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Ecological Engineering 68 (2014) 143–154

Contents lists available at ScienceDirect

Ecological Engineering

jou rn al hom ep age: www.elsev ier .com/ locate /eco leng

creening of Ca- and Fe-rich materials for their applicability ashosphate-retaining filters

leksandar Klimeski ∗, Risto Uusitalo, Eila TurtolaTT Agrifood Research Finland, E-building, Soils and Environment, 31600 Jokioinen, Finland

r t i c l e i n f o

rticle history:eceived 16 August 2013eceived in revised form 29 January 2014ccepted 29 March 2014

eywords:low-through testshosphorus retention media

retention mechanismeathering

hosphorus release

a b s t r a c t

Ecology of a large number of lakes and estuaries is affected by external phosphorus (P) loading. Many stud-ies have investigated the applicability of different materials as P-filters for treating runoff or wastewaters.We performed flow-through retention tests and water extractions to estimate the P retention and releaseproperties of potential Ca- and Fe-rich filter materials. In addition to the fresh Ca-rich materials, the Pretention tests involved their weathered counterparts. During weathering, the materials were immersedinto water to leach out soluble species (e.g., Ca2+ and OH−), thus mimicking aged filters. For the mostefficient P-retainers (Filtra P®, steel slag, Sachtofer PR® and mine drainage residual), the cumulative Pretentions ranged between 12 and 24 mg P/g. The weathering reduced the P retentions of Filtra P® andof slag by 70–80%, but did not affect the abilities of other materials to retain P. No clear relationshipexisted between the P retentions and water-extractable Ca or oxalate-extractable Fe and Al amounts in

the materials. During one month of extraction with a large volume of water, the maximum release ofP from the P-saturated materials was 35% of the total P. To recognize the P retention mechanism (pre-cipitation of Ca-phosphates or sorption onto metal hydroxides) and thus predict possible P release, weperformed equilibration of P-saturated materials in solutions with variable pH. We suggest incorporationof weathering and P release tests in the screening scheme for identifying potential P-retaining filters.

© 2014 Elsevier B.V. All rights reserved.

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. Introduction

Phosphorus (P) fuels primary production in many watercourses,hus deteriorating their ecological status (Schindler, 1977; Bairdnd Cann, 2005). Wastewater treatment plants in many countriesfficiently remove P originating from point sources, through chem-cal precipitation, to prevent eutrophication of surface waters.onsequently, non-point P sources (e.g., agricultural fields, animal

eedlots) have become significant contributors to P loading in bod-es of water (Penn and Bryant, 2006). For example, 45% of the total Pnput into the Baltic Sea in 2006 originated from diffuse sources thatave proved problematic and difficult to control (HELCOM, 2011).

possible solution for mitigating P inputs from localized sourcesight be the application of solid P-retaining materials (Penn et al.,

007).Removal of the entirely bioavailable P, the orthophosphate, from

astewaters is considered the most beneficial treatment method

∗ Corresponding author. Tel.: +358 443386462.E-mail addresses: aleksandar.klimeski@mtt.fi (A. Klimeski), risto.uusitalo@mtt.fi

R. Uusitalo), eila.turtola@mtt.fi (E. Turtola).

tdfeotmm

ttp://dx.doi.org/10.1016/j.ecoleng.2014.03.054925-8574/© 2014 Elsevier B.V. All rights reserved.

Ekholm and Krogerus, 1998). Solid materials capable of retainingrthophosphate usually contain significant amounts of amorphousetal hydroxides or soluble alkaline earth metals and they retain

either by sorption to Fe/Al hydroxide surfaces or via precipi-ation as Ca/Mg-phosphates (Penn et al., 2007; Sigg and Stumm,981; Moore and Miller, 1994). Although rarely used at munici-al wastewater treatment plants (see Shilton et al., 2006), some Porbing materials have been utilized in constructed wetland sys-ems (Dobbie et al., 2009) or as soil amendments (Penn and Bryant,006).

Prior to larger scale application, the first step in material char-cterization involves bench-scale experiments. In a recent review,e discussed the P retention and release mechanisms as well as the

dvantages and limitations of laboratory batch and flow-throughests as common ways for estimating the P retention potential ofifferent materials (Klimeski et al., 2012). These types of tests dif-er in nature and thus frequently provide unequal results (Ádámt al., 2007; Penn et al., 2011). Batch tests enable the accumulation

f dissolved species, whereas flow-through set-ups are open sys-ems and more realistically resemble field-scale applications and

ay thus provide closer estimates for the P retention capacity of aaterial in field conditions (Drizo et al., 2002). Rarely, laboratory

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ests assess the susceptibility of P-saturated materials to changes inolution chemistry (pH, P and Ca concentrations or redox potential),hat frequently occur in field set-ups.

The aim of this study was to improve laboratory protocols forstimating the P retention potentials of solid materials. Our pro-ocol involves laboratory flow-through tests with a set of Ca- ande-rich materials. The tests with Ca-rich materials also includedheir weathered counterparts obtained after immersion into watero remove the most soluble compounds. The weathering mimicseld conditions where pools of Ca2+ and OH−, the ions capablef controlling dissolved P concentration in water solutions, mayapidly decline (e.g., during spring snowmelt or storm events). Inhe retention tests, the applied P concentration of 50 mg/l is hardlyncountered when treating agricultural runoff, but the high con-entration served to estimate their ability of retaining substantialmounts of P. In addition, we performed pH-manipulation experi-ents to understand the P retention mechanisms and investigated

he ability of the materials to hold previously retained P by per-orming desorption/solubilization tests.

. Materials and methods

Our study included one natural material (a sand sample fromhe Bs horizon of a forest soil), three by-products of industrialnd mine activities (steel slag, mine drainage residual and Kemiraiotite), and three commercial products (Sachtofer PR®, Filtra P®,nd Filtralite P®).

