U. T. Hammer

Oxidation and sorption of arsenite by manganese dioxide as influenced by surface coatings of iron and aluminum oxides and calcium carbonate

Manganese dioxide (birnessite) was coated with two levels of Fe and Al oxides and CaCO3, and the influence of these coatings on the surface features and the reactivity of MnO2 with respect to the oxidation and sorption of As(III) (arsenite) was examined.For all untreated and coated MnO2 samples, the depletion (oxidation plus sorption) of As(III) by the samples follows first-order kinetics. The rate constants are smaller for the samples with the high levels of coating of Fe and Al oxides and CaCO3 on MnO2 than they are for the untreated MnO2 and the MnO2 with the low levels of coating. The extent of masking of the electron-accepting sites on the MnO2 for converting the toxic As(III) to the less toxic As(V) significantly varies with the kinds and levels of coatings.Coatings of Fe and A1 oxides and CaCO3, on MnO2 distinctively affect the sorption of As. Manganese oxide evidently catalyzes the sorption of As by Al oxide through oxidation of As(III) to As(V). The relative affinities of the oxides of Mn, Fe, and Al and CaCO3, toward As(III) and As(V) account for the coating effects.

Distribution and abundance of littoral benthic fauna in Canadian prairie saline lakes

The littoral benthos of 18 lakes in Alberta and Saskatchewan ranging in salinity from 3 to 126‰ (g1−1 TDS) were investigated twice, in the spring and in the summer of 1986. Multiple Ekman dredge samples were taken at water depths of about 0.5, 1.0 and 2 metres in each transect. Two to three transects were used in each lake according to its estimated limnological diversity for a total of 114 stations. A total of 76 species was present varying from 29–31 species in the three lakes of lowest salinity (means of 3.1–5.55‰) to only 2 species in lakes exceeding 100‰. Species richness decreased rapidly in salinities greater than 15‰.Biomass maximum mean of 10.91 g m−2 dry weight (maximum 63.0 g m−2) occurred in culturally eutrophic Humboldt Lake (3.1‰) but one third as great in other low salinity lakes. However, biomass again increased to about 4.5 gm−2 in two lakes of 15‰ As the salinity increased still further biomass declined steadily until a minimum of 0.0212 g m−2 was recorded in most saline Aroma Lake (mean 119‰). Summer biomass (11 lakes) was greater than spring biomass (4 lakes) because some groups such as amphipods, corixids and ostracods became more abundant in summer. Wet weight biomass averaged 15.8‰ of dry weight biomass.Seasonality (spring or summer), sediment texture and organic matter content, water depth, pH, salinity (TDS) and the presence of aquatic plants (‰ plant cover) were considered in the matrix involving species dry weight biomass at each of 117 stations. TWINSPAN classification of the samples yielded a dendrogram with 18 indicator species. Successive dichotomies divided these indicator species into four main lake groups based on salinity, i.e., Group I: 3–10‰ (Gammarus, Glyptotendipes I, Chironomus cf. plumosus), Group II: 10–38%. (Hyalella, Enallagma,Bezzia), Group III: 38–63‰ (Hygrotus salinarius, Cricotopus ornatus), Group IV: >63‰ (Dolichopodidae, Ephydra hians). Each of these main groups was subdivided into smaller groups of lakes based on factors such as pH, seasonality (spring or summer species dominance), ‰ organic matter and ‰ plant cover. Depth of samples played no apparent role.

INFLUENCE OF CITRIC ACID AND GLYCINE ON THE ADSORPTION OF MERCURY (II) BY KAOLINITE UNDER VARIOUS pH CONDITIONS

This investigation was carried out to study the effect of different concentrations of citric acid and glycine, which are common in freshwaters, on the kinetics of the adsorption of Hg by kaolinite under various pH conditions. The data indicate that Hg adsorption by kaolinite at different concentrations of citric acid and glycine obeyed multiple first order kinetics. In the absence of the organic acids, the rate constants of the initial fast process were 46 to 75 times faster than those of the slow adsorption process in the pH range of 4.00 to 8.00. Citric acid had a significant retarding effect on both the fast and slow adsorption process at pHs of 6.0 and 8.0. It had a significant promoting effect on the fast and slow adsorption process at pH 4.00. Glycine had a pronounced enhancing effect on the rate of Hg adsorption by kaolinite during the fast process. The rise in pH of the system further increased the effect of glycine on Hg adsorption. The magnitude of the retarding/promoting effect upon the rate of Hg adsorption was evidently dependent upon the pH, structure and functionality of organic acids, and molar ratio of the organic acid/Hg. The data obtained suggest that low-molecular-weight organic acids merit close attention in studying the kinetics and mechanisms of the binding of Hg by sediment particulates and the subsequent food chain contamination.

