Laura Sigg

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.

Dissolution of Fe(III) (hydr) oxides by metal-EDTA complexes

Abstract The dissolution of Fe(III)(hydr)oxides (goethite and hydrous ferric oxide) by metal-EDTA complexes occurs by ligand-promoted dissolution. The process is initiated by the adsorption of metal-EDTA complexes to the surface and is followed by the dissociation of the complex at the surface and the release of Fe(III)EDTA into solution. The dissolution rate is decreased to a great extent if EDTA is complexed by metals in comparison to the uncomplexed EDTA. The rate decreases in the order EDTA CaEDTA ⪢ PbEDTA > ZnEDTA > CuEDTA > Co(II)EDTA > NiEDTA. Two different rate-limiting steps determine the dissolution process: (1) detachment of Fe(III) from the oxide-structure and (2) dissociation of the metal-EDTA complexes. In the case of goethite, step 1 is slower than step 2 and the dissolution rates by various metals are similar. In the case of hydrous ferric oxide, step 2 is rate-limiting and the effect of the complexed metal is very pronounced.

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.

Zinc speciation in lake waters and its determination by ligand exchange with EDTA and differential pulse anodic stripping voltammetry

Abstract A technique to determine free zinc ion concentration and zinc speciation in lake water was developed. The technique involves ligand exchange of added EDTA with natural organic ligands and measurement by differential pulse anodic stripping voltammetry (DPASV); pretreatment of samples and separation are not necessary. The validity of the ligand exchange approach was demonstrated in model waters containing glycine and CDTA (or EDTA), as representatives of weak and strong organic ligands. The ligand exchange and DPASV measurements were applied to water samples from Lake Greifen, a small eutrophic lake in Switzerland. The average fractions of dissolved zinc species in this lake were 7.8% free zinc ions, 33.5% weak organic complexes and 50.5% strong organic complexes. Total dissolved Zn in the lake water column ranged between 18 to 25 nM, free zinc ion concentrations 1.3–1.8 nM, with an average pZn of 8.8 and an average ratio of total Zn to [Zn 2+ ] ≈ 13. The extent of complexation of Zn in the lake was much less than in some sea waters (ratios 60–100 in the North Pacific), probably due to higher ratios of total Zn concentration relative to organic ligands in the lake.

Extracellular Polymeric Substances (EPS) of Freshwater Biofilms Stabilize and Modify CeO2 and Ag Nanoparticles

Streams are potential receiving compartments for engineered nanoparticles (NP). In streams, NP may remain dispersed or settle to the benthic compartment. Both dispersed and settling NP can accumulate in benthic biofilms called periphyton that are essential to stream ecosystems. Periphytic organisms excrete extracellular polymeric substances (EPS) that interact with any material reaching the biofilms. To understand the interaction of NP with periphyton it is therefore crucial to study the interaction of NP with EPS. We investigated the influence of EPS on the physicochemical properties of selected NP (CeO2, Ag) under controlled conditions at pH 6, 7.6, 8.6 and light or dark exposure. We extracted EPS from five different periphyton communities, characterized the extracts, and exposed CeO2 and carbonate-stabilized Ag NP (0.5 and 5 mg/L, both 25 nm primary particle size) and AgNO3 to EPS (10 mg/L) over two weeks. We measured NP size distribution, shape, primary particle size, surface plasmon resonance, and dissolution. All EPS extracts were composed of biopolymers, building blocks of humic substances, low molecular weight (Mr) acids, and small amphiphilic or neutral compounds in varying concentrations. CeO2 NP were stabilized by EPS independent of pH and light/dark while dissolution increased over time in the dark at pH 6. EPS induced a size increase in Ag NP in the light with decreasing pH and the formation of metallic Ag NP from AgNO3 at the same conditions via EPS-enhanced photoreduction. NP transformation and formation were slower in the extract with the lowest biopolymer and low Mr acid concentrations. Periphytic EPS in combination with naturally varying pH and light/dark conditions influence the properties of the Ag and CeO2 NP tested and thus the exposure conditions within biofilms. Our results indicate that periphytic organisms may be exposed to a constantly changing mixture of engineered and naturally formed Ag NP and Ag+.

Cadmium speciation and accumulation in periphyton in a small stream with dynamic concentration variations

Accumulation of cadmium in periphyton was investigated under field conditions while Cd concentration and speciation were dynamically varying in a small stream during rain events. Speciation in water was determined in situ by diffusion gradient in thin-films (DGT) and by modeling of complexation with fulvic acids. During the rain events, dissolved Cd concentrations increased from 0.17 nM to 0.27-0.36 nM, and 70-97% were DGT-labile. Cd content in periphyton closely followed Cd concentrations in water, despite higher concentrations of Zn and Mn, and may be controlled by either free or DGT-labile Cd concentrations. Decrease of Cd content in periphyton after the rain events was slower than the decrease of Cd concentration in water. Concentrations of Zn, Mn, Cu, Pb and Fe in periphyton also followed the dynamic variations of metal concentrations in water. Repeated exposure of periphyton to elevated dissolved Cd may lead to Cd accumulation.

