Formation pathways for iron oxide minerals and

geochemical conditions for phosphate retention in iron

enhanced sand filters

Final report to the Minnesota Stormwater Research Council

on the funded project

“Determining which iron minerals in iron-enhanced sand filters

remove phosphorus from stormwater runoff”

Respectfully submitted to MSRC on November 25, 2019

by

Beth A. Fisher, University of Minnesota

Joshua M. Feinberg, University of Minnesota

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We would like to express our appreciation and acknowledge the cooperation, sampling access,

data sharing, and camaraderie of the following individuals who made this work possible:

Anthony Aufdenkampe, Keith Pilgrim, Diane Spector, Lu Zhang, Ed Matthiesen, Sarah Nalven,

Chris Meehan, John Lenth, Curtis Hinman, Dylan Ahearn, Carlos Herrera, Karen Kill, Ryan

Fleming, Britta Belden, Joe Sellner, Mike Trojan, and Anette von der Handt.

This project was supported by the University of Minnesota Water Resources Center and by the

Minnesota Stormwater Research Council with financial contributions from:

● Capitol Region Watershed District

● Mississippi Watershed Management Organization

● Ramsey-Washington Metro Watershed District

● South Washington Watershed District

● Valley Branch Watershed District, and

● City of Edina

● Minnesota Cities Stormwater Coalition

For more information about the Center and the Council, visit

https://www.wrc.umn.edu/projects/storm-waste-water

Any opinions, findings, conclusions, or recommendations expressed in this publication are those

of the author(s) and do not necessarily reflect the view of the Water Resources Center or

Minnesota Stormwater Research Council.

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Table of Contents

Summary of Findings 4

Engineered Iron-Phosphorus Traps 5

Chemistry of Iron Source 7

Chemistry of IESF components 7

Binding Mechanisms for Fe and P 8

Mineral Precipitation 8

Adsorption 9

Mineral surfaces for adsorption 10

Sorption capacity depends on surface area and pH 10

Study Sites 11

Champlin Biochar IESF (2017) 11

North Lions Park IESF (2017) 12

Olson Middle School Biochar IESF (2017) 12

William Street IESFs (2011) 12

Hwy 36 & 61 Cloverleaf IESF (2013) 12

Beam Ave IESF (2007) 12

Maplewood Mall Rain Garden IESF (2011) 12

Settlers Glenn IESF (2013) 13

Results 13

Electron microscopy and magnetic mineralogy 14

Analytical methods 14

Summary of results 14

Zero Valent Iron (Fe0) 16

Magnetite (Fe3O4) 16

Ferrihydrite 17

Goethite, Lepidocrocite, Akaganeite (FeOOH) 17

Hematite 18

Column aging study 18

Environmental Monitoring 23

Recommendations and Research Needs 27

References 29

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Summary of Findings

Our overall goal for this project was to better understand the geochemical and mineralogical

processes within Iron Enhanced Sand Filters (IESFs) that lead to effective removal of

phosphorus. Our specific objectives were to (1) identify the most effective iron minerals for

phosphorus trapping, (2) provide a screening method to optimize the success of Iron Enhanced

Sand Filters (IESFs) to trap iron, (3) determine if iron minerals within filters get used up or

remain active over time, and (4) use real time monitoring to understand IESF chemical

conditions and dynamics.

We thus structured our research activities and findings around answering these four questions:

1. What are the most effective minerals for phosphorus trapping?

a. We identified the sequence of minerals that form as Zero Valent Iron (ZVI)

corrodes in the environment (Figure 3) and observed that IESFs that successfully

remove P also contain goethite and hematite.

b. We determined that iron-phosphate minerals did not precipitate in IESFs, and

suggest that the primary mechanism for trapping phosphate is adsorption to the

surfaces of iron oxide minerals formed during ZVI corrosion.

c. The natural variability of pH in IESFs in the Twin Cites region has the potential to

cause important P-adsorbing minerals to periodically change their net surface

charge from positive to negative and thus lose their ability to electrostatically

attract negatively charged phosphate ions. This could result in the release of

previously adsorbed P.

2. Is there a method to screen iron sources prior to acquisition?

a. No, there is no way to screen sources of iron media to engineer corrosion to a

specific iron mineral or minerals, because any ZVI (e.g. steel, cast iron, etc.) will

corrode in response to the pH, oxygen and other geochemical factors of the

surrounding waters.

b. We found that nearly all studied IESFs utilize the same source of ZVI (from

Connelly GPM).

3. Do the iron minerals in the filter get used up over time?

a. Yes. ZVI is converted to high-surface-area iron oxide minerals that have large

capacity for P-adsorption. Nevertheless, the iron oxide surfaces can “fill up” and

lose their capacity to trap P over time. New iron oxides will form as long as ZVI is

present (and geochemical conditions remain favorable), but eventually all ZVI will

be converted after which new P-adsorption will cease.

b. Our column aging study demonstrates that magnetic susceptibility decreases

with each inundation of iron-sand filter media. This magnetic susceptibility

decrease is tracking the conversion of high susceptibility materials (metallic iron

and magnetite) to lower susceptibility minerals, such as goethite and hematite.

c. Magnetic susceptibility is easily measured in the field. If practitioners measure an

initial magnetic susceptibility of newly installed IESF media, continued

measurements will indicate the remaining capacity of the media to provide

additional corrodible iron.

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4. Monitoring: Is the water and filter chemistry conducive for effective iron-

phosphorus bonding?

a. Studied IESFs exhibited different chemical conditions, which also varied with

time over daily cycles, storms and the season.

b. Oxygen: Perpetual inundation was not the only cause of low oxygen in filter

media. A layer of fine particles or biofilm in the upper layer of the filter can also

result in low oxygen in filter media. Impermeable liners at the base of filters may

also result in perpetual inundation and low oxygen along the bottom of the filter

media.

c. Redox: Daily fluctuations in redox, even in inundated media, indicate that redox

species are active and responding to biological activity in the filter media. Redox

values of inundated media do not favor the formation of oxidized iron minerals

such as goethite and hematite. Redox values vary with depth and the lowest

levels had redox conditions that do not favor the formation of any oxidized iron

minerals, even while the upper layers of the same filter had favorable redox

values (positive).

d. Conductivity: High conductivity sites had low oxygen, so current monitoring is not

conclusive about the potential for competing ions to influence filter effectiveness.

Engineered Iron-Phosphorus Traps

Excess phosphorus (P) is a nutrient pollutant that enters water bodies when rain and snow flow

over streets, parking lots, and roofs collecting excess phosphorus from lawn and garden

fertilizers, detergents, pet and yard waste before entering into a storm drain or water body.

