# Surface modified mesoporous g-$C_{3}N_{4}$@FeN$i_{3}$ as prompt and
proficient magnetic adsorbent for crude oil recovery

Meenakshi Talukdar, Sushant Kumar Behera, Kakoli Bhattacharya and Pritam Deb
Corresponding Author<EMAIL_ADDRESS>Advanced Functional Material
Laboratory, Department of Physics, Tezpur University (Central University),
Tezpur-784028, India.

###### Abstract

Efficient oil adsorption and recovery is a generous universal importance for
future energy demand and environmental protection. Adsorbents based on 2D
flatland with engineered surfaces can overcome the limitations of conventional
methods for selective oil absorption. Here, we report magnetic
hydrophobic/oleophilic graphitic $C_{3}N_{4}$ nanosheets that exhibit
excellent oil sorption performance and ready removal of adsorbed oil using
magnetic field. Combining porous and nanosheet structure along with magnetic
FeN$i_{3}$ and fatty acid surface functionalization make the system an ideal
adsorbent for adsorbing and separating viscous crude oil from water. The
nanocomposite can be further recycled and reused in an ecofriendly manner for
oil adsorption and recovery. The graphitic sheets selectively absorb a wide
range of drilled oils with enhancement of thickness upto 9 folds than the
pristine one. Oil can be collected and recovered with high efficiencies once,
it gets adsorbed by the adsorbent.

keywords

graphitic carbon nitride . hydrophobicity and oleophilicity . magnetic
nanocomposite .
oil-water sorbent . oil separation.

Crude oil is important and predominant energy resource in present human
societies. The availability of crude oil needs to be increased to match the
rapidly growing consumption. Hence, the proficient recovery of spilled oil
from oily-water is an importance concern. During crude oil exploration, a vast
amount of oil in mud is generated and wasted causing soil pollution L. Lee
(2010). The wastage of oil during drilling not only leads to compromisation on
future fuel consumption, but also creates significant threats to environment
and human health. On the other side, oil spills often causes immediate and
long term environmental damage Jernelov (2010). The oil spillage can be
cleaned traditionally by mechanical collection Keller and Broje (2006),
absorbent materials Choi and Cloud (1992), chemical dispersants E. B.
Kujawinski (2011a), bioremediations H. A. Aziz (2010), in situ burning I.
Buist (1999), dispersants E. B. Kujawinski (2011b), solidifiers A. Vidyasagar
and Sureshan (2011), skimmers Broje and Keller (2006), etc. These conventional
oil-removal technologies fail to meet the required efficiency without
affecting the ecosystem. In order to sustain the crude oil sources for future
generations, it is required to adopt corrective means for maximum utilization
and minimum wastage.

While developing an effective treatment method, it is essential to choose an
appropriate adsorbing material to yield oil from water surface. Worldwide
efforts are underway for developing sorption material with optimum water
repelling property (i.e. hydrophobicity) as well as oil adsorption capacity
(i.e. oleophilicity) for practical utilization. Oil adsorption generally
follows three primary steps, like the diffusion of oil molecules into the
sorbent surface, capillary entrapment of the same molecules in the sorbent
structure and oil droplet agglomeration in the porous structure of the sorbent
D. L. Tian (2011). In this regard, few of the synthetic and natural sorption
materials, with both hydrophobic and oleophilic properties, have been
investigated and tested for oil-removal purpose H. Sun (2013). An active area
of research in this regard is to optimize adsorption performance, where the
surface area plays major role Y. Si (2014). Primarily, they should be
developed with high specific surface area and large proportion of meso-pores.

Figure 1: Surface atomic configuration of (a) g-$C_{3}N_{4}$ layer structure
with a interplanner spacing of 12 $\AA{}$, (b) composite of g-$C_{3}N_{4}$ and
FeN$i_{3}$ where the FeN$i_{3}$ is stabilized on surface of the layer
structure, monolayers of (c) g-$C_{3}N_{4}$ and (d) composite. Color codes of
the respective atoms are marked in the figure. Dotted red circle shows the
presence of FeN$i_{3}$ on g-$C_{3}N_{4}$ surface.