The sand served as a complementary material, possibly facili-ating the recognition of the P retention and release mechanisms.iltra P® is a granular product, consisting of lime and Fe-rich gyp-um, manufactured (currently unavailable) by Ab Nordkalk OyParainen, Finland). Steel slag is an industrial by-product obtainedrom Rautaruukki Oy (Raahe, Finland). During iron ore processing,

olten slag and Fe form after the addition of alkaline carbonateslimestone, dolomite), and oxides of Ca, Mg, Al and Si are usually the

ajor constituents of such slags. Filtralite P® (here referred to asiltralite) is a light-weight aggregate obtained from Saint-Gobaineber (Oslo, Norway). Sachtofer PR® (referred to as Sachtofer)

riginated from a factory of Sachtleben Pigments Oy (Pori, Finland)hich produces TiO2. The granular material is produced by mixing

he by-product of the process, Fe2(SO4)3, with CaO and water. Theyhäsalmi mine (Pyhäjärvi, Finland) of Inmet Mining CorporationOntario, Canada), which processes ores rich in Cu and Zn, suppliedhe acidic mine drainage residual (MDR). The Fe-rich residual formshen mine drainage water comes into contact with air, whereby

e(II) oxidizes to Fe(III) and subsequently precipitates as ferricydroxides. Kemira biotite originates from the Siilinjärvi complex

n Eastern Finland (mined by Yara Suomi Oy, Helsinki, Finland)nd results from the processing of phlogopite-rich mine tailingsbtained during the beneficiation of apatite ore.

.1. Material weathering

In field conditions, after establishing a P-retaining barrier,he amounts of soluble species delivered by the reactive mate-ial decline due to continuous contact with water. For example,nowmelt or stormwater runoff may rapidly dilute the pore waterithin a P-filter and thus consume the stock of soluble elements.

herefore, in addition to the tests with fresh Ca-rich materials, westimated the P retention capacities of their weathered counter-

arts.

To weather the Ca-rich materials (Filtra P, slag, Filtralite,achtofer) prior to the retention tests, 2–4 subsamples wereeighed into separate nylon bags (250-�m openings) and then

m�a8

eering 68 (2014) 143–154

laced in buckets filled with tap water (0.005 mg P/l, 19 mg Ca/l and.3 mg Mg/l) to allow the dissolution of CaO, Ca(OH)2 and/or CaSO4.he water was changed daily and electrical conductivity (EC) waseasured before and after the change. The first batches were

emoved for drying when we registered a considerable decreasen EC of the water in the buckets (for example, from 2500 downo 1500 �S/cm). The remaining subsamples were further subjectedo leaching until we registered only a slight increase in the EC ofhe tap water after an additional water change. At that point, theasily soluble species were considered to have dissolved and, afterrying, we recorded the mass losses from the materials.

Depending on the weathering time the tests with the slag, Filtra and Filtralite included fresh (W0) and two weathered materi-ls (W1 and W2), whereas for Sachtofer we differentiated fourypes of weathered batches (W1–W4); the higher number signifiesreater weathering time and mass loss (Table 1). The higher num-er of weathered Sachtofer subsamples is associated with both theignificant loss of mass and a gradual decline in EC.

.2. Chemical characterization

To determine the pH and EC of the materials, material-wateruspensions (1:2.5, v/v) were first mixed and left to stand for about8 h. Then, the EC of the supernatants and, after stirring, the pH ofhe suspensions were measured with a pH/EC meter (HI 98129 –anna Instruments, Ann Arbor, MI, USA).

The amounts of Fe, Al, Ca, Cd, Cr, Cu, Ni, Pb and Zn, soluble inqua regia, in the fresh and weathered materials were determinedccording to ISO 11466:1995(E). Two grams of dried samples werextracted with a mixture of concentrated HCl and HNO3 for 16 ht room temperature followed by boiling for 2 h on a hot plate.fterwards, the extracts were analyzed for the above mentionedlements with an inductively coupled plasma-atomic emissionpectrometer (ICP-AES – Thermo Jarrel Ash, Franklin, MA, USA).or Hg determination, SnCl2 was added to the aqua regia digests toeduce the Hg to its elemental form. Vaporized Hg was then ana-yzed with a Varian M-6000A (CETAC, Omaha, NE, USA) mercurynalyzer.

The water-soluble Ca, Caw and Mg contents were estimated in a-h extraction with deionized water (material-to-solution ratio of:200) on an end-over-end shaker, followed by filtration through

filter paper (Grade 589/2: 4–12 �m – Whatman, Maidstone, UK)nd analysis with an atomic absorption spectrometer (AAS 4000

PerkinElmer, Waltham, MA, USA). To determine the amounts ofmorphous Fe and Al hydroxides, 0.5 g of a ground material wasxtracted in darkness with 25 ml of ammonium oxalate-oxalic acid,NH4)2C2O4/H2C2O4, buffered at pH 3 (Schwertmann, 1964). Thextracts were analyzed for Fe and Al with the ICP-AES.

.3. Phosphate retention tests

Phosphate retention tests were performed with a SampleTechacuum extractor (Science Hill, KY, USA). Each P application lastedor 30 min so that 50 ml of solution with a concentration of 50 mg P/lassed through the materials. The relatively high P concentrationerved to complete the experiments within a reasonable period ofime. In the tests with Sachtofer, Filtralite and MDR, the sampleeight was 6 g. Since the bulk densities of the slag, Filtra P and

iotite were higher, the sample size was increased to 12 g to obtainimilar bed volumes in all of the columns. The P retention testsncluded six replicates of Sachtofer and four replicates of the other

aterials. All effluents from the columns were passed through 0.2-m Nuclepore membranes (Whatman, Maidstone, UK) and thennalyzed for dissolved molybdate-reactive P (DRP) with a Lachat000 flow injection analyzer (Lachat Instruments, Wisconsin, MI,

A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154 145

Table 1Physical and chemical properties, mass losses during weathering, as well as total, water or oxalate-extractable amounts of Ca, Mg, Fe and Al in the fresh (W0) and weatheredmaterials (W1–W4).

