Pore-water sulphide

McKee, K.L., Mendelssohn, I.A. and Hester, M.W. (1988) Reexamination of pore water sulphide concentrations and redox potentials near the aerial roots of Rhizophora mangle and Avicennia germinans. American Journal of Botany, 75, 1352-1359. 

In natural environments, abiotic reduction may also be effected by a range of natural reductants including sulphide and methane. In the sulphide producing sediments in the Long Island Sound and the Mississippi Delta, for example, Fe oxides have been transformed to Fe sulphides (FeS and FeS2) (Canfield Berner, 1987 Boesen Postma, 1988). As a result, when there was a sufficient supply of reactive Fe oxides in the sediments, hardly any dissolved sulphide was found in the pore water. The reduci-... 

Figure 6.4 Iron, manganese and sulphur at Stiffkey, North Norfolk (1990-1991) (Ruddy, 1993). (a) Pore-water iron profiles, (b) Pore-water manganese profiles, (c) Solid sulphide (predominantly pyrite) profile.

Siderite precipitates from reducing, non-sulphidic pore waters that evolve in the suboxic and microbial methanogenesis zones of all depMDsitional environments. These geochemical conditions occur in organic-rich sediments containing appreciable amounts of reactive iron minerals and in which the pore waters are S04 -poor meteoric or brackish (Postma, 1982). 

Keywords Acid-volatile sulphide Bioavailability Pore water Redox Sediment Trace metals... 

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

Respiratory sulphate reduction ideally takes place when all other electron acceptors are exhausted, but significant overlap may occur between the zones of microbial Fe(III) reduction and sulphate reduction due to kinetic constraints, as discussed before. Sulphate concentrations typically decrease to zero within the upper sediment layer (Fig. 1). In freshwater sediments, reduced S formed mainly by reduction of pore-water sulphate, is predominantly present as inorganic S in the form of AVS. Although pyrite is the most stable sulphide mineral, its formation in permanently submerged freshwater sediments is subject to controversy (Rickard et al., 1995). Because, contrary to marine sediments (S-dominated), there is an excess of Fe liberation over HS production in freshwater sediments (Fe-dominated), FeC03 as well as FeS may control pore-water Fe concentrations in the anoxic sediment layer. 

Thus, pore water studies provide evidence that some trace elements (As, Co, Cr) become more soluble on redox dissolution of Mn-Fe oxides in marine and sediments. However, these studies have failed to detect a parallel release of trace elements (other than Co) with Fe and Mn during redox events (Sakata, 1985 Morfett et al., 1988 Achterberg et al., 1997). It appears that in spite of the loss of labile organic matter and oxides of Fe and Mn, the trace elements remain relatively immobile. In oceanic or estuarine sediments it has been proposed that sulphides have a role to play in fixing the trace elements under reducing conditions. This has also been demonstrated in sulphate-rich freshwater systems (Huerta-Diaz et al., 1998). However, Cu and Pb, at least, clearly remain firmly attached to freshwater sediment even in the absence of measurable sulphides, and where Mn and Fe are being actively released via reduction (Sakata, 1985). 

Pore-water data from dredged material from Hamburg Harbour indicate typical differences in the kinetics of proton release from organic and sulphidic sources (Table 10.2). Recent deposits are characterized by low concentrations of nitrate, cadmium and zinc. When these low-buffered sediments are oxidized during a time period of a few months to years, the concentrations of ammonia and iron in the pore water typically decrease, whereas those of cadmium and zinc increase (with the result that these metals are easily transferred into agricultural crops ). 

Type 4A sieves. The pore size is about 4 Angstroms, so that, besides water, the ethane molecules (but not butane) can be adsorbed. Other molecules removed from mixtures include carbon dioxide, hydrogen sulphide, sulphur dioxide, ammonia, methanol, ethanol, ethylene, acetylene, propylene, n-propyl alcohol, ethylene oxide and (below -30°) nitrogen, oxygen and methane. The material is supplied as beads, pellets or powder. 

The pore size of a molecular sieve can be modified by exchange of the cations incorporated into the aluminosilicate skeleton. In general, molecular sieves of small pore size (ca. 4 A 0.4 nm) are the most suitable for dehydration (e.g., the Linde 0.4 nm product of Union Carbide, or products equivalent to this). In addition to water molecules, this molecular sieve is suitable for the binding of carbon dioxide, hydrogen sulphide, sulphur dioxide, ammonia, methanol and other molecules of similar size. For chemical reasons, however, its affinity for water is so much higher than that for the other molecules of similar size that it may even be used for the dehydration of methanol. As an example of the effectiveness of this sieve, it can reduce the water content of ethanol from 0.5% to 10 ppm. 

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