Facile removal of Fluoride Ions from water using Triethylamine
Modified Polyethylene Adsorbent
Gerald Mbugua1, Isaac Mwangi1,
Ruth Wanjau1, Moses Abednego Ollengo2, Esther Wanja
Nthiga2, Jane Catherine Ngila3
1 Department
of Chemistry, Kenyatta University, P.O. Box 43844, Nairobi, Kenya
2 Department
of Chemistry, Dedan Kimathi University of Technology, Private bag, Nyeri, Kenya
3Department
of Chemistry, University of Johannesburg, Doornfontein Campus, Doornfontein,
South Africa
*Corresponding Author E-mail:
Highly
fluoridated water is hazardous to human and animal health and effects are not
limited to dental and skeletal fluorosis,
diarrhea and impaired brain development. Traditional water treatment methods have limitations
such as high production cost, ineffectiveness and are not reusable. In this
work, the use of waste polyethylene materials derived from municipal solid waste as a
green water treatment technique that affords low price, value addition and
ecological friendly technology has been presented. The polyethylene wastes were
dispersed in vegetable oil and modified using triethylamine. Modification was confirmed by Fourier transform
infrared spectrometry (FTIR) and scanning electron microscopy (SEM). The effect
of pH, initial fluoride concentration, contact time and adsorbent dose were
investigated and optimized. adsorption was found to prescribe to Langmuir adsorption with an adsorption
capacity of 10. 30 mgg-1. The removal process was found to be optimal at pH
6.0. This study showed that triethylamine modified polyethylene adsorbent has a
potential application for removal of fluoride from contaminated waters.
INTRODUCTION:
Plastics are very light, cheap, and dangerous
materials. They are generally used only once and are then thrown away as they
are easily made. The fact that plastics are light and non- biodegradable, they
litter and gather in the environment and are transported from source areas to
the ocean (Hammer et al.. 2012). The throwing away of plastic based materials into
the environment has become an increasingly serious form of environmental
pollution in recent years (Yu et al.. 2019). These materials last for long and are persistent especially in
developing countries like Kenya (Krueger et al.. 2015).
Many
challenges are experienced regarding plastic waste management which degrades
the beauty of the environment including littering towns, rural areas, shores
and even oceans in many developing countries.
Source: Author
These
materials may also be eaten by animals during grazing thus providing a way by
which poisonous polychlorinated biphenyls enter into the animal body (Chua et
al., 2014). As much as plastic is harmful and degrade the environment when
unwanted after its first utility as seen in image 1.
Since
the release of polyethylene into the market, addition of functional groups on
their surface is a concern to both academic and industrial communities (Hu et al. 2018). Some approaches which change the surface of plastics are engaged to
alter this polymer surface, which include use of chemicals, heat, mechanical or
plasma usage (Navaneetha Pandiyaraj et al.. 2008).The material has quaternary ammonium compounds which
represent a group of polymers that have special properties along its backbone (Bshena et al.. 2011). In this study, improvement of polyethylene waste was
done..
MATERIALS
AND METHODOLOGY:
Modification
of the polyethylene material:
Five
hundred milliters of vegetable oil was heated to boiling in a three neck flask.
To this, clean dry plastic wastes cut to small pieces were added. A 10 mL
strongly basic hydrogen peroxide was then added drop wise. The mixture was
allowed to boil for 15 minutes for complete epoxidation process. Epoxidated
plastic material was refluxed for six hours in an isomantle. The hot epoxidated
plastic sample was allowed to cool and then ground to fine powder. Ground epoxy
was activated at 100°C for 72 hours. Activated solid sample was
ground into fine powder to increase surface area for fluoride adsorption.
Batch
experiments:
Removal
studies were carried out on a Lab-line mechanical reciprocating shaker model
(DKZ-1NO.1007827) using plastic screw cap bottles. Batch experiments were done
to investigate the effects of pH, initial fluoride concentration, adsorbent
dose and contact time. Fluoride ions removed per unit mass were calculated using
the equations 1 and 2 respectively (Mbugua et al.. 2017).
………………..……….………………….1
……………………….…………………2
Where
C0 and Ce are the initial and equilibrium concentration
(mgl-1) of fluoride ions in solution respectively, V is the volume
of solution (in liters) and W (g) is the mass of the solid material.