Particle size (mm) Mass loss (%) pH EC (�S/cm) Ca (mg/g) Mg (mg/g) Fe (mg/g) Al (mg/g)

Catot Caw Mgw Fetot Feox Altot Alox

Filtra P W0 4–9 NA 11.8 640 232.8 6.9 0.01 42.8 1.7 3.9 0.18W1 4–9 5.5 11.4 390 233.4 9.7 0.03 45.1 2.9 4.2 0.32W2 4–9 4.2 11.3 370 226.2 12.2 0.02 44.9 3.3 4 0.35

Slag W0 0–6 NA 11.6 7800 298.9 26.8 0.02 147.8 12.4 7.3 0.26W1 0–6 2.3 9.4 130 275.1 1.4 0.03 142.9 6.2 9.5 0.19W2 0–6 1.7 9.6 140 271.8 1.8 0.04 176.8 4.6 8.7 0.13

Filtralite W0 0.5–4 NA 9.4 270 25.2 1.1 0.51 7.5 0.7 7.2 0.5W1 0.5–4 6.2 9.4 160 23.4 1.2 0.46 7.6 0.9 7.6 0.74W2 0.5–4 3.9 9.3 160 20.7 1.2 0.30 7.5 0.8 7 0.75

Sachtofer W0 2–5 NA 8.3 2470 154.4 72.9 0.65 93 2.4 2.9 0.12W1 2–5 21.1 8.7 1190 151.4 51.5 1.19 113.8 23.5 3.8 0.89W2 2–5 37.1 8.4 990 143.9 35.7 1.82 118.5 65.9 3.9 2.67W3 2–5 54.1 8.3 850 119.3 18.8 2.39 175 83.8 5.4 4.53W4 2–5 63.5 8.3 620 100.4 3.3 1.89 229.3 79.5 7 5.37

MDR W0 0–4 NA 2.5 3230 0.5 0.4 0.27 487.7 130.3 0.9 0.46Sand W0 0–1 NA 5.9 60 3 0.1 0.01 21.5 7.9 18.6 12.3

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arware to determine the saturation indices (SI) for selected solids. TheSI value may indicate the formation of precipitates in the columns(see supplemental information).

Fig. 1. Phosphorus retention by sand, exhibiting a two-phase association. The sorp-

Biotite W0 0–1 NA 8.0 14

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SA). In addition to monitoring DRP, pH and EC of the effluents afterach application, Ca and sulphur (S) concentrations in the filtered0.2 �m) effluents were determined with an ICP-AES after aboutvery tenth application.

The amount of P retained during a single application was calcu-ated as follows:

retention (mg/g) = Vs × Cin − Cout

m(1)

here Vs = solution volume (0.05 l); Cin and Cout = P concentrationsn the influent and effluent, respectively (mg/l); m = initial material

ass (6 or 12 g).The cumulative P retention for each material was calculated

y summing up the amounts of retained P during each applica-ion. In addition, after finishing the flow-through tests, the total

contents in two replicate samples of each material were deter-ined by extracting 0.25 or 0.5 g (depending on the available mass)ith 5 ml H2SO4, 3 ml H2O2 and 1 ml HF, followed by heating on aot plate (at about 150 ◦C), as described by Bowman (1988).

.3.1. Estimation of P retention maximaThe maximum P retention capacities (Smax) were assessed from

retention curves by fitting the data to exponential associationodels with the GraphPad Prism 4.03 software Motulsky (2005)

GraphPad Inc., La Jolla, CA, USA). For most of the materials, ane-phase association seemed to fit well, but three materials (Fil-ralite, MDR and sand) apparently exhibited different behavior. Forxample, Fig. 1 presents the P retention of sand, and the curveepicts phosphate accumulation on two planes or different reten-ion affinities. The binding strength of phosphate was assumed toiffer between the two planes, with a higher bonding strength onhe near-surface plane. The equations for fitting the data were asollows:

ne-phase association : Y = Smax × (1 − exp(−k × X)) (2)

wo-phase association : Y = Smax 1 × (1 − exp(−k1 × X)) + Smax 2

× (1 − exp(−k2 × X)) (3)

where Y = cumulative P retention (mg/g); X = cumulative P inputn relation to the initial material mass introduced into the columns

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mg/g); Smax = estimates of the P retention maxima (mg/g); k, k1 and2 = constants which describe the kinetics of P retention (g/mg).

The Smax1 rather than the Smax2 values of the two-phase asso-iation model probably provide more realistic estimates of theaximum sorption capacities, and the former estimates later

erved to calculate the P saturation degrees of the materials. Evenhough the Smax values have little practical significance since theyere estimated from the P retention curves, based on high P con-

entrations in the feed solution, they allow comparison betweenhe materials.

.3.2. Saturation indices for different solids in effluents froma-rich materials

To evaluate whether some effluents from the Ca-rich materi-ls were undersaturated, in equilibrium or supersaturated withespect to Ca-phosphates and -carbonates, we used MINEQL+ soft-

ion curve presents the accumulation of phosphate ions on the particle surface inwo separate layers. The dotted line, indicated by Smax1, shows the end-point esti-

ate for single-layer P retention on the near-surface plane, and the dashed linehows the end-point when additional phosphate ions accumulate in the seconddsorption layer. The triangle markers indicate measured values.

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.4. Phosphate release tests

.4.1. Impact of pH on P releaseAmbient pH was manipulated to identify the P retention mecha-

isms for the fresh and most weathered materials. We added 40 mlf deionized water into test tubes containing 0.4 g of a P-saturatedaterial (material-to-solution ratio of 1:100, three replicates). The

H of the suspensions was then lowered or raised by adding 1 MCl (0–1000 �l) or 1 M NaOH (0–180 �l). The suspensions werequilibrated on a shaker at 70 rpm for a period of 18 h, after whichhe supernatants were filtered (0.2 �m) and analyzed for DRP andH.

