Redox effects

Oxidation-reduction reactions in soils, e.g. as a result of soil flooding or drainage, may not only affect the speciation of red-ox-active metals, like Cr or Mo, but also of red-ox-stable metals, like Zn, Cu or Ni. Reductive dissolution of Fe and Mn hydroxides under sub-oxic conditions may release these trace metals into soil pore-water, while persistence of sub-oxic or anoxic conditions may lead to their subsequent partitioning into sulphides or carbonates.

Dissolved metal concentrations in terrestrial systems also vary significantly with seasonal changes, which are triggered by concomitant light and temperature variations, which in turn affect the activity of microbes. In fact, the release of metals from soil particulates often coincides with a decline in red-ox potential and an increase of organic carbon in the soil solution, induced by microbial activity.

Acidic sulphate-rich soils are formed as a result of the oxidation of anoxic, sulphide-rich soils, which may be caused by human (e.g. digging) or natural activities (e.g. land-rise). The production of sulphuric acid and subsequent Fe precipitation significantly changes natural soil characteristics. Acid soil conditions cause the release of metals from various soil minerals and contribute to increased concentrations of metals, like Cd, Ni, Zn and Cu, in soil run-off and receiving surface waters.

Like in sediments (compare section 5.4.9), redox potential is one of the critical factors regulating the speciation and bioavailability of metals in soils. In the following, some examples from the most recent literature are given to document some new results. Oxidation-reduction reactions may not only affect the partitioning of redox-active trace metals, like Cr or Mo, but also of redox stable metals like Zn, Cu or Ni, in soil or aquatic environments. For example, the reductive dissolution of Fe and Mn hydroxides under suboxic conditions may release redox-stable trace metals into the aqueous phase, whereas persistence of suboxic or anoxic conditions may lead to their partitioning into sulphides or carbonates. But slow transformation rates and fluctuations in physicochemical conditions can alter the predicted species changes.

Bostick et al. (2001) determined the speciation of Zn within a seasonally saturated mining-impacted wetland soil by using XAS (see section 5.4.5), in relation to the soil redox status. Each Zn-containing species was spectroscopically identified by its unique set of interatomic distances and their presence evaluated by comparing the bond distances found in the soil sample with those of model compounds (compare to Table 5.8 in section 5.4.5). The XAS data suggested at least the presence of four primary Zn species within these wetland soils, namely ZnO, ZnCO3, Zn sorbed to hydroxide, and ZnS. Zn speciation seemed to be strongly correlated with the soil waterdepth. In dry oxidized areas almost all Zn was sorbed on hydroxides or present as discrete ZnO phase. On the other hand, ZnS and ZnCO3 was formed in response to a lowered redox potential associated with soil flooding. In soils under intermediate suboxic shallow water conditions, Zn occurred as ZnO, sorbed or as ZnS. Thus waterdepth and redox status appeared to be the most important variables for the speciation of Zn in these wetlands.

To verify the effects of depth, duplicate soil cores were taken from various sites within the wetland (see Table 5.14 below). The cores collected at different sites contained different Zn species, even when their water level was similar. But despite these differences, Zn was mainly determined by the redox stability of each particular phase. Therefor, in oxic soil cores, either ZnO or Zn adsorbed onto hydroxides were dominant, whereas in anoxic environments, ZnS or ZnCO3 dominated, indicating that the water level and corresponding redox status were indeed most relevant for Zn speciation in these wetland soils.

Table 5.14. Influence of water depth and redox status on Zn speciation in wetland soils (from Bostick et al., 2001)

Water depth

core section

% ZnO

% sorbed Zn

% ZnS

% ZnCO3

(cm)

(cm)

on hydroxide

0

0-5

0 ± 0a

97 ± 2

3 ± 2

0

0

5-10

0 ± 0

96 ± 2

4 ± 2

0

36

0-5

0 ± 0

38 ± 2

62 ± 2

0

36

5-10

0 ± 0

36 ± 4

64 ± 4

0

39

0-5

43 ± 2

0 ± 0

57 ± 3

0

39

5-10

43 ± 5

0 ± 0

57 ± 5

0

89

0-5

0

0

85

15

89

5-10

0

0

80

20

105

0-5

12

0

0

88

105

5-10

0

0

0

90

"each value represents the average of duplicate cores, ± standard deviation. Where single values are given, only one core was analysed

"each value represents the average of duplicate cores, ± standard deviation. Where single values are given, only one core was analysed

