Environ. Sci. Technol. 2001, 35, 4126-4131
Stability of Phosphorus within a
Wetland Soil following Ferric
Chloride Treatment To Control
Eutrophication
LINDSAY J. SHERWOOD AND
ROBERT G. QUALLS*
Department of Environmental and Resource Sciences,
MS/370, University of Nevada, Reno, Reno, Nevada 89557
Addition of iron and aluminum compounds has become
an increasingly popular method to regulate phosphorus
eutrophication in lakes and reservoirs. It has been proposed
that ferric chloride addition to agricultural runoff entering
the northern Everglades could provide a means for
enhancing natural mechanisms of phosphorus removal
from the wetland. In this study we added ferric chloride
to Everglades water spiked with 32PO4, incubating the resulting
precipitates in microcosms simulating the Everglades
ecosystem. 32P activity and reduction-oxidation (redox)
potentials were monitored to determine if the 32P was released
into the overlying water column due to iron reduction.
Results of redox potential measurements and 32P activity
indicate that although reducing conditions exist in the soil,
on average less than 1% of the added 32P was measured
in the water column during the 139-day incubation.
Ferric chloride addition thus might prove an effective
means of long-term phosphorus retention in the Florida
Everglades and perhaps other wetland systems.
Introduction
Ferric chloride and aluminum sulfate (alum) have been widely
used in wastewater treatment processes to reduce phosphorus concentrations (1). The addition of these iron and
aluminum compounds has also become an increasingly
popular method to regulate phosphorus availability and
control eutrophication in lakes and reservoirs (2). Alum and
ferric chloride have been added to river water or stormwater
entering lakes (3, 4) or directly to the lake (5, 6), leading to
a decrease in phosphorus concentrations. The main mechanism of phosphorus removal upon addition of ferric chloride
and alum involves the precipitation of metal oxyhydroxides
and subsequent adsorption of phosphorus by ligand exchange (7). In the cases of lakes and reservoirs, the water
body itself serves as a settling basin, not only removing
phosphorus from the water column but also forming a blanket
of precipitated metal oxyhydroxides covering the top layer
of sediment, blocking the release of phosphorus from the
sediment.
Natural water in the Everglades ecosystem characteristically contained relatively low nutrient levels, with especially
low levels of phosphorus (8). Since the late 1960s, nutrients
in water draining from the Everglades Agricultural Area have
periodically flowed into the water conservation areas of the
northern Everglades, creating a nutrient enrichment gradient
* Corresponding author phone: (775)327-5014; fax: (775)784-4789;
e-mail: qualls@equinox.unr.edu.
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in Water Conservation Area 2A (WCA-2A) extending roughly
8-10 km downstream of inflow structures (9-11). The South
Florida Water Management District (SFWMD) has constructed a demonstration-scale wetland, the 1545 ha Everglades Nutrient Removal Project, to address the problem of
nutrient loading from stormwater runoff associated with the
Everglades Agricultural Area (12). Based on the first 3 years
of operation, the Everglades Nutrient Removal Project
achieved approximately an 80% reduction in total P levels
to an average of about 50 µg/L total P (13). However, much
lower concentrations (planning target of 10 µg/L total P)
may be necessary to meet future restoration goals for the
Florida Everglades (South Florida Water Management District, personal communication).
It has been proposed that addition of ferric chloride or
aluminum sulfate to water within the marshes of the
stormwater treatment areas will improve performance and
reduce phosphorus concentrations in the effluent to the
desired threshold levels (14). In this scheme, ferric chloride
or alum would be added to water traversing a control structure
and the marsh itself would serve as the settling basin, greatly
increasing the retention time and enabling smaller doses to
be applied. The synergistic effect between chemical and
wetland methods for phosphorus removal would provide a
relatively low cost solution, and the settling and potential
burial of the phosphorus complexes in the soil could provide
a mechanism for long-term phosphorus removal from the
water column. However, the reducing conditions present
just below the soil surface of the northern Everglades could
present a significant limitation on the long-term effectiveness
of iron addition.
