st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Plasma polymerised 1,7-octadiene coated particles
for hydrophobic matter removal from water
Behnam Akhavan, Karyn Jarvis, Peter Majewski
School of Engineering, Mawson Institute, University of South Australia
Mawson Lakes, SA 5095, Australia
Abstract: In this study we outline a novel method for hydrophobic matter removal by functionalizing silica particles via plasma polymerization. 1,7-octadine was plasma polymerised
onto silica particles to produce hydrophobic adsorbents. Surface chemistry and hydrophobicity of particles were studied via X-ray photoelectron spectroscopy and Washburn capillary
rise measurements, respectively. To evaluate the effectiveness of developed adsorbents in removal of hydrophobic matter, motor oil was used as targeted pollutant. The oil removal efficiency of plasma polymer coated silica particles was found to be closely linked with surface
chemistry and hydrophobicity of particles. Developed adsorbents could remove approximately 100% of motor oil from water.
Keywords: Plasma polymerisation, Hydrophobicity, Silica particles, Oil removal
1. Introduction
Petroleum hydrocarbons are among the major pollutants in water [1]. Removal of petroleum hydrocarbons
from water is of significant importance due to their high
persistency and long environmental half-lives [2]. Gravity
separation, chemical coagulation, flotation and adsorption
are the main processes applied in petroleum hydrocarbon
removal [3-4]. Among these processes, adsorption has
attracted considerable interest as an efficient, low-cost
and simple method which is specifically suitable for
de-centralised water treatment scenarios for rural areas
and regions with sparse population. Petroleum hydrocarbon molecules in water can easily be adsorbed onto any
hydrophobic material due to hydrophobic interactions [5].
Synthetic hydrophobic adsorbents are conventionally
produced via wet-chemistry methods. These adsorbents
are often hydrophobised via introduction of non-polar
functional groups, e.g. –CH3 [1] and –CF3(CH2)2 [6], onto
the surface. Wet chemistry methods are however complex
and highly surface dependent. Moreover, due to their high
rate of waste production, they are not environmentally
friendly. Applying plasma polymerisation to produce hydrophobic adsorbents can, however, overcome the mentioned hindrances. Plasma polymerisation is a simple,
solvent-free process which does not virtually produce any
waste [7]. In contrast to wet chemistry methods, plasma
polymer films can be deposited onto almost any solid
substrate regardless of its shape and chemical composition. Deposition of plasma polymers onto particulate surfaces is not however as simple as deposition onto planar
surfaces because of the high surface area which is in contact with plasma [8]. Plasma surface modification of particles requires specific reactors to obtain homogenous
coatings. Fluidised bed [8] and rotating reactors [9] are
the two most common designs that have been applied in
this field.
The aim of this investigation was to develop hydrocarbon functionalised particles (Figure 1) via plasma
polymerisation technology for the removal of hydrophobic contaminants. Silica particles were hydrophobised via
deposition of 1,7-octadiene plasma polymer films using a
rotating barrel plasma reactor. The plasma polymerisation
parameters, i.e. radio frequency (RF) input power, monomer flow rate and deposition time, were controlled to
optimise the surface chemistry, hydrophobicity
/oleophilicity and oil removal efficiency of silica particles.
Surface chemistry of plasma polymerized 1,7-octadiene
(ppOD) coated silica particles were studied via X-ray
photoelectron spectroscopy (XPS), while Washburn capillary rise measurements were undertaken to determine
the hydrophobicity of particles. To evaluate the effectiveness of devolved particles in hydrophobic matter removal,
motor oil was used as targeted pollutant in water purification tests. Plasma polymerisation technology has shown
to be a promising method for production of hydrophobic
adsorbents for hydrophobic contaminant removal.
Figure 1. Schematic illustration of a hydrocarbon functionalised particle.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
2. Experimental
Plasma polymerised 1,7-octadiene was deposited
onto 30 g of silica particles (average size = 400 µm)
using a radio frequency (13.56 MHz) plasma polymerisation reactor equipped with a rotating chamber. The
reactor was pumped down to the base pressure of approximately 7 × 10-3 mbar. RF input power and
1,7-octadiene flow rate were varied in a range of 2080 W and 2 – 10 sccm, respectively, to achieved power
to monomer flow rate ratios (W/F) of 0.24 – 2.4 kJ.cm-3.
For energy dependent samples, plasma polymerisation
time was kept constant at 5 minutes. For plasma
polymerisation time variable samples, deposition time
was varied in a range of 5 – 60 minutes, while W/F
ratio was kept constant at 1.2 kJ.cm-3.
