Manganese arsenites

Case a, i is well illustrated by the arsenite-induced oxidation of manganese(II) by chromic acid, studied by Lang and Zwerina. The overall equation of this induced reaction is... 

Arsenite can be oxidized by manganese dioxides in soils. The rate constants for the depletion of As(III) by bimessite and cryptomelane are much higher than those by pyrolusite due to the difference in the crystallinity and specific surfaces of the Mn oxides (Oscarson et al., 1983). The ability of the Mn dioxides to sorb As(III) and As(V) is related to the specific surface and the point-of-zero charge of the oxides. The one-to-one relationship between the amount of As(III) depleted and the amount of As(V) appearing in solution was reported by Oscarson and colleagues (1983). 

Oscarson DW, Huang PM, Liaw WK, Hammer UT (1983) Kinetics of oxidation of arsenite by various manganese dioxides. Soil Sci Soc Am J 47 644-648 Pang LP, Close M, Flintoft M (2005) Degradation and sorption of atrazine, hexazinone and pro-cymidone in coastal sand aquifer media. Pest Man Sci 61 133-143 Paris DF, Lewis DL (1976) Accumulation of metoxychlor by microorganisms isolated from aqueous systems. BuU Environ Contam Toxicol 13 443-450 Parr JF, Smith S (1976) Degradation of toxaphene in selected anaerobic soil environments. Soil Science 121 52-57... 

After an extensive study of the adsorption of arsenious oxide by metallic hydroxides,3 Sen concluded that this type of adsorption resembles that of cations by manganese dioxide, and that the chemical affinity between the adsorbent and the substance adsorbed plays an important part, thus differing from adsorption by charcoal. It has been observed that soils having a high absorption capacity for bases also absorb the arsenite ion from solutions of 0-001 to 0-01X concentration.4 The absorption increases with time, without reaching an end-point, and the process follows the normal adsorption equation C1=kC1Jn. The addition of ferric oxide or calcium carbonate to the soil considerably increases the capacity for absorption, but such salts as calcium sulphate or copper sulphate have no effect. 

Oscarson, D.W., Huang, P.M., Liaw, W.K. and Hammer, U.T. (1983) Kinetics of oxidation of arsenite by various manganese dioxides. Soil Science Society of America Journal, 47, 644-48. 

Oscarson, D. W., Huang, P. M., and Hammer, U. T. (1983). Oxidation and sorption of arsenite by manganese dioxide as influenced by surface coatings of iron and aluminum oxides and calcium carbonate. Water, Air, Soil Pollut. 20, 233-244. 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

The speciation of arsenic in lakes does not always follow thermodynamic predictions. Recent studies have shown that arsenite predominates in the oxidized epilimnion of some stratified lakes, whereas arsenate may persist in the anoxic hypolimnion (Kuhn and Sigg, 1993 Newman et al., 1998 Seyler and Martin, 1989). Proportions of arsenic species can also vary according to the availability of particulate iron and manganese oxides (Kuhn and Sigg, 1993 Pettine et al., 1992). Sunlight could promote oxidation in surface waters (Voegelin and Hug, 2003). 

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

Oxidation of As(III) has also been observed in groundwater. Arsenite was injected into oxic [200-300 pM (micromoles per liter) O2] and suboxic (1-6 pM O2) zones of groundwater at a site on Cape Cod, Massachusetts, USA. Significant oxidation of As(III) to As(V) along a 2.2 meter flow path was observed in both the oxic and suboxic zones (Kent et al., 2001). Manganese oxides are present in the mixutre of oxides that coat these aquifer solids (Fuller et al., 1996) and are the most likely electron acceptor in both the low O2 suboxic zone and possibly the oxic zone. 

DW Oscarson, PM Huang, WK Liaw. Role of manganese in the oxidation of arsenite by freshwater lake sediments. Clays Clay Miner 29 219-225, 1981. 

---