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A three-step test of phosphate sorption efficiencyof potential agricultural drainage filter materials

G. Lyngsie*, O.K. Borggaard, H.C.B. Hansen

University of Copenhagen, Department of Plant and Environmental Sciences, Thorvaldsensvej 40, DK-1871

Frederiksberg C, Denmark

a r t i c l e i n f o

Article history:

Received 17 August 2013

Received in revised form

24 October 2013

Accepted 27 October 2013

Available online 8 November 2013

Keywords:

Water quality

Eutrophication

Iron oxides

Carbonates

Farmland drainage

* Corresponding author.E-mail addresses: gryly@life.ku.dk, grylyn

0043-1354/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.watres.2013.10.061

a b s t r a c t

Phosphorus (P) eutrophication of lakes and streams, coming from drained farmlands, is a

serious problem in areas with intensive agriculture. Installation of P sorbing filters at drain

outlets may be a solution. Efficient sorbents to be used for such filters must possess high P

bonding affinity to retain ortho-phosphate (Pi) at low concentrations. In addition high P

sorption capacity, fast bonding and low desorption is necessary. In this study five potential

filter materials (Filtralite-P�, limestone, calcinated diatomaceous earth, shell-sand and

iron-oxide based CFH) in four particle size intervals were investigated under field relevant P

concentrations (0e161 mM) and retentions times of 0e24 min. Of the five materials exam-

ined, the results from P sorption and desorption studies clearly demonstrate that the iron

based CFH is superior as a filter material compared to calcium based materials when tested

against criteria for sorption affinity, capacity and stability. The finest CFH and Filtralite-P�

fractions (0.05e0.5 mm) were best with P retention of �90% of Pi from an initial concen-

tration of 161 mM corresponding to 14.5 mmol/kg sorbed within 24 min. They were further

capable to retain �90% of Pi from an initially 16 mM solution within 1½ min. However, only

the finest CFH fraction was also able to retain �90% of Pi sorbed from the 16 mM solution

against 4 times desorption sequences with 6 mM KNO3. Among the materials investigated,

the finest CFH fraction is therefore the only suitable filter material, when very fast and

strong bonding of high Pi concentrations is needed, e.g. in drains under P rich soils during

extreme weather conditions.

ª 2013 Elsevier Ltd. All rights reserved.

1. Introduction and ditches collect and direct the diffuse P contribution (loss)

Soils in intensively farmed countries may be sources of

phosphorus (P) due to decades of surplus application of P in

organic and inorganic fertilisers (Heal et al., 2005; Delgado and

Scalenghe, 2008; Buda et al., 2012). However, a P-enriched soil

only becomes an environmental problem when connected to

the aquatic environment by an effective transport pathway

such as artificial drains (Heathwaite et al., 2003). Tile drains

gsie@hotmail.com (G. Ly

ier Ltd. All rights reserve

from fields to recipient waters and act as highways for both

soluble and particulate P (Ulen et al., 2007). Despite substantial

efforts over many years to reduce this transport, leaching of P

from agricultural land to the aquatic environment is still a

serious and costly problem in many parts of Europe and

elsewhere (Delgado and Scalenghe, 2008; Ballantine and

Tanner, 2010; Buda et al., 2012). Thus, to reach the goal of

good water quality as stated in the EU Water Framework

ngsie).

d.

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5 257

Directive (WFD) requires a substantial reduction of the diffuse

P loss from farmland in many parts of Europe (Søndergaard

et al., 2005; Kaasik et al., 2008).

While a range of P mitigation options have been tested for

surface transport (Hoffmann et al., 2009), this is not the case

for subsurface transport (Kroger et al., 2008). However, by

installing a filter construction at the end of a drainage pipe,

Penn et al. (2007) demonstrated an immediate reduction of

drainage P leaching. This end-of-pipe approach could be car-

ried out in connection with a small-scale constructed wetland

(Reinhardt et al., 2005) or with flow-through filter structures in

ditches (Penn et al., 2007). A great variety of different types of

filters and filter materials for P retention have been described

for retention of high P concentrations in wastewaters

(Johansson Westholm, 2006; Penn et al., 2007; Cucarella and

Renman, 2009; Vohla et al., 2011). However, it is question-

able to directly transfer the experience obtained from these

high P concentration studies to the low P concentrations in

drainage water as the filter materials may behave differently

at high and low solution concentrations (Agyei et al., 2002;

Adam et al., 2007). Therefore, the filter materials need to be

tested at low P concentrations and short reaction times rele-

vant for cleaning P contaminated drainage water.

P in drainage water may consist of P in dissolved

organic matter, particulate P and dissolved ortho-phosphate

(Pi). The focus in this investigation will be on Pi, which

denotes inorganic phosphate irrespective of the species

ðH2PO�4 ; HPO2�

4 and=or PO3�4 Þ In natural, unpolluted areas in

Denmark, the Pi concentration in base flow drainage water is

typically �1.6 mM but the concentration can be up to 42 mM in

farmland drains (Andersen et al., 2006) and Penn et al. (2007)

found more than four times this concentration in Maryland,

US. The highwater flow during rainstorms and fast frost-thaw

transitions is critical because of high or very high P leaching

during such peak flows (Grant et al., 1996; Johnes, 2007). To

effectively remove the Pi during peak flows with high Pi con-

centrations, the filter material must react fast and possess

high Pi sorption stability and capacity. Even though fast and

substantial Pi sorption is mandatory for the practical use of

the filter materials, it is also important with a stable bonding

of sorbed Pi to ensure that Pi is not desorbed when the sorp-

tion condition changes, e.g. because of decreasing Pi concen-

tration in the drainage water (Grant et al., 1996). This,

however, is often overlooked in studies focussing on sorption

capacity (Klimeski et al., 2012). The Pi removal efficiency of a

filtermaterial is closely related to the content of various Al, Ca,

Fe andMg (hydr)oxides and carbonates (JohanssonWestholm,

2006; Ballantine and Tanner, 2010; Vohla et al., 2011). In

addition to the elemental composition, the specific surface

area (SSA) is important as sorption normally increases at

increasing SSA. Although it is outside the scope of this study,

high hydraulic conductivity of the filter material is also

essential for use in high-flow drainage filters.

