Kinetics and Thermodynamic Adsorption of Pb(II) and Cd(II) Ions from Used Oil onto Thevetia neriifolia Nutshell Active Carbon
Friday O. Nwosu,
Bamidele I. Olu-Owolabi
Kayode O. Adebowale
Active carbon was prepared from Thevetia neriifolia nutshell by carbonization at 480°C in N2 and activation with CO2 at 840°C for 3 h. The values of 822.2 mg g-1, 0.113 cm3 g-1 and 551 m2 g-1 were obtained for iodine number, micropore volume and BET surface area, respectively. First, second and Pseudo-second orders kinetic models described kinetic data at various temperatures for adsorption of Pb(II) and Cd(II) ions from used oil onto the carbon. Pseudo-second order kinetic model fitted better and was controlled by film diffusion. Enthalpy and entropy were found to be negative for Pb(II) and positive for Cd(II). Adsorption process was non-spontaneous for Pb(II) and spontaneous for Cd(II) metal ions. However, low enthalpy value makes the adsorption process energetically stable. Thus, the active carbon could be adopted as a potential adsorbent for remediation of environment.
Received: November 04, 2011;
Accepted: February 18, 2012;
Published: May 22, 2012
Over the last two decades, the safety of our environment has been continuously
threatened by increase in industrialization and human activities (Rivera-Utrilla
et al., 1988). There had been increase in the production of vehicles,
heavy duty machineries that make use of oil as lubricant. Unfortunately, used
oil types that are often disposed contain toxic metals. Some researchers have
reported that virgin lubricating oils contain about 30% addictives that include
viscosity index improvers, pour point depressants, defoamers, antiwear and anti-friction
compounds whose contents are mainly metals (Ciora and Paul,
2000). They have also described steps in removing ash and colour from used
oils with aid of membrane filtration.
Despite the existence of various refining processes such as cracking, upgrading,
hydro treatment, chemical treatment and addition of addictives, United States
Environmental Agency reported that 24% of used oil is still disposed. Certain
heavy metals have been found present in used automobile engine oils (McKenzie,
1981) and are injurious to human health whenever their concentrations in
the environment exceed certain limit. The management of these heavy metals is
of global concern because heavy metals contaminate soils, water and air which
in turn accumulate and affect plants and animals. Some major sources of these
heavy metals include wastes from industries like electroplating, metal finishing,
metallurgy, tannery operations, chemical processing, mine drainages, battery
manufacturing leachates from land fills, disposal of automobile oils and domestic
wastes (Reed et al., 1994; Gallagher
et al., 1990).
The effects of heavy metals on human health are enormous due to their accumulation
in living organisms which eventually reach humans via food chain (Viessman
and Hamner, 1993; Bailey et al., 1999). The
oral uptake of cadmium through food and beverages tend to cause kidney failure,
cancer and hypertension. Its acute intoxication through dust, results in dyspnea,
cough, fever and chronic pneumonitis (Pier and Bang, 1980;
Anderson, 2003; Dekhil et al.,
2011). The intake of lead poisoned food or its intoxication cause break
down of central nervous system, kidney failure and affect red blood cells resulting
into death (Silva et al., 1993). It affects liver
and bone marrow cause reduction of intelligence and stunted growth in children
(Chang, 1990; Low et al.,
2000). Active sites of enzymes are also attacked by these heavy metals (Olayinka
et al., 2007).
Many techniques like reverse osmosis, chemical precipitation, ion exchange
and electro- flotation had commonly been used for removal of heavy metals. Likewise,
commercial active carbon had been used for adsorption and removal of heavy metals
from effluent waste water and the environment. Some work on removal of colour
and odour from lubricating oils had been reported (Ciora
and Paul, 2000). Among the various methods of metal ion removal from the
environment, adsorption with active carbon seem to be advantageous in that it
combines large scale application, simple technology and cost effectiveness of
cheap local agricultural raw materials. Moreover removal of trace quantities
of metals or lowering of concentration of adsorbates in effluents (Bello
et al., 2011) is feasible when adsorption process is adopted.
