Gary G. Leppard

Metal sorption to diagenetic iron and manganese oxyhydroxides and associated organic matter: Narrowing the gap between field and laboratory measurements

Diagenetic Fe and Mn oxyhydroxides were isolated in situ by vertically inserting inert collectors into the sediments of two geochemically different lakes located near Sudbury, Ontario. X-ray diffraction and electron microscopic analyses indicated that the Fe-rich material collected was predominantly ferrihydrite and poorly crystallized lepidocrocite, while the Mn-rich material was a mixture of poorly crystallized Mn oxyhydroxides. Conditional adsorption constants (KF-M and KMn-M) were calculated using the concentrations of metals (Ca, Cd, Cu, Mg, Ni, Pb, Zn) associated with the Fe- and Mn-rich material and the measured dissolved concentrations of these metals. Comparison of these in situ derived KFe-M and KMn-M values were made with: (1) the hydrolysis constants of the metals; (2) laboratory-derived intrinsic surface complexation constants obtained for adsorption of these metals on well-characterized Fe and Mn oxyhydroxides, and (3) predicted KFe-M and KMn-M values determined using the surface complexation model under the geochemical conditions observed in the lakes. Complexation of these metals with adsorbed natural organic matter was also compared to metal complexation with dissolved natural organic matter. The results are consistent with the scenario that trace metals bind directly to the OH groups of the Fe and Mn oxyhydroxides in circumneutral McFarlane Lake and to the functional groups of organic matter adsorbed on Fe oxyhydroxides in the more acidic (pH = 4.8) Clearwater Lake. Alkaline earth metals Ca and Mg bind, presumably as outer-sphere complexes, to the organic coatings. Our results provide support for the argument that laboratory-derived adsorption datasets may be useful for predicting metal adsorption in the field.

Characteristics of lacustrine diagenetic iron oxyhydroxides

Abstract Diagenetic iron oxyhydroxides formed by the oxidation of iron (II) were allowed to deposit onto inert collectors vertically inserted into the sediments of eight lakes chosen to represent a range of lake pH values. Particles comprising the iron-rich deposits have been characterized at macroscopic (composition; X-ray diffraction) and microscopic (transmission electron microscopy; energy dispersive spectroscopy; electron diffraction) levels. The only crystalline forms of Fe(III) oxyhydroxides identified were poorly ordered ferrihydrite and lepidocrocite. Morphologies of lepidocrocite (lath-type) and presumably of ferrihydrite (spherical and ellipsoidal particles) are similar to those reported previously as being formed in the water column of seasonally anoxic lakes. Deposition of iron oxyhydroxides in the lake sediments also appears to occur on bacterial cells and on their exopolymers. Silicon, sulfate, chloride, phosphate, manganese, calcium, and aluminum represented minor components of the diagenetic Fe(III) oxyhydroxides, whereas organic carbon was a more abundant component. Calculations with a diffuse layer version of the surface complexation model using the ambient dissolved concentrations, and comparison of the predicted and measured compositions of the iron-rich particles suggest that sulfate, phosphate, and probably calcium were sorbed on the iron oxyhydroxides, whereas silicon was probably more firmly bound at the surface.

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.

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.

Colloidal organic fibrils of acid polysaccharides in surface waters: electron-optical characteristics, activities and chemical estimates of abundance

Abstract Many organisms in fresh waters secrete carbohydrate-rich substances which facilitate growth and survival. Some of these substances are acid polysaccharides of high molecular mass; they are multifunctional and their molecular architecture can be engineered by many organisms (e.g. algae, bacteria, plants) to combat specific environmental stresses (e.g. high toxicant levels, low nutrient levels). These polyanionic extracellular polymers represent families of macromolecules which aggregate to form colloidal fibrils; these can reside at the organism-water interface, participate in biofilm formation or enter the bulk water phase as colloids. Their manifold activities include the natural decontamination of water bodies; in the form of flocs, fibril aggregates are implicated in the functioning of engineered water treatment systems. The decontamination activity begins with the binding of toxicants by fibrils, followed by aggregation processes which produce suspended particles (1 μm). Dense suspended particles can aggregate further to form settling particles, thus removing the “bound” toxicants to the sediments. While various fibril activities have been described by many laboratories encompassing many scientific disciplines, fibril impacts on surface water quality are only vaguely understood. The vagueness is a result of technical difficulties in classifying fibrils and in quantifying both abundance and native activities. The problem of quantifying the chemical features of polyanionic extracellular polymers can now be overcome by technology transfer. A strategy to combine separation technology, biochemical concepts and the tools of analytical chemistry is outlined in this paper. A specific feature essential to the use of the strategy is the use of transmission electron microscopy to monitor and “tune” the isolation and purification methods for quantitative analyses of extracellular polymers. An improved quantitative understanding of fibril chemistry and activities should lead not only to a better understanding of colloid impacts, but also to an improved capacity to “engineer” flocs for the treatment of polluted waters.