As a practical test of high Pi removal efficiency also under

extreme conditions, we suggest the following three sorption/

desorption criteria: (i) A capacity to retain �90% Pi from

161 mM solution within 24 min; (ii) A reactivity resulting in

retention of �90% Pi from 16 mM solution within 1½ min; (iii)

A stability resulting in dissolution of <10% of this retained Pi

after four desorptions with artificial drainage water (6 mM

KNO3). A detailed discussion of these test criteria is given

later in the paper. This concentration range is in line with the

findings in natural drainage water as stated above and the

contact time is based on field findings (Penn et al., 2007; Penn

and McGrath, 2011).

Accordingly, the aim of the present investigation is to

assess the sorption parameters of five manufactured filter

materials (so-called Filtralite-P�, limestone, calcinated diato-

maceous earth CDE, shell-sand and iron oxide based CFH).

The assessment includes a characterization of the materials

and a batch-mode testing of the Pi removal efficiency ac-

cording to above-mentioned criteria of the materials in four

particle sizes at low concentrations of Pi in artificial and nat-

ural drainage waters. To the best of our knowledge, this is the

first studywhere various potential phosphorus filtermaterials

are tested under the same field-relevant conditions, i.e. both

at low to rather low Pi concentrations and very short to semi-

short sorption (reaction) times. The materials chosen for this

study have either been used or tested as wastewater filters or

as an adsorbent/absorbent in other respects.

2. Materials and methods

2.1. Filter materials

Filtralite-P� is a Light Expanded Clay Aggregates (LECA)-

resembling material calcinated at 1200 �C that was provided

by Weber, Norway. The porous material contains granules of

Ca/Mg oxides, which is the active sorbent. Limestone consists

of a mixture of bryozo and coral chalk from the Danian for-

mation at Faxe. The dried product was provided by Faxe Kalk

A/S, Denmark. Calcinated diatomaceous earth (CDE) from the

Fur formation calcinated at 750 �C was provided by Damolin

A/S, Denmark. Shell-sand consisting of crushed sea shells was

provided by DanShells Aps, Denmark. CFH-12 (CFH) consists

of dried iron oxides, which was provided by Kemira Oyj,

Finland.

2.2. Filter material characterization

Bulk samples of the five materials were analysed for pH,

mineral composition, carbonate, oxalate- and citrate-

bicarbonate-dithionite-extractable Al and Fe as well as total

Al, Fe, Ca, Mg and Pi, while the specific surface area (SSA) was

determined on all the particle size fractions. In addition, the

total composition of the CFH and Filtralite-P� fractions was

determined.

All analyses were carried out on ball-milled bulkmaterials.

pH was measured potentiometrically in 0.01 M CaCl2 using a

solid:solution ratio of 1:2.5. The mineralogy of the materials

was assessed by X-ray diffraction analysis on unoriented

samples using a Siemens 5000 instrument equipped with Co-

Ka radiation and a diffracted beam monochromator. Dif-

fractograms were recorded from 10 to 90� 2q using 0.03� 2q

steps and a step speed of 2 s. Diffraction peak positions were

used to calculate d-values for mineral identification. The

carbonate content was determined volumetrically by a

calcimeter (Allison and Moodie, 1965). Oxalate-extractable

aluminium (Alox) and iron (Feox) were determined by

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5258

extraction with 0.2 M ammonium oxalate for 2 h at pH 3 in the

dark (Schwertmann, 1964). Citrate-bicarbonate-dithionite-

extractable Al and Fe (AlCBD, FeCBD) were determined by

three sequential extractions for 15min at 70 �C as described by

Mehra and Jackson (1960). Total Al, Fe, Ca and Mg (Altotal,

Fetotal, Catotal, Mgtotal) were determined after dissolution of the

materials in a mixture of concentrated nitric acid, hydrogen

peroxide, hydrochloric acid and hydrofluoric acid (EPA 3052).

Al and Fe in oxalate and CBD extracts were determined by

atomic absorption spectroscopy (AAS) using a Perkin Elmer

3300 and the concentrations of total Al, Fe, Ca and Mg were

determined by inductive coupled plasma mass spectroscopy

(ICP-MS) on an Agilent 7500C instrument. Total Pi (Ptotal) was

determined by extracting the material with 6 M H2SO4 for

10 min at 70 �C (Mehta et al., 1954). The concentration of P in

the extract was determined by the molybdenum-blue method

(Murphy and Riley, 1962).

To assess the influence of particle size on Pi sorption, the

filter materials were fractionated into particle size intervals of

2e4mm, 1e2mm, 0.5e1mmand 0.05e0.5mmby sieving. The

(external) SSA of the different fractions was determined by

applying the BET equation (Brunauer et al., 1938) to N2

adsorption data obtained by means of a Micromeritic Gemini

VII 2390a instrument. Total element composition of the CFH

and Filtralite-P� fractions was determined by X-ray fluores-

cence at Actlabs, Canada.

2.3. Phosphate sorption

2.3.1. Influence of Pi concentration and background electrolyteThesorption testswere carriedout as follows:Nine 1 g samples

of each of the different particle sizes of the five filter materials

were shaken (175 rpm) in open beakers for 24minwith 100mL

6 mM KNO3 and 7 initial KH2PO4 concentrations ranging be-

tween 0 and 161 mM adjusted to pH 6 with 0.1 M NaOH. After

shaking, pH was measured and an aliquot of the solution was

withdrawnwith a syringe andfiltered through 0.2 mmMillipore

syringe-filter. The filtrate was added 20 mL 2 M sulphuric acid

per mL for preservation, and stored (<1 week) at 5 �C until P

determination. Before performing Pi sorption, the filter mate-

rials were soaked in the background electrolyte for 24 h.

