Biological reduction processes, sediments

Biological reduction processes in sediments may be viewed as the oxidation of carbohydrate (in its simplest form CH2O) with accompanying reduction of an oxygen carrier. In the first instance, dissolved molecular oxygen is used. The reaction is thermodynamically favoured, as reflected by the strongly negative AG . 

The sediment N cycle consists of oxidative and reductive processes that are frequently coupled in space and/or time (Flerbert, 1999) and are strongly influenced by chemical, physical, and biological factors (Figs. 19.1 and 19.2). We present... 

Sulphates are transported into the seas and oceans directly from the atmosphere and from surface waters. One third of sulphates in surface waters comes from the soil and rocks, two thirds from the atmosphere. As compared to the amount in surface water, sulphur is present in sea water in concentrations higher by a factor of 240. In coastal waters, which are characterized by a remarkable input of sulphur, the biological reduction occurs with a simultaneous H2S release. This gas escapes into the atmosphere, when a mud layer is exposed in the course of the tidal variation. Since the process also occurs in the depths of seas, hydrogen sulphide reacts with the iron compounds present, producing sulphides, which are deposited continuously in sea sediments. 

The activated sludge process, depicted in Fig. 1, involves basically the aeration and agitation of an effluent in the presence of a flocculated suspension of micro-organisms which are supported on particulate organic matter. After a predetermined residence time (usually several hours) the effluent is passed to a sedimentation tank where the flocculated solids are separated from the treated liquid. A reduction of BOD from 250-350 mg IT1 to a final value of 20 mg L-1 is achieved under typical operating conditions. Part of the settled sludge is usually recycled to the aeration tank in order to maintain biological activity. 

All four dissolution procedures studied were found to be suitable for arsenic determinations in biological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitricperchloric-hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation inductively coupled plasma atomic emission spectrometry requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements. 

The primary abiotic and biological processes that transform uranium in soil are oxidation-reduchon reactions that convert U(VI) (soluble) to U(IV) (insoluble). Reduction of U(VI) to U(IV) can occur as a result of microbial action under anaerobic soil or sediment conditions, thereby reducing the mobility of uranium in its matrix (Barnes and Cochran 1993 Francis et al. 1989). Further abiotic and biological processes that can transform uranium in the environment are the reactions that form complexes with inorganic and organic ligands (see Section 5.3.1). 

---