Distribution of colloid particles onto interfaces in partially saturated sand

Colloids have long been known to facilitate the transport of contaminants in soils, but few direct observations have been made of transport and retention in unsaturated porous media. Studies have typically been limited to evaluation of column breakthrough curves, resulting in differing and sometimes conflicting proposed retention mechanisms. We carried out pore scale visualization studies of colloid transport in unsaturated quartz sand to directly observe and characterize colloid retention phenomena. Synthetic hydrophilic (0.8, 2.6, and 4.8 microm carboxylated polystyrene latex) and relatively hydrophobic (5.2 microm polystyrene latex) colloidal microspheres were added to steady-state water flow (0.15 mm min(-1)) applied to an inclined infiltration chamber. Bright field microscopy was used to determine the positions and movement of water and colloids. Confocal laser scanning microscopy was used to determine water film geometry in an unsaturated horizontal chamber. We determined mechanisms of hydrophilic colloid retention at what is generally termed the air/water/solid (AWS) interface. Based on our observations, the AWS interface is here more accuratelytermed the air/water meniscus/solid (AWmS) interface, denoting the region where between-grain water meniscii diminish to thin water films on the grain surfaces. Colloids were retained at the AWmS interface where the film thickness approximately equaled colloid diameters. The greater retention for hydrophilic colloids at this interface (compared to elsewhere in the solid/water interface) can be explained by the additional surface tension capillary potentials exerted on colloids at the AWmS interface. While some 0.8-microm colloids were observed in thin water films, film straining played no significant role in the retention of larger colloids. Mechanisms for slightly hydrophobic colloids differed slightly. In addition to primary retention at the AWmS interface, hydrophobic colloids attached to others already present atthat interface resulting in apparent retention at the air/water (AW) interface. Attachment of hydrophobic colloids was also observed at water-solid interfaces, as hydrophobicity impelled the colloids to avoid water. Factors contributing to retention of slightly hydrophobic colloids were sand grain roughness and possibly a tendency for these colloids to flow near surfaces and interfaces, consonant with the enhanced retention of hydrophobic colloids (relative to hydrophilic colloids) observed in the literature.     

Manganese in natural mineral waters from Germany

154 commercially available natural mineral waters were analysed for their content of dissolved manganese (Mn(II)). 114 bottles were marked “deironized” (geogenic iron removed). Unexpectedly, waters rich in mangenese were more frequent among the deironized waters than among the non-deironized ones. Almost 20% of all waters contained more than 0,20 mg/l Mn(II), a toxicologically justified limit value with respect to formula-fed infants. Two waters which were declared as “adapted for the preparation of infant food” contained even more than 1,0 mg/l Mn(II). It is proposed to legally prescribe the declaration of manganese on the bottle. For reasons of preventive public health care, waters with more than 0,20 mg/l Mn(II) are not considered as suitable for the preparation of infant food.     

On the relationship between D(ow) and K(ow) in natural waters

The relationship between the overall octanol-water partition coefficient of a mixture of related chemical species, D(ow), and the octanol-water partition coefficients of its components, (K(ow))i, is explored. One form of the relationship (model 1) is generally applicable but relies on definition of aqueous phase speciation at equilibrium with octanol. An alternative form of the relationship (model 2) circumvents this requirement but assumes that related species are conserved during the partitioning process and is explicitly dependent on the water to octanol volume ratio, Vw/Vo. The potential applications and limitations of each model for defining the hydrophobic characteristics of chemical species in natural waters are examined in the light of experimental partition results for dissolved Cu and Pb in river waters. Given the general difficulties in accurate speciation modeling of trace metals in natural samples, model 1 was only able to estimate a K(ow) (typically in the range 0.03-0.3) for a computed organically complexed fraction of metal (generally > 90%). However, by conducting partition "isotherms" as a function of Vw/Vo and, because of the buffering capacity of natural waters, by treating a sample as two distinct hydrophilic and hydrophobic "pools", model 2 was able to estimate both the abundance and K(ow) of a more specific group of species. Parameter values derived from the latter approach indicated that river waters comprise a relatively small pool (about 4-20%) of metal whose octanol-water partitioning is in the region of 15-150. Given that the free ion activity of strongly binding metals in natural waters is extremely small, the hydrophobic fraction may, in many cases, representthe most biologically and environmentally significant component of metal. Accordingly, the experimental and modeling approaches described herein could be of great significance to an improved understanding of the fate and impacts of trace metals in the aquatic environment.     

