The purpose of this investigation is to access the suitability and applicability of the biomass from the non-useful yet abundantly available Caladium bicolor plant as metal ion adsorbent under the influence of temperature. The sorption capacity of Cd²⁺ was higher than Pb²⁺ because of differences in their ionic sizes. The sorption equilibra and thermodynamic treatment of experimental data were evaluated to indicate that the sorption process is spontaneous and exothermic in nature and that lower solution temperatures favours metal ion removal by the biomass. The results indicate that optimal temperature of adsorption in utilizing Caladium bicolor biomass for the removal of metals in aqueous solutions is about 40ºC.

A recent screening (Horsfall and Spiff, 2004) 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 have made C. bicolor to have great potential as an adsorbent for metal ions in aqueous solutions. 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. However, temperature has not been studied as relevant variable in biosorption experiments. The tests are usually performed at approximately 25-30ºC. Only very few reports has been given in the range 4 to 55^oC (Tsezos and Volesky, 1987; Kuyucak and Volesky, 1989 and Aksu and Kutsal, 1991).

This paper reports 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 30-80ºC.

The effect of temperature was studied by taking specified volume of standard metal ion solution of Pb²⁺ and Cd²⁺ to prepare varying initial metal ion concentrations. An accurately weighed Caladium bicolor biomass sample of definite particle size was then added to the solution to obtain a suspension. The suspensions were adjusted to pH 5.0, shaken at constant speed in a bath at temperatures ranging from 30-80ºC. After 2 hrs, the suspensions were filtered and centrifuged. The metal content in the supernatants at each temperature range was determined using flame atomic absorption spectrometer model A300.

The Freundlich and Langmuir models were used to fit the experimental data in order to assess the maximum adsorption capacity corresponding to biomass surface saturation and the adsorption intensity of the sorbent towards the biomass. Thermodynamic evaluation was also made to assess the spontaneity of the sorption process.

The data obtained 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 (Ho, 2003). However, the magnitude of such increase continues to decline as temperatures are raised from 30 to 80ºC.

The decrease in adsorption with increasing temperature, suggest weak adsorption interaction between biomass surface and the metal ion, which supports physisorption.

Sorption data were fitted by Freundlich adsorption isotherm at all temperatures (r² were greater than 0.94). The sorption intensities were found to be more than unity at all temperatures except 30 and 40ºC, indicating that desorption occurs at above 40ºC. 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 was calculated from the isothermal data by substituting the required equilibrium concentration in the Freundlich equation. The value of K_F, which is a measure of the degree of adsorption, decreases with increase in temperature (Figure 1). The higher K_F values at lower temperatures indicate that more sorption would be expected at these temperatures.

The most probable temperature of adsorption was predicted using the Langmuir maximum adsorption, X_m, for a monomolecular surface coverage and the adsorption equilibrium constants, K_F, at the different temperatures.

The thermodynamic treatment of the sorption data indicates that ΔGº values were negative at all the temperatures investigated. The negative values of ΔGº indicate the spontaneous nature of adsorption of metal ion by the biomass. The ΔGºvalues obtained in this study for both metal ions are lee than - 10 KJ gmol^-1, indicative that physical adsorption is the predominant mechanism in the sorption process. The computed values of (ΔHº) and (ΔSº) for Pb²⁺ and Cd²⁺ on to the biomass further confirm the exothermic nature of the adsorption process. The positive values of ΔSº 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.

The sticking probability, S*, which, was evaluated to indicate the measure of the potential of an adsorbate to remain on the adsorbent indefinitely using an Arrhenius type equation:

The results indicated that the probability of metal ion sticking to the C. bicolor biomass surface is very high as S*. These values confirm that, the sorption process is physisorption.

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. 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 recyclable and the recovered biomass is biodegradable and therefore environment friendly, reduce the huge amount of indiscriminate effluent discharges all around the small industry concerns in Nigeria and may provide an affordable technology for small and medium-scale industry in Nigeria.

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