Psyllium-g-Poly-(acrylamide-co-acrylonitrile) atmosphere in the presence of ceric ammonium


Psyllium-g-Poly-(acrylamide-co-acrylonitrile)
has been synthesised from psyllium under N2 atmosphere in the
presence of ceric ammonium nitrate and ascorbic acid couple (CAN/AA) initiator
for adsorption of mercuric ions from synthetic solution of HgCl2.
The synthesised samples were optimised by varying various synthetic parameters viz.
monomer concentration, reaction time, temperature, initiator concentration etc.
for maximum yield and adsorption of ionic mercuric.  The optimised sample
has been characterized by using FTIR spectroscopy, SEM analysis, X-Ray
diffraction and thermal studied (TGA/DTA/DTG). The mercury adsorption was
studied onto the optimum sample and found maximum at temperature (30°C), dose
(30 mg), pH (6), time (60 min) and initial mercury concentration 100 ppm. Equilibrium
isotherm data were analyzed using the Langmuir and Freundlich isotherms both.
The Langmuir model was more fitted (R2=0.9976) which indicates the
monolayer sorption. The kinetics of the
rate of sorption of mercury (II) were also analysed using the first order (R2=
0.9971), second order (R2=
0.9887), pseudo-first order (R2=
0.9971), pseudo-second-order (R2=
0.9481), intra-particle
diffusion (R2= 0.9958) and Elovich equation (R2=0.9624). Second order rate kinetics has best linearly fitting,
which follows chemisorption mechanism.

Key words –
Psyllium mucilage, Synthesis, Graft copolymer, Mercury, Adsorption.

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1.     
INTRODUCTION

Mercury
occurs in liquid state at ambient conditions, and mercuric salts (Hg++)
are much common. The toxicity of these salts are due to the capacity of
bio-accumulation in living species and their own compounds to bio-concentrate
in organisms and even to biomagnifies through the food chain 1. Mercury has very high tendency to binds
protein and severely disturbs the nervous and renal  system 2. Mercury continuously effluenting in
the aquatic systems and creating long term water contamination problem 3-5.         

Removal of mercury from the aqueous solution is
great challenge for scientific community for few past decades. Mercury is
highly concern at current time because of its persistence, toxicity and
volatility   into the environment 6-9.  The minor
concentration of the mercuric ions is toxic to the atmosphere and human health
(higher permissible 
limit recommended WHO 2.0 ?g/L) 10, and all species of mercury
are damaging to human beings and can be simply absorbed through human beings
and injury our nervous system, kidneys, other organs
and also several environmental pollutions 11-13. 

Many methods have been used for
waste water treatment and heavy metal removal from aqueous solutions such as
coagulation, ion exchange, advanced oxidation, reverse osmosis, adsorption, chemical
precipitation and absorption and biomass adsorption14, 15. Among of these techniques, an adsorption method is widely
used for the removal of heavy metal ions from aqueous solutions. Many adsorbents including graft
copolymer have been established for heavy metal adsorption, due to their unique
chemical, mechanical, electrical, thermal and rheological properties.

The present article deals with
the synthesis and characterization of a binary grafted copolymer on Psyllium with
a mixture of acrylamide (Am) and acrylonitrile (An) for efficient mercury
removal application. The graft copolymers were synthesized with the free radical
polymerization technique using ceric ammonia nitrate/ascorbic acid (CAN)/AA
couple as the free radical initiator by conventional method. The graft
compound was characterized by FTIR spectroscopy, SEM, XRD and thermal studies
and also investigated the Hg(II) adsorption capability of the adsorbent
Psy-g-Poly (Am-co-An) through batch adsorption studies.