As background electrolyte so-called artificial drainage

water (ADW), i.e. 6 mM KNO3 was used for all Pi sorptions

instead of natural drainage water (NDW). ADW has the same

electric conductivity (ionic strength) as was found in NDW

(0.6 dS/m) collected from a drainage well at Tastrup, Denmark

under a cultivated soil (Typic Argiudoll) developed on calcar-

eous morainic material from the Weichelian Glaciation. This

NDW had pH 6.9 and contained 2.5 mM Ca2þ, 0.3 mM Mg2þ,0.4 mM Naþ, 1.6 mM Kþ and 1.6 mM Pi; Cl� and NO�

3 were

identified but not quantified. ADW was chosen to avoid

interfering reactions such as formation of Ca-precipitates if

using NDW. Furthermore, use of NDW throughout would be

inconvenient as drainage water composition can exhibit great

temporal variation, which would necessitate collection and

long-time storage of very large volumes of drainage water.

However, sorption by the 1e2 mm fractions of CFH, Fil-

tralite-P� and limestone was tested with the above-

mentioned NDW as background electrolyte using the same

Pi concentrations and procedure as described above.

2.3.2. Pi sorption and desorption kineticsThe importance of contact time was addressed in two turns: (i)

short-time sorption covering the time span of 1½-24minand (ii)

semi long-term sorption running from 0.75 to 48 h. For short-

term Pi sorption 0.7 g samples of the different materials were

shaken end-over-end (175 rpm) with 70 mL 6 mM KNO3 con-

taining 16 mMKH2PO4adjusted to pH 6 for 90 s, 180 s, 360 s, 720 s

and 1440 s (24 min). The kinetics of semi long-term Pi sorption

was determined for the 1e2 mm fractions of CFH, Filtralite-P�

and limestone in the same way as for the short-term sorption

but the sorption times were extended to 45 min, 1.5 h, 3 h, 6 h,

12 h, 24 h and 48 h. After shaking and filtration, the extracts

were treated as described in Section 2.3.1.

Immediately after the 24-min sorption from 16 mM Pi so-

lution, desorption was carried out on all samples by means of

four successive extractions, each by end-over-end shaking of

the sample (175 rpm) for 15 min with 50 mL of 6 mM KNO3,

centrifugation of the suspension and replacement of the

extract with a fresh portion of 6 mM KNO3. After extraction,

the extracts were acidified as described in Section 2.3.1.

The phosphate concentrations in the filtrates and extracts

were determined by themolybdenum blue method using flow

injection analysis on a FIAstar 5000 instrument (Ruzicka and

Hansen, 1988). Sorbed Pi in mmol/kg was calculated from the

difference between the Pi concentrations before and after

shaking with the filter materials. In the desorption investiga-

tion, desorbed Pi in each desorption step was calculated from

the Pi concentration in the extract corrected for left-over from

the previous desorption.

All experiments were carried out in triplicate. The glass-

and plastic wares were acid-washed, the chemicals were pro

analysis or of better quality and triple deionized water was

used throughout.

2.4. Data handling

Sorbed Pi was plotted against the solution concentration

resulting in the isotherms. Sorbed Pi corresponding to initial

concentrations of 16 mM and 161 mM were read on the iso-

therms for the various materials and particle sizes (S16, S161).

The sorption versus time data from the short-term

(90e1440 s) and semi long-term (0.75e48 h) sorption in-

vestigations were fitted to the hyperbola equation:

SK ¼ Smax$tKþ t

(1)

SK is the time-dependent amount of sorbed Pi (mmol/kg) at

time t (min). Smax is the maximum Pi sorption (mmol/kg) and K

(min) is a fitting parameter determined by the shape of the

sorption curve. The curves were fitted with SigmaPlot (v. 12.0,

Systat Software, Inc.).

3. Results and discussion

3.1. Filter material characteristics

The filters comprise three kinds of minerals including car-

bonates (limestone, shell-sand), iron oxides (CFH) and mixed

minerals (CDE, Filtralite-P�) (Table 1). According to XRD, the

Table 1 e Chemical composition of the five potential filter materials (CDE, CFH, limestone, Filtralite-P� and shell-sand)based on analyses of bulk samples.

Element/characteristic

Form Unit CDE CFH Limestone Filtralite-P� Shell-sand

Fe Oxalate mmol/kg 26.0 � 0.4 7310 � 90 5.0 � 0.1 22.0 � 0.2 5.0 � 0.1

CBD mmol/kg 218 � 11 7790 � 150 5.0 � 0.0 24.0 � 0.3 4.0 � 0.1

Total mmol/kg 815 � 30 7360 � 420 8.0 � 0.1 737 � 30 11 � 1

Al Oxalate mmol/kg 26 � 3 15.0 � 0.2 15.0 � 0.1 31.0 � 0.4 15 � 1

CBD mmol/kg 31 � 1 6.0 � 2.8 ndb 21.0 � 1.6 nd

Total mmol/kg 629 � 27 32 � 2 nd 2350 � 160 23 � 2

Ca Total mmol/kg 381 � 10 658 � 23 9590 � 50 1720 � 60 9280 � 120

Mg Total mmol/kg 100 � 18 1390 � 100 103 � 1 1560 � 110 565 � 26

Pi Total mmol/kg 12.0 � 0.5 8.0 � 0.3 1.0 � 0.1 3.0 � 0.1 4.0 � 0.1

Carbonatea % 0.0 � 0.0 5.0 � 0.1 101 � 3 6.0 � 0.2 102 � 2

pH 4.7 � 0.1 8.2 � 0.1 7.9 � 0.1 11.9 � 0.2 8.1 � 0.1

Minerals Mixed calcined

clay silicates

Two line

Fe oxides

Calcite Calcite, Ca/Mg

oxides, clay silicates

Calcite, aragonite,

dolomite

a Carbonate expressed as CaCO3.b Not determined.

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5 259

limestone consists of pure calcite (CaCO3), while shell-sand

contains two calcium carbonates, i.e. aragonite and calcite

(both CaCO3 polymorphs), together with dolomite

(CaMg(CO3)2). Both materials contain small amounts of Al and

Fe oxides. CFH consists of poorly ordered Fe oxides (Table 1) as

also indicated by XRD. In addition to Fe, CFH contains Ca and

Mg carbonates. CDE and Filtralite-P� are calcinated at high

temperature, which makes the clay mineral structures

collapse and turn into amorphous solids. Together with the

elemental analyses it seems, however, rather clear that they

consist of mixtures of poorly ordered Al, Ca, Fe and Mg sili-

cates. In addition, Filtralite-P� undoubtedly also contains Ca/

Mg oxides as indicated by the very alkaline pH (Table 1).