Excess P in water bodies causes algal blooms, which makes lakes green and negatively affects

fish and wildlife. Phosphorus occurs in stormwater in several forms, including particulate and

dissolved (orthophosphate) forms, which originate from organic and manufactured sources,

including condensed phosphates (polyphosphates (linear), metaphosphates (cyclic), which

occur naturally and in manufactured products). For brevity, unless we need to specify a specific

form of phosphorus, we will refer to the forms of phosphorus as “P”.

Among the interventions to remediate P pollution are iron minerals, which are expected to

chemically trap P. The first testing of iron as an engineered P trap for urban stormwater was

done by Carlos Herrera in Washington state in the 1980’s (personal communication on February

14, 2018, data no longer available, so the recounting here is from his memory). Herrera first did

a column experiment using sand and steel wool shavings (5% by volume) in columns that were

15 ft high by 8 in diameter. He used 4 ft of filter media in a column with iron-sand media and

synthesized stormwater to monitor the quantity of P retained in the column. Herrera recalls that

the columns removed 95-100% of total P and ortho-P. Following this successful laboratory test,

Herrera designed the first large-scale installation of a stormwater treatment system using iron

media for Lakemont Stormwater Treatment Facility in Bellevue, WA

(https://www.werf.org/liveablecommunities/studies_bell_wa.htm). The Lakemont iron-sand filter

was successful for three years before it began to fail and release P due to the formation of

impervious zones created by secondary iron oxide minerals that slowed water movement

6

through the filter. Herrera believes that the impervious zones resulted in anaerobic conditions

(not monitored). He attributed the failure to either inadequate mixing of iron with sand upon

installation or translocation of iron oxides within the media, noting that the clogging occurred in

the top six inches of the filter. Initially the iron-sand filter had vegetation that died when the

infiltration rate slowed (WA DNR, 1998).

Use of iron as a chemical trap has a long history in wastewater treatment (ferric chloride, e.g.

Carnduff, 1976) and groundwater remediation (zero valent iron particles, e.g. Kaplan, Cantrell, &

Wietsma, 1994). In the 1990’s “reactive mixtures”—such as Fe and Al compounds, pure iron

minerals, and carbonates—were tested for P removal in 10-hour laboratory batch experiments

as an application for multi-stage septic systems (Baker, Blowes, & Ptacek, 1998). The

experiment used a mix of sand (silica, carbonate, limestone, or a mix) with 5% of the column

containing Al or Fe media. One of the iron compounds that successfully removed P was “slag”,

which also contained portlandite (Ca(OH)2) and immediately increased solution pH from 5.5 to

12. Baker et. al. (1998) found that limestone sand, goethite, hematite, and magnetite all

removed P in the batch experiments, and they found the most promising removal from activated

alumina.

The use of reactive media to remove P from stormwater expanded considerably throughout the

2000’s (Arias, Del Bubba, & Brix, 2001; Banchand & Heyvaert, 2005; Barr Engineering, 2010;

Brix, Arias, & Del Bubba, 2001; Erickson, Gulliver, & Weiss, 2007). Several testing efforts by

researchers and consultants evaluated a range of active media options, usually mixed with C-33

sand to overcome issues with clogging. The media included limestone, calcareous sand, steel

wool, aluminum oxide, blast oxygen furnace (BOF) byproducts, and scraps of cast iron. Cast

iron scraps, larger particles of the zero-valent iron (ZVI) used in groundwater, became the most

frequently used iron reactive media due to its effectiveness at removing P and its low cost

(Erickson, Gulliver, & Weiss, 2012).

Currently, iron-enhanced sand filters (IESFs) are being installed in field-scale remediation

efforts with installation costs from tens to hundreds of thousands of dollars each, depending on

size and technological factors such as pumps and sampling devices. Two IESF designs are

outlined in the Minnesota Stormwater Manual. Basin type IESFs can be infiltration basins with

the addition of iron media designed to chemically capture phosphorus, but some conditions call

for an impermeable liner (to prohibit infiltration) and function as filtration devices that direct

water through the filter and to an outlet. Bench type IESFs are placed along one or more edges

of a wet pond and are designed to be inundated only during storm events. Bench type IESFs

filter only a portion of the water that enters a stormwater pond. Despite wide success, some

currently installed iron-enhanced sand filter systems are not working effectively.

In this work we will review the geochemical mechanisms of P retention by iron minerals, detail

our laboratory characterization of the iron mineral species found in IESF media from eight sites,

describe their response to inundation and aging, and present results of our monitoring of the

chemical conditions in IESFs. We will compare our results from several IESFs with their

observed P-removal effectiveness.

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Chemistry of Iron Source

The favored (or at least most commonly used) source of iron for iron-enhanced sand filters is

from a single Chicago-based company, Connelly GPM. The iron media is composed of shavings

(or chips, turnings, bushy wads, etc.) of cast iron that have been removed during the cutting,

shaping, and smoothing of cast iron products. The raw shavings contain a variety of sizes and

shapes of iron scraps, along with chemicals that are used as a coolant and lubricant during the

machining process. Connelly GPM receives iron shavings from numerous companies and

composites them for processing and distribution to minimize variability. Connelly GPM cuts and

sieves the shavings to consistent size groupings and cleans the iron using an undisclosed

cleaning process to remove cutting oils and other chemicals. The final iron product (Connelly

GPM product ETI CC‐1004) contains between 87-93% metallic iron along with several other

elements as reported in Table 1.

Table 1 Elemental concentration of constituents found in Connelly GPM iron (2015 ETI CC‐1004 specifications

report), which is commonly used as iron-sand filter media.

Element Concentration (%)

Metallic Iron 87-93

Total Carbon 2.85-3.23

Manganese 0.14-0.60

Sulphur 00067-0.107

Phosphorus 0.000-0.132

Silicon 1.0-1.85

Nickel 0.05-0.21

Chromium 0.03-0.23

Vanadium ND

Molybdenum 0.08-0.15

Titanium 0.004-0.1

Copper 0.11-0.20

Aluminum 0-0.005

Cobalt ND

Magnesium 0.01

Boron 0.01

Zinc 0.01

Zirconium 0.01

Chemistry of IESF components

Phosphorus occurs in the environment in solid (associated with particulates) and dissolved

forms, which are frequently measured separately, but regulated as total phosphorus (TP).

Regulatory standards for TP vary depending on water body type and region. Minnesota has

criteria ranging from 0.02 mg/L TP for trout streams to a maximum of 0.09 mg/L TP for shallow

lakes in the Western Corn Belt Plains region of the state.