Simulated solar light irradiation of mesotrione in natural waters

Photolysis is expected to be a major degradation process for pollutants in surface waters. We report here the first photodegradation study on mesotrione, a new triketone herbicide for use in maize. In a first step, we investigated the direct photolysis of mesotrione at 365 nm from a kinetic and analytical point of view. Mesotrione sensitizes its own oxidation through singlet oxygen formation and sensitizes the oxidation of H-donors through electron or H-atom transfer. In a second step, irradiation experiments were performed under conditions prevalent in the aqueous environment. Mesotrione in submicromolar concentrations was exposed to simulated sunlight, in addition to Suwannee River natural organic matter and/or nitrates. Suwannee River natural organic matter sensitizes the oxidation of mesotrione through the intermediacy of singlet oxygen, and the rate of mesotrione transformation is significantly enhanced for Suwannee River natural organic matter concentrations equal to or above 10 mg/L. Nitrates played a negligible role in SRNOM solutions.     

Hydrophobicity and octanol-water partitioning of trace metals in natural waters

The hydrophobicities of dissolved Al, Cu, Mn, and Pb have been determined in various contaminated natural water samples by 1-octanol extraction and C18 column retention. Octanol extraction varied among the metals studied and between the environments sampled but, in general, was greatest for Pb, whose conditional octanol-water partition coefficient, Dow, exceeded unit value in some samples. In most cases, metal partition into octanol either increased with increasing pH or exhibited a maximum under near-neutral conditions. Although the order and pH-dependence of metal retention by the C18 columns was consistent with these observations, the extent of retention was generally greater than the extent of metal extraction by octanol, possibly because of interferences effected by the C18 column matrix. Speciation calculations and results of controlled experiments employing metals in the presence of model ligands suggest that metals may become hydrophobic either by neutralizing relatively hydrophilic ligands or by combining with ligands that are intrinsically hydrophobic themselves. Given that octanol solubility affords an upper estimate of lipophilicity, the results of this investigation may have important implications regarding our understanding of metal bioavailability and toxicity in natural waters.     

Quantifying the dimensions of nanoscale organic surface layers in natural waters

Nanoscale surface films are known to develop on surfaces exposed to natural waters and have potential impacts on many environmental processes. A new method using atomic force microscopy is presented which physically removes the developed film in a defined area and then quantifies the difference in height between the film and the area where the film has been removed. The difference gives the absolute thickness of the surface film, which has not previously been measured. Suwannee River humic acid was exposed to substrates, and the surface film thickness as a function of pH and exposure time was measured. Discrete and very small colloids in the range 1-5 nm were observed as expected, and these sat on a coherent surface film, notthe original mica substrate. Low pH values of 2 gave rise to relatively thick surface films of about3 nm, although these films were not continuous at higher pH values. At pH 4.8, the film thickness increased with exposure time up to about 5 h and did not subsequently increase. The maximum film thickness measured was about 1 nm at that pH. The method is applicable to the measurement of many environmental surfaces, although resolution will depend on the substrate and film roughness.     

Fouling of microfiltration and ultrafiltration membranes by natural waters

Membrane filtration (microfiltration and ultrafiltration) has become an accepted process for drinking water treatment, but membrane fouling remains a significant problem. The objective of this study was to systematically investigate the mechanisms and components in natural waters that contribute to fouling. Natural waters from five sources were filtered in a benchtop filtration system. A sequential filtration process was used in most experiments. The first filtration steps removed specific components from the water, and the latter filtration steps investigated membrane fouling by the remaining components. Particulate matter (larger than 0.45 microm) was relatively unimportant in fouling as compared to dissolved matter. Very small colloids, ranging from about 3-20 nm in diameter, appeared to be important membrane foulants based on this experimental protocol. The colloidal foulants included both inorganic and organic matter, but the greatest fraction of material was organic. When the colloidal fraction of material was removed, the remaining dissolved organic matter (DOM), which was smaller than about 3 nm and included about 85-90% of the total DOM, caused very little fouling. Thus, although other studies have identified DOM as a major foulant during filtration of natural waters, this work shows that a small fraction of DOM may be responsible for fouling. Adsorption was demonstrated to be an important mechanism for fouling by colloids.     

Photooxidation of Hg(0) in artificial and natural waters

The oxidation of volatile aqueous Hg(0) in aquatic systems may be important in reducing fluxes of Hg out of aquatic systems. Here we report the results of laboratory and field experiments designed to identify the parameters that control the photooxidation of Hg(0)(aq) and to assess the possible importance of this process in aquatic systems. The concentrations of elemental and total Hg were measured as a function of time in both artificial and natural waters irradiated with a UV-B lamp. No change in Hg speciation was observed in dark controls, while a significant decrease in Hg(0) was observed in UV-B irradiated artificial solutions containing both chloride ions and benzoquinone. Significant photooxidation rates were also measured in natural samples spiked with Hg(0)(aq); the photooxidation of Hg(0) then follows pseudo first-order kinetics (k = 0.6 h(-1)). These results indicate that the previously observed Hg(II) photoreduction rates in natural waters could represent a net balance between Hg(0) photoreduction and Hg(0) photooxidation. As calculated from Hg(0) photooxidation rates, the dominant Hg(0) sink is likely to be photooxidation rather than volatilization from the water column during summer days.     