It has been shown empirically that Fe3+ is reduced to the
Fe2+ form in the 100-200 mV range of measured redox
potential in a variety of soils (15, 16). Native phosphorus has
been shown to be released as soils are reduced under
anaerobic conditions after flooding (17-19). The reduction
of Fe3+ to Fe2+ has been demonstrated as the mechanism for
the release of phosphorus and reduction in the phosphorus
sorption capacity of the soil (17, 18).
The objectives of this study were to evaluate the potential
for phosphorus retention by ferric chloride addition in
eutrophic areas of the northern Everglades and to determine
if the precipitated ferric phosphate or phosphate adsorbed
to iron oxyhydroxides complexes was subsequently released
by iron reduction over the course of the study period. To
accomplish these objectives, radiolabeled precipitates formed
by the addition of ferric chloride to Everglades water spiked
with 32PO4 were incubated in microcosms simulating the
natural Everglades ecosystem. Consequently, we ensured that
any 32P activity appearing in the water column of the
microcosms must have originated from P previously bound
to the iron precipitate. Throughout the course of the
experiment we measured redox potentials at fixed depths
within the microcosms and monitored 32P activity to determine if reducing conditions existed, and if a significant
amount of phosphorus was released into the overlying water
column due to iron reduction. At the end of the experiment,
saturated soil samples were withdrawn from replicate one
of each treatment microcosm and subjected to a sequential
fractionation procedure to determine the fate of the phosphorus not released into the water column.
Materials and Methods
Sampling Location. The 1545 ha Everglades Nutrient Removal project is located near West Palm Beach, FL, bordering
the northwest corner of Water Conservation Area 1. Water
10.1021/es0106366 CCC: $20.00
 2001 American Chemical Society
Published on Web 09/06/2001
FIGURE 1. Diagram of treatments used in microcosm experiments.
from the Everglades Agricultural Area runoff is diverted
from the West Palm Beach Canal and enters into the
Everglades Nutrient Removal (ENR) portion of Water Conservation Area 1. Total phosphorus concentrations of influent
water into the ENR over a 3-year study period (August 1994August 1997) ranged from 66 to 201 µg/L total P, with an
average concentration of 108 µg/L total P (13). Soil and water
samples were taken from a location in the western portion
of the ENR (longitude 26°38′44′′ north/latitude 80°26′00′′
west) in May of 1999. Composite soil samples were collected
from the top 6-12 in., sealed in coolers, and shipped to the
laboratory. Water was collected at the same time from
approximately the same locations. Before addition to the
microcosms, the soil was homogenized to ensure uniform
composition.
Microcosm Construction. The objective of the microcosm
experiments was to determine what proportion of radiolabeled phosphorus was released back into the water column
under reducing conditions. Microcosms were constructed
from PVC pipe sections 15 cm in diameter and 60 cm in
length and were filled with a 20 cm depth of Everglades soil
collected from the sampling site (0.34 mg Fe/cm3 soil,
approximately 44% C content, soil pH ) 7.2). Four treatment
levels were investigated in triplicate to examine 32P release
under different levels of burial (Figure 1). Ferric oxyhydroxide
with adsorbed phosphate precipitates deposited on the soil
surface might be subject to various fates. Precipitates might
simply be buried under more iron precipitates (Fe-precipitate
treatment), or they might be buried in peat soil, which
accumulates at the rate of approximately 6 mm per year in
the P enriched portions of the Everglades (9). Three and 6
cm burial depths were chosen to ensure that at least one
treatment would be below any aerobic zone.
Radiolabeled precipitates were prepared in 250 mL bottles
prior to addition to the microcosms. Everglades water (pH
) 7.5-8.2, DOC ) 35 mg/L) was adjusted to an initial orthophosphate concentration of 50 µg/L and then spiked with
92.6 µCi of H332PO4 (omitted for the three control microcosms). Then 800 µL of 0.1 M FeCl3 were added to each bottle,
and the solutions were stirred for 2 min and then allowed
to sit for 24 h as ferric oxyhydroxide-phosphate precipitates
formed.
