A STUDY ON HEAVY METAL UPTAKE AND ITS EFFECT ON ELECTROKINETIC PROPERTIES OF MICROORGANISMS

Abstract

It is well documented that a wide variety of micro-organisms is capable of removing and accumulating trace levels of heavy metal ions and other pollutants from their external aqueous environment. The metal uptake capacities of these micro-organisms are known to be highly influenced by their environmental conditions. The uptake of copper and nickel ions by autoclaved Saccharomyces cerevisiae was examined in batch cultures. It was found that the uptake of copper and nickel was a fast and energy-independent process. The uptake capacity for copper and nickel ions was directly related to the pH of solution, with the optimum between pH 7-8. A maximum copper uptake of 0.41 mmol/g-cell was obtained with an initial copper/biomass ratio at 13 mmol/g-cell, while a maximum nickel uptake of 0.22 mmol/g-cell was achieved with an initial nickel/biomass ratio at 8 mmol/g-cell. Adsorption isotherms of copper and nickel uptake obeyed both the Freundlich and Langmuir models, although the Freundlich model gave a better fit. No significant effect of temperature on copper and nickel uptake was observed over the range 4 to 55°C. In addition, it was also found that a competitive effect existed in multi-ion systems. The degree of inhibition of copper uptake by other cations followed the order: Cd2+>Ni2+>Zn2+>Co2+, and the degree of inhibition of nickel uptake by co-ions followed the order: Cu2+>Cd2+>Zn2+>Co2+. Physically/chemically treated yeast cells gave rise to changes in the copper and nickel uptake capacity. The uptake capacity was enhanced for NaOH-, HCHO-treated and autoclaved yeast cells, but was lowered for HNO3-treated cells. The kinetics of copper and nickel uptake by living cells were also investigated. It was found that the kinetics followed a two-stage process; an initial rapid and energy-independent sorption and subsequent slow and energy-dependent uptake due to membrane transport of metal ions into the cells. The initial rate of copper or nickel uptake increased with increasing initial metal ions concentration. An analysis of the relationship between initial uptake rate and initial metal concentration showed that the nickel uptake kinetics was possibly first order, while the copper uptake kinetics was not. Copper and nickel uptake by living cells is directly related to pH; maximum uptake occurred at about pH 7 for both copper and nickel ions. Temperature significantly affected the copper and nickel uptake; increased temperature over the range 10 to 50°C resulted in increased uptake capacity of both copper and nickel. It was also found that the copper uptake was significantly enhanced in the presence of equimolar concentration of nickel; a 46% increase in copper uptake occurred within 24h. However, no significant effect of copper on nickel uptake was observed. Electrophoretic mobility (EPM.) of both autoclaved and living cells was measured to determine any relationship between the EPM and the metal uptake capacity of yeast cells. It was found that the uptake of metal ions can be ascribed to the change in surface charge as evidenced by the change in the EPM. Increasing pH resulted in increasing EPM due to the ionization of carboxyl and phosphate groups. This result is consistent with the change in the uptake of copper and nickel with increasing pH. It was also found that the effect of multi-metal ions on the EPM is different between dead and living cells. No significant effect of the presence of multi-metal ions on the EPM of dead cells was observed. However, the change in the EPM of live cells in the presence of multi-ions is dependent on the metal species involved. Increased nickel concentration (at a fixed copper concentration) gave rise to a decreased EPM. However, increase copper concentration (at a fixed nickel concentration) did not cause significant change in the EPM.