The roles of active carbon include removal of heavy metals (Teker
and Imamoglu, 1999; Qadeer and Akhtar, 2005) decolourization
of dyes (Gomez-Serrano et al., 1998; Netzer
and Hughes, 1984; Gokturk and Kaluc, 2008) and deodourization
of effluents from industrial waste waters (Basar, 2006;
Maleki et al., 2007; Al-Jlil,
2009). Over the years, the use of active carbon has proved to be inevitable
in the control of environmental pollution.
Since activities of human and industrial development are major sources of contamination
of our environment (Igwe et al., 2005), the use
of automobiles and heavy duty machines in industries have contributed significantly
to contamination of the environment. Improper disposal of used oils especially
in Africa and some parts of third world countries abound. Lubricating oils play
the role of reduction of friction between engine parts thereby protecting the
engine parts from being corroded. If the oils are properly managed, they can
be used again as base stock for new lubricating oils or as fuels for other machines.
As these heavy metals cause low life expectancy and high mortality rate, their adsorption from used oils becomes necessary. Active carbon prepared from Thevetia neriifolia (bush milk) nutshell which is a renewable source is utilized as the main precursor biomass in this study.
Therefore, this study investigated the applicability of T. neriifolia nutshell active carbon prepared by activation of its char for kinetic and thermodynamic studies in removal of Pb(II) and Cd(II) metal ions from digested used oil. The effect of temperature on adsorption of Cd(II) and Pb(II) metal ions from used oil as well as fitting of adsorption data into various kinetic equations was studied.
MATERIALS AND METHODS
Materials: The used automobile engine oil samples were collected at
the mechanic site from Nigeria, during servicing of motor car. The brand of
used engine oil investigated is AZ engine oil. Their corresponding virgin engine
oil was bought directly from petrol filling station. The virgin and used engine
oils of AZ crown super high performance engine oil SAE 20 W 50-AP1 SG/CD were
separately collected in tightly corked plastic containers and stored in the
laboratory at room temperature.
Preparation of activated carbon: Active carbon utilised for the study
was obtained from activation of T. neriifolia (oleander yellow) char
with CO2. The Char nutshell was prepared from Thevetia neriifolia
nutshells via carbonization at 480°C in N2 flowing at 1000 mL
min-1 followed by activation of 12 g of the char with CO2
at flow rate of 500 mL min-1 and temperature of 840°C for 3 h
in horizontal tubular reactor furnace (Carbolite tube furnace, CTF 12/65/550
Model, Italy). Prior to activation, the air in the quartz tube was first evacuated
by flow of N2 gas at 500 mL min-1 for 30 min (Nwosu
et al., 2009).
Characterization of activated carbon: The prepared Active Carbon (AC)
was characterized using adsorption of iodine molecules to ascertain its iodine
number and its BET surface area was obtained by aid of Gemni 2375 Sorptometer.
Its micropore and mesopore volumes were determined using n- hexane adsorption
(Bayer et al., 1995). FT-IR and Scanning Electron
Microscopy (SEM) were carried out using a Nicolet Awatar 330 FT-IR spectrometer
model (Thermo Electron Corporation, USA) and scanning electron microscopy instrument
(model DSM 982, Germany) respectively.
Physicochemical properties of lubricating oils: The specific relative
density of both used and virgin AZ oils was carried out using dried relative
density bottles (McKenzie, 1981) while their viscosity
was determined using Ostwald viscometer (Atkins and Paula,
2002). Wet and dry ash methods were adopted for determination of concentration
of Pb(II) and Cd(II) metal ions. The dry ash method (Milner
et al., 1952) for determination of heavy metals involved the separate
treatment of 2.5 g of each of the oil samples with 200 mL of concentrated H2SO4
acid in a crucible. It was thoroughly stirred and heated gradually on a thermostatic
hot plate to dryness for about 5 h. The crucibles containing the dried oil samples
were then transferred to a muffle furnace and held at 550°C for 1 h. They
were then cooled in desiccators. The ash of each of the oils were then dissolved
with 6 M HNO3 in a 20 mL standard volumetric flask and made up to
mark. The Pb(II) and Cd(II) metal ions were determined using Atomic Absorption
The wet digestion method described by the Analytical methods
Committee (1959) was also used. 15 mL of concentrated HNO3 was
added to 150 mL of the oil sample and heated for 10 min and cooled. Five milliliter
each of concentrated HNO3 and HClO4 acids were added to
the mixture and heated. The intermittent additions of 5 mL concentrated HNO3
and 5 mL concentrated HClO4 acids continued during the digestion
process till clearer solutions of the oil samples were observed. The digested
oil samples were then separately transferred to 1 L volumetric flask and made
up to mark using methyl isobutyl ketone (MIBK) which formed the stock solution.