Kinetics of the desorption of mercury from selected freshwater sediments as influenced by chloride

The kinetics of Hg desorption from selected freshwater sediments in the Canadian Prairies as influenced by a range of chloride concentrations (0, 10−4, 10−3, 10−2, 2×10−2, 4×10−2, 6×10−2, 8×10−2, 10−1M) were studied. The extent of the influence of Cl− concentrations on the increase of the rate of Hg release from the sediments in the fast desorption and slow desorption processes varied from 2.5 to 10.5 times and 2.0 to 8.5 times, respectively. An abrupt increase in the Hg release from the sediments was observed when the Cl− concentration was 2×10−2M and higher. The increase of Hg release with increasing Cl− concentrations was attributed to the dissolution of the adsorbed Hg through its complexation with Cl−. The release of Hg was not affected by the ionic strength and the Na concentrations in the systems studied. The influence of Cl- concentrations on the kinetics of the release of the sediment-bound Hg varied with the nature and properties of the sediments. The data indicate that short-range ordered oxides of Al, Fe, and Mn and their complexes with organic components merit close attention in studying the influence of Cl−, whose sources include deicing salts, fertilizers, animal wastes, and sewage effluents, on the rate of the dispersion of Hg from freshwater sediments.

Influence of selected cation and anion species on the adsorption of mercury (II) by montmorillonite

Abstract The influence of Cl − , NO 3 − , ClO 4 − , Na + , and Ca 2+ on the kinetics of the Hg adsorption by montmorillonite was studied. The data indicate that the Hg adsorption process in different salt solutions obeyed multiple first order kinetics. In the absence of salts, the Hg adsorption in the fast process was 1.6, 2.0, 2.0, 4.6, and 4.6 times faster than that in the presence of NaClO 4 , NaNO 3 , Ca(NO 3 ) 2 , NaCl, and CaCl 2 , respectively. Similarly, after the initial rapid adsorption, the system in the presence of the strong complexing ligand, Cl − , had a slower adsorption rate than the systems in the absence of salts or in the presence of weaker ligands, NO 3 − and ClO 4 − . However, the bulk of the Hg adsorption by montmorillonite took place in the initial rapid adsorption process. The magnitude of the reduction in the rate of the Hg adsorption was evidently dependent upon the ability of the ligands to complex with Hg. No significant differences between Na + and Ca 2+ in their ability to suppress the Hg adsorption were observed. When the Cl/Hg and ClO 4 /Hg molar ratios were higher than 50 and 500, respectively, their suppressing effects on the adsorption of Hg by montmorillonite were evident.

Dynamics and mechanisms of arsenite oxidation by freshwater lake sediments

There has been increasing concern over As in freshwater environments from sources such as arsenical pesticides, smelters, coal-fired power plants, and erosion caused by intensive land use. Arsenic in the reduced state, As (III) (arsenite), is much more toxic, more soluble and mobile, than when in the oxidized state, As (V) (arsenate). This paper summarizes the dynamics and mechanisms involved in the oxidation of As (III) to As (V) by freshwater lake sediments. Sediments from selected freshwater lakes in southern Saskatchewan oxidize As (III) to As (V) predominantly through an abiotic process. Solution analysis of As (III) and As (V) by colorimetry, and examination of the oxidation state of surface-sorbed As species by X-ray photoelectron spectroscopy, indicate that Mn present in the sediment is the primary electron acceptor in the oxidation of As (III). The transformation of As (III) to As (V) by carbonate and silicate minerals, common in sediments, is not evident. The heat of activation, ΔHa, for the depletion (oxidation plus sorption) of As (III) by the sediments, varies from 3.3 to 8.5 kcal mole−1, indicating that the process is predominantly diffusion-controlled. The Mn present in a series of particle size fractions ( 20 µm) of the sediments may potentially detoxify As (III) in aquatic systems, by converting it to As (V).