Infoscience

Thesis

Linking chemical speciation to metal bioavailability: role of dissolved organic matter on cadmium and lead bioavailability by green microalgae

It is recognized that dissolved organic matter (DOM) plays an utmost role in aquatic ecosystems by binding trace metals and influencing their speciation, bioavailability and detrimental effects. DOM is considered to protect the aquatic microorganisms from heavy metal stress by decreasing free metal ion concentration and thus decrease their bioavailability. Nonetheless, a fundamental understanding of the relationship between trace metal speciation, bioavailability and effects is still lacking for many environmental systems, although progresses have been made in recent years. The main objective of the present thesis was to link Pb(II) and Cd(II) speciation to its bioavailability to green micro algae in presence of DOM. In particular, (i) to characterize metal binding properties of different DOM components (e.g. EPS), (ii) to determine their effect on metal bioavailability and (iii) to quantify as rigorously as possible the relationship between chemical speciation and bioavailability in presence of different DOM components. The proton, Pb2+ and Cd2+ binding capacities of extracellular polymeric substances (EPS), in particular alginate and Rhizobium meliloti exopolysaccharides were quantified by determination of complex stability constants and the concentration of binding sites using ion selective electrodes in static or dynamic titration. The influence of ionic strength, pH and metal to EPS ratio was determined over large concentration ranges. Metal binding and conditional stability constants increased with increasing pH due to a decrease of competition with the proton. In contrast, an increase in the metal to EPS ratio or ionic strength resulted in a decrease in the average conditional stability constants as could be expected by the polyelectrolytic and polyfunctional character of the EPS. A non ideal competitive adsorption isotherm (NICA) combined with a Donnan electrostatics approach previously developed for humic substances, takes into account the intrinsic polyfunctionality and polyelectrolytic properties and can reasonably account for electrostatic and competition effects. It was successfully applied to describe Pb(II) and Cd(II) complexation by EPS. Results demonstrated that EPS played a non neglected role in Pb and Cd speciation. The influence (both direct and indirect) of different DOM components on Pb bioavailability by green algae Chlorella kesslerii was studied. Results demonstrated that some of the studied EPS (e.g. alginate) may exhibit similar influences on Pb bioavailability as humic substances. DOM components had several simultaneous effects on Pb bioavailability including complexation in the medium, adsorption to algal surfaces, and modification of membrane permeability. An important discrepancy between Pb bioavailability in the presence of HS and alginates (as individual components or mixtures) and that predicted for similar free Pb ion concentrations in the presence of synthetic ligands (NTA, citrate) was obtained. In contrast, the experimental results of Cd and Cu uptake were in good agreement with that predicted from free metal concentration. An improved fit between experimental observations and Pb speciation was obtained by extending the free ion activity model (FIAM) when the formation of a ternary complex between metal, DOM and the algal surface is taken into account. The contribution of ternary complexes on metal biouptake was thus dependent on (i) the metal affinity to microorganisms, (ii) the concentration of different components that constituted DOM, (iii) their capacity to adsorb onto the surface of the microorganisms and (iv) their affinity to bind the metal. By simultaneously examining metal speciation and biological availability in the presence of different DOM components and their mixtures, this work provided new information with respect to the applicability and limitation of existing equation models. An extension of existing models will further improve a site specific predictive capacity by avoiding underestimation of metal biouptake in presence of high DOM concentration.

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