Aging and weathering

Effects of soil aging on trace metal fluxes can be the enhanced retention via sorption, precipitation/co-precipitation, occlusion or incorporation into reservoir minerals, leading to stronger metal adsorption and decreasing metal extractability. Metal sorption can become irreversible due to long term diffusion into the crystal lattice of reservoir minerals, like goethite. In addition to irreversible adsorption, diffusion through pores in the organic matter or pores coated with organic material may slow down metal desorption. Indeed, most empirical studies show a higher portion of mobile metal fractions in recently contaminated soils than in old ("aged") contaminated or unpolluted soils.

Also weathering processes, like the formation of secondary minerals or the degradation/mineralization of organic compounds, have profound effects on metal mobility and bioavailability in soils. However, aging and weathering act differently for different metal forms. For example, Fe hydroxides known as an important sink for long-term metal retention, may sometimes fail to reduce activities of trace metals in the soil solution, to reach permitted metal loadings (e.g. for sludge application).

Regarding the bioavailability and extractability of persistent organic contaminants, we know that both will decrease in general with increasing soil-pollutant contact time. In how far this may be also valid for trace metals in terrestrial systems is still not very well established (see also following section). Effects of aging are described as enhanced retention via sorption, precipitation/coprecipitation processes, occlusion and incorporation into reservoir minerals. For instance, it is experimentally observed that metal sorption is irreversible due to long term diffusion into the crystal lattice of e. g. goethite. In addition to irreversible adsorption, diffusion through pores in organic matter or pores coated with organic material may cause the observed slow desorption (Ahlf and Forstner 2001). Two recent review paper provide an excellent background of processes inherent to aging, and possible consequences on metal bioavailability/toxicity and appropriate extraction techniques (by Reid et al., 2000; McLaughlin et al., 2002). Most empirical studies show indeed that mobile metal fractions in recently contaminated soils are frequently higher than in old contaminated or in unpolluted soils. Without a model addressing the "aging" of metals, risk assessment can deal only with soils that have been polluted for a long time, and hence have reached an equilibrium (McGrath, 2002).

Natural weathering processes in soils, like the formation of secondary minerals or organic degradation/mineralization, have a profound effect on the mobility and bioavailability of trace metals. Fujikawa et al. (2000) observed a downward movement of Cr and Ni (and Fe and Mn) in a natural soil probably induced by long-term weathering processes. Highest water extractability was obtained for Mn, Zn and Ba, whereas extractability of Zn and Pb in near-surface soil layers was higher than in deeper layers, indicating weathering and aging effects.

Schaaf et al. (1999) pointed out that the development of a scientifically sound soil solution chemistry in different mine soils (lignite and pyrite substrates) needs a clear knowledge of processes like the weathering of primary and the formation of secondary minerals, and of the size of the acid production and buffering capacity. Soil solution analyses showed that pyrite oxidation occurred in these soils causing the observed extremely low pH values, and high Fe(n+) and SO/2"' levels. The strong acid production accelerated the weathering of Al silicates and caused high Al levels in the solution of the soil. In addition, rather low leaching rates enabled the formation of secondary phases, which in turn control the composition of the soil solution.

Martinez and McBride (1998) presented investigations on the solubility of trace metals after simultaneous coprecipitation with Fe (hydr)oxides and after long-time aging of the precipitate. Controls on soluble metals by clays and oxides could be attributed to ion-exchange (non-specific adsorption) and surface complexation (specific adsorption) to hydroxyl groups, coprecipitation (solid solution, i. e. isolated atoms within a large oxide structure), and precipitation as discrete oxides or hydroxides. According to the authors, our current knowledge on processes involved in metal retention obtained so far from adsorption analysis and solution chemistry, or by spectroscopic and microscopic techniques, suggests a 'continuum', formed when metals adsorb to clays and oxides, extending from specific adsorption to precipitation. 'Extended X-ray absorption fine structure analysis' (EXAFS) demonstrated the direct adsorption of metal ions (Cd2+) onto the surface of hydrous ferric oxides and a-FeOOH as mononuclear surface complex. But also multinuclear surface complexes were evidenced by the performed X-ray absorption studies (XAS). Formation of a surface precipitate from Ni2+ sorption onto pyrophyllite was shown by transmission electron microscopy (TEM). Subsequent XAFS analysis indicated the formation of a bidentate inner-sphere complex and of a mixed Ni-Al-hydroxide on the clay and on the Al-hydroxide surface. The use of 'Electrophoretic Mobility' (EM), 'Electron Spin Resonance' (ESR) and 'Scanning Force Microscopy' studies indicated the coexistence of adsorbed metals at isolated sites on oxide surfaces (e. g. of Cu2+, Mg2+, Mn2+), the formation of 'surface clusters' (for Cr3+) at isolated areas, but also of evenly distributed precipitates (for Cr3+) on the surface (of geothite). These different surface morphologies resulted in the observed different extractabilities (by oxalate).