A 250 mL of the stock solution was measured and made up to mark with MIBK solution
in a standard 500 mL volumetric flask to obtain 50% oil: MIBK solution.
Kinetics and thermodynamic studies of adsorption: The kinetic and thermodynamic
studies of adsorption of Pb(II) and Cd(II) were conducted using batch method
(Kadirvelu and Namasivayam, 2003; Dekhil
et al., 2011; Maleki et al., 2007).
About 0.2 g of the CO2-based active carbon of T. neriifolia nutshell
was added to 20 mL portions of 50% oil: MBIK solution in a plastic container
and agitated at 150 rpm for various periods of 2-120 min on a thermostatic shaker.
The kinetic studies were carried out at temperatures of 27 and 35°C; and
at pH of 4.8 buffered by 2 mL of a mixture of equal volume of 1 M CH3COONa
and 1 M CH3COOH (Johns et al., 1999;
Toles et al., 1999). Aliquots of the supernatant
solution were withdrawn and subjected to atomic absorption analysis for Pb(II)
and Cd(II) using Buck scientific 205 Atomic Absorption Spectrophotometer with
the aid of air acetylene flame. Plastic containers were used to ensure that
no adsorption onto container occurred.
RESULTS AND DISCUSSION
Properties of activated carbon: The properties of the prepared T.
neriifolia Active Carbon (AC) are shown in Table 1. Its
percentage yield, bulk density, ash content and pH were found to be 21.63%,
0.50 g mL-1, 1.33% and 4.3, respectively. The T. neriifolia
CO2- based AC exhibited iodine number value of 822.2 mg g-1,
micropore volume of 0.113 cm3 g-1, mesopore volume of
0.017 cm3 g-1 and BET surface area of 551 m2
g-1. The FT-IR of the prepared AC showed stretch vibrations of C
= O in ketones and aldehydes in the range of 1700-1735 cm-1. The
C = O functional group may be attributable to aliphatic ketones, aldehydes and
carboxyls formed (Petrov et al., 2000). The C
= O stretch vibrations which is corresponding to vibrations of C = O in carboxylic
acids (-COOH) might be assigned stretching vibrations between 1637-1685 cm-1.
Its SEM micrograph revealed numerous pores and crevices all over the surface
of the activated carbon (Fig. 1) (Dekhil
et al., 2011).
Properties of lubricating oils: Some properties of the virgin and used
oils as well as their metal ion concentrations are presented in Table
2. It could be observed from Table 2 that the virgin oil
with relative specific density of 0.950 exhibited a higher density than its
corresponding used oil (0.923). The value (73.27 cp) of the virgin oil was found
to be more viscous than its corresponding used oil brand (70.27 cp).