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Aqueous phosphorus, whether contributing to a measurement of total or dissolved phosphorus,

is typically measured using colorimetry with ascorbic acid (blue) or molybdovanadate (yellow),

and is reported in mg/L as P. P may also be measured using inductively coupled plasma optical

emission spectroscopy (ICP-OES), but this is unusual for water quality applications. Soluble

reactive phosphorus (SRP) methods are designed to quantify dissolved orthophosphate (PO43-),

the form most readily available to algae and plants, but the reagents detect other forms of P and

overestimate orthophosphate (Yi, Song, Liu, Maruo, & Ban, 2019). TP is measured on unfiltered

water samples after they have undergone digestion (heating and acidifying), which converts

particulate forms to dissolved phosphate. TP reagents will detect phosphate, condensed

phosphate, and organic phosphate (dissolved and suspended). Total dissolved phosphate

(TDP) is measured on water samples that have been filtered (0.45μm), so this measurement

detects only the fraction of phosphorus in solution in the water.

In dissolved form, the speciation of the orthophosphate ion is a function of the ambient pH. At

any given pH, all forms of phosphate may be present, but the major form of phosphate between

pH 2.12-7.21 occurs as H2PO4-. Between pH 7.21-12.67 the phosphate ion loses an additional

proton and occurs as HPO42-. As we consider how this anion reacts with other ions in an

aqueous environment, including iron minerals, it is important to recognize the charge

contribution from P.

Binding Mechanisms for Fe and P

Adding iron to a sand filter can chemically trap P by two primary mechanisms: mineral

precipitation and adsorption of phosphate to the surface of iron minerals.

Mineral Precipitation

Potential iron-phosphate minerals that could form and serve as a phosphate trap, are vivianite

(Fe22+(PO4)2·8H2O) and strengite (Fe23+PO4·2H2O). The equilibrium formation for vivianite and

strengite is based on the concentration of phosphate, pH and Eh (Figure 1), where Eh is a

measure of the electron potential available for reduction-oxidation (redox) reactions. High values

of Eh, measured in volts, represent more oxidizing conditions, whereas low (negative) values

represent more reducing conditions.

Figure 1 demonstrates that equilibrium formation of both minerals would require quantities of

phosphate that are several orders of magnitude higher than Minnesota’s phosphate limits for

natural waters (e.g. HPO42- concentrations of 10-5 ≈ 0.3 mg/L TP). According to the stability

diagram, vivianite is expected to form in negative Eh conditions, which represents reducing

environments (low dissolved oxygen). Vivianite formation in anoxic lake settings has been

observed throughout the literature (Frederichs, von Dobeneck, Bleil, & Dekkers, 2003; J.O.

Nriagu & Dell, 1974; Rothe, Frederichs, Eder, Kleeberg, & Hupfer, 2014); however, we have not

detected vivianite in IESF media. Furthermore, the formation conditions for vivianite would be

considered failure conditions for IESFs. In short, the conditions for the formation of iron-

phosphate minerals in IESFs are extremely unlikely and neither vivianite nor strengite have

been detected to our knowledge in IESF media.

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Figure 1. 3D equilibrium diagram representing the relationships between iron, phosphates, and hydroxides in an aqueous setting without sulfides. The chemical concentrations of carbonate and iron are given on the right hand side of the diagram (Jerome O. Nriagu, 1972).

Adsorption

Dissolved inorganic phosphorus (DIP) species—including the bio-available orthophosphate ion

and also condensed phosphates (i.e. pyrophosphate, metaphosphate, and polyphosphate)—

have a strong affinity for binding to inorganic and organic particles through a variety of

adsorption and/or surface complexation reactions. These sorption processes can occur

relatively rapidly and explain the observation that typically more than half of aquatic total

phosphorus (TP) is found in the particulate form. This proportion typically increases as water

travels downstream from runoff sites, to streams, ponds and lakes, likely as a result of

increasing contact with mineral particles (Kayhanian, Suverkropp, Ruby, & Tsay, 2007; Pitt et

al., 2003; Yi et al., 2019).

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Mineral surfaces for adsorption

Particulate phosphorus is typically in the form of inorganic phosphorus species adsorbed to the

surfaces of minerals such as alumino-silicates, aluminum oxides, and iron oxides via anion

exchange, ligand exchange, and cation bridging mechanisms. Organic phosphorus, which can

be present in stormwater systems, is also able to bind with mineral surfaces, but through

additional binding mechanisms. Organic phosphorus is not to be confused with detrital plant

particles, which can contain phosphorus. Detrital plant particles may be trapped by the sand

filter, and are not reactive with mineral surfaces. Such detrital plant particles trapped within the

filter may degrade and release dissolved organic or inorganic phosphorus into the system,

which may associate with mineral surfaces.

The phosphorus content (in weight percent) of total suspended solids/sediments (TSS) is thus

largely determined by the concentration of reactive surface binding sites in the TSS, which is in

turn a function of the mineralogy of the TSS and its mineral surface area. Smaller particle sizes

are loosely correlated with increased reactive mineral surface area, but for the same mineral

size the mineral surface area can vary by one to two orders of magnitude. Generally, the

highest reactive surface areas are found on the various aluminum oxide and hydroxide

minerals—such as gibbsite (Al(OH)3 and its polymorphs that are either naturally formed in soils

or are precipitated from soluble alum (Al2(SO4)3·nH2O), boehmite (γ-AlO(OH)), and diaspore (α-

AlO(OH), alumina (Al2O3))—and various iron(II & III) oxide and hydroxide minerals—such as

goethite (α-FeOOH), akaganéite (β-FeOOH), lepidocrocite (γ-FeOOH), ferrihydrite

(~FeOOH•1.8H2O), hematite (Fe2O3) and magnetite (Fe3O4, or Fe2+Fe3+2O4). These aluminum

and iron oxides exhibit a wide range of variability in surface reactivity that depends both on

mineralogy and formation conditions.

Sorption capacity depends on surface area and pH

The capacity for stabilizing phosphate by adsorption is highly dependent on the surface area of

the minerals, which varies based on mineral type and degree of crystallinity. Published ranges

of the phosphate capacity of several minerals are in Figure 3d.

The pH of the aqueous environment surrounding hydroxide and oxyhydroxide minerals has a

critical influence on sorption because the surface charge of minerals will swing from positive to

negative as a function of pH. The point where the mineral surface charge changes from positive

to negative is known as the point of zero charge (PZC), and this point occurs at a different pH

for each mineral (Figure 3c).

Phosphate is a suite of anions that increase in charge number with increasing pH (H2PO4-,

HPO42-, PO43-). For iron minerals to have a positive surface charge to attract phosphate

anions, the pH must be below the PZC (Figure 3d). In pH ranges below each iron mineral’s PZC

(positive mineral surface charge), adsorption of phosphate can occur most securely through

ligand exchange, which creates a covalent bond in the inner sphere of the mineral surface,

meaning that the phosphate anion attaches to the mineral (by electron donation). If the pH is

above about 7.2, where H2PO4- converts to HPO42-, then a bidentate ligand-exchange bond can

11

form, where the phosphate anion is covalently bonded to the mineral surface in two places.

Sulfate (SO4-) can compete with phosphate for ligand exchange sites (Parfitt & Smart, 1978).

