We have evaluated the effect of pH on the removal of cationic pollutants in wastewater using a non-useful plant material as naturally occurring biosorbents. The experimental results have been analysed in terms of Langmuir, Freundlich and Flory-Huggins isotherms. The data showed that the maximum pH (pH_max) for efficient sorption of Pb²⁺ was 7.0 and for Cd²⁺ 5.0. Surface characterization of acid and base treated C. bicolor biomass indicates a physiosorption as the predominant mechanism for the sorption process. The pH study is particularly important for determining the ligands that may be involved in the reaction for optimization in the design of sorption process units.

BIP Article

Due to the bioaccumulating tendency and toxicity of heavy metals in the environment, it has been consistently desired that their levels be considerably reduced in industrial effluents to meet regulatory standards before final repository in the ecosystem. Conventional technologies for effluent treatment such as reverse osmosis, electroplating, etc are not economically feasible for small-scale industries. Several low-cost and environment friendly naturally occurring or agricultural by-products has been tested (Gardea-Torresdey et al. 1998; Ho et al 1998; Quek et al. 1998; Horsfall and Abia, 2003). The adsorbent used in this study is Caladium bicolor (Wild Cocoyam) Biomass, which is wild and abundantly available.

This paper reports the effect of pH on the ability of the biomass of the corm of Caladium bicolor (Wild Cocoyam) to remove Pb²⁺ and Cd²⁺ ions from single metal ion solutions.

Materials and Methods

Materials

The C. bicolor (Wild Cocoyam) were cut into 5 cm pieces and oven-dried to constant weight and screened through a 100 μm-mesh to obtain a fine biomass. The proximate composition (AOAC, 1990) andsome surface characteristics (Miguel et al. 2001; Horsfall and Abia, 2003) of the C. bicolor biomass samples were also determined. The effect of pH was determined by suspending 1.0 ± 0.01 g C. bicolor biomass in 50 mL of Pb²⁺ and Cd²⁺ metal ion solutions in triplicate. The pH of these suspensions was adjusted to 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 and the mixtures agitated for 1 h at 30ºC. At the end of agitation, the metal content at each pHwas determined using Buck Scientific Flame Atomic Absorption Spectrometer (FAAS).

Results and Discussion

The data for proximate compositions of the corm of C. bicolor showed that, the corm ofC. bicolor (Wild Cocoyam) is a complex material containing mainly organic residues made of several polar functional groups. The influence on the surface properties was evaluated by measuring the surface area, particle density, porosity, pore volume, cation exchange capacity (CEC) and surface charge density (SCD). The results showed that treatment of the pure biomass with either acidic or basic medium enhances the occurrence of physisorption so that cationic reaction may take place on the biomass surface, indicating an ion exchange process between the biomass and the metal ion.

Furthermore, it was observed that acid treated biomass had enhanced surface properties than base treated biomass. This differential behaviour in acidic and basic treatment may be an artifact of the presence of high affinity ligands in the biomass such as cyano and amino groups on the biomass surface. Hence, complexation and ion exchange processes are both playing a part in the sorption mechanism.

The efficiency of metal ion sorption by the biomass is controlled by the initial pH of the reaction mixture. The optimal binding by C. bicolor occurred at pH 7 for Pb²⁺ with 82.9% sorption and pH 5.0 for Cd²⁺ with 65.9% sorption. The applicability of the sorption processes as a unit operation was evaluated using isotherm models of Langmuir, Freundlich and Florry-Huggins equations. The R² values suggested that the three equations provided a good model for the sorption system. Furthermore, the equilibrium constant, K_a, obtained from the Flory-Huggins isotherm was used to compute the apparent Gibbs free energy of change. The apparent Gibbs free energy of sorption, ΔG^o, is the fundamental criterion of spontaneity. Reactions occur spontaneously at a given temperature if ΔG^o is negative quantity. ΔG^o (KJ mol^-1 K^-1) was evaluated using the following equation:

where R is the universal gas constant, 8.314 J/mol K and T is absolute temperature.

The negative values of ΔG^o  (Table 6) confirm the feasibility of the process and the spontaneous nature of sorption with high preference at pH values of 7.0 and 5.0 for Pb²⁺ and Cd²⁺ respectively.

In conclusion, the data has shown that, the sorption process of Pb²⁺ and Cd²⁺ ions on to C. bicolor biomass is feasible and spontaneous in nature. The metal ions binding capacity of the biomass was shown as a function of initial pH of the aqueous solution. The equilibrium data fitted the Langmuir, Freundlich and the Flory-Huggins isotherms very well. The three models confirmed that maximum removal of the two metal ions in an effluent using C. bicolor biomass could be achieved by pH adjustment between 5.0 and 7.0. The data showed that, C. bicolor is a successful biosorbent for treating heavy metal contaminated wastewater and may serve as an alternative adsorbent to conventional means. Hence, not only is C. bicolor readily available, it also has the potential for metal removal and recovery from contaminated waters. This process will be environment friendly and convert the non-useful plant into an economic crop for local farmers. It may also provide an affordable technology for small and medium-scale industries.

References

ASSOCIATION OF OFFICIAL ANALYTICAL CHEMISTS (AOAC). Official Methods of Analysis, 15^th ed., Arlington, VA. 1990, p. 331. ISBN 0-85226-141-1.

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.

HO, Y.S.; JOHN WASE, D.A. and FORSTER, C.F. Batch nickel removal from aqueous solution by Spagnum Moss Peat. Water Research, 1995, vol. 29, no. 5, p. 1327-1332.

HORSFALL, M. Jnr and ABIA, A.A. Sorption of Cd(II) and Zn(II) ions from aqueous solutions by cassava waste biomass (Manihot sculenta Cranz). Water Research, 2003, vol. 37, no. 20, p. 4913-4923.

MIGUEL, G.S.; FOWLER, G.D.; DALL'ORSO, M. and SOLLARS, C.J. Porosity and surface characteristics of activated carbons from waste tyre rubber. Journal of Chemical Technology and Biotechnology, 2001, vol. 77, p. 1-8.

QUEK, S.Y.; WASE, D.A.J. and Forster, C.F. The use of Sago waste for the sorption of lead and copper. Water S.A.,1998, vol.24, no. 3, p. 251-256.

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