Mobility of Iron and Manganese

Wetlands exhibit distinct redox gradients between the soil and overlying water column and in the root zone (Chapter 4), resulting in aerobic interfaces. For example, the aerobic layer at the soil-floodwater interface is created by a slow diffusion of oxygen and the rapid consumption at the interface. The thin aerobic layer at the soil-floodwater interface and around roots functions as an effective zone for aerobic oxidation of Fe(ll) and Mn(II). Below this aerobic layer there exists the zone of anaerobic oxidation of Fe(ll) and Mn(ll) and reduction of Fe(III) and Mn(IV). The juxtaposition of aerobic and anaerobic zones creates conditions of intense cycling of iron and manganese mediated by both biotic and abiotic reactions. 

FIGURE 10.26 Depth profile of dissolved oxygen, Fe(lll), and Fe(ll) concentrations in wetland soil. (Redrawn from Roden and Wetzel, 2002.) 

FIGURE 10.27 Mobile (Fe +) and immobile (Fe-oxides) forms of iron in a flooded soil profile. (Redrawn from Howler and Bonldin, 1971.) 

FIGURE 10.28 Depth profile of dissolved iron in a flooded organic soil. (Redrawn from D Angelo and Reddy, 1994a.) 

FIGURE 10.29 Vertical profile of pore water Mn(II) and manganese extracted with CDB at the Skagerrak site. (Redrawn from van der Zee and van Raaphorst, 2004.) 

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The ability of organisms to create conditions to form organic Fe or Mn complexes and increase the mobility of iron and manganese... 

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

Mobile pools of iron and manganese are present in water-soluble or dissolved forms in soil pore water. Immobile forms include solid phases such as insoluble precipitates and mineral phases (amorphous and crystalline forms) present both in aerobic and anaerobic soil layers. The flux of dissolved iron and manganese is typically from anaerobic soil layers to aerobic soil layers, where it is oxidized to insoluble precipitates. This results in the establishment of concentration gradients across the aerobic-anaerobic soil interface. Mobilization is also regulated by pH and CEC. Manganese is more soluble in moderately acidic conditions (between pH 5 and 6) than iron. 

Zinc ligands are soluble in neutral and acidic solutions, so that zinc is readily transported in most natural waters (USEPA 1980, 1987), but zinc oxide, the compound most commonly used in industry, has a low solubility in most solvents (Elinder 1986). Zinc mobility in aquatic ecosystems is a function of the composition of suspended and bed sediments, dissolved and particulate iron and manganese concentrations, pH, salinity, concentrations of complexing ligands, and the concentration of zinc (USEPA 1980). In freshwater, zinc is most soluble at low pH and low alkalinity 10 mg Zn/L of solution at pH 6 that declines to 6.5 at pH 7, 0.65 at pH 8, and 0.01 mg/L at pH 9 (Spear 1981). Dissolved zinc rarely exceeds 40 pg/L in Canadian rivers and lakes higher concentrations are usually associated with zinc-enriched ore deposits and anthropogenic activities. Marine... 

York and New England are devoid of fish due to the effects of acid rain. Indirect effects of the low pH values associated with acid rain also affect organisms. As noted in Table 13.1, one of the properties of an acid is the ability to dissolve certain metals. This has a profound effect on soil subjected to acid rain. Acid rain can mobilize metal ions such as aluminum, iron, and manganese in the basin surrounding a lake. This not only depletes the soil of these cations disrupting nutrient uptake in plants, but also introduces toxic metals into the aquatic system. 

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