Effects of temperature on the sorption of Pb2+ and Cd2+ from aqueous solution by Caladium bicolor (Wild Cocoyam) biomass
Michael Horsfall Jnr*
International Foundation Science,
biosorption, Cocoyam, heavy metals removal, temperature, thermodynamics
of sorption, waste management.
This report is based
on the investigation of the effect of temperature on the removal of
Pb2+ and Cd2+ in aqueous effluent using C.
bicolor biomass in a batch sorption process. The result showed
that the most suitable sorption temperature was
is a crucial parameter in adsorption reactions. According to the adsorption
theory, adsorption decreases with increase in temperature and molecules
adsorbed earlier on a surface tend to desorb from the surface at elevated
temperatures. But for activated carbon, a different trend is noticed
where decreasing viscosity and increasing molecular motion at higher
temperature allows the uptake of molecules into the pores more easily,
causing adsorption to increase as temperature increases. However,
temperature has not been studied as relevant variable in biosorption
experiments. The tests are usually performed at approximately 25-
Heavy metals in the environment have become a major threat to plant, animal and human life due to their bioaccumulating tendency and toxicity and therefore must be removed from municipal and industrial effluents before discharge. It is therefore necessary that there are technologies for controlling the concentrations of these metals in aqueous discharges/effluents. The conventional technologies, which have been used, ranged from granular activated carbon to reverse osmosis (Gardea-Torresdey et al. 1998). However, these processes are not economically feasible for small-scale industries prevalent in developing economies due to huge capital investment. It is therefore necessary to search for alternative adsorbents, which are low-cost, often naturally occurring biodegradable products that have good adsorbent properties and low value to the inhabitants.
A range of products has been examined. These include pillared clay (Vinod and Anirudhan, 2001), sago waste (Quek et al. 1998), cassava waste (Abia et al. 2003), banana pith (Low et al. 1995), peanut skins (Randall et al. 1974), Medicago sativa (Alfalfa) (Gardea-Torresdey et al. 1998) and spagnum moss peat (Ho et al. 1995) just to mention a few.
proximate composition and some surface characteristics essential in
assessing the ability of C. bicolor as an adsorbent, and the
effect of pH on the sorption of Pb2+ and Cd2+
using C. bicolor (Wild Cocoyam) biomass has been reported elsewhere
(Horsfall and Spiff, 2004). The data showed that
C. bicolor is an excellent adsorbent for metal ions in aqueous
solutions. In this paper, we report the effect of temperature on the
sorption of Pb2+ and Cd2+ from single metal
ion solution using the biomass of C. bicolor (Wild Cocoyam)
in a temperature range of 30-
The plants were harvested and carefully prepared to obtain the biomass as previously reported in our work elsewhere (Horsfall and Spiff, 2004a). A recent screening (Horsfall and Spiff, 2004b) for chemical composition and surface characterization has shown that the major functional groups on C. bicolor biomass are polar hydroxyl, aldehydic and carboxylic groups. These groups has made C. bicolor to have great potential as an adsorbent for metal ions in aqueous solutions.
volume of 50 mL of metal ion solution [Pb2+ (from Pb(NO3)2
and Cd2+ (from Cd(NO3)2.4H2O)]
with varying initial metal ion concentrations of 10 - 100mg/L was
placed in a 125 mL conical flask in triplicates. An accurately weighed
Caladium bicolor biomass sample (250 ± 0.01mg) with particle
size of 100 μm was then added to the solution to obtain a suspension.
The suspensions were adjusted to pH 5.0. A series of such conical
flasks was then shaken at a constant speed 100 x g in a shaking water
bath at temperatures of 30, 40, 50, 60, 70, and
The mean metal ion sorbed by the biomass at each temperature was determined using a mass balance equation expressed as
where qe = metal ion adsorption per unit weight of biomass (mg/g biomass) at equilibrium, Ce = metal ion concentration in solution (mg/L) at equilibrium, Co = initial metal ion concentration in solution (mg/L), ν = volume of initial metal ion solution used (L), m = mass of biomass used (g).