|| Properties of CO2-based AC and ultimate analysis
data of T. neriifolia nutshell
|BET, Vmi and Vme: Brunaur Emmett Teller surface area, microporous
volume and mesoporous volume, respectively
|| Some properties and metal composition of used and virgin
|*Indicate percentage of metal ion present among all the metal
ions in lubricant oil investigated
|| Micrograph of T. neriifolia activated carbon
The trend in specific density could be attributed to combustion of some components
of the lubricating oil as it was heated up in various sections of automobile
engine leading to contamination (McKenzie, 1981). Thus,
the low viscosity of oils obtained from used oil became less viscous due to
contamination of the oil with fuel and other impurities from the engine. Table
2 also showed that the concentrations of metal ions with values of 0.8104
and 0.0017 mg g-1 for Pb(II) and Cd(II), respectively for used oils
are higher than their corresponding concentrations in the virgin oil. Preliminary
investigation of wear metals in lubricating oil revealed the presence of 16
metal ions in both used and virgin lubricating oil. Pb(II) constituted 20.64%
and Cd(II) 0.03% for the used oil while 0.25 and 0.30% were obtained for Pb(II)
and Cd(II), respectively in the case of the virgin oil. This is dangerous for
the environment as there is possibility of its bioaccumulation that eventually
gets to human and become injurious. The permissible WHO threshold limit value
of Pb(II) ion in water is 0.01 mg L-1 (Bulut
and Baysal, 2006) which is less than Pb(II) value of 0.81 mg g-1
found in the used oil. Similarly, the Cd(II) value found in the oils is greater
than World Health Organization (WHO) and America Water Works Association (AWWA)
drinking water guideline value of 0.005 mg g-1 for Cd (Mohan
and Singh, 2002).
Surface adsorption kinetic studies: The surface kinetic study for the
adsorption of Pb(II) and Cd(II) were investigated at optimum pH of 4.8 using
0.1 M sodium acetate and 0.1 M acetic acid buffer (Toles
et al., 1999; Johns et al., 1999).
This is because Mohan and Singh (2002) and Brown
et al. (2000) noted that Cd(II) ions do not easily hydrolyse at pH
= 8 but existed in their hydroxo complex forms at pH>11. It was suggested
that a pH lower than 8 is essential for adsorption to be significant while pH>3
is needed to avoid metal ions being exchanged with H+ ions that causes
leaching of the metals from the adsorbent (Randall et
al., 1974; Coupal and Lalancette, 1976). In
general, Brown et al. (2000) suggested that optimum
pH for meaningful sorption of divalent metal ions from solution should range
from 3.5 to 6.5. Thus, pH of 4.8 was chosen for adsorption process in this study
in line with literature.
Four kinetic adsorption linear rate law expressions were applied to the adsorption data obtained in this study. The linear forms of the integrated rate law expressions are as follows:
||Second order adsorption rate law used by Ho
and McKay (1998) for sorption of Pb(II) ion onto peat:
the symbol Qe represent the derived maximum adsorption capacity (mg g-1)
of adsorbate for Pb(II) or Cd(II) metal ions on active carbon adsorbent and
ks is pseudo- second order rate constant (g mg-1 min-1).
The Q0 and Qt symbols are initial and residual concentrations (mg
g-1) of metal ions respectively at various times, t (minutes). k1
is the rate constant of first order adsorption (min-1), k2
is the rate constant of second order adsorption (g mg-1 min-1)
while kp is the intraparticle diffusion rate constant (mg g-1 min-1/2)
and C is the intercept. The linear plots of t/Qt versus time, t obtained for
the pseudo-second order adsorption process (Fig. 2-3)
of Pb(II) and Cd(II) at two temperatures onto T. neriifolia CO2-based
AC gave slope, intercept and correlation coefficients, R2 from which
their rate constants and maximum adsorption capacities were determined (Table
The non-fitting of first order and second order kinetic adsorption data using
residual concentrations revealed that the adsorption of Pb(II) and Cd(II) from
used oil at pH 4.8 and at the temperatures of 27 and 35°C may not be feasible
as their linear regression correlation coefficient; R2 values were
found to be less than 0.300 (graphs not shown) (Gokturk
and Kaluc, 2008). From Table 3, it could be observed that
the pseudo-second order rate constant, ks for Pb(II) increased from 0.122 to
0.327 g mg-1 min-1 as temperature increased from 27 to
|| Kinetic adsorption parameters of pseudo-second order sorption
|ks represent pseudo-second order adsorption rate constant
(g mg-1 min-1) Qe: Maximum adsorption capacity (mg
g-1), R2: Correlation coefficient, BF:
Boltzmann factor, SR: Initial sorption rate (mg/g min) and C0:
represent initial concentration (mg L-1)
||Plot of t/Qt versus time t for adsorption of Pb(II) at 27
and 35°C from used lubricating oil, (a) 27°C, (b) 35°C
The linear regression correlation coefficients, R2 equal 0.979 and
0.742 obtained for the Pseudo-second order rate constant at 27 and 35°C,
respectively for Pb(II) were found to be high. The ks for Cd also increased
from 1.558 to 2.179 g mg-1 min-1 when temperature was
increased from 27 to 35°C with high correlation coefficients that fell within
0.922-0.940. The increase in pseudo- second order rate constant for Pb(II) and
Cd(II) ions with increase in temperature could probably be attributed to the
intrinsic nature of the metal ions in that they are divalent and have also gained
kinetic energy as temperature increased and exhibited greater adsorption rate.
