Adsorption of trace metals

Greater adsorption of trace metals is found at higher pH and C02(g) concentrations. Sites available for Zn2+ sorption are less than 10% of the Ca2+ sites on the calcite surface, and Zn adsorption is independent of surface charge. This indicates a surface complex with a covalent character (Zachara et al., 1991). Furthermore, the surface complex remains hydrated and labile because Zn2+ is rapidly exchangeable with Ca2+, Zn2+ and ZnOH. At the dolomite-solution interface, the carbonate(C03)-metal (Ca/Mg) complex dominates surface speciation at pH > 8, but at pH 4-8, hydroxide (OH) -metal (Ca/Mg) dominates surface speciation (Pokrovsky et al., 1999). Calcite has an observed selectivity sequence Cd > Zn > Mn > Co > Ni > Ba = Sr, but their sorption reversibility is correlated with the hydration energies of the metal sorbates. Cadmium and Mn dehydrate soon after adsorption to calcite and form a precipitate, while Zn, Co and Ni form surface complexes, remaining hydrated until the ions are incorporated into the structure by recystallization (Zachara et al., 1991). 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

Metallic corrosion IV. V. Corrosion of iron and mild steel. Proc. R. Soc. A134 494 Benjamin, M.M. Bloom, N.S. (1981) Effects of strong binding adsorbates on adsorption of trace metals on amorphous iron oxyhydrox-ide. In Tewari, P.H. (ed.) Adsorption from aqueous solutions. Plenum Press, New York,... 

Gerritse, R.G. and Van Driel, W. (1984) The relationship between adsorption of trace metals, organic matter and pH in temperate soils. J. Environ. Qual., 13, 197-204. 

Wenzel, W. W., Sletten, R. S., Brandstetter, A., Wieshammer, G., and Stingeder, G. (1997). Adsorption of trace metals by tension lysimeters Nylon membrane vs. porous ceramic cup. J. Environ. Qual. 26(5), 1430—1434. 

The acidification of seawater samples is required for the storage of seawater in order to prevent the precipitation, adsorption of trace metals on the container walls or volatilization loss from solution. Generally, nitric acid is added to seawater to bring the pH of the solutions to below 1.5. The addition of acids immediately after collection on board ship may be preferable for sample storage rather than addition before analysis in the laboratory. 

Figure 22 Adsorption of trace metals by the Meuse River bottom sediments measured data in NaN03 0.01 M and interpretation (curves) in terms of adsorption.

The chemical and biochemical behaviors of humic substances can also be changed by GPC. Frimmel and Sattler (1982) studied the complexation/ adsorption of trace metals by dissolved humic substances and discovered that the affinity of humic substances for metals markedly increased following GPC. Similarly, Stewart and Wetzel (1982) observed that all Sephadex G-lOO fractions of dissolved humic material obtained from the aquatic macrophyte Typha were more stimulatory to C assimilation by algae than were the same humic substances that had not been fractionated. The observations indicated that the gel, eluent, or processing procedure (e.g., lyophilization, reconstitution, cleavage during separation) either reduced the toxicity of the humic substances or enhanced its stimulatory nature or affinity toward trace substances. 

In our work we concentrate on the interactions between di- and tri-valent metal radionuclides which enter estuarine and seawater in ionic forms and humic and fulvic acids of different origins. However, as humic substances react with metals, not only in dissolved but also in undissolved states, we report some data on the adsorption of trace metal radionuclides on suspended humic acids. 

Davis, J.A., Adsorption of trace metals and complexing ligands at the oxide/water interface, Ph.D. Thesis, Stanford University, Stanford, CA, 1977, cited after [10, 807]. 

The pH-dependent chemisorption seen in Figure 4.5 provides an explanation for the general observation in soil clays that specific adsorption of trace metal cations increases with pH. That is ... 

M.D.A. Bolland, A. M. Posner, and J. P. Quirk, Zinc adsorption by goethite in the absence and presence of phosphate, Aust. J. Soil Res. 15 279 (1977). M. M. Benjamin and N. S. Bloom, Effects of strong binding of anionic adsorbates on adsorption of trace metals on amorphous iron oxyhydroxide, in Adsorption from Aqueous Solutions (P. H. Tewari, ed.). Plenum, New York, 1981. 

Most chemical and reaction path models currently account only for the aqueous carboxylic acids and their cation complexes, amino acids, some liquid hydrocarbons, alcohols, and certain other compounds entered into the data base for project-specific purposes. Adsorption of trace metals onto solid humic substances, for example, requires the user to create a fictitious solid and use an empirical adsorption coefficient. Scattered reports of carboxylic acid solids such as calcium oxalate (Marlowe 1970 Naumov et al. 1971 Galimov et al. 1975 Graustein et al. 1977 Campbell and Roberts 1986) emphasize the necessity to perform sensitivity analyses on the formation of such solids and indicate another area of uncertainty in the interpretation of chemical and reaction path model results. 

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