Jacques Buffle

The use of ultrafiltration for the separation and fractionation of organic ligands in fresh waters

Abstract The effects of various experimental conditions on the results obtained by using membrane filtration for the separation and fractionation of organic matter in fresh waters (mainly fulvic and humic substances) are described. The technique used (washing or concentration) and the initial concentration of the organic matter to be filtered are the most critical factors. The technique is used for the fractionation of eight water samples, one sample of peat interstitial water, five water extracts of soil and four water samples obtained by decomposition of leaves. The results are compared. A comparison is also made with results cited in the literature.

CHARACTERISTIC FEATURES OF THE MAJOR COMPONENTS OF FRESHWATER COLLOIDAL ORGANIC MATTER REVEALED BY TRANSMISSION ELECTRON AND ATOMIC FORCE MICROSCOPY

Organic biopolymers such as humic substances and polysaccharides account for the majority of freshwater NOM. Their role in natural systems is largely dependent upon their supramolecular microscopic structure which cannot be determined by bulk chemical measurements alone. Microscopic techniques were developed so as to permit the systematic observation of several colloidal-sized organic macromolecules with variable structures. This paper describes the characteristic structures of some reference compounds representative of the major organic components of natural waters. Polyacrylic acid, alginic acid and schizophyllan in addition to humic substances and polysaccharides isolated from natural freshwaters were examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The techniques were optimized for the observation of the aquatic biopolymers. Colloidal organic matter isolated from natural freshwaters was also observed by TEM and AFM and compared with the standard images of colloidal organic matter.

Complexation by natural heterogeneous compounds: Site occupation distribution functions, a normalized description of metal complexation

Abstract This paper presents a new conceptual approach to interpreting titration curves of metal complexation by physically and chemically heterogeneous natural complexants such as humic acids, clays, complete soils, or sediments. The physico-chemical and analytical difficulties encountered with such systems are reviewed by comparison with a system containing only a few simple ligands, followed by discussion of the new approach on the same basis. It is shown that interpretation of heterogeneous complexant properties necessitates a preliminary transformation of experimental raw data into a function sufficiently normalized so as to allow comparison of results obtained under different conditions. A normalized function called a Site Occupation Distribution Function (SODF) and its potential usefulness is described here. The SODF is a readily computable function which relates the complexation buffer intensity of the system to the differential free energy of the complexation sites present. Its major interest is that it enables one to obtain both a rigorous mathematical description of the complexant properties (even when highly heterogeneous) at the macroscopic level and, in certain cases, an estimation of the molecular-scale behavior of particular site types. The relationship of the SODF to other distribution functions proposed in the literature is discussed and applications are exemplified using simulated and real natural systems. In particular, its utility is discussed in detail for 1. (1) discriminating between different site types (major, minor, dominant, background), 2. (2) evaluating the degree of heterogeneity of an unknown complexant system, 3. (3) estimating the nature and true thermodynamic constants of complexes, and 4. (4) yielding a rigorous definition of “complexation capacity.”

Size Effects on Diffusion Processes within Agarose Gels

To investigate diffusion processes in agarose gel, nanoparticles with sizes in the range between 1 and 140 nm have been tested by means of fluorescence correlation spectroscopy. Understanding the diffusion properties in agarose gels is interesting, because such gels are good models for microbial biofilms and cells cytoplasm. The fluorescence correlation spectroscopy technique is very useful for such investigations due to its high sensitivity and selectivity, its excellent spatial resolution compared to the pore size of the gel, and its ability to probe a wide range of sizes of diffusing nanoparticles. The largest hydrodynamic radius (R(c)) of trapped particles that displayed local mobility was estimated to be 70 nm for a 1.5% agarose gel. The results showed that diffusion of particles in agarose gel is anomalous, with a diverging fractal dimension of diffusion when the large particles become entrapped in the pores of the gel. The latter situation occurs when the reduced size (R(A)/R(c)) of the diffusing particle, A, is >0.4. Variations of the fractal exponent of diffusion (d(w)) with the reduced particle size were in agreement with three-dimensional Monte Carlo simulations in porous media. Nonetheless, a systematic offset of d(w) was observed in real systems and was attributed to weak nonelastic interactions between the diffusing particles and polymer fibers, which was not considered in the Monte Carlo simulations.

Coagulation of colloidal material in surface waters: the role of natural organic matter

Abstract Organic matter has a great influence over the fate of inorganic colloids in surface waters. The chemical nature and structure of natural organic matter (NOM) will be an important factor in determining whether colloids will be stabilised or destabilised by NOM. Under environmentally relevant conditions, the ubiquitous fulvic acids are likely to be responsible for coating and imparting a negative charge to colloids. If the adsorbed polyelectrolyte coating produces an increase in absolute surface potential, it will act to stabilise colloids in the water column. On the other hand, colloidal organic carbon, especially chain-like structures, has been shown to be involved in the aggregation of inorganic colloids through the formation of bridges. It is highly probable that both adsorption and bridging flocculation are occurring simultaneously in the natural aquatic environment. The importance of each process depends directly on the nature and concentration of organic matter in the system and indirectly on the productivity of the lake, its hydrological pathways, temporal variations, temperature, etc. The present paper reports such results and emphasises the need to discriminate the different kinds of NOM.