2.     
EXPERIMENTAL

2.1.Materials

Psyllium husks were procured from Sidhpur Sat-Isabgol Factory India and acrylamide (Am),
acrylonitrile (An), sodium hydroxide (NaOH), hydrochloric acid (HCl), methyl
alcohol (MeOH), acetone (MeCoMe), gelatine, mercuric chloride (HgCl2)
were supplied by E-Merck Ltd. Mumbai, India. Rhodamine
6 G, Potassium iodide (KI) were supplied by
SD Fine chem Ltd. Mumbai, India.  Double
distilled deionised water was used for the synthesis of binary grafted
copolymer and water analysis.

2.2.Synthesis of
Psy-g-Poly (Am-co-An)

Grafted
psyllium Psy-g-Poly (Am-co-An) was synthesized by using the previously
reported method. 1.0 g of psyllium mucilage (Psy) was dissolved in double
distilled water (100 mL) in two necks round bottom flask. Require amount of
acrylamide and acrylonitrile monomers were dissolved in distilled water (10 mL)
in conical flask and this solution was added to the Psy solution in the two
necks round bottom flask. This solution was stirred with the help of magnetic
stirrer. The round bottom flask wrapped by septum stopper and nitrogen gas
flushed into the solution by using a hypodermic needle. Solution was
continuously stirred, while being N2 bubbled. They require an amount
of CAN was injected in solution by the stopper through hypodermic syringe. The
nitrogen gas flushing was continued for again 20 min, and the needle was taken
out from the flask and was further wrapped with teflon tape and the reaction
mixture was continuously stirred  at 30°C
for require time followed by adding 0.5 mL of saturated aqueous hydroquinone
solution for reaction termination16, 17 . The reaction product Psy-g-Poly
(Am-co-An) was precipitate in methanol and washed with acetone and dry at
40°C.

 

The
%  was calculated by the equation (1)-

          

2.3.Characterizations

Psyllium,
graft Psy-g-Poly (Am-co-An) copolymer was characterized by Fourier
transform infrared (FTIR) (Nicole – 6700). FTIR spectrum offers the necessary
information about the molecular structure such as functional group and chemical
bonding in the region between 4000 and 450 cm?1.
The Samples were kept absent from light formerly
and dried for 1 hour 50°C temperature in a drying oven. The dried samples were
ground with KBr and formed the KBr pellets containing 1 % (w/w) of samples
before analysis. X-ray Diffraction analysis was performed using a Bruker D 8
advance (Shimadzu, Japan), XRD spectra is
widely used for quantitative analysis and identification
of different crystalline forms of molecules. The samples were scanned from 2? = 5o to 50o
with a steep angle of 6o/min. Thermogravimetric
analysis, differential thermal analysis,
and differential thermogravimetric analysis were performed with an SII 6300 EXSTAR TG-DTA (Japan).  5.0 mg Samples were placed in the TGA furnace
and the measurements were carried out under a nitrogen
atmosphere with a heating rate of 10°C/min from 0 to 800°C. Weight losses of
the samples were determined with respect to temperature. Scanning electron
microscopy (SEM) was used to obtain the morphological
images of Psyllium, graft Psy-g-Poly(Am-co-An) copolymer18.
The instrument (JSM, 6490) was used to investigate the surface morphology of
Psyllium, graft Psy-g-Poly (Am-co-An) copolymer in powder form at 15 KV and with a resolution of 10
nm. The pH of the solution was measured with Digital pH meter (Globe instrument
auto pH meter). The concentration of Hg (II) was determined by Systronics
double beam UV visible spectrophotometer 2203.

2.4.Determination
of pHZPC

A solution of 5.0×10-3 molar calcium chloride
(CaCl2) was boiled to remove the dissolved carbon dioxide (CO2)
and then cooled to room temperature. The pH was set to a value between 2 and 10
by the using 2.5 M hydrochloric acid or 5.0 M sodium hydroxide. 20 mg grafted
psyllium was added in 20 mL of the pH solution in conical flask for 60 min and
measure the final pH, and plotted graph between the initial pH and final pH. The
pH at which the both curve crosses  to
each other ( pH initial/pH final lines) that point is the pHZPC 19, 20. (Figure. was not given)

