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.     

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.     

The process of methylmercury accumulation in rice (Oryza sativa L.)

Recent studies have shown that rice consumption can be an important pathway of methylmercury (MeHg) exposure to humans in Hg mining areas and also in certain inland areas of Southwestern China. The seed of rice has the highest ability to accumulate MeHg compared to other tissues. The main objective of this study was to investigate the process of (MeHg) accumulation in rice seed (Oryza sativa L.) by monitoring MeHg levels in specific tissues of rice plants experiencing various levels of Hg multisource pollution during a full rice growing season. Four groups of experimental plantations were utilized, distributed among a rural artisanal Hg production site and a regional background control site. Our results suggest that the newly deposited Hg is more readily transformed to MeHg and accumulated in rice plants than Hg forms with an extended residence time in soil, and soil is the potential source of MeHg in the tissues of rice plants. MeHg in soil was first absorbed by roots and then translocated to the above-ground parts (leaf and stalk). During the full rice growing season only a very small amount of MeHg was retained in the root section. In the premature plant, the majority of MeHg is located in the leaf and stalk; however, most of this MeHg was transferred to seed during the ripening period.     

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.     

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.     

Oxidation of nanomolar levels of Fe(II) with oxygen in natural waters

The oxidation of Fe(II) by molecular oxygen at nanomolar levels has been studied using a UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5-8.2), NaHCO3 (0.1-9 mM), temperature (3-35 degrees C), and salinity (0-36) on the oxidation of Fe(II) are presented. The first-order oxidation rates at nanomolar Fe(II) are higher than the values at micromolar levels at a pH below 7.5 and lower than the values at a higher pH. A kinetic model has been developed to consider the mechanism of the Fe(II) oxidation and the speciation of Fe(II) in seawater, the interactions between the major ions, and the oxidation rates of the different Fe(II) species. The concentration of Fe(II) is largely controlled by oxidation with O2 and O2.- but is also affected by hydrogen peroxide that may be both initially present and formed from the oxidation of Fe(II) by superoxide. The model has been applied to describe the effect of pH, concentration of NaHCO3, temperature, and salinity on the kinetics of Fe(II) oxidation. At a pH over 7.2, Fe(OH)2 is the most important contributing species to the apparent oxidation rate. At high levels of CO3(2-) and pH, the Fe(CO3)2(2-) species become important. At pH values below 7, the oxidation rate is controlled by Fe2+. Using the model, log k(i) values for the most kinetically active species (Fe2+, Fe(OH)+, Fe(OH)2, Fe(CO3), and Fe(CO3)2(2-)) are given that are valid over a wide range of temperature, salinity, and pH in natural waters. Model results showthatwhen H2O2 concentrations approach the Fe(II) concentrations used in this study, the oxidation of Fe(II) with H2O2 also needs to be considered.     

Chemical speciation and toxicity of nickel species in natural waters from the Sudbury area (Canada)

Metal complexation properties of dissolved organic carbon (DOC) in freshwaters are recognized but poorly understood. Here, we investigated the release of free nickel from Ni-DOC complexes using nickel-polluted freshwaters from Sudbury (Canada). We used the Competing Ligand Exchange Method with Chelex-100 as the competing ligand to measure the rate of free Ni2+ ion released by the dissociation of Ni-DOC complexes. The kinetic studies showed that the fastest kinetically distinguishable component representing approximately 30-95% of the total nickel had a dissociation rate coefficient similar to that reported for [Ni(H20)6]2+. High concentrations of Ca2+ and Mg2+ caused a larger amount of the DOC-bound nickel to be released as free Ni2+ ion. Growth inhibition of the freshwater alga Pseudokirchneriella subcapitata was highly correlated with the Ni/DOC ratio, the free plus labile nickel concentration, and the dissociation rate coefficient. While the levels of metals were not sufficient to kill Daphnia magna, these test organisms were immobilized in the same samples that showed algal growth inhibition. Only one sample caused 22% death of Hydra attenuata. The algal toxicity tests were consistent with the kinetic speciation results and are consistent with the hypothesis that dissolved [Ni(H20)6]2+ plus other labile nickel species are toxic forms of Ni present.     

Photodegradation of dimethyl sulfide (DMS) in natural waters: laboratory assessment of the nitrate-photolysis-induced DMS oxidation

The interaction of sunlight and dissolved chromophoric matter produces reactive chemical species that are significant in the removal of dimethyl sulfide (DMS) in the surface ocean. Using artificial solar radiation, we examined the role of several inorganic components of seawater on the kinetics of NO3- -photolysis-induced DMS removal in aqueous solution. This study strongly suggests that NO3- photolysis products react significantly with DMS in aqueous solution possibly via an electrophilic attack on the electron-rich sulfur atom. This supports previous field observations that indicate that NO3- photolysis has a substantial control on DMS photochemistry in nutrient-rich waters. A key finding of this research is that the oxidation rate of DMS induced by NO3- photolysis is dramatically enhanced in the presence of bromide ion. Moreover, our results suggest that bicarbonate/carbonate ions are involved in free radical production/scavenging processes important for DMS photochemistry. These reactions are pH dependent. We propose that DMS removal by some selective free radicals derived from bromide and bicarbonate/carbonate ion oxidation is a potentially important and previously unrecognized pathway for DMS photodegradation in marine waters.     

