ABSTRACT

Batch bio sorption experiments were carried out to investigate the effects of particle size, bio sorbent dose/weight, pH, temperature, contact time, and metal ion concentration on the uptake levels of Cu2 , Cd2and Pb2ions by unmodified and modified cocoa pod husk (CPH) and breadfruit seed hull (BSI-I). Results of the equilibrium sorption studies revealed optimum adsorption capacity of the biosorbents using particle size of 425 tm (by modified BSH); biosorbent dose/weight of 10 g (by unmodified CPH); pH 2.5 (by unmodified BSH); temperature of 303 K (30°C) (by modified BSJ-I); contact time of 60 mm (by unmodified CPH); and initial metal ion concentration of 100 mg/L (by unmodified CPH) at the various conditions under investigation. Maximum sorption capacity of the biosorbents showed Pb2(99.963 6 mg/g) by unmodified CPH, Cd2 (99.9159 mg/g) by modified BSH and Cu2(99.8496 mg/g) also by modified BSH. The trend of adsorption for the metal ions was always in the pattern: Pb2 > Cd2 > Cu2for all parameters investigated. Thiolation modification of the biosorbents increased the sorption capacity of breadfruit seed hull and decreased that of cocoa pod husk. Further investigation was done with evaluation and modelling by kinetic/thermodynamic models as well as adsorption isotherms. Van't Hoff thermodynamic model confirmed physisorption dominant for the sorption process as a result of more positive ASacis values and chemisorption as well due to few negative ASadsvalues. Negative Alladsvalues indicated the exothermiç nature of the sorption process. Sticking probability model also suggested the sorption process an exothermic reaction and to be diffusion controlled. Pseudo-second order kinetic model favoured the sorption process with R2value ranging from 0.9999 to 1.0 thus providing a better fit than the Pseudo-first order model. Intraparticle diffusion models by McKay & Poots and Weber & Morris all fitted with experimental data and confirmed the sorption process to be particle diffusion controlled. Evaluation by adsorption isotherms, showed Freundlich isotherm with R2value ranging from 0.3965 to 0.8575 as the best fit for the sorption process, followed by Dubinin- Radushkevich isotherm and then Langmuir isotherm.