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# A facile synthesis of porous graphene for efficient water and wastewater treatment

## Abstract

The use of two-dimensional graphene-based materials in water treatment has recently gained significant attention due to their unique electronic and thermal mobility, high surface area, high mechanical strength, excellent corrosion resistance and tunable surface chemistry. However, the relatively expensive, poor hydrophobicity, low adsorption capacity and recyclability, and complex post-treatment of the most pristine graphene frameworks limit their practical application. Here, we report a facile scalable method to produce highly porous graphene from reduced graphene oxide via thermal treatment without addition of any catalyst or use of any template. Comparing to conventional graphene counterparts, as-prepared porous graphene nanosheets showed evident improvement in hydrophobicity, adsorption capacity, and recyclability, making them ideal candidate materials for water treatment. Superhydrophobic and superoleophilic porous graphene prepared in this work has been demonstrated as effective absorbents for a broad range of ions, oils and organic solvents, exhibiting high selectivity, good recyclability, and excellent absorption capacities > 90%. The synthesis method of porous graphene reported in this paper is easy to implement, low cost and scalable. These attributes could contribute towards efficient and cost-effective water purification and pollution reduction.

## Introduction

In this study, we present a facile cost-effective production route for this novel superhydrophobic PG based on thermal treatment of rGO, so as to unlock the current main bottlenecks in its commercial application. Porous nanosheets with a surface area of 652 m2/g was achieved by introducing and disintegrating a small number of edge defects into the PG by thermal treatment of rGO. Additionally, the temperature range involved in this treatment process was (190–200 °C), which was lower than that previously reported for the synthesis of porous rGO (800 °C30). Chemically synthesized GO was reduced using hydrazine to rGO, which was then thermally treated to produce PG. Surface area of as-prepared PG was among the highest reported for porous rGO to date (652 m2 g−1). Moreover, graphene’s physiochemical, mechanical, electrical and thermal characteristics including high conductivity, high surface area, porous morphology and chemical inertness make them not only ideal candidates for water treatment but also potentially viable alternatives for other applications such as for batteries, photocatalysis, DNA sequencing, and enzyme modulation. As-prepared PG provided abundant ion/porous channels, showing an excellent selective adsorption capacities and fast adsorption kinetics for the elimination of arsenic, fluoride, nitrate, oil, methylene blue and rhodamine B. Systematic studies on the adsorption performances of PG were carried out, using different thermodynamic and kinetic models, and Fourier transform infrared (FTIR), X-ray diffraction (X-Ray) and elemental mapping analysis were carried to in order to elucidate the adsorption mechanisms. PG was revealed to be highly effective for water decontamination, with superior adsorption capacity, kinetics, recovery, regeneration and recyclability.

## Materials and Methods

### Synthesis of porous graphene

Exfoliated graphite oxide flakes, with ~0.5–20 μm lateral size and ~1.5 nm thickness, were prepared following the modified Hummers method previously reported by us36. Graphite was initially oxidized to form graphite oxide, which was further exfoliated and chemically reduced to graphene sheets. The thickness of GO sheets varied between 0.8–1 nm. Before the reduction, 500 ml aqueous solution of graphite oxide with concentration of 1 mg/ml was synthesized, and further exfoliated for 2 h in an ultrasonic bath (Bandelin Sonorex RK-100H). The pH values were found to be in the range 9–11. 150 ml GO (1 mg/ml) was combined with 1.5 ml of hydrazine (35 wt%) under magnetic stirring in a flask which was heated to 100 °C. After 12 h reaction, hydrazine (1.5 ml) was again added to perform the reaction for additional 2 h to ensure full reduction of the GO. The rGO was allowed to settle, washed with distilled water and filtered until the supernatant became clear. To obtain porous nanosheets, the filtered product was oven-dried in vacuum overnight and then thermally treated at 200 °C in Ar for 12 h under a slow ramp rate of 3 °C min−1. The preparation scheme is schematically shown in Fig. S1 of supporting information (SI).

