Environmental Biotechnology

Process Biotechnology

Electronic Journal of Biotechnology ISSN: 0717-3458 Vol. 8 No. 2, Issue of August 15, 2005
© 2005 by Pontificia Universidad Católica de Valparaíso -- Chile Received November 12, 2004 / Accepted March 15, 2005
DOI: 10.2225/vol8-issue2-fulltext-4  

Effects of temperature on the sorption of Pb2+ and Cd2+ from aqueous solution by Caladium bicolor (Wild Cocoyam) biomass 

Michael Horsfall Jnr*
Department of Pure and Industrial Chemistry
University of Port Harcourt
Uniport P.O.Box 402, Choba
Port Harcourt, Nigeria
Tel: 234 803 507 9595
E-mail: horsfalljnr@yahoo.com

Ayebaemi I. Spiff
Department of Pure and Industrial Chemistry
University of Port Harcourt
Uniport P.O.Box 402, Choba
Port Harcourt, Nigeria
Tel: 234 803 309 5183
E-mail: emispiff@yahoo.com

*Corresponding author 

Financial support: The International Foundation Science, Sweden supported this work through grant number W/3624-1 to Dr M. Horsfall Jnr.

Keywords: biosorption, Cocoyam, heavy metals removal, temperature, thermodynamics of sorption, waste management.

Abstract Reprint (PDF)

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 40șC with maximum sorption capacities of 49.02 mg/g and 52.63 mg/g for Pb2+ and Cd2+ respectively. Various thermodynamic parameters, such as ΔGo, ΔHo, ΔSo and Ea have been calculated. The data showed that the sorption process is spontaneous and exothermic in nature and that lower solution temperatures favours metal ion removal by the biomass. The findings of this investigation suggest that physical sorption plays a role in controlling the sorption rate. The sticking probability model was further employed to assess the applicability of the C. bicolor biomass as an alternative adsorbent for metal ion contaminants in aqueous system.

Materials and Methods
  • Biomass preparation
  • Sorption study as a function of temperature
  • Theory and data evaluation
    Results and Discussion
  • Effect of temperature
  • Thermodynamic treatment of the sorption process
    Concluding Remarks
    Table 1
    Table 2
    Table 3
    Figure 1
    Figure 2
    Figure 3
    Figure 4
    Figure 5
  • Temperature 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 25-30șC. However, Tsezos and Volesky, (1987); Kuyucak and Volesky (1989) and Aksu and Kutsal (1991) reported a slight increase in cation uptake by seaweed in the range of 4 to 55șC.

    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.

    The 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 30-80șC.

    Materials and Methods

    Biomass preparation

    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.

    Sorption study as a function of temperature

    A 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 80șC respectively. After shaking the flasks for 2 hrs, the suspension was filtered using Nș 45 Whatman filter paper and then centrifuge at 2800 x g for 5 min. The supernatants were collected in separate clean test tubes. The metal content at each temperature range was determined using flame atomic absorption spectrometer model A300.

    Theory and data evaluation

    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 [2] 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 [3] 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+".

    Results and Discussion

    Effect of temperature

    The 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 80șC. The data presented in Figure 1a and 1b showed that adsorption of metal ion by the C. bicolor biomass increased with increase in temperature, which is typical for the biosorption of most metal ions from their solution (Manju et al. 1998; McKay et al. 1999; Ho, 2003).

    However, the magnitude of such increase continues to decline as temperatures are increased from 30 to 80șC. This is because with increasing temperature, the attractive forces between biomass surface and metal ions are weakened and the sorption decreases. Careful examination of the figures revealed that most of the metal ions were removed between the temperatures of 30 to 50șC. The temperature increases were observed to be in two phases for lower temperatures and three phases for higher temperatures. For lower temperatures, equilibrium sorption occurs rapidly at lower metal ion concentrations in the first phase and becomes relatively constant at higher concentrations. The equilibrium concentration was obtained at 50 mg/L for Pb2+ and 70 mg/L for Cd2+. As temperature increased above 50șC, an initial slow sorption was observed followed by a rapid sorption process to reach equilibrium and a relatively constant sorption process in the third and final phase.The equilibrium concentrations for higher temperatures (60-80șC) were not significantly different for those of lower temperatures. This indicates that increasing the initial metal ion concentrations above the equilibrium concentrations of 50 - 70 mg/L may not have any significant increase in the sorption of metal ions by C. bicolor biomass.

