Direct Oxidation,
Precipitation, and Particle Removal
A
commonly practiced Mn treatment approach is to chemically oxidize dissolved Mn(II)
to particulate Mn(IV) and then physically separate this solid from solution
through clarification and filtration processes. The kinetics of oxidation of Mn(II) by oxygen (O2) or
free chlorine (Cl2, present in water as HOCl and OCl−, depending on pH) are very slow relative to the
hydraulic retention times typically encountered in drinking water treatment
systems when pH < 9, with the estimated
half-life of Mn(II) in the presence of O2 and Cl2 on
the order of years and hours, respectively, for these conditions [20]. Therefore, strong oxidants such
as chlorine dioxide (ClO2), permanganate (MnO4 −), and
ozone (O3) are required [21]. Ferrate (Fe(VI)), a
strong oxidant, has been evaluated for drinking water treatment [22•] and
is likely to be effective for Mn(II) oxidation. Hydrogen peroxide (H2O2) has
been shown to be ineffective for Mn(II) oxidation [23].
Chlorine Dioxide
ClO2 oxidation
of Mn(II) follows a stoichiometry of 2.45 mg ClO2 per mg of Mn(II) and
proceeds via a rapid
second order reaction with a k 2 of 1 × 104 M−1 s−1 at pH
7 [21, 23]. However, over twice the stoichiometric
dose was required to achieve full oxidation in that study. In contrast,
ClO2 was found to be the most effective oxidant for Mn(II) in a
reservoir with 3.5 mg/L of total organic carbon (TOC) [24]. The use of ClO2 yields
the regulated by-product chlorite, and chlorate, an unregulated product of some
concern, which limit the total dose of ClO2 that can be added
to water. Therefore, ClO2 may
not be appropriate for utilities treating relatively high amounts of Mn(II)
co-occurring with other oxidant demands, such as reduced iron or organic carbon
[23].
Ozone
O3 is
another strong oxidant that is also used for Mn(II) oxidation. O3 oxidizes
Mn(II) at a 0.87-mg O3 to
1.0 mg Mn(II) ratio, in the absence of other oxidant demands. The rate of
reaction between O3 and Mn(II) is relatively rapid with a rate constant of 2 × 104 M−1 s−1 at pH
7 [25]. In the
presence of moderate amounts of organic matter, much more O3 is required to
achieve complete oxidation of Mn(II) than stoichiometry would predict.
Bench-scale experiments indicated O3 was not successful at oxidizing
Mn(II) in a river water with approximately 4 mg/L of TOC [26]. In
addition, overdosing of O3 in the presence of Mn(II)
leads to the in situ formation of permanganate (Mn(VII)), which can cause
downstream water quality problems
Permanganate
Potassium and sodium permanganate are also used
to oxidize Mn(II). The stoichiometric dose for oxidation of Mn(II) with KMnO4 is 1.92 mg KMnO4 per
mg Mn(II) [23]. This
oxidation reaction occurs rapidly,
with a reaction rate constant of 1 × 105 M−1 s−1 at pH
7 [27]. Studies
have demonstrated that oxidation of Mn(II) by Mn(VII) is much less impacted by
the presence of natural organic matter (NOM) than is oxidation by O3 or ClO2, with only
small (10–30 %) increases in oxidant dose above stoichiometry required for
adequate treatment [23, 26]. However, overdosing of KMnO4 can
lead to increased levels of dissolved Mn (and pink water), and
dosages must be monitored and optimized frequently [24]. In addition, the reduction of
Mn(VII) (permanganate) results in the formation of additional particulate
Mn(IV) that must be removed.
Oxidation and Precipitation at High pH
Mn(II) in solution can be rapidly oxidized by free chlorine when the pH is
increased to greater than approximately 9. This is not a common practice although the
authors are aware of at least one large surface water plant (Providence, RI)
that historically has used lime and free chlorine addition just prior to media
filtration that results in removal of particulate Mn as was confirmed by
assessment of Mn fractions in the filter influent and the lack of any MnO x coating on
long-used filter media. A
more common situation where elevated pH results in the removal of Mn(II) is in
the process of high pH lime-soda precipitative softening for removal of
hardness. At the
elevated pH of this process (∼>10 to 11), Mn2+ and
CO3 2− combine to form the relatively insoluble
MnCO3(s) precipitate, thus achieving Mn removal along
with hardness removal, without oxidation of the Mn(II).
Sorption and Catalytic Oxidation
Dissolved Mn(II), a divalent cation, can be
removed from solution by sorption to a solid surface, typically a metal oxide,
and most often a manganese oxide, typically in the pH range of 6 to 9 ([34, 35]). Mn-oxide surfaces used for Mn
removal have manganese in the Mn(III) or Mn(IV) oxidation state, or
both, and are often referred to as “MnO x(s)” with x between
1.5 and 2.0. A natural ion
exchange mineral, glauconite, a green-colored material (called greensand)
that does not contain Mn, was among the first materials coated with a Mn-oxide
surface and then used for Mn(II) removal by adsorption, with the black Mn-oxide
coated glauconite also referred to as “greensand” or “manganese greensand.”
