Elements in seawater

Thus, the chemical reactivity of the elements in seawater is reflected by the residence time. It is important to note, however, that while residence times tell us something about the relative reactivities, they also tell us nothing about the nature of the reactions. The best source of clues for imderstanding these reactions is to study the shape of dissolved profiles of the different elements. When we do this we find that there are six main characteristic types of profiles as described in Table 10-8. Notice that most of these reactions occur at the phase discontinuities between the atmosphere, biosphere, hydrosphere, and lithosphere. 

By definition a minor element in seawater is one has a concentration less than Ippm(m). It is experimentally challenging to determine the total concentrations, much less their major chemical forms. Development of new analytical techniques has greatly extended our knowledge (Johnson et al, 1992). Because early data (prior to about 1975) was so erratic, the principle of oceanographic consistency was proposed as a test for the data (Boyle and Edmond, 1975). According to this principle the analyses of minor elements should ... 

The chemical reactivity of minor elements in seawater is strongly influenced by their specia-tion (see Stumm and Brauner, 1975). For example, the Cu ion is toxic to phytoplankton (Sunda and Guillard, 1976). Uranium (VI) forms the soluble carbonate complex, U02(C03)3, and as a result uranium behaves like an unreactive conservative element in seawater (Ku et ah, 1977). 

Klinkhammer, G., Elderfield, H. and Hudson, A. (1983) Rare earth elements in seawater near hydrothermal vents. Nature (London), 362, 185-188. 

Iieser et al. [628] studied the application of neutron activation analysis to the determination of trace elements in seawater. The rare earths included in this study were cerium and europium. The element concerned were adsorbed onto charcoal. Between 75% and 100% of the elements were adsorbed onto the charcoal which was then subjected to analysis by neutron activation analysis. Cerium (300 p,g/l) and europium (0.00082 pg/1) were found in North Sea water by this method. 

Elderfield and Greaves [629] have described a method for the mass spectromet-ric isotope dilution analysis of rare earth elements in seawater. In this method, the rare earth elements are concentrated from seawater by coprecipitation with ferric hydroxide and separated from other elements and into groups for analysis by anion exchange [630-635] using mixed solvents. Results for synthetic mixtures and standards show that the method is accurate and precise to 1% and blanks are low (e.g., 1() 12 moles La and 10 14 moles Eu). The method has been applied to the determination of nine rare earth elements in a variety of oceanographic samples. Results for North Atlantic Ocean water below the mixed layer are (in 10 12 mol/kg) 13.0 La, 16.8 Ce, 12.8 Nd, 2.67 Sm, 0.644 Eu, 3.41 Gd, 4.78 Dy, 407 Er, and 3.55 Yb, with enrichment of rare earth elements in deep ocean water by a factor of 2 for the light rare earth elements, and a factor of 1.3 for the heavy rare earth elements. 

Tian-Hong Zhang et al. [652] have reported a new ion exchange chelating fibre with aminophosphonic and dithiocarbamate groups, based on polyacrylonitrile for the preconcentration of rare earth elements in seawater prior to their determination by inductively coupled plasma mass spectrometry. Rare... 

Wen et al. [950] used 8-hydroxyquinoline immobilised on a polyarylonitrile hollow fibre membrane to achieve a 300-fold concentration factor for rare earth elements in seawater. 

Orren [663] used atomic absorption spectrometry to determine these elements in seawater in both their soluble and insoluble forms. The alkali metals are determined directly, but the other elements are first concentrated by solvent extraction. The particulate matter content is derived by dissolving the membranes used to filter the sample and determine the metals in the resulting solution. For organic standards of known metal content, the efficiency of the technique was almost 100%. 

Segar and Gonzalez [431] carried out a direct determination of these elements in seawater using a graphite atomiser and a deuterium background connector. Sea salts are volatilised at a lower temperature than is required for the volatilisation of the above elements. 

Grobenski et al. [709] have reviewed methodology for the determination of these elements in seawater. Zeeman-effect background correction using an AC magnet around the graphite furnace corrects for nonspecific attenuation up to 2.0 absorbance and corrects for structured background. 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

Berman et al. [735] and McLaren [738] attempted to determine the foregoing nine elements in seawater by a combination of ion exchange preconcentration on Chelex 100 [129,736-738], and ICP-AES using ultrasonic nebulisation. Preconcentration factors of between 25 and 100 were obtained by this technique. 

Chong et al. [742] have described a multielement analysis of multicomponent metallic electrode deposits, based on scanning electron microscopy with energy dispersive X-ray fluorescence detection, followed by dissolution and ICP-MS detection. Application of the method is described for determination of trace elements in seawater, including the above elements. These elements are simultaneously electrodeposited onto a niobium-wire working electrode at -1.40 V relative to an Ag/AgCl reference electrode, and subjected to energy dispersive X-ray fluorescence spectroscopy analysis. Internal standardisation... 

Brihaye et al. [787] have described a procedure for the determination of these elements in seawater. 

Holzbecker and Ryan [825] determined these elements in seawater by neutron activation analysis after coprecipitation with lead phosphate. Lead phosphate gives no intense activities on irradiation, so it is a suitable matrix for trace metal determinations by neutron activation analysis. Precipitation of lead phosphate also brings down quantitatively the insoluble phosphates of silver (I), cadmium (II), chromium (III), copper (II), manganese (II), thorium (IV), uranium (VI), and zirconium (IV). Detection limits for each of these are given, and thorium and uranium determinations are described in detail. Gamma activity from 204Pb makes a useful internal standard to correct for geometry differences between samples, which for the lowest detection limits are counted close to the detector. 

This technique has been used to determine a number of elements in seawater, including lithium [826], barium [74], lead [827], rubidium [840], uranium [828], and copper [298,299]. It has not been extensively applied. 

Nagaosa et al. [839] simultaneously separated and determined these elements in seawater by high-performance liquid chromatography (HPLC) using spec-trophotometric and electrochemical detectors. 

Bruland KW (1983) Trace elements in seawater. In Riley JP, Chester R (eds) Chemical oceanography, vol 8. Academic Press, London... 

H. Louie, M. Wu, P. Di, P. Snitch, and G. Chappie. Determination of Trace Elements in Seawater Using Reaction Cell Inductively Coupled Plasma Mass Spectrometry. J. Anal. Atom. Spectrom., 17(2002) 587-591. 

More recently, Wells and Goldberg (1991) report that very small marine colloids (d < 120 nm) are, by at least three order of magnitudes, more abundant. A vertical stratification of these particles was found (very high concentrations in the thermodine and season-dependent in the bottom-waters). This stratification indicates that these very small colloidal particles are reactive. The apparent close association of metals with these colloids suggests that they may play an important part in the transport and fate of trace elements in seawater (Wells and Goldberg,... 

The concentrations of these elements in seawater are extremely low. Radionuclides are primarily used as tracers to study water circulation, par-... 

At equilibrium, the reactant concentrations and products can be used to define a mass ratio called an equilibrium constant (A). This constant can then be used to predict the equilibrium concentrations of the reactants and products from the total amount of C or from either the equilibrium concentration of the products or the reactants. Although K is referred to as an equilibrium constant, it is a function of salinity, temperature, and pressure. With the appropriate value of K, calculations can be made to predict the equilibrium speciation of elements in seawater. The procedure for doing this is provided in the next section along with an expansion of K to multicomponent chemical systems. 

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