INTRODUCTION
Recovery of toxic heavy metals in the environment by adsorption process has
gained the attention of many researchers since the conventional methods available
nowadays are costly due to high maintenance and operation. One of the most common
heavy metals found in wastewater is Pb(II) which originates from industrial
wastes such as batteries, paint, metal plating and automotive (Balaria
and Schiewer, 2008; Naiya et al., 2009).
Pb(II) ions need to be recovered before reaching the water bodies to ensure
that the toxic properties of Pb(II) does not pose any threat to aquatic lives
and more importantly, the human health.
As the environment is faced with solid waste disposal problem, agricultural
by-products have the potential to be used as adsorbents. Plant leaves for instance,
have been reported to be capable of removing various heavy metal ions from wastewater
(Wan Ngah and Hanafiah, 2008). Rubber trees are widely
planted in Malaysia and have become one of the major plantation crops after
oil palm. Rubber related industries are now expanding to meet the demands from
consumers. However, the large area of rubber plantation in Malaysia generates
a huge amount of ‘waste’ rubber leaves especially during dry season
(February to March) every year. Numerous studies have been performed on the
application of unmodified and chemically modified rubber leaves to sequester
Cu(II), Cd(II) and Pb(II) ions (Hanafiah and Wan Ngah, 2009;
Hanafiah et al., 2006a, b).
However, the recorded adsorption capacities were found to be quite low or could
hardly achieve greater than 0.3 mmol g-1.
This study highlighted the introduction of sulphur groups on rubber leaf powder, a process called xanthation for removing Pb(II) from aqueous solutions. The main objective of this study was to determine the effect of physicochemical parameters particularly adsorbent dosage, shaking rate and temperature on adsorption efficiency of Pb(II). Behavior of Pb(II) adsorption in a continuous downflow packed-bed column was also studied.
MATERIALS AND METHODS
Chemicals and adsorbent preparation: All the chemicals used were of
analytical reagent grade. Lead(II) nitrate stock solutions (1000 mg L-1)
were purchased from Spectrosol (England) and the desired concentrations of Pb(II)
were obtained by successive dilutions with deionized water. The fresh mature
(brownish color) rubber leaves were collected from Universiti Teknologi Mara
Pahang rubber plantation in Pahang, Malaysia. The rubber leaves were washed
thoroughly with tap water to remove dirt and particles before drying in the
oven at 105°C for 24 h. The leaves were later ground using a mechanical
grinder and sieved to obtain particle size of 180 μm. This Untreated Rubber
Leaf Powder (URLP) was stored in a tight container for further study.
Xanthation process was performed according to the previous method by Liang
et al. (2009) with some modification. Fifteen grams of URLP and 200
mL (4.0 M) sodium hydroxide were mixed in a 250 mL conical flask. The mixture
was stirred for 3 h at room temperature (30±0.5°C) and for another
3 h after the addition of 10 mL carbon disulfide. After allowing the mixture
to settle for 30 min, the supernatant was decanted. The Xanthated Rubber Leaf
powder (XRL) was extensively washed with 200 mL of deionized water for 20 times
to remove excess base. Finally, the leaf powder was washed with acetone followed
by drying in an oven at 50°C for 24 h. Thermal stability of XRL was determined
by performing thermogravimetric analysis (TGA, Perkin Elmer Pyris 1, USA). A
weight of 5.90 mg of XRL was placed onto platinum crucible and the analysis
was carried out under N2 flow at the heating rate of 20°C min-1
with the temperature ranging from 50 to 900°C.