In such situation, two dimensional (2D) materials have great potential towards
adsorption processes due to its high surface to volume ratio unlike, its bulk
form S. Barg (2014). Besides, for decent adsorption outcome, high content of
carbon and nitrogen are preferred to present as a prerequisite in the
adsorbent moiety. The strategy here is to employ graphitic $C_{3}N_{4}$, which
provides high surface area owing to its 2D sheet structure. Nontoxic iron
nickel (FeN$i_{3}$) magnetic nanoparticles (MNPs) have been mixed with
graphitic carbon nitride (g-$C_{3}N_{4}$) to form the composite material
(shown in toc Fig. 1). The layers in the g-$C_{3}N_{4}$ sheet are connected
via tertiary amines in a stacked fashion and separated by weak van der Waals
(vdW) forces. The vdW force hardly regulate the uniform delocalization (i.e.
the distribution of magnetic spheres) of FeN$i_{3}$ MNPs on the sheet surface
enhancing the sorption capability of the host flakes. Besides, the
nanocomposite exhibits hydrophobic and oleophilic properties after surface
engineering with stearic acid $(CH_{3}(CH_{2})_{16}$COOH), which makes the oil
extraction easy and effective. Moreover, the nanocomposite efficiently removes
variety of oils from water surface under an external magnet.

The minimum energy based structures of the composite system are fixed using
geometry optimization via Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm W.
Lei (2013). The geometry optimization is fixed via affecting the organizing
atoms in the supercell to minimum total energy position for stable geometry
(Figure 1) with its electronic structure to verify its stability.

Figure 2: (a)TEM micrograph of surface modified g-$C_{3}N_{4}@FeNi_{3}$
nanocomposite before oil absorbtion with SAED pattern embedded at the left
top, AFM 2D images of the surface modified nanocomposite (b) before oil
absortion and (c) after oil absorption embeded with their respective 3D image
on the left top. (d) hydrophilic/oleophobic surface of nanocomposite showing
contact angle below 9$0^{0}$ and (e) surface modified to
hydrophobic/oleophilic showing contact angle above 9$0^{0}$. (f) The final
surface topography of the material is shown after oil absorption corroborating
AFM image.

A representative TEM image of the composite system before surface modification
was shown in Figure 2 (a) with its SAED pattern and schematic structure with
contact angle (Figure 2 (d)). The image revealed uniformity in shape and size
of MNPs, with a diameter around 70 nm over the sheet surface in the composite
system. A stacked graphitic structure was noticed with the dispersion of MNPs
on the carbon nitride sheets impeding the formation of composite system. SAED
pattern indicated that the nanoparticles are polycrystalline in nature with
(002), (111) and (100) planes of both FeN$i_{3}$ and g-$C_{3}N_{4}$ identified
in the composite. AFM images of the system were shown in Figure 2 (b) and (c))
to visualize the surface topography before and after oil removal, respectively
along with their schematic structure (Figure 2 (e) and (f)) for easy
visualization. AFM images provided convincing evidence regarding post-oil
absorption enhancement in MNP size indicating the formation of finite number
of nanoparticle clusters over the sheet surface. Also, the images revealed the
uniform increase of nine times in thickness of the nanosheets after oil
absorption (Figure 2 (c)) compared to the pristine nanosheets (Figure 2(b)).

Micro-structural study of the composite was carried out through x-ray
diffraction (Figure S2-4). The diffraction peaks observed at 2θ=27.40, 44.10,
51.30 and 75.70 in the composite system indexed to (002), (111), (200) and
(220) lattice planes respectively. The above results endorsed the formation of
g-$C_{3}N_{4}@FeNi_{3}$ composite. However, the peak intensity became weaker
and the width of the diffraction peak became broader indicating interactions
between MNPs and host sheet due to presence of Fe in the composite system.