Positive mineral surface charges also support anion exchange between phosphate and iron

mineral surfaces, which occur in the outer sphere or diffuse swarm of the hydrated mineral

surface, meaning that these bonds are further from the mineral surface and are insecure bonds

that are extremely mobile. In the context of stormwater, other anions in the system, such as Cl,

N, and S species, may compete with phosphate for surface exchange sites (Chitrakar et al.,

2006).

Another adsorption mechanism, cation bridging, can occur when a polyvalent-cation acts as a

bridge to enable an anion to electrostatically adsorb to a negatively charged surface. When pH

values are higher than the PZC of the iron mineral, or when the mineral is bonded with an

organic compound, and the surface has a net negative charge, cation bridging can thus facilitate

adsorption with phosphate. The affinity of a metal to cation bridging decreases proportionally to

the ionic radius of the cation.

The combined effect of all of these processes is for phosphate adsorption to iron oxide and

oxyhydroxide minerals to decrease with increasing pH, but that the presence of polyvalent

cations might reduce this effect (Spiteri, Cappellen, & Regnier, 2008; Zeng, Li, & Liu, 2004).

Study Sites

We collaborated with several consulting firms and watershed districts to access sites with a

monitoring history, which in most cases meant that the sites had water samples taken following

storm events of water from the stormwater pond or impoundment (pre-filter) and water sampled

at the post-filter outlet. We sampled nine IESFs in collaboration with four watershed districts

(and/or their affiliated consulting firms). Below we list each of the sampled sites with a brief

description of notable characteristics of the sites, moving from northwest to southeast on the

site map (Figure 2).

Champlin Biochar IESF (2017)

This IESF was retrofitted as a bench type filter to an existing stormwater pond and contains a

layer of biochar in addition to iron and sand media. Shortly after the design and construction of

the IESF, the adjacent highway went under two years of road construction. The road

construction appears to have changed the drainage dynamics of the pond because during the

course of the study the pond outlet was continuously in overflow mode and the IESF was

always inundated. Despite inundation, the IESF sampling indicated that the filter was removing

phosphorus during the 2017 sampling season (data from August-November). In 2018 the IESF

shifted and after trapping phosphorus in May, it began exporting phosphorus in June 2018.

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North Lions Park IESF (2017)

North Lions Park IESF is a bench that was retrofitted to an existing stormwater pond. Most

measurements of phosphorus in 2017 and 2018 indicate that this IESF is successfully filtering.

Olson Middle School Biochar IESF (2017)

Olson Middle School Biochar IESF was designed as a retrofit to an existing stormwater pond.

The intent was for half of the filter bench to consist of iron+biochar+sand media and half as

iron+sand media, but it was not constructed as designed. Instead half had only biochar+sand

and half had iron+sand. Additionally, this IESF had clearly never been inundated by a storm

event and only experienced wetting and drying cycles during direct rainfall. We sampled the

iron+sand media from this filter bench to analyze as an example of unused media.

William Street IESFs (2011)

William Street Pond has two IESF filter benches retrofitted to the same stormwater pond. The

monitoring history of these filters indicates successful removal of phosphorus, but the constant

flow at the IESF outlet raised concern that groundwater intrusion may be diluting water in the

IESFs, which was part of our investigation.

Hwy 36 & 61 Cloverleaf IESF (2013)

This basin type IESF was constructed in the cloverleaf of a highway interchange. To our

knowledge this IESF has been successfully filtering phosphorus since construction, and the site

has an extensive monitoring station, but the data has not been shared with us.

Beam Ave IESF (2007)

Anecdotally referred to as “Stonehenge”, this is to our knowledge the first field-scale pilot project

of an IESF in Minnesota. This IESF is a basin type system in which portions of the filter are

clogged. However, to our knowledge this IESF has been successfully filtering phosphorus since

construction, but the data has not been shared with us.

Maplewood Mall Rain Garden IESF (2011)

This site is an intentional deviation from the Minnesota Stormwater Manual’s IESF design

specifications because this system has soil overlying the IESF and supports vegetation.

Because this is a rain garden, it is designed to infiltrate as much water as possible. The deepest

point in the rain garden is where the IESF is located and was unvegetated, which suggests that

this has been intentionally maintained. To our knowledge this IESF has been successfully

filtering phosphorus since construction, but the data has not been shared with us. We are told

that monitoring of dissolved oxygen at this site indicates anoxia during inundation.

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Settlers Glenn IESF (2013)

Water from a stream tributary collects in a harvest pond (along the stream), and is pumped to a

sediment basin that also collects runoff from the nearby residential streets. The sediment basin

overflows into the IESF, which returns the filtered water to the same tributary. Performance

monitoring has been consistent since 2014 and indicates that the system is achieving consistent

phosphorus removal of 80-90%. Following construction the system has been attentively

maintained and extensively optimized for flow rates, hours pumping, and hours the filter is

allowed to dry. The system was designed to reduce 118 pounds/year of P, but influent

concentrations never reached those pre-construction levels, and removal goals have been

dialed back to 50-60 pounds/year. The system began year-round (winter) pumping during 2018.

Figure 2. Site map of eight locations for this study, where we sampled from nine IESFs.

Results

Using electron microscopy and magnetic measurements, we identified that metallic iron

undergoes corrosion in a sequence that was common among all of the sites studied. Where

sites have a record of successful phosphate removal, we identify the presence of goethite and

hematite, in addition to magnetite and zero valent iron. In the site that was shifted to phosphate

export, goethite and hematite were present in media sampled before IESF failure, and were

absent in media specimens after failure. These results suggest that chemical conditions that

favor the formation and presence of goethite and hematite may be necessary for IESFs to

successfully trap phosphates.

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Electron microscopy and magnetic mineralogy

Analytical methods

We characterized iron mineral makeup within filter media to determine which iron minerals are

bonding with phosphate using an established series of geophysical magnetic techniques that

allow us to preserve the sampled state of iron minerals. IESF media was sampled using 1-inch

soil recovery probes with plastic liners. Samples were immediately sealed and frozen in liquid

nitrogen in the field, followed by freeze-drying in the laboratory to minimize potential for redox

sensitive species to be altered. Analytical techniques on each sample included the following:

• Measured total concentration of magnetic material in sample and determine provenance

of magnetic minerals, i.e. quantity of iron in original filter material and quantity of iron that

has formed within the filter, including the forms that have bonded with phosphorus

(Vibrating Sample Magnetometer, <100 ppm sensitivity).

• Determined the composition (mineralogy) and grain size (related to reactive potential) of

iron (magnetic susceptibility, <100 ppm sensitivity).

• In samples where the magnetic properties of one mineral obscure the properties of less

abundant minerals (e.g. metallic iron and magnetite obscure goethite and hematite), we

sequentially removed the characteristic magnetization signals of the dominant minerals

to enable characterization of the concentration, composition, and grain size of all

minerals in the filter system (Magnetic Properties Measurement System-room

temperature saturated isothermal remanent magnetization, <100 ppm sensitivity).