Batch and fixed-bed column adsorption experiments: All adsorption experiments were carried out in duplicate and the data was reported as average. Batch adsorption studies were performed in 100 mL conical flasks with 50 mL (40 mg L-1) Pb(II) solutions shaken in a thermostat water bath shaker at 120 stroke min-1 for 90 min at room temperature, 30°C (unless state otherwise). The initial pH of Pb(II) solutions was 4. The effect of adsorption dosage on Pb(II) removal was studied by using different weight of adsorbent (0.01 to 0.10 g). The effect of shaking rate was studied by using different shaking rates from 30 to 150 stroke min-1. Thermodynamic study was investigated by mixing 0.02 g of XRL with different Pb(II) concentrations (20 to 150 mg L-1) at three different temperatures (303, 313 and 323 K). The mixture was shaken for 90 min at room temperature to ensure that equilibrium was achieved. After adsorption, the mixture was separated by using Whatman No. 42 filter papers and the filtrates were analyzed for remaining Pb(II) ions using atomic adsorption spectrophotometer (Perkin Elmer, AAnalyst 800 model, USA) at a wavelength of 283.3 nm.
In the fixed-bed column study, a glass column with the internal diameter of
2 cm was used. One gram of XRL was soaked in deionized water before slowly being
poured into the column. This technique was done to avoid the air gaps in the
column. Glass wool was placed at the bottom of the column and on the top of
XRL to avoid the adsorbent from floating. The bed height of the column was 2
cm and inlet concentration of 100 mg L-1 of Pb(II) solution was loaded
into the column at a flow rate of 12 mL min-1 using a peristaltic
pump (Cole Parmer, USA). The column operation was stopped when XRL reached the
exhaustion stage. After analysis, the amount of Pb(II) adsorbed, qe (mg
g-1) and the percentage removal were calculated by using Eqs.
1 and 2, respectively:
where, Ci and Ce are Pb(II) concentration before and after adsorption (mg L-1), respectively; V is the volume of Pb(II) solution (L) and m is the weight of XRL (g).
RESULTS AND DISCUSSION
Thermogravimetric (TGA) analysis: A biomass such as plant leaf consists
of cellulose, hemicelluloses and lignin as the major components (Williams
and Besler, 1996). TGA analysis was performed to determine the thermal degradation
of XRL. The decomposition of XRL as a function of temperature under nitrogen
atmosphere is shown in Fig. 1. According to Yang
et al. (2007), hemicelluloses start to decompose at 220-315°C,
while the temperature range of 315-400°C corresponds to cellulose. Lignin
however, decomposes in a wider temperature range (200-720°C) (Williams
and Besler, 1996).
|
| Fig. 1: |
TGA curve of XRL, Temperature: 50-900°C, Heating rate:
20°C min-1 |
|
| Fig. 2: |
Effect of shaking rate on Pb(II) adsorption by XRL, Adsorbent
weight: 0.02 g, Volume: 50 mL, Shaking rates: 30-150 stroke min-1,
Equilibrium time: 90 min, Initial Pb(II) concentration: 40 mg L-1 |
As shown in Fig. 1, the mass loss of 7.9% in the region
50 to 188°C indicates the loss of moisture and adsorbed water. The weight
loss of about 54.3% at temperature range of 188-429°C is attributed to decomposition
of hemicellulose, cellulose and lignin. Higher molecular weight of lignin decomposed
at a much higher temperature range (429-603°C) with 27.7% weight loss. The
adsorbent still undergoing decomposition at higher temperature >700°C
with 3.6% weight loss before finally produced char at 900°C.
Effect of shaking rate: The effect of shaking rate on Pb(II) adsorption
onto XRL was studied at different rates (30, 60, 90, 120, 150 stroke min-1).
As shown in Fig. 2, the amount of Pb(II) adsorbed onto XRL
increased as the shaking rate increased. The lowest amount of Pb(II) adsorbed
occurred at the lowest shaking rate which was 30 stroke min-1 (58.28
mg g-1) and the highest amount at the rate of 150 stroke min-1
(82.53 mg g-1 ). At the shaking rates of 60, 90 and 120 stroke min-1,
the amounts of Pb(II) adsorbed were 65.18, 74.08 mg g-1 and 80.54
mg g-1, respectively. This adsorption characteristic can be explained
in term of external film diffusion. An increase in shaking rate would favor
higher adsorption rate by decreasing the mass transfer resistance between the
adsorbate and adsorbent surface (Ponnusami and Srivastava,
2009). However, beyond 120 stroke min-1, there was no difference
in the amount of Pb(II) adsorbed. Hence, shaking rate of 120 stroke min-1
was selected in subsequent experiments.