After the settling period, the contents of each bottle were
filtered using Gelman GN-6 0.45 µm membrane filters, and
the precipitates were washed with deionized water to remove
soluble unbound phosphate. Precipitates were carefully
scraped off the filter paper and added to small beakers.
Radioactivity of the filtrate and that remaining on the filter
paper was measured to calculate by mass balance the 32P
activity actually added to each microcosm. The 32P activity
remaining in the dissolved phase was only a small percentage
of the total activity added to each bottle, on average 0.14%
for all treatment microcosms.
To make the radiolabeled precipitate part of a larger layer
of precipitate, a 46 mL aliquot of nonradiolabeled bulk
precipitate (50 µg/L PO4 previously precipitated with 0.1 M
FeCl3 and centrifuged to concentrate the precipitate flock)
was added to the radiolabeled precipitate. The combined
precipitate was then added over the top of the 20 cm of
Everglades soil already present in each microcosm. Precipitates were then covered by an additional layer of soil
simulating burial by cattail litter and soil deposition (controls,
3 and 6 cm burial depth treatments), or in the case of the
Fe-precipitate treatment, an additional layer of nonradiolabeled precipitate to simulate the proposed continuous
addition of ferric chloride to the stormwater treatment areas.
Measurements of Redox Potential. Redox potentials were
measured in the soil using platinum electrodes (20) permanently installed in each microcosm and a portable
voltmeter. Prior to installation, redox probes were tested in
a standard Eh buffer (21). Redox potential measurements
were taken using a portable Orion model 250A mV/pH meter
and an Orion double junction Ag/AgCl reference electrode
that was inserted in the top few centimeters of the water
column. All mV readings were corrected to the standard H
electrode by adding +244 mV. Redox potentials for each day
were averaged over the three replicates of each treatment
level.
Three redox probes were installed in each microcosm
(except in control microcosms which contained two redox
probes per microcosm) at appropriate levels for the burial
depths (Figure 1). After addition of the redox probes, a 25 cm
depth of Everglades water (ortho-P ) 27 µg/L, pH ) 7.5-8.2,
DOC ) 35 mg/L) was added to each microcosm. The top of
each microcosm was covered with aluminum foil and allowed
to settle for 24 h before aeration began. The water was aerated
via airstones for 10 hours a day, simulating the natural diurnal
dissolved O2 cycle of the Everglades surface water (22).
Microcosms were maintained at approximately 23 °C.
Water Sampling and Analysis. Over the course of the
experiment, water samples were withdrawn close to the soilwater interface from each treatment microcosm to determine
32P activity, P concentration (see Figure 1, Supporting
Information), and Fe3+/Fe2+ concentration (see Figure 2,
Supporting Information) in the water column.
Radioactivity. Unfiltered and 0.45 µm filtered water
samples in Ecolite (+) scintillation fluid (ICN) were counted
using a Beckman LS60001C liquid scintillation counter to
analyze for 32P activity. Background activity in a water/
scintillation fluid blank was subtracted from both the filtered
and unfiltered sample 32P activity, and the resulting activity
corrected for decay to the time it was added to each
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FIGURE 2. Average particulate (A) and dissolved (B) 32P released
into the overlying water column in microcosms. Activities were
averaged over the three replicates of each treatment level for each
sampling period and are represented as a percentage of the total
32P added to each microcosm with standard error bars.
microcosm. The final filtered and particulate (filtered unfiltered activity) percentages of the added 32P were averaged
over the three replicates of each treatment level during each
sampling period.
Soil Sampling and Analysis. Saturated soil samples were
taken from replicate one of each treatment microcosm on
day 164, 165, or 166 from treatment addition and placed into
plastic bottles that were immediately purged with N2 gas.
Sampling increments for the three treatment microcosms
were as follows: Fe-precipitate treatment: every 0.5 cm from
the surface to 3 cm, then every 1 cm from 3 to 6 cm; 3 cm
burial treatment: every 0.5 cm from the surface to 4 cm,
then a 4-6 cm increment; 6 cm burial treatment: a surface
to 0.5 cm and a 0.5 cm to 1 cm increment, then every 1 cm
from 1 to 8 cm, giving a total of nine sampling increments
for each treatment.