The XPS survey spectra of uncoated and ppOD
coated silica particles were measured using a SPECS
electron
spectrometer
equipped
with
a
non-monochromatic Mg Kα (200 W) and a hemispherical analyser (Phoibos 150). The electron take-off angle
was 90o, while pass energy of 30 eV and a resolution of
0.5 eV over the energy range of 0-1000 eV was applied.
Analysis of survey spectra and chemical composition
calculations were undertaken using CasaXPS software.
Water contact angle (WCA) of uncoated and ppOD
coated particles were measured via Washburn capillary
rise method. A capillary rise tube was packed with 2 g of
particles and was placed in contact of water. The penetration rate of water into the pack of particles was measured
and plotted as a function time. WCA of particles was calculated according to Washburn equation [10].
Oil removal tests were carried out in batches and at the
natural pH of water. Motor oil-water mixtures of 20 g/L
were prepared by adding 2 g of a commercial motor oil
(viscosity = 393 cP) to 100 mL of milli-Q water. The influence of plasma polymerisation parameters on oil removal efficiency was evaluated by adding 4 g of ppOD
particles, coated at different polymerisation parameters, to
100 mL of oil-water mixture, while the stirring time was
kept constant at 10 min for all samples. ppOD coated particles, coated at the optimised parameters (plasma power
= 40 W, monomer flow rate = 2 sccm and polymerisation
time = 45 minutes), were used in interaction time variable
and adsorbent dose variable tests. The influence of interaction time was studied by adding 40 g.L-1 of ppOD
coated particles to the oil-water mixture, while the interaction time was varied from 5 to 60 minutes. The influence of adsorbent dose was evaluated by adding different
masses of ppOD particles (10 - 60 g.L-1) to the oil-water
mixture, while the interaction time of 10 minutes was kept
constant. The concentration of residual oil in the effluent
was measured using the solvent extraction method followed by gravimetric measurements. The residual oil was
dissolved into 100 ml of petroleum ether (analytical reagent, 60 – 80oC) and was separated from water via sepa-
ration funnel. The solvent was evaporated at ~ 70oC and
the residual oil was weighed using an electronic micro
balance (Mettler Toledo with an accuracy of 10-4 g).
3. Results and discussions
3.1 Influence of plasma specific energy
Plasma power and monomer flow rate are the two crucial
parameters which define the plasma conditions in a plasma polymerisation process. The plasma conditions are
closely linked with the deposited film properties such as
surface chemistry and hydrophobicity. Plasma power (W)
to monomer flow rate (F) ratio (W/F) is known as plasma
specific energy, and represents available energy per unit
volume of the monomer [11]. The influence of W/F ratio
on surface chemistry, water contact angle (WCA) and oil
removal efficiency (ORE) of ppOD particles is shown in
Figure 2. As observed, W/F ratios lower than 1 kJ.cm-3,
do not have a significant effect on surface chemistry. This
was to be expected as at such low energies, the
1,7-octadiene monomer is less likely to be fragmented
and consequently less polymerised film is deposited onto
silica particles. By increasing specific energy past 1
kJ.cm-3, the concentration of carbon increases, while that
of silicon and oxygen decreases. Such a variation in surface chemistry is attributed to the deposition of hydrocarbon functionalities (CxHy) onto surfaces which has
increased the carbon signals and decreased the oxygen
and silicon signals originating from underlying substrates.
As observed, at W/F values greater than 1.5 kJ.cm-3 the
concentration of carbon decreases and oxygen and silicon
increase. Such a variation in surface chemistry may imply
that the ablation process is predominant at high input specific energies. In this process, the deposited plasma polymer is etched as a result of high energy ions bombardments [12]. Such behaviour causes a lower deposition rate,
which results in deposition of thinner films, thus also detecting lower carbon and greater oxygen and silicon signals.
As shown in Figures 2b and 2c, the changes of WCA
and ORE as a function of input specific energy follow a
similar trend to that observed for carbon concentration.
These results demonstrate that the more hydrocarbon
functionalities are deposited onto the surface, the more
hydrophobic surface is achieved. Such a consistency is
due to the deposition of non-polar hydrocarbon groups
which mask the underlying polar functionalities. The inverse correlation of hydrophobicity and surface polarity is
well documented in the literature [13]. The higher ORE
observed at higher WCA suggests that hydrophobic interactions between the petroleum hydrocarbon chains and
ppOD coated surfaces are increased due to the higher hydrophobic character of the particles. According to these
results, it can be concluded that the maximum ORE is
achieved at optimal input specific energy of ~ 1.2 kJ.cm-3
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
(plasma power = 40 W, 1,7-octadiene flow rate = 2 sccm),
where the highest concentration of carbon and the highest
WCA is observed.
Figure 3. (a) XPS survey elemental composition, (b)
WCA and (c) ORE as a function of plasma polymerisation
time. W/F = 1.2 kJ.cm-3.
