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2003.01
Literature Review
Potential Application of Adsorptive Media to Enhance Phosphorus
Uptake in Stormwater Basins and Wetlands at Lake Tahoe
Final Revision
By Philip A.M. Bachand, Ph.D.
Bachand & Associate
submitted to the
University of California Davis
Tahoe Research Group
In fulfillment of Deliverable 1a of Contract No. 02-00700ICR
November 2003
Adsorptive Media to Enhance P Uptake in Stormwater Basins and Wetlands at Lake Tahoe Bachand & Associates
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Table of Contents
Literature Review............................................................................................................................ 1
Potential Application of Adsorptive Media to Enhance Phosphorus Uptake in Stormwater Basins
and Wetlands at Lake Tahoe........................................................................................................... 1
Abstract........................................................................................................................................... 1
1. Introduction............................................................................................................................. 1
2. Examples of Different Substrates Used for Phosphorus Removal ......................................... 2
2.1 Natural and local mineral soils. ...................................................................................... 2
2.2 Industrial or wastewater byproducts............................................................................... 3
2.3 Engineered Substrate ...................................................................................................... 4
3. Chemical Mechanisms for P removal..................................................................................... 5
3.1 Iron and aluminum rich minerals.................................................................................... 5
3.2 Calcium rich minerals..................................................................................................... 6
3.3 Lanthanum based substrates ........................................................................................... 7
4. Other factors affecting performance: Specific surface Area, Porosity, pH, hydraulic
conductivity..................................................................................................................................... 7
4.1 Physical factors: Size distribution, Specific Surface Area, Porosity and Hydraulic
Conductivity................................................................................................................................ 7
4.2 pH.................................................................................................................................... 8
5. Case Studies and Comparisons Between Substrates............................................................... 8
6. Other issues when considering substrates.............................................................................10
6.1 Environmental considerations....................................................................................... 10
6.2 Application logistics .....................................................................................................11
6.3 Costs.............................................................................................................................. 11
6.4 Cementification............................................................................................................. 12
7. Conclusion ............................................................................................................................ 12
References.....................................................................................................................................17
List of Tables
Table 1. Chemical characteristics selected substrates. Shaded region shows aluminum, iron and
calcium based materials........................................................................................................ 13
Table 2. Qualitative comparison of adsorptive media used for phosphorus removal................... 14
Table 3. Phosphorus removal in subsurface flow experimental systems...................................... 15
Table 4. Langmuir Isotherm results for various adsorptive media.............................................. 16
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Abstract
Phosphorus removal by wetlands and basins in Lake Tahoe may be improved through designing
these systems to filter stormwater through media having higher phosphorus removal capabilities
than local parent material. Substrates rich in iron, aluminum and calcium oftentimes have
enhanced phosphorus removal. These substrates can be naturally occurring, byproducts of
industrial or water treatment processes, or engineered. Phosphorus removal fundamentally
occurs through chemical adsorption and/or precipitation and much of the phosphorus can be
irreversibly bound. In addition to these standard media, other engineered substrates are available
to enhance P removal. One such substrate is locally available in Reno and uses lanthanum coated
diatomaceous earth for arsenate removal. This material, which has a high positive surface
charge, can also irreversibly remove phosphorus. Physical factors also affect P removal.
Specifically, specific surface are and particle shape affect filtration capacity, contact area
between water and the surface area, and likelihood of clogging and blinding. A number of
substrates have been shown to effectively remove P in case studies. Based upon these studies,
promising substrates include WTRs, blast furnace slag, steel furnace slag, OPC, calcite, marble,
Utelite and other LWAs, zeolite and shale. However, other nonperformance factors such as
environmental considerations, application logistics, costs, and potential for cementification
narrow the list of possible media for application at Tahoe. Industrial byproducts such as slags
risk possible leaching of heavy metals and this potential cannot be easily predicted. Fly ash and
other fine particle substrates would be more difficult to apply because they would need to be
blended, making them less desirable and more costly to apply than larger diameter media. High
transportation costs rule out non-local products. Finally, amorphous calcium products will
eventually cementify reducing their effectiveness in filtration systems. Based upon these
considerations, bauxite, LWAs and expanded shales/clays, iron-rich sands, activated alumina,
marble and dolomite, and natural and lanthanum activated diatomaceous earth are the products
most likely to be tested for application at Tahoe. These materials are typically iron, calcium or
aluminum based; many have a high specific surface area; and all have low transportation costs.
1. Introduction
Phosphorus removal by wetlands and basins in Lake Tahoe may be improved through designing
these systems to filter stormwater through certain types of media having high phosphorus
removal capabilities than local parent material. Studies have shown using substrate rich in iron
(Fe), aluminum (Al) or calcium (Ca) concentrations enhances phosphate removal in experimental
subsurface wetlands beyond that which can be achieved by using native soils (Arias et al. 2001;
Pant et al. 2001; Grüneberg and Kern 2001; Mæhlum and Stålnacke 1999; Cerezo et al. 2001;
Comeau et al. 2001). Substrates studied include fly ash, steel furnace slag, blast furnace slag,
red mud, limestone, zeolite, iron-rich sands, laterite, bauxite, burnt oil shale, shale, light
expanded clay aggregate (LECA), crushed marble, vermiculite and calcite (Comeau et al. 2001;
Sakadevan and Bavor 1998; López et al. 1998; Ugurlu and Salman 1998; Wood and McAtamney
1996; Drizo et al. 1999; Brix et al. 2001). Thus, utilizing media with higher P uptake potential
may make these basins more effective at P removal.
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A number of physical and chemical characteristics affect phosphorus uptake potential and rates
including specific surface charge, likelihood to form precipitates, and pH. This literature review
discusses different substrates studied for phosphorus removal and their removal mechanisms.
