Dames & Moore, 1999 - USDA Forest Service

summer. Efflorescence occurs when water becomes saturated with respect to minerals and nucleation of
crystals can occur. Chemical saturation is defined by the solubility product (k,), which is the equilibrium
constant for reactions such as:
Whether a mineral can be expected to form by efflorescence is determined by the saturation index (SI):
where:
IAP is the ion activity product. If SI is greater than 1, the mineral is expected to
precipitate from the solution. As indicated above, there are kinetic considerations which
prevent precipitation, so solutions can appear over-saturated. SIs are usually expressed as
logs so that the critical value is 0, rather than 1.
Co-precipitation describes the mechanism by which trace metal ions are incorporated into the structure of
precipitating solids (Figure 6.3-7). It is a very important process during pH andlor Eh changes, which
cause precipitation of aluminum, manganese and iron oxyhydroxides. The trace metals may be present at
concentrations well below those required to precipitate their own hydroxides but the rapid precipitation
processes allow the elements to be incorporated into the structure due to their similar physica-chemical
properties. Evidence of the process is commonly observed in the analysis of iron precipitates. Examples
of elements affected by co-precipitation include copper, zinc, cadmium, cobalt, manganese and arsenic.
Sorption
Sorption describes surface charge effects that allow trace elements to be precipitated at lower
concentrations than predicted based purely on the solubility of their oxides and hydroxides. The results
are often indistinguishable from co-precipitation. However, sorption involves the trace elements
attaching to the surface of iron and manganese oxyhydroxides after the oxyhydroxides have formed rather
than during their formation. The process is common in stream beds where iron and manganese
oxyhydroxides coat sediments. It is commonly observed that iron and manganese concentrations in fine
stream sediments are very strongly correlated with concentrations of trace metals due to sorption.
Sorption refers to numerous complex surface processes, which are specific to each substrate. However,
the main control is pH because the sorptive surfaces undergo charge reversal at a certain PH refeked to as
the zero-point-of-charge (zpc). The surfaces are positively charged as pHpH,. Above pH, cations are therefore attracted to the surfaces. Figure 6.3-8 illustrates some
typical curves showing amount adsorbed on goethite versus pH for several elements. The upper part of
the curve represents near 100 percent adsorption. As shown, the transition from negligible to near
complete adsorption occurs over a pH range of about 2 units. Sorption occurs onto organic materials and
organisms (plants and animals). Intake of trace elements by plants and bacteria may occur.
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17693-005-0Irmuly 27,1999,4:11 PM;DRAFT FINAL it1 REPORT