirred for 1 min. This process was done to obtain nearly neutral aqueous mixture, and was continuously repeated with centrifugation and vortex, alternatively. The graphite oxide pellets were then added to N-methyl pyrrolidone (NMR), followed by sonication. A tip sonicator, was used for the exfoliation, of graphite oxide to GO. The ultrasonication also included an ice water bath for 1 h. Subsequently, the preparation of reduced graphene oxide (rGO), was achieved by reducing dispersed GO in the resultant homogeneous GO mixture using hydrazine (N₂H₄, Sigma-Aid rich) in the ratio of 0.7 mg/mg of GO. Then, the mixture stirred for 10 hat 80°C [23].

To prepare the respective membranes, N-methyl pyrrolidone (NMR) was used for the exfoliation of graphene oxide with the sonication process. To obtain various GO concentrations (0.02, 0.05, 0.14, 0.20 and 0.39 wt%) an NMR mixture of polysulfone (PSf) (15 wt% PSf and 85 wt% of NMR) was used to disperse the nanoplatelets of GO. The solution was then stirred to 60°C and kept at room temperature overnight. The mixture was sonicated for 1 h so that the bubbles in the solution would disappear. Then, an Elcometer 3570 (Micrometric Film Applicator) was used to cast the polymer mixture on a nonwoven polyester fabric. A 24-h water bath was used for the immersion of the produced membranes to achieve a complete liquid - liquid demixing. The yielded PSf/GO membranes had GO:PS ratios of 0.16, 0.32, 0.92, 1.30, and 2.60 wt% [23].

Synthesis of Fe3О4/rG0

In another study, Wang et al. [24] used 120 mg of graphite oxide under the ultrasonication process for 2 h to be dispersed in deionized water. The yielded exfoliated GO nanosheets were stuck to Fe₃0₄ nanoparticles with the application of the postoxidation method. A solution of 1 M FeCl.vbhbO was added to GO and extrasonicated for 1 h. Then, with the use of a dropping funnel, a KBH₄ solution (1.65 M) was injected to the dispersion solution of GO with vigorous stirring. The reaction was finished after 4 h and the mixture was cooled down to room temperature. The synthesized black solution was filtered and washed several times with water and ethanol to remove residual acid and dissociative Fe(ll), To obtain Fe₃0₄/rG0, the yielded solid was soaked for 60 min with the addition of anhydrous ethanol. Finally, the yielded solid residues were dried at 60°C in vacuum atmosphere [24].

The synthesis of Ppy-decorated Fe^OVrGO was done by using ammonium persulfate (APS) as the oxidant Ppy was embedded on the Fe₃0₄/rG0 composite surface. The process conducted by in situ polymerization under nonacidic conditions. In order to form dispersion solution, the magnetic Fe₃0₄/rG0 nanoparticles were first sonicated for 3 h into 100 mL hexadecyltrimethylammonium bromide (СТАВ) solution. A 1 mL pyrrole monomer was inserted and dissolved into this solution. Then, 20 mL of APS solution was dropped slowly, after cooling to 273 K. and was stirred at this temperature for 8 h. The yielded product was then washed with water and anhydrous ethanol and was finally dried at 60°C in a vacuum oven. The modification process of the Ppy-Fe₃0₄/rG0 composite is schematically illustrated in Figure 4.1 [24].

Preparation of GO with One-step Ultrasonication

Zhibin Wu et al. [25] also followed the modified Hummers method for the preparation of GO from graphite powder (particle size<30 pm). Briefly, a beaker 250 mL was used to place 1 g of sodium nitrate and 2 g of graphite. During the stirring process in ice bath, 46 mL of H₂S0₄ (98%) was added; 1 g of sodium nitrate and 6 g potassium permanganate were slowly dropped under vigorous stirring (283 K) to the suspension. After vigorous stirring for 2 h in ice bath, the mixture was stirred for 30 min to 303 K. As the reaction process continued, the color of the solution gradually transformed to brownish paste. The new yielded paste was dissolved into 92 mL of ultrapure water under vigorous agitation for 30 min at 368 K. This had as a consequence, to change the color of suspension to bright yellow. 10 mL of hydrogen peroxide (30 wt%) was added to the solution to terminate the reaction process, and was then stirred at room

FIGURE 4.1 Schematic illustration of the ternary composites preparation. Reprinted with permission from Hou Wang et al. [24]. Copyright (2015) Elsevier.

temperature for 2 h. This process was achieved, when the suspension temperature was 333 K. The precipitate after the centrifugation process was washed repeatedly with 5% hydrochloric acid to remove residual metal ions, and was then washed with deionized water to remove the sulfate ions. Finally, the yielded precipitate- GO was sonicated and dried at 338 К under vacuum [25].

