Speciation natural waters

METHODOLOGICAL APPROACHES TO THE INVESTIGATION OF THE METAL SPECIATION IN THE NATURAL WATER... 

A development of the study of adsorption currents is referred to as Tensammetry, and a useful review50 refers to the application of this technique to the examination of natural waters as an aid to speciation , the procedure for deciding how a metal ion is distributed between the various species in which the metal may be present. 

Table 15-6 Model results for metal speciation in natural waters ...

Equilibrium complexation constants for Cu reactions with natural organic matter and the details of Cu speciation are bound to remain somewhat uncertain, since the composition of the complexing molecules varies from site to site. What is not in dispute is that the fraction of dissolved copper present as free aquo Cu is probably very small in any natural water. In extremely pristine waters, hydroxide and carbonate complexes may dominate, but organic complexes usually dominate in waters containing more than a few tenths of a mg/L organic carbon. 

TBTO is a colorless liquid of low water solubility and low polarity. Its water solubility varies between <1.0 and >100 mg/L, depending on the pH, temperature, and presence of other anions. These other anions determine the speciation of tributyltin in natural waters. Thus, in sea water, TBT exists largely as hydroxide, chloride, and carbonate, the structures of which are given in Figure 8.5. At pH values below 7.0, the predominant forms are the chloride and the protonated hydroxide at pH8 they are the chloride, hydroxide, and carbonate and at pH values above 10 they are the hydroxide and the carbonate (EHC 116). 

Ra. Exceptions to this are environments where Rn is lost from the system by degassing (e.g., see Condomines et al. 2003), or aqueous systems where the insoluble nature of °Pb leads to its preferential removal. The speciation of Pb in natural waters is rather complex and heavily depends on the availability of organic complexing agents for which Pb has the highest affinity. In the oceans, Pb has a very short residence (30-150 yrs) and is rapidly scavenged by particles. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

Ball, J. W, E. A. Jenne and D. K. Nordstrom, 1979, WATEQ2 - a computerized chemical model for trace and major element speciation and mineral equilibria of natural waters. In E. A. Jenne (ed.), Chemical Modeling in Aqueous Systems, American Chemical Society, Washington DC, pp. 815-835. 

Ball, J. W. and D. K. Nordstrom, 1991, User s manual for WATEQ4F, with revised thermodynamic data base and test cases for calculating speciation of major, trace, and redox elements in natural waters. US Geological Survey Open File Report 91-183. 

Most measurements of silver concentrations in natural waters prior to the use of clean techniques are considered inaccurate. Until analytical capabilities that exceed the dissolved-particulate classification are developed, it will be necessary to rely on laboratory and theoretical modeling studies to fully understand chemical speciation of silver in natural waters (Andren et al. 1995). 

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. 

Simple organic molecules such as small carboxylic acids (oxalate, acetate, malonate, citrate, etc.), amino acids and phenols are all ligands for metals. Such compounds may all occur as degradation products of organic matter in natural waters. The complexes formed are typically charged hydrophilic complexes. The stability of the metal complexes with these ligands is, however, moderate in most cases. Model calculations including such compounds at realistic concentrations indicate that their effects on speciation are relatively small [29],... 

Town, R. M. and Filella, M. (2000). Dispelling the myths is the existence of LI and L2 ligands necessary to explain metal ion speciation in natural waters Limnol. Oceanogr., 45, 1341-1357. 

Hart, B. T. (1982). Trace metals in natural waters. I. Speciation, Chem. Aust., 49, 260-265. 

Bioaccumulation is a complicated process that couples numerous complex and interacting factors. In order to directly relate the chemical speciation of an element to its bioavailability in natural waters, it will be necessary to first improve our mechanistic understanding of the uptake process from mass transport reactions in solution to element transfer across the biological membrane. In addition, the role(s) of complex lability and mobility, the presence of competing metal concentrations and the role(s) of natural organic ligands will need to be examined quantitatively and mechanistically. The preceding chapter... 

Pei, J., Tercier-Waeber, M.-L. and Buffle, J. (2000). Simultaneous determination and speciation of zinc, cadmium, lead and copper in natural waters with minimum handling and artefacts by voltammetry on gel-integrated microelectrode arrays, Anal. Chem., 72, 161-171. 

The solid-water interface, mostly established by the particles in natural waters and soils, plays a commanding role in regulating the concentrations of most dissolved reactive trace elements in soil and natural water systems and in the coupling of various hydrogeochemical cycles (Fig. 1.1). Usually the concentrations of most trace elements (M or mol kg-1) are much larger in solid or surface phases than in the water phase. Thus, the capacity of particles to bind trace elements (ion exchange, adsorption) must be considered in addition to the effect of solute complex formers in influencing the speciation of the trace metals. 

Van den Berg, C. M. G., and J. R. Kramer (1979), "Conditional Stability Constants for Copper Ions with Ligands in Natural Waters", in E. Jenne, Ed., On Chemical Modeling Speciation, Sorption, Solubility and Kinetics in Aqueous Systems, ACS Symp. Series. 

In freshwater, Mn(II) oxidation is slightly slower than in 0.1M NaClO. The difference between the Mn(II) oxidation rate in freshwater and 0.1M NaCIO, is greatest at pH 8.5, at this pH the rate of Mn(II) oxidation is only 40% lower in the freshwater than in 0.1M NaClO. In the estuarine-water at pH 8.5 the rate of Mn(II) oxidation is 20 times slower than in 0.1M NaCIO,. The speciation calculations indicate why the model predicts the oxidation is slower than in natural waters (see, for example Table VII). 

Matthews A, Morgans-Bell HS, Emmanuel S, Jenkyns HC, Erel Y, Halicz L (2004) Controls on iron-isotope fractionation in organic-rich sediments. Geochim Cosmochim Acta, in press Millero FJ, Yao W, Aicher J (1995) The speciation of Fe(II) and Fe(III) in natural waters. Marine Chemistry 50 21-39... 

Table 5.4 Examples of Commercially Available and Free Software for Computing Aqueous Chemical Speciation in Natural Waters.

A USGS model for computing the major and trace element speciation and mineral saturation for natural waters. 

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