Complexation in natural waters

Nordstrom, D.K. and Ball, J.W., Chemical models, computer programs and metal complexation in natural waters, in Complexation of Trace Metals in Natural Waters, Kramer, C.J.M. and Duinker, J.C., Eds., Martinus, 1984. 

The ferrocyanide complex is not easily biodegradable (Belly and Goodhue 1976 Pettet and Mills 1954). However, when an aqueous solution of potassium ferrocyanide was seeded with pure culture of Pseudomona aeruginosa, E. coli, or a mixture of the two bacteria, formation of free cyanide was observed after a delay period of 2 days (Cherryholmes et al. 1985). The rate of free cyanide formation increased with addition of nutrient in water, and a free cyanide concentration <4,000 pg/L was detected at the end of 25 days. It was shown that the free cyanide formation was due to biodegradation and not to either photolysis or hydrolysis. The relevance of this study to the fate of ferrocyanide complexes in natural water or industrial effluents is difficult to assess because cyanide concentrations used in these experiments (3,300 mg/L) are rarely encountered in these media. 

Recent reviews on chemical speciation are published by e.g. Stumm and Brauner (1975), Florence and Batley (1980) and Leppard (1983) sometimes, with special reference to metal-organic interactions (Mantoura, 1982) or complexation in natural waters (Kramer and Duinker, 1984b). Bruland (1983) summarized the distribution and behaviour of trace elements in ocean waters. The occurrence of certain species is largely dependent on the environmental conditions. There exists a strong competition of trace metals with H+ or major cations like Ca2+ and Mg2+ in seawater, but also with other trace metals which might form more stable complexes with the ligand in question on the other side, many potential ligands or chelators compete for one trace element. 

Nurnberg, H.W., 1984. Potentialities of voltammetry in the study of physicochemical aspects of heavy metal complexation in natural waters. In C.3.M. Kramer and 3.C. Duinker (eds), Complexation of Trace Metals in Natural Waters. Nijhoff/3unk Publ., the Hague, pp. 95-115. 

Lerman, A. and Childs, C.W. Metal-organic complexes in natural waters control of distribution by thermodynamic, kinetic and physical factors, p. 201-236, in Singer, P.C., ed., "Trace Metals and Metal-Organic Interactions in Natural Waters". Ann Arbor Science, Ann Arbor, Michigan, 1973. 

Gaffney, J. S., Marley, N. S., and Orlandini, K. A., 1992, Evidence for thorium isotopic disequilibria due to organic complexation in natural waters, Environ. Sci. Technol. 26 1248. 

Analytical Difficulties Although we can make many chemical equilibrium models that predict the existence of complexes in natural waters, analytically one encounters difficulties in identifying unequivocally the various solute species and in distinguishing between dissolved and particulate concentrations. The analytical task is rendered veiy difficult because the individual chemical species are often present at nano- and picomolar concentrations. The ion-selective electrode (ISE), if it were sufficiently sensitive, would permit the measurement of free metal-ion activity. 

Many environmentally important chemicals are transported as complexes in natural waters. Complexes may increase or decrease the toxicity and/or bioavailability of elements. Complexation increases the solubility of minerals and may increase or decrease the adsorption of elements. The major monovalent and divalent cations and anions (especially > 10 m) form outer-sphere complexes or ion pairs, in which the bonding is chiefly long-range and electrostatic. Ion pairs are unimportant in dilute fresh waters, but become important in saline waters such as seawater. Minor and trace ions such as Cu, Fe +, Pb +, and Hg are usually complexed, and occur in inner-sphere complexes, which are usually much stronger complexes than the ion pairs. Written in terms of Gibbs free energy. 

Which actinide cations form important organic complexes in natural waters and which do not. Why ... 

For natural waters and/or effluent applications, some interference may occur with the presence of natural or anthropogenic chelating agents. For example, humic substances can compete with dithizone for metallic constituent complexation in natural waters [60]. In this case, the degradation of organometallic complexes must be effective before the analytical determination. A photodegradation step, with a simple device as the one already proposed in this chapter for N and P compounds determination, can be used... 

Because ground waters, like other natural waters, are dilute solutions of many compounds, metal speciation measurements are difficult. Therefore, the metal-complexing properties of natural waters are operationally defined by many factors, including the analytic method used for speciation, the conceptual and mathematical models used to analyze the data, the range of titrant metal concentrations used, and conditions such as pH, ionic strength, and temperature. The analytical methods used to determine metal speciation all have inherent assumptions and limitations. Most published studies of metal complexation in natural waters have used one analytical method. However, confirmation of results (e.g. stability constants, ligand concentrations) by independent methods would add confidence to such results. In the present work, three independent methods were used. 

Some published Cu complexation studies have been performed at a pH of 6 (9,10), probably to avoid hydrolysis and carbonate complexation, and at an ionic strength of 0.1. The pH and ionic strength both affect metal complexation by natural organic matter (11). Therefore, for reahstic studies of metal complexation in natural waters, it is desireable to keep both parameters as close to their natural values as possible. In this work, the experiments were performed at the natural pH values of the ground waters (approximately 8.0) as well as at lower values for comparison with published results. The ionic strength used was 0.02 eq L which was as close as possible to the natural values of approximately 0.01 eq L ... 

Organo-metallic complexes in natural waters have mainly been characte ised using five operational classifications size, solubility, stability, degradability and lability of the complexes with respect to a wide variety of analytical conditions (Stumm and Brauner, 1975 Whitfield, 1975). 

Biomaterials such as natural gums are extracted from living matter. The molecules forming these biomaterials are known to be very complex in nature. Water content in biomaterials is an essential characteristic of them. The water content plays a crucial role in its physical properties like electrical conduction through it. Since these materials are either a covalent or a hydrogen bonded system they cannot be used and tested at temperatures above 120°C. It is apparent, therefore, that not all conventional methods of material characterization can be applied. Thus, as a method of material characterization, some of the conventional methods are used in a restricted way so as to retain the biomaterial characteristics. The characterization method used in the study of natural gum Arabica is summarized in the following sections. 

The cyanide is not detected by the conductivity detector of the ion chromatograph due to its low dissociation constant (pK = 9.2) [64]. An ion chromatography procedure has been used for the determination of free cyanide and metal-cyanide complexes in natural water and wastewater samples using oxidation of cyanide ion by sodium hypochlorite to the cyanate ion (pK = 3.66) and a conductivity detector. So, cyanide ions can now be measured indirectly by the conductivity detector. In this procedure, optimum operating conditions were examined. 

An alternative HPLC method for separation of metal ions is ion-pair or ion-interaction chromatography, working with a reversed-phase column [269,310]. Furthermore, the use of gel filtration chromatographic method was opportunely reported, with sequential UV adsorption and ICP-MS detection, to determine large organic complexes in natural waters [311,312]. 

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