Important is to note that all materials contain minor to small

but significant amounts of Pi, especially the CDE and CFH

materials. The contents of Al and Fe (hydr)oxides are very

important in relation to Pi sorption, as Al and Fe oxides (AlCBD,

FeCBD), especially the poorly ordered, oxalate extractable

forms (AlOx, FeOx), are well known effective Pi sorbents

(Borggaard et al., 2005; Johansson Westholm, 2006; Cucarella

and Renman, 2009; Vohla et al., 2011; Penn et al., 2011).

Accordingly, CFH with very high FeCBD and FeOx contents is

expected to be a very strong Pi sorbent. As Ca and Mg car-

bonates and oxides are important Pi sorbents, Filtralite-P�,

limestone and shell-sand may also be considered to act a Pi

sorbents. On the other hand, CDE will probably be a rather

poor sorbent as most silicates even tiny clay silicates possess

limited Pi sorption capacity (Ballantine and Tanner, 2010).

The different particle size fractions had somewhat different

specific surface areas, SSAs (Table 2). Except for CDE, the

smallest particles had e as expected e the highest SSAs.

Consequently, the order of decreasing Pi sorption permass unit

isexpected tobe0.05e0.5mm>0.5e1mm> 1e2mm>2e4mm

as Pi sorption is normally found to increasewith increasing SSA

for the samematerial (Ballantine and Tanner, 2010; Penn et al.,

2011). SSA also differs among the materials with CFH and CDE

having much larger SSAs than the other materials. The poorly

ordered Fe oxides are responsible for the substantial CFH SSAs,

whereas CDE has high SSA due to larger internal (N2 accessible)

pores generated by the diatomite framework. The total element

composition of the Filtralite-P� and CFH fractions (Supple-

mental data, Table S1) showed the finer Filtralite-P� fractions

were somewhat enriched in Ca and Mg but not in Al and Fe,

whereas the three CFH fractions had the same composition.

CaeMg enrichment in Filtralite-P� finer fractions reflects the

heterogeneity of the material with overrepresentation of the

active sorbent (CaeMg compounds) in the smaller white parti-

cles at the expense of the Al/Fe-bearing matrix.

3.2. Phosphate retention

3.2.1. Phosphate sorption capacityThe 24-min Pi sorption isotherms for the five materials are

very different with limited sorption to CDE, limestone and

shell-sand; in fact CDE releases Pi at low concentration (Fig. 1).

In contrast, CFH and Filtralite-P� possess much higher Pi

sorption capacity. Except CFH, the shape of the isotherms

comprises two steps.

For soils and various materials such as Al and Fe oxides, Pi

sorption isotherms can often be fitted with the Langmuir

equation resulting in sorption maximum and stability

e.g. (Borggaard et al., 2005; Johansson Westholm, 2006; Vohla

et al., 2011). This is not the case for the 2-steps isotherms in

Fig. 1, i.e. Langmuirian sorption capacity and stability cannot

be determined. Similar non-Langmuirian shaped sorption

isotherms have also been reported for similar and other ma-

terials tested for their suitability as P filters and the shape of

the curves has been ascribed to a combination of adsorption

and precipitation reactions at low and high Pi concentrations,

respectively (Adam et al., 2007; Kaasik et al., 2008; Cucarella

and Renman, 2009). Therefore, it was decided to characterize

the Pi retention capacity of all materials by means of Pi sorbed

at initial Pi concentrations of 16 mM (S16) and 161 mM (S161).

Sorption (S161) at the highest Pi concentration (161 mM) was

chosen to show sorption at a high filter inlet concentration

comparable to Pi concentrations reported for high Pi drainage

waters (Penn et al., 2007; Chardon et al., 2012). The first test

criterion was the material ability to remove at least 90% of Pi

from the highest initial concentration within 24 min corre-

sponding to sorption of �14.5 mmol/kg. As indicated in Fig. 1,

Table 2 e Phosphate sorption (mean and SD) by the particle size fractions of the five filter materials from solutions withinitially 16 mM Pi (S16) and 161 mM Pi (S161) together with pH and the specific surface area (SSA) of the various size fractions.Pi sorption expressed as mmol/kg and percent of added Pi. S16 and S161 were taken from isotherms in Fig. 1; sorption time24 min and solution:solid ratio 100.

Material Particle size S16 S161 pH SSAa

mm mmol/kg (%) mmol/kg (%) m2/g

Artificial drainage water (ADW) as background electrolyte

CDE 0.05e0.5 142 � 15 (9) 4020 � 325 (25) 5.3 26 � 1

0.5e1 102 � 30 (6) 3870 � 222 (24) 5.2 26 � 1

1e2 �21 � 60 (�1) 2150 � 952 (13) 5.5 32 � 4

2e4 �157 � 32 (�10) 2440 � 141 (15) 5.5 30 � 2

CFH 0.05e0.5 1580 � 51 (98) 16,100 � 50 (100) 8.2 44 � 7

0.5e1 1570 � 32 (97) 11,000 � 195 (68) 6.9 32 � 10

1e2 1100 � 51 (68) 7930 � 140 (49) 6.7 32 � 2

Limestone 0.05e0.5 1320 � 208 (82) 4340 � 1860 (68) 8.9 1.3 � 0.2

0.5e1 258 � 34 (16) 2030 � 1410 (13) 8.8 <0.1

1e2 188 � 19 (12) 2670 � 251 (17) 8.8 <0.1

2e4 195 � 76 (12) 2230 � 276 (14) 8.6 <0.1

Filtralite-P� 0.05e0.5 1590 � 52 (99) 16,200 � 12 (100) 11.7 3.2 � 0.2

0.5e1 1130 � 365 (70) 14,900 � 839 (92) 10.8 1.2 � 0.0

1e2 571 � 136 (35) 10,200 � 2100 (63) 10.1 0.9 � 0.2

2e4 524 � 213 (33) 9430 � 21,400 (59) 9.8 0.9 � 0.1

Shell-sand 0.05e0.5 479 � 28 (30) 3330 � 461 (21) 9.1 <0.1

05e1 358 � 12 (22) 2800 � 45 (17) 8.6 <0.1

1e2 283 � 12 (18) 2560 � 65 (16) 8.6 <0.1

2e4 250 � 11 (16) 2450 � 60 (15) 8.3 <0.1

Natural drainage water (NDW) as background electrolyte

CFH 1e2 894 � 133 (47) 5550 � 273 (37) 8.2 32 � 2

Limestone 1e2 299 � 85 (16) 1970 � 85 (12) 8.3 <0.1

Filtralite-P� 1e2 418 � 33 (22) 1980 � 96 (12) 8.3 0.9 � 0.2

a Limit of detection was 0.1 m2/g.