This literature review also discusses the logistics of using different media at Lake Tahoe and
recommends media to test at Lake Tahoe.
2. Examples of Different Substrates Used for Phosphorus Removal
Typically, substrates studied for enhanced phosphorus removal have elevated aluminum (Al),
iron (Fe) or calcium (Ca) concentrations. Aluminum, iron and calcium rich materials have been
shown to enhance P removal as previously discussed and the mechanisms for removal are
discussed in Section 2. Different substrates have been studied in different parts of the country
and world. One major reason for this different testing preference by region is transportation
costs. Transportation costs can be the highest costs associated with these different media,
especially in the case of natural materials and industrial byproducts. Thus, source location of the
substrate is very important in determining which material to consider for an application and this
issue is reflected in the studies that have been done to date by others.
Many of the substrates tested are naturally occurring. Others are byproducts of industrial or
wastewater processes. Finally, some are engineered. Of these different substrates, most are
based upon the principal that aluminum, iron and calcium enhance dissolved phosphorus
removal through precipitation or adsorption of dissolved phosphorus species. Table 1 shows the
chemical characteristics of selected substrates for which there was published data. For many of
the substrates discussed here, data in the form shown in Table 1 were not available.
2.1 Natural and local mineral soils.
Zeolite, bauxite, laterite, dolomite, shale, limestone, calcite, vermiculite and iron-rich sands are
naturally occurring minerals tested for phosphorus removal. Some of these are iron and/or
aluminum rich and some of these are calcium rich.
Zeolite, bauxite, laterite and shale are aluminum- and/or iron-rich. Zeolite is a hydrous
aluminum silicate. It is composed primarily of silica oxides and secondarily of aluminum oxides
(Sakadevan and Bavor 1998). Calcium, ferric, magnesium and titanium oxides are also found in
zeolite at much lower concentrations. Bauxite, a naturally occurring mixture of hydrous
aluminum oxides and aluminum hydroxides, is the principle source of aluminum and is also high
in ferric oxides. Laterite, a low grade bauxite (Wood and McAtamney 1996), is red in color and
high in iron oxides and aluminum hydroxides. Shale is a fissile rock formed by the consolidation
of clay, mud, or silt. It has a laminated structure, and is composed of minerals essentially
unaltered since deposition. Shale has high concentrations of iron and aluminum (Pant et al.
2001).
Limestone, marble, wallastonite and dolomite are calcium-rich. Limestone is a rock formed
primarily by the accumulation of shells, coral and other calcium rich organic matter and consists
mainly of calcium carbonate. Marble is essentially crystallized limestone. Dolomite is mineral
consisting of calcium magnesium carbonate. Wollastonite is a native calcium silicate.
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2.2 Industrial or wastewater byproducts
Several industrial and wastewater byproducts have also been tested by others. These include
blast furnace slag, steel furnace slag, red mud, Lightweight Aggregates (LWA) of which Light
Expanded Clay Aggregates (LECA) is a subset, Fly ash, HiClay Alumina (HCA), and aluminum
and iron based water treatment residuals (WTR). All these materials are either rich in aluminum,
iron and/or calcium.
Blast furnace slag has tested in several studies (Johansson 1999; Agyei et al. 2002; Grüneberg
and Kern 2001; Sakadevan and Bavor 1998) and is an industrial by-product of iron ore
processing. Its composition varies with the ore used. Blast furnace slag is rich in calcium oxide
(33 – 43%) and is also relatively high in aluminum oxide (9 – 16%; Table 1). Blast furnace slag
can be granulated by rapid quenching (Sakadevan and Bavor 1998). It has been tested for use in
wastewater treatment and in constructed wetlands.
Sakadevan and Bavor (1998) tested steel furnace slag, which is rich in both iron and calcium
(Table 1). Steel furnace slag results form the conversion of pig-iron to steel and consists of iron,
aluminum and calcium oxides.
Lopez et al. (1998) investigated using red mud to remove phosphorus from secondary effluent
for the removal of phosphorus and heavy metals. Red mud is a byproduct of bauxite refining and
composed primarily of iron, calcium and titanium oxides, though also having nearly 5% of
calcium oxide as well (Table 1; Lopez et al. 1998). It has a high initial pH and forms stable
suspensions in water. When pretreated with 8% anhydrite, it forms aggregates stable in aqueous
media and has a lower pH, just above neutral (Lopez et al. 1998).
HiClay Alumina is a byproduct of the commercial alum production, consisting of bauxite
impurities that did not react with sulfuric acid used to produce alum. It has a similar elemental
composition to that of highly weathered soils (Haustein et al. 2000) and is approximately 1.5%
aluminum.
Water Treatment Residuals (WTR) are byproducts of potable water treatment. Haustein et al.
(2000) investigated WTR produced from water treatment with alum. It is typically composed of
coagulated aluminum compounds and other materials like sand, silt, clay, bacteria and color-
forming compounds found in raw water. WTR are typically similar to natural soils in both the
metal concentrations, like Al, as well as in trace element concentrations unless the source water
is grossly contaminated (Haustein et al. 2000). Aluminum concentrations are approximately
three times higher than in HiClay Alumina at roughly 5%.
WTR can also be iron based. Brown (1999) studied blending iron-rich WTR with sand to
improve fertilizer uptake and retention for use on golf greens. This iron-rich WTR results from
using ferric sulfate to treat drinking water. The Ferric Sulfate reacts with the natural organic
material (NOM) in colored surface waters to form Iron Humate®, a registered product of
Kemiron.