Phytochelatin formation kinetics and toxic effects in the freshwater alga Chlamydomonas reinhardtii upon short- and long-term exposure to lead(II).

Phytochelatins (PC) are metal-binding ligands synthesized by algae in response to elevated concentrations of various metals, such as Pb. Kinetics of PC synthesis and Pb accumulation in Chlamydomonas reinhardtii were investigated as a function of [Pb(2+)]=10(-11)-10(-7)M (pPb11-pPb7.1) in the exposure medium for up to 6h. The role of PC in Pb detoxification was explored by relating PC synthesis to the effects of Pb on growth and photosynthetic yield upon exposure to pPb9 and pPb8.3 for up to 72h. Pb accumulation increased with increasing [Pb(2+)], reaching a maximum concentration of 596±77amol/cell (intracellular concentration 2.98mM) at pPb7.1. Low concentrations of PC(2)-PC(4) were present in C. reinhardtii grown in control media without Pb addition. Upon short-term exposure, PC(2) and PC(3) synthesis was induced within minutes at [Pb(2+)]≥pPb8 and PC(4) synthesis after a lag phase at pPb7.1. Cellular PC(2)-PC(4) concentrations increased with time over 6h and with increasing [Pb(2+)]. PC concentrations after 6h exposure to pPb7.1 were 28.5±0.2amol/cell (142μM) PC(2), 2.8±0.05amol/cell (14μM) PC(3) and 0.30±0.01amol/cell (1.5μM) PC(4). Upon long-term exposure, induction of PC synthesis was detected at pPb9 and synthesis of PCs with a higher degree of polymerization was observed (PC(5)). PC concentrations were lower than intracellular Pb and were thus not present at sufficiently high concentrations to immobilize accumulated Pb. Inhibition of photosynthesis and growth up to 100% was observed upon long-term exposure, whereas in short-term experiments no inhibitory effects were detected.

Dry deposition measurements using water as a receptor : a chemical approach

The field measurement of dry deposition still represents a difficult task. In our approach, a 1 to 2 cm thick layer of water in a petri dish with a diameter of 22 cm, serves as a surrogate surface. The atmospheric constituents taken up by the water can be analyzed chemically by the same procedure as for the wet deposition samples. In contrast to solid surrogate surfaces, water exhibits the following advantageous properties: low and constant surface resistance, high sticking coefficient for aerosols, and predictable Sorption behavior for gases. Consequently, the deposition rates measured to the wet surface are generally higher, by up to a factor of 4 for NH4+, Cl−, NO3− and SO42−, than those to a dry surface, but still smaller than the concurrent wet deposition rates. We observed the following average dry deposition rates in μmol m−2 d−1∶ NH4+ 48.3, Ca2+ 40.7, Na+ 15.8, Mg2+ 8.4, K+ 4.2, H-Aci 36.4; SO42− 57.2, Cl− 39.2, NO3− 34.5, HSO3− 5.7, formate 4.0; acid soluble metals: Fe 2.8, Zn 0.60, Cu 0.11, Pb 0.073, Cd 0.0022. The soluble fraction of Zn, Cd, Cu, Pb and Fe in the dry deposition varied with the pH of the water phase corresponding to the adsorption tendency of these metals to oxide surfaces. The sampling method also allows tracing of regionally and locally emitted atmospheric pollutants. In our study the local pollution sources included road salting, construction work and a refuse incinerator. Finally, chemical reactions occurring in the atmosphere, such as the conversion of Cl− to HCl by HNO3 or the oxidation of SO2, can be identified by evaluating the data. The method proposed is relevant to measure reproducibly the dry deposition of a variety of compounds to water bodies and moist vegetation.

The composition of settling particles in Lake Zürich

Settling particles in the Lake Zürich were collected at different depths. Water samples and particles have been analyzed for heavy metals and for organic matter elements. Samples were investigated by transmission electron microscopy, scanning electron microscopy, energy dispersive X ray and X ray. The elemental composition of the settling particles was found to be almost constant. Special remarks are made on iron and phosphate and on manganese. Manganese oxidizing microorganisms were found near the bottom of the lake.

Chemical Processes at the Particle-Water Interface; Implications Concerning the Form of Occurrence of Solute and Adsorbed Species

In recent years it has become more obvious that the reactions of solutes with particles are of importance in regulating the solute concentrations of many reactive elements and controlling the geochemistry of most trace elements.