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5260

S161 strongly depends on material and particle size (Table 2).

Apart from shell-sand, sorbed Pi generally increases with

decreasing particle size or increasing SSA in agreement with

most other studies (Cucarella and Renman, 2009; Ballantine

and Tanner, 2010; Vohla et al., 2011). For the same particle

size, S161 decreases in the order: CFH z Filtralite-

P� > limestone z CDE > shell-sand. In fact, the 0.05e0.5 mm

and 0.5e1 mm Filtralite-P� fractions and the smallest CFH

fraction sorbs all or almost all added Pi, i.e. these samples fulfil

the test criterion of �14.5 mmol/kg sorption within 24 min.

However, SSA cannot adequately explain the big differ-

ences between the various fractions of, in particular Filtralite-

P� and CFH. The differences can neither be explained by the

similar (CFH) or rather similar (Filtralite-P�) total element

compositions (Table S1) indicating that other material char-

acteristics are responsible for the differences. The much

higher sorption by the finer particle size fractions (Fig. 1, Table

2) may be due to more internal hydrophilic sorption sites in

the finer than coarser fractions, which are accessible to Pi but

not to the hydrophobic N2, and hence not accounted for by the

N2 determined SSA (Makris et al., 2004). Comparison with

previously published data for Pi sorbed to similar materials

such as iron oxide sludge, Filtralite-P�, limestone and shell-

sand (Johansson Westholm, 2006; Vohla et al., 2011), is diffi-

cult as in general much higher P concentrations and reaction

times have been used. However, for comparable solution Pi

concentrations our short term S161 for Filtralite-P� is in the

same range as sorbed amounts after reaction for 24 h found by

Adam et al. (2007), while for iron oxides our S161 is more than

10 times lower than the sorbed amounts after 1e21 d (Chardon

et al., 2012). It demonstrates that iron oxides may have very

high sorption capacity that increases strongly with time

(Makris et al., 2004).

In the field, however, efficient filters must react fast to

retain drainage water P during fast leaching events such as

rainstorms or when the transition between frost and thaw is

very short but they must also be able to reduce Pi to very low

concentrations as Pi concentrations in lakes higher than

w50 mg/L (1.6 mM) may cause eutrophication (Smil, 2000).

Consequently, the Pi threshold at the filter outlet is set at

1.6 mM. The determination of sorbed P (S16) at an initial Pi

concentration of 16 mM was used to test the capacity of the

materials to substantially sorb Pi even at low Pi concentration.

In fact, the criterion was 90% retention (1440 mmol/kg) from

the 16 mM Pi solution. The sorption criterion of 1440 mmol/kg

(S16) is fulfilled by the two finest CFH particle size fractions and

the 0.05e0.5 mm Filtralite-P� fraction (Table 2), whereas the

other materials and size fractions are less effective sorbents;

although the 1e2 mm CFH, the finest limestone and the

0.5e1 mm Filtralite-P� fractions are rather close to the

threshold.

CFH,Filtralite-P�and limestone,whichperformedbest in the

ADWexperiments,were further testedwithNDWaselectrolyte.

As also seen in Table 2, sorbed Pi depends on the background

electrolyte even though the ADW and the NDW have similar

electric conductivities, i.e. similar ionic strengths. However, the

Fig. 1 e Pi sorption isotherms for the 3e4 particle size fractions of the five potential drainage water filter materials. The

initial Pi concentrations covered the range 0e161 mM, the solution:solid ratio was 100, the sorption time was 24 min and the

background electrolyte was 6 mM KNO3. The values are averages of the triplicate and error bars represent standard

deviation. Red arrows indicate possible change in sorption mechanisms. (For interpretation of the references to colour in

this figure legend, the reader is referred to the web version of this article.)

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5 261

ion composition of the two electrolytes is different as NDW is

dominated byCa2þ andHCO�3 ,whereasKþ andNO�

3 are theonly

ions in ADW. The influence of the background electrolyte on

sorbed Pi (S16 and S161) differs for the three materials tested.

Thus, CFH seems more effective with ADW as background

electrolyte (sorbs20e30%lesswithNDW),while limestonesorbs

more (60% increase) from the initial 16 mM Pi solution but less

(25%) from the 161 mM solution with NDW as background elec-

trolyte instead of ADW. The biggest difference is seen for Fil-

tralite-P�, where especially S161 is substantially reduced (80%)

with NDW as background electrolyte. This difference may be

explained by a substantial drop in pH from10.1 inADW to 8.3 in

NDWbecause ofHCO�3 inNDW,which forms CaCO3 by reaction

withCa2þ in Filtralite-P� changing the sorptionS161 to be similar

to limestone (Table2).Theobserved influenceof thebackground

electrolyte on Pi sorption is in accordancewith previous studies

showing that the composition and concentration of the various

background electrolytes may affect the sorption (Johansson

Westholm, 2006; Adam et al., 2007; Sø et al., 2011).