Fly ash has been the subject of many studies including those focusing on wastewater treatment
(Agyei et al. 2002; Ugurlu and Salman) , constructed wetlands (Sakadevan and Bavor 1998;
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Drizo et al. 1999, Mann 1997) and infiltration (Cheung and Venkitachalam 2000). Fly ash is a
residue of combustion from coal power plants (Ugurlu and Salman 1998). Fly ash is categorized
as Type F which is produced when either anthracite, bituminous or sub-bituminous coal is
burned or Type C which normally comes form lignite or sub-bituminous (Rinker Materials
2003). The amount of calcium, silica, alumina and iron content in the ash differentiates these
two classes. Type F fly ash, typically has total calcium ranging from 1 – 12% with it in the form
of calcium hydroxide, calcium sulfate and glassy components combined with silica and alumina
(Turner-Fairbank 2003). Type C can have calcium oxide as high as 30 – 40% and has higher
sulfate concentrations. Silica and aluminum oxide concentrations are higher in Type F fly ash,
and calcium oxide concentrations are higher in Type C (Grubb et al. 2000). Type F fly ash are
more acidic. Approximately sixty million tons of fly ash are produced annually in the United
States and only 25 – 30% of the material is reused (Grubb et al. 2000).
Lightweight aggregates (LWAs) are factory made filter media used mainly for construction
material such as building blocks but recently tested for phosphorus removal (Zhu et al. 1999).
Some LWAs are clay-based and some are shale-based; thus their chemical composition varies
according to the parent material. LECA is one such material, produced in Norway. It is clay
aggregate formed by expanding clay minerals at high temperatures (Drizo et al. 1999).
2.3 Engineered Substrate
Some substrates have been engineered for phosphorus removal and are proprietary. Some of
these substrates are based upon the principle that iron and aluminum rich materials enhance P
uptake. One such substrate is activated alumina. Activated alumina is used commercially from
several manufacturers and is used for removal of phosphorus and other pollutants. Activated
alumina is characterized by high aluminum content, high specific surface area and high
macroporisity. LWAs can also be considered engineered substrates in some respects. These
substrates are engineered for other purposes but because of their relatively high specific surface
area and high aluminum content some of these substrates are also good for phosphorus removal.
Other engineered substrates rely on other technologies. Phoslock is a modified clay developed
by CSIRO Land and Water. Laboratory batch studies by CSIRO have shown that Phoslock
effectively reduces filterable reactive phosphorus and in field mesocosm studies this removal has
been shown to be approximately 90% (Douglas et al, unpublished). There is very little
information on the mechanisms for phosphorus removal by Phoslock as the product is pending
patents.
Another such proprietary product is modified or activated diatomaceous earth (DE). Modified
diatomaceous earth (MDE) utilizes lanthanum oxide and lanthanum-alumina oxide to remove
phosphorus. This technology was first developed for arsenic and arsenate removal (Misra and
Lenz, 2003) but has been recently been shown to effectively remove phosphorus as well (Misra,
unpublished data). The mechanisms for phosphorus removal are discussed in Section 2. This
technology was developed at the University of Nevada Reno Lackey School of Mines and
products based on this technology are available locally to Lake Tahoe from Eagle-Picher
Minerals, Inc., located outside of Reno, Nevada.
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3. Chemical Mechanisms for P removal
Two mechanisms lead to phosphorus removal, adsorption and precipitation. Under appropriate
pH conditions, all the above media have been tested and found to have some success in
phosphorus removal. However, pH is an important consideration when predicting which
substrate is likely to be among the more effective and the associated mechanisms controlling
phosphorus removal. In general, alkaline environments favor phosphorus (P) removal by
calcium through adsorption and precipitation whereas low pH and acidic environments favor
removal by iron and aluminum (Ugurlu and Salman 1998).
3.1 Iron and aluminum rich minerals
In acidic to neutral environments, phosphate ions are chemically adsorbed onto Fe and Al oxide
surfaces through ligand exchange (Wood and McAtamney 1996). In many substrates, P
adsorption correlates better with the amount of oxalate extractable aluminum and iron than with
other independent factors such as total aluminum or iron, or specific surface area (Sakadevan and
Bavor 1998; Reddy et al. 1995). Oxalate extractable iron or aluminum represents amorphous
and poorly crystalline oxides of Al and Fe. This fraction, rather than the crystalline phases, is
often more critical for P removal in iron and aluminum rich minerals. This explains why
Baskaran et al. (1994) found that allophanic soil adsorbed 7 times more phosphate than non-
allophanic soil. As pH rises above 7 – 8, P adsorption by amorphous iron and aluminum
decreases (Grubb et al. 2000).
However, this fraction may also not be thermodynamically stable (Misra, personal
communications). As the amorphous structure of these substrates evolves into more ordered and
crystalline structures, P may be released.
Hongshao and Stanforth (2001) use a two-phase model to describe P uptake onto iron oxides.
They propose in their model that adsorption initially occurs rapidly through formation of a
surface precipitate. This precipitate is non-exchangeable. These sites then act as sorption sites for
dissolved iron, which in turn sorbs more phosphate slowly from solution, forming an amorphous
iron phosphate precipitate. This model is only one model used to describe this process and is not
the definitive model describing this process.
During P uptake, other ions compete with phosphate for adsorption. Arsenate (AsO4) competes
with phosphate for uptake onto the iron hydrogen oxide goethite (Hongshae and Stanforth 2001).
Sulfate, arsenate and silicic acid compete with phosphate for uptake by both allophane, an
amorphous aluminum crystal, and gibbsite, an amorphous iron oxide (Gustafsson 2001). This
competitive adsorption may be caused by either direct competition for adsorption sites or
through changes in surface charge upon adsorption (Geelhoed et al. 1997). Regardless, this
competitive adsorption is likely a factor in why red mud, laterite and other iron and aluminum
rich substrates can effectively removal heavy metals like manganese, chrome, lead, cadmium,
copper, nickel and zinc in addition to P (Lopez et al. 1998; Wood and McAtamney 1996).