• Refined composition and grain size characterization of nanoparticles requires the

particles to be cooled to cryogenic temperatures to slow down particle motion to facilitate

identification. This step is important because the smallest grain sizes carry the most

reactive surface potential. (Magnetic Properties Measurement System-zero-field cooled

and field cooled series, <100 ppm sensitivity).

• On polished cross-sections of filter media, we used electron microprobe to both capture

images of mineral associations and assess the elemental composition of the

assemblages. This enabled us to confirm the sequence of corrosion of zero valent iron in

the filter media and visualize the mineral forms and their occurrence relative to sand

particles within the media.

Summary of results

When stormwater flows through IESFs, which generally occurs only during inundation following

a storm event, it encounters abundant iron oxide minerals. If oxygenated conditions are

maintained within the IESF media, the process of inundation generates several stages of

oxidation reactions of the metallic (or zero valent) iron contained within the filter. We determined

that metallic iron in IESFs progressively oxidizes to form an assemblage of iron minerals with

increasing surface area (Figure 3).

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Figure 3. Summary of cast iron corrosion (a,b) and chemical context for iron surface charge (c) and phosphate sorption capacity for each mineral (d). Column (a) displays mineral structures observed by electron microscopy, shown alongside column (b), which summarizes the observed cast iron corrosion sequence based on data from magnetic mineralogy and electron microprobe. Column (c) illustrates ranges of surface charge as a function of pH for each iron mineral observed in IESFs and phosphate phases, demonstrating that iron minerals change from positive to negative surface charge as pH increases, and the pH value where this conversion occurs (the point of zero charge) is different for each iron species. Column (c) has shading to indicate the range of pH for rain water and water measured in IESFs. Column (d) summarizes published values for sorption capacity of phosphate for each iron mineral.

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Zero Valent Iron (Fe0)

The metallic iron used in IESFs begins as a zero-valent iron with pearlitic texture (Figure 4),

which has intergrowths of body centered cubic (BCC) iron (light) and cementite (Fe3C, dark).

This texture is common in cast steels cooled at air temperature and pressure from above 725°C.

The intergrowths optimize the mechanical properties of steel with the soft-ductile iron and the

hard, brittle cementite. Chemically C, Mn and Cr tend to partition in the cementite, Al and Si

partition into the iron.

Figure 4. Electron microprobe images from North Lions Park IESF illustrating full corrosion sequence of metallic iron (Fe0). In the left image, the lightest gray color is metallic iron with pearlitic structure. The metallic iron is bounded by magnetite, which is a darker gray that still retains the pearlitic structure. On the far left edge of the image are intergrowths of hematite and goethite. The image on the right is a magnified view of the pearlitic metallic iron located in the white box.

Magnetite (Fe3O4)

Electron microprobe images of magnetite indicate that it forms within the texture of the metallic,

pearlitic iron (Figure 4). Magnetite forms as an early oxidation product within the structure of

metallic iron without mobilizing; however, the volume change associated with the oxidation can

cause cracking and increases mineral surface area within the original metallic iron grain.

Magnetite also occurs in IESFs in a spheroidal form (Figure 5), likely from nucleation and

growth on the edges of metallic iron.

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Figure 5. The oblong mass in this electron microprobe image is a cluster of oxidized iron minerals that appear to retain the initial structure of a particle of metallic iron from Champlin Biochar IESF in November 2017, prior to export of phosphate from this IESF. Taking a closer view of the white box in the left image, the right image reveals that the lightest gray forms are magnetite in composition and appear to mostly occur in a mass of spheroidal shapes. On the bottom edge of the oblong mass is a darker gray area that is poorly crystalline intergrowths of hematite and goethite.

Ferrihydrite

Ferrihydrite was not detected in the specimens using magnetic measurements, most likely

because its magnetic signal is overwhelmed by that of metallic iron and magnetite, rather than

due to a lack of presence. We expect that ferrihydrite is present in IESFs, but as a flow-through

component of these systems that does not remain stable in IESFs (or as a mineral in the

environment). Instead, it likely acts as a means for transporting iron from one point in the IESF

to another, where it is transformed into a more stable iron-oxyhydroxide. Ferrihydrite is also

likely a component of the orange bacterial slime observed on the export pipes of several filters,

but this inference has not been tested. Synthetic ferrihydrite was shown to co-precipitate with

organic matter and form a complex more stable than ferrihydrite alone (Chen, Kukkadapu, &

Sparks, 2015). This co-precipitation mechanism may play a role in IESFs, but ferrihydrite found

in environmental contexts is much less stable and less crystalline than synthesized ferrihydrite.

Although ferrihydrite is most likely present in IESFs, its presence is probably ephemeral, and

although it may have binding capacity with phosphates, we do not expect it to be a key player

for long-term trapping of phosphates.

Goethite, Lepidocrocite, Akaganeite (FeOOH)

Goethite, lepidocrocite, and akageneite have the same chemical formula, but different

crystalline morphologies. Goethite occurs as radial needles, lepidocrocite forms fibrous crystals,

and akaganeite is spindlelike, which lends to differences in surface area and capacity to adsorb

phosphate (Table 1). Goethite is present in IESFs with effective removal of phosphorus (as

measured by water chemistry of effluent in and out of IESFs by partner organizations). Our

methods did not differentiate between the three forms, but such differentiation may be

warranted because Chitrakar et al., 2006 found that goethite did not continue to adsorb

phosphate after repeated cycles of high (addition of NaOH) to low (addition of HCl) pH, shifting

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the fluid chemistry across the pH of PZC for each mineral, but akaganeite continued to resorb

phosphate even after ten cycles of pH changes. In the context of stormwater, pH ranges could

shift across the PZC for akageneite (~7), but is unlikely to cross the PZC for goethite (~9).

Hematite

Hematite (Figure 6) occurs in abundance in successful IESFs. Hematite is on the high end of

PZC pH of the iron minerals detected in IESFs (8.5) and has a low probability to cycle between

positive and negative surface charge under stormwater pH conditions (Figure 3c).

Figure 6. Images from Champlin Biochar IESF in November 2017, prior to export of phosphate from this IESF, reveal how hematite and goethite have mobilized within the IESF and have adhered to a grain of quartz.

Column aging study

To observe how IESF media responds over time to repeated wetting (inundation) and drying

cycles, we set up three simple flow-through columns (Figure 7) that allow us to measure

magnetic susceptibility as a function of time after repeated exposure to water. Our magnetic

measurements did not disturb the material during the experiment, allowing us to observe how

magnetic susceptibility changed as the media altered during 85 inundation cycles over 150

days.