### Basic Characterization

Microstructures of PG, graphite flakes and GO samples were taken on a Philips XL-30 scanning electronic microscope (SEM) under high vacuum conditions with accelerating voltage 20 kV and the samples were mounted onto carbon sticky tape. High resolution microstructural images were also taken, along with elemental mapping analysis, on a JEOL-2100 transmission electron microscope (TEM) operating at an accelerating voltage 200 kV. The powder sample was dispersed in acetone, after which the sample was dropped on the centre of a carbon Cu grid using a micro pipet. The X-ray photoelectron spectrometer (XPS) spectra were recorded on an ESCALab 250 XPS using monochromated Al Kalph X-ray as the excitation source, and the Raman spectra were collected using a 532 nm laser excitation operating at 6 mW power. The power of the laser was kept at 6 mW. X-ray diffraction (XRD) analysis was performed using Cu Kα radiation (at 40 kV and –40 mA). Fourier-transform infrared (FTIR) spectra were obtained in the wavenumber range of 2000–500 cm−1 using a Bruker Optics Tensor-27 FTIR spectrometer. UV–Vis absorbance were obtained by using a Jenway 6715 UV/Vis spectrophotometer. Nitrogen gas sorption analysis was conducted using a Quantachrome Autosorb-iQ gas sorptometer. Prior to the sorption measurements, sample was heated at 200 °C under vacuum conditions for 3 h. Surface area was calculated by using Brunauer–Emmett–Teller (BET) theory method. The total pore volume (Vt) was measured from the amount of adsorbed nitrogen (at P/Po = ca. 0.99). The wettability of PG samples was characterized by using a contact angle goniometer. Images of each droplet were captured on a digital camera and the contact angle measurements were based on the PolyPro software package.

### Removal of heavy metal and other contaminant ions

All of the chemicals used in the experiments were of the highest purity commercially available and were obtained from sigma Aldrich (detail is given in SI). Three stock solutions of 1000 mg/L arsenic, fluoride and nitrate were prepared by mixing appropriate amounts of sodium (meta) arsenite, sodium fluoride and potassium nitrate salt respectively in distilled water. Standard working solutions at the required concentrations with 130, 150 and 200 mg/l of arsenic, fluoride and nitrate were again prepared by diluting stock solution. 10 mg of PG was added to 25 ml of As (III), fluoride and nitrate working solutions of 130, 150 and 200 mg/l respectively, and the suspension was stirred and separated by filtration through a 0.2 micron membrane filter over different time scale from 0–60 mins. At these times, UV–vis absorption spectra were recorded at 280, 464 and 410 nm to monitor the adsorption processes of arsenic, fluoride and nitrate respectively. To verify the adsorption of fluoride, a fluoride kit was also used. The adsorption isotherms of arsenic on the PG were shown to the best fit to pseudo-first and pseudo-second order kinetic models and an intra-particle diffusion model with the experimental data of this study.

Linear transformations of pseudo-first order and pseudo-second order kinetic models37 are, respectively,

$$\mathrm{ln}({q}_{e}-{q}_{t})=\,\mathrm{ln}({q}_{e})-{k}_{1}t,$$
(1)

and

$$\frac{t}{{q}_{t}}=\frac{1}{{k}_{2}{q}_{e}^{2}}+\frac{t}{{q}_{e}},$$
(2)

where t, qe, qt, k1 and k2 represent time, amount of arsenic uptake per unit mass of adsorbent at a particular time, pseudo-first and -second order rate constants, respectively. The values of qe, k1 and k2 were calculated from the slopes of their respective graphs. In addition to these models, Fick’s second law was used to find out if intraparticle diffusion is a rate-controlling step during the adsorption experiment37,38:

$${q}_{t}={k}_{id}\sqrt{t}+I,$$
(3)