2.5.Hg (II) adsorption method

A stock solution of 1000 ppm of Hg (II) was prepared by
dissolving 1.354 g of HgCl2 in deionized double distilled water. All
mercury (II) adsorption experiments were investigated at ambient temperature. The
impact of various factors like adsorbent amount, contact time, pH and Hg (II)
concentration were investigated by batch adsorption experiment. The impact of pH
on mercury (II) adsorption was investigated at various pH and adjust pH the by 0.1 M HCl or 0.1 M NaOH. 20 mL Hg (II) solution (100 ppm) was taken in 50 mL beaker and added
the 20 mg adsorbent and stirred with magnetic stirrer for the desired time
period, and filtered the solution by a Whatman 0.45mm filter paper. After
appropriate dilution, the remaining quantity of Hg (II) was projected by a
double beam UV spectrophotometer (?-575nm) by using  the rhodamine 6G dye and iodine buffer
solution20, 21. The quantity of Hg++ adsorbed by grafted
copolymer in ppm was calculated using the following equation (2),

Where qe = the amount of the metal adsorbed
(ppm) onto the adsorbent, Qo = the initial concentration solution
(ppm), Qe = equilibrium concentration of solution (ppm), V= volume
and W= adsorbent weight.

2.6.Kinetic
Studies

In order to investigate kinetic data, the contact time
was various from 10  to 120 min and the
kinetic studies were completed the using 100 ppm Hg(II) concentration, 30 mg
adsorbent dose at pH 6 and temperature of 25°C.

2.7.Optimization
of Various Adsorption Conditions

The adsorption Hg (II) by Psy-g-Poly
(Am-co-An) has been investigated by varying only one adsorption parameter at a
time while others keep the fixed. Varied adsorption parameters and their range
pH ( 4 to 10), adsorbent dose (10 mg-70 mg), temperature (15°C to 50°C),
contact time 60 minutes and contact volume 10 mL at 100 ppm mercury(II)
concentration were studied.

2.8.Adsorption
Isotherm Studies

For isotherm investigation, the adsorption equilibrium
data were originated at different initial Hg (II) concentrations ranging from
50 ppm to 400 ppm using a 30mg adsorbent, pH 6, 10 mL Hg (II) solution, 60 min
interaction time at room temperature.

 

3.     
RESULT AND
DISCUSSION

3.1.Effect
of various parameters variation onto the grafting

3.1.1.      Monomers
concentration

The effect the monomer
concentration on to
grafting was shown in figure1 (a) and it carried out from the binary mixture of vinyl
monomers (Am and An). The concentration of Am was varied from 0.07 to 0.28
mol/L in different sets of experiments while keeping the concentration of a
fixed at 0.01 mol/L. The grafting increased with the increasing in Am (0.07
mol/L to 0.21mol/L), but as Am was increased beyond 0.21mol/L, grafting
decreased. The initial augmentation of the monomer concentration gradually increased
grafting with the diffusion of Am to the backbone psyllium as acrylonitrile was
further increased, grafting decreased due to more homo polymerization 22, 23.

3.1.2.      Initiator
(CAN/AA) Concentration

The effect of initiator concentration (CAN/AA)
on the percentage of grafting is shown in figure (1b) with increasing the
concentration of initiator (1.8×10-3 
to 7.2x 10-3 mol/L), increase the grafting yield due to the
availability of more free radicals to initiate grafting at higher CAN/AA
concentration and further increase initiator concentration decreased grafting
yield due to the favoured homo polymerization at high radical availability24.

3.1.3.     
Reaction
Time

The influence of time on grafting is shown in
figure (1c). The grafting enhance with enhance the
time up to 124 min and then a slight decline or constancy in grafting content
was observed. The quick increase of grafting
between 60 and 102 min enhances due to rate of initiation and propagation and the decline or constancy of grafting after 120 min
is a clear remark on the depletion of monomer
from the solution 16, 25.