The amounts of P released during each extraction were calcu-ated with the following formula:

release (mg/g) = Vs × Cs

m(4)

here Vs = solution volume (0.04 l); Cs = measured P concentrationn the supernatant (mg/l); m = material mass (0.4 g).

.4.2. Water extractionTo assess the P release potential, P-saturated materials (fresh

nd the most weathered) were immersed in a large volume of tapater for an extended period of time. For this purpose, 2 g of eachaterial (1.5 g of MDR because no sufficient amount was available)ere enclosed in nylon bags (250-�m openings) and then placed

nto buckets containing 5 l of tap water (material-to-solution ratio:2500 or 1:3333 for the MDR). No replicate samples were used

n the extraction procedure. The water was changed and sampledvery two or three days, and analyzed for DRP. The experimentasted for one month, and the water was changed a total of 11imes. We calculated the cumulative P release from each mate-ial by adding up the amounts of P released in each bucket (the Pontent in the tap water was subtracted). After the extraction, theotal P content in the materials was also determined according toowman (1988).

.5. Statistical analysis

GraphPad Prism 4.03 and Excel were used to calculate the meanf the replicates and the standard error of the mean (SEM) forhe cumulative and estimated P retention parameters. Addition-lly, the goodness of the fits and p-values of the linear regressionodels describing the correlations between the Caw vs. cumulative

retention or Smax, and between Fe + Alox vs. cumulative Pretention ormax were determined.

. Results

.1. Physico-chemical properties of the materials

The physical and chemical properties of the materials differedonsiderably from each other (Table 1). The sand and biotiteonsisted of fine, granular and platy particles, respectively, whileachtofer and Filtra P consisted of coarser spheres. MDR produced atrongly acidic solution, whereas the slag and Filtra P strongly alka-ine ones. The weathering reduced the pH of only the fresh Filtra Pnd the slag, by one-half and two units, respectively.

The EC of fresh materials varied from 60 �S/cm for the sand to800 �S/cm for the slag, and for all weathered materials, the val-es were lower than for their counterparts. The most substantial

ecrease in EC (to less than 2% of the initial value) as a result ofhe weathering, was observed for the slag. Of the most weathered

aterials, Sachtofer W4 possessed the highest EC (620 �S/cm). Dueo the leaching of soluble species, Sachtofer lost a considerable part

cPab

eering 68 (2014) 143–154

f its initial mass. This was associated with a high loss of water-xtractable Ca, while the actual Fe mass remained unchanged,eading to a strong enrichment of the relative Fe contents in the

aterial (Table 1). The mass losses from the other materials wereonsiderably smaller, resulting in a slight alteration of the chemicalomposition.

The slag contained the highest total concentration of Ca298 mg/g), the dominant element in all the materials except MDRnd sand. Fresh and Sachtofer W3 contained the largest amountsf water-extractable Ca and Mg (72.9 mg/g and 2.4 mg/g, respec-ively); the slag and Filtra P were also rich in soluble Ca (Table 1).

The total Fe content in the MDR was 487.7 mg/g, whereashe Filtralite contained only 7.5 mg/g. As for weathered Sachtofernd MDR, significant Fe amounts were extracted with the oxalateuffer, ranging from 20 to 55% of the total Fe contents. Filtralite,and and biotite contained comparable total Fe and Al concentra-ions, whereas in the other materials the content of total Al was

inor to that of Fe.Because Finland, or other EU countries, have no guideline values

or heavy metal concentrations in P-filters, we compared the valuesith the maximum permissible concentrations in soil amendments

n Finland (supplemental Table 1). All metal concentrations wereelow the limits, except for Cr in Sachtofer and in the slag. How-ver, the Finnish legislation includes an alternative limit, less than

mg/kg, which applies to the water-soluble toxic form Cr(VI); theoncentrations of Cr in the weathered materials indicate that noeaching of Cr occurred, suggesting low Cr(VI) concentrations.

.2. Phosphate retention tests

Table 2 lists the number of P applications as well as the cumula-ive and estimated P retention capacities of the materials. Sachtofer,he slag, Filtra P and the MDR possessed high P retention poten-ials, whereas Filtralite and biotite possessed considerably lowernes (see Fig. 2). The number of P applications needed to completehe tests differed considerably because the materials showed vari-ble P retention capacities. Since the sample weight was either 6 or2 g, the daily loading rates for the columns were either 0.025 l/gnd 0.0125 l/g, respectively.

.2.1. Filtra PAmong all materials used in this study, the cumulative P reten-

ion by fresh Filtra P was the highest (24.6 mg P/g). The weatheringesulted in negligible loss of mass from the fresh material, but didignificantly lower its P retention during the test (Fig. 2). At the end,he cumulative P retentions of the weathered Filtra P reached 11.4nd 4.9 mg/g for W1 and W2, respectively.

Due to the time limit, we terminated the test at a point wheniltra P still removed about 40% of the phosphates and the satura-ion curves showed a significant retention that correlated linearlyith the amount of added P. The Smax estimates are therefore highlyncertain and no value could be estimated for W2 (see Table 2).

During the retention experiments white precipitates formedn all of the Filtra P columns, resulting in significant occasionalecreases in the water flow. To re-establish the flow, the columnsere flushed with water and the filters installed below the materi-

ls were replaced. The extraction procedure for total P, performedfter the retention tests, involved granular material only (exclud-ng the precipitates) and provided lower values than the calculated

retentions (see Table 2).Fig. 3 shows the discrete removal of P, as well as pH and the Ca

oncentrations in the effluents in relation to the amount of added. At a pH above 11, the fresh Filtra P removed virtually all of the P,nd at the same time the Ca concentrations in the effluents variedetween 30 and 290 mg/l. At a highly alkaline pH the substantial

A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154 147

Table 2Number of P applications, mean cumulative and estimated P retentions; and other selected parameters for the fresh (W0) and weathered materials (W1–W4). The numbersin parentheses represent the standard errors of the means (SEM).