Determination of Fe2+ and Fe3+ in Soil Extracts. Oxalate
extractions of soil samples were performed in a glovebag
under anaerobic conditions to determine concentrations of
Fe3+ and Fe2+ (23).
32P Extraction of Soil Samples. Sequential extraction of
P with NaHCO3 and NaOH in 5 mL soil samples was
performed in a glovebag under anaerobic conditions using
techniques modified from Chang and Jackson (24). To extract
any 32P still bound to iron in the soil samples, an additional
extraction procedure was performed in which 30 mL of
ammonium oxalate/oxalic acid extract solution was added
to each of the soil pellets remaining after the NaOH extraction
procedure. Samples were shaken for 12 h, centrifuged, and
filtered. The supernatant of each of the NaHCO3, NaOH, and
oxalate extract were analyzed for 32P activity.
An additional extraction was performed for selected depth
increments with high 32P activity in which hydroxylamine
hydrochloride was added to unextracted soil samples to
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 20, 2001
FIGURE 3. Average redox potentials for control microcosms. Redox
potential measurements were averaged over the three replicates
and depth level for each sampling date and bars indicate standard
error. *One malfunctioning probe was excluded form the average
of the 0-0.9 cm below peat surface data between day 103 and day
110.
convert any Fe3+ to Fe2+ (26) and indicate the 32P potentially
released by complete Fe3+ reduction. The supernatant was
later analyzed for 32P activity.
Analysis of 32P in Ashed Residues. Total 32P activity of
wet soil from the original sampling bottles containing each
depth increment were determined by ashing at 500 °C and
solubilizing the residues in 0.1 M HCl (25). A 200 µL aliquot
of a 100 g L-1 hydroxylamine hydrochloride solution was
also added to the residue to facilitate reduction of any Fe3+
present in the ash. Ash residues were heated for 1 h at 90 °C
and analyzed for 32P activity.
Results and Discussion
32P
Released into the Water Column. Results from the
analysis of filtered and unfiltered water samples from each
treatment microcosm indicate that on average less than 1%
of the total 32P activity added to each microcosm appeared
in the overlying water column at any time throughout the
139-day study (Figure 2). The activity of each water sample
on a particular sampling date was corrected for decay to the
activity at the day of 32P addition to each microcosm and
then expressed as a percentage of actual 32P added to each
microcosm. Results of this radio-tracer experiment indicate
that phosphorus mobilization did not occur to any significant
extent regardless of the burial depth of the radiolabeled
precipitates. Note that values after day 100 were not
significantly different from zero due to the extremely low
percentage value and the increasing variability as readings
approached background levels. This evidence could indicate
that ferric chloride addition provides an effective barrier
against phosphorus release.
Redox Potential Measurements at Fixed Depths. Redox
potential measurements at fixed depths indicate that conditions favorable for Fe3+ reduction to Fe2+ do exist both at the
3 and 6 cm depths as well as in the first few millimeters
below the soil surface (Figures 3-6). Reducing conditions
FIGURE 4. Average redox potentials for Fe precipitate treatment
microcosms. Redox potential measurements were averaged over
the three replicates for each sampling date and bars indicate
standard error. *Average is for all probes, except one which was
removed after day 86, one which was removed after day 60 and
returned after day 106, and one which was removed after day 100
and returned after day 114 due to loss of function.
FIGURE 6. Average redox potentials for 6 cm burial treatment
microcosms. Redox potential measurements were averaged over
the three replicates and depth level for each sampling date and
bars indicate standard error. *One malfunctioning probe was
excluded from the 0-0.9 cm below peat surface average over the
course of the experiment.