For iron and aluminum substrates, P uptake rate will decrease over time as the P concentrations
increase in the substrate (Ugurlu and Salman 1998; Haustein et al. 2000). The rate of decrease is
dependent upon the P loading rate and at low loading rates may be negligible over time
(Grüneberg and Kern 2001). Much of the bound P is likely to be irreversibly bound. Mann
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(1997) showed in laboratory desorption studies that about 85% of the adsorbed phosphate
remained adsorbed to both blast furnace and steel furnace slags during the desorption studies.
However, some of the P will likely be adsorbed reversibly and as substrate are used and become
more saturated with respect to P, higher phosphate concentrations are to be expected in the
outflow under equilibrium conditions (Pant et al. 2001). The increase in equilibrium phosphate
concentration in the outflow will depend upon the chemistry of the media and its P loading
history.
Finally, uptake will be pH dependent. Oh et al. (1999) found that optimum pH for iron and
aluminum based materials were at about 5.4 and decreased with increasing pH.
3.2 Calcium rich minerals
Ugurlu and Salman (1998) conclude that at higher pH, phosphate removal occurs through
calcium phosphate precipitation. This is supported by a decrease in Ca2+ ions and a decrease in
pH values found by Ugurlu and Salman (1998) during adsorption studies on fly ash as well as the
formation of a white-looking precipitate at the top of the fly ash bed in column studies.
Precipitates were originally amorphous calcium phosphate as shown in XRD studies. Johansson
and Gustafsson (2000) also found that precipitation was the primary mechanism removing P
from solution but calcium and P speciation data ruled out formation of amorphous calcium
phosphates. Instead evidence existed that precipitation to hydroxyapatite was the primary
mechanism. Hydroxyapatite has low solubility and may tightly bind P.
Regardless of the exact mechanism, Johansson and Gustafsson (2000) concluded the strongest P
removal is obtained from substrate from which calcium easily leach to supersaturate solutions
with regard to calcium, and for substrate that already contain seeds for hydroxyapatite or other
apatite which can act as seeds for precipitation. The rate of calcium desorption may be a primary
factor in sorption experiments which show that there is an initially rapid removal of P followed
by a lower but more sustained P removal rate. The initially high removal rate is attributed to
rapid desorption of calcium from the substrate leading to precipitation (Cheung and
Venkitachalam 2000).
For these calcium rich substrates, calcium content and its form is the primary factor determing P
sorption capacity (Brix et al. 2001, Johannson 1999). Johannson (1999) found that amorphous
forms of calcium outperformed crystalline forms.
P removed initially by calcium-rich minerals appears to be relatively insoluble. Mann (1997)
tested sandstone, blast furnace slag, granulated blast furnace slag, steel slag, fly ash bottom ash,
coal wash and two local gravels. He found P removal was significantly related to Ca, Mg, S and
Si concentrations as well as pH. He concluded that high and stable P removal of the slags
occurred because stable calcium-P complexes occurred at elevated pH. Effluent pH levels were
found to range from 8.5 – 9.5. Slags removed 4 – 8 times as much P as local gravels and
desorbed less. O’Reilly and Sims (1995) found similarly that as the percentage of fly ash
amended to soils increased, the soils had higher P uptake rates and desorbed less P. O’Reilly and
Sims (1995) found that over time the adsorbed and precipitated P removed by fly ash became
more stable and was less likely to desorb.
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However, P removed by these materials may also later become bound to iron or aluminum
complexes or hydroxides. Johansson (1999) found in P fractionation experiments that 50-63% of
P removed by blast furnace slag, a substrate high in calcium, was loosely bound while another
34-46% was iron bound even though iron makes up less than 1% of the substrate. Thus, there
may not be such a clean break between calcium based processes and iron and aluminum based
processes as once P is adsorbed or precipitated onto a substrate, it may migrate to another
mineral that might more tightly bind to it.
3.3 Lanthanum based substrates
The University of Nevada Reno and Eagle-Picher Minerals, Inc. have developed a new
technology to remove arsenic from drinking water. Naturally occurring DE is coated with
nanocrystals of lanthanum (Misra and Lenz 2003). DE is a naturally occurring siliceous
sedimentary material resulting from the accumulation of an enormous number of fossil diatoms.
Its microscopic structure traps sub-miron particles while maintaining its permeability.
Lanthanum is coated onto the DE in a crystalline hydroxy-gel. The hydroxy-gel has a high
positive surface charge up to a pH of 11 and this enables negatively charged arsenate to be
tightly bonded to the substrate (Misra and Lenz 2003). Arsenate and phosphate compete for
adsorption sites. Thus, data shows that the modified DE (MDE) also removes phosphate (Misra,
unpublished data).
4. Other factors affecting performance: Specific surface Area, Porosity, pH,
hydraulic conductivity
A number of other factors may affect P uptake by different substrates and this includes specific
surface area, porosity, cation exchange capacity (CEC), pH and hydraulic conductivity (Drizo et
al. 1999). Drizo et al. (1999) in a study of several different substrates which included bauxite,
shale, burnt oil shale, limestone, zeolite, LWA and fly ash found that none of these factors
significantly affected phosphate adsorption capacity as determined through the Langmuir
equation. This represents the results of laboratory adsorption isotherm batch studies and not that
of an adsorption column. In systems in which adsorption and filtration is occurring through
columns of some sort, such as in sand filters or subsurface wetlands, these other characteristics
can affect P uptake.
