Chemical speciation, ocean

Recent reviews on chemical speciation are published by e.g. Stumm and Brauner (1975), Florence and Batley (1980) and Leppard (1983) sometimes, with special reference to metal-organic interactions (Mantoura, 1982) or complexation in natural waters (Kramer and Duinker, 1984b). Bruland (1983) summarized the distribution and behaviour of trace elements in ocean waters. The occurrence of certain species is largely dependent on the environmental conditions. There exists a strong competition of trace metals with H+ or major cations like Ca2+ and Mg2+ in seawater, but also with other trace metals which might form more stable complexes with the ligand in question on the other side, many potential ligands or chelators compete for one trace element. 

The ocean is the penultimate repository for most chemical substances, natural and anthropogenic, prior to their incorporation and burial in marine sediments. In spite of an enormous annual input of terrestrially derived chemicals, with respect to many reactive chemical species, the pelagic ocean is a relatively clean environment. This is due, in large part, to the intensity of near-shore sedimentation. This chapter highlights chemical speciation in the vast, and relatively clean, interior region of the ocean. 

The Elemental Content of Human Diets and Excreta The Elemental Constituents of Soils Mycotoxins Occurrence, Distribution, and Chemical Speciation of some Minor Dissolved Constituents in Ocean Waters. 

Physico-chemical speciation refers to the various physical and chemical forms in which an element may exist in the system. In oceanic waters, it is difficult to determine chemical species directly. Whereas some individual species can be analysed, others can only be inferred from thermodynamic equilibrium models as exemplified by the speciation of carbonic acid in Figure 9. Often an element is fractionated into various forms that behave similarly under a given physical (e.g., filtration) or chemical (e.g., ion exchange) operation. The resulting partition of the element is highly dependent upon the procedure utilised, and so known as operationally defined. In the following discussion, speciation will be exemplified with respect to size distribution, complexation characteristics, redox behaviour and methylation reactions. 

Copper was selected as the first metal for which to attempt to optimize the shipboard analyses because considerable information is available about the marine chemistry of copper, and because this new analytical capability would greatly enhance our ability to study copper in the ocean. The concentration of copper in the ocean varies from 0.5 to 5 nmol/kg in response to biological and geochemical processes (Table I). The chemical speciation of copper has received considerable attention because the biological effects of copper depend on its chemical form (i-3). The principal forms of copper include inorganic complexes such as CUCO3, CuHCO , CuOH, and organically bound copper (4, 5). 

Studies carried out to evaluate the uptake of Fe by phytoplankton showed that only the dissolved metal is bioavailable and that a thermal or photochemical treatment is necessary for the colloidal Fe to become bioavailable (163). Moreover, the chemical form in which Fe is present can also affect its availability for plankton. The distribution of Fe(II) in the euphotic layer of the equatorial Pacific Ocean was examined by O Sullivan et al. (164). Its concentration is regulated by the balance between production and removal Fe(II) can be produced by microbial and chemical reduction, while the loss in surface water is controlled by biological uptake and by oxidation to Fe(III), subsequent hydrolysis, ageing and settling. The results showed maximum concentration near the surface and at the depths with higher chlorophyll a levels, the concentration ranging between 0.12 and 0.53 nM. Laboratory experiments carried out by the same authors showed that photoreduction can be an important source of Fe(II). Considering the different chemical speciation observed at various depths, different bioavailability can be expected in the examined zone. 

Zirino, A. and Yamamoto, S., 1972. A pH-dependent model for the chemical speciation of copper, zinc, cadmium and lead in sea water. Limnol. Oceanogr., 17 661—671. Zsolnay, A., 1977. Inventory of non-volatile fatty acids and hydrocarbons in the oceans. Mar. Chem., 5 465—475. 

Here D is the molecular diffusivity of CO2, z is the film thickness, a, is the solubility of i, V and A are the volume and surface area of the ocean, and X is the decay coefficient. Use of pre-industrial mean concentrations gave a global boundary layer thickness of 30pm (D/z 1800m y = piston velocity). The film thickness is then used to estimate gas residence times either in the atmosphere or in the mixed layer of the ocean. For CO2 special consideration must be made for the chemical speciation in the ocean, and for " C02 further modification is necessary for isotopic effects. The equilibration times for CO2 with respect to gas exchange, chemistry, and isotopics are approximately 1 month, 1 year, and 10 years, respectively. 

Since iodine in these processes is present in various chemical forms, the chemical speciation of iodine is important for understanding its geochemistry in oceans. Further, iodine is redox sensitive and the most abundant biophilic minor element in the oceans. Here, the behaviors of various iodine species in seawater will be described in regard to biological and abiological processes. 

Only a small fraction of the atmospheric flux of iron to the oceans ever becomes bioavailable, largely because of the low solubihty of the particulate and colloidal Fe(lll) phases that comprise the bulk of aerosol iron species. Nonetheless, bottle incubation experiments have shown that aerosol addition is an efficient stimulator of chlorophyll and biomass production in phytoplankton cultures [105,106], emphasizing the need to understand the factors and processes controlling the chemical speciation and solubihty of aerosol iron before and after deposition. 

Solution speciation exerts important controls on chemical behavior. Speciation is known to influence solubility, membrane transport and bioavailability, adsorptive phenomena and oceanic residence times, volatility, oxidation/reduction behavior, and even physical properties of solutions such as sound attenuation. In recogni-tion of such influences, substantial efforts have been made to characterize the chemical speciation of elements in seawater. While assessments of organic speciation have dominantly been obtained using modern voltammetric procedures and, as such, have a relatively short history, assessments of inorganic speciation typically involve a wide variety of analytical procedures that have been employed over many decades. 

Adsorption on manganese oxide, followed by oxidation at the surface, helps considerably to convert Cr(III) into the thermodynamically stable Cr(VI). However, because of the low concentration of suspended MnOa in the oceans, it is not clear whether this catalyzed oxidation is quantitatively more important than the direct oxidation by dissolved oxygen. Thus, chemical speciation of chromium in seawater is still an important issue in marine chemistry. 

A sample of sediment from a given depth below the water/sediment interface can be dated from its ( Th/ Th) activity ratio if this has stayed constant for at least several hundred thousand years in the water body next to the sediment in any given ocean basin and also if there was no migration of the isotopes in the sediments. In addition, their chemical speciation must have been the same in seawater with no isotopic fractionation between this and the mineral phases involving thorium in the sediment. Furthermore, the ionium and thorium found in detrital mineral particles must be excluded from the analysis. Assuming that the contribution made by uranium-supported ionium is negligible, the activity of a sample given as an activity ratio may be related to its age as... 

Surface and bottom seawater samples from three locations in the Kattegat and Baltic Sea were analyzed for and in both iodide and iodate species, and total inorganic iodine by neutron activation analysis using nuclear reactions of l(n,y) I and I(n,y) I (Hou et al. 2001). Levels of 2.3 x 10 mol/1 for and 7 x 10 mol/1 for are obtained. Therefore, these isotopes of iodine are promising tracers of physical and biochemical process in the ocean, and research about the chemical speciation of and in ocean will be developed. ... 

M. Boye, A.P. Aldrich, C.M.G. van den Berg, J.T.M. de Jong, M. Veldhuis, and H.J.W. de Baar. Horizontal gradient of the chemical speciation of iron in surface waters of the northeast Atlantic Ocean. Marine Chemistry 80 129-143,2003. 

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