RESULTS
AND DISCUSSION:
Characterization
of solid sample:
FTIR
for unmodified and modified sample:
The
FTIR spectra of liquid vegetable oil dispersed polyethylene bags, epoxidated
samples, activated, fluorine treated adsorbent and regenerated adsorbent were
characterized using FTIR (8400 Shimadzu model) and the resulting spectra are
presented in Fig.1.
Disappearance
of the peaks at 1741.6 cm-1, 1074.7 cm-1, 609.5 cm-1,
538.1 cm-1, 486.0 cm-1 and 846.7 cm-1 in epoxy
was observed upon activation with amines. This was as a result of - NH
attachment on the surface of the epoxide that occurred during curing reaction.
The resulting functional group which is permanently charged, is capable of
interacting with the with the negatively charged fluoride ion to form a strong
bond (Mwangi et al.. 2019).
Fig 1: Modified polyethylene material (Temperature= 373 K,
500 mL liquid vegetable oil, 10 mL basic hydrogen peroxide, 10mLbtriethylamine
and 72 hours).
Image 2: Scanning electron microscope images for unmodified
and modified polyethylene sample.
Scanning
Electron Microscopy:
The
SEM images of the surface morphology of the unmodified and modified
polyethylene material as shown in image 2
Scanning
Electron microscopy images for unmodified appeared rough with few cracks while
in modified material, the surface appeared rough with few large pores.
Appearance of pores on the surface could increase the adsorption capacity of
the product the three images showed a clear illustration that amine groups had
interacted with the epoxide.
Optimization
experiments:
Effect
of pH:
Monitoring
the pH of an sorbate is important in adjusting
sorption process as reported (Pérez-Marín et al.. 2007). In this study, a 50 mL fluoride solution was put in
100 mL plastic bottle. The pH of solution was adjusted using 0.1 M Nitric acid
and 0.1 M sodium hydroxide solutions. The results were presented in Fig 2.
Fig 2: Effect of pH on the removal of fluoride ions by triethylamine
modified plastic material (Initial solution concentration = 10 mgl-1,
temperature = 298 K, amount of adsorbent = 0.1 g and contact time = 30 minutes,
agitation speed = 150 rpm).
It was observed that at higher pH values, the amount
of fluoride ions removed from water was low. Decrease in fluoride removal at
higher pH may be due to gradual increase in number of negatively charged
hydroxide ions on the adsorbent surface causing electrostatic repulsion of
fluoride ions (Kim et al.. 2015). This corroborates the high surface charge
characteristic observed in SEM images. The sharp decrease in fluoride removal
efficiency was probably due to the competition between hydroxide and fluoride
ions for adsorption sites and the increase in diffusion resistance of fluoride
caused by abundant hydroxide ions. The OH- ions could have displaced
fluoride ions from the solid decreasing the efficiency of the modified material
(Iriel et al.. 2018). At lower pH values, removal of fluoride increased
significantly. This could be due to increase in the positively charged H+
ion on the surface of the adsorbent from which reacts with fluoride ions to
form hydrofluoric acid that leads to a greater decrease of fluoride ions in
solution. To increase efficiency of the modified polyethylene adsorbentin this
study, the pH of solution was maintained 6.0.
Effect
of initial fluoride ions concentration on removal of fluoride ions:
Concentration
of fluoride ions on the percentage removal of fluoride was studied at different
initial fluoride concentrations by keeping other parameters constant. The
effect of initial concentration on removal of fluoride was shown in Fig3.
Fig 3: Effect of varying the initial concentration of fluoride
solution from 0.5 to 20 mgl-1on percentage removal of fluoride ions by
triethylamine modified plastic material (pH= 6.0, dosage = 0.1 g, temperature =
298 K, contact time = 30 minutes, agitation speed = 150 rpm).
The
results show that when fluoride initial concentration was increased, the
adsorption capacity of fluoride ions removal of fluoride ions increased as
shown in Fig 7 because of higher concentration gradient acting as a driving
force overcame mass transfer resistance between bulk solution and adsorbent
surface (Srivastava et al.. 2006). At higher concentration (.>10 mgl-1)
the efficiency of the material declines, this could be because at a fixed
adsorbent dose, the total available adsorption sites are few and this becomes
saturated at higher concentrations (Bhaumik and Mondal 2016).
