W. K. Liaw

Role of manganese in the oxidation of arsenite by freshwater lake sediments

The importance of various sediment components in the oxidation of As(III) (arsenite) to As(V) (arsenate) by freshwater lake sediments in southern Saskatchewan was examined. Treating the sediments with hydroxylamine hydrochloride or sodium acetate to remove Mn greatly decreased the oxidation of As(III). Furthermore, synthetic Mn(IV) oxide was a very effective oxidant with respect to As(III): 216 µg As(V)/ml was formed in solution when 1000 µg As(III)/ml was added to suspensions of 0.1 g of the oxide. These results indicate that Mn in the sediment was probably the primary electron acceptor in the oxidation of As(III). The conversion of As(III) to As(V) by naturally occurring carbonate and silicate minerals common in sediments was not evident in the system studied. Sediment particles >20 µm in size are the least effective in oxidizing As(III); the oxidizing ability of the 5–20-, 2–5-, and <2-µm particle size fractions varies depending on the sediment. The concentration of As(V) in equilibrated solutions after adding increasing amounts of As(III) (as much as 100 µg/ml) to 1 g of the three sediments ranged from approximately 3.5 to 19 µg/ml. Because As(III) is more toxic and soluble than As(V), Mn-bearing components of both the colloidal and non-colloidal fractions of the sediments may potentially detoxify any As(III) that may enter aquatic environments by converting it to As(V). This is very important in reducing the As contamination and in maintaining the ecological balance in aquatic environments.РезюмеИсследовалось значение различных компонентов осадка при окислении As(III) (мышьяковистокислой соли) в As(V) (мышьяковокислую соль) при помощи осадков из свежей озерной воды в нижном Саскачеване. Воздействие гидроксиламинового гидрохлорида или натриевого ацетата на осадки, чтобы удалить Mn, значительно уменьшает окисление As(HI). Кроме этого, синтетические окиси Mn(IV) были очень эффективными окислителями по отношению к As(IIl): 216 µг AS(V)/MJI было образовано в растворе при добавке 1000 µг As(V)/мл в суспензиях 0,1 г окиси. Эти результаты указывают, что Mn в осадке был, вероятно, акцептором первичных электронов при окислении As(III). Переход As(III) в As(V) при помощи натурально выступающих карбонатовых и кремниевых минералов, часто находящихся в осадках, не быд очевиден в изучаемой системе. Частицы осадка размером >20 µM были наименее эффективны при окислении As(III); окислительная способность частиц размером 5–20, 2–5, и 2 изменялась в зависимости от осадка. Концентрация As(V) в равновесных растворах после добавки увеличивающегося количества As(III) (до 100 µг/Mл) к одному грамму трёх осадков находилась в диапазоне приблизительно от 3,5 до 19 µг/Mл. Так как As(III) более токсический и растворяемый, чем As(V), Mn содержащие компоненты коллоидальных и неколлоидальных фракций осадков могут потенциально обезвредить любой As(III), который может поступать в водную среду, путём превращения её в As(V). Это очень важно при уменьшении загрязнения As и при сохранении экологического баланса водной среды. [Е.С.]ResümeeDie Bedeutung der verschiedenen sedimentären Komponenten bei der Oxidation von As(III), Arsenit, zu As(V), Arsenat, durch Süßwassersedimente wurde im südlichen Saskatchewan untersucht. Die Behandlung der Sedimente mit Hydroxylaminhydrochlorid oder Natriumacetat—um Mangan weitgehend zu entfernen—führte zu einer Abnahme der Oxidation von As(III). Darüberhinaus war synthetisches Mn(IV)-Oxid ein sehr wirksames Oxidationsmittel im Hinblick auf As(III): 216 µg As(V)/ml bildete sich in der Lösung bei einer Zugabe von 1000 µg As(III)/ml zu Suspensionen von 0,1 g Oxid. Diese Ergebnisse deuten darauf hin, daß Mangan in den Sedimenten wahrscheinlich der wichtigste Elektronenakzeptor bei der Oxidation von As(III) war. Die Umwandlung von As(III) in As(V) durch natürlich auftretende Karbonate und Silikatminerale, die gewöhnlich in Sedimenten auftreten, wurde in den untersuchten System nicht beobachtet. Die Kornfraktionen größer als 20 µm spielen bei der Oxidation von As(III) die kleinste Rolle. Die Oxidationskraft der Fraktionen 5–20, 2–5, und <2 µm variiert je nach Sediment. Die Konzentration von As(V) in den Gleichgewichtslösungen, die sich einstellten, wenn zunehmende Mengen As(III) (bis zu 100 µg/ml) zu jeweils 1 g der 3 Sedimente hinzugefügt wurde, reichte von etwa 3,5 bis 19 µg/ml. Da As(III) giftiger und leichter löslich als As(V) ist, können Mangan-haltige Komponenten der kolloidalen und nichtkolloidalen Fraktionen der Sedimente als mögliches Entgiftungsmittel für As(III) gelten, das in aquatisches Milieu gelangt, indem sie es in As(V) umwandeln. Dies ist von großer Bedeutung für die Verringerung der As-Kontamination und für die Erhaltung des ökologischen Gleichgewichtes im aquatischen Milieu. [U.W.]RésuméL’importance de composants sédimentaires variés dans l’oxidation d’As(III) (arsenite) à As(V) (arsenate) par des sédiments de lac d’eau douce dans le Saskatchewan du sud a été examinée. Le traitement des sédiments à l’hydrochloride hydroxylamine ou à l’acétate de sodium pour retirer Mn a fortement décru l’oxidation d’As(III). De plus, l’oxide Mn(IV) synthétique s’est montré un oxidant très éfficace en ce qui concerne As(III): 216 µg As(V)/ml a été formé en solution après que 1000 µg As(III)/ml avait été ajouté à des suspensions de 0,1 g de l’oxide. Ces résultats indiquent que Mn dans le sédiment était probablement l’accepteur d’électrons primaire dans l’oxidation d’As(III). La conversion d’As(III) à As(V) par des minéraux carbonates et silicés courants dans les sédiments n’était pas évidente dans le système étudié. Les particules sédimentaìres de taille >20 µm sont les moins éfficaces pour oxyder As(III); l’abilité d’oxydation des fractions de particules de taille 5–20, 2–5-, et <2-µm varie selon le sédiment. La concentration d’As(V) dans des solutions équilibrées, après avoir ajouté des quantités croissantes d’As(III) (jusqu’ à 100 µg/ml) à 1 g des trois sédiments s’étageait d’approximativement 3,5 à 19 µg/ml. Des composants portant Mn des fractions colloïdales et noncolloïdales des sédiments peuvent partiellement détoxifier tout As(III) qui pénètre les environements aquatiques en le convertissant en As(V), parcequ’ As(III) est plus toxique et plus soluble qu’As(V). Ceci est très important pour la réduction de la contamination par As et pour le maintient de l’équilibre écologique dans les environements aquatiques. [D.J.]

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.

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).