Soil ligands

The value of - the slope of the metal solubility as a function of pH - is often found to be between -0.5 and -0.7 for Cd + and (McBride et al., 1997 Salam and Helmke, 1998 Gray et al., 1999 Sauve et al., 2000). If we take —0.67 as a reasonable value for Cd, then metal solubility will show a 2 3 dependence on soil solution pH. This can be handled by having GEOCHEM-PC consider the formation of a mixed solid phase with a mythical soil ligand, the MSL ... 

Important parameters used to develop the mythical soil ligand (MSL) for input into Geochem-PC (Parker et al, 1995)... 

To test the ability of Fe(IIl) to compete for DMA with other metals, two-metal scenarios were modeled. The findings from this suggest that while Fe(III) is able to effectively compete with Zn, Cd, Cu or Ni at low pH and complex the majority of the DMA present, at higher pHs, the other metals predominate DMA complexation (see Fig. 3). Further GEOCHEM-PC simulations with 25 tiM concentrations of DMA, Fe(III), and Cd/Zn/Cu/Ni, and with neither precipitation nor mythical soil ligand, were of the same general shape as those in Fig. 1 but had... 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

Reabsorption of the ligand plus its metal partner is a necessary requirement of processes like Fe acquisition by phytosiderophores (32). However, whether or not reabsorption of diffusates, which undoubtedly occurs in solution cultures (45), has a significant role to play is uncertain, largely because in soil most diffusates (sugars, amino acids, and other organic acids) are readily utilized by microorganisms or adsorbed by soil colloids. 

Americium will occur in soil in the trivalent state. The transformations that may occur would involve complexation with inorganic and organic ligands (see Section 6.3.1) and precipitation reactions with anions and other substances present in the soil solution. The 241 Am occurring as an ingrowth progeny of 241Pu and trapped in a plutonium matrix will exhibit solubility and biokinetic characteristics of the plutonium, rather than americium. 

Organic complexed Cd is not important in arid soil solution. Hirsh and Banin (1990) observed 5-10% of Cd bound to organic ligands in Israeli arid soil solution. Emmerich et al. (1982) found that organic-Cd complexes constituted 1-4% of Cd in California arid soil solution. However, Villarroel et al. (1993) reported that in a California sludge-treated soil, Cd was mainly present in both free ion and organic complex forms (each accounted for 32-40% and 30-45% of total Cd in soil solution, respectively), followed by the chloride complexes (8-20%), S04-complex (3-10%), and P04-Cd complex (1.5-7.7%). The nitrate Cd complexes were the lowest. Cadmium activities and speciation is not significantly affected by P and N treatments. 

Soil solution to soil ratios also strongly affect distribution of some trace elements such as Zn speciation in arid and semi-arid soils. Fotovat et al. (1997) reported that the proportion of free hydrated Zn2+ to total Zn ranged from 20-65% at field capacity soil water content and decreased with increases in solution to soil ratios, while the proportion of Zn complexed with organic ligands increased dramatically in soils. However, solution to soil ratios do not strongly affect the distribution of Cu speciation in soil solution since Cu primarily occurs as organic complexes in these soil solutions. 

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