Céline Guéguen

Binding interactions of algal-derived dissolved organic matter with metal ions

The nature and composition of dissolved organic matter (DOM) strongly influences its binding properties to heavy metals and thus their fate, mobility and toxicity in aquatic environments. Fluorescence spectroscopy with parallel factor analysis (PARAFAC) was used to characterize DOM exuded by the cosmopolitan freshwater green algae Scenedesmus acutus during early exponential growth phase. One protein-like (peak T; C2) and two humic-like components (peaks A+C and A+M, C1 and C3, respectively) were split half validated on 122 emission-excitation matrices (EEMs). Our data show that both humic-like could be associated with biological activities. Unlike Cd, Pb and Zn, Cu strongly binds to algogenic DOM with conditional stability constants (logK) averaging 5.26±0.29 (from 4.85 to 5.36). Significant differences in logK values were found between humic-like PARAFAC components, indicating clear differences in the binding properties of humic-like components with copper.

Partitioning of trace metals between particulate, colloidal and truly dissolved fractions in a polluted river: the Upper Vistula River (Poland)

Metal partitioning depends on the physical–chemical conditions of a system and can be affected by anthropogenic inputs. In this study, the authors report the results of trace metal partitioning between particulate (>1.2 μm), colloidal (1.2 μm–1 kDa) and truly dissolved (<1 kDa) fractions in the polluted section of the Upper Vistula River compared with the non-polluted headwaters. It was found that the salt input in the Vistula River induced a decrease of colloid concentration and the increase of suspended particulate matter. Compared with upstream from the polluted section, the metal concentrations (Co, Cu, Cr, Mn and Zn) in the colloidal fraction were lower. It was mainly due to the rapid colloid coagulation at increased salinity, the competition with ligands and major ions (Ca and Mg) and the weak mobility of metals associated with particles at the pollution sources.

Characterization of aquatic dissolved organic matter by asymmetrical flow field-flow fractionation coupled to UV–Visible diode array and excitation emission matrix fluorescence

Flow field-flow fractionation (FlFFF) with on-line UV/Visible diode array detector (DAD) and excitation emission matrix (EEM) fluorescence detector has been developed for the characterization of optical properties of aquatic dissolved organic matter (DOM) collected in the Otonabee River (Ontario, Canada) and Athabasca River (Alberta, Canada). The molecular weight (MW) distribution of DOM was estimated using a series of organic macromolecules ranging from 479 to 66,000 Da. Both the number-average (M(n)) and weight-average (M(w)) molecular weights of Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA) determined using these macromolecular standards were comparable to those obtained using polystyrenesulfonate (PSS) standards, suggesting that organic macromolecules can be used to estimate MW of natural organic colloids. The MW of eight river DOM samples determined by this method was found to have an M(n) range of 0.8-1.1 kDa, which agrees with available literature estimates. The FlFFF-DAD-EEM system provided insight into the MW components of river DOM including the optical properties by on-line absorbance and fluorescence measurement. A red-shift in emission and excitation wavelength maxima associated with lower spectral slope ratios (S(R)=S₂₇₅₋₂₉₅:S₃₅₀₋₄₀₀) was related to higher MW DOM. However, DOM of different origins at similar MW also showed significant difference in optical properties. A difference of 47 and 40 nm in excitation and emission peak C maxima was found. This supports the hypothesis that river DOM is not uniform in size and optical composition.