The derived maximum equilibrium adsorption capacity, Qe value for Pb(II) was
small and decreased as temperature increased (Table 3) from
27 to 35°C because of matrix effect due to simultaneous adsorption of more
than 16 metal ions contained in the used oil at same time (Nwosu
et al., 2008). The attachment of Cd(II) and Pb(II) metal ions onto
the AC via physical adsorption process may be attributed to the aid of weak
van der Waals forces. At higher temperatures, the adsorbed Pb(II) ions tend
to detach, thereby reducing amount of adsorption capacity. This finding agrees
with trend reported in literature that adsorption capacity reduces with increase
in temperature for some divalent metal ions. This may be due to the weakening
of sorption forces between the T. neriifolia AC sites and the metal ions
as well as between the adsorbed metal ions of the sorbed phase (Viraragharan
and Dronamraju, 1995; Gokturk and Kaluc, 2008).
||Plot of t/Qt versus time, t for adsorption of Cd(II) at 27
and 35°C from used lubricating oil, (a) 27°C, (b) 35°C
In the case of Cd(II) ions, a slight increase in equilibrium adsorption capacity
was observed with increase in temperature (Table 3) and is
in agreement with finding of Bulut and Baysal (2006)
and Raji and Anirudhan (1997). Moreover, Mohan
and Singh (2002) asserted that adsorption in multi-component systems is
complicated because of solute-surface and solute-solute interactions that are
Despite the multi-component system of metal ions in used oils investigated,
the pseudo-second order kinetic model of Pb(II) and Cd(II) ions fitted well
for description of these metal ions that were selectively adsorbed onto the
CO2-based AC (Joseph et al., 2009).
Since adsorption of Pb(II) ions are observed to follow exothermic process, (ΔH=
-16.68 kJ mol-1) , then it could be expected that their adsorption
rate would decrease with increase in temperature in line with the findings of
some researchers (Atkins and Paula, 2002; Tsai
et al., 2004; Ho and McKay, 1999a; Singh,
1998). However, the adsorption of Cd(II) ion followed exothermic process
as its ΔH value was found to be -105.72 kJ mol-1 and as such
a slight increase in its maximum equilibrium adsorption capacity.
In another study, Murgulescu et al. (1981) and
Mihaly-Cozmuta et al. (2005) referred e-Ea/RT
factor in Arrhenius equation as Boltzmann factor, BF. They
defined it as fraction of active sites on adsorbent surface with adequate potential
energy to adsorb adsorbates. The values of Boltzmann factor obtained for both
Pb(II) and Cd(II) ions is within range of 10-1 (Table
3). From these values, it could be suggested that the CO2-based
AC from T. neriifolia nutshells had equal potential to adsorb Pb(II)
ion than Cd(II) metal ions. It further indicated that unoccupied active sites
are equally available to both metal ions at conditions of sorption process considered
(Murgulescu et al., 1981; Mihaly-Cozmuta
et al., 2005).
Thermodynamic quantities of adsorption of Pb(II) and Cd(II) ions: The practical application of a process is dependent on values of thermodynamic parameters obtained. Equations 5, 6,7, 8 and 9 were utilized for calculation of activation energy of adsorption (Ea), equilibrium constant (Kc), enthalpy change of adsorption (ΔHad), Gibbs free energy of adsorption (ΔGad) and entropy change of adsorption (ΔSad), respectively.
