Arsenic from iron-oxide phases

Geochemical analyses of the contaminated sediments in the root zone using sequential chemical extractions showed that greater than half of the arsenic is strongly adsorbed (Keon et al. 2000, 2001). A mixture of arsenic oxidation states and associations was observed and supported by bulk XANES and EXAFS data collected at the SSRL. Arsenic in the upper 40 cm of the wetland, which contains the peak corresponding to maximum deposition, appears to be controlled by iron phases, with a small contribution from sulfidic phases. The results suggest that iron oxide phases may be present in the otherwise reducing wetland sediments as a substrate onto which arsenic can adsorb, perhaps due to cattail root plaque formation. 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

The mobilization of arsenic from the tailings material seems to be a slow and continuos process attributed to reduction of iron phases. The seepage water of the middle source contains arsenite as well as arsenate in high concentrations and seems to be the only water source in contact with the tailings material. The concentrations of arsenic downstream are still high and the immobilization process by precipitation of iron hydroxide and coprecipitation or sorption of arsenic is incomplete. A reason for this may be the slow kinetics of the oxidation process and the transport of fine grained hydroxide particles. These particles are mobile and can bind the arsenic (mainly as arsenate) too. 

Waste piles from former bedrock mining in the area are found to contain up to 30% As, the majority in secondary arsenate minerals, particularly scorodite (Williams et ah, 1996). Alluvial soils also contain up to 5000 mg kg As. In these, Fordyce et al. (1995) concluded that some 20% of the As was present in crystalline iron oxides, with the remainder assumed to be in sulphate mineral phases or other oxidised products. 

The aqueous chemistry of iron is also important in a number of other settings. Iron can be the dominant cation released in acid rock drainage, due to the oxidation of pyrite (FeS2(s)) when it becomes exposed to air and water. This process is catalysed by bacteria which cycle ferrous iron back to ferric iron which, in turn, can oxidise further pyrite. Thus, the rate of oxidation will depend on the aqueous concentration of ferric iron. If insufficient iron (and acid) is produced or the iron is removed by the inherent neutralisation capacity of the material, the rate of oxidation will be substantially reduced. The precipitation of iron oxyhydroxide phases and their ability to adsorb other aqueous elements have also been studied in detail (Dzombak and Morel, 1990). The removal of arsenic from drinking water by hydrous iron oxides is one example of these adsorption reactions. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

The exposure of sulfide minerals contained in mine wastes to atmospheric oxygen results in the oxidation of these minerals. The oxidation reactions are accelerated by the catalytic effects of iron hydrolysis and sulfide-oxidizing bacteria. The oxidation of sulfide minerals results in the depletion of minerals in the mine waste, and the release of H, SO4, Fe(II), and other metals to the water flowing through the wastes. The most abundant solid-phase products of the reactions are typically ferric oxyhydroxide or hydroxysulfate minerals. Other secondary metal sulfate, hydroxide, hydroxy sulfate, carbonate, arsenate, and phosphate precipitates also form. These secondary phases limit the concentrations of dissolved metals released from mine wastes. 

Of particular interest was the way in which detailed information could be derived from voltammetric studies of clay minerals treated aerobically with iron (11) solutions. This caused the precipitation of a thin, active layer of iron (HI) oxi(hydroxides) which could later be used for the sorption of arsenate(V) ions [69]. By employing the voltammetry of immobilized microparticles, it was possible to distinguish different iron species, namely (i) ion-exchangeable, labile, or sorbed iron (HI) ions (ii) ferrihydrite or lepidocrocite and (iii) crystalline hematite or goethite. Cepria et al. subsequently employed the voltammetry of immobilized microparticles in the phase analysis of iron (III) oxides and oxi(hydroxides) in binary mixtures, as well as in cosmetic formulations [70]. 

Phenol can be produced directly from benzene by use of N2O as an oxidizing agent in the gas phase. Catalysts are modified ZSM-5 or ZSM-11 materials, containing elements such as antimony, arsenic, beryllium, boron, cobalt, chromium, copper, gaUium, indium, iron, nickel, scandium, vanadium or zinc. (208)... 

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