Organic colloid separation in contrasting aquatic environments with tangential flow filtration

The use of tangential flow filtration (TFF) for size fractionation of natural dissolved organic matter was investigated. The performance of regenerated cellulose membrane with a nominal molecular weight cut-off of 1 kDa was examined on 20 samples from lake, river and estuary systems, characterised by contrasting dissolved organic carbon (DOC) contents and conductivity. The evaluation was based on absorbance, fluorescence and DOC measurements. Detailed protocols of membrane cleaning and conditioning nation are proposed. The ultrafiltration membrane can efficiently be cleaned to provide low carbon blank (<0.01 mg/L). Fluorescence measurements confirmed that the higher molecular weight compounds were isolated in the retentate and the lower molecular weight remain in the permeate. Mass balance for natural samples show good recovery for DOC (109 +/- 12%, n = 20) and fluorescence measurements (106 +/- 9%, n = 13). No relation between factors of concentration (fc) and mass balance quality was observed for the fc range 1.5-11. Moreover, high ionic strength and high DOC contents did not enhance membrane fouling. These findings demonstrate that reliable fractionations by TFF of natural organic colloids in aquatic systems can be achieved.

Water toxicity and metal contamination assessment of a polluted river: the Upper Vistula River (Poland)

In aquatic systems, the bioavailability of an element to microorganisms is greatly influenced by its chemical speciation. The goal of this work was to assess metal toxicity to a green algae (Pseudokirchneriella subcapitata) and a bacterium (Vibrio fisheri) as a function of size fractionation and chemical speciation (using the program MINTEQA2) in contaminated water of the Upper Vistula River. Water samples were collected at 1 reference site, 4 polluted sites and one polluted site on the Vistulas main tributary, the Przemsza River. Toxicity measurements were performed on unfiltered samples and, total dissolved (<1.2 μm), and truly dissolved (<1 kDa) fractions. Trace metal (Cd, Co, Cr, Cu, Mn, Pb, Zn) concentrations were measured in these samples and also in the colloidal fraction (1 kDa–1.2 μm). At the reference site, the low metal concentrations were in agreement with the absence of measurable toxicity. In the polluted section of the river, free metal concentrations were largely below the potential toxic levels for bacteria, which was in agreement with the absence of toxicity. Although Zn2+ was at potentially toxic-level concentrations in total dissolved and truly dissolved fractions in the polluted riverine section, toxicity for algae was observed, only in truly dissolved fractions from two stations. The absence of toxicity in most samples was related to metal association with particles and with low molecular weight ligands as well as the presence of organic ligands (phenol). The reason for toxic effects in two ultrafiltered samples is not clear, but may be related to the elimination of the colloidal organic fraction and thus the eradication of its protective effect occurring in natural samples.

Pb and Sr isotope measurements by inductively coupled plasma–mass spectrometer: efficient time management for precision improvement

One of the factors limiting the precision of inductively coupled plasma mass spectrometry is the counting statistics, which depend upon acquisition time and ion fluxes. In the present study, the precision of the isotopic measurements of Pb and Sr is examined. The time of measurement is optimally shared for each isotope, using a mathematical simulation, to provide the lowest theoretical analytical error. Different algorithms of mass bias correction are also taken into account and evaluated in term of improvement of overall precision. Several experiments allow a comparison of real conditions with theory. The present method significantly improves the precision, regardless of the instrument used. However, this benefit is more important for equipment which originally yields a precision close to that predicted by counting statistics. Additionally, the procedure is flexible enough to be easily adapted to other problems, such as isotopic dilution.

Influence of water chemistry and dissolved organic matter (DOM) molecular size on copper and mercury binding determined by multiresponse fluorescence quenching

The effects of water chemistry (i.e. pH and Ca(2+) concentration) dissolved organic carbon (DOC) concentration) and DOM quality (i.e. composition and molecular weight) on metal complexation were successfully investigated by a combination of tangential flow filtration, excitation-emission matrix fluorescence, parallel factor analysis (PARAFAC), and fluorescence quenching on four freshwater samples and one extracted Suwannee River fulvic acid (SRFA). Two terrestrial and one microbial humic-like components were found in this study. Despite strong correlation between the Ryan-Weber model and the multiresponse model, the latter is more appropriate for the calculation of binding parameters in multiple-ligand DOM system. Decreasing pH from 6 to 4 significantly reduced logK-Cu(2+) from 5.22±0.24 to 4.60±0.30 at pH 6 and 4, respectively (p<0.001), while the impacts of Ca(2+) and DOC were not discernible at concentrations<100ppm and<2.06ppm, respectively. For natural freshwater DOM binding, the three humic-like components had similar logK values for both metals. High molecular weight (>1kDa) DOM generally had higher logK and binding fluorophore abundance than bulk (unfractionated) and low molecular weight (<1kDa) DOM for both metals. This trend however was not always true for Hg(2+) where the binding parameters were quite variable. Overall the combined results provide evidence that binding parameters are not only affected by water chemistry, but also depend on DOM molecular weight.