where I represents the boundary layer effect (a large value corresponds to a larger boundary layer thickness37,39) and kid is the intraparticle rate constant. The adsorption capacity, qe, was calculated (as a percentage) using qe = Co-Cf/Co × 100, where Co (mg l−1) is solute concentration at equilibrium, and Cf is the maximum amount of solute adsorbed corresponding to monolayer exposure. To evaluate the stability and reusability of PG, regeneration cycles were repeated five times. 0.2 M HCl solution was used as the desorption agent to recover As (III), fluoride and nitrate from the absorbed PG. The regenerated adsorbent was used for subsequent adsorption cycles under similar reaction conditions as carried out with fresh PG. The initial pH of stock solution was adjusted to neutral (pH 7) using NaOH or HCl solutions. All tests were conducted in triplicate and the average values were used for data analysis. The pH of the solution was maintained at neutral level. We measured the pH of the solution after treatment and found no change in the pH.

### Oil sorption experiment

The oil sorption capacity was achieved and measured by following Standard Test Method for adsorbent performance (ASTMF726–99). Five grades of oils were used in this study; vegetable, engine, pump, used engine and used pump oil. For oil sorption tests, oils were poured into petri dishes. The absorbent was pre-weighed and then weighed again instantly after the experiment. PG were squeezed gently to remove and drained for about 15 mins. 10 mg of PG were dipped in oil and distilled water (50 ml) mixture during this experiment. PG were dropped in a portable folded porous sheet and were removed after a given immersion time before being weighed. To assess the reusability/recyclability and regeneration of the used absorbent, PG were heated up to the boiling point of adsorbate to remove the oil prior to the next round of adsorption test. The regenerated absorbent was used for subsequent sorption cycles under similar conditions as carried out with fresh PG. Each experiment was repeated three times independently and average values were taken. The sorption capacity (Q) was determined using the weight (wt) of adsorbent before and after the experiment:

$$Q=\frac{(w{t}_{{\rm{after}}}-w{t}_{{\rm{before}}})}{w{t}_{{\rm{before}}}}$$
(4)

Stock solutions of dyes were prepared by dissolving precisely weighted amounts of methylene blue (MB) and rhodamine b (RB) in distilled water. Working solutions at desired concentrations were prepared by serially dissolving more water in stock solution.10 mg of PG were added to 25 ml of RB and MB solution (of concentration 150 mg l−1) and continuously stirred. UV–vis absorption spectrophotometer was used to record their respective spectra at different time intervals to monitor the adsorption at 535 and 496 nm respectively. The adsorption isotherms are fitted by the pseudo first order, second order kinetic and intraparticle adsorption models as described above37,38,39. Regeneration cycles were repeated five times. The adsorbed dyes were eliminated from the obtained adsorbent by heating at 400–450 °C in air for 2–3 h. The regenerated adsorbent was used for subsequent adsorption cycles under similar conditions as carried out with fresh PG.

## Results and Discussion

### Basic characterization

Figure 1 shows surface morphology of as-prepared PG. Scanning electron micrographs enable the visualization of wrinkles and corrugations in the graphene sheets40, and induce the formation of nano-sized channels or pores on the surface (Fig. 1A). As seen in Fig. 1A, the PG had an irregular, folded structure with sheets entangled with each other. This is further seen in the high-resolution transmission electron micrographs shown in Fig. 1(B,C). These images show folded edges of the flakes with few, irregularly stacked layers41 and the formation of pores over the sheets. The SEM images of graphite flakes and GO were shown in Figs S2 and S3. Furthermore, XPS study of the PG revealed that the C 1 s and O 1 s peaks appeared at ~281.18 eV and 528.77 eV, respectively (see Fig. 1D). High-resolution C 1 s and O 1 s scans were carried out in order to investigate the nature of functional groups. Figure 1(E,F) presents the deconvoluted C1s and O 1 s XPS spectra. The intensity of the C 1s spectra corresponding to sp2-hydridized carbon revealed the reduction of GO, thermal treatment of rGO and the restoration of graphene sheets28. PG (Fig. 1D) mainly was observed to have a peak at around 284.7 eV corresponding to the C=C network due to the sp2-carbon, which reveals the predominantly graphitic nature of carbon42. Deconvolution of the O 1 s spectra was observed to designate into two peaks, centred at 528.7 and 523.05 eV, which arose mainly from the oxygen containing functional groups (carboxylic, epoxy, carbonyl and hydroxide groups) at the surface of the basal plane of graphene43. To further clarify the growth process of the PG structure, the raw material and intermediate product were investigated by XRD, FTIR and Raman spectroscopy.