3.2.2. Phosphate sorption kineticsThe sorption kinetic data were fitted by Eq. (1) (Fig. 2) and

Table 3 shows the sorptionmaxima (Smax), the time parameter

(K), and sorption after 1½min (S1½) calculated by Eq. (1). Smax is

the maximum sorption under the current experimental con-

ditions, in casu sorption from a 16 mM Pi solution with a sol-

ution:solid ratio of 100, i.e. Smax in Table 3 cannot exceed

1600 mmol/kg. The above-mentioned threshold sorption of

1440 mmol/kg corresponding to 90% of the initially added Pi is

only fulfilled by the two finest CFH size fractions and the finest

Filtralite-P� size fraction. The fitting parameter K is deter-

mined by the shape of sorption versus time curve and in-

creases as the curve becomes more straight-lined, i.e. a low K

value corresponds to a strongly bending curve as seen for the

CFH sample in Fig. 2 with K ¼ 5.8 min compared to the more

smoothly bending Filtralite-P� curve with K ¼ 160 min

(Table 3). In other words, K is a measure of the sorption rate,

the smaller the K, the faster the sorption. Fast reaction of CFH

is in line with reported initial fast sorption of Pi by various

Table 3 e Influence of time on phosphate sorption by theparticle size fractions of the five filter materials divided inshort-term sorption, 1½-24 min, and semi long-termsorption, 3/4e48 h. Sorption data were fitted to Eq. (1)resulting in sorptionmaximum (Smax) and sorption curveshape factor (K). SK1½ is sorbed Pi after 1½min calculatedby Eq. (1). Initial Pi concentration 16 mM, material:solidratio 100 and background electrolyte ADW (6 mM KNO3).

Material Particle size Smax K SK1½

mm mmol/kg min mmol/kg

Short-term kinetic series 1½-24 min

CDE 0.05e0.5 8 � 25 nda nd

0.5e1 9 � 36 nd nd

1e2 �84 � 9 nd nd

2e4 �134 � 14 nd nd

CFH 0.05e0.5 1610 � 10 0.02 � 0.01 1590 � 10

0.5e1 1550 � 8 0.66 � 0.01 1100 � 20

1e2 1140 � 30 1.68 � 0.20 590 � 45

Limestone 0.05e0.5 1170 � 10 0.13 � 0.01 1080 � 20

0.5e1 343 � 26 0.56 � 0.01 252 � 7

1e2 219 � 7 0.12 � 0.01 201 � 10

2e4 99 � 10 2.3 � 0.9 44 � 7

Filtralite-P� 0.05e0.5 1570 � 10 0.00 � 0.00 1570 � 10

0.5e1 913 � 50 0.8 � 0.3 620 � 95

1e2 412 � 10 2.0 � 0.2 195 � 17

2e4 476 � 81 1.5 � 0.1 257 � 108

Shell-sand 0.05e0.5 569 � 46 2.3 � 0.7 283 � 64

0.5e1 208 � 24 0.4 � 0.5 142 � 36

1e2 237 � 9 1.3 � 0.2 149 � 15

2e4 233 � 11 2.0 � 0.4 120 � 13

Semi long-term kinetic series 3/4-48 h

CFH 1e2 1620 � 30 5.8 � 0.1 e

Limestone 1e2 630 � 41 320 � 66 e

Filtralite-P� 1e2 1610 � 50 160 � 18 e

a Not determined as the CDE graphs could not be fitted by Eq. (1).

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5262

pure (synthetic) Fe oxides (Strauss et al., 1997; Chardon et al.,

2012). Even though the kinetics of Pi sorption by soils and

similar materials have been investigated for more than half a

century (Chardon et al., 2012), short-time kinetics is less

studied. An exception is the work by Heal et al. (2003), who

reported fast sorption of Pi by ochre from mine water treat-

ment plants able to reduce the Pi concentration from161 mM to

<0.3 mM within 8 min. The fast sorption by Filtralite-P� is

assumed due to rapid reaction between Pi and hydrated Ca/Mg

oxides as well as between Pi and reactive Al and Fe com-

pounds in the Filtralite-P� sorbent (Table 1). Even more

important is to note that after as little as 1½ min, the finest

CFH and Filtralite-P� size fractions have passed this threshold

showing that these samples are fast reacting even at this low

Pi concentration (Table 3). Extending sorption time up to 48 h,

the 1e2 mm CFH and Filtralite-P� samples can sorb all added

Pi, whereas the limestone sorbs less than half of the added Pi.

3.2.3. Phosphate sorption stabilityAs a test criterion for the stability of Pi retention, desorption of

no more than 10% of the Pi sorbed during 4 successive

desorption steps from the initial 16 mM solution was adopted.

After four desorptions by ADW, less than 10% was desorbed

from the three CFH size fractions indicating high stability of

the bonding of Pi by this material (Fig. 3). Considering the high

content of Fe oxides in this material (Table 1), strong Pi

retention is expected as Fe oxides are known to form strong

bonds with Pi (Strauss et al., 1997; Heal et al., 2003; Borggaard

et al., 2005; Chardon et al., 2012). All the other samples lost

more than 10% in the desorption test. In fact, more than 10%

was desorbed already after the first desorption (Fig. 3), and for

CDE (data not shown) more Pi was desorbed than sorbed from

the initial 16 mM Pi solution, which may be ascribed to the

original Pi content of this material (Table 1, Fig. 1). Apart from

Fig. 2 e Pi sorption kinetics for the 1e2mm size fractions of

CFH, Filtralite-P� and limestone. The initial Pi

concentration was 16 mM, the solution:solid ratio was 100

and the background electrolyte was 6 mM KNO3. The data

were fitted by the Eq. (1) and shown as curves; the dashed

line corresponds to sorption of 1440 mmol/kg. The values

are averages of triplicates and error bars represent

standard deviation.

CFH, which showed very little Pi release, the relative desorp-

tion seen in this study (Fig. 3) is high compared to the relative

desorption of 10e32% of sorbed Pi seen for six industrial by-

products (Penn et al., 2011), which may be attributed to

different reaction times. However, the high desorption seen

for especially the Ca-based sorbents emphasizes the impor-

tance of desorption studies in regard to Pi sorbing materials.

3.3. Suitability of the materials as phosphate filters

The five materials in 3e4 particle size fractions have been

tested with respect to Pi sorption efficiency, i.e. sorption ca-

pacity, sorption reactivity and sorption stability. Emphasis has

been put on test conditions considered important for possible

use of the materials as filters to reduce Pi in drainage water to

a non-eutrophication level before it enters open water bodies,

especially streams and lakes, also under extreme drainage

water flow conditions. Consequently, high, fast and strong Pi

retention was the test criteria in this investigation. Obviously,

only the 0.05e0.5 mm CFH fraction fulfils all three criteria.

