In this research, the following biosorption processes were investigated: 1) single and ternary metal solutions removal with non-living cells of S. Cerevisiae in a batch system. For this purpose, the capacity of yeast cells to adsorb metal ions was studied with respect to process parameters, namely, pH, sorbent dose, and initial metal concentration; 2) Cu2+ and Pb2+ ions treatment by a mixture of living and non-living yeast cells in a self-contained novel continuous system, where biomass generation and metal adsorption are combined; and 3) Cu2+ ions removal by Acer saccharum leaves in a flow-through packed-bed column.
Batch biosorption system in phase (I) of this study provided the basis for tackling the continuous mode systems in the subsequent phases. Large-scale applicability of biosorption relies mostly on the results of continuous-flow systems offered in phase (II) and (III) of this research. In the last chapter of the thesis, conflicting and confusing reports in the literature on the use and interpretation of the Langmuir model constants for quantifying and contrasting the performance of biosorbents were addressed, and Langmuir isotherm was derived based on a dynamic equilibrium between the metal species and biosorbent’s active sites for adsorption.
In all experiments, MINEQL+ speciation modeling was used for the tested metals to find out their solution chemistry at different environmental conditions. This ensured us that metal removal was predominantly due to biosorption but not due to any metal precipitation. The optimum process parameters for multi-metal removal were determined using statistical tools such as design of experiment technique (academic
version of Minitab-16). Applying design of experiment in biosorption studies helped to reduce the number of experimental runs considering the interactions among the process parameters (variables), and evaluate the individual and relative importance of the parameters as well as their cumulative effects.
For biomass characterization, the FTIR analysis, EDX spectra, and zeta-potential measurements were performed on untreated and metal-treated biomass, which revealed the chemical environment responsible for biomass-metal interactions. In the column
studied in phase (III) of experiments, modeling of biosorption was essential for process
design and optimization due to dynamic nature of column sorption where sorption equilibrium, diffusion, and bulk flow coexist.
Figures 1.1 and 1.2 show the biomaterials chosen for biosorption work in this research. High surface-to-volume ratio of the cells of S. cerevisiae (Baker’s yeast) provided a large metal-biomass contact interface for biosorption.
(a) (b)
Figure 1.1. (a) Optical microscopy of hydrated yeast cells (40x); and (b) scanning electron micrograph of the dried yeast biomass.
Figure 1.2. Fallen roadside maple tree leaves collected and used for biosorption studies as a locally available waste.
The experimental set-up used in phase (II) of experiments is shown in Figure 1.3. The ability of yeast cells to remove copper and lead ions from aqueous solutions was examined in a continuous system, where biomass is produced using inexpensive medium in a bioreactor to perform on-line metal biosorption in an airlift fluidized column. The unique specification of this set-up was that the biomass production and the metal biosorption were performed in the same system where they were connected in series, and yeast cells were used without any pretreatment of heat-inactivation steps.
Figure 1.4 shows the experimental set-up for the phase (III) of biosorption work using maple tree leaves as biosorbent. Maple tree wood and sap are of high economical value, but maple tree leaves are of none or low economical value (e.g. composting) till now though they have created waste management issues for municipalities.
In this study, we make use of waste maple leaves by evaluating their capability to adsorb heavy metal ions from aqueous solutions. The use of packed-bed columns is one of the most suitable ways for heavy metal treatment as it allows more effective utilization of the biomass capacity by making the best use of concentration difference between sorbent and sorbate known to be the sorption driving force (Sag et al., 2001; Aksu and Gönen, 2004). The outcome of the dynamic performance evaluation of a continuous-flow column packed with biosorbent - similar to ion-exchange columns
packed with synthetic resins – to remove heavy metals can be correlated to design and scale up of the process.
(c) (d) (e) (e) (b) (a)
Figure 1.3. Experimental set-up used for continuous-flow bioreactor-biosorption system for removal of heavy metals: (a) bioreactor system, (b) nutrients tank for biomass production, (c) biosorption vessel, (d) pH-controller, (e) settling tanks.
Figure 1.4. Experimental set-up for removal of heavy metals by maple leaves biomass in packed-bed columns.
Generally, biomass recovery is an uneconomical method as the fresh biomass is abundant and inexpensive. Also, the number of times that the biomass can be reused could be limited. However, recycling biomass is a useful option and may increase the process economy by recovering adsorbed metals. Biomass regeneration was investigated in continuous-flow column packed with maple leaves for removal of copper ions. Recovery of the desorbed metals from the concentrated eluent solutions is another step to enhance the process economy and reduce the environmental footprint of biosorption. Electrowinning (electroextraction) is often the feasible potential metal recovery method (Volesky, 2001), which was not the focus in this research.