4.1 Physical factors: Size distribution, Specific Surface Area, Porosity and Hydraulic
Conductivity
Physical factors that compromise water flow and cause short-circuiting and blinding can reduce
P removal efficiencies. Hydraulic conductivity, size distribution, porosity and specific surface
area are different variables quantifying the physical characteristics of the media. Hydraulic
conductivity is dependent upon size distribution, porosity and specific surface area. Porosity is a
function of size distribution, specific surface area and particle shape. Thus, size distribution and
specific surface area are the key variables defining these physical factors and these two
characteristics are discussed below.
Size distribution. Size distribution is expected to affect both physical filtration and adsorption
efficiency. In specifying size distribution, a balance exists between too small of particles that
will cause cl0gging and blinding, and too large of particles providing inefficient filtering.
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Danish EPA guidelines recommend an effective grain size d10 in the range of 0.3 – 2.0 mm, a d60
between 0.5 and 8.0 mm, and a uniformity coefficient (d60/d10) less than four (Arias et al. 2001).
These recommendations are to help ensure adequate hydrologic conductivity and to reduce
blinding during filtration. Substrates with smaller particle size distribution are expected to
increase the potential for direct reaction between the water and the substrate because these
substrates are expected to have the highest surface area. Drizo et al. (1999) showed P removal
data supporting this expectation with substrates with smaller particles size distribution typically
having higher P adsorption capacity as derived from a Langmuir equation. Stevik et al. (1999)
support these recommendations finding that finer grain media enabled stable unsaturated flow
through a greater area of a filter because of the higher capillary forces in finer particles. This
reduces diffusions limitations and improves the exchange between mobile and less mobile water.
Specific surface area. Specific surface area is the surface area of the substrate normalized
against its mass. It is a function of both the size of the particles but also the structure of the
particles and is expressed as m2/g. Porous or fractured media will have a higher specific surface
area than solid media. Thus, porous media such as activated alumina (>300 m2/g), diatomaceous
earth (> 2 m2/g), modified diatomaceous earth (lanthanum coated; up to 90 m2/g) and LWA have
higher specific surface area than solid media such as sands. Stevik et al. (1999) showed that
specific surface area can enhance pollutant uptake (e.g. bacteria) because of an increased surface
area available for adsorption sites. However, the density of charged adsorption sites on the
available specific surface area will also affect pollutant uptake. Drizo et al. (1999) show an
example of this in which zeolite has poor P uptake capacities despite a high specific surface area
because of the low density of adsorption sites. Conversely, Misra and Lenz (2003) show MDE
with a high arsenate removal capacity because of both its high specific surface area and high
charge density.
4.2 pH
During chemical precipitation or adsorption of P by iron, aluminum and calcium, the process
affects pH. P removal by iron and aluminum lowers pH and has an optimum of around 5.4 (Oh
et al. 1999). For P removal by calcium-based substrates in which P is removed by precipitation,
pH increases. pH can be increased to as high as 10, as is shown when P is removed by Utelite,
an expanded shale. For more crystalline calcium substrate, such as marble or metamorphic
limestone such as found in California, pH changes are smaller though the P removal is also less
effective. Thus, under acidic conditions, iron and aluminum based substrates tend to be more
effective than calcium based substrates. Under basic conditions, the reverse is true. Lanthanum
based substrates do not affect pH and P removal with lanthanum based substrates is effective
over a wide range of pH, showing similar P removal rates up to a pH of 10.
5. Case Studies and Comparisons Between Substrates
Case studies have shown that adsorptive media can be used successfully as a substrate to
improve P removal in subsurface flow wetlands. Grüneberg and Kern (2001) showed blast
furnace slag removed P in a subsurface constructed wetland mesocosm under both aerobic and
anaerobic conditions. In these systems, most P was removed by the substrate rather than by plant
uptake. Under aerobic conditions, the P fraction removed was attributed to loosely bound P or P
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associated with Ca or Mg carbonates. This mechanism is very pH dependent. Under anaerobic
conditions, slag also stored P but more as a fraction associated with the formation of amorphous
iron oxides. This compartment appeared redox-insensitive, forming under both aerobic and
anaerobic conditions.
Wood And McAtamney conducted pilot scale constructed wetland tests (5m long x 1 m wide x
0.5 m deep) in systems with Phragmites spp. planted in crushed granite gravel with laterite
strips. These systems were operated under detention times of 4 – 8 days and achieved 95% P
removal, achieving P concentrations of 0.1 mg/L from an inflow of 1.46 mg/L. Cadmium,
chrome and lead were also removed.
Several studies have compared substrates:
• Haustein et al. (2000) found that Water Treatment Residuals (WTRs) removed 20 times
more P than does HiClay Alumina and attributed the higher removal rates primarily to the
WTR having aluminum concentrations three times higher. Runoff for WTRs contained
higher aluminum concentrations than for HiClay Alumina.
• Johansson and Gustafsson (2000) found blast furnace slag more efficient at retaining P
than opoka, a mineral with a high calcium content and not insignificant iron and
aluminum concentrations. Though opoka has high calcium concentrations, it was
ineffective at P retention and this was attributed to opoka not being able to supersaturate
the solution with calcium ions and thus not enable the initiation of precipitation.
• Mann (1997) compared 2 gravels, hawkesbury sandstone, granulated blast furnace slag,
blast furnace slag, steel slag, fly ash, bottom ash, and coal wash. He found calcium and
magnesium concentrations in the substrates best correlated with P removal performance.
• Agyei et al. (2002) compared OTC (ordinary Portland cement), steel slag, and fly ash and
found OPC the most effective and fly ash the least effective. CaO concentrations best
predicted performance.
• Sakadevan and Bavor (1998) compared six soils, blast furnace slag, steel furnace slag and
zeolite. Blast furnace slag was found to have the highest P adsorption capacity and this
factor best correlated with the oxalante extractable iron and aluminum concentrations.
• Arias et al. (2001) compared different sands and found that calcium content best
predicted P removal performance.