Removal of heavy metals from mine waters by natural zeolites

In this study, we investigated the removal of Fe, Pb, Cd, and Zn from synthetic mine waters by a natural zeolite. The emphasis was given to the zeolite's behavior toward a few cations in competition with each other. Pb was removed efficiently from neutral as well as from acidic solutions, whereas the uptake of Zn and Cd decreased with low pH and high iron concentrations. With increasing Ca concentrations in solution, elimination of Zn and Cd became poorer while removal of Pb remained virtually unchanged. The zeolite was stable in acidic solutions. Disintegration was only observed below pH 2.0. Forward- and back-titration of synthetic acidic mine water were carried out in the presence and absence of zeolite to simulate the effects of a pH increase by addition of neutralizing agents and a re-acidification which can be caused by subsequent mixing with acidic water. The pH increase during neutralization causes precipitation of hydrous ferric oxides and decreased dissolved metal concentrations. Zeolite addition further diminished Pb concentrations but did not have an effect on Zn and Cd concentrations in solution. During re-acidification of the solution, remobilization of Pb was weaker in the presence than in the absence of zeolite. No substantial differences were observed for Fe, Cd, and Zn immobilization. The immobilization of the metals during pH increase and the subsequent remobilization caused by re-acidification can be well described by a geochemical equilibrium speciation model that accounts for metal complexation at hydrous ferric oxides, for ion exchange on the zeolite surfaces, as well as for dissolution and precipitation processes.     

Spectrofluorometric method for determination of the total mercury content in natural waters

A simple and highly selective spectrofluorometric method for the determination of the total mercury (Hg) in natural waters is described. Fluorescence quenching of rhodamine B with Hg(II) in the presence of iodide, after a concentration step, has been made the basis of this sensitive method. Organic and inorganic Hg compounds in water samples were decomposed with sulphuric acid and potassium permanganate. Precipitated manganese dioxide and excess oxidants (permanganate) were destroyed with hydroxylammonium chloride. Hg(II) was reduced by tin(II) chloride and Hg vapour driven by an air stream into an absorption solution containing potassium permanganate and sulphuric acid, using a closed, recirculating air stream. In this solution fluorescence quenching of rhodamine B at an excitation wavelength of 485 nm and emission wavelength of 586 nm was measured. This method is suitable for the determination of total Hg in drinking and surface waters. The method gives reliable results down to a concentration of 0.5 g Hg per litre of water. The water samples analysed by this method gave a mean recovery and standard error of 97.3±2.39 for added Hg.

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Spektrofluorometrische Methode zur Bestimmung des Gehalts an Gesamtquecksilber im Wasser

Zusammenfassung Es wurde eine einfache, schnelle und hochselektive spektrofluorometrische Methode zur quantitativen Bestimmung des Gehalts an Gesamtquecksilber in Wasser beschrieben. Diese empfindliche Methode begründet sich auf der Fluorescenzlöschung in Rodamin-B-Lösung mittels Hg(II) bei Anwesenheit von Jodid und nach einem Verfahren, das den Zweck einer Quecksilberanreicherung hat. In Wasserproben wurden die organischen als auch anorganischen Quecksilberverbindungen mit H₂SO₄ aufgeschlossen und mit KMnO₄ oxydiert. Nach der Beseitigung des entstandenen MnO₂-Niederschlags bzw. des überschüssigen Permanganats mit HONH₃Cl wurde das Quecksilber mit SnCl₂ reduziert. Dann wurde das flüchtige Quecksilber durch einen Luftstrom in wässerige Absorptionslösung mit H₂SO₄ und KMnO₄ eingeleitet und damit angereichert. In dieser Meßlösung wurde die Fluorescenzlöschung des Rodamin-B bei der Anregungswellenlänge von 485 nm und Emissionswellenlänge von 586 nm ermittelt. Das entwickelte Untersuchungsverfahren ist für die Bestimmung des Gehalts an Gesamtquecksilber in Trinkwasser und Oberflächengewässern anwendbar. Die Nachweisgrenze liegt bei etwa 0,5 g Quecksilber/L Wasser. Zusätze von Quecksilber zu untersuchten Wasserproben ergaben eine Wiederfindungsrate und Standardfehler von im Durchschnitt 97,3±2,39.