Sampleweight (g)

P app. Added P(mg/g)

Cumul.retained P(mg/g)

Total P,Bowman(mg/g)

Estim. P retent.,Smax (mg/g)

Retent.constant, k(g/mg P)

P retent. in thetotal P input (%)

P retent., lastapplication (%)

Filtra P W0 12 159 33.1 24.6 (0.39) 3.9 (0.28) 53.2 (0.8) 0.019 (0.0004) 74.2 (1.19) 40 (2.35)W1 12 74 15.4 4.9 (0.12) 2.9 (0.1) 69 (33.24) 0.005 (0.003) 32.4 (0.79) 24.4 (1.17)W2 12 121 25.2 11.7 (1.32) 4.6 (0.16) NA NA 46.6 (5.23) 43.8 (5.56)

Slag W0 12 134 27.9 16.2 (1.41) 9.6 (0.24) 19.8 (0.43) 0.06 (0.002) 57.9 (5.05) 10 (2.97)W1 12 74 15.4 6.5 (0.39) 5.6 (2) 9.9 (0.41) 0.07 (0.005) 42.2 (2.56) 16 (2.04)W2 12 74 15.4 4.9 (0.33) 3.8 (0.23) 6.1 (0.18) 0.12 (0.007) 32 (2.1) 8.5 (0.46)

Filtralite W0 6 44 18.3 1.1 (0.07) 1.6 (0.04) 1.1 (0.03) 0.18 (0.014) 6.3 (0.8) 1.4 (0.24)W1 6 44 18.3 1.4 (0.17) 1.9 (0.12) 1.6 (0.1) 0.1 (0.012) 7.6 (0.96) 3.4 (1)W2 6 44 18.3 1.3 (0.05) 1.9 (0.05) 1.6 (0.04) 0.09 (0.004) 7.1 (0.28) 3.5 (0.5)

Sachtofer W0 6 40 16.5 6.8 (0.16) 7.1 (0.12) 9.9 (0.2) 0.07 (0.002) 41.3 (0.95) 25 (2.94)W1 6 92 38.3 19 (0.33) 34.6 (0.04) 55.2 (2.2) 0.01 (0.0005) 49.7 (0.86) 38.5 (0.43)W2 6 92 38.3 20.7 (0.19) 31.5 (0.02) 53.9 (1.04) 0.01 (0.0003) 54 (0.51) 30.9 (0.31)W3 6 92 38.3 19.4 (0.47) 20.6 (0.09) 31 (0.57) 0.03 (0.0007) 50.6 (1.24) 24.3 (1.19)W4 6 92 38.3 15.2 (0.12) 11.1 (0.02) 20.8 (0.13) 0.03 (0.0003) 39.6 (0.32) 21.7 (0.11)

MDR W0 6 90 37.4 12.4 (0.28) 14 (0.02) 18.6 (0.7) 0.03 (0.002) 33.2 (1.95) 6.9 (0.37)Sand W0 12 13 2.7 1.3 (0.005) 2.8 (0.05) 1.39 (0.03) 0.82 (0.03) 48.8 (0.18) 11.2 (0.29)

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mounts of soluble species increased the EC up to 2400 �S/cm.he weathered Filtra P (W2) initially removed considerably less

than did the fresh material. However, with further P addition,he efficiency of the fresh material decreased whereas the weath-red material sustained its initial efficiency, and during the finalpplications, all Filtra P materials removed equal amounts of theiscrete P additions. The weathered materials produced effluentsith a lower pH, and the Ca concentrations and EC ranged from 10

o 52 mg/l and 240 to 500 �S/cm, respectively.

.2.2. SlagThe fresh (W0) and weathered slag (W2) retained 16 mg P/g

nd 4.9 mg P/g, respectively (see Fig. 2). Although the weatheringtrongly affected the P retention potential of the slag, the losses ofass after the weathering were small, ranging from 1.7 to 2.3%.Throughout the experiment, similarly to Filtra P, white precip-

tates formed in the columns filled with fresh slag, clogging thelters below the materials and impairing the water flow. The filtersad to be replaced in these columns, whereas no filter replacementas necessary for the columns with the weathered slag. As with Fil-

ra P, some difference existed between the cumulative retentionsnd the total amounts of P determined after the retention tests,otably for the fresh slag (Table 2). This was associated with theemoval of precipitates while replacing the filters.

During the experiment, the saturation curves began leveling off;t the end of the experiment, all slag materials reached almost com-lete saturation, with a discrete removal of less than 10% of the P

n the influent. The modeled Smax values were slightly higher thanhe calculated cumulative P retentions, ranging from 6.1 vs. 4.9 mg/g for W2 to 19.8 vs. 16.2 mg P/g for the fresh slag.

The fresh slag removed all P as long as the pH remained above1.5; at that pH the amounts of Ca in the effluents varied between0 and 600 mg/l (Fig. 3). Initially, the fresh slag supplied significantmounts of soluble compounds to the solutions, increasing the ECver 4000 �S/cm (above the upper detection limit of the instru-ent; the samples were not diluted). The EC gradually decreased

ver time, finally landing at 200 �S/cm; at the same time, the dis-rete P removal was only 8%.

The weathered slag initially reduced the P concentration byp to 65%, while the EC remained relatively low (between 200

dv

u

) 0.016 (0.003) 2.2 (1.06) 0.8 (0.26) 0

nd 270 �S/cm). During the last application, however, the mate-ial removed only 4% of the phosphate introduced into the column.or the slag W2, the maximum concentration of Ca in the effluentas 17 mg/l.

.2.3. Filtralite PThe weathering did not affect the P retention potential of Fil-

ralite and both the fresh and the weathered materials retainedow P amounts, varying between 1.1 and 1.3 mg P/g. The materialsemoved up to 7.6% of the total introduced P masses, ultimatelyeaching complete saturation.