Thus, the equation that defines activation energy, Ea is:
and equilibrium constant is defined as (Murgulescu et
al., 1981; Mihaly-Cozmuta et al., 2005):
while enthalpy change of adsorption is defined by Vant Hoffs equation
(Atkins and Paula, 2002):
The determination of Gibbs free energy change (ΔG) and entropy change (ΔS) of adsorption were calculated with the aid of the following equations:
where, ks1, ks2, Kc, R, represent pseudo second order rate constant at 27°C (T1), pseudo-second order rate constant at 35°C (T2), equilibrium constant and gas constant respectively. Co and Ce are initial and equilibrium concentrations. ΔH, ΔS and ΔG indicate enthalpy change, entropy change and Gibbs free energy of adsorption, respectively.
From Eq. 5, activation energy, Ea values of the adsorption
process was found to be 94.44 and 32.21 kJ mol-1 for Pb(II) and Cd(II)
metal ions respectively (Table 4). Hence energy barrier for
Cd(II) adsorption is lower than that of Pb(II) ion. Furthermore, Morais
et al. (1999) characterized low Ea energies (5-40 kJ mol-1)
as physical adsorption while higher Ea energies (50-800 kJ mol-1)
as chemical adsorption. The low value of Ea obtained for Cd(II) ions indicated
low energy barrier for adsorption of Cd(II) and was found to be within range
obtained by Nollet et al. (2003).
|| Thermodynamics parameters using pseudo-second order rate
|Where Kc, A0, Ea, ΔGad, ΔHad, ΔSad
are symbols that indicate equilibrium constant, pre- exponential factor,
activation energy, Gibbs free energy change, enthalpy change and entropy
change of adsorption, respectively
Table 4 summarized the values of the thermodynamic quantities
obtained for pseudo second adsorption kinetic data for Pb(II) and Cd(II) metal
ions. Entropy change of adsorption of the metal ions, ΔSad which measured
degree of randomness was found to be negative for Pb(II) ion and implied decrease
in entropy and it was found to be negative for Cd(II) ion indicating decrease
in entropy. In line with Mohan and Singh (2002), negative
values of entropy change, ΔSad obtained for adsorption of Pb(II) ion onto
the nutshell AC showed that no significant change occurred in the internal structure
of the active carbon. Similar negative values have been obtained by other researchers
(Gupta et al., 1997a, b;
Moham et al., 2001). The positive entropy change
obtained indicated that significant change in the internal structure of the
active carbon surface might have occurred at the solid/solution interfaces.
It also shows the affinity of the active carbon for the Cd(II) ion (Ajmal
et al., 2003; Bulut and Baysal, 2006). Moreover,
the values of enthalpy change of adsorption, ΔHad obtained described the
pseudo-second order adsorption kinetic model for Pb(II) and Cd(II) ions from
used oil with multi-component metal ions as exothermic for Pb(II) ion and Cd(II)
ion. The large negative value of enthalpy change of adsorption of -16.68 obtained
for Pb(II) ion suggested that the process of adsorption for the Pb(II) metal
ions onto the nutshell AC might tend towards chemical adsorption (Ozean
and Ozean, 2005). Gibbs free energy change, ΔG is important indices
for spontaneity of adsorption process. In general, Jaycock
Parfitt (1981) assigned free energy change of adsorption, ΔGad values
that ranged from -20 to 0 kJ mol-1 as physisorption while chemisorption
was assigned values that ranged from -80 to -400 kJ mol-1. In this
study, the spontaneity of adsorption process, ΔGad of Pb(II) were found
to be 0.78 and 6.71 kJ mol-1 at 27 and 35°C, respectively while
-1.61 and -2.55 kJ mol-1 were obtained for Cd(II) ions at 27 and
35°C, respectively. It could be suggested that adsorption of Pb(II) ions
onto nutshell of T. neriifolia CO2-based AC is non-spontaneous.