     

Changes in bacterial community structure after exposure to silver nanoparticles in natural waters

Silver nanoparticles (AgNPs) are widely used in commercial products as antibacterial agents, but AgNPs might be hazardous to the environment and natural aquatic bacterial communities. Our recent research demonstrated that AgNPs rapidly but temporarily inhibited natural bacterioplankton production. The current study investigates the mechanism for the observed bacterial reaction to AgNPs by examining how AgNPs impact bacterial abundance, metabolic activity (5-cyano-2,3-ditolyl tetrazolium chloride (CTC+) cells), and 16S rRNA community composition. Natural bacterioplankton communities were dosed with carboxy-functionalized AgNPs at four concentrations (0.01-1 mg-Ag/L), incubated in triplicate, and monitored over 5 days. Ionic silver (AgNO(3)) and Milli-Q water treatments were used as a positive and negative control, respectively. Four general AgNP exposure responses, relative to the negative control, were observed: (1) intolerant, (2) impacted but recovering, (3) tolerant, and (4) stimulated phylotypes. Relationships between cell activity indicators and bacterial phylotypes, suggested that tolerant and recovering bacteria contributed the most to the community's productivity and rare bacteria phylotypes stimulated by AgNPs did not appear to contribute much to cell activity. Overall, natural bacterial communities tolerated single, low level AgNP doses and had similar activity levels to the negative control within five days of exposure, but bacterial community composition was different from that of the control.     

Production of hydrated electrons from photoionization of dissolved organic matter in natural waters

Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is electron ejection to produce hydrated electrons (e-aq). The efficiency of this process has been studied in both fresh water and seawater samples with both steady-state scavenger (S-SS) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of e-aq for the same samples using the two methods differ by as much as a factor of 100, necessitating a closer re-examination of how the process is measured. We developed a highly sensitive multipass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, we measured the AQY of e-aq from Laurentian fulvic acid as 1.3 x 10(-4), and set the upper limit for other CDOM samples at 6 x 10(-5), bringing the LFP results into agreement with those from S-SS methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments, but well above those likely to occur at the earth's surface. This multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.     

Automated analysis of nanomolar concentrations of phosphate in natural waters with liquid waveguide

Concentrations of phosphate in natural waters are often below the detection limits of conventional nutrient autoanalyzers, by either gas-segmented continuous-flow analysis orflow injection analysis. A liquid waveguide capillary flow cell has been used to extend the sensitivity of a conventional autoanalyzer for the automated analysis of nanomolar concentrations of phosphate in natural waters. Total reflection of light can be achieved within the liquid core of the flow cell because the refractive index of a cell wall coated with Teflon 1600 is lower than that of water. This property allows the manufacturers to construct long liquid waveguide capillary flow cells in a helical, rather than a linear shape, with compact dimensions. A small sample volume is required because the internal volume of a 2-m long capillary flow cell is only approximately 0.5 cm3. Adaptation of this long flow cell to autoanalyzers significantly enhances the sensitivity of automated colorimetric analysis of phosphate with a molybdenum blue method, allowing for the accurate and precise determination of nanomolar concentrations of phosphate in natural waters. The advantages of this technique are a low detection limit (0.5 nM), a small sample volume (2 mL), high precision (2% at 10 nM levels), and automation for the rapid analysis of a large number of samples.     

Oxidation of Fe(II) in natural waters at high nutrient concentrations

The Fe(II) oxidation kinetic was studied in seawater enriched with nutrients as a function of pH (7.2-8.2), temperature (5-35 °C), and salinity (10-36.72) and compared with the same parameters in seawater media. The effect of nitrate (0-1.77 × 10(-3) M), phosphate (0-5.80 × 10(-5) M) and silicate (0-2.84 × 10(-4) M) was studied at pH 8.0 and 25 °C. The experimental results demonstrated that Fe(II) oxidation was faster in high nutrient concentrations affecting the lifetime of Fe(II) in nutrient rich waters. Silicate displayed the most significant effects on the Fe(II) oxidation rate with values similar to those determined in seawater enriched with all the nutrients. A kinetic model was applied to the experimental results in order to account for changes in the speciation and to compute the fractional contribution of each Fe(II) species to the total rate constant as a function of pH. FeH(3)SiO(4)(+) played a key role in the Fe(II) speciation, dominating the process at pH over 8.4. At pH 8.0, FeH(3)SiO(4)(+) represented 18% of the total Fe(II) species. Model results show that when the concentration of silicate is 3 × 10(-5) M as in high nutrient low chlorophyll areas, FeH(3)SiO(4)(+) contributed at pH 8.0 by 4% increasing the rate to 11% at 1.4 × 10(-4) M. The effect of nutrients, especially silicate, must be considered in any further study in seawater media cultures and eutrophic oceanic areas.