Determination of the biosorption activation energies of heavy metal ions on Zoogloea ramigera and Rhizopus arrhizus
Introduction
Accumulation of metals by microorganisms has been known for a few decades but has received more attention in recent years because of its potential application in environmental protection or recovery of precious or strategic metals. Biological processes for removal of metal ions from solution can be divided into three general categories: (i) biosorption (adsorption) of metal ions onto the surfaces of a microorganism, (ii) intracellular uptake of metal ions, and (iii) chemical transformation of metal ions by microorganisms. The latter two processes require living organisms [1], [2], [3], [4]. The active mode of metal accumulation by living cells is designated as bioaccumulation. Non-viable microbial biomass frequently exhibits a higher affinity for metal ions compared with viable biomass probably due to the absence of competing protons produced during metabolism. To avoid the problems of toxicity of metals for microbial growth, or inhibition of metal accumulation by nutrient or excreted metabolites, the decoupling of the growth of the biomass from its function as a metal-sorbing material is seen as a one of the major advantages of biosorption [5], [6], [7].
Biosorption is caused by a number of different physicochemical mechanisms, depending on a number of external environmental factors as well as on the type of a metal, its ionic form in the solution, and on the type of a particular active binding site responsible for sequestering the metal. Temperature, adsorption pH, initial metal ion concentration, biomass concentration, and concentrations of other interfering ions are the environmental influences which are important in the biosorption of heavy metal ions [8], [9], [10]. The influence of pH and the metal/biosorbent ratio on heavy metal removal by biosorption have been widely recognized [2], [5], [11], [12], [13], but information on the effect of temperature is still scanty. The binding of most metals to microorganisms by biosorption is observed to enchance as temperature is increased [13], [14].
Although the search for new and innovative treatment technologies has focused attention on the metal binding capacities of various microorganisms, the exact interactions between the ligands on the cell walls and the heavy metal ions, the kinetics of the metal uptake process and the description of the thermal properties of the biosorption remain essentially unknown. Although the magnitude of the heat effect for the biosorption process is the most important criterion to develop a thermodynamic and kinetic relationship for the metal–microorganism interaction process, little attention seems to have been given to the study of the evaluation of the heat change and/or activation energy of biosorption process.
The Langmuir model, the most simple used for adsorption phenomena of one component, has a theoretical basis, which relies on a postulated chemical or physical interaction (or both) between solute and vacant sites on the adsorbent surface [15], [16], [17]. The adsorption rate isthe desorption rate iswhere C is the unadsorbed solute concentration in solution, k and k′, respectively the adsorption and desorption rate constants and θ, the fraction of surface covered by adsorbed solute. At equilibrium, the equality of these two rates leads to the Langmuir adsorption isotherm:where the adsorption equilibrium constant is K=k/k′. Combining Eq. (1) and Eq. (3), the Langmuir–Hinshelwood adsorption equation modified for monolayer adsorption is obtained and the rate of adsorption is given as follows [15], [16], [17]:
In an experimental data plot of rate versus C, the rate of adsorption is proportional to the first power of the concentration of metal ion at lower bulk metal ion concentrations and can be given using Eq. (5):
At higher bulk metal ion concentrations, the rate of adsorption becomes independent of bulk metal ion concentration. Eq. (4) can describe the rate of adsorption very accurately in both of these situations. This kind of rate equation is also defined as ‘saturation type rate’. This rate equation can be linearized by plotting 1/r versus 1/C to determine the rate and equilibrium constants of adsorption from the slope and the intercept, 1/k and K/k, respectively.
The rate of adsorption depends on the temperature, through variation of the rate coefficient. According to the Arrhenius equation [15], [17]:where E is activation energy and A0 is a constant called the frequency factor. Consequently, when ln k is plotted versus 1/T, a straight line with slope −E/R is obtained.
Section snippets
Microorganisms and preparation of the microorganisms for biosorption
Zooloea ramigera, an activated sludge bacterium, and Rhizopus arrhizus, a filamentous fungus, were obtained from the US Department of Agriculture Culture Collection. Z. ramigera and R. arrhizus were grown aerobically in batch cultures at 25 and 30°C, respectively as described previously [18], [19]. In the stationary phase of growth (120 h), Z. ramigera cells were centrifuged at 5000 rev min−1 for 5 min, washed twice with distilled water and then dried in an oven at 60°C for 24 h. After the
Results and discussion
Upon contact between the biosorbent and the solution containing the metal species, an equilibrium is established at a given temperature whereby a certain amount of the metal species sequestered by the biosorbent is in equilibrium with its residue left free in the solution containing then the residual, final, or equilibrium concentration of that metal species. Equilibrium considerations of the biosorption process have been extensively investigated [10], [21], [22], [23], [24]. In addition to
Conclusions
Despite the quite extensive literature available on heavy metal biosorption, little attention seems to have been given to the temperature dependence of the biosoption process. In this study, batch adsorption experiments were performed at different temperatures in the range 15–45°C and the effect of initial metal ion concentration on the initial biosorption rates was investigated. The initial uptake kinetics of heavy metal ions on Z. ramigera and R. arrhizus was shown to be represented by
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