As presented in Figure 4.2, the synthesized RL-GO hybrid composite was prepared by a one-step ultrasonication process. 200 mg of GO were used and dissolved into 100 mL of dimethylformamide (DMF), followed by 1 h sonic- ation. Then, 600 mg of rhamnolipid were added to the GO suspension and sonicated under vigorous stirring until complete dissolution. After that, 1 g of N-(3-dimethylaminopropyl-N-ethylcarbodiimide) hydrochloride and 200 mL of 4-(dimethylamino) pyridine were added to the suspension. The reaction process under stirring and ultrasonication was allowed to progress for over 3 h. Then, the procedure continued under vigorous stirring by adding methanol to the precipitation of the suspension. The black solid precipitate yielded with the centrifugation process. The process contained washing of black solid precipitate five times with anhydrous ethanol and then twice with ultrapure water. Finally, freeze-drying under vacuum was used to the yielded rhamnolipid- functionalized GO composite. The obtained GO composite was suitable for adsorption applications [25].

Synthesis of Magnetic Chitosan Functionalized with GO

A very promising approach regarding the use of nanohybrid GO in wastewater treatment is the combination with magnetic chitosan detailed by Fan et al. [26].

FIGURE 4.2 Schematic depiction of the formation of RL - GO and application for removal of MB. Reprinted with permission from Zhibin Wu et al. [25]. Copyright (2014) Elsevier.

25 mL of double distilled water, 1.7312 g of FeCl₂ 6 H₂0 and 0.6268 g of Fe СЬ 4 H₂0 were inserted to ammonia solution, which was purged with N and stirred in a bath of water for 3 h at 90°C. Magnetic separation yielded the magnetic particles that were used in the chitosan coating. In order to give a final content of 1.5 % (w/v), 0.3 g chitosan was diluted in 30 mL 3% of acetic solution. A four-neck rounded bottom flask was used to add the chitosan mixture and 0.1 g magnetic particles. After that, 2.0 mL neat glutaraldehyde was inserted into a reaction flask to blend with the mixture and was stirred for 2 h at 60°C. The obtained precipitate was purged with petroleum ether, ethanol, and distilled water until pH was about 7, and was then dried at 50°C in a vacuum oven. The resulting product was magnetic chitosan [26].

On the other hand, GO was synthesized from purged natural graphite according to the modified Hummers method. Briefly, potassium permanganate and natural graphite were agitated for 12 h at 60°C with the mixed acid (HN0₂:H₂S04=1:9), and were then added to the mixed liquor hydrogen peroxide and agitated for 1 h, to yield a bright yellow material. This obtained bright yellow material was pureed with 2 M HCI to remove bisulfate ions and was then additionally washed with abundant amount of water to become the solution neutral. In addition, the GO was obtained by centrifuging, and was then dried in a vacuum desiccator [26].

To prepare the functionalized final material, a special procedure was followed. Ultrapure water was used to sonicate GO for 3 h to obtain a GO dispersion. Then, to activate the carboxyl groups of GO, a mixture of 0.05 M NHS and 0.05 M EDC was inserted into the GO dispersion under continuous stirring for 2 h. Then, in order to maintain the pH of the obtained solution at 7.0 was used dissolved sodium hydroxide. After that, the activated GO mixture and 0.1 g of magnetic chitosan (MC) were inserted in a flask and dispersed by ultrasonic dispersion in distilled water for 10 min, and the blended solutions were agitated for 2 h at 60°C. The yielded precipitate in turn until pH was about 7.0, was washed with 2 % (w/v) NaOH and distilled water. Furthermore, the yielded material was collected with the use of a magnet, and in order to obtain the final MCGO product dried at 50°C in a vacuum oven. Figures 4.3 and 4.4 present the synthesis of magnetic chitosan and GO and their application, respectively [26].