Figure 2. (a) XPS survey elemental composition, (b)
WCA and (c) ORE as a function of W/F. plasma
polymerisation time = 5 minutes.
3.1 Influence of plasma polymerisation time
The optimum deposition of ppOD coatings was
achieved at a W/F ratio of approximately 1.2 kJ.cm-3.
Plasma polymerisation time, however, also influences
the chemical composition and hydrophobicity of the
surface through changing the film thickness. To investigate the influence of polymerisation time on these
properties and also on ORE, silica particles were coated at constant input specific energy of 1.2 kJ.cm-3,
while the polymerisation time was varied in the range
of 5 – 60 minutes. As observed in Figure 3a, it is apparent that by increasing the polymerisation time, the
atomic concentration of carbon increases, and that of
oxygen and silicon decreases. The deposition of thicker
plasma polymer films at longer deposition times adds
more hydrocarbon fragments to the surface, while obscures the underlying signals of substrate, i.e. silicon
and oxygen.
From Figure 3b, it can be observed that the variation
of WCA as a function of plasma polymerisation time is
consistent with that of carbon concentration. By increasing the polymerisation time, the WCA increases
from approximately 36o to more than 90o for uncoated
and silica particles coated for 60 minutes, respectively.
The particles coated for 60 minutes did not adsorb any
water in Washburn capillary rise test, which indicates a
WCA of more than 90o. Since Washburn capillary rise
measurements are restricted to the measurement of
WCA below 90o [14], absolute values of WCA higher
than 90o cannot be calculated via this method. The
variations of WCA once again correlate with the deposition of more hydrocarbon fragments (CxHy) onto the
surface which render the surface more hydrophobic.
The higher surface hydrophobicity results in greater
hydrophobic interrelations and thus more oil molecules
are adsorbed onto the particles as observed in Figure
3c. Although the surface hydrophobicity increases by
increasing the polymerisation time past 10 minutes, the
ORE does not significantly change. Such behaviour
implies that ppOD coated particles showing a WCA of
~ 85o are sufficiently hydrophobic to adsorb oil molecules, hence increasing their hydrophobicity to WCA
of more than 90o does not further affect the magnitude
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
of adsorption. According to the obtained results, the
optimum ORE is achieved at plasma polymerisation
times of longer than 30 minutes.
3.1 Influence of interaction time and adsorbent dose
The adsorbent dose and interaction time are the two important parameters in water purification tests which
strongly influence the ORE. Silica particles coated at optimum parameters (W/F = 1.2 kJ.cm-3, plasma polymerisation time = 45 minutes) were applied in water purification tests to investigate the influence of these two parameters on ORE. From Figure 4 it can be observed that with
an increase in the interaction time, the ORE increases and
reaches ~ 92% in 5 minutes. Further increasing the interaction time decreases the ORE by ~ 10%. This reduction
of ORE at long agitation times was found to be due to
desorption of adsorbed oil as a result of the mechanical
abrasion and breaking of oil-loaded agglomerates.
Figure 4. ORE as a function of interaction time. Adsorbent dose = 40 g.L-1
The influence of adsorbent dose on ORE is shown in
Figure 5. As observed, at the optimum interaction time of
10 minutes, by increasing the mass of ppOD particles the
ORE increases and reaches ~ 100% for the adsorbent
doses of greater than 50 g.L-1. Such an increase of ORE
was expected and is simply attributed to the increase of
hydrophobic adsorption sites. Measuring almost no oil in
the effluent indicates that hydrophobic ppOD coated particles are highly efficient in the removal of hydrophobic
matter. Application of a more precise oil measurement
technique, such as UV-Vis spectrometry of the dissolved
oil, will assist determining the concentration of the removed oil more accurately.
Figure 4. ORE as a function of adsorbent dose. Interaction time = 10 minutes.
4. Conclusions
Deposition of plasma polymerised 1,7-octadiene onto
silica particles increased the hydrophobicity of particles
due to the replacement of substrate polar groups (Si-OH)
by non-polar hydrocarbon functionalities (CxHy). The
water contact angle of silica particles increased from 36o
to more than 90o by deposition of hydrocarbon fragments.
Such a hydrophobic character developed a hydrophobic
force towards hydrophobic motor oil molecules in water,
and approximately 100% of motor oil was adsorbed in
less than 10 minutes of interaction time. This investigation demonstrated the great potential of plasma polymerisation technology for the development of a new class of
materials for hydrophobic matter removal.
Acknowledgments
Financial support of Government of South Australia,
through the Premier Science and Research Fund (PSRF),
and National Centre of Excellence in Desalination Australia (NCEDA) is gratefully acknowledged.
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