Figure 3: (a) GC-MS plot of the surface modified composite before and after
oil absorptions. (b) Nitrogen adsorption-desorption isotherm, where the pore
size distribution of the same is in the inset. (c) Light microscope image
showing the oil presence on the surface of the composite.

We first used a laser with 514 nm wavelength with hardly detectable Raman
peaks. Fluorescence background of the composite suppressed the Raman peaks at
lower wavelength excitation source of 514 nm A. D. Liao (2011). Lastly, 699
c$m^{-1}$ was due to bending and stretching mode on ring atoms of NCN and CN
(shown in Table S1). Due to breaking of the cyano group, no further Raman
modes were obtained beyond 1650 cm-1 (Figure S6). With rise in temperature
from 45$0^{0}$ C to 55$0^{0}$ C, the crystalline phase transforms to complete
amorphous state corroborating our XRD results T. Rojalin (2016).

The GC-MS characterization (Figure 3(a)) was performed to confirm the
adsorption of the viscous oil and its removal efficacy. It was observed that
several chromatographic peaks were present revealing the existence of crude
oil in the oil-water mixture before oil removal. The water, left-over in the
Petri dish after oil adsorption, was tested again and found no such peaks. The
absence of aforementioned peaks indicated the absence of oil in the oil water
mixture showing efficient oil removal by the composite.

The FTIR spectra of g-$C_{3}N_{4}$ sheet were plotted for 45$0^{0}$ C and
55$0^{0}$ C and 50$0^{0}$ C for comparison with the composite system. The
adsorption band of composite system at 1605 c$m^{-1}$ were assigned to C=N.
Similarly, the absorption bands at 1257, 1311 and 1424 c$m^{-1}$ were assigned
to aromatic C-N stretching heterocycles. The presence of MNPs suppressed the
peak intensity in case of composite suggesting weak interaction between the
other carbon and nitrogen bonds X. Gao (2016). The textural properties of
carbon nitride based catalysts were studied by nitrogen sorption. Figure 3(b)
displayed the absorption-desorption isotherm curves of g-$C_{3}N_{4}$ sheet
showing a characteristic of type IV isotherm pattern. The surface area was
found to be 41 $m^{2}g^{−1}$ for g-$C_{3}N_{4}$ from the Brunauer-Emmett-
Teller (BET) curve. Barrett-Joyner-Halenda (BJH) curve indicated the pore-size
distribution of the flake reflecting the existence of mesopore in the flake
structure (inset of Figure 3(b)). The pore-size distribution curve showed the
mesoporous structure with the pore size in the range of 4-40 nm. The textural
property of the g-$C_{3}N_{4}@FeNi_{3}$ is basically consistent with that of
g-$C_{3}N_{4}$ support, suggesting that the FeN$i_{3}$ nanoparticles do not
block the pore distribution on the sheet surface. Figure 3(c) gives the light
microscopic image of the oil absorbed g-C3N4@FeNi3 surface. The spotted green
colour dotted line reveals the presence of crude oil on nanocomposite surface.

Figure 4: Particle size distribution pattern of (a) surface modified
g-$C_{3}N_{4}@FeNi_{3}$ nanocomposite (i) before oil absorbtion embe dded
(right hand top position of the image) with contact angle and (ii) after oil
absorption inset with contact angle showing hydrophobic and oleophilic
behaviour. (b) Removal process of crude oil ((i)-(iv)) from water surface by
nanocomposite under magnetic ﬁeld.