    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).

    The 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 10-40șC belong to the subgroup III of Giles classification. On the other hand, the sorption isotherms at 50-60șC belongs to subgroup II, while above 70șC is subgroup I. In this investigation temperatures of 30 and 45șC in Figure 1 tend to define a plateau; therefore it seems reasonable to support the proposal that for the experimental conditions used, the formation of a complete monolayer of metal ion covering the biomass surface belongs to subgroup III, meaning that saturation of the biomass surface seems to be reached at 10 to 40șC and the optimal temperature of adsorption for Pb2+ and Cd2+ could be obtained within this range.

    To facilitate the estimation of the adsorption capacities at various temperatures, experimental data were fitted into equilibrium adsorption isotherm models of Freundlich and Langmuir.

    Sorption 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 40șC, indicating that desorption occurs at above 40șC. This implies that significant adsorption took place at low temperatures, which becomes less significant at higher temperatures. The ultimate adsorption capacity of the biomass at the different temperatures can be calculated from the isothermal data by substituting the required equilibrium concentration in the Freundlich equation. The value of KF, which is a measure of the degree of adsorption, decreases with increase in temperature (Figure 2). The higher KF values at lower temperatures indicate that more sorption would be expected at these temperatures.

    The 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 40șC. The Langmuir fits at all temperatures show slight curvatures (Figure 3), which suggest that the surface adsorption is not a single monolayer with single sites. Two or more sites with different affinities and maximum may be involved in metal ion sorption. After 40șC, the values of Xm and K decreased with increase in temperature, showing that adsorption capacity and intensity of adsorption are enhanced at lower temperatures.

    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 .

    Thermodynamic treatment of the sorption process

    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.

    The effect of temperature on the sticking probability was evaluated throughout the temperature range from 30 to 80șC by calculating the surface coverage at the various temperatures. The plot of against 1/T gave linear plots with intercept of  and slope of  as shown in Figure 5. The apparent activation energy (Ea) and the sticking probability (S*) are estimated from the plot with reasonable good fit for the two metal ions on the biomass (r2 > 0.97). The Ea values calculated from the slope of the plot were found to be -16.14 KJ gmol-1 and - 7.92 KJ gmol-1 for Pb2+ and Cd2+ respectively. The negative values of Ea indicate that lower solution temperatures favours metal ion removal by adsorption onto the Caladium bicolor biomass and the adsorption process is exothermic in nature. Relatively low values of Ea suggests that metal ion adsorption is a diffusion controlled process. The results as shown in Table 2 indicate that the probability of metal ion sticking to the C. bicolor biomass surface is very high as S* << 1 for both metals (Table 2). These values confirm that, the sorption process is physisorption.

    Concluding Remarks

    In conclusion, 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 10-45șC with optimal temperature at 40șC. This temperature range is favourable for solubility of chemicals in wastewater treatment systems and will also enhance the reaction rates. The activation energy further supports lower solution temperatures and an excellent sticking of metal ions on to C. bicolor biomass. The sorption process is spontaneous and exothermic and the mechanism is physisorption. The equilibrium data agrees with the Langmuir isotherm. The sorption capacity of Cd2+ is higher than Pb2+ because of the ionic sizes of the ions; however, there is no significant difference in the sorption potential of the biomass towards the two metal ions in aqueous solution. Caladium bicolor is a non-useful plant growing in the wild. Its use as an adsorbent may eventually encourage cultivation of the plant and enhance the economies of local farmers and generate employment. The biomass from C. bicolor may be recycled and the recovered biomass is biodegradable and therefore environment friendly. Hence, not only is C. bicolor waste inexpensive and readily available, it also has the potential for metal removal and recovery of metal ions from contaminated waters. This process will be environment friendly and reduce the huge amount of indiscriminate effluent discharges all around the small industry concerns in Nigeria. It may also provide an affordable technology for small and medium-scale industry in Nigeria.


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