Because the Ca content in Filtralite was considerably lower thann Filtra P and slag, a different type of relationship existed betweenhe Ca concentrations in the effluents and the corresponding Pemoval efficiencies (Fig. 3). At pH 10.4, the fresh Filtralite exhibited

discrete P removal of 50%; at the same time, the Ca concentra-ion in the effluent was low (0.8 mg/l). In the later stages of theest the concentrations of Ca and P increased in the effluent as theH decreased, accompanied by low retention. At a neutral pH, theaterial achieved complete saturation. The weathered materials

emoved 40% of the phosphates during the first application, butfter a few P applications, the discrete removal decreased to 15%.he EC of the effluents from the fresh and weathered Filtralite wereimilar, varying between 250 and 400 �S/cm.

.2.4. Sachtofer PRThe test with fresh Sachtofer indicated P retention of 6.8 mg/g,

nd the saturation curve began leveling off when the mass ofntroduced P into the column was 16.5 mg/g. The weathered mate-ials exhibited cumulative P retentions between 15 and 20 mg/gFig. 2). By the end of the test, Sachtofer W1 and W4 removed0–50% of the total P input, respectively and the granules had noteached complete saturation, retaining 20–40% of the P in the feed.

By visual examination, the granules remained intact after theeathering even though the loss of mass was as high as 64%. How-

ver, due to the leaching of CaSO4, the density of the material

ecreased, and the same mass (6 g) of material occupied a largerolume in the case of W4.

The total P analyses of Sachtofer W1 and W2 yielded higher val-es than the respective cumulative retentions. This probably stems

148 A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154

F nes inB throuf ed da

ftecittAu0

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ig. 2. Retention of P by Filtra P, slag, Filtralite, Sachtofer, MDR and biotite. Solid lioth P additions and retentions relate to the material mass introduced into the flow-

or). The R2 values refer to the agreement between the fitted values and the measur

rom the losses of mass that these materials also exhibited duringhe retention tests (significant amounts of Ca were present in theffluents), while mostly retaining the introduced masses of P in theolumns. Since W3 and W4 were obtained after longer weather-ng, resulting in smaller losses of mass during the retention tests,he amounts of P extracted from them after finishing the reten-ion test closely matched the cumulative retentions (see Table 2).s assessed by the total P and Fe analyses, W1 seemed more sat-rated than W4, exhibiting a two-fold P/Fe molar ratio (0.32 vs..16).

The saturation curves for W1 and W2 clearly show that theaterials still had the potential to remove additional P (Fig. 2);

ccording to the model, the Smax values were about 2.5–fold thealculated cumulative P retentions. As for W3 and W4, the satura-ion curves began leveling off, and the estimated Smax values closely

atched the calculated retentions.All Sachtofer materials, except W4, initially provided the solu-

ion with significant amounts of Ca (160–320 mg/l), but at the endf the test, we recorded values lower than 50 mg/l (Fig. 3). Initially,epending on the weathering grade, the EC ranged between 800nd 2500 �S/cm and decreased to less than 500 �S/cm by the end of

ts

d

dicate fresh materials, while dashed and dotted lines signify weathered materials.gh columns (i.e., possible mass losses during the material weathering not accountedta.

he experiment. Throughout the test, the most weathered materialW4) produced effluents containing less than 20 mg Ca/l, possess-ng a neutral pH and one unit higher than that of the W1 effluents,robably due to the greater supply of Ca2+ ions from the latteraterial. This resulted in reduced buffering capacity (likely due

o removal of carbonates) through precipitation as Ca-carbonatesnd/or the formation of soluble complexes. Regardless of the dif-erences in the pH and Ca concentrations of the effluents, the freshnd weathered materials efficiently retained P.

.2.5. Mine drainage residualThe MDR demonstrated a high cumulative retention of

2.4 mg/g, retaining 33.2% of the total mass of P introduced into theolumn (Fig. 2). Since the material did not reach complete satura-ion, the modeled Smax (18.6 mg/g) was higher than the calculatedumulative retention. However, the total amount of P extractedrom the material after the test was 12.6 mg/g. Calculated from the

otal extracted amounts of P and Fe, the P/Fe molar ratio in the Paturated material was 0.046.

The discrete P removal by MDR, initially 85%, graduallyecreased to 7% by the end of the experiment. The pH of the

A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154 149

Fig. 3. Discrete removal of P, pH and Ca concentration in the effluents in relation to the added P mass for fresh (W0) and weathered Filtra P, slag and Filtralite (W2) andSachtofer (W1 and W4).

150 A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154

on to

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3

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3

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3

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Fig. 4. Discrete removal of P and pH in relati

ffluents increased from 2.2 to 4.8 with the removal of acidity (seeig. 4) due to introduction of the P solution which had a pH ofbout 5.5.

Although high EC (2300 �S/cm) characterized the first effluent,C ultimately decreased to 400 �S/cm. The concentrations of S, Cand Fe in the first effluent were 260 mg/l, 30 mg/l and 60 mg/l,espectively. After the fifth P application the effluents were freef Ca and Fe and contained only small amounts of S.

.2.6. SandDuring the first three P applications, the discrete removal of P

aried between 84 and 96% (Fig. 1). Over the course of the test, theand retained around 50% of the total P input, achieving a cumu-ative P retention of 1.32 mg/g which was in good accordance withhe modeled Smax. According to the analyses of total P, the sandriginally contained 1.4 mg P/g; adding the cumulative retentionields a total P sorption of 2.7 mg/g. At the saturation point, the/Fe molar ratio was 0.23.

During the test, we noticed an increase in the pH of the efflu-nts from 4.8 to 6.3, while the EC remained low and stable, varyingetween 150 and 180 �S/cm. The effluents collected from the sand-ontaining columns were free of Ca and initially contained onlymall amounts of S (up to 11 mg/l).

.2.7. BiotiteFor this material, we calculated a negligible cumulative P reten-

ion of 0.013 mg P/g (Fig. 2). The material already reached totalaturation after the first P application, and the influent P concen-ration was reduced by only 1%. A neutral pH characterized theffluents (6.9–7.1). The biotite provided the solution with moder-te amounts of Ca; for example, the Ca concentration at the start ofhe test was 30 mg/l, which decreased to 20 mg/l in the last efflu-nt. Similarly, the EC decreased with time, ranging between 400nd 250 �S/cm.