Effect of adsorbent dosage: The dependence of adsorbent dosage in the
removal of Pb(II) ions was studied at different adsorbent dosages (0.01 to 0.10
g).
|
| Fig. 3: |
Effect of adsorbent dosage on Pb(II) adsorption by XRL, Adsorbent
weight: 0.01 to 0.10 g, Volume: 50 mL, Shaking rate: 120 stroke min-1,
Equilibrium time: 90 min, Initial Pb(II) concentration: 40 mg L-1 |
The concentration of Pb(II) was fixed at 40 mg L-1 and the results
are presented in Fig. 3. The amount of Pb(II) adsorbed decreased
from 156.66 to 14.54 mg g-1 with increased adsorbent dosage. This
was due to metal concentration shortage in solution at high dosage (Qaiser
et al., 2007). Another possible explanation is due to the availability
of more surface area and adsorption sites which is the results of plenty unadsorbed
sites as dosage is increased (Gong et al., 2009;
Kamal et al., 2010). In this study, the optimum
dosage chosen was 0.02 g because of the high amount of Pb(II) adsorbed (81.06
mg g-1) and high percentage of Pb(II) removal (81.06%). Low dosage
but good percentage of metal removal means less amount of adsorbent has to be
used, thus treatment process will be more economical.
Effect of temperature and adsorption thermodynamics: For all Pb(II)
concentrations tested, it was found that the amount of Pb(II) adsorbed decreased
as the temperature increased (figure not shown). This would suggest that the
interaction between Pb(II) ions and the XRL surface is exothermic. Excess energy
promotes desorption of adsorbate instead of adsorption, thus decreases the amount
of adsorbate adsorbed (Gupta and Bhattacharyya, 2008).
The trend also explains that with the increase in temperature, the solubility
of Pb(II) ions in the aqueous phases would increase, thus decreases the Pb(II)
concentration in the solid phase (Gupta and Bhattacharyya,
2008).
The thermodynamic parameters, ΔH°, ΔS°, ΔG° were computed from the Van’t Hoff equation and Gibbs-Helmholtz equations (Eq. 3 and 4):
| Table 1: |
Thermodynamic parameters of Pb(II) adsorption onto XRL |
|
|
|
| Fig. 4: |
Van’t Hoff plot of Pb(II) adsorption onto XRL |
where, qe/Ce is the equilibrium constant (mL g-1), ΔS° is standard entropy change (J mol-1 K-1), ΔH° is standard enthalpy change (kJ mol-1), T is absolute temperature (K) and R is the gas constant (8.314 J mol-1 K-1). ΔG° is the standard free energy change (kJ mol-1). ΔH° and ΔS° can be determined from the slope and intercept of linear plot of ln (qe/Ce) versus 1/T, respectively (Fig. 4). From Table 1, the negative values of ΔH° explain the exothermic nature of interactions between Pb(II) ions and XRL surface. The adsorption of Pb(II) was spontaneous as ΔG° values were negative. The low negative values of ΔH° indicated that one of the main mechanisms of Pb(II) adsorbed on XRL was a physical adsorption.