FIGURE 5. Average redox potentials for 3 cm burial treatment
microcosms. Redox potential measurements were averaged over
the three replicates and depth level for each sampling date and
bars indicate standard error. *One malfunctioning probe was
excluded from the 0-0.9 cm below peat surface average over the
course of the experiment.
were also found in the top 1 cm of Everglades soil in a ferric
chloride application experiment performed in the field at
the same site from which the soil samples were taken (27).
On average, redox potentials at the 3 cm depth in both control
and treatment microcosms as well as at the 6 cm depth in
treatment microcosms were found to be at or slightly below
-150 mV, well into the range at which iron reduction might
occur. Average redox potentials a few millimeters below the
soil surface for control, Fe-precipitate treatment, and 3 cm
burial treatment microcosms were also at or slightly below
-150 mV. Average redox potentials at the surface of the 6 cm
burial treatment were slightly more variable; however, the
individual measurements were less than +120 mV (with seven
exceptions), indicating that reducing conditions were present
at the 6 cm depth level.
Iron hydroxides have been shown to be very sensitive to
changes in redox potential (17, 28). Under anaerobic condi-
tions, phosphate adsorbed to iron oxyhydroxide complexes
or precipitated ferric phosphate complexes may redissolve
as Fe3+ to Fe2+ reduction occurs. However, the potential
release of phosphorus under reducing conditions depends
not only on redox conditions under the surface of the soil
but also on the solubility of the various iron oxyhydroxidephosphate complexes formed. Lack of easily mineralized
organic matter could retard the development of reduced
conditions and subsequent Fe reduction; however, Everglades
soil is easily mineralized (29), and redox potentials indicated
reducing conditions were achieved within days in the
microcosms.
Fe2+ has been shown to predominate at redox potentials
below +120 mV, the boundary of Fe3+ to Fe2+ reduction (30).
Classic experiments by Mortimer (31, 32) have shown an
increase in the release of phosphorus and iron with a decrease
in redox potential as oxygen is reduced near the soil-water
interface in lakes. With the reduction of ferric hydroxidephosphate complexes in Mortimer’s experiments, mobilization of phosphate was seen in the water column. However,
in water treatment plants where ferric chloride treatments
are used to reduce phosphorus concentrations of effluent in
anaerobic digesters, phosphate release has not been shown
(33). A possible explanation for the lack of phosphorus release
in some studies is that under anaerobic conditions, a form
of ferrous phosphate (vivianite) with very low water solubility
was formed (34). Consideration of the solubility product of
vivianite and the observed concentrations of Fe2+ and
inorganic P and the pH suggest that precipitation of vivianite
was possible (35). However, the unknown extent of complexation of Fe2+ by humic acids means the concentration
of free Fe2+ cannot be known exactly.
Analysis of 32P in Soil Extracts. Results from the analysis
of 32P activity in the water samples and redox potential
measurements indicate that conditions favorable for Fe3+
reduction to Fe2+ did occur in the soil contained within the
microcosms, and no significant 32P release was observed in
the overlying water column. Therefore, the next question we
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TABLE 1. 32P Activity Remaining in the Soil (pCi/cm3 Soil) at the End of the Incubationsa
sequential extraction
procedure
treatment
depth
increment
NaHCO3
extract
NaOH
extract
oxalate
extract
total P
(ash)
hydroxylamine
hydrochloride
Fe ppt
Fe ppt
Fe ppt
Fe ppt
Fe ppt
Fe ppt
Fe ppt
Fe ppt
Fe ppt
3 cm burial
3 cm burial
3 cm burial
3 cm burial
3 cm burial
3 cm burial
3 cm burial
3 cm burial
3 cm burial
6 cm burial
6 cm burial
6 cm burial
6 cm burial
6 cm burial
6 cm burial
6 cm burial
6 cm burial