The effect of freshwater UV-irradiation prior to resin preconcentration of trace metals

Abstract Trace metal concentrations in natural freshwater samples are often below detection limits for determination by inductively coupled plasma-mass spectrometry (ICP-MS). This is especially so following size fractionation of the sample. Preconcentration is therefore necessary, and sorption on a chelating resin such as Chelex-100 is often used. In order to chelate metals strongly complexed with organics, the metal–organic association has to be severed. In this study, ultra violet (UV)-irradiation was used to oxidize organic matter in a humic acid reference solution, and in a natural sample spiked with Ag, Cd, Cu and Pb, where the recoveries of the metals were calculated. Except for Ag, recoveries of Cd, Cu and Pb increased significantly up to almost 100% after irradiation. If preconcentration on Chelex-100 is used, UV-irradiation appears to be an essential step in achieving accurate concentrations of organically chelated trace metals in freshwater.

Characterization of biochar-derived dissolved organic matter using UV-visible absorption and excitation-emission fluorescence spectroscopies

In recent years, biochar has become of considerable interest for a variety of environmental applications. However, the feasibility of its application is entirely dependent on its physical and chemical properties, including the characteristics of biochar-derived dissolved organic matter (DOM). The goal of this study was to assess the use of optical analysis for the purpose of characterizing biochar-derived DOM. Three different biochars (slow pyrolysis birch and maple; fast pyrolysis maple) were produced and leached in distilled water over 17d. Samples were taken on days 3, 10, 13 and 17, filtered, and analyzed for DOC content. Samples were also subjected to optical analysis using UV-visible absorption and excitation-emission matrix (EEM) fluorescence spectroscopies. EEM fluorescence data were further analyzed using parallel factor analysis (PARAFAC). Absorbance and fluorescence results were combined and examined using principal component analysis (PCA). Significant differences in the water soluble organic carbon content were observed for all biochar types. The estimated aromaticity (SUVA254) and mean molecular weight (S275-295) of biochar-derived DOM were also found to differ based on biochar type. PARAFAC analysis identified three humic-like components and one protein-like component. Distinct DOM signatures were observed for each biochar type. Transformations in biochar DOM characteristics over time were also observed. The PCA showed a clear delineation in biochar types based on their optical properties. The results of this study indicate that optical analysis may provide valuable information regarding the characteristics of biochar-derived DOM.

Determination of relative molecular weights of fluorescent components in dissolved organic matter using asymmetrical flow field-flow fractionation and parallel factor analysis.

Dissolved organic matter in aquatic systems is of variable structure and composition. Asymmetrical flow field-flow fractionation coupled to UV/vis diode array and fluorescence detectors (AF4-DAD-EEM) was used to assess the size and optical properties of dissolved organic matter. The results were analyzed using parallel factor analysis (PARAFAC) and statistical fractogram deconvolution to correlate fluorescing components with molecular weight fractions. This coupling, which is shown for the first time in this work, is a powerful method capable of revealing novel information about the size properties of PARAFAC components. Tyrosine/polyphenol-like fluorescence (peak B) was significantly correlated (p<0.05) with the smallest size group (relative molecular weight=310±10 Da), microbial humic-like and terrestrial visible humic-like fluorescence (peaks M, C, A) with the intermediate size group (1600±150 Da), and terrestrial fulvic-like and tryptophan/polyphenol-like fluorescence (peaks A and T) with the largest size group (4300±660 Da).