However, the low values of ΔGad of Pb(II) indicated that its adsorption
should preferably be carried out at lower temperature. Furthermore, adsorption
of Cd(II) ions onto the nutshell AC indicated spontaneous and its adsorption
process could be classified as physisorption. These findings agreed with other
works reported in literature in support of non-spontaneity of adsorption process
due to positive ΔGad values obtained (Ozean and Ozean,
Sorption mechanism: Sorption mechanism is important in industrial application
especially after a particular kinetic model has been identified. According to
Vadivelan and Kumar (2005), sorption mechanisms between
solid-liquid solution systems follow certain stages:
||Movement of solutes to the exterior surface of adsorbent which
implies boundary surface diffusion (external mass transfer or film diffusion)
||Movement of solute from external surface of the adsorbent
which is intra particle diffusion
It has been noted that the plot of qt versus t1/2 represents multi-linearity
which characterizes two or more steps occurring in sorption process (Gokturk
and Kaluc, 2008). If intraparticle diffusion is rate determining step in
an adsorption process, then the plot of qt versus t1/2 will give
linear graph that will pass through the origin (Kannan and
Sundaram, 2001; Bhattacharyya and Sharma, 2004;
Chen et al., 2003). Figure 4
depicts the linearity of plot of qt (amount adsorbed) versus t1/2 (time)
1/2 that does not pass through the origin within first 15 min. It implied
that the adsorption process of Pb(II) metal ion at 27 and 35°C for 15 min
was controlled by film diffusion (Vadivelan and Kumar, 2005).
||Plot of Qt versus (time)1/2 at various temperature
for adsorption of Pb(II) ions from used lubricating oil
||Plot of Qt versus (time)1/2 for adsorption of Cd(II)
ions at various temperature from used lubricant oil
Figure 5 also showed that the mechanism of adsorption of
Cd(II) ion onto T. neriifolia nutshell CO2-based AC at 27
and 35°C followed film diffusion mechanism as plot of qt against t1/2
did not pass through the origin. Their correlation coefficient, R2
obtained fell within the range of 0.624-0.899. However, some factors have been
attributed to be responsible for rate determining step of adsorption of particular
adsorbate (Moham et al., 2001). The factors that
assign rate determining step mechanism as film diffusion or external transport
mechanism have been reported to be poor mixing, small particle size, dilute
concentration of adsorbate and high affinity of the adsorbate for the adsorbent.
The factors that include good mixing, large particle size, high concentration
of adsorbate and low affinity of adsorbate for adsorbent are assigned intraparticle
diffusion mechanism as the rate determining step. Consequently, the prepared
active carbon has high affinity for the metal ions and as such followed film
The first and second order rates of kinetic parameters with their linear regression
correlation coefficient, R2 of adsorption of these metal ions were
also obtained (Data is not shown). It was found that none of these kinetic adsorption
model fitted well as their correlation coefficients were very low (R2
= 0.5). This is not unexpected as other works have supported pseudo- second
order as the best fit model for metal ion adsorption (Bulut
and Baysal, 2006; Ozean and Ozean, 2005; Vadivelan
and Kumar, 2005; Ho and McKay, 2000; Ho
and McKay, 1999b; Joseph et al., 2009). Moreover,
this study has looked into real life situations in nature whereby the used oil
contained more than 16 metal ions (Nwosu et al.,
2008). Thus, low values of correlation coefficient were obtained for first
and second order kinetic rate equations. However, the correlation coefficients
for pseudo-second order for the investigated metal ions were high enough for
adsorption from a multi-component metal ions system as in used oil.
This study has revealed that the adsorption capacity of Pb(II) ions decreases with increase in temperature at pH of 4.8 while a slight increase in adsorption capacity was observed for the Cd(II) metal ion. The adsorption process for Pb(II) ion was found to be exothermic and that of Cd(II) was also exothermic. However, the free energy change of adsorption of Pb(II) ion at 27 and 35°C showed to be non-spontaneous while spontaneous adsorption process was obtained for Cd(II) ion. The adsorption process fitted well into pseudo-second order kinetic equation for both metal ions but did not fit well into first and second order kinetic equations. The mechanism of the pseudo-second order adsorption kinetics of Pb(II) and Cd(II) metal ions onto the CO2-based nutshell AC was found to be controlled by film diffusion.
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