Hanbin Xue

The binding of heavy metals to algal surfaces

Abstract Biological particles can profoundly influence the distribution of heavy metals in natural waters because the functional groups on the cell surfaces are able to bind metal ions. The interaction of Cu(III) and Cd(II) was evaluated from titration of algal suspensions ( Chlamydomonas rheinhardii ) (i) at a constant pH with increments of metal ions and (ii) in the presence of the metal ions with increments of acid. A voltammetric methodology was developed to measure (without prior separation of the solid phase) the metal ions in solution in the presence of algae and to assess the binding of metals to the surfaces of algae. The surfaces of algal cells have a high affinity for Cu(II) and Cd(II), even in the presence of 10 −3 M Ca 2+ ; their functional group ligands can compete with soluble complex formers typically present in natural waters. The adsorption of metals is readily interpreted in terms of surface complex formation equilibria or—mathematically equivalent—Langmuir type adsorption equilibria. The “average” conditional equilibrium constant extracted from the experimental data permits a generalization valid for a certain range of metal surface loading and can be used in multi-metal multi-ligand speciation calculations. Under-standably, the data cannot be fitted over a large range of free metal ion concentrations or metal loading of the surface because the tendency to form surface complexes decreases with increasing metal loading, because there are a variety of surface ligands, and the metal ions bind first to the surface groups with highest affinity and subsequently to groups with lower affinity. A much better fit of the data is obtained if models are used with more than one adjustable constant such as the constant capacitance or the Fowler Guggenheim Frumkin model or a two-site Langmuir isotherm. The kinetics of adsorption and uptake of Cu(II) to Chlamydomonas is characterized by the establishment of a relatively fast pseudo-adsorption equilibrium with the surface that is followed by slow diffusion-controlled uptake into the inside of the cell.

Speciation of EDTA in natural waters : exchange kinetics of Fe-EDTA in river water

In order to understand the speciation of EDTA, the exchange kinetics of Fe(III)-EDTA with other cations (Zn, Ca) are studied under the conditions of natural river water. Measurements in sewage treatment plant effluents indicate that EDTA is partly present as Fe(III)-EDTA. At equilibrium, Fe-EDTA introduced into river water is expected to exchange with Zn(II) and Ca to form Zn-EDTA or Ca-EDTA. The exchange of Fe-EDTA with Zn(II), which was followed by voltammetric measurement of labile Zn, as well as the disappearance of Fe-EDTA, which was measured as a photolabile species, occur on a time scale of days

Binding of Cu(II) to algae in a metal buffer

Abstract The interactions of Cu(II) with algal surfaces and exudates were studied in metal-NTA buffers by a combination of several analytical techniques. Suspensions of living algae in the presence of NTA were titrated at constant pH with Cu(II). The various Cu species were determined as follows: a copper ion selective electrode was used reliably in the pCu range 9–12; differential pulse polarography was used to measure separately Cu(II)-NTA complexes and labile Cu(II) species and to evaluate the complexation of copper by ligands in solution; copper bound to the algal surfaces was extracted by acid treatment and measured by AAS. Thus, we determined both the binding of Cu to the algal surfaces and to exudates excreted by the algae. The results were interpreted in terms of conditional equilibrium constants valid at a given pH; the conditional constants, both for the binding to the surfaces and with the exudates increase in the pH range 5.0–6.5. Simple equilibrium models using the experimentally determined binding capacities and equilibrium constants were able to simulate the results and to evaluate the speciation of copper. Under the experimental conditions used, the binding of Cu(II) to algal exudates has a more significant effect on copper speciation than the binding to the algal surfaces. These extracellular ligands may play an important role in decreasing the concentration of free copper ion and thus mitigating the potential toxic effects in organisms.