Mobilisation of metals

A further area in which sequential extraction continues to be applied successfully is in assessment of the likelihood of mobilisation of metal contaminants from sediment-derived soil. When dredged sediment is used to reclaim land from the coastal margins or applied to arable soil to improve fertility, there is concern that potentially toxic elements accumulated under reducing conditions may be released on exposure to an oxygen-rich environment. Sequential extraction can be used to characterise the sediment prior to application, or to monitor changes in metal availability in the soil with time (e.g. Singh et al, 1998). 

When flooding an abandoned mine processes of oxidation and reduction are of great importance. Due to the supply of oxygen, protons and sulfate are formed changing the chemistry of ground water fundamentally, e.g. mobilisation of metals as a result of the decrease of the pH value ( acidification ). 

Furan hardeners Toluene-, Xylene-, Benzene-sulphonic acids Could leach into groruidwater or alter soil properties, e.g. mobilisation of metals... 

Speciation in solution is considered a major factor in the mobilisation and leaching of metal cations (DeKoninck, 1980 Bloomfield, 1981 Stevenson and Fitch, 1986). Complexation increases the total soluble concentration of a metal and hence increases its potential to be leached. Organic ligands (e.g. humate, ful-vate, citrate, polyphenols) are the major complexers involved in this mechanism, but they are effective only if the soluble organic complex does not become saturated and precipitate (DeKoninck, 1980). 

In assessing environmental risks, however, the lack of specificity can be not so crucial. It might be more interesting to get information on the possibility of mobilisation or on bioavailability of PTMs than to identify the exact chemical species of metals in soil. It could be not so important to know whether the metals come from sulphides or organic matter, but could be more useful to know that they can be released under reducing or oxidising conditions . 

Sternbeck, J., Skei, J., Verta, M. and 0stlund, P. (1999) Mobilisation of sedimentary trace metals following improved oxygen conditions - an assessment of the impact of a lowered primary productivity on trace metal cycling in the marine environment. TemaNord, 594, 65 pp. 

The environmental impact of metallic contaminants in soils is dependent both on the chemical speciation of the metal and the response of the matrix to biological and physicochemical conditions. These factors are responsible for the mobilisation of the metal from the solid into the aqueous phase hence transport within the immediate vicinity, has an impact on the rate of dispersal, dilution, uptake and transfer into living systems. The impact of changing environmental conditions on the contaminant inventory can be to enhance or moderate these phenomena, with subsequent consequences for the broader risk assessment of the contaminants. During the last ten years, extensive research in analytical chemistry was initiated to develop highly specific and sensitive methods for measuring potentially harmful substances in various... 

Capacity controlling properties. Both pH and redox potential in sediment/water systems are significant parameters for mobilisation and transformation of metals or phosphorus. Criteria for prognosis of the middle- and long-term behavior of these and other substances should, therefore, include the abilities of sediment matrices to produce acidity and to neutralise such acid constituents (Kersten and Forstner, 1991 Salomons, 1995). 

Cadmium is of particular note partly because there is more understanding regarding its mobilisation mechanism/ but also because of its toxicity and ability to accumulate up the food chain The mobilisation of cadmium is dependent upon the pH of the soil solution/ and unlike other heavy metals/ cadmium does not become fixed within the soil structure. Research has shown that the divalent cadmium ion will be displaced by hydrogen ions when the pH is lowered by one unit from pH 5 to 4. Concentrations of up to 1-2 g/1 have been obtained. 

These considerations of the general behaviour of metals in aquatic and soil systems should be used to assess and control accumulation and mobilisation processes of trace metal ions. To evaluate the environmental impact of mining activities with regard to chemical contaminants, the following main steps of geochemical activities are recommended (1) estimate the extent of the local and regional contamination, (2) quantify the sources of heavy metals, chlorides, sulphates and thus the total acidity and salinity, as well as other contaminants and (3) define and control major sources, i.e. waste rock heaps, tailings, waste dumps and air pollution, etc. 

Fraction 1 is likely to define the actual concentration of metals in soil solution. Fractions 2-4 might be mobilised in the short and medium term by changes in soil chemistry. Fraction 2 is supposed to be easily mobilised from the surface of soil constituents (soil sorptive complex) by changes in pH. Fractions 3 and 5, which represent metals bound to Mn oxides and amorphous Fe oxides, should be sensitive to changes in redox potential. Fraction 4, reflecting the organically bound metals, will probably be mobilised by the decomposition of soil organic matter. Fractions 6 and 7 are expected to be relatively stable, particularly in well-aerated soils, as reaction kinetics of iron oxides and silicates are slow. 

A high contribution of easily mobilisable fractions of Cu, Pb and Zn in the examined soils confirmed the enhanced risk of metal mobilisation. 

Changes in soil redox properties, in particular due to soil flooding, may result in the dissolution and mobilisation of heavy metals from the polluted soils. 

The relationship between the solubility of cadmium, lead, mercury and zinc, and pH has been discussed by Hahne and Krootje [304], and these authors have also reported results which indicate that chloride complexes may contribute to the mobilisation of ions of these metals in the soil. In areas of naturally high salinity, or where soil chloride levels have been enhanced as a result of the use of chloride-containing fertilisers, or from drainage waters containing salt, there is a likelihood of increased availability of heavy metals. 

Bourg, a. C. M Loch, J. P. G. 1995. Mobilisation of Heavy Metals as Affected by pH and Redox Conditions. In Salomans, W. Forstner, A. U. (eds) Biogeodynamics of Pollutants in Soils and Sediments Risk Assessment of Delayed and Non-Linear Responses. Springer, Berlin, 213-238. 

A Factor in Hcemoglobin SyrUhesis.—Copper salts administered in micro-dosage are effective in raising the haemoglobin content of mammahan blood in some conditions of anaemia. The metal is believed to aid in the synthesis of the porphyrin nucleus of haemoglobin, and the mobilisation of stored iron. 

Van den Berg GA, Meijers GGA, Van der Heijdt LM, Zwolsman JJG (2001) Dredging-related mobilisation of trace metals a case study in the Netherlands. Water Res 35 1979-1986... 

Foerstner, U. (1983) Types of binding of heavy metals in sediments and sludges sorption/mobilisation, chemical extraction and bioavailability. Fresenius Z.Anal. Chem., 316, 604-611. 

Gupta, S.K., Vollmer, M.K. and Krebs, R. (1996) The importance of mobile, mobilisable and pseudototal heavy metal fractions for 3-level risk assessment and risk management. Sci. Total Environ., 178, 11—20. 

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