Speciation salinity

When TBTO is released into ambient water, a considerable proportion becomes adsorbed to sediments, as might be expected from its lipophilicity. Studies have shown that between 10 and 95% of TBTO added to surface waters becomes bound to sediment. In the water column it exists in several different forms, principally the hydroxide, the chloride, and the carbonate (Figure 8.5). Once TBT has been adsorbed, loss is almost entirely due to slow degradation, leading to desorption of diphenyl-tin (DPT). The distribution and state of speciation of TBT can vary considerably between aquatic systems, depending on pH, temperature, salinity, and other factors. 

Ferguson, E.A. and C. Hogstrand. 1998. Acute silver toxicity to seawater-acclimated rainbow trout influence of salinity on toxicity and silver speciation. Environ. Toxicol. Chem. 17 589-593. 

In natural waters, dissolved zinc speciates into the toxic aquo ion [Zn(H20)6]2+, other dissolved chemical species, and various inorganic and organic complexes zinc complexes are readily transported. Aquo ions and other toxic species are most harmful to aquatic life under conditions of low pH, low alkalinity, low dissolved oxygen, and elevated temperatures. Most of the zinc introduced into aquatic environments is eventually partitioned into the sediments. Zinc bioavailability from sediments is enhanced under conditions of high dissolved oxygen, low salinity, low pH, and high levels of inorganic oxides and humic substances. 

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. 

Effect of temperature, pressure, and salinity on speciation of the dissolved inorganic carbon for 2co2 = 2mmol/kg. Source After Zeebe, R.E. and D. Wolf-Gladrow (2001) Elsevier Oceanography... 

Vertical concentration profiles of (a) temperature, (b) potential density, (c) salinity, (d) O2, (e) % saturation of O2, (f) bicarbonate and TDIC, (g) carbonate alkalinity and total alkalinity, (h) pH, (i) carbonate, ( ) carbon dioxide and carbonic acid concentrations, and (k) carbonate-to-bicarbonate ion concentration ratio. Curves labeled f,p have been corrected for the effects of in-situ temperature and pressure on equilibrium speciation. Curves labeled t, 1 atm have been corrected for the in-situ temperature effect, but not for that caused by pressure. Data from 50°27.5 N, 176°13.8 W in the North Pacific Ocean on June 1966. Source From Culberson, C., and R. M. Pytkowicz (1968). Limnology and Oceanography, 13, 403-417. 

The solubilized metals form complexes with organic and inorganic anions. The chemical speciation of these complexes changes as the metal moves seaward through the estuary due to increasing salinity. These shifts can be predicted from equilibrium speciation calculations as described in Chapter 5.7. Two examples are shown in Figure 28.23 for... 

Dissolved inorganic trace metal (e.g., Hg, Ag, Cd, Ni, Zn) speciation and the tendency to form stable soluble complexes in saline water was considered by Turner... 

Demianov, P D. DeStefano, A. Gianguzza, and S. Sammartano, Equilibrium studies in natural waters Speciation of phenolic compounds in synthetic seawater at different salinities , Environ. Toxicol. Chem, 14,767-773 (1995). 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Table VI. Salinity Indices (Mass Ratios) Based on S Speciation in Lake Sediments...

Inorganic speciation in solution can also affect the mobility of metal ions (Doner, 1978). The formation of an ion-pair with Cl can more than double the mobility of Cd in the presence of 200molm 3NaCl. At the same chloride concentration, however, the mobilities of Cu2+ and Ni2+ are only increased slightly (5-10%), presumably because of very weak complexation with Cl. This mechanism could increase the leaching of Cd from saline soils but it may not be effective in non-saline soils because the ratio of the total concentrations of Cd Cl must be >1 106 before >50% of total Cd is complexed by Cl (estimated using the computer model TITRATOR (Cabaniss, 1987), which considered the chloro and hydroxy complexes of Cd at pH 5.0 and a total Cd concentration of 0.1 mmolm-3 equilibrium constants were taken from Lindsay (1979)). 

In a review of available data relating to the physico-chemical speciation of plutonium in the Irish Sea and western Mediterranean, Mitchell et ai. (1995) concluded that a high percentage of the plutonium is present as Puv and not retained by a 1 kD filter, thus demonstrating that plutonium in the oxidised state is in true solution. The data also indicate that a significant proportion of plutonium in the reduced state is associated with colloids and that the size of the colloidal particles or aggregates involved (<10kD) is considerably smaller than those observed in non-saline waters. 

Computer simulation is now used extensively as a tool to help to understand and predict the transport of radionuclides through environmental systems. Most models relate to waste disposal and are based on measured parameters such as water movements, salinity, suspended load and the radionuclide concentration in the solute, suspended particulate matter and bottom deposits. Comparatively few attempts appear to have been made to include chemical speciation into this type of model, presumably because of the added complexity involved. Some modellers have attempted to take into account the characteristics of the major chemical phases such as those present in different particles or coatings (e.g. Martinez-Aquirre et al., 1994). Others have noted the importance of including details of particular chemical species present in industrial waste releases when constructing models to predict dispersion (Abril and Fraga, 1996). 

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