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Figure 7. Experimental design for the aging of iron enhanced sand filter media. Approximately 50 mL of tap was allowed to drain through the filter media each business day. Samples were allowed to dry over weekends. This design allows for periodic measurements of magnetic susceptibility without disturbing the filter media. At the end of the experiment, samples were embedded with epoxy to create thin sections for microscopic analysis.

We used fresh media that had never been exposed to wetting or environmental conditions,

sourced from Herrera Environmental Consultants, who had questions about filter clogging when

they used this media in the Pacific Northwest. The media was composed of gravelly sand with a

mixture of mafic and felsic fragments (Figure 8, left image, in contrast to the quartz sand used in

Minnesota IESFs in core) and metallic cast iron fragments (ETI CC-1004 from Connelly GPM in

Chicago). Manually separating the metallic iron from the pre-mixed media using a magnet, the

iron fragments were 26% by weight and the gravelly sand was 74%. This media is notably not

compliant with the Minnesota Stormwater Manual design specifications of C33 type sand (or

comparable size distribution) and 5-8% iron by weight.

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Figure 8. Image of IESF media, with Herrera media used in aging column experiment in dish on left and C-33 sand plus iron from a Minnesota IESF in a core on the right for comparison.

Within the first 20 days the columns developed a rind of orange-colored alteration that covered

many of the grains in each column (Figure 9), and the time for water to flow through the media

took longer than it did at the beginning of the experiment. The production of alteration minerals

was reducing the permeability of the media with iron media components at this high proportion.

At day 112, one of the columns had not completely drained after 24 hours, which persisted for

the remainder of the experiment. The clogging column drained within four days when given the

opportunity to dry for that duration (e.g. between day 139-143). The two other columns did not

experience clogging.

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Figure 9. Close-up image of aging experiment columns from study day 25 after 18 inundations. Note the rinds of orange-colored alteration covering many grains in the column.

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Figure 10. Close-up image of columns at the end of the aging experiment (day 150).

The aging experiment reveals that every wetting cycle of IESF media results in a decrease in

magnetic susceptibility (Figure 11). This decrease represents loss of high susceptibility materials

(metallic iron and magnetite). This loss can be interpreted as export from the column, which is

unlikely given the 5-10 µm filter, or dissolution of higher susceptibility materials such as

magnetite or metallic iron to form lower susceptibility minerals, such as goethite and hematite.

We use magnetic susceptibility (𝜒) as a tracer of the metal in these filters. For reference, the

metal separate from the filter media used in the aging experiment has a magnetic susceptibility

of 6.2x10-4 m3kg-1, which is approximately triple the susceptibility of magnetite (2 x 10-4 m3kg-1)

and three orders of magnitude higher than hematite and goethite (ranging between 1 to 20 x 10 -

7 m3kg-1). The gravelly sand used in the column experiment had a magnetic susceptibility of 4.3

x 10-7 m3kg-1, which is higher than pure quartz and feldspar (-0.5 x 10-8 m3kg-1) due to the iron-

rich igneous fragments within this aggregate. Thus, we can see that as the iron in the media

dissolves or is lost, the magnetic susceptibility will decrease. Over the course of the 150-day

experiment, the columns experienced a 40-70% decrease in magnetic susceptibility.

We therefore recommend that measurements of magnetic susceptibility, which are simple and

inexpensive to make in the field, are indicators of corrosion in IESFs. To make such

measurements useful to practitioners, the protocol must include an initial measurement upon

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IESF installation. Then subsequent measurements through the life span of the IESF will indicate

the progression of corrosion of high susceptibility metals to the much lower susceptibility iron

oxide minerals. Additional studies are needed to establish threshold levels of change to be used

for decisions regarding IESF management.

Figure 11. Changes in magnetic susceptibility for three columns over the duration of the experiment reveal that each column had a different starting quantity of magnetic material but they followed a similar decrease in susceptibility over time. The pattern of decrease continued over the duration of the experiment.

Figure 12. The percentage change in the rate of decrease of magnetic susceptibility as a function of the number of days between inundation. When the column was allowed to dry for additional days, the change (loss) in magnetic susceptibility was greater. (Error bars are 1-sigma.)

Environmental Monitoring

Custom-built Arduino-framework data loggers monitored conductivity, dissolved oxygen, redox

and other parameters at three IESFs. The generalized meaning of each of these properties as

they pertain to IESFs include:

• Conductivity was measured in each pond and within the filter media. This property is

related to the total ionic activity in the water, which is a proxy for nutrients, salts, and

metals dissolved in stormwater. Continuous monitoring reveals when nutrient pulses

occur and the conditions when the filter is being required to trap P. High conductivity can

mean that competing ions are abundant and may compete for adsorption sites on ion

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mineral surfaces, which may hinder P filtration, but more testing is required to confirm

this concern.

• Redox potential was logged in the filter media at multiple depths, and this property

indicates the activity of the electrons in the IESF, measured in millivolts, which in turn

determines the favorability of a reduction-oxidization reaction to occur. A redox reaction

involves the exchange of electrons between chemical species. For iron minerals,

reducing (Fe2+) conditions are generally negative voltage ranges and oxidizing (Fe3+)

conditions are generally positive. Iron is not the only redox species in stormwater; nitrate

and sulfate are among the redox-sensitive molecules that may occur in IESFs. However,

in our engineered context with abundant iron, we expect that iron species play a

dominant role in the redox reactions of IESFs.

• Dissolved oxygen was logged in the pond and filter media at each site to indicate

anoxic conditions, which favor dissolution of iron oxide minerals. Dissolved oxygen also

provides an estimate of microbial oxygen consumption or biological oxygen demand

through the stages of inundation in IESF systems.

Champlin Biochar IESF (45.172690, -93.392550). This pond was perpetually inundated

throughout the duration of the study, including our first visit in November 2017. According to

water chemistry measurements by Wenck Associates, the pond began exporting phosphorus

between May and June of 2018. We installed monitoring stations during the summer of 2018.

Redox monitoring at this site remained in negative millivolt values during the entire monitoring

season, which is expected in anoxic conditions. Despite perpetual inundation the redox values

responded to daily biogeochemical processes within the IESF filter bench with fluctuations

ranging on the order of ~25-50 mV. The seasonal fluctuation range was ~200 mV. These redox

values appear to be within the range of published values for redox in other contexts (Vorenhout,

van der Geest, & Hunting, 2011; Wallace, Sawyer, & Barnes, 2019), and more monitoring in

engineered iron media is needed to fully relate these values to IESF success or failure.

Dissolved oxygen ranged from 0-6 mg/L in the pond adjacent to the filter bench and remained

under 0.3 mg/L in the IESF media, which is a low concentration of oxygen and may not be ideal

to maintain oxidized forms on iron in the IESF. Conductivity readings ranged between 250-1500

µS/cm just below the surface of the pond, with the readings typically closer to 1500 µS/cm. The

IESF media recorded conductivity ranging between 200-800 µS/cm. These conductivity values

indicate a high degree of dissolved ions in the system. (Monitoring stations for this site were

funded by EPA Section 319 Project: Shingle Creek Biochar/Iron Sand Bacteria Filters.)