The surface engineering enabled in wettability transition of the
nanocomposite, which was ensured by contact angle measurements before and
after oil absorption (shown inset of Figure 4a (i) and (ii) respectively). The
increase in contact angle upto 140° implied the hydrophobic and oleophilic
nature of the prepared nanocomposite material. The shape of the water droplet
above the sample was uniform and indicating the surface to be highly
hydrophobic. The contact angle showed hydrophobic behavior after reusing the
composite for oil removal for the second time (shown inset of Figure 4a (ii)).
The nanocomposite retained its hydrophobic and oleophilic behavior after its
first use which showed the transition in wettability nature due to good
stearic acid based surface engineering of the nanocomposite. The dynamic light
scattering (DLS) measurements are shown in Figure 4a (i) and (ii), which
evaluated the hydrodynamic diameters. The hydrodynamic diameter was obtained
to be almost 1.2 $\mu$m before oil extraction, while this diameter value
increased significantly after oil adsorption. The hydrodynamic diameter,
measured by DLS, was affected by the viscosity and concentration of the
medium. As a result, the value was larger than that the value obtained from
TEM image of the same system (Figure 2a).

The highly hydrophobic and oleophilic nanocomposite exhibited a selective
absorbance for oil recovery. When brought into contact with a layer of crude
oil in oil-water mixture, the nanocomposite adsorbed the oil instantaneously
repelling the water. Interestingly, the oil-adsorbed nanocomposite could be
removed from oil-water mixture with an external magnet with a very quick
response. The process of oil absorption and removal is shown in Figure 4b (i)
to (iv) in stepwise manner.

The outcome efficacy is measured using the formula C. Li (2017)

$n=(m_{2}-m_{1})/c_{0}$

where $m_{1}$ is the weight of nanocomposite before extraction and $m_{2}$ is
the weight of nanocomposite after oil extraction. $c_{0}$ is the weight of the
added crude oil. The outcome efficiency of crude oil extraction is found to be
92 % with the prepared magnetic nanocomposite. Apart from crude oil
separation, other oils such as Mobil, Petrol and Mustard oil were also used in
extraction process. It has been found that surface modified 2D
g-$C_{3}N_{4}@FeNi_{3}$ can also be used to separate these variety of oils.
Apart from crude oil separation, it showed noted removal efficiency in case of
other oils.

The nanocomposite can be further recycled and reused for oil adsorption and
separation in an ecofriendly manner as shown in Figure 5. The graphitic
magnetic nanocomposite once used can be recycled and reuse again. The system
can be further reused easily by washing the composite with ethanol and then
drying at a temperature of 6$0^{0}$ C. The removed oil can be recovered
successfully, apart from its reusability. Figure 5(c) shows the recovery of
oil after separation.

Figure 5: The collected surface modified g-$C_{3}N_{4}@FeNi_{3}$ nanocomposite
(a) is washed (b) and oil is collected (c).

In summary, we develop a novel and smart surface engineered sorbent material
to realize magnetic separation of oil from oily water mixtures. The stearic
acid based surface functionalized porous graphitic flakes exhibit not only
wettability transition from hydrophilicity to hydrophobicity, but also
selective oleophilicity in oil removal from water body. 2D graphitic flatland
with precise surface area and high proportion of mesopore achieves near-
absolute adsorption efficiency of crude oil. As a result, the sheet thickness
enhanced upto 9 folds compared to the pristine one. The developed system
efficiently recovers crude oil from water surface under an external magnet
with a quick response. Apart from the crude oil, the composite system
selectively adsorbs a wide range of oils. Moreover, we present density
functional theory DFT calculations to validate the stability and sorption
activity of the composite system. Thus, the prepared g-$C_{3}N_{4}@FeNi_{3}$
nanocomposite acts as promising absorbent material for efficient oil recovery
and hence provides feasible solution towards the upcoming oil consumption.

###### Acknowledgements.

PD would like to acknowledge UGC research award grant. MT acknowledges SAIC,
Tezpur University and SAIF, NEHU, Shillong for few characterization analyses.
MT acknowledges Tezpur University for providing financial support. SKB
acknowledges CSE, Tezpur University for providing HPCC facility and DST, Govt.
of India for INSPIRE Fellowship. MT gratefully acknowledges Dr. Pabitra Nath
and Prof. Ashok Kumar for their valuable inputs.

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