3.1.4.     
Reaction
Temperature

The
effect of reaction temperature on grafting parameters has been studied in the
temperature range of 20–50°C shown in figure (1d) the enhanced in reaction
temperature (up to 50°C) increased the grafting. The increase in the grafting
yield with increase in grafting temperature can be attributed to  enhanced mobility of the initiator as well as
monomer and increased the number of reactive sites  26.

3.2.Characterization

3.2.1.     
FTIR
spectra

The
FTIR spectra of pure psyllium and Psy-cl-Poly (Am-co-An) are shown in figure 2a
and 2b, respectively. The FTIR spectra of purified psyllium shows
characteristic peak 3392 cm-1, is due to stretching vibration of OH and
Smaller peak at 2923 cm-1 is assigned to the C-H stretching
vibrations. The band at 1043 cm-1 is attributed to the C-O-C stretching
vibrations. Whereas, in case of Psy-cl-Poly
(Am-co-An) one peaks at 2240.78 cm-1 (C?N stretching of
nitrile), 1726, 1673.25 cm-1 (C=O stretching of amide-I) 27, 1423 cm-1 (N-H in plane
bending of amide-II) and 1251 cm-1 (C-N stretching of amide-III)
were observed in addition to the peaks 
observed with IR of psyllium.

3.2.2.     
SEM

It is evident from the SEM
micrographs of psyllium figure 3(a) and
that of the best grade of Psy-g-Poly (Am-co-An) figure 3(b) that profound morphological change has taken place. The
homogeneous surface of psyllium was convert into heterogeneous (fibrillar)
surface after the modification. The homogenous morphology of psyllium was lost
after grafting with acrylic acid and acrylamide and converted into
heterogeneous morphology.

3.2.3.      XRD analysis

The XRD investigation of any material provides evidence
about its crystalline nature. The XRD patterns of Psyllium and Psy-g-Poly
(Am-co-An) are shown in figure (4a) and
(4b), respectively. The XRD patterns of Psyllium demonstrations a broad a peak
at 2? = 22°, indicating amorphous nature 28, 29. Whereas, in case
of Psy-cl-Poly
(Am-co-An) one peaks at 2? = 22°, also show amorphicity,
on studying both XRD spectra it was found that the diffraction
peak intensity of psyllium are not significantly decreased after grafting at
all angles but the XRD of grafted spectrum present slightly broad spectrum as
compared to pure psyllium means slightly increase the amorphous nature of
crystals.

3.2.4.      Thermal behaviour

Thermal
study (TGA/ DTA/DTG) of pure psyllium and Psy-g-Poly (Am-co-An) were displayed
in figure (5a-b). TGA curves of psyllium and Psy-g-Poly (Am-co-An) were
achieved to scan the both samples 0°C to 800°C. We previously reported that TGA curves of
psyllium (figure 5a) show two weight loss step. The initial weight loss is 12.2 % between
temperatures 30-250°C which is due to the traces of moisture present. Again weight loss of 83.8
% is detected up to 442°C which is due to break of psyllium backbone. Psy-g-Poly
(Am-co-An) is also showing two thermal degradation stages. The first stage ranged from 25°C to 250°C (14.5%) corresponds to the remove of
water. In the second stage, the weight loss is 79.3%
in the temperate region of 350-500°C. Decomposition of Psy-g-Poly (Am-co-An) started at 100°C and 50% weight loss of Psy-g-Poly (Am-co-An) at the temperature 350°C. 
Finally, the polymer backbone of Psy-g-Poly
(Am-co-An) was
completely degraded at 500°C shown in figure 5b, which is slightly higher compared
to original psyllium (Table 1.) 30. It is also observe form
in which DTA and DTG curve, grafted psyllium comparatively more stable to pure
psyllium, as one major peak in both the appears at 416 (98.0 uV) in figure 5a
and at 413 (71.1 uV) in figure 5b ie the grafted material has enough good
stability up to 400°C while the psyllium itself almost decomposes upto 300°C as
shown in figure 5a-b.