Specific characteristics of the nanostructured morphology have a great impact on the wettability of a surface53. The wettability of PG surface was studied by water contact angle measurements, and the results are given in Fig. 3(A–C). The PG was found to be superhydrophobic, with water droplet contact angles of 160° formed within 60 seconds (Fig. 3A). In contrast, oils were found to be fully absorbed on PG after 30 s (Fig. 3B), with contact angles of less than 5°, demonstrating that the PG were superoleophilic. The extent of the hydrophobicity of graphene depends on the ethyl functional groups on its surface. The wetting transparencies can be engineered by altering the functional groups, specific surface area, surface roughness and wrinkling network of graphene sheets54. The stabilization mechanism and electrostatic interactions are equally important factors that prevent graphene sheets from aggregating in an aqueous solution. The carboxyl and hydroxyl functional groups on the surface of PG indicated that these groups were not reduced by hydrazine55,56, as confirmed by the FTIR analysis (see Fig. 2B).

### Removal of heavy metal and other contaminant ions

Figure 4B and C show the adsorption capabilities of PG for the elimination of fluoride and nitrate. The adsorption capacity of fluoride (Fig. 4B) followed a similar trend to that of As (III), and was much higher than that reported by Gupta et al.62. When fluoride adsorption was tested using a fluoride kit, complete elimination of fluoride from aqueous solution was achieved, as shown in Fig. S4 in SI. The adsorption capability of as-prepared PG for all the tested ions can be ordered as As > Fluoride > Nitrate. The prominent single atom thick sp2 hybridized feature of graphene was responsible for its appreciable and rapid interaction with arsenic ions. Electrostatic interaction between PG and contaminant was also responsible for the fast adsorption kinetics of arsenic. The capacity for regeneration and reusability of PG for the removal of these micro pollutants was also examined. The adsorbent can subsequently be recycled after desorption of PG. Figure 4(D) shows the recyclability of PG in terms of the adsorption capacity of these metal ions. It was observed that the adsorption capabilities of regenerated PG remained considerably high, > 80% after 5 cycles, suggesting an environmentally friendly recyclability of PG for the decontamination of a variety of target contaminant ions.

### Sorption of Oil

As-prepared PG were used as a sorbent to examine their sorption efficiency for a variety of oils. Figure 5(A) shows that, upon their immersion, the oils were immediately absorbed and completely taken up within just 20 seconds. They exhibited oil absorbing capacities over a range between 54–165 times their own weight (Fig. 5B), indicating their superior oil-sorption capacity to other absorbents previously reported63. In a further test, as-prepared PG samples were wrapped in a combined oil and water system in a beaker (see SI Fig. S5). It was observed that they took up all the oil, and the oil uptake capacity from the mixture was very close to that from pure oil. Figure 5(C) illustrates the recyclability of PG in the oil uptake process. In the second cycle, the sorption capacity decreased, which was most likely due to an incomplete regeneration process: repeated cycling showed that the capacity was overall consistent and above 90% even after 15 cycles. All of these results indicated that as-prepared PG can be used as an effective, simple filter for the decontamination of large quantities of oils directly from wastewater.

### Adsorption mechanism and structural evolution

Based on the above analysis, we find five distinct reasons for the high adsorption capacities of the PG:

1. 1)

The synthesis of PG produces several types of active sites.