Filtralite-P� has a rather high sorption capacity and the finest

fraction (0.05e0.5 mm) sorbs >90% Pi from the initially 16 mM

solution but all Filtralite-P� fractions fail to retain >90% of the

sorbed Pi in the desorption test. In the test, CDE came out as

the least suitablematerial as it has a low sorption capacity and

Fig. 3 e Pi desorption by means of 6 mM KNO3 from the 3e4 particle size fractions of the five potential drainage water filter

materials loaded with Pi from 16 mM solution. The P-loaded material was shaken 4 times, each time for 15 min using 6 mM

KNO3 (ADW).

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5 263

more Pi was released during desorption than originally sorbed

(data not shown). The carbonate-based materials, limestone

and shell-sand possess characteristics between those of Fil-

tralite-P� and CDE and they are all not suitable as fast reacting

filtermaterials. The remarkable behaviour of Filtralite-P�with

high capacity at high pH but limited capacity at neutral pH,

when NDW was used as background electrolyte (Table 2),

limits the practical suitability of this material.

The criteria set in this study may be considered very

rigorous, and possibly less strict criteria may occasionally be

justified when for example drainage water from an agricul-

tural field is diluted with water from an unpolluted area, e.g.

old forests, resulting in ecosystem-tolerable Pi concentrations

in receiving open water bodies. Furthermore, the result does

not exclude that some of the other tested materials can be

used (and are maybe superior to CFH) in remediation of

certainwastewaters, as landscape barriers, as amendments to

control Pi release in very P-rich soils and other uses than as

filter material for drainage water. Finally it must be empha-

sized that despite the favourable phosphate sorption charac-

teristics of CFH, assessment of the suitability of this material

in drainage water filters requires consideration of additional

factors such as hydraulic properties, costs, availability,

toxicity, long-term performance and use of the P-saturated

(used) filter material as P fertilizer (Ballantine and Tanner,

2010; Vohla et al., 2011). Especially CFH strongly bound Pi

might limit its reuse potential as a direct soil amendment and

other recycling strategies should be looked into. However,

assessment of these additional and important factors is

outside the scope of the present study.

It can be argued that investigation of the finer fractions is

not relevant for field application as this material most likely

will be clogged rather fast but as the fine factions are a part of

the commercial available products and have shown to be the

most reactive part (Table 2) it is considered relevant to study

the behaviour of the different particle size fractions.

4. Conclusions

Materials proposed as efficient drainage water filters must

possess high capacity, reactivity and stability to retain Pi at

rather low and very low concentrations. To address these re-

quirements also under extreme subsurface runoff conditions,

a 3-step test was proposed:

1. Capacity: Retention of �90% of Pi from 161 mM solution

corresponding to 14.5 mmol/kg within 24 min with sol-

ution:material ratio ¼ 100.

2. Reactivity: Retention of �90% of Pi from 16 mM solution

decreasing solution Pi to �1.6 mM (50 mg/L) within 1½ min

(representing high flow situations) and 24 min (represent-

ing low flow situations).

3. Stability: Retention of �90% of Pi sorbed from 16 mM solu-

tion within 24 min against 4 times desorption with 6 mM

KNO3.

Application of the test to sorption and desorption results

obtained for the four particle size fractions of the five po-

tential filter materials showed that criteria 1 and 2 were

passed by the finest CFH and Filtralite-P� fractions, while all

CFH samples fulfilled the third criterion. Consequently, only

the finest (0.05e0.5 mm) CFH sample fulfilled all criteria of

this very strict test. The choice of background electrolyte,

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5264

whether ADW or NDW, affected the sorption, in particular for

Filtralite-P�.

Acknowledgement

The project was carried out in the frames of SupremeTech

project funded by The Danish Council for Strategic Research

(grant no. 09-067280). Special thanks to Bente Postvang for

laboratory assistance.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.watres.2013.10.061.

r e f e r e n c e s

Adam, K., Krogstad, T., Vrale, L., Søvik, A.K., Jenssen, P.D., 2007.Phosphorus retention in the filter materials shellsand andfiltralite P�dbatch and column experiment with synthetic Psolution and secondary wastewater. Ecol. Eng. 29 (2),200e208.

Agyei, N.M., Strydom, C.A., Potgieter, J.H., 2002. The removal ofphosphate ions from aqueous solution by fly ash, slag,ordinary Portland cement and related blends. CementConcrete Res. 32 (12), 1889e1897.

Allison, L.E., Moodie, C.D., 1965. Carbonate. In: Black, C.A.,Evans, D.D., White, J.L., Ensminger, L.E., Clark, F.E. (Eds.),Methods of Soil Analysis: Part 2. Chemical and MicrobiologicalProperties. Soil Science Society of America, Madison,Wisconsin, pp. 1392e1395.

Andersen, H.E., Larsen, S.E., Kronvang, B., Hansen, K.M.,Laubel, A., Windolf, J., Muus, K., 2006. Fosfat i drænvand. Vandog Jord 13 (4), 152e156 (in Danish).

Ballantine, D.J., Tanner, C.C., 2010. Substrate and filter materialsto enhance phosphorus removal in constructed wetlandstreating diffuse farm runoff: a review. N.Z. J. Agric. Res. 53 (1),71e95.

Borggaard, O.K., Raben-Lange, B., Gimsing, A.L., Strobel, B.W.,2005. Influence of humic substances on phosphateadsorption by aluminium and iron oxides. Geoderma 127(3e4), 270e279.

Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases inmultimolecular layers. J. Am. Chem. Soc. 60 (2), 309e319.

Buda, A.R., Koopmans, G.F., Bryant, R.B., Chardon, W.J., 2012.Emerging technologies for removing nonpoint phosphorusfrom surface water and groundwater: introduction. J. Environ.Qual. 41 (3), 621e627.

Chardon, W.J., Groenenberg, J.E., Temminghoff, E.J.M.,Koopmans, G.F., 2012. Use of reactive materials to bindphosphorus. J. Environ. Qual. 41 (3), 636e646.

Cucarella, V., Renman, G., 2009. Phosphorus sorption capacity offilter materials used for on-site wastewater treatmentdetermined in batch experiments e a comparative study. J.Environ. Qual. 38 (2), 381e392.