3.3.Effect of various parameters onto
the adsorption

3.3.1.      Adsorbent
dose

The effect of adsorbent (Psy-g-Poly (Am-Co-An) dose on the Hg
(II) adsorption was studied in the 10 to 50 mg, keeping other parameters
affecting the adsorption and the result is shown in
figure 6a. It was observed that removal of Hg (II)
increases from 56.5% to 89.9% with the increase in adsorbent dose from 10 mg to
30 mg due to the availability of extra binding sites
at higher doses and further increase the adsorbent dose 30 to 50 mg found the
slightly nominal increase removal of Hg (II).
30 mg adsorbent dose was designated for further optimization and kinetic
studies as there was only slightly increase (in adsorption) beyond 50 mg
adsorbent dose.

3.3.1. Effect
of pH

The impact of pH on the Hg (II) removal was investigated in the pH
range 2-8 under constant other parameters affecting the adsorption. The result
is shown in figure 6b. It was
observed that the percentage removal of Hg (II) increases from 41 % to 92 %
with the increase in adsorbent dose from 2 to 6, because at low pH mercury
exist as positive ions (Hg++) 
further increase the pH 7 to 8, decrease the percentage
removal of Hg(II) due the formation of Hg(OH)2 31 and pH 6 was
selected for further optimization and kinetic studies because at  pH 6 mercury exist as positive ion (Hg+) 32. 

 

3.3.2.     
Contact time

The study of the removal of Hg (II) was
performed with fixed adsorbent dose at various time intervals (10–120 min). The
result is shown in figure 6c. It was investigated that the
percentage removal of Hg (II) increases from 53.5% to 94.9% with the increase
in adsorption time from 10 to 60 min due to increase the metal binding time
with vacant adsorbing sites. Further increase the time beyond 60 min did not approvable
increase the adsorption due to deposition of Hg++ ion on the
available adsorption sites 33. 

3.3.3.      Temperature

The influence of temperature on the Hg (II)
adsorption was performed in the range 20–50 °C under
constant parameters at equilibrium condition shown in figure 6d. The mercury adsorption
continuously increased with the increase the temperature 20-30 °C; increase the
active surface centre sites for sorption. Further increase the temperature,
decrease the adsorption was observed indicating some desorption taking place
above 30 °C.

3.3.4.      Initial
Hg (II) ion concentration

Effect of initial concentration of Hg (II)
ion on adsorption when initial concentration of Hg (II) ion was varied from 50
to 400 ppm at a particular time, particular pH and at a fixed temperature was shown
in figure 6e. With the increase
in the initial concentration of Hg (II) from 50 to 400 ppm, then increase
mercury adsorption due to the availability of extra mercury (II) for the
binding.

3.4.Adsorption isotherms and
models

Adsorptions isotherms give the
information how molecules subjected to adsorption distribute themselves between
adsorbate and adsorbent phases at equilibrium time{Bao, 2017 #307;Aly, 2013 #798}. They offer some insight into the adsorption mechanism, surface
properties and affinities of the adsorbent. The commonly used adsorption models
are the Langmuir model (corresponding to monolayer homogeneous adsorbent surface)
and the Freundlich model (corresponding to heterogeneous adsorbent surface). The
equilibrium sorption of the Hg2+ ions was carried out by contacting
30 mg of the Psy-g-Poly (Am-co-An) with 20 mL of 100 ppm of different
concentrations from 50 to 400 ppm in 25 mL conical flasks for 60 minutes on the
orbital shaker. The mixture was filtered and the filtrate analyzed for metal
ions concentration using a UV spectrophotometer. The data were fitted into the Langmuir
as well as Freundlich adsorption34.

3.4.1.     
Langmuir
Adsorption Isotherm

The Langmuir adsorption isotherm is highly effective for
monolayer sorption due to a surface of a finite number of identical sites and
expressed in the linear form as equation (3).

