Environmental Engineering Reference
In-Depth Information
down to an acceptable level (the remediation target). For a given contaminant
and reactive material the required residence time is a function of the reaction
rate and the equilibrium constant. As a PRB is basically a flow-through cell
there is a continual reestablishment of equilibrium (or at least the tendency
toward reestablishing equilibrium) as the groundwater passes through the
barrier. For example, a given reactive material could reach equilibrium very
rapidly with a contaminant, but if the initial concentration is high and the
equilibrium constant is low, then a long low path may be required to reduce
the contaminant concentration to an acceptable level. Alternatively, a slow
reacting material with a high equilibrium constant may reach an acceptable
exit concentration in a relatively short residence time, without ever reaching
equilibrium. As a range of factors may affect reaction rate, the retention time
required to treat the groundwater at a particular site with a specific reactive
material should always be determined in a feasibility study (e.g., by column
experiments).
9.2.1 Removal of Metals from Groundwater
Cationic metals usually have limited mobility in soil and groundwater with
high clay and organic content, high alkalinity, and low permeability (Fetter
1993). However, complexing agents such as carbonates, hydroxides, sulfates,
phosphates, fluorides, and possibly silicates which are present in natural
waters increase the solubility of metals (Langmuir 1978). Precipitation is a
possible reaction for heavy metal contamination to lower the concentrations
in the groundwater. Other possibilities are sorption, precipitation subse-
quent to a chemical reduction, or a combination of the different processes.
Precipitation of heavy metals is a commonly used process in wastewater
treatment plants. Precipitation is used in PRBs in the same way to lower the
heavy metal concentrations in groundwater. Chemical precipitation is a pro-
cess by which a soluble substance is converted into an insoluble form by a
reaction with the precipitant. Frequently used precipitants are hydroxides,
sulfides, phosphates, and carbonates. The solubility of heavy metal hydrox-
ides, sulfides, and carbonates is pH-dependent. Metal hydroxides exhibit
amphoteric behavior, that is, solubility is high both at low pH (removal of
hydroxide anions, see Reaction 9.2, with Me as a generic bivalent heavy metal)
and high pH (formation of soluble hydroxo complexes, see Reaction 9.3). A
minimum in solubility for most heavy metals can be observed between 9
and 11 (Chung 1989).
Me
2+
+ 2 OH
−
→ Me(OH)
2
(s)
(9.1)
Me(OH)
2
(s) + 2 H
+
→ Me
2+
+ 2 H
2
O
(9.2)
2
+ →
(
)
−
3
−
Me(OH)
OH
Me OH
(9.3)
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