Physico-chemical speciation

Size range Metal species Examples Phase state 

1-10 nm Inorganic ion pairs Inorganic complexes Low molecular mass organic complexes Nice HgCh4- Zn-fulvates Soluble 

10-100 nm High molecular mass organic complexes Pb-humates Colloidal 

100-1000 nm Metal species adsorbed onto inorganic colloids Metals associated with detritus Co-Mn02 Pb-Fe(OH)3 Particulate 

Complex formation is an equilibrium process. Ignoring charges for the general case of a metal, M and ligand, L, complex formation occurs as 

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Berthet, B., J.C. Amiard, C. Amiard-Triquet, M. Martoja, and A.Y. Jeantet. 1992. Bioaccumulation, toxicity and physico-chemical speciation of silver in bivalve molluscs ecotoxicological and health consequences. Sci. Total Environ. 125 97-122. 

Laxen, D.P.H. and Harrison, R.M. (1981) A scheme for the physico-chemical speciation of trace metals in freshwater samples. Sci. Total Environ., 19, 59-82. 

Tanizaki, Y., Shimokawa, T. and Yimazaki, M. (1992a) Physico-chemical speciation of trace elements in urban streams by size fractionation. Water Res., 26, 55-63. 

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. 

Mitchell, P.I., Battle, J., Vives I., Downes, A.B., Condren, O.M., Leon Vintro, L. and Sanchez Cabeza, J.A. (1995) Recent observations on the physico-chemical speciation of plutonium in the Irish Sea and the western Mediterranean. Appl. Radiat. Isot., 46(11), 1175-1190. 

Laxen, D.RH. and Harrison, R.M. (1981) The physico-chemical speciation ofCd, Pb, Cu, Fe and Mn in the final effluent of a sewage treatment works and its impact on speciation in the receiving river. Water Res., 15, 1053-1056. 

Physico-chemical speciation refers to the various physical and chemical forms in which an element may exist in the system. In oceanic waters, it is difficult to determine chemical species directly. Whereas some individual species can be analysed, others can only be inferred from thermodynamic equilibrium models as exemplified by the speciation of carbonic acid in Figure 9. Often an element is fractionated into various forms that behave similarly under a given physical (e.g., filtration) or chemical (e.g., ion exchange) operation. The resulting partition of the element is highly dependent upon the procedure utilised, and so known as operationally defined. In the following discussion, speciation will be exemplified with respect to size distribution, complexation characteristics, redox behaviour and methylation reactions. 

A component can undergo considerable physico-chemical speciation alterations in an estuary. With respect to dissolved constituents, the composition and concentration of available ligands changes. Depending upon the initial pH of the riverine water, OH may become markedly more important down the estuary. Similarly, chlorocomplexes for metals such as Cd, Hg and Zn become more prevalent as the salinity increases. Conversely, the competitive influence of seawater derived Ca and Mg for organic material decreases the relative importance of humic complexation for Mn and Zn. 

Level 2 In the second level (Level 2), physico-chemical speciation models are introduced in order to correct the toxicity data for chemical availability. Indeed, NOEC and/or ECm values that are used in the effects assessment are generally generated in test media with varying physico-chemical characteristics (e.g. pH, hardness, DOC) known to alter metal availability and toxicity. In case metal concentrations are reported and appropriate speciation models (e.g. WHAM, MINTEQA2, etc.) and relevant input data (i.e. main physico-chemical parameters driving the availability of a metal such as pH, DOC, etc.) are available, NOEC and/or ECm values should be expressed on the basis of the metal species of concern in order to reduce uncertainty. Eor regulatory compliance purposes, the dissolved exposure concentrations should also be translated at the same level of availability (expressed in the same units) as the effects assessment. 

The major part of the biosphere is aerobic and consequently priority has been given to the study and assessment of biodegradability under aerobic conditions. Nevertheless, there are environmental compartments that can be permanently (e.g. anaerobic digesters) or temporarily anaerobic (e.g. river sediments and soils) and surfactants do reach these. The majority of surfactants entering the environment is exposed to and degraded under aerobic conditions. This is the predominant mechanism of removal even in cases of absence of wastewater treatment practices (direct discharge) and it is estimated that less than 20% of the total surfactant mass will potentially reach anaerobic environmental compartments [1]. Only in a few cases, however, will the presence of surfactants in these compartments be permanent. The presence of surfactants in anaerobic zones is not exclusively due to the lack of anaerobic degradation. Physico-chemical factors such as adsorption or precipitation play an important role as well as the poor bioavailability of surfactant derivatives (chemical speciation) in these situations. 

Many different separation and detection systems have been used for speciation. For example, size fractionation and ultra-filtration have been used for separation with the separated species then being determined by neutron activation (Tanizaki et al., 1992). These physico-chemical separation processes are, however, time consuming and the species have to be collected and then determined separately. Although the techniques are invaluable for certain types of speciation where the interaction of the species with colloids and sediments is important, hybrid or coupled techniques are usually preferred. 

Plasma MS is usually based on quadrupole mass analysers. The atmospheric ICP, optimised for ion formation, is placed on its side facing a sample cone (Fig. 4.3). The mass spectrometer operates at reduced pressure and therefore a two- or three-stage differentially pumped interface is needed to transfer the ions from the plasma to the mass analyser. The interface for GC-ICP-MS is generally the same as for ICP emission systems. In one of the earliest GC-MS speciation studies (Chong and Houk, 1987) a packed GC column was used to obtain mass spectra of organic compounds with detection limits in the range 0.001-500 ngs The effects of isotopic fractionation by natural physico-chemical processes were also studied. 

Those synthetic and mechanistic studies led inevitably to investigation of the complex ionisation processes in H2S04 and the oleums and thence to characterisation of speciation of solutes in these media. This work has been reviewed by R.J. Gillespie [1] who led much of the pioneering work on inorganic and physico-chemical investigation of H2S04—S03 solvent systems and of later superacid systems. 

The modeling results suggest that the physico-chemical state of the oxide/water interface has a significant effect on the speciation of ions at the surface. For example, adsorbed metal ions may be more easily hydrolyzed (. . at lower pH) than aquo metal ions in bulk solution. Consider the following surface equilibria of Cu(II) (2), i.e.. 

Refinement of the model awaits further experimental work on the physico-chemical nature of surface bonding of ions within the compact layer of the EDL. At present our conclusions concerning the speciation of adsorbed ions are supported by 1) enthalpy/ entropy arguments for analogous reactions in solution, and 2) a limited knowledge of the solvent medium of the compact layer of the EDL. 

The speciation of trace metals in soils is related to their biogeochemical reactivity and to several physico-chemical conditions of the soil. The determinations of various fractions of these elements are broadly... 

The physico-chemical characterisation of the species in the case of, for example, the determination of the form in which it is present, represents a substantially more demanding field of trace analysis than total trace contents. According to Florence speciation analysis of an element in a water sample may be defined as the determination of the concentrations of the different physico-chemical forms of the element which together make up its total concentration in the sample . 

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