Otero (2002) investigated the occurrence of metal sulphides in salt marsh soils in NW Spain and the interaction between seasonal and spatial variations and the Fe and S redox cycle. In high salt marsh soils (with suboxic redox conditions), iron sulphides were low (both as AVS and pyrite) indicating a low degree of trace metal pyritization (DTMP) in surface layers. However, metals associated with the pyrite fraction increased with soil depth. In turn, low salt marsh soils (anoxic conditions) showed maximum metal sulphide contents already at the soil surface, which was colonized by Spartina maritima. It is known that the roots of S. maritima stimulate the activitiy of SO4-reducing bacteria in strongly reduced soils, but also favour partial soil oxidation and so the formation of polysulphides, which may immediately precipitate with Fe2+ as pyrite. In the deep permanently anoxic layers, pyrites form in a reaction with FeS probably as an intermediate, according to: FeS + H2S = FeS2 + H2O. The authors observed also seasonal variations in metal sulphide concentrations with lowest levels in summer due to a net loss of metal sulphides (e. g. by evapotranspiration of H2S and release of oxygen from plant roots). In contrast, highest concentrations of AVS and pyritic metals occurred in summer in the low marsh soils, which were colonized by vascular plants with an increased activitiy of sulfur-reducing bacteria, resulting in an accumulation of soil metal sulphides, probably also due to the higher ambient temperature.

Soils not colonized by plants had highest pH values and lowest porewater metal concentrations during summer, suggesting that the higher temperature increased the activitiy of SO4-reducing bacteria, leading to increasing alkalinity and sulphide concentrations in the porewater. Under these conditions metals are precipitated and removed from porewater, increasing AVS and pyrite in soil. In contrast, sulphide oxidation in soils colonized by Spartina maritima, showed at the same time a decrease in pH and a concomitant release of trace metals in the porewater (Otero and Macias 2002). Although comparatively low (only 0.43% of pyrite-Fe and 1.38% of AVS-Fe, and trace metals Cu, Ni, Zn, Mn and Cr only about 301000 times lower), measured metal porewater concentrations were consistent with metal concentrations in the pyrite, but not in the AVS (1 N HCl) or total metal fraction. From their field studies, the authors suggest that pyrite oxidation may be a final step in the precipitation of authigenic Fe oxide minerals, an assumption which may be confirmed by the observed high Fe oxyhydroxide concentrations in the upper soil layers, a process removing again dissolved metals from the porewater.

Quantin et al. (2001) studied the bacterial reduction of Fe and Mn oxides in a Ferrasol by laboratory batch experiments. They found that the anaerobic Fe and Mn reducing bacterial activity (measured as anaerobic respiration or fermentation) in that soil was responsible for the observed Fe and Mn oxide solubilization. Oxide reduction increased when more organic C (as electron-donator) was available. It was also evident that Mn oxide was the major reducible phase and metal source before goethite. Both Ni and Co solubilized with Fe and Mn oxides, but decreased again at the end of the experiment, probably due to adsorption or precipitation processes.

Fortin et al. (2002) assessed the role of SO4-reducing bacteria (SRB) on the fate of Fe and SO4 in Cu-Zn and Au tailings by combining SRB analysis, solid-phase mineralogy, and porewater geochemistry. The oxidized surface of the Cu/Zn tailings showed a low pH, high redox potential and high Cu, Zn and SO4 porewater concentrations, and depletion of pyrite. In contrast, Au tailings revealed a more neutral pH, slightly anoxic conditions, low Fe and porewater metal concentrations and little pyrite oxidation. The authors assume that SRB are involved in SO4 reduction occuring in the Cu/Zn tailings because SO4 solubility was not controlled by SO4-rich minerals. On the other hands, in the reduced tailings zone the occurrence of soluble Fe indicated the presence of iron reducing bacteria (IRB). The observed SO4 decline and release of soluble Fe into porewaters was paralleled by an increase in pH and alkalinity. In contrast to the Cu/Zn tailings, SO4 and Fe solubility in the Au tailings was supposed to be partly controlled by jarosite (a K-Fe-SO4-OH mineral) and Fe oxides.

Dissolved metal concentrations in terrestrial systems vary significantly with seasonal changes, which are triggered by changing light and temperature conditions. Olivie-Lauquet et al. (2001) observed marked seasonal variations for soluble metals in soils, except for Zn and Cu. Release of metals from soil particulates coincided with a decline in redox potential and an increase of organic carbon in the soil solution. The decline in Eh was inversely correlated with ambient temperature, dissolved Fe and Mn, and organic carbon. The authors suggest that microorganisms using soil Fe and Mn oxyhydroxides as e-acceptors catalyse redox changes and induce the observed increase of dissolved organic carbon.

Acid sulfate soils are a rather widespread phenomenon in various parts of the world (e. g. in Scandinavia, Northern America) and mainly the result of the oxidation of O2-free, sulphide-rich soils due to human (e. g. digging) or natural activities (e. g. landrise). The production of sulfuric acid and subsequent Fe precipitation significantly can change natural soil characteristics. As an example, in Sweden, acid sulfate soils, which are used for agrocultural purposes, occur, according to Sohlenius and Oborn (2000), by and large along the northern part of the Swedish Baltic Sea coast, and around the big lakes Malaren and Hjalmaren. Resulting acid soil conditions are releasing metals from various soil minerals and cause increased concentrations of metals, like Cd, Ni, Zn and Cu in surface waters draining these acid sulfate-rich soils.

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