• Brix et al. (2001) compared 13 sands as well as LECA, crushed marble, diatomaceous
earth, vermiculite and calcite. Calcium content was the most important criteria
determining performance. Order of performance were calcite, marble, diatomaceous
earth, LECA, vermiculite and quartz sand.
• Drizo et al. (1999) compared a suite of natural and industrial products, which included
bauxite, shale, burnt oil shale, limestone, zeolite, LECA and fly ash. Fly ash and shale
had the highest P adsorption values, with bauxite, limestone and LECA with the next
highest values. In column studies, the order of effectiveness for adsorbing P was shale,
fly ash, bauxite and LECA.
• Zhu et al. (1997) compared several LWAs which included LECA with iron-rich sands.
They found P removal capacity of the different LWAs varied greatly with higher uptake
rates by those LWAs with a higher total metal concentration (i.e. iron, aluminum,
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calcium). Poor performing LWAs removed about one order of magnitude less P than
iron rich sands. Good performing LWAs removed about one order of magnitude more.
• Oh et al. (1999) compared synthetic hematite, goethite and allophane with alumina.
Alumina had the second highest adsorption capacity of the four tested compounds and is
commercially available.
• Pant et al. (2001) found that over time media performance decreased for dolomite, sand
and shale. Shale had the highest adsorptive capacity after a period of extended use and
thus had the highest capacity of media tested. Over the time period tested, shale had taken
up between 71 and 88% loaded P as compared to 56 – 68% with dolomite.
• Phillips (1998) compared zeolite, red mud and calcium phosphate as soil amendments
and found when red mud and zeolite were added to sandy soil at a 10% (w/w) ratio,
phosphate uptake improvements were negligible. This was attributed to the high pH and
predominantly negative charge of the red mud and lack of sorption sites of zeolite.
Tables 2 and 3 summarize these findings. Based upon these comparisons, a number of substrates
would seem more promising and just as importantly a number seem not very promising. LECA,
vermiculite, uncoated sands, gravels, coal wash bottom ash, Opoka, and HiClay alumina are not
promising being either the worst or poorer performers in one of the reported studies (Table 2).
More promising substrates are WTRs (iron or aluminum based), blast furnace slag, steel furnace
slag, OPC, calcite, marble, UTELITE, and shale. In general, these were either best performers in
a reported study (e.g. WTR, blast furnace slag, fly ash, OPC, calcite, UTELITE, shale) or in
another study outperformed a substrate that was the best in one study (e.g. steel furnace slag).
Table 4 presents Langmuir isotherm data for the various media. There is great variation in these
results. For instance, blast furnace slag has Smax values that vary by several orders of magnitude.
This variation could be due to several factors including chemical differences in the tested
substrate or laboratory methodology differences. Nonetheless, based upon these results,
activated alumina is a commercially available substrate that should be added to the list of more
promising substrates.
6. Other issues when considering substrates
Several other issues should be considered when selecting coagulants to test. These include
potential to environmental considerations, application logistics, transportation costs and
cementification.
6.1 Environmental considerations
Heavy metals. Industrial byproducts such as blast furnace slags and steel furnace slags and fly
ash have inherently high concentrations of metals. Thus, there may be a higher likelihood with
these products than with more natural products or less processed products to leach heavy metals
into the environment (Misra, personal communications; Patel, personal communications).
Conversely, several substrates may actually remove heavy metals. Lopez et al. (1998) found
that red mud removed nickel, copper and zinc. Ugurlu and Salman (1998) reported that fly ash
has been found to removal cadmium, chromium, arsenic and can be substituted for activated
carbon when blended with coal in the removal of chrome dye. Wood and McAtamney (1996)
showed that laterite can reduce cadmium, chromium and lead concentrations to undetectable
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levels. Notably, red mud and fly ash are high in calcium whereas lateriate is high in aluminum
and iron, suggesting that both classes of adsorptive media, iron and aluminum rich media vs.
calcium rich media, can effectively remove heavy metals.
Thus, heavy metal leaching and uptake are both possible and may not be easily predicted. For
instance fly ash is an industrial byproduct and would be expected to leach heavy metals but has
also been shown to remove cadmium, chromium and arsenic. This issue needs to be considered
when initially selecting substrates for consideration and at some point tested for when finalizing
substrate selection.
Aluminum and iron content in runoff. Several aluminum based material can result in
elevating aluminum concentrations above background levels. Haustein et al. (2000) found that
WTRs removed P more effectively than HiClay Alumina but also resulted in more greatly
elevating aluminum concentrations in runoff. Patel (personal communications) found higher
aluminum concentrations in waters experimentally treated with Utelite and with activated
alumina than in non-treated waters. These increases may be incidental or negligible under many
situations but still should be considered when deciding upon and testing different substrates for P
removal.
6.2 Application logistics
Depending upon the selected site, the expected design, and available expertise and budget,
certain substrates may be eliminated. For instance, if P removal is expected to occur in a system
with subsurface flow, the ideal substrate will have a good hydraulic conductivity, not blind easily
and require minimal replacement and maintenance. Thus, substrates will have a specific size
distribution typical of substrates such as fine sand, but also have high uptake P uptake
capabilities. Several of the substrates discussed would not be ideal for this application. For
instance, more fine substrate such as fly ash would need to be blended with sand or other media
as it is likely too fine for good filtration. Blending adds additional costs and expertise. Under
such an application, porous media such as DE, activated alumina, LWA and expanded shales
would seem more ideal than non-porous media such as sands.
6.3 Costs
Costs standardized against performance is an important consideration. Activated alumina is very
effective at P removal and is widely used in water treatment systems, but it is also very
expensive as compared to natural products and industrial and wastewater treatment byproducts.
Thus costs is a very important consideration.
