Hydrous oxides heavy metals

The concentrations of other metals attenuate when the metals sorb onto the surfaces of precipitating minerals (see Chapter 10). Hydrous ferric oxide, the behavior of which is well studied (Dzombak and Morel, 1990), has a large specific surface area and is capable of sorbing metals from solution in considerable amounts, especially at moderate to high pH HAO may behave similarly. The process by which hfo or HAO form and then adsorb metals from solution, known as coprecipitation, represents an important control on the mobility of heavy metals in acid drainages (e.g., Chapman etal., 1983 Johnson, 1986 Davis etal., 1991 Smith et ai, 1992). 

Ford, R.G., Bertsch, P.M., Farley, K.J. 1997. Changes in transition and heavy metal partitioning during hydrous iron oxide aging. Environmental Science and Technology, 31, 2028-2033. 

Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminum. Soil Sci. Soc. Am. J. 40 796-799... 

Manceau, A. Charlet, L. Boisset, M.C. Didier, B. Spadini, L. (1992) Sorption and specia-tion of heavy metals on hydrous Fe and Mn oxides. From microscopic to macroscopic. Appl. Clay Sd. 7 201-223 Manceau, A. Combes, J.-M. Calas, G. (1990) New data and a revised structural model for ferrihydrite Comment. Clays Clay Min. 38 331-334... 

Isocracking A hydrocracking process, developed by Chevron and now licensed by Chevron Lummus Global. The catalyst contains a mixture of hydrous oxides for cracking, plus heavy metal sulfides for hydrogenation. First commercialized in 1962 and now widely licensed worldwide. See also Isomax. 

Manganese is relatively abundant, constituting about 0.085% of the earth s crust. Among the heavy metals, only Fe is more abundant. Although widely distributed, it occurs in a number of substantial deposits, mainly oxides, hydrous oxides, or carbonate. It also occurs in nodules on the Pacific seabed together with Ni, Cu, and Co. 

Single Ion Activity Predictions. In lieu of an attempt to test the carbonate system predictive capability of the model, use was made of the fact that in most aerobic natural aquatic systems the solution concentration of heavy metal cations in the absence of interfacial phenomena is often controlled by the carbonate, basic carbonate or oxide (hydrous) solid phase form of the metal (, 33, 34). 

Thus, the ability of the model to predict the chemistry of heavy metals in brine in a sense was used to test the validity of the carbonate subroutine. The general procedure was to assume that the trace metal solubility in brine was controlled by either the carbonate, basic carbonate or hydrous oxide form of the metal. The heavy metal and carbonate ion activities were determined by the model. The resultant calculated solubility of the heavy metals in brine was then compared with experimentally determined values. 

In simple adsorption from aqueous solution, Hg has features in contrast and in common with the base metals. The hydroxy-cation is the active species in the model for heavy-metal adsorption and this also appears to be true for Hg. However, in contrast with Cu, Pb and Zn, the adsorption is less efficient and is strongly inhibited by the formation of halide complexes, as has been shown by Forbes et al. (1974) (Fig. 12-1). These authors also demonstrate that the adsorption of Hg to goethite is effective at pH as low as 4, allowing it to be trapped subsequent to sulphide oxidation. Whilst many minerals in weathered rocks and soils may each adsorb Hg, the relative efficiency of the hydrous iron oxides (Andersson, 1979) implies that these phases will be the dominant host in most exploration samples. However, the soil organic matter is also of importance and, although the association with Hg has been described as adsorption, it seems more... 

Unfortunately, many precipitates cannot be formed as crystals under practical laboratory conditions. A colloidal solid is generally encountered when a precipitate has such a low solubility that S in Equation 12-1 always remains negligible relative to Q. The relative supersaturation thus remains enormous throughout precipitate formation, and a colloidal suspension results. For example, under conditions feasible for an analysis, the hydrous oxides of iron(lll), aluminum, and chromium(III) and the sulfides of most heavy-metal ions form only as colloids because of their very low solubilities.- ... 

Regardless of the method of treatment, a coagulated colloid is always contaminated to some degree, even after extensive washing. The error introduced into the analysis from this source can be as low as 1 to 2 ppt, as in the coprecipitation of silver nitrate on silver chloride. In contrast, coprecipitation of heavy-metal hydroxides on the hydrous oxides of trivalent iron or aluminum can result in errors as large as several percent, which is generally intolerable. 

Reprecipitation A drastic but effective way to minimize the effects of adsorption is reprecipitation. In this process, the filtered solid is redissolved and reprecipitated. The first precipitate ordinarily carries down only a fraction of the contaminant present in the original solvent. Thus, the solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original, and even less adsorption occurs during the second precipitation. Reprecipitation adds substantially to the time required for an analysis but is often necessary for such precipitates as the hydrous oxides of iron(III) and aluminum, which have extraordinary tendencies to adsorb the hydroxides of heavy-metal cations such as zinc, cadmium, and manganese. 

Crawford, R.J., Harding, LH., and Mainwaring, D.E., The zeta potential of iron and chromium hydrous oxides during adsorption and coprecipitation of aqueous heavy metals, J. Colloid Interf. Sci., 181, 561, 1996. 

Crawford, R.J., Mainwaring, D.E., and Harding, I.H., Adsoiption and coprecipitation of heavy metals from ammoniacal solutions using hydrous metal oxides. Colloids Surf. A, 126, 167, 1997. 

Gadde. R.R. and Laitinen, H.A., Studies of heavy metal adsorption by hydrous iron and manganese oxides. Anal. Chem., 46, 2022, 1974. 

Loganathan, P. and Burau, R.G., Sorption of heavy metal ions by a hydrous manganese oxide, Geochim. Cosmochim. Acta, 37, 1277, 1973. 

Mishra, S.P, Dubey, S.S., and Tiwari, D., Inorganic particulates in removal of heavy metal toxic ions. IX. Rapid and efficient removal of Hg(II) by hydrous manganese and tin oxides, J. Colloid Interf. Sci., 219, 61, 2004. 

The cycling of iron in natural environments is of great importance to the geochemical cycling of other reactive elements. The oxidation of Fe(II) to Fe(III) (hydr)oxides is accompanied by the binding of reactive compounds [heavy metals, silicates, phosphates, and other oxyanions of metalloids such as As(III,V) and Se(III,V)] to the hydrous Fe(lII) oxide surface, and the reduction of the hydrous Fe(III) oxides to dissolved Fe(II) is accompanied by the release of these substances. 

Manceau A, Charlet L, Boisset MC, Didier B, Spadini L (1992a) Sorption and speciation of heavy metals on hydrous Fe and Mn oxides. From microscopic to macroscopic. Applied Clay Sci 7 201-223 Manceau A, Chateigner D, Gates WP (1998) Polarized EXAFS, distance-valence least-squares modeling (DVLS), and quantitative texture analysis approaches to the structural refinement of Garfield nontronite. Phys Chem Minerals 25 347-365... 

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