Two models were used to fit the experimental data: Langmuir model and the Freundlich model. The Langmuir equation was chosen for the estimation of maximum adsorption capacity corresponding to biomass surface saturation. The linealised form of the above equation after rearrangement is given below:
where KL (dm3 g-1) is a constant related to the adsorption/desorption energy, and qmax is the maximum sorption upon complete saturation of the biomass surface.
The experimental data were fitted into equation  for linearisation by plotting against Ce.
The Freundlich model was chosen to estimate the adsorption intensity of the sorbent towards the biomass and the linear form is represented by equation 3:
where; qe = the metal ion uptake per unit weight of biomass (mg of metal ion adsorbed/g biomass); Ce = Conc. of metal ion in solution at equilibrium (mg dm-3); KL and n are the Freundlich constants. The value of n indicates the affinity of the sorbent towards the biomass. A plot of ln Ce against ln qe in equation  yielding a straight line indicates the confirmation of the Freundlich adsorption isotherm. The constants and ln KL can be determined from the slope and intercept respectively.
In these systems, the Gibbs free energy change is the driving force and the fundamental criterion of spontaneity. Reactions occur spontaneously at a given temperature if ΔGo is a negative quantity. The free energy of the sorption reaction, considering the sorption equilibrium constant, Ko, is given by the following equation:
Where ΔGo is standard free energy of change, J/gmol; R is universal gas constant, 8.314 J/(gmol K); Ko is the thermodynamic equilibrium constant and T is absolute temperature, K. Values of Ko for the sorption process may be determined by plotting against qe at different temperatures and extrapolating to zero qe according to the method of Khan and Singh (1987). The other thermal parameters such as enthalpy change (ΔHo), and entropy change (ΔSo), may be determined using the relationships:
The surface coverage (θ) for studying the sticking probability was calculated from the relation
where Co and Ce are the initial and equilibrium metal ion concentrations respectively.
Statistical analyses were performed using Data Analysis Toolpak Microsoft Excel for Windows 2000 with level of significance maintained at 95% for all tests. One-way analysis of variance (ANOVA) without replication was further used to test the null hypothesis of "no significant differences in the applicability of the C. bicolor biomass towards the sorption of Pb2+ and Cd2+".
purpose of this research is to ascertain the effect of temperature
on the sorption of metal ion by the non - useful C. bicolor biomass
plant. The effect of temperature on the removal of Pb2+ and
Cd2+ in aqueous solution by C. bicolor biomass was
studied by varying the temperature between 30 and
the magnitude of such increase continues to decline as temperatures
are increased from 30 to
At high temperature, the thickness of the boundary layer decreases, due to the increased tendency of the metal ion to escape from the biomass surface to the solution phase, which results in a decrease in adsorption as temperature increases (Aksu and Kutsal, 1991).
decrease in adsorption with increasing temperature, suggest weak adsorption
interaction between biomass surface and the metal ion, which supports
physisorption. According to Giles classification as reported by Vinod
and Anirudhan (2001), the adsorption isotherms for all temperatures
may be further classified into several subgroups of I, II, III, etc
according to the shape of the curves. The sorption isotherms at 10-
To facilitate the estimation of the adsorption capacities at various temperatures, experimental data were fitted into equilibrium adsorption isotherm models of Freundlich and Langmuir.
