Biosorption of cadmium (II), copper (II) and lead (II) ions by citric acid modified and unmodified cocoa pod shell: Equilibrium, kinetics and thermodynamics

Abstract

Chemical modification of biosorbent is a common practice in wastewater treatment when using adsorption technology.
Thioglycollic acid has been used for the modification of cocoa pod without any significant improvement in its adsorption
capacity for lead, cadmium and copper. This study is aimed at improving the sorption of these metal ions from aqueous
solution using citric acid as the modifying agent for cocoa pod shell. The dried cocoa pod shell was ground and reacted
with citric acid according to the method in the literature. Batch adsorption studies were conducted to examine the
influence of pH, temperature, initial metal ion concentration; sorbent dose and contact time on the biosorption of Pb2+,
Cu2+and Cd2+ions by modified and unmodified cocoa pod shell. Langmuir, Freundlich, Temkin and Dubinin-Radushkevich
models were used for fitting the equilibrium data. The amount of metal ion sorbed was determined using flame atomic
absorption spectrophotometer (FAAS). The result for the adsorption of the metal ions under varying pH showed that
sorption was pH dependent. The trend in biosorption was in the order of Cd2+> Cu2+> Pb2+ on unmodified cocoa pod shell
(UCPS); while the trend in biosorption of the ions onto modified cocoa pod shell (MCPS) was in the order of
Pb2+ > Cu2+> Cd2+. The amount of Pb2+ adsorbed increased from 4.62-33.56 mg/g after modification as shown by values
obtained using the Langmuir equation to fit the biosorption data. Langmuir isotherm gave R2 values which shows that it
satisfactorily describe the biosorption process for Pb2+, Cu2+and Cd2+ ions on UCPS and MCPS. The kinetics of the
sorption process was best described by the pseudo-second order kinetic model (R2>0.9922). Thermodynamic parameters
showed that biosorption process was feasible and spontaneous for Pb2+, Cu2+and Cd2+ions onto UCPS and MCPS. The
process is exothermic for Cu2+ and Cd2+ ions; but endothermic for Pb2+. This study has shown that citric acid modified
cocoa pod shell is a good biosorbent especially for Pb2+ ions