Characterization of Iron-Oxides Formed by Oxidation of Ferrous Ions in the Presence of Various Bacterial Species and Inorganic Ligands

The oxidation of ferrous ions in the presence of an excess of dissolved oxygen at neutral pH generally leads to the formation of lepidocrocite. The effect of inorganic ligands (PO4, SO4, or Si(OH)4) in concentrations typical of those in sediment pore waters, and of microorganisms (Escherichia coli K12, Pseudomonas aeruginosa PA01, Bacillus subtilis or Bacillus licheniformis) on the mineralogy, chemical composition, morphology and spatial distribution of the iron-oxides were examined using various complementary techniques, including TEM, XRD, and EXAFS. The presence of inorganic ligands during the oxidation can affect the mineralogy as well as the size and structure of the Fe-oxide particles. While the presence of sulfate (SO4/Fe = 0.5) had little effect on the outcome of the Fe-oxide synthesis, low quantities of phosphate (PO4/Fe = 0.05) inhibited lepidocrocite and large quantities of aqueous silica (Si/Fe = 5) favored the formation of 2-line ferrihydrite. The presence of any of the four representative species of bacterial cells in the various systems did not modify the mineralogy of the Fe-oxides. However, the size of the Fe-oxide particles tended to be reduced, and the presence of the cells also affected the spatial organization and the morphology of the particles. In addition, in some systems, some of the iron remains adsorbed on the cells and does not contribute to the formation of mineral phases.

Effect of the presence of bacterial surfaces during the synthesis of Fe oxides by oxidation of ferrous ions

Natural iron-oxides are often found in close association with bacterial cells in aquatic environments, but the effect of bacteria on their formation is still under investigation. The present study was undertaken to assess the effect of two common bacteria, Bacillus subtilis and Escherichia coli , on the morphology and mineralogy of Fe oxides. All Fe oxides were synthesised by oxidation of Fe(II) (2 × 10 −4 M) at pH = 7. Three systems were studied, i.e. , abiotic Fe oxides, Fe oxides formed in the presence of bacteria (which we call “biogenic” Fe oxides) and abiotic Fe oxides mixed with bacterial cells. Samples were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fe oxide particles in all systems showed a needle-like morphology, with many needles seeming to be attached to a sheet, and were identified as lepidocrocite. However, the biogenic lepidocrocite crystals were generally shorter than the abiotic ones, and the crystals were found in association with the bacterial cell-wall, especially with B. subtilis , a Gram-positive bacterium. Biogenic lepidocrocite crystals also displayed an attenuation of the XRD 120 line, which is indicative of a low crystallinity. Growth limitation and poor crystalline order are then likely to affect the surface area of Fe oxides and indirectly, their sorptive capacity.

Transmission electron microscopy of a phosphate effect on the colloid structure of iron hydroxide

Abstract Transmission electron microscopy (TEM) has been used to study phosphate association with iron hydroxide colloid and consequent alteration of aggregated colloid structure. Three complementary sample preparation procedures were tested and images derived from them were correlated. A Nanoplast preservation and embedding procedure was found to minimize particle aggregation artifacts. With Nanoplast, primary particle size down to 1 nm was detected. Solvent dehydration followed by epoxy resin embedding produced similar results to Nanoplast embedding, although the use of epoxy resin resulted in a more time consuming procedure and image resolution was not as good. The TEM results show that aggregated iron hydroxide colloid has a ramified chain structure, extending in three dimensions to produce a highly porous, sponge-like precipitate. The size of the primary particles visualized was in the 1–4 nm range. Submicron scale elemental analysis for Fe and P in ultrathin sections of embedded colloid was done using energy dispersive X-ray spectroscopy (EDX) in a scanning transmission electron microscope (STEM). An increased P Fe ratio in the starting solution for producing iron-rich colloids led to an increased P Fe ratio in the resulting coloids; it also caused an alteration of the chain structure of iron hydroxide colloid, leading to large agglomerates. This provides evidence of a phosphate effect on the aggregation behaviour of iron hydroxide. A study of field samples from a wastewater treatment plant suggests that phosphate incorporation by iron hydroxide colloid might be the main mechanism for chemical phosphorus removal in a municipal wastewater treatment process using iron salts.

Transmission electron microscopy of the natural organic matter of surface waters.

Abstract Some components of aquatic natural organic matter (NOM) can be analysed effectively by methods of particle analysis employing transmission electron microscopy in conjunction with multi-method analytical approaches in the field, minimum perturbation techniques for sample handling and technology transfer from the biomedical sciences. The NOM components, include fulvic acids, colloidal fibrils and organic polymers of MW > 30 000. The use of a water-compatible embedding resin permits shape and size analyses of colloidal NOM (1–1000 nm) in ultrathin sections which minimize the misleading dehydration artifacts of the past. Experimentally induced perturbations allow one to follow aggregation/coagulation events at 1 nm resolution, while permitting the analyst to relate some components of coagulum structure to chemical entities. This review presents the current status of attempts to optimize a combination of analytical chemistry and transmission electron microscopy for describing NOM and its behaviour in surface waters.

Copper Resistance in Anabaena variabilis: Effects of Phosphate Nutrition and Polyphosphate Bodies

A copper-resistant Anabaena variabilis strain was obtained after repeated culturing in progressively higher concentrations of Cu(NO3)2. This strain maintained its resistance even after a year of repeated subculturing in copper-free medium. The resistant strain differed from the sensitive parent strain with respect to filament length, cell shape and size, and control of heterocyst formation. The resistant strain was also more resistant to cadmium, zinc, and nickel. Copper distribution studies conducted with atomic absorption spectroscopy revealed that at low copper levels the sensitive strain bound significantly more metal than the resistant strain. At higher copper levels, however, the resistant strain bound large amounts of the metal. Phosphate-loaded resistant cells could grow in higher copper concentrations than phosphate-starved resistant cells. Toluidine blue staining showed that the resistant strain contained more polyphosphate bodies than the sensitive strain; the resistant cells also had higher internal phosphate levels. X-ray microanalysis, however, did not show a direct localization of copper on polyphosphate bodies. More than one mechanism of resistance may exist in this A. variabilis strain.