Anoxic systems

Reduction by Fe(ll) results in an increase in the amount of iron oxides, which favor further reaction. Such autocatalytic behavior characterizes the oxidation of Fe(II) by and explains C Cl NO reduction by Fe(ll) in the absence of an iron mineral phase. Generalizing this behavior, it can be assumed that Fe(III) colloids derived from Fe(ll) oxidation in subsurface anoxic systems, together with other colloids, affect the environmental persistence of nitroaromatic contaminants. Colon et al. (2006), for example, elucidate factors controlling the transformation of nitrosobenzenes and N-hydroxylanilines, which are the two intermediate... 

The predicted product, 1,1 -dichloro-2,2-di(4-chlorophenyl)ethane (DDD), is indeed found to be formed from DDT in a variety of anoxic systems like sewage sludges and sediments (Montgomery, 1997). 

Anoxic systems—i.e., those in which no free oxygen exists—which commonly represent stagnant conditions, can in general because of their slow flushing be expected to be closer to equilibrium than oxygenated systems. Hence, we will emphasize mainly anoxic natural and model systems. 

The situation may be comparable to deep groundwater, which is also an anoxic system. The change of pe + pH in the present experiment is shown in Figure 2. 

Lee, C. (1992) Controls on organic carbon preservation the use of stratified water bodies to compare intrinsic rates of decomposition in oxic and anoxic systems. Geochim. Cosmochim. Acta 56, 3323-3335. 

Redox processes are important for elements which can exist in more than one oxidation state in natural waters, e.g. Fe and Fe, Mn, and Mn. These are termed redox-sensitive elements. The redox conditions in natural waters often affect the mobility of these elements since the inherent solubility of different oxidation states of an element may vary considerably. For example, Mn is soluble whereas Mn is highly insoluble. In oxic systems, Mn is precipitated in the form of oxyhydr-oxides. In anoxic systems, Mn predominates and is able to diffuse along concentration gradients both upwards and downwards in a water column. This behaviour gives rise to the classic concentration profiles observed for Mn (and Fe) at oxic-anoxic interfaces as illustrated in Figure 2. 

The oxidation of pyrite can occur when the mineral surface is exposed to an oxidant and water, either in oxygenated or anoxic systems, depending on the oxidant. The process is complex and can involve chemical, biological, and electrochemical reactions. The chemical oxidation of pyrite can follow a variety of pathways involving surface interactions with dissolved O2, Fe, and other mineral catalysts (e.g., Mn02). Oxidation of pyrite... 

Anoxic systems are divided into those with or without measureable sulfide, which Berner termed sulfidic and nonsulfidic. Nonsulfidic environments themselves are described as postoxic if too oxidized to permit sulfate reduction and methanic if strongly reduced with sulfate reduction and methane formation. Berner suggests that the presence or absence of specific iron and manganese minerals in Table 11.5 can be used to distinguish these different redox environments. 

Diagenesis and elemental cycling in oxic or suboxic sediments in Saguenay Fjord is to some extent governed by different processes than in fully anoxic systems, where organic matter oxidation and sulfate reduction have been taking place without interruption for a considerable length of time. This has led to a situation where sulfide is present in the water column. 

As reviewed in detail by Schoonen (2004) these different conversion mechanisms - and in particular the H,S pathway - have received controversial discussion. However, field studies have shown that hydrogen sulfide can indeed sulfidize amorphous FeS and form pyrite. Rickard (1997) found that the H,S process is by far the most rapid of the pyrite-forming reactions hitherto identified and suggested that it represents the dominant pyrite forming pathway in strictly anoxic systems. In addition, Morse (2002) discussed that the oxidation of FeS by hydrogen sulfide is the faster process compared with the oxidation by elemental sulfur. Berner (1970) suggested that, in the presence of zero-valent sulfur, a complete transformation of FeS to pyrite should be possible on a time scale of years. An incomplete conversion of FeS to pyrite, as often observed, e g. in... 

Fig. 15.4 Geochemical model calculation using the program PHREEQC. In an anoxic system (state at the end of the model calculation from Fig. 15.3), the gradual addition of organic matter to the redox reaction is continued, whereby the system is kept open for calcite equilibrium and sealed from the gaseous phase. Initially, the dissolved sulfate will be consumed, in the course of which low amounts (logarithmic scale) of methane will emerge. Only after the sulfate concentration has become sufficiently low, will the generation of methane display its distinct increase.

An anoxic system, as obtained from the previous reaction sequence, will be capable of further ongoing reaction, if the supply of organic matter is continued and the reaction is held under the same boundary conditions. The result consists in the continuation of the processes shown in Figure... 

The mobility of Mo is strongly influenced by redox conditions. Increased mobility of Mo occurs in oxic systems relative to anoxic systems, as predicted by thermodynamic calculations and as observed in environments with different redox conditions. 

Although Mo can exist in the oxidation states II, III, IV, V, and VI, its dominant oxidation states in nature are IV and VI (Adriano, 1986). Thermodynamic calculations indicate that for oxic and neutral pH conditions, Mo exists in the VI oxidation state as a mobile, dissolved oxyanion (Mo04 "). In anoxic systems. Mo is predicted to be in the IV oxidation state as the insoluble sulfide mineral molybdenite (MoSj) (Brookings, 1987). 

Lee C (1992) Controls on organic carbon preservation The use of stratified water bodies to compare intrinsic rates of decomposition in oxic and anoxic systems. Geochim Cosmochim Acta 56(8) 3323-3335 Lerat Y, Lasserre P, le Corre P (1990) Seasonal changes in pore water concentrations of nutrients and their diffusive fluxes at the sediment-water interface. J Exp Mar Biol Ecol 135(2) 135-160... 

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