Three Mn dioxides—birnessite, cryptomelane, and pyrolusite—were examined for their ability to deplete the concentration of As(III), a highly toxic pollutant, in solution. The depletion [oxidation of As(III) to As(V) and sorption of As(III)] of As(III) by all three Mn dioxides follows first‐order kinetics. The rate constants for the depletion of As(III) by birnessite and cryptomelane at 298 K are 0.267 and 0.189 h−1, respectively. On the other hand, the depletion rate of As(III) by pyrolusite is much slower: the rate constant at 298 K is 0.44 × 10−3 h−1. This difference in the rate of depletion is largely attributed to the crystallinity and specific surfaces of the Mn dioxides. Pyrolusite is highly ordered and has a low specific surface of 0.8 hm2/kg (7.9 m2/g); conversely, birnessite and cryptomelane are poorly crystalline and have relatively high specific surfaces of 27.7 and 34.6 hm2/kg (277 and 346 m2/g), respectively. The energies of activation for the depletion of As(III) by the Mn dioxides range from 26.0 to 32.3 kJ/mol. The reaction appears to be predominantly diffusion‐controlled. The ability of the Mn dioxides to sorb As(III) and As(V) appears to be related to the specific surface and the point‐of‐zero charge of the oxides. The data indicate that, after the systems have reached equilibrium with respect to the sorption of total As, the depletion of As(III) by the oxides is still progressing. This is apparently because of the one‐to‐one relationship between the amount of As(III) depleted and the amount of As(V) appearing in solution.

Sediments from five lakes in southern Saskatchewan, Canada, oxidize As(III) (arsenite) to As(V) (arsenate). The oxidation is not affected by flushing N2 or air through the sediment suspensions, nor does the addition of HgCl2 to the system eliminate the conversion of As(III) to As(V). The oxidation is an abiotic process with microorganisms playing a relatively minor role in this system. Because As(III) is more toxic and sorbed to a lesser extent by sediments than As(V), the suspended and bottom sediments may potentially alleviate the toxicity of As(III) through abiotic oxidation in aquatic environments.

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(liD: 216 t~g As(V)/ml was formed in solution when 1000 txg 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 ~.m in size are the least effective in oxidizing As(III); the oxidizing ability of the 5-20-, 2-5-, and <2-/~m 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/~g/ml) to 1 g of the three sediments ranged from approximately 3.5 to 19 p.g/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(Ill) 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.

Abstract--Surface diffusion, or migration within the electrical double layer next to mineral surfaces, is often invoked as a significant contributor to the overall diffusion coefficient in compacted clays, particularly where model predictions underestimate measured diffusion coefficients. The potential for surface diffusion of Sr 2 § Ca 2+ and Na + on three clays compacted to dry bulk densities of 1.25 and 1.60 Mg/m 3 was examined. The clays were a bentonite, an illite/smectite, and a glacial lake clay (composed mainly of smectite, illite, kaolinite and quartz). The clays were saturated with a Na-Ca-Cl-dominated synthetic groundwater solution with an effective ionic strength of 220 mol/m 3. Total intrinsic diffusion coefficients for the cations were determined from their steady-state flux through compacted clays, and apparent diffusion coefficients were obtained from the time-lag technique. Models of diffusive transport in compacted clays, based only on diffusion in the pore solution, adequately described the diffusion data for all clays and diffusants, and there was no need to invoke other transport mechanisms, like surface diffusion. The data indicate that surface diffusion is not a significant transport mechanism in compacted clays at least to a clay density of 1.60 Mg/m 3.

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