Dissolved species, speciation

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. 

The chemical speciation and transport of the dissolved species involved in... 

Figure 2. Eh-pH diagram of dissolved Mo speciation in the system M0-H2O-S. ZMo = 10 M ES = 10 M. Modified after Manheim and Landergren (1974), using molybdate protonation constants from Baes and Mesmer (1986). H2M0O4 is related to Mo(OH)g by addition of two water molecules (see text). MoO +, included in earlier Eh-pH diagrams, is omitted because this and other Mo(V) species are typically unstable except as dimers (e.g., Mo20/ ) at higher EMo than common in natiwe. Speciation at Eh below fiie SO/ - H2S ftansition is not well characterized and is commonly out of equilibrium. The boundary between MoS/ and MoO/ is based on Erickson and Helz (2000) intermediate oxythiomolybdates are metastable and hence not indicted.

MINTEQA2 http //www.epa.gov/ceampubl/mmedia/minteq/index.htm MINTEQA2 is an equilibrium speciation model that can be used to calculate the equilibrium composition of dilute aqueous solutions in the laboratory or in natural aqueous systems. The model is useful for calculating the equilibrium mass distribution among dissolved species, adsorbed species, and multiple solid phases under a variety of conditions including a gas phase with constant partial pressures. 

At the pH and ionic strength of seawater, the dominant dissolved species of silicon is orthosilicic acid [H4Si04(aq) or Si(OH)4(aq)]. The speciation of silicic acid is shown in Figure 5.19. At the pH of seawater, a minor amount of dissociation occurs, such that about 5% of the dissolved silicon is in the form of HjSiO faq). Dissolved organic complexes of silicon do not occur naturally. 

Hydrolysis reactions of Am(in) and Pu(VI) ions in CO2-free solutions of 0.1 M NaC104 were studied by means of solubility experiments using the oxide or hydroxide of 241 Am and 238Pu. The pH of solutions was varied from 3 to 13.5. All experiments were carried out under an argon atmosphere. The speciation of dissolved species was determined as far as possible by spectrophotometry. Various ultrafiltration membranes were applied to examine the proper phase separation. Stability constants of all possible hydrolysis products are presented and compared with literature data. 

Speciation of Dissolved Species. Since there is no straightforward method to identify the dissolved species in solubility experiments, indirect approaches were applied in this experiment. 

A number of studies have been reported in the literature in which theoretical aqueous speciation of key radionuclides has been determined from available thermodynamic data (4.9.21-25). In general, where comparisons are possible, one finds reasonably close agreement in the calculated dominant dissolved species, the solids controlling solubility, and computed total solubilities for this study and those listed above. 

In addition to the soluble chemical species and possible solid phase species described in the previous sections no discussion on speciation can be complete without the consideration of surface species. These include the inorganic and organic ions adsorbed on the surface of particles. Natural systems such as soils, sediments and waters abound with colloids such as the hydrous oxides of iron, aluminium, manganese and silicon which have the potential to form surface complexes with the various cationic and anionic dissolved species (Evans, 1989). 

Speciation encompasses both the chemical and physical form an element takes in a geochemical setting. A detailed definition of speciation includes the following components (1) the identity of the contaminant of concern or interest (2) the oxidation state of the contaminant (3) associations and complexes to solids and dissolved species (surface complexes, metal-ligand bonds, surface precipitates) and (4) the molecular geometry and coordination environment of the metal.5 The more of these parameters that can be identified the better one can predict the potential risk of toxicity to organisms by heavy metal contaminants. Prior to the application... 

Another example of aqueous speciation that includes redox can be shown with the arsenic pe-pH diagram shown in Figure 1. Arsenic can exist in several oxidation states including As(-lll) as in arsine gas (ASH3), As(0) as in elemental arsenic, As(ll) as in realgar (AsS), As(lll) as in orpiment (AS2S3) and dissolved arsenite, and As(V) as in dissolved arsenate. Figure 1 shows the dominant dissolved species, arsenate and arsenite, and their hydrolysis products as a function of redox potential and pH based on the thermodynamic evaluation of Nordstrom and Archer (2003). These results show the dominance of hydrolysis for arsenate species, but it is of minor consequence for the arsenite species. 

ASV and DPASV are still useful speciation methods because of their sensitivity and their ability to distinguish labile metal species from strong (non-labile) organic complexes. Furthermore, these techniques are very useful to distinguish dissolved species from species bound to colloidal or particulate phases (Gonsalves et al., 1985). For recent applications see Bruland (1989, 1992), Donat and Bruland (1990), and Muller and Kester (1990, 1991). 

The speciation, concentrations and residence times of dissolved substances in natural waters are dependent on many factors and processes. Important factors Include temperature, pH, redox potential, ionic strength and the concentrations of other dissolved species such as organic and Inorganic ligands as well as the presence of suspended particulate and colloidal matter. Important processes in addition to rate of input, and biochemical cycling include precipitation, complexatlon, coagulation and adsorption onto suspended particulate matter. 

Leckie [14] emphasized the advantage of chemical speciation over overall distribution coefficients in adsorption modeling. On the other hand, in many theoreticar studies of adsorption even the speciation in solution is neglected and only the total concentration of dissolved species is taken into account. One probable reason of paying no attention to well-known experimental facts is that some authors use adsorption equations borrowed from gas adsorption, and obviously these equations are not suitable to deal with multiple solution species involving the adsorbate. 

Many different complexes of elements in a given oxidation state may exist in water. The amphoteric nature of Al(III) and Fe(III) results from the formation of a series of dissolved species, MOH, M(OH)J, M(0H)3, and other forms, in addition to the more common M ". The speciation of soluble A1 and Fe is thus a sensitive function of pH. 

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