Sorption of heavy metals by green macro alga, Caulerpa lentillifera and modified zeolite from coal fly ash

Abstract

This dissertation comprised two main parts which are the investigations of the biosorption of heavey metal ions by agricultural waste (green macroalga, Caulerpa lentillifera) and by Zeolite X modified from coal fly ash (CFA). The biosorption of Cu[superscript 2+], Cd[superscript 2+], and Pb[superscript 2+] by a dried green macroalga Caulerpa lentillifera was investigated. The sorption kinetic data could be fitted to the pseudo second order kinetic model. The governing transport mechanisms in the sorption process were both external mass transfer and intra-particle diffusion. Isotherm data followed the Sips isotherm model with the exponent of approximately unity suggesting that these biosorption could be described reasonably well with the Langmuir isotherm. The maximum sorption capacities of the various metal components on Caulerpa lentillifera biomass could be prioritized in order from high to low as: Pb[superscript 2+] > Cu[superscript 2+] > Cd[superscript 2+] . The sorption energies obtained from the Dubinin-Radushkevich model for all sorption systems were in the range of 4 6 kJ mol[superscript -1] indicating that a physical electrostatic force was potentially involved in the sorption process. The sorption capacity decreased with a decrease in pH. The sorption of Cu[superscript 2+], Cd[superscript 2+], and Pb[superscript 2+] by a dried green macroalga Caulerpa lentillifera in binary components systems was investigated. The partial competitive binary isotherm model was proven to be effective in describing the experimental data. The model and experimental data of each binary component system demonstrated that the presence of the secondary metal ion always reduced the total sorption capacity of the biomass. This implied that there existed the same pooled binding sites for the sorption of all of these heavy metal ions. Pb[superscript 2+] was demonstrated to be the most uptaken species. Followed by Cu[superscript 2+] and Cd[superscript 2+]. The presence of Pb[superscript 2+] significantly suppressed the sorptions of Cu[superscript 2+] and Cd[superscript 2+] than it was by the presence of Cu[superscript 2+] and Cd[superscript 2+]. Similarly the sorption of Cd[superscript 2+] was more disturbed by the presence of Cu[superscript 2+] than that of Cu[superscript 2+] by Cd[superscript 2+]. The effect of pH on metal sorption could also be described using similar fundamentals with the sorption of binary metal component, and the partial competitive model could also be applied to predict the effect of pH on the sorption of these metals. It was demonstrated that a decrease in pH resulted in a reduction in the sorption capacity and sorption affinity. Carboxyl, hydroxyl, sulfonate, amine, and amide functional groups in the alga could be responsible for the sorption of all heavy metals while Cd[superscript 2+] could mainly form bond with amine and amide groups and had difficulty in forming bonds with the other groups. Thomas model could well describe the breakthrough data from column experiments. Ca[subscript 2+] , Mg[superscript 2+]. and Mn[superscript 2+] were the major ions released from the algal biomass during the sorption which revealed that ion exchange was one of the main sorption mechanisms. The CFA was modified to zeolite X using the fusion method with cationic exchange capacity (CEC) of about 140 meq/100 g. The zeolite was used as an effective sorbent for removing of Cu[superscript 2+], Cd[superscript 2+], and Pb[superscript 2+]. The pseudo second order kinetic model was appropriate for the description of the kinetic performance of the sorption. Although it took longer to reach equilibrium for experiments at higher initial metal concentration and lower sorbent dose, all sorption reached equilibrium within 120 minutes. External mass transfer step seemed to take part as a rate limiting step for the sorption of Pb[superscript 2+] at low initial concentration and high sorbent dose. In the contrary, the process was controlled more significantly by intraparticle diffusion step at high initial concentration and low sorbent dose. However, Cu[superscript 2+] was found to be generally controlled by intraparticle diffusion step at all concentration range examined in this work. The sorption of Cd[superscript 2+], on the other hand was controlled both by external mass transfer and intraparticle diffusion steps at all range of initial concentration. Langmuir and Dubinin-Radushkevich isotherms were applied to describe equilibrium fata. The order of maximum sorption capacity in a unit of mol kg[superscript -1] was: Pb[superscript 2+] (2.03)>Cu[superscript 2+] (1.43) >Cd[superscript 2+] (0.870). The sorption energy fell in the range of physic-sorption. Equilibrium sorption capacity and removal percentage were governed by both initial concentration and sorbent dose. A general mathematical model was developed for describing the sorption under the variations in initial metal concentration and zeolite doses. This model was proven to be reasonable accurate with additional sets of experiments.