Delgado, A., Scalenghe, R., 2008. Aspects of phosphorustransfer from soils in Europe. J. Plant Nutr. Soil Sci. 171 (4),552e575.

Grant, R., Laubel, A., Kronvang, B., Andersen, H.E., Svendsen, L.M.,Fuglsang, A., 1996. Loss of dissolved and particulate

phosphorus from arable catchments by subsurface drainage.Water Res. 30 (11), 2633e2642.

Heal, K., Younger, P.L., Smith, K., Glendinning, S., Quinn, P.,Dobbie, K., 2003. Novel use of ochre from mine watertreatment plants to reduce point and diffuse phosphoruspollution. Land Contam. Reclam. 11 (2), 145e152.

Heal, K., Dobbie, K., Bozika, E., McHaffie, H., Simpson, A.,Smith, K., 2005. Enhancing phosphorus removal inconstructed wetlands with ochre from mine drainagetreatment. Water Sci. Technol. 51 (9), 275e282.

Heathwaite, L., Sharpley, A., Bechmann, M., 2003. The conceptualbasis for a decision support framework to assess the risk ofphosphorus loss at the field scale across Europe. J. Plant Nutr.Soil Sci. 166 (4), 447e458.

Hoffmann, C.C., Kjaergaard, C., Uusi-Kamppa, J., Hansen, H.C.B.,Kronvang, B., 2009. Phosphorus retention in Riparian buffers:review of their efficiency. J. Environ. Qual. 38 (5), 1942e1955.

Johansson Westholm, L., 2006. Substrates for phosphorusremovaldpotential benefits for on-site wastewatertreatment? Water Res. 40 (1), 23e36.

Johnes, P.J., 2007. Uncertainties in annual riverine phosphorusload estimation: impact of load estimation methodology,sampling frequency, baseflow index and catchmentpopulation density. J. Hydrol. 332 (1e2), 241e258.

Kaasik, A., Vohla, C., Motlep, R., Mander, U., Kirsimae, K., 2008.Hydrated calcareous oil-shale ash as potential filter media forphosphorus removal in constructed wetlands. Water Res. 42(4e5), 1315e1323.

Klimeski, A., Chardon, W.J., Turtola, E., Uusitalo, R., 2012.Potential and limitations of phosphate retention media inwater protection: a process-based review of laboratory andfield-scale tests. Agric. Food Sci. 21, 206e223.

Kroger, R., Holland, M.M., Moore, M.T., Cooper, C.M., 2008.Agricultural drainage ditches mitigate phosphorus loads as afunction of hydrological variability. J. Environ. Qual. 37,107e113.

Makris, K.C., Harris, W.G., O’Connor, G.A., Obreza, T.A., 2004.Phosphorus immobilization in micropores of drinking-watertreatment residuals: implications for long-term stability.Environ. Sci. Technol. 38 (24), 6590e6596.

Mehra, O.P., Jackson, M.L., 1960. Iron oxide removal from soilsand clays by dithionite-citrate system buffered withsodium bicarbonate. In: Proceedings of 7th NationalConference of Clay and Clay Minerals. Washington, 1958,pp. 317e327.

Mehta, N., Legg, J., Goring, C., Black, C., 1954. Determination oforganic phosphorus in soils: I. Extraction method. Soil Sci. Soc.Am. J. 18 (4), 443e449.

Murphy, J., Riley, J.P., 1962. A modified single solution method forthe determination of phosphate in natural waters. Anal. Chim.Acta 27 (0), 31e36.

Penn, C.J., Bryant, R.B., Kleinman, P.J.A., Allen, A.L., 2007.Removing dissolved phosphorus from drainage ditch waterwith phosphorus sorbing materials. J. Soil Water Conserv. 62(4), 269e276.

Penn, C.J., Bryant, R.B., Callahan, M.P., McGrath, J.M., 2011. Use ofindustrial by-products to sorb and retain phosphorus.Commun. Soil Sci. Plant Anal. 42 (6), 633e644.

Penn, C.J., McGrath, J.M., 2011. Predicting phosphorus sorptiononto steel slag using a flow-through approach withapplication to a pilot scale system. J. Water Res. Prot. 3,235e244.

Reinhardt, M., Gachter, R., Wehrli, B., Muler, B., 2005. Phosphorusretention in small constructed wetlands treating agriculturaldrainage water. J. Environ. Qual. 34 (4), 1251e1259.

Ruzicka, J., Hansen, E.H., 1988. Homogeneous and heterogeneoussystems: flow injection analysis today and tomorrow. Anal.Chim. Acta 214 (0), 1e27.

wat e r r e s e a r c h 5 1 ( 2 0 1 4 ) 2 5 6e2 6 5 265

Schwertmann,U., 1964.Differenzierungder EisenoxidedesBodensdurch Extraction mit Ammoniumoxalat-Losung. Zietschr.Pflanzenernahr. Dung. Bodenkun. 105, 194e202 (in German).

Smil, V., 2000. Phosphorus in the environment: natural flows andhuman interferences. Annu. Rev. Energy Environ. 25 (1), 53e88.

Sø, H.U., Postma, D., Jakobsen, R., Larsen, F., 2011. Sorption ofphosphate onto calcite; results from batch experiments andsurface complexation modeling. Geochim. Cosmochim. Acta75 (10), 2911e2923.

Søndergaard, M., Jeppesen, E., Jensen, J.P., Amsinck, S.L., 2005.Water framework directive: ecological classification of Danishlakes. J. Appl. Ecol. 42 (4), 616e629.

Strauss, R., Brummer, G.W., Barrow, N.J., 1997. Effects ofcrystallinity of goethite: II. Rates of sorption and desorption ofphosphate. Eur. J. Soil Sci. 48 (1), 101e114.

Ulen, B., Bechmann, M., Folster, J., Jarvie, H.P., Tunney, H.,2007. Agriculture as a phosphorus source foreutrophication in the north-west European countries,Norway, Sweden, United Kingdom and Ireland: a review.Soil Use Manag. 23, 5e15.

Vohla, C., Koiv, M., Bavor, H.J., Chazarenc, F., Mander, U.,2011. Filter materials for phosphorus removal fromwastewater in treatment wetlandsda review. Ecol. Eng. 37(1), 70e89.