Where

Ce = Equilibrium concentration

Qe = Amount adsorbed at equilibrium

Qm = Langmuir constants

KL =
Heat of adsorption.

The vital characteristics of
Langmuir model are explain through means of RL (dimensionless
constant) and RL is calculated the  equation (4).

Where

C0 = Hg (II)
concentration (mg/L).

If RL values lie between 0
and 1, the adsorption is favourable.

The value of Qm
(57.47 mg/g) was calculated from Langmuir model, indicating that the
adsorbent showed high capacity to remove mercuric ions (figure 7a). RL and KL were
calculated to be 0.01493 and 0.01649 ml/mg respectively, thus adsorption
is favourable.

3.4.2.     
Freundlich
Adsorption Isotherm

Freundlich isotherm defines the heterogeneous surface
energy through multilayer adsorption 35 and indicates the linear form as equation (5).

Where

 Kf = Adsorption capacity of adsorbent

 Value
of Freundlich parameters (Kf), Correlation constant (R2)
and rate constant were calculated by Freundlich isotherm (figure
7b) given in Table-2. The
equilibrium data fitted to Langmuir (R2
= 0.9976) model better than Freundlich
model (R2 = 0.9434) indicating surface homogeneity of adsorbent and monolayer
adsorption.

3.5.Kinetic
studies

The removed
rate of mercuric ions from aquatic system by the adsorbent is a significant
variable for the application of the real process in treatments. In order to
determine the equilibrium time, effect of interaction time for removal of Hg (II)
by the using many initial Hg(II) concentrations was illustrated in figure 6c. It is observed that the adsorption reaction initially fast,
and at equilibrium time, become slows 36 because great number of unoccupied
surface sites are presented for adsorption in the initial phase. Six of the most widely
used kinetics models: first order, Elovich equation, second order, pseudo-second-order,
intra-particle diffusion and pseudo-first order models 37 were used to evaluate kinetic mechanism of
the Hg (II) adsorption onto Psy-g- Poly (Am-co-An).

3.5.1.     
First
-order kinetics equation

The linear form of first
order kinetics equation is given
as equation (6).

……………(6)

Where
Qo (mgL-1) and Qt (mgL-1) are
concentration at the time zero (initial) and a given time ‘t’ concentration of
metal ions in solution respectively. K1 (min-1) is the
first order rate constant. The regression R2 obtained by the linear
plot of ln (Qo/Qt) Vs t (figure 8a), is shown in Table 3.
For mercury, R2 was more than 0.9, which shows a good fit the
experimental data.

3.5.2.     
Second
order rate equation

The
linear form second order kinetics eqution is given in equation (7) below:

Where
K2 Lmg-1min-1 is the second order rate
constant for the sorption process, determined from the linear plot of (1/Qt
– 1/Qo) against t, shown in figure (8b) for mercury see the Table-3
above for the value of the constants.

3.5.3.     
Pseudo-first-order kinetic equation

Linear form
pseudo-first-order equation is given in equation
(8).

where,

Qt, Q0
and k1 are adsorbate at time t, adsorption capacity at
equilibrium, and rate constant respectively.  All parameters of this kinetic equation were calculated
by figure 8c result
shown in Table 3.

3.5.4.      Pseudo second order kinetics equation

The
pseudo-second-order kinetic rate was studied by equation (9) 31.

Where k2
is the rate constant. The plot for the equation
(9) was shown
in figure 8d which shows the data was perfectly fitted to the model and value
of all parameters were given in table 3. R2 for the second-order
kinetic model exceeded 0.99 which indicated adsorption system highly follows
second-order kinetic mechanism compare to other kinetic mechanisms. So it
supports the assumption behind the model and suggests that the overall rate of
Hg(II) adsorption by psy-g-Poly(Am-co-An) appeared to be controlled by
physicochemical process.

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