6 cm burial
surf-0.5 cm
0.5-1 cm
1-1.5 cm
1.5-2 cm
2-2.5 cm
2.5-3 cm
3-4 cm
4-5 cm
5-6 cm
surf-0.5 cm
0.5-1 cm
1-1.5 cm
1.5-2 cm
2-2.5 cm
2.5-3 cm
3-3.5 cm
3.5-4 cm
4-6 cm
surf-0.5 cm
0.5-1 cm
1-2 cm
2-3 cm
3-4 cm
4-5 cm
5-6 cm
6-7 cm
7-8 cm
14
ndc
ndc
ndc
ndc
ndc
b
ndc
ndc
6
8
1
0.3
1
ndc
2
ndc
1
ndc
ndc
5
ndc
ndc
ndc
ndc
ndc
2
23
0.2
0.9
ndc
ndc
ndc
ndc
ndc
2
9
4
ndc
2
ndc
0.1
8
1
1
0.8
3
ndc
0.7
ndc
0.8
ndc
0.2
ndc
23
2
ndc
ndc
ndc
ndc
ndc
ndc
ndc
8
6
2
0.3
ndc
2
ndc
ndc
ndc
ndc
2
ndc
ndc
3
ndc
2
6
ndc
109
27
7
4
b
2
ndc
b
b
60
26
13
14
8
10
16
8
7
10
10
b
b
b
ndc
12
10
2
88
b
b
b
b
b
3
b
b
25
b
b
b
b
15
b
5
b
4
b
b
b
b
b
8
b
b
a NaHCO , NaOH, and oxalate extractions were performed sequentially on the same soil sample, while total P and hydroxylamine hydrochloride
3
extractions were both performed on fresh soil samples. b Indicates analysis not performed. c nd indicates activity not detected above background.
addressed was whether the 32P was mobilized and diffused
through the soil in the 3 and 6 cm depth increments, or
whether ferric chloride addition formed an effective barrier,
trapping the radiolabeled precipitates at the depth of
treatment addition.
The activities of the soil extracts (reported in pCi/cm3
soil) indicate that there was some translocation of 32P into
depth increments surrounding the site of treatment addition
(Table 1). For the Fe-precipitate treatment, the majority of
the activity was found in the surface to 0.5 cm depth
increment, although some activity diffused into lower depth
increments. The amount of activity in the NaHCO3 extract
sample represents the amount of readily exchangeable 32P
that might be available for plant uptake or recycling and was
generally only between 0 and 30% of the total 32P activity in
ashed samples. The activity obtained from the ashing of soil
samples represents the amount of total 32P activity in the
soil, giving the largest activity in the surface to 0.5 cm depth
increment.
Results of 32P activity in the 3 and 6 cm depth increments
indicate movement of phosphorus toward the surface from
the site of treatment application. The translocation of 32P
from the depth at which it was placed could have been caused
by two processes: (1) release of 32P to the dissolved phase
followed by diffusion to the surface where it was again bound
or (2) mass flow of the low-density colloidal precipitates
through the soil. The first process is the classical mechanism
proposed by Mortimer for release and trapping of phosphorus
at the surface of reduced lake sediments. Although this
migration of 32P activity raises the question of whether
phosphorus would be mobilized into the water column over
a longer period of time, the results from the Fe-precipitate
treatment seem to indicate that even at the surface to 0.5 cm
depth increment, 32P is not mobilized into the overlying water
column to any significant extent. In addition, only a small
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portion of 32P in the soil was readily exchangeable, which
seems to indicate little was released to the dissolved phase
for diffusion. Bioturbation, which can be a means of turning
over surface layers of sediments in lakes, is unlikely to occur
in this soil. Sharply defined 137Cs profiles in the soils of Water
Conservation Area 2A suggest bioturbation has an insignificant role (9).
X-ray diffraction analysis of soil samples from field
applications in the same soil, using the same ferric chloride
dose, indicates that the iron was in an amorphous state, and
there was no evidence of any crystalline iron compounds
(such as goethite) in the iron precipitates (27). Further study
could determine if the iron was reduced and then reprecipitated with phosphate to form an insoluble complex, or
if the iron remained in the ferric form in a stable complex
even under reducing conditions.
The 32P that was released by iron reduction and dissolution
using hydroxylamine hydrochloride averaged about 74% of
the total P activity obtained by ashing (Table 1).