Effect of natural antioxidants in controlling alkaline contaminant materials (ACM) in heated palm olein

A spectrophotometric-based method was developed to determine the changes in alkaline contaminant materials (ACM) of natural antioxidant-treated, refined, bleached and deodorized (RBD) palm olein during 5-day deep-fat frying of potato chips. Results showed that the ACM contents in RBD palm olein could be determined spectrophotometrically at 540 nm. Oleoresin rosemary extract, sage extract and citric acid significantly (P<.05) lowered the ACM content in oil during 5-day deep-fat frying of potato chips. A combination of 0.087% oleoresin rosemary extract, 0.072% sage extract and 0.025% citric acid was required to achieve the minimum level of ACM after 5-day frying.     

Factors controlling the bioavailability of ingested methylmercury to channel catfish and Atlantic sturgeon

The bioavailability of ingested methylmercury (CH3Hg(III) was investigated in vitro using the gastric and intestinal fluids of channel catfish, Ictalurus punctatus, and Atlantic sturgeon, Acipenser oxyrinchus. Gastric fluid collected from each species was incubated with CH3Hg(II)-spiked sediment or bloodworms, after which the intestinal fluid of each species was added and incubated further. The proportion of CH3Hg(II) solubilized from bloodworms and sediment appeared to be controlled by complexation to amino acids in both the stomach and the intestinal fluids during the digestive process,with the more thorough digestion of bloodworm organic material enhancing CH3Hg(II) solubilization. A greater proportion of CH3Hg(II) was solubilized by the sturgeon fluids compared to the catfish fluids, especially for the sediment incubations. These differences corresponded to the relative amount of amino acids in the fluids of these fish. A comparison of the catfish gastrointestinal solubilization incubations and a CH3-Hg(II) bioaccumulation experiment with bloodworms revealed that the solubilization incubations may be a reasonable surrogate measurement of the bioavailability of CH3Hg(II) to fish. Overall, it appears that digestive processes is the most important controlling factor in the bioavailability of CH3Hg(II) to fish.     

Succession of bacterial and fungal communities during natural coffee (Coffea arabica) fermentation

Bacteria, yeasts and filamentous fungi were isolated during natural coffee processing. Bacteria were isolated in greater numbers at the beginning of the fermentation, when the moisture of the coffee beans was around 68%. Gram-positive bacteria represented 85.5% of all bacteria isolated, and Bacillus was the predominant genus (51%). Gram-negative species of the genera Serratia, Enterobacter and Acinetobacter were also found. Approximately 22% of 940 randomly chosen isolates of microorganisms were yeasts. Debaryomyces (27%), Pichia (18.9%) and Candida (8.0%) were the most commonly found genera, and these three genera tended to appear more often as the fruit was fermented and dried. Aspergillus was the most abundant genus besides Penicillium, Fusarium and Cladosporium, with 42.6% of the total fungi isolates. The genera and species identified included members known to have pectinase and cellulase activities. Of the 10 organic acids analyzed and quantified in coffee beans, acetic and lactic acids may have been generated by microbial activity. Butyric acid was not detected in any sample.     

Cloud-point extraction and preconcentration of cyanobacterial toxins (microcystins) from natural waters using a cationic surfactant

A new cloud-point extraction and preconcentration method using a cationic surfactant, Aliquat-336 (tricaprylylmethylammonium chloride), has been developed for the determination of cyanobacterial toxins, microcystins, in natural waters. Sodium sulfate was used to induce phase separation at 25 degrees C. The phase behavior of Aliquat-336 with respect to concentration of Na2SO4 was studied. The cloud-point system revealed a very high phase volume ratio compared to other established systems of nonionic, anionic, and cationic surfactants. At pH 6-7, it showed an outstanding selectivity in analyte extraction for anionic species. Only MC-LR and MC-YR, which are known to be predominantly anionic, were extracted (with averaged recoveries of 113.9 +/- 9% and 87.1 +/- 7%, respectively). MC-RR, which is likely to be amphoteric at the above pH range, was not detectable in the extract. Coupled to HPLC/UV separation and detection, the cloud-point extraction method (with 2.5 mM Aliquat-336 and 75 mM Na2SO4 at 25 degrees C) offered detection limits of 150 +/- 7 and 470 +/- 72 pg/mL for MC-LR and MC-YR, respectively, in 25 mL of deionized water. Repeatability of the method was 7.6% for MC-LR and 7.3% for MC-YR. The cloud-point extraction process can be completed within 10-15 min with no cleanup steps required. Applicability of the new method to the determination of microcystins in real samples was demonstrated using natural surface waters collected from a local river and a local duck pond spiked with realistic concentrations of microcystins. Effects of salinity and organic matter (TOC) content in the water sample on the extraction efficiency were also studied.     

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.     

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.     

Role of bacterial community succession in flavor formation during Sichuan sun vinegar grain (Cupei) fermentation

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