Other materials used for Mn(II) sorption include naturally occurring Mn
minerals such as pyrolucite (MnO2(s)),
engineered oxide and/or ceramic materials coated with an MnO x surface, and
traditional particle filtration media such as anthracite coal or silica sand
that are coated with MnO x either
intentionally or unintentionally (i.e., naturally).
Adsorption of Mn(II) to MnO x surfaces is fast and is
accompanied by the release of H+, as occurs
with adsorption of cations to oxide surfaces . The extent of adsorption is a
function of MnO x coating level (mg Mn/g dried media),
oxidation state of the Mn in the MnO x , and pH (a more alkaline pH promotes
adsorption). When the adsorption capacity of the Mn-oxide-coated media
is exhausted, breakthrough of dissolved Mn(II) occurs. The bed of media can be regenerated using an
oxidant in the backwash, typically permanganate (MnO4 −), to
oxidize the adsorbed Mn(II) and form MnO x(s). Much, if
not most, of the MnO x(s) formed by oxidation of the
adsorbed Mn(II) is removed during backwash such that the media can be utilized
for a long period of repeated sorption and intermittent regeneration cycles. In
addition, MnO x(s) material that is not removed
from the media provides adsorption sites for additional Mn(II) uptake.
Catalytic Oxidation by Chlorine
As noted above, the homogeneous oxidation of
Mn(II) by free chlorine in solution (HOCl, OCl−) is
relatively slow at low pH (less than about 8 to 8.5) and low temperature [23]. However, free chlorine oxidation of
Mn(II) that has adsorbed to an oxide-coated surface is very rapid (less than
seconds to minutes) and can occur at pH as low as 6 and at low temperatures
[38]. The
Mn-oxide surface thus catalyzes the oxidation of adsorbed Mn(II) by free chlorine,
creating new MnO x(s) for additional Mn(II) removal,
a continuous regeneration process. Media from particle removal filters that
have Mn(II) and free chlorine in the influent often develop a MnO x coating over
time, even if no intentional initial MnO x surface was
created, producing a so-called natural greensand effect [34]. If iron
(Fe) or aluminum (Al) are in the influent, these can be incorporated into the oxide
coating [39]; elemental
analysis (Al, Mn, Fe) of the oxide coating is performed after digestion by
reductive dissolution [40, 41].
Mn(II) removal by sorption and surface catalyzed
chlorine oxidation is effective, with consistently low effluent Mn
concentrations (below detection limit to 0.02); maintenance of a free chlorine
residual throughout the media is necessary [18••]. Particle removal media, such as
anthracite, silica sand, or other materials, can be conditioned in situ with a
MnO x(s) coating to provide Mn(II) removal capability at
start-up. One conditioning
method involves soaking the media in a Mn(II)-rich solution (e.g., manganous sulfate),
draining, and then soaking in an oxidant (e.g., potassium permanganate)
[34]; another method involves soaking
overnight in a permanganate solution, possibly in the presence of free chlorine
[42]. As noted
above, sand, anthracite coal, and pyrolusite (MnO2(s)) are among
the media types used in filters for Mn(II) removal by catalytic oxidation [38]. For
filter media from various drinking water treatment plants (DWTPs), MnO x coating
levels ranged from 0.01 to >100 mg Mn/g media, and Mn(II) uptake was
found to increase nonlinearly with MnOx coating level [4]. MnOx
coatings are typically greater at the top of stratified media beds as the
majority of Mn(II) adsorption occurs within the first 10 in. or so of the
media bed [4, 40]. Over
time, MnO x is removed during backwash of media
filters, again allowing for long-term use of MnO x -coated
media for both particle and dissolved Mn(II) removal.
In principle, continuous regeneration of MnO x surfaces by
oxidation of adsorbed Mn(II) by oxidants other than chlorine can also occur.
However, addition of strong oxidants such as permanganate, ozone, or chlorine
dioxide most often is likely to result in oxidation of dissolved Mn(II) to
particulate form prior to the filter media such that Mn removal would occur by
particle deposition, not sorption of dissolved Mn(II) and subsequent surface
catalyzed oxidation. Pre-filter oxidation with permanganate is not uncommon,
and continuous regeneration may occur, especially at lower pH. However, if a permanganate
residual is in the filter influent, there is a risk of having permanganate
residual (pink water) or MnO x colloids in
the filter effluent, an undesired result. Thus, continuous regeneration with
permanganate can be operationally challenging.
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