North Lions Park IESF (45.065479, -93.367688). The IESF at North Lions Park was inundated

only after storm events. According to water chemistry measurements by Wenck Associates, this

IESF is successfully removing phosphate. In November 2017 the IESF had no vegetation, but

had a 0.5-1 cm thick layer of very fine material, perhaps an organic-rich film deposited on the

filter during inundation, coating the top of the filter media. Based on the lack of orange oxidized

iron in the filter media at this time, this film may have been inhibiting oxygen penetration into the

media. Beginning in summer 2018, this film was still present and volumetric water content of the

media was between 20-45%, which approaches nearly saturated media. With volumetric water

content maintaining near 45%, the soil moisture sensor failed by end of June. By the middle of

25

summer 2018, the IESF was covered in tall (>3 feet high) vegetation, to the extent that we

needed to trim the vegetation so sunlight could reach the solar panel for the monitoring station.

Redox monitoring indicated positive values but the 500 mV swings in data seemed unusual and

may indicate that the sensors did not maintain an electrical connection with the embedded

reference electrode. Dissolved oxygen ranged between 0-3 mg/L in media and pond, which may

not be ideal to maintain oxidized forms on iron in the IESF. Conductivity near the surface of the

pond ranged between 250-500 µS/cm and 100-200 µS/cm in the IESF media, and these values

are within the range of conductivity of healthy natural waters. (Monitoring stations for this site

were funded by EPA Section 319 Project: Shingle Creek Biochar/Iron Sand Bacteria Filters.)

William Street Pond IESF (45.001729, -93.111139) has two filter benches along the western

edge of the pond (called the north bench) and along the southern edge of the pond (south

bench), with the outlet for both IESFs located between the filter benches at the southwest

corner of the pond. Both filters have a long record of measurements (by Capitol Region

Watershed District) of phosphorus concentrations that are lower in the post IESF outlets than in

the stormwater pond, which is the primary measure of successful IESF performance. Capitol

Region Watershed District is supporting a second season of monitoring in summer 2019.

(Monitoring stations for this site were funded by MSRC: Determining which iron minerals in iron-

enhanced sand filters remove phosphorus from stormwater runoff.)

In summer 2018, redox values from both IESF filters were positive, which indicates oxidation of

iron species, at depths of 10 and 25 cm below the surface, and redox was negative (indicating

reduction of iron species) at 40 cm deep, were the IESF filters appear to be perpetually

saturated.

Capitol Region Watershed District was concerned that groundwater intrusion into the filter

media might be diluting the water that exports from the IESF, possibly influencing the

measurement of filter performance. Their observation that the IESF outlets always had water

running out of them was a source of their concern. They created a 2-inch diameter piezometer

for each filter bench and we added water level and a multi-depth temperature profile to the

sensor stations.

Figure 13 shows the water level and temperature profile measurements through September and

October 2018 to reveal that in mid-September water level began increasing regardless of rain

events. For most of the summer season, water level only responded to rain events. We

conclude that groundwater does appear to be leaking through the membranes below the William

Street Pond IESFs, which were designed to be impermeable. Given that groundwater did not

accumulate in the piezometers during the summer monitoring season, this intrusion may not be

influencing measurements of IESF performance. However, our monitoring indicates that only

the upper portions of the WSP filters are functioning as designed, which may result in a

shortened life expectancy.

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Figure 13. Groundwater intrusion was monitored in William Street Pond IESFs using water level and temperature profiles (daily precipitation from MSP airport included to approximate storm events). For most of the summer season, water level only responded to rain events, but in mid-September water level began increasing regardless of rain events.

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Recommendations and Research Needs

Our overall goal for this project was to better understand the geochemical and mineralogical

processes within Iron Enhanced Sand Filters (IESFs) that lead to effective removal of

phosphorus.

Our initial proposal sought to discover which iron minerals in IESFs remove phosphorus from

stormwater runoff most effectively. We found that the metallic iron particles in IESFs undergo a

clear progression of corrosion ultimately to the oxidized iron minerals hematite, goethite,

lepidocrocite, and akaganéite, and these minerals are present in successful IESFs, but absent

in the failing IESFs in our study.

We recognize that the chemical conditions required for iron to trap phosphorus are not fully

outlined in the scope of the Minnesota Stormwater Manual design specifications for IESFs. We

do not believe that our study is extensive enough to yield immediate recommendations to

practitioners, but based on our contact with numerous practitioners, we can confidently make

the following clarifying statements and recommendations:

1) The mechanism for trapping phosphorus in IESFs is by adsorption on the surfaces of

iron oxide and oxyhydroxide minerals, not through precipitation of iron-phosphate

minerals. This strength of this mechanism changes with pH and mineral surface area of

the iron minerals that form within the filter.

2) Chemical evaluation of sites needs to be included in pre-IESF feasibility studies. We

recommend evaluation of dissolved oxygen and pH of incoming waters through multiple

seasons, if possible. If dissolved oxygen is too low, the conditions would not favor the

formation of oxide minerals, or inundation could shift the filter media chemistry to anoxic

conditions that could result in soluble iron (Fe2+) and export of trapped phosphate. If pH

oscillates or is consistently above neutral (such as in carbonate bedrock areas, or from

dissolution of concrete), the site may not be ideal for adsorption of phosphate with iron

minerals.

3) Iron will occur in soluble form in oxygenated media (as Fe3+). This is a pathway for iron

oxide coating on the silica sand portion of IESF media, which lends abundant iron oxide

surface area for adsorption of phosphate. Soluble iron may export from IESFs, and this

can occur in both oxic and anoxic (as Fe2+) settings.

4) Using magnets to pull “used-up” iron out of filter media to reclaim iron and adsorbed

phosphorus is not a realistic option for renewal of IESFs. Most of the “used-up” iron

occurs as goethite and hematite, which are weakly magnetic and will not be attracted to

common magnets (e.g. Strehlau, Hegner, Strauss, Feinberg, & Penn, 2014). Further,

oxide coatings on silica sand grains will contribute only a small fraction to the mass of

these grains, which are otherwise nonmagnetic. Metallic iron and magnetite would be

picked up by magnets, but would not be abundant in “used-up” media, nor would ZVI or

magnetite be associated with much phosphorus.

5) Magnetic susceptibility measurements are a practical measure of IESF life span. This

simple field measurement is an indicator of corrosion stage in IESFs. The protocol must

include an initial measurement of filter media upon IESF installation. Subsequent

28

measurements will indicate the progression of corrosion of high susceptibility metals to

the much lower susceptibility iron oxide minerals that trap P (including goethite and

hematite), and ultimately the end of new oxide mineral formation. Additional studies are

needed to establish a calibration curve.