For many products, there will be costs associated with their manufacturing or mining, and costs
associated with their transportation. For many products, such as aggregates (e.g. sand,
limestone), transportation cost will be the major factor determining cost to the customer.
Transporation costs is a major factor in determining what substrates have been tested where.
For an application at Lake Tahoe, transportation costs can be minimized by considering substrate
local to California and Nevada. Metamorphic (not amorphous) limestone and dolomite; iron-rich
sands; sands; diatomaceous earth and modified diatomaceous earth; bauxite; and expanded shale
are natural substrates found in either or both California and Nevada. Local producers of
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industrial byproducts such as fly ash, blast furnace slag and steel furnace slag are not easily
found in California or Nevada. Utelite can be found relatively locally in Utah. We have not
been able to find local producers of other promsing substrate discussed in this literature review.
6.4 Cementification
Cementification is an issue for amorphous calcium based substrates. In these substrates, P
removal occurs through precipitation. This eventually results in cementification of the substrate
(Misra, personal communication). Cementification will eventually reduce hydraulic conductivity
in filtration or subsurface flow applications.
7. Conclusion
Substrates rich in iron, aluminum and calcium oftentimes have enhanced phosphorus removal. P
removal by iron and aluminum is more prevalent under pH conditions below neutral and removal
by calcium is more prevalent under more alkaline pH conditions. These substrates can be
naturally occurring, byproducts of industrial or water treatment processes, or engineered.
Phosphorus removal fundamentally occurs through chemical adsorption and/or precipitation.
Phosphorus removal is typically greater for amorphous materials because of the greater number
of adsorption sites. However, this removal process may not be thermodynamically stable and
could release phosphorus as the structure becomes less amorphous and more crystalline.
Nonetheless, in many substrates, most the P removed is irreversibly bound.
Other options have become available for P removal and these are engineered substrates. One
such substrate is locally available in Reno and uses lanthanum impregnated diatomaceous earth
for arsenate removal. This material, which has a high positive surface charge, can also
irreversibly remove phosphorus.
Other factors affect P removal and these are mainly physical. Specific surface are and particle
shape are the likely key variables. These variables affect filtering capacity, contact area between
water and the surface area, and likelihood of clogging and blinding.
A number of substrates have been shown to effectively remove P in case studies. Based upon
these studies, promising substrates include WTRs, blast furnace slag, steel furnace slag, OPC,
calcite, marble, Utelite and other LWAs, zeolite and shale. However, other nonperformance
factors such as environmental considerations, application logistics, costs, and potential for
cementification further narrow the list of possible media for application at Tahoe. Industrial
byproducts such as slags risk possible leaching of heavy metals and this potential cannot be
easily predicted. Fly ash and other fine particle substrates would be more difficult to apply
because they would need to be blended, making them less desirable and more costly to apply
than larger diameter media. High transportation costs rule out non-local products. Finally,
amorphous calcium products will eventually cementify reducing their effectiveness in filtration
systems. Based upon these considerations, bauxite, LWAs and expanded shales/clays, iron-rich
sands, activated alumina, marble and dolomite, and natural and lanthanum activated
diatomaceous earth are the products most likely to be tested for application at Tahoe. These
materials are typically iron, calcium or aluminum based; many have a high specific surface area;
and all have low transportation costs.
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Table 1. Chemical characteristics selected substrates. Shaded region shows aluminum, iron and calcium based materials.
Substrate Reference SiO
2
Al
2
O
3
Fe
2
O
3
FeO CaO CaCO
3
MnO MgO SO
3
Na
2
OK
2
OTiO
2
LOI
2
Natural and local minerals
zeolite Ash Meadows
2001.
Natural Clinoptilolite 66.9 10.5 0.9 1.2 < 0.1 0.6 3.0 4.1 0.1 9
bauxite IMPASCO 2003 Depends upon grade >12 - 22 57 - 71 2 - 35 < 1 < 1 3 - 3.8 13 - 15
laterite Wood and
McAtamney 1996
0.7 39.8 26.7 3.6
shale Krishisworld 58.9 15.6 4.1 2.5 3.2 2.5 1.3 3.3 0.7
wollastonite Dada 2003 42.1 6.4 2.6 3.1 6.3
dolomite -
magnesium
limestone
Dada 2003 0.6 <0.1 <0.1 30.3 Nil 20.7 1.6 Nil 46.2
opoka Johansson and
Gustafsson 2000
34.1 8.8 5.2 50.7
calcium
carbonate
(marble,
limestone)1
CMC SRL 2003 Typical 1.7 0.6 0.3 53.4 0.7
Industrial and Wastewater Byproducts
blast furnace slag Sakadevan and
Bavor 1998
32-37 13-16 <1.3 38-43 <1 5-8 <0.5 <1
Johansson and
Gustafsson 2000
35 9.6-11.4 0.3-0.5 33.4-
35.0
0.4-0.5 13.7-
14.3
steel furnace slag Sakadevan and
Bavor 1998
10-15 1-5 20-30 35-45 2-5 7-12 <0.5 <1
red mud 20.1 31.8 4.8 22.6
Fly ash Turner-Fairbanks
2003
Bituminous 20-60 5-35 10-40 1-12 0-5 0-4 0-4 0-3 0-15
Subbituminous 40-60 20-30 4-10 5-30 1-6 0-2 0-2 0-4 0-3
Lignite 15-45 10-25 4-15 15-40 3-10 0-10 0-6 0-4 0-5
Ugurlu and Salman
1998
26.4 10.0 3.4 33.8 1.7 0.2 1.0 0.4
1. Marble is approximately 97% calcium carbonate and limestone ranges from about 93 - 97% calcium carbonate
2. Loss on ignition
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Table 2. Qualitative comparison of adsorptive media used for phosphorus removal.