Forms of Fe in Soil Samples. Results of oxalate extractions
of soil from the 3 and 6 cm treatment microcosms indicate
that the iron added to each microcosm (in the form of ferric
chloride) was not distinguishable from the iron native to
Everglades soil. It was expected that an observable change
in the ratio of Fe3+ to Fe2+ would be seen at the depth of
treatment addition for the 3 and 6 cm treatments; however,
this change was only observed in the Fe-precipitate treatment
microcosm where the additional nonradiolabeled iron
precipitate was added over the top of the radiolabeled
precipitate. In the surface to 0.5 cm depth increment of the
Fe-precipitate treatment microcosm, the total oxalate extractable iron (Fe3+ and Fe2+) was 2.43 mg/cm3 soil and the
Fe3+ to Fe2+ ratio was 26. In the depth increments far below
the treatment additions, the average oxalate extractable iron
was 0.34 mg/cm3 soil and the average Fe3+ to Fe2+ ratio was
7, which can be taken to approximate native iron levels in
the Everglades soil of the sampling area. Fe3+ was also found
to persist in the soil for at least several weeks after ferric
chloride addition in an Everglades field study using the same
site from which our soil was taken (27).
The main objective of this experiment was to test the
stability of iron oxyhydroxide with adsorbed phosphate
precipitates that were buried under a layer of soil or under
more iron precipitate. In this experiment, iron precipitates
were not exposed to light after burial; however, during the
settling process of the application of ferric chloride to water
within the stormwater treatment areas, photochemical redox
reactions might occur. This variable was omitted in our
experiments by covering microcosms to exclude light;
however, further study would be necessary to determine the
effect of these potential reactions on iron speciation and
phosphorus retention in the proposed application of ferric
chloride to stormwater treatment areas within the Everglades.
Based on behavior in laboratory microcosms simulating
the Everglades ecosystem, it is believed that the addition of
ferric chloride to water within the STAs would prove a viable
method of reducing phosphorus concentrations. Ferric
chloride addition has the ability to reduce PO4 (32P) concentrations to less than a few µg/L in conjunction with natural
mechanisms of phosphorus removal, such as uptake by
microbial communities and aquatic vegetation, and seems
to provide an effective barrier preventing the release of
phosphorus into the overlying water column even after
exposure to several months of reducing conditions occurring
after burial of the precipitates. Results of this research support
the initiation of a larger scale longer-term field trial to test
the viability of long-term P storage and retention in the STAs
of the northern Everglades. This research also presents a
method that might be used to study the fate of phosphorus
bound under specific conditions in lake sediments.
Although this research was concerned with the potential
application of ferric chloride to reduce phosphorus levels in
the northern Everglades, addition of aluminum sulfate (alum)
is also a potential alternative. Unlike iron, aluminum
compounds do not undergo reduction-oxidation reactions
under anaerobic conditions. However, there is some concern
regarding the effect of aluminum and sulfate ions on the
plant and animal communities. Further investigation would
be required to test the potential use of alum for phosphorus
reduction and retention in the northern Everglades.
Acknowledgments
This project was funded by a grant from the Everglades
Agricultural Area Environmental Protection District and the
Florida Department of Environmental Protection. We would
like to thank Curtis Richardson and Phillip Bachand and for
valuable suggestions regarding this project, Lea Karppi and
Sean Cimilluca for collecting soil and water samples from
the ENR, Jeff Johnson for information regarding the sampling
site, Myun Chul Jo for the use of the scintillation counter,
and Ilka Dinkelman for laboratory assistance.
Supporting Information Available
Methods and data for ortho-P, dissolved organic P +
particulate P, and Fe2+ and Fe3+ concentrations in the water
column of the microcosms. This material is available free of
charge via the Internet at http://pubs.acs.org.
Literature Cited
(1) Narasiah, K. S.; Morasse, C.; Lemay, J. Water Pollut. Res. J. Can.
1994, 29(1), 1-18.
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Received for review February 12, 2001. Revised manuscript
received July 18, 2001. Accepted July 23, 2001.
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