Reductive dissolution of iron and manganese oxy hydr oxides

11 Reductive dissolution of iron and manganese (oxy)(hydr)oxides 

The reductive dissolution of solid compounds in anaerobic soils, sediments, and waters begins with the reduction of prominent cations within the compounds. Many Fe(III) (oxy)(hydr)oxide compounds are especially susceptible to reductive dissolution. The reduction process converts Fe(III) into more water-soluble Fe(II). The formation of Fe(II) causes the (oxy)(hydr)oxides to decompose in water. In some cases, the Fe(II) rapidly precipitates as new solid compounds, such as siderite (FeCCT) or magnetite (FesCL). 

Extensive reductive dissolution of Fe(III) compounds in water would release arsenic that might have sorbed or coprecipitated with the compounds. However, the arsenic may immediately sorb or coprecipitate with any Fe(II) compounds that might form. As discussed throughout this chapter, reductive dissolution is an important and ubiquitous process that controls the mobility of arsenic in many sediments, soils, wetlands, estuaries, seas, groundwaters, and stagnant lakes. 

The chronological order of the reductions of As(V) and Fe(III) in field and laboratory studies shows some variation. Although both species reduce before sulfate, the reduction of As(V) to As(III) often starts after the initiation of Fe(III) reduction (Smedley and Kinniburgh, 2002), 536 (Pedersen, Postma and Jakobsen, 2006), 4120 (Islam et al., 2004). Inskeep, McDermott and Fendorf (2002), 196-197 further conclude that in most cases As(V) is released into solution by reductive dissolution before being reduced. On the other hand, Campbell et al. (2006) found that two strains of the microorganism Shewanella sp. reduced As(V) before or simultaneously with Fe(III) reduction in iron (oxy)(hydr)oxides. 

Sometimes clay layers are inserted into mine tailings piles (e.g. Globe and Phoenix Mine in Zimbabwe) or piles are capped or surrounded with clay or plastic liners. The liners and caps are installed to reduce the infiltration of air and oxygenated water, and hinder the formation of mine drainage (Williams, 2001), 275. 

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