.3. Phosphorus release tests

.3.1. Manipulation of pHIn this section, we present relative P release based on total

analyses. The materials’ buffering capacities varied; thus, the

Aoft

the introduced mass of P for MDR and sand.

ntroduced spikes of HCl or NaOH resulted in a different pH of thequilibrium solutions. In the pH range 5–9, the largest amount ofeleased P (about 30%) occurred for Filtralite at pH 5.5, whereasiltra P released only 1.7% of the total P at pH 6.2. Filtra P, the slag,iltralite and fresh Sachtofer maintained alkaline solutions in theest tubes when no acid was added to the deionized water. These

aterials also released more of the retained P with decreasing pH,hich corresponds to increased solubility of Ca-phosphates upon

cidification (Fig. 5).The contrasting P release from the fresh and Sachtofer W4

pon pH changes implies the existence of different P retentionechanisms. While the fresh Sachtofer released more P upon acid-

fication, Sachtofer W4, MDR and sand, liberated more P withncreasing pH. For these three materials the largest amounts of

were solubilized in highly alkaline solutions. This agrees withhe fact that the sorption of P to Fe-hydroxides reaches maximumevels in reasonably acidic environments, whereas alkaline condi-ions favor P desorption from metal hydroxides. The MDR releasedbout 21.4% of the total mass of P at pH 8.5, while the release fromachtofer W4 was only 2.4% at pH 8.9.

.3.2. Water extractionsDuring the extended immersion in tap water, the materi-

ls liberated variable amounts of P, with a difference of onerder of magnitude (0.1–1.6 mg/g, Table 3). The analyses of theotal P content in the materials was in good agreement withhe cumulative releases of P calculated by summing up themounts of P measured in water during the prolonged extrac-ion.

For the slag, Filtra P and Filtralite, the amounts of released ranged from 0.1 to 0.66 mg/g, and small differences existedetween the fresh and weathered materials. In relation to the totalmount of P present in the materials, Filtralite released about 35%,hereas the respective value for the slag and Filtra P was below 4%.

s for the Fe-rich materials, Sachtofer W4 liberated 1.2 mg/g (11%f the P initially present), while the relative share of P releasedrom the MDR was one half of that, representing about 4.8% of theotal P.

A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154 151

Fig. 5. Release of P from the saturated materials as affected by pH. Black and

Table 3Released P amounts during the water extraction from the saturated fresh (W0) andweathered (W2–W4) materials.

Total release(mg/g)

Relative to the total P amountin the material (%)

Filtra P W0 0.14 3.5Filtra P W2 0.18 3.8Slag W0 0.31 3.2Slag W2 0.1 2.7Filtralite W0 0.43 26.7Filtralite W2 0.67 35Sachtofer W0 1.6 24Sachtofer W4 1.24 11.2MDR W0 0.67 4.8

4

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Sand W0 0.24 8.6Biotite W0 0.11 11.7

. Discussion

.1. Phosphorus retention controlled by Ca-phosphaterecipitation

In a P retention study with blast furnace slag, Johansson andustafsson (2000) noted that while hydroxyapatite was the most

hermodynamically stable Ca-P association, initially less stable pre-ipitates, such as amorphous calcium phosphates or octacalciumhosphate, are formed. In our study, the positive mineral saturation

ndices (SI) for hydroxyapatite, aragonite and calcite in the pH range

cf

w

white markers indicate fresh and weathered materials, respectively.

rom 10.4 to 11.6 may suggest the precipitation of these solids (seeupplemental Table 2). The high SI of 6.2 and 9.4 for hydroxyapatitet an alkaline and neutral pH are somewhat uncertain and probablyesult from the simplicity of the systems considered by MINEQL+.upersaturated systems may be stable without causing the for-ation of solids until attaining a certain supersaturation limit

Valsami-Jones, 2004). Johansson and Gustafsson (2000) arguedhat at the supersaturation limit, the SI for hydroxyapatite equals.5 and is affected by pH as well as contact time and the initial pres-nce of Ca-phosphate seed crystals. An important process whichay have impeded the formation of Ca-phosphates in our systems

s the precipitation of Ca-carbonates that also occurs at pH 8–11Jenkins et al., 1971; Henze et al., 2002).

For fresh Filtra P, the slag and Sachtofer the precipitation of Ca- compounds was probably the main retention mechanism dueo the initial production of alkaline effluents containing substan-ial amounts of Ca. These materials buffered the solution pH wellbove 6.5 which is essential for enabling efficient precipitation ofa-phosphates (see Stoner et al., 2012). Moreover, a clear indicationf the presence of Ca-P associations in the saturated materials washeir behavior in the pH-manipulation test: release of more P withecreasing pH. Diaz et al. (1994) reported that in stream watersrom the Everglades region (FL, USA) containing dissolved P con-

entrations between 0.08 and 1.18 mg/l, Ca-phosphates evidentlyormed at Ca concentrations higher than 100 mg/l at pH 8.