2. 2)

Surface complexation occurs between contaminants and the surface hydroxyl groups.

3. 3)

Co-exchanges occur between arsenite and fluoride anions as well as the surface hydroxyl and carboxyl groups. Arsenic and fluoride can react with PG adsorbent as given in equations (58):

$$PG-O{H}_{2}^{+}+{F}^{-}\to PG-OH-{F}^{-}+O{H}^{-}$$
(5)
$$PG-COO{H}^{+}+{F}^{-}\to PG-COO{H}^{+}-{F}^{-}$$
(6)
$$PG-O{H}_{2}^{+}+{H}_{2}As{O}_{4}^{-}\to PG-COO{H}_{2}^{+}-{H}_{2}As{O}_{4}^{-}$$
(7)
$$PG-COO{H}_{2}^{+}+{H}_{2}As{O}_{4}^{-}\to PG-COO{H}_{2}^{+}-{H}_{2}As{O}_{4}^{-}$$
(8)

where PG is representing porous graphene. Reactive functional groups react quite readily with most of the organic pollutants and also cleave the bonds in organic molecules. Free radical generation in adsorbents also play a vital role in adsorption. As a large number of free radicals may provide more active and chemically reactive sites for chemisorption where organic molecules or metal ions might extensively be adsorbed. Free radicals contain a strong initiation source of adsorption and can overwhelm the majority of the organic compounds and contaminated ions.

4. 4)

The abundant channels (porous architecture) on the PG surface are favourable to the adsorption of contaminants having different sizes.

5. 5)

Surface area introduces stacking sites and physiochemical partition to adsorb molecules because of the availability of significant transformation sites. Generally, the affinity in the adsorption process between adsorbent and adsorbate depends on the high surface area.

## Conclusion

In summary, we investigated facile, scalable and novel synthesis of PG and its application for the treatment/removal of pollutants present in water and wastewater. As-prepared PG material exhibited highly selective absorption and adsorption capacities of about 99% for a wide range of ecologically important pollutants, including heavy metal ion (arsenic) and other contaminated ions (fluoride and nitrate), oils (vegetable, engine, pump, used engine and used pump oil) and dyes (methylene blue and rhodamine B). The dominant mechanisms of surface complexation and co-exchanges with hydroxyl and carboxyl groups of graphene nanosheets were revealed. PG was nontoxic and environmentally friendly, and was demonstrated to not only have highly efficient adsorptive properties but also have superior regeneration and cycling efficiency (above 90% after 5 cycles). Therefore, the application of developed PG materials could contribute towards efficient water/wastewater treatment, reduce pollution load and improve the access to safe drinking water in areas where groundwater is contaminated. The conventional energy intensive treatment processes employed are expensive and unsustainable and show low efficiency in contaminant removal. An improved understanding of complex interactions and interferences from contaminants to the treatment of water and wastewaters under controlled conditions can provide insights into pollutant specific treatment kinetics and support the development of compact optimal treatment strategies.

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## Acknowledgements

The authors acknowledge support from the EPSRC Centre for Doctoral Training in Metamaterials at the University of Exeter [Grant no. EP/L015331/1].

## Author information

### Affiliations

1. #### College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, United Kingdom

• Tanveer A. Tabish
• , Fayyaz A. Memon
• , Diego E. Gomez
• , David W. Horsell
•  & Shaowei Zhang

### Contributions

F.A.M., S. Z., D.E.G. and T.A.T proposed and designed this project together. T.A.T. completed most of the experimental works. T.A.T. and D.W.H. wrote the main manuscript and SI text and prepared the Figures. All authors reviewed and approved the manuscript.

### Competing Interests

The authors declare that they have no competing interests.

### Corresponding authors

Correspondence to Fayyaz A. Memon or Shaowei Zhang.

## Electronic supplementary material

### DOI

https://doi.org/10.1038/s41598-018-19978-8