Seawater redox state

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Figure 8.35 shows the redox state and acidity of the main types of seawaters. The redox state of normal oceanic waters is almost neutral, but they are slightly alkaline in terms of pH. The redox state increases in aerated surface waters. Seawaters of euxinic basins and those rich in nutrients (eutrophic) often exhibit Eh-pH values below the sulfide-sulfate transition and below carbonate stability limits (zone of organic carbon and methane cf figure 8.21). We have already seen (section 8.10.1) that the pH of normal oceanic waters is buffered by carbonate equilibria. At the normal pH of seawater (pH = 8.2), carbonate alkalinity is 2.47 mEq per kg of solution. 

Goldberg has presented impressive information (given that seawater concentrations are as low as 1.5 pg for Ir and 2 pg for Ru) on the speciation, including redox state, of platinum group metals as a means of interpreting distributions in seawater and marine sediments. Pt and Pd are stabilised in seawater as tetrachloro-divalent anions. Their... 

The redox conditions, of course, have to be considered also. A number of elements with several redox states have a very different speciation under oxic and anoxic conditions. We will discuss inorganic seawater speciation in Section 6.8. 

Species of the more soluble and kineticaUy labile Fe(II) redox state are intermittently present in seawater as a result of Fe(III) reduction by a variety of processes in different ocean environments. Chemical and/or microbial reduction of Fe(III) occurs on a large scale in anoxic basins and sediments (Sections 3.1.4 and 3.3.4) and on a microscopic scale within the fecal pellets of zooplankton. In the surface ocean reduction occurs via absorption of high visible-low UV light (photo-reduction) [51,59-65], and via biologically-mediated reactions at cell surfaces [12,66-68]. 

Aquatic chemists have defined their own electrochemical standard state to fecilitate calculation of redox speciation in aqueous solutions. In this standard state, all reactions are conducted at pH 7.0, 25°C, and 1 atm. The concentrations of all other solutes are 1 molal (unless otherwise specifically noted). Values so obtained are designated with the subscript w. The pe s for the most important redox couples in seawater are given in Table 7.4. 

If ammonium concentrations in seawater are low, phytoplankton will assimilate nitrate and nitrite using chemical-specific permeases. Once inside the cell, these DIN species are transformed into ammonium via redox reactions in which nitrogen is reduced to the -III oxidation state ... 

The kinetics of the oxidation of Cr(III) and Cu(I) have been discussed before. Cr(VI) is reduced by dissolved organic matter, the slow re-oxidation resulting in a large enough ti for an existence of Cr(III). Also the existence of Cu(I) in seawater is a steady state between the reduction- and back-oxidation reactions. The lifetime is dependent on pH, PC>2, complexing ligands and redox intermediates such as H2O2 (Moffet and Zika, 1983). 

Thallium (Tl), which appears to exhibit conservative behaviour in seawater, has two potential oxidation states. As Tl1, thallium is very weakly complexed in solution. In contrast, Tl111 should be strongly hydrolysed in solution ([T13+]/[T13+]t — 10 20 5) with Tl(OH)3 as the dominant species over a very wide range of pH. The calculation of Turner et at. (1981) indicated thatTl111 is the thermodynamically favoured oxidation state at pH 8.2. Lower pH and p()2 would be favourable to Tl1 formation. Within the water column, pH can be considerably less than 8.2 and /)( )2 lower than 0.20 atm. In view of these factors, and the observation that redox disequilibrium in seawater is not uncommon, the oxidation state of Tl in seawater is somewhat uncertain. The existence of Tl in solution as Tl+, a very weakly interactive ion, would reasonably explain the conservative behaviour of Tl in seawater. However, the extremely strong solution complexation of Tl3+ suggests that Tl3+ may be substantially less particle reactive than other Group 13 elements (with the exception of boron). 

The data in Figures 1 and 2 are consistent with the existence of Pu(V) as the dominant oxidation state in solution in seawater. It is not possible to state whether this represents thermodynamic equilibrium or some balance of opposing redox conditions which results in a steady state concentration of Pu(v). Pu(Vl) can be rapidly reduced to Pu(V). However, in the presence of humic materials a significant fraction of the Pu(Vl) is reduced directly to Pu(lV) which hydrolyzes and sorbs to the walls and to particulate matter. Apparently there is competition between reduction of Pu(Vl) to Pu(V) by seawater and complexatlon of Pu(Vl) by humic acid (16). The latter results in rapid reduction of Pu(Vl) to Pu(lV) with subsequent hydrolysis. Complexatlon of Pu(V) by humic acid should be much weaker and may account for the slow reduction of Pu(V) in the dark. Photolysis of the humic material apparently results in some oxidation of Pu(lV) to Pu(V), to provide a metastable Pu(V) concentration. 

Like iron complexes, copper complexes have been shown to be an important sink for photochemically-generated superoxide in seawater and, based on the high reactivity of Cu(ii) complexed by cyanobacterial-derived ligands, it is likely that redox reactions with superoxide significantly influence Cu redox speciation in the ocean [221,222]. These reactions also have important effects on the steady state concentrations of superoxide in seawater, reducing the concentration by at least an order of magnitude compared to previous estimates that ignored the reactions with copper complexes [222]. 

The most stable and predominant form of dissolved phosphate in marine sediments is orthophosphate (henceforth referred to as phosphate) with an oxidation state of -E5. Phosphate, unlike nitrogen (oxidation state varying between — 3 and -E 5) and sulfur (oxidation state —2 to -E6), is not directly involved in redox reactions. Phosphate in solution is chiefly present as ion pairs with the major cations of seawater (Kester and Pytkowicz, 1967). In sediments it is also found in organic matter, adsorbed on hydrous ferric oxides (Mortimer, 1941, 1971 Mackereth, 1966 Stumm and Leckie, 1970), adsorbed on clay minerals (Chen, 1972), and as various forms of apatite. 

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