Chemical and Water Quality Kinetic Characteristics and Processes - Hydro-Environmental Analysis: Freshwater Environments - page 406

concentration or CCC) for sulides be 2 μg L -1 . Excessive concentrations may be a particular prob-
lem in reservoir tailwaters receiving untreated outlows from reservoir anaerobic bottom waters.
Unionized hydrogen sulide (H 2 S) is also only slightly soluble in water (Table 14.2). So, hydrogen
sulide may be volatilized from the water surface or from reservoir releases, often creating odor
problems in tailwaters.
Sulides are also important as they impact the bioavailability of many metals, thereby impact-
ing metal toxicity. Sulides form insoluble precipitates with a number of metals, including mercury.
Therefore, the presence of sulfur is often used to assess the toxicity of sediments. A common ana-
lytical method is to acidify a sample, convert the sulfur to gaseous hydrogen sulide (H 2 S), which
is then purged from the sample, trapped in an aqueous solution and measured; the result is referred
to as acid volatile sulide (AVS; Allen et al. 1993). AVS concentrations are then used to predict the
toxicity in the sediments of divalent metals, including copper (Cu), cadmium (Cd), nickels (Ni), lead
(Pb), and zinc (Zn) (Di Toro et al. 1992, 2005).
14.9 IRON AND MANGANESE
Oxides of manganese (e.g., MnO 2 ) follow nitrate in terms of being thermodynamically attractive oxi-
dants under anaerobic conditions, and in terms of producing biologically usable energy. Manganese
is followed thermodynamically by iron oxides (e.g., FeO(OH)) as TEAs. The end products of the
microbial reduction of oxides of manganese and iron are the ionic forms Mn 2+ and Fe 2+ (Table 14.3).
As with other end products of microbial reduction, iron and manganese may accumulate in
anaerobic waters and sediments, such as in or below the hypolimnion of lakes and reservoirs. This
accumulation is often impacted by pH and other materials, such as sulides, with which they may
form insoluble precipitates. For example, ferric iron may form Fe(OH) 2 and FeCO 3 , which are insol-
uble, and FeS, which is very insoluble.
When reduced forms of iron and manganese are introduced into oxic waters, such as across a
redox boundary, they both form insoluble oxyhydroxides. The redox potential and the oxidation rate
of manganese are slower than those of iron, therefore it will remain in solution longer in the oxic
epilimnion or in reservoir releases. The oxidation is typically estimated using kinetic rates rather
than equilibrium chemistry. For example, Dortch et al. (1992) modeled the oxidation of iron and
manganese in reservoir tailwaters using irst-order rates.
Iron and manganese are generally not toxic, but are considered nuisance chemicals for water
supply. The oxidation of the reduced forms also results in a loss of oxygen.
One of the important aspects of the formation of iron and manganese oxyhydroxides is that they
commonly form high surface area locs to which a number of other chemicals sorb. Generally, iron
is considered the more important of the two. The importance of iron was noted by Mortimer (1941),
who postulated the role of an iron hydroxide precipitate in controlling releases of phosphorus from
sediments. While the scavenging of phosphorus from the water column by iron oxyhydroxides is
well known and important, the oxyhydroxides sorb and remove a number of other metals as well.
It is generally accepted that the sorption of metals to iron oxyhydroxides is the dominant chemical
process regulating the dissolved concentrations of metals in waters and sediments (Horowitz 1991;
Dzombak and Morel 1990). Because of its importance, models of metal speciation, for example,
equilibrium chemical models, such as the U.S. EPA's MINTEQA2 (Allison et al. 1991), or the metal
transport, kinetic, and speciation models, such as META4-WASP (Martin et al. 2012), generally
include oxyhydroxide formation and sorption, such as using the hydrous ferric oxide double layer
(HFO DLM) sorption model initially of Dzombak and Morel (1990).
14.10 METHANE
Methane in lakes is typically produced by bacteria (methanogens) using one of several meta-
bolic pathways and substrates (e.g., acetate, formate, hydrogen, and carbon dioxide), in a process