Fixed-bed column studies: As industries generate a large quantity of
wastewater, it is more practical to remove heavy metal ions by applying continuous
column method. In a column operation, solution continuously enters and leaves
the column until the adsorbent surface becomes fully saturated. The overall
performance of the column is judged by the time the adsorbed heavy metal penetrates
the column bed and is detected in the effluent (Hanafiah
et al., 2010).
| Table 2: |
Adsorption breakthrough data and Thomas and Yoon-Nelson parameters
for a fixed-bed column experiment |
|
| Pb(II) concentration: 100 mg L-1, pH: 4, column
internal diameter: 2 cm, bed height: 2 cm, adsorbent weight: 1.0 g, flow
rate: 12 mL min-1 |
A breakthrough curve can be obtained by plotting effluent concentration (Ceff)
versus treated volume (V) or service time (t). The ideal breakthrough curve
is the ‘S’ shape, which indicates favorable adsorption process (Al-Degs
et al., 2009). A breakthrough curve (figure not shown) can give important
parameters such as breakthrough time (tb), breakthrough volume (Vb),
exhaustion volume (Vexh) or column exhaustion time (texh),
which occurs at 95% of inlet concentration (Ci) and breakthrough
capacity (qb, mg g-1). The total effluent volume and breakthrough
capacity can be calculated from Eq. 5 and 6,
respectively:
where, Q is the volumetric flow rate (mL min-1), t is the time (min), t10% is the time taken (min) to reach 10% of the inlet concentration, Ci is the initial Pb(II) concentration (mg L-1) and m is the weight of XRL (g).
All the data from fixed-bed column study are presented in Table
2. The behavior of Pb(II) adsorption under column operation was further
analyzed by using two column models: Thomas and Yoon-Nelson. Thomas model is
based on several assumptions such as adsorption follows Langmuir isotherm, no
axial dispersion and second order adsorption kinetics (Qaiser
et al., 2009). Yoon-Nelson model is based on the assumptions that
the rate of decrease in the probability of adsorption for each adsorbate molecule
is proportional to the probability of adsorbate adsorption and the probability
of adsorbate breakthrough on the adsorbent (Aksu et al.,
2007). The linear forms of the Thomas and Yoon-Nelson model are given by
Eqs. 7 and 8, respectively:
where, Ct is the Pb(II) concentration (mg L-1) at time t (min), kTh is the Thomas rate constant (mL min-1 mmol-1), qo is the column adsorption capacity (mmol g-1), Q is the volumetric flow rate (mL min-1), Veff is the effluent volume (mL), m is the weight of XRL in column (g), kYN is the Yoon-Nelson rate constant (min-1) and is measured off the slope of the breakthrough curves. Steeper breakthrough curves will have higher values of kYN. τ is the time required for 50% adsorbate breakthrough (min). A linear plot of ln ((Ci/Ct)-1) versus Veff/Q (or t) was employed to determine the values of kTh and qo, which were obtained from the slope and intercept, respectively (plot not shown). The column capacity based on the Yoon-Nelson model (qoYN) can be computed from the Eq. 9:
Both column models (plots not shown) showed good correlation coefficient values with R2>0.96. High values of correlation coefficients were obtained and the time required to achieve 50% of adsorbate breakthrough (τ) was closed to the experimental data (t50%,exp). As such this would strongly suggest that adsorption of Pb(II) onto XRL in column process agreed well with these models.
CONCLUSION
Based on the experimental data presented in this study, XRL could be used as an alternative adsorbent in removing Pb(II) ions from aqueous solution since the amount of Pb(II) adsorbed was high even though only a low dosage (0.02 g) was used. The effect of dosage, shaking rate and temperature has a great influence on the amount of Pb(II) adsorbed. TGA analysis showed that XRL has good chemical stability since the adsorbent decomposed at a higher temperature. Thermodynamic study proved the adsorption process of Pb(II) ions was exothermic. XRL is also suitable to be used under column operation.
ACKNOWLEDGMENTS
The authors would like to express sincere gratitude to the Malaysian Ministry
of Higher Education for financial support (Grant No. 011000100005). One of the
authors (Wan Khaima Azira Wan Mat Khalir) is thankful to the Malaysian Ministry
of Science, Technology and Innovation for providing National Science Fellowship
(NSF).
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