Citation Comment
Haustein et al. 2000 WTR HiClay Alumina Depended upon aluminum content.
Johansson and Gustafsson
20001
FC and CC Blast
Furnace Slag
FA Blast Furnace
Slag
CA Blast furnace
slag
Opoka Opoka not supersaturate Ca in solution
Mann 19972 Fly ash G. Blast Furnace
Slag
steel slag, blast
furnace slag
Hawkesbury
sandsone
coal wash bottom
ash
gravels. Correlated primarily to Ca and Mg
concentration
Agyei et al. 20023 OPC steel slag fly ash Correlated with CaO concentration in substrate
Sakadevan and Bavor
19984
Blast Furnace
Slag
Some soils, steel
furnace slag,
zeolite
Some soils Phosphorus removal best correlated with
oxalate extractable iron and aluminum
Brix et al 2001 Calcite Marble Diatomaceous
earth
LECA, vermiculite Calcium content best predictor of performance.
Brown 1999 Iron Humate Iron rich sands Uncoated sands Iron Humate is a WTR. pH was not
depressed as much with iron rich sands.
Zhu et al. 1997 UTELITE Other LWAs Iron rich sands LECA Performance depend upon total metal content.
Drizo et al 1999 Shale Fly ash Bauxite LECA No specific property best predict performance
1. FC = Fine crystalline, FA = Fine amorphous; CC = Course Crystalline; CA = Course Amorphous
2. G=Granulated
3. Ordinary Portland Cement
4. No clear rating given
Substrate in order of performance (best to worst)
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Table 3. Phosphorus removal in subsurface flow experimental systems
media Size Influent Effluent Removal HRT Length of
study
Comment Citation
mg/L mg/L %days months
Blast furnace slag Buckets >40 198 20 3w/o aeration Gruneberg and Kern 2001
Blast furnace slag Buckets >40 0.6 98 20 3w/aeration Gruneberg and Kern 2001
laterite pilot scale reed bed 5 - 10 <1 96 310 Wood and McAtamney 1996
Shale wetland mesocosm 10 ~0.2 98 510 Drizo et al 1997
Marble Column 10 ~2.5 75 0.5 3Brix et al 2001
Vermiculite Column 10 ~7.0 30 0.5 3Brix et al 2001
LECA Column 10 ~7.0 30 0.5 3Brix et al 2001
Diatomaceous earth Column 10 ~3.5 65 0.5 3Brix et al 2001
Dolomite wetland mesocosm 56 45 Pant et al 2001
Queenstone shale wetland mesocosm 88 40 Pant et al 2001
Fonthill sand wetland mesocosm 77 - 82 33 Pant et al 2001
Fly ash wetland mesocosm 20 0.8 - 4.0 80 - 96 0.04 0.1 Ugurlu and Salmon 1998
15-30% Fly ash in
spearwood sand
Column ND 3Cheung and Venkitachalam
2000
Blast furnace slag Column 10 ~0.5 >95 2Johansson 1999
Red mud Column 5.2 ~3.4 38 0.13 2Lopez et al. 1998
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Table 4. Langmuir Isotherm results for various adsorptive media.
Adsorption Media Sorption maxima Binding strength Equilibrium P concentration(b) Shaking time Notes Citations
Smax kEPCo
mg kg
-1
l mg
-1
mg L
-1 hr
LWA - Arkansas Lightweight Corp 37(a) 24 Zhu et al 1997
LWA - Chandler, OK 39(a) 24 Zhu et al 1997
LECA 46 - 565(a) 24 Varies with type Zhu et al 1997
Queenston shale 192 0.07 0.48 24 Pant et al. 2001
Evesboro Loamy sand + 10% fly ash 203 0.55 24 O'Reilly and Sims 1995
FILTRALITE 209 - 2210(a) 24 Zhu et al 1997
Evesboro loamy sand 213 0.49 24 O'Reilly and Sims 1995
Evesboro Loamy sand + 20% fly ash 242 0.44 24 O'Reilly and Sims 1995
Evesboro Loamy sand + 30% fly ash 280 0.28 24 O'Reilly and Sims 1995
Lockport dolomite 303 0.16 0.05 24 Pant et al. 2001
Steel Furnace Slag 380 30 Mann 1997
Blast Furnace Slag 400 30 Mann 1997
Foothill sand 417 0.07 0.02 24 Pant et al. 2001
LECA 420 0.1 24 Drizo et al 1999
Sand 439-443(a) 24 Zhu et al 1997
Zeolite 460 0.03 24 Drizo et al 1999
Burnt Oil Shale 580 0.06 24 Drizo et al 1999
Bauxite 610 0.26 24 Drizo et al 1999
Fly Ash 625 30 Mann 1997
Shale 650 0.61 24 Drizo et al 1999
Limestone 680 0.1 24 Drizo et al 1999
Lagoon fly ash 860 0.07 24 Aged Drizo et al 1999
Steel Furnace Slag 1430 0.046 48 Sakadevan and Bavor 1998
Zeolite 2150 0.0442 48 Sakadevan and Bavor 1998
hematite 2200 Synthesized Oh et al 1999
UTELITE 3496(a) 24 Zhu et al 1997
Red mud 7027 24 Lopez et al 1998
Precipitator Fly ash 13766 0.28 24 Cheung and Venkitachalam 2000
goethite 16400 Synthesized Oh et al 1999
alumina 17100 Oh et al 1999
Blast Furnace Slag 44247 0.367 48 Sakadevan and Bavor 1998
Allophane 51000 Synthesized Oh et al 1999
Notes
a. At equilibrium with P concentration of 320 mg/L, below level required for Langmuir calculation of Smax
b. Solution P concentration in equilibrium with media.
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