Effect
of contact time:
The
speed of fluoride ions uptake is related to the efficiency of the adsorption
sites to hold the ion, as well as the electro negativity of the halide, thus
controlling the residence time of the ions between sorbate and the adsorbent (Demirbas et al.. 2004). This led to the investigation of the effect of
contact time by varying the equilibrium time from 0 to 60 minutes while keeping
all the other conditions constant. The results obtained were presented in Fig
4.
Fig 4: Effect of contact time on the removal of fluoride ions using
modified plastic material (initial solution concentration = 10 mgl-1,
temperature = 298 K, dosage = 0.1 g, pH= 6.0, agitation speed = 150 rpm).
The
general observation was that there was an initial rapid rate within the first
30 minutes followed by a subsequent slower rate. This was probably due to the
availability of high number of complexation sites on the surface of the
material at the initial stage. Upon saturation of the binding sites, the amount
of fluoride complexed at a given time decreased due to limited complexation
sites (Hiremath and Theodore 2016). Between 20 to 30 minutes, the removal of fluoride
decreased slightly, this could have been contributed by a steep concentration
gradient between the solution and the adsorbent, making the fluoride ion to
migrate back to the sorbate until equilibrium is established.
Effect
of dosage:
The
removal efficiency of an adsorbent increases with increase in adsorbent dose (Mbugua et al.. 2014). This is due to available adsorption sites increase
by increasing the adsorbent dosage. In this study, effect of dosage on fluoride ions removal, initial
solution concentration of 10 mg L-1, 30 minutes contact time, temperature of 298 K and
agitation speed of 150 rpm and the results were as shown in Fig 5.
Fig 5: Effect of dosage (initial solution concentration = 10 mgl-1,
pH= 6.0, temperature = 298 K, contact time = 30 minutes, agitation speed = 150
rpm).
The
results show an increase in complexation capacity as the dosage was increased
from 0.01g to 0.1g. This was due to the large number of binding sites resulting
from increased adsorbent dosage and availability of more effective surface area
of the adsorbent. Similar results were also obtained by Suneetha et al.. (2015) in removal of fluoride from polluted waters using
active carbon derived from barks of vitexnegundo plants.
Adsorption
capacity:
To determine the efficiency of modified
polyethylene material adsorption isotherms models were used to provide the
nature and physico-chemical interactions involved in the adsorption. In this
study, Langmuir and Freundlich isotherm models were used to determine the
maximum removal capacity of modified plastic adsorbent (Foo and Hameed, 2010). The
study was done by varying the initial concentration of fluoride solution from 4
to 50 mgl-1 in a water bath shaker for 30 minute and the results
presented in Table 1.
Table 1: Langmuir and Freundlich constants
|
Adsorbent
|
Langmuir constants*
|
Freundlich constants*
|
|
Modified plastic material
|
Qmax (mgg-1)
|
KL (L/mg)
|
R2
|
(Kf) (mgg-1)
|
1/n
|
R2
|
|
10.30 mgg-1
|
0.0348
|
0.8638
|
7.86
|
0.6745
|
0.4836
|
*Langmuir and Freundlich adsorption isotherm constants Initial
concentration 4 to 50 mgl-1, dosage = 0.1 g, pH = 6.0,
Temperature = 298 K, agitation speed = 150
rpm).
From
the table, it was observed that the data fitted well in the Langmuir adsorption
isotherm with a removal capacity of 10.30 mgg-1 and a correlation
coefficient R2 of 0.8638 compared to that of Freundlich isotherm
model which gave a lower value. The 1/n value obtained was 0.6745 and Kf value
was 7.86 mgg-1a lower value of 1/n indicated that the adsorption
process was favorable to a homogenous surface. Therefore, there is a clear
indication that adsorption of fluoride ions using modified polyethylene
material follows a monolayer adsorption process on homogeneous surface.
CONCLUSIONS:
This
study successfully modified plastic wastes through modification with
triethylamine. The modified adsorbent confirmed removal of 10.30 mgg-1 in
less than 30 minutes at pH 6.0 indicating that plastic wastes can be modified
and used as an alternative cheap adsorbent for treating water of with high
fluoride content.
ACKNOWLEDGMENT:
The
author acknowledges assistance from the staff at water resources management
authority, members of the water, the technical members of staff in the
Chemistry department of Kenyatta University and African Development Bank.
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