6) Microbiology of IESFs is a critical topic for future research. Microbial activity can override

chemically expected mineral formation (kinetics). Given the abundant organic content of

stormwater, the oxygen demand from biota in stormwater and in IESFs may promote

cycles of anoxia within the filter media. Microbial activity and biological oxygen demand

have not been comprehensively evaluated in IESFs.

7) IESFs would benefit from pretreatment of the pond-water entering the IESF, to remove

organic debris, fine sediment, and fine organic matter that might lead to increased

biological oxygen demand within the filter media or that might create layers on top of

IESFs that inhibit the penetration of oxygen into filter media. Anoxia creates an

opportunity for iron and phosphorus export.

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References

Arias, C. A., Del Bubba, M., & Brix, H. (2001). Phosphorus removal by sands for use as media in subsurface flow constructed reed beds. Water Research. https://doi.org/10.1016/S0043-1354(00)00368-7

Baker, M. J., Blowes, D. W., & Ptacek, C. J. (1998). Laboratory development of permeable reactive mixtures for the removal of phosphorus from onsite wastewater disposal systems. Environmental Science and Technology. https://doi.org/10.1021/es970934w

Banchand, P. A. M., & Heyvaert, A. (2005). Adsorptive Media Investigations and Testing for Improved Performance of Stormwater Treatment Systems in the Tahoe Basin: Prepared for Placer County Department of Public Works, Auburn, CA and California Tahoe Conservancy, South Lake Tahoe, CA.

Barr Engineering. (2010). Iron-enhanced sand filter monitoring: Report to Ramsey-Washington Metro Watershed District.

Brix, H., Arias, C. A., & Del Bubba, M. (2001). Media selection for sustainable phosphorus removal in subsurface flow constructed wetlands. Water Science and Technology.

Carnduff, J. A. (1976). PRETREATMENT OF THE METROPOLITAN TORONTO WATER SUPPLY UTILIZING FERRIC CHLORIDE. Journal / American Water Works Association.

Chen, C., Kukkadapu, R., & Sparks, D. L. (2015). Influence of Coprecipitated Organic Matter on Fe2+(aq)-Catalyzed Transformation of Ferrihydrite: Implications for Carbon Dynamics. Environmental Science and Technology. https://doi.org/10.1021/acs.est.5b02448

Chitrakar, R., Tezuka, S., Sonoda, A., Sakane, K., Ooi, K., & Hirotsu, T. (2006). Phosphate adsorption on synthetic goethite and akaganeite. Journal of Colloid and Interface Science. https://doi.org/10.1016/j.jcis.2005.12.054

DNR, K. C. (1998). Lake Sammamish Water Quality Management Project, Final Reprot. Erickson, A. J., Gulliver, J. S., & Weiss, P. T. (2007). Enhanced Sand Filtration for Storm Water

Phosphorus Removal. Journal of Environmental Engineering, 133(5), 485–497. https://doi.org/10.1061/(ASCE)0733-9372(2007)133:5(485)

Erickson, A. J., Gulliver, J. S., & Weiss, P. T. (2012). Capturing phosphates with iron enhanced sand filtration. Water Research. https://doi.org/10.1016/j.watres.2012.03.009

Frederichs, T., von Dobeneck, T., Bleil, U., & Dekkers, M. J. (2003). Towards the identification of siderite, rhodochrosite, and vivianite in sediments by their low-temperature magnetic properties. Physics and Chemistry of the Earth. https://doi.org/10.1016/S1474-7065(03)00121-9

Kaplan, D. I., Cantrell, K. J., & Wietsma, T. W. (1994). Formation of a barrier to groundwater contaminants by the injection of zero-valent iron colloids: suspension properties. Hanford Symposium on Health and the Environment, 33rd.

Kayhanian, M., Suverkropp, C., Ruby, A., & Tsay, K. (2007). Characterization and prediction of highway runoff constituent event mean concentration. Journal of Environmental Management. https://doi.org/10.1016/j.jenvman.2006.09.024

Nriagu, J.O., & Dell, C. I. (1974). Diagenetic Formation of lron Phosphates in Recent Lake Sediments. American Mineralogist.

Nriagu, Jerome O. (1972). Stability of vivianite and ion-pair formation in the system fe3(PO4)2-H3PO4H3PO4-H2o. Geochimica et Cosmochimica Acta. https://doi.org/10.1016/0016-7037(72)90035-X

Parfitt, R. L., & Smart, R. S. C. (1978). The Mechanism of Sulfate Adsorption on Iron Oxides1. Soil Science Society of America Journal, 42(1), 48. https://doi.org/10.2136/sssaj1978.03615995004200010011x

Pitt, R., Maestre, A., Morquecho, R., Brown, T., Swann, C., Cappiella, K., & Schueler, T. (2003). Evaluation of Npdes Phase 1 Municipal Stormwater Monitoring Data. National Conference on Urban Stormwater: Enhancing the Programs at the Local Level. EPA/625/R-03/003., 306–327.

Rothe, M., Frederichs, T., Eder, M., Kleeberg, A., & Hupfer, M. (2014). Evidence for vivianite formation and its contribution to long-term phosphorus retention in a recent lake sediment: A novel analytical approach. Biogeosciences. https://doi.org/10.5194/bg-11-5169-2014

Spiteri, C., Cappellen, P. Van, & Regnier, P. (2008). Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers. Geochimica et Cosmochimica Acta, 72(14), 3431–3445.

30

https://doi.org/10.1016/j.gca.2008.05.003 Strehlau, J. H., Hegner, L. A., Strauss, B. E., Feinberg, J. M., & Penn, R. L. (2014). Simple and Efficient

Separation of Magnetic Minerals From Speleothems and Other Carbonates. Journal of Sedimentary Research. https://doi.org/10.2110/jsr.2014.89

Vorenhout, M., van der Geest, H. G., & Hunting, E. R. (2011). An improved datalogger and novel probes for continuous redox measurements in wetlands. International Journal of Environmental Analytical Chemistry. https://doi.org/10.1080/03067319.2010.535123

Wallace, C. D., Sawyer, A. H., & Barnes, R. T. (2019). Spectral analysis of continuous redox data reveals geochemical dynamics near the stream–aquifer interface. Hydrological Processes. https://doi.org/10.1002/hyp.13335

Yi, R., Song, P., Liu, X., Maruo, M., & Ban, S. (2019). Differences in dissolved phosphate in shallow-lake waters as determined by spectrophotometry and ion chromatography. Limnology. https://doi.org/10.1007/s10201-019-00574-2

Zeng, L., Li, X., & Liu, J. (2004). Adsorptive removal of phosphate from aqueous solutions using iron oxide tailings. Water Research, 38(5), 1318–1326. https://doi.org/10.1016/j.watres.2003.12.009