data were fitted by Freundlich adsorption isotherm at all temperatures
(r2 were greater than 0.94). The Freundlich adsorption
isotherm parameters, 1/n and KF, were then plotted against
temperature (Figure 2). The values of 1/n were
found to be more than unity at all temperatures except 30 and
most probable temperature of adsorption was further evaluated by the
Langmuir isotherm. The Langmuir maximum adsorption, Xm,
for a monomolecular surface coverage and the adsorption equilibrium
constants, KF, at the temperatures investigated were obtained
from the plot (Figure 3) for the prediction
of the probable temperature of adsorption. Relevant parameters values
as shown in Table 1 indicate that optimal temperature
of adsorption in utilizing Caladium bicolor biomass for the
removal of metals in aqueous solutions is about
Furthermore, the coefficients of determination, R2, from the Langmuir model was subjected to the one-way analysis of variance (ANOVA) at α = 0.05 to test the null hypothesis (ho) of no significant difference in the applicability of C. bicolor to remove Pb2+ and Cd2+ from aqueous solution. The statistical data obtained showed that Fcal(0.47) << Fcrit, (4.39), hence, we accept the ho, which indicates that the C. bicolor had similar sorption potential for the removal of Pb2+ and Cd2+ from aqueous effluent .
The thermodynamic treatment of the sorption data indicates that ΔGo values were negative at all the temperatures investigated. The negative values of ΔGo (Table 2) indicate the spontaneous nature of adsorption of metal ion by the biomass. It is of note that ΔGo up to - 20 KJ gmol-1 are consistent with electrostatic interaction between sorption sites and the metal ion (physical adsorption) while ΔGo values more negative than - 40 KJ gmol-1 involve charge sharing or transfer from the biomass surface to the metal ion to form a coordinate bond (chemical adsorption). The ΔGo values obtained in this study for both metal ions are < - 10 KJ gmol-1, indicative that physical adsorption is the predominant mechanism in the sorption process. The values of (ΔHo) and (ΔSo) were obtained from the slope and intercept of plots of ln Ko vs 1/T (Figure 4) and are shown in Table 2. The negative values of (ΔHo) for Pb2+ and Cd2+ on to the biomass further confirm the exothermic nature of the adsorption process. The positive values of ΔSo (Table 2) show that the freedom of metal ions is not too restricted in the biomass confirming a physical adsorption, which is further confirmed by the relatively low values of ΔGo.
In order to further support the assertion that physical adsorption is the predominant mechanism, the values of activation energy (Ea) and sticking probability (S*) were estimated from the experimental data. They were calculated using a modified Arrhenius type equation related to surface coverage as expressed in equation 7
The sticking probability, S*, is a function of the adsorbate/adsorbent system under consideration but must lie in the range 0 < S* < 1 and is dependent on the temperature of the system. The parameter S* indicates the measure of the potential of an adsorbate to remain on the adsorbent indefinitely. It can be expressed as in Table 3.
effect of temperature on the sticking probability was evaluated throughout
the temperature range from 30 to
the results clearly establish that the sorption of Pb2+
and Cd2+ onto C. bicolor is favoured at lower solution
temperatures. The range of temperatures which favours the adsorption
process was 10-
ABIA, A.A.; HORSFALL, M. Jnr and DIDI, O. The use of chemically modified and unmodified cassava waste for the removal of Cd, Cu and Zn ions from aqueous solution. Bioresource Technology, 2003, vol. 90, no. 3, p. 345-348.
GARDEA-TORRESDEY, J.L.; GONZALEZ, J.H.; TIEMANN, K.J.; RODRIGUEZ, O and GAMEZ, G. Phytofiltration of hazardous cadmium, chromium, lead and zinc ions by biomass of Medicago sativa (Alfalfa). Journal of Hazardous Materials, 1998, vol. 57, no. 1-3, p. 29-39.
HORSFALL, M. Jnr and SPIFF, A.I. Adsorption of metal Ions from mixed metal solutions on Caladium Bicolor (wild cocoyam) biomass surface. Journal of Corrosion Science and Technology, 2004a, vol. 2, p. 12-18.
HORSFALL, M. Jnr and SPIFF, A.I. Studies on the effect of pH on the sorption of Pb2+ and Cd2+ ions from aqueous solutions by Caladium bicolor (wild cocoyam) biomass. Electronic Journal of Biotechnology [online]. 15 December 2004b, vol. 7, no. 3. Available from Internet: http://www.ejbiotechnology.info/content/vol7/issue3/full/8/index.html. ISSN 0717 3458.
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