Electron microscopy of aquatic colloids: Non-perturbing preparation of specimens in the field

Abstract This paper describes a simple and powerful procedure for the specimen preparation of aquatic colloidal particles prior to direct observation by transmission electron microscopy (TEM). Four preparation schemes are described. For waters containing sufficiently large concentrations of colloids, aqueous solutions of particles are mixed with Nanoplast, a water-soluble embedding resin, and centrifuged over specimen grids placed on a horizontal disc; after curing, the resulting film has optimum quality for studies of particles at ultra-high resolution (⩾ 1 nm). For waters with low concentrations of colloids, or when size fraction is desired, particles are collected directly on a TEM grid placed at the bottom of a centrifugation tube; after deposition and curing of a film of Nanoplast over the specimen, the grid can be used directly for TEM observation. The advantages of these methods, and of two other applications of Nanoplast for aquatic and sediment particles, are compared to those of classical specimen preparation schemes. A large number of specimens may be quickly prepared in the field, using the above procedures; at the same time, most of the preparation artefacts linked to classical procedures are avoided. The new procedures should help to make TEM a semi-routine analysis method for studying the nature and behaviour of aquatic colloids.

Submicron particles in the rhine river—I. Physico-chemical characterization

Abstract This paper describes a complete sampling, fractionation and characterization scheme for submicrometer particles isolated from the Rhine River. Decreasing particle size fractions were obtained by means of gravitational sedimentation, cascade centrifugation/ultracentrifugation and cascade filtration. These size fractions were analyzed by photon correlation spectroscopy (PCS), micro-electrophoresis (ME), transmission electron microscopy (TEM), light scattering (LS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and total organic carbon (TOC), which gave complementary results concerning the nature of the particles. The data indicated that submicron particles contribute only a small proportion of the total particle mass and volume, but an important proportion of the total particle number. Moreover, their specific surface area may be quite large. Associations of submicrometer particles with organic macromolecules and fibrils, which may have maintained such particles in suspension, were observed.

Mercury-plated iridium-based microelectrode arrays for trace metals detection by voltammetry: optimum conditions and reliability

An amperometric microsensor for the detection of trace metals in the low or sub nanomolar range is presented. It is obtained by successively evaporating iridium (2000 A) and Si3N4 (2000 A) on a silicon wafer, followed by a photolithographic pattering procedure. It consists of an array of 10 x 10 iridium microdisc electrodes with a recessed depth of 0.2 μm, separated by 50 or 150 μm. The electrical contacts are isolated by a layer of Agolit or Epoxy resin. Scanning electron microscopy and Atomic force microscopy have been used to control the regularity of the microelectrode array geometry and morphology. For the analysis of trace metals, mercury is deposited on the iridium-based microelectrode array. A given array is able to sustain the operations of Hg deposition/dissolution at least 10 times. The reliability of the mercury-plated iridium-based microelectrode arrays has been tested by a series of systematic Square Wave Anodic Striping Voltammetry (SWASV) analyses in synthetic solutions of lead and cadmium ions in the concentration range 1-10 nM. Repeated measurements over long periods of time on a given mercury layer showed good stability when the Epoxy resin was used and good reproducibility (± 4%) for at least 5 h. A good reproducibility was also found between different arrays. Finally, the mercury-plated iridium-based microelectrode arrays were applied to the lead and cadmium speciation in river water, by direct SWASV measurements, without any separation. A detection limit of 50 pM was established for a preconcentration time of 15 min. The results were compared with other techniques, in particular a similar procedure using a single Hg-plated Ir-based microelectrode.

Hollow fiber based supported liquid membrane: a novel analytical system for trace metal analysis

Abstract A new versatile tool for speciation of trace metals, in particular Cu, Pb and Cd based on a hollow fiber supported liquid membrane (HFSLM) consisting of 1,10-didecyl diaza 18-crown-6, fatty acid and a mixture of toluene and phenylhexane (1 + 1) has been studied for application in natural water conditions. Two types of single hollow fiber modules i.e. non flow and flow systems for free metal ion separation and preconcentration were used. The effect of target elements, sample volume, target metal concentration, and flow rates of sample solution in the case of the flow system on the preconcentration factor for a given receiver solution volume (ca 50–60 μl) placed in the lumen of the membrane) have been studied. High preconcentration factors in the range 100–3000 for Cu, Pb and Cd, for preconcentration times in the range 5–120 min were found using sample and receiver solution volumes of 250 ml and 60 μl, respectively. Preconcentration factors were found to be independent of the initial concentration of target elements in the sample and dependent on initial sample volume. Long term stabilities of these membranes showed that they are stable for at least seven days of continuous use. The results of these studies suggest that by combining SLM with sensitive analytical detectors e.g., voltammetric ones, detection limits of below 10 −10 mol l −1 can be achieved.

Circulation of pedogenic and aquagenic organic matter in an eutrophic lake

Abstract Measurements of total and dissolved organic carbon (respectively TOC and DOC), absorbance (A) and fluorescence (F) were used to characterize natural pedogenic (soil derived) and aquagenic (produced in lake) organic matter (NOM) in Lake Bret (Vaud, Switzerland). Profiles were established and settling matter was collected in the centre of the lake every 2–4 weeks from March to November 1981. In addition, water samples were collected at 1–2 week intervals at the lake inlet and outlet, in order to compute and compare the mass balances of pedogenic and aquagenic NOM. The results showed that NOM in lake water consisted mostly of pedogenic refractory matter (PROM), which originates from the leaching of soils and is brought into the lake by its affluent and runoff. The variations in DOC, A and F in the river water indicate that leaching of PROM is strongly related to the amount of rainfall received by the watersheds the days preceding sampling. The NOM mass balances reveal that quasi totality of the aquagenic organic matter (AOM) not mineralized in the water column is lost by sedimentation, whereas about 75% of PROM is eliminated at the outflow and does not sediment. Chemical analyses of settling matter suggested that the sedimenting NOM are mainly bound to clays and to Fe(III) hydroxides but very little to calcite, the precipitation of which probably only causes a dilution of other sedimenting particles.