Redox effects

The geochemistry of trace metals in anoxic sediments is different from the geochemistry of metals in oxidized sediments. In particular, the upper 3 cm of aquatic sediments represent a highly reactive zone of biogeochemical transformation. Sediment red-ox potential (expressed as Eh value) can be considered as a bulk electrochemical measure for oxidized and reduced metal-binding forms occurring in the sediment pore-water, which may integrate all relevant metabolic and transport processes and effects in the sediment. Red-ox characteristics may influence behavioural processes of sediment dwelling organisms and, thus, metal bioavailability. Moreover, these metabolic and behavioural processes advect particles and water between bottom water and the deep sediment, and finally define the overall structure of the pore-water chemistry. In this way, a full spectrum of biogeochemical conditions of solubility, reactivity and microbial metabolism of organic matter and compounds is created, which finally defines metal solubilities, and steadily pushes the Eh horizon in the sediment profiles towards more oxidizing conditions.

The geochemistry of trace metals in anoxic sediments obviously is different from the geochemistry of metals in oxic sediments, with corresponding implications for metal speciation and bioavailability. According to Griscom et al. (2000) metal partitioning and bioavailability seems most complex in oxic sediments. As demonstrated before, anoxic sediment conditions with metal sulphide formation alone do not appear to eliminate metal uptake from the diet by benthic invertebrates (see above).

Davis et al. (1998) considered the sediment redox potential (expressed as Eh value) as a bulk electrochemical measure of oxidized and reduced metal-binding compounds existing in sediment porewater, which integrates all consequences of metabolic and transport processes occuring within benthic communities. The authors discuss and speculate about the importance of rather simple but often overlooked behavioural processes of sediment dwelling organisms, which may influence redox characteristics and so metal bioavailability (compare section 5.4.6). For example, motile benthic macrofauna ingest and transport particles, ventilate deep burrows in anoxic sediments with the overlying oxic water, while sedentary suspension-feeding organisms deposit suspended organic matter onto the sediment surface. All these metabolic and behavioural processes advect particles and water between bottom water and the deep sediment and finally define the overall structure of porewater chemistry. In this way these processes create a full spectrum of biogeochemical conditions of solubility, reactivity and microbial metabolism mineralizing organic matter and compounds, which finally define metal solubility, and push vertical Eh profiles steadily towards more oxidizing conditions. The authors stress that the use of standard Eh probes inserted downward through the bioturbation zone of a sediment can provide a general measure of porewater chemistry appropriate for screening purposes.

In the following, some recent examples are given to underline once again the importance of the prevailing redox conditions for the speciation of trace metals in aquatic sediments.

Donahoe and Liu (1998) found a good spatial corrrelation between trace metals and Fe and Mn in wetland porewater and sediments suggesting that adsorption and desorption reactions from Fe and Mn oxyhydroxides are controlling dissolved metal distribution. However, equilibrium adsorption modelling alone did not explain the observed distribution. On the other hand, redox kinetic modelling was suggesting that relative rates of ferrous Fe oxidation and reductive dissolution of ferric iron in sediment may coexist and so determine the final porewater distribution of metals. Also Xue et al.

(1997) could confirm the relevance of redox-controlled processes at the sediment-water interface for the cycling of Cu and Zn in eutrophic lakes. Hypolimnetic oxygenation enhanced the release of sediment-Cu (from sulphide oxidation), but at the same time accelerated entrapment and deposition of Cu and Zn by freshly formed Mn and Fe oxides. Astrom

(1998) studied the behaviour of trace metals in sulphide-bearing fine-grained sediments exposed to atmospheric oxygen. On sediment oxidation high portions of Co, Mn and Ni, intermediate of Cu and low portions of Fe, Al, Cr and V were released from the sediment. The extent of release seemed to depend on the drop in pH (as a consequence of sulphide oxidation), the actual amount of metals associated with (H2O2-extractable) metal sulphides, and on the sum of the amount of metals associated with reduced sediment phases and those adsorbed on soil compounds. The author stressed that any artificial drainage of these sulphide-bearing wetland sediments may cause extensive leaching in particular of Co, Mn and Ni.

That the upper 3 cm of sediments taken from a mining lake represent indeed a highly reactive zone of biogeochemical transformation processes (for example, expressed by enhanced concentrations of lipid phosphate, degradable organic matter, DOC and Fe2+ in the porewater and a drastic change in sulfur isotope ratios) was likewise documented by Friese et al. (1998). The occurrence of Fe and SO4-reducing processes resulted in a distinct pH increase with sediment depth.

In another study, Gianmarco et al. (1997) assessed the seasonal variation of the redox potential represented by the mobility of sulphide in a dystrophic lagoon sediment. In general, they observed high reactive Fe (II, III) concentrations and low SO4-reduction rates in the sediment porewater. Particularly in winter and spring, reactive Fe3+-levels were high, whereas in summer free sulphides prevailed in porewater although most of the reactive Fe2+ was not sulphide-bound, indicating that the reactive Fe pool is not as a whole available to buffer against sulphide release. For this reason, the authors judge the reactive Fe pool not as a good measure of the true redox buffering capacity in sediments.

Lee et al. (1997) used distribution coefficients (kd) to assess the relative mobilities of metals in retention pond sediment along a motorway. They found a vertical decrease of kd for Zn (and Cd), and an increase for Pb. Based on established k^-values the investigated sediments showed different relative mobilities in the upper (Mn>Zn>Cd>Pb) and lower sediment layer (Mn>Cd>Zn>Pb). If and to what extent the observed vertical distribution of metal mobilities (expressed as kd value) in the sediment follow prevailing redox and pH gradients was unfortunately not demonstrated by the authors.

Like in sediments, metals in soils are part of a complex mixture of solid phase compounds of varying particle size and morphology, including discrete mineral phases, coprecipitated and sorbed species associated with soil minerals and organic matter, and dissolved species complexed by a variety of inorganic and organic ligands, including the vast variety of soil

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