Both the increased metal adsorption and decreased extractabilities with time (aging) may be explained by these surface phenomena rather than by solid-state diffusion processes. From this it seems that the metal sorption 'continuum' begins with specific adsorption at discrete sites with multinuclear complexes or mixed cation hydroxide complexes forming at the surface at high metal loadings. These complexes may distribute evenly over the surface or may form clusters.

More intriguing, different metal forms may coexist and their solubility determined by their chemical and morphological/structural characteristics, will vary with time. Surface complexation constants for high affinity binding sites on hydrous ferric oxide can be calculated from adsorption experiments and are based on the reaction: FeOH0 + M2+ U FeOM+ + H , where FeOH0 is a high affinity binding site on the oxide surface, and M2+ a divalent cation. The sorption experiments by Martinez and McBride (1998) gave log Kiint *of 0.47 for Cd2+, 0.99 for Zn2+, 2.89 for Cu2+ and 4.65 for Pb2+. From this the solubility of metals decreases in the order: Cd2+ > Zn2+ > Cu2+ > Pb2+. Solubility lines drawn for discrete Cu precipitates (hydroxide, carbonate, oxide) showed a higher solubility than for coprecipitates. Franklinite had the lowest solubility of all Zn species. The solubility of Cu and Zn as calculated from the intrinsic surface complexation constants for hydrous ferric oxide (as taken from literature data) predicted a lower solubility than was found for the amporphous coprecipitates established here.

According to thermodynamic principles it is suppposed that coprecipitation ("true" solid solution) results in the lowest solubility. However, recent spectroscopic and microscopic evidence suggests that formation of multi-nuclear and mixed-metal phases at mineral surfaces are more common and that adsorption at discrete sites ("coprecipitation") may be less prevailing than previously thought. Consequently, so the authors conclude, metal solubilities and hence their bioavailability calculated from surface complexation models may underestimate metal solubilities in real systems. The data presented by Martinez and McBride may indeed not indicate discrete-site adsorption or formation of true solid solutions at least under the experimental conditions. Instead, polymerization of Cu2+ hydroxy species (and to a lesser extent of Zn2+), and their segregation/adsorption within/on the Fe hydr(oxide) may explain the observed phenomena. The adsorbed metal hydroxy polymer may have a higher solubility than the metal adsorbed to isolated sites. Also pH changes have to be considered as important factor controlling the observed metal solubility behaviour.

When the soil metal retention capacity is overloaded, or when metals are solubilized, e. g. at low pH, a significant downward transport of metals from the soil surface occurs. Metals will travel downward with the leaching water. Little attention has been given in the examined literature to the actual concentration remaining in soil solution after pH has changed. These concentrations can still be high enough to be toxic to biological systems. The results presented by Martinez and McBride (1998) show that metals coprecipitating with Fe hydroxides can have solubilities, which result in free

* Kiint = intrinsic surface complexation constants metal ions (Cd, Cu and Zn) that are toxic to sensitive crops (shown in hydroponic experiments; references given), like alfalfa (pZn = 5.0-5.4) or wheat (pCu = 8.2). In summarizing their results, the authors conclude that although Fe hydroxides may be an important sink for long-term metal retention and have been successfully used to remediate metal-contaminated soils, they may fail to lower activities of some trace metals, including Cu, to reach the U. S. EPA permitted metal loadings (e. g. for sludge aplication).

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