Stoner et al. (2012) suggested that water-extractable Ca coupledith the ability of a material to buffer the solution pH above 6.5 may

152 A. Klimeski et al. / Ecological Engineering 68 (2014) 143–154

Fig. 6. (a) Relationships between the water-extractable Ca and calculated P retentions (figure on the left) or Smax estimates (figure on the right) for Filtra P (fresh = W0;weathered = W1, W2), the slag, Sachtofer W1 and Filtralite. The non-denoted points represent the weathered slag (P retention between 5 and 7 mg/g), fresh and two weatheredF ted foA the r

sIoSgrspi3fttwiCTr

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iltralites (P retention between 1.1 and 1.4 mg/g). No Smax estimate could be calculal, and the calculated P retention (figure on the left) or the Smax estimates (figure on

erve as indicators for the P retention potential of Ca-rich materials.n our study, no significant correlations exist between the amountsf water-extractable Ca and cumulative P retention capacities ormax values (Fig. 6a). Taking into account linear correlations, theoodness of the fits (R2) and p-values for Caw vs. cumulative Petention equaled 0.38 and 0.056, respectively. For Caw vs. Smax

till poorer fits were calculated, R2 = 0.26 and p = 0.16. For exam-le, a relatively low amount of water-extractable Ca was present

n Filtra P that, nevertheless, exhibited high P retention. The single-h extraction, however, could only partly predict the release of Carom this material, since Filtra P delivered additional Ca2+ ions tohe solution when the granules disintegrated. On the other hand,he delivery of Ca from the fresh slag steeply declined upon theeathering process (see Fig. 3). Ziemkiewicz (1998) described that

n steel slags, a soluble alumino-silicate glassy matrix envelopesaO, which initially enables free delivery of Ca2+ to water solutions.he high cumulative retention of P by the fresh slag therefore solelyesulted from the efficient retention in the early phases.

Our results concerning the P retention by the slag are compa-able to those obtained by Kostura et al. (2005) for amorphousnd crystalline steel slags. In contrast, Jourak et al. (2011), whoonducted column studies with Filtra P and applied the same Poncentration as in our tests, reported P retention of only 8 mg/g.he P retention presented by Jourak et al. (2011) is thus about one-hird of our result. This is probably partly due to the use of coarserranules (4–13 mm) than in our study (4–9 mm). Coarser parti-les commonly retain less P due to their smaller reactive surfacerea (see Cucarella and Renman, 2009), but such a high discrep-ncy between the results likely stems from reasons that remain

nclear.

The weathering of Filtralite only faintly affected its P removalapacity, suggesting the presence of an additional retention mech-nism besides precipitation. In favor of this, Ádám et al. (2006)

tC1d

r Filtra P W2. (b) Relationships between the content of oxalate-extractable Fe andight) for Sachtofer W3 and W4, MDR and sand.

tudied the performance of Filtralite, and after conducting sequen-ial extractions with P-saturated material the authors concludedhat, in addition to Ca-P precipitates, loosely bound P and Al-P werelso important pools. Ádám et al. (2007) reported a P retentionapacity of about 0.5 mg/g for Filtralite, one-third of our result. Theysed secondary wastewater containing one-tenth of the P amountsed in our study, which may well explain the difference.

Whereas the materials discussed above retained P mostly viarecipitation, Sachtofer seems to promote two separate retentionechanisms: precipitation of Ca-phosphates in the early phases ofeathering and sorption of P to Fe-hydroxides after the soluble Caas leached out from the material. This implies that the materialould be less sensitive to changes in the ambient Ca concentra-

ion and pH than the other Ca-rich materials. Bastin et al. (1999)eported a retention capacity of 14.4 mg P/g for a chemically simi-ar product prepared in a laboratory (gypsum-Fe oxide compound),

hich is twice the retention capacity of fresh Sachtofer but closelyatches the retentions by the weathered material.Our results may greatly overestimate the retention potentials of

he materials in field conditions, especially when treating watersith low concentrations of P. Firstly, the relatively high P concen-

ration in our study influenced the cumulative retention capacitiesf the materials such that greater masses of P were captured as pre-ipitates, due to the increased ionic products for Ca-phosphates.econdly, either when treating agricultural runoff or secondaryastewater from constructed wetlands, the presence of humic sub-

tances may decrease the precipitation rate of Ca-phosphates dueo the formation of soluble complexes with Ca2+ ions. In batchxperiments with organic-rich water (dissolved organic carbon up

o 20 mg/l), Song et al. (2006) observed that the consumption ofa increased in solutions with DOC concentrations higher than0 mg/l. Thirdly, the efficiency of field filters may be greatly reducedue to the blockage of reactive sites on particles’ surfaces as a result

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A. Klimeski et al. / Ecologica

f accumulation of suspended solids and microbial biomass (seeirkkala et al., 2012; Uusitalo et al., 2013).

.2. Phosphorus retention controlled by sorption toe/Al-hydroxides

During the P retention experiments, the MDR produced acidicffluents, whereas Sachtofer W4 and the sand produced neutralnes. These materials were rich in oxalate-extractable Fe and/orl and most likely retained P through sorption onto Fe- and/orl-hydroxides, as indicated by the pH-manipulation tests (Fig. 5).

n acidic environments, protonated surfaces easily attract phos-hates, but as the pH rises, phosphate species shift toward theore negatively charged ones, and at the same time deprotonation

f the metal hydroxide surfaces occurs, hence increasing repul-ion forces. The concentration of hydroxyl ions in solution alsoncreases, leading to increased competition for the anion exchangeites on the metal hydroxide surfaces (Sigg and Stumm, 1981; Bohnnd McNeal, 1983).

An increase in the concentration of P in a feed solution com-only affects the sorption of phosphates onto metal oxide surfaces

y increasing the ionic strength of the solution. This results inompression of the electrical double layer around the Fe/Al hydrox-de particles, leading to more efficient sorption (Yli-Halla andartikainen, 1996; Antelo et al., 2005). This most likely contributed

o the high P retentions by Sachtofer, MDR and the sand, with a dis-inctive two-phase sorption behavior (see Fig. 1). Such retentionsould be hardly achievable in field conditions, where significantly

ower amounts of P in the influent exist. In addition, similarly to thea-rich filters, the retention may be greatly reduced by the pres-nce of DOC, suspended solids and microbial biomass. In P retentionxperiments with granular ferric hydroxide treating wastewaterrom membrane bioreactors, Genz et al. (2004) reported that sig-ificant amounts of DOC were retained by the material.

Phosphorus retentions similar to those of the MDR and Sachtofer4 have been reported in studies employing flow-through set-

ps. For example, Chardon et al. (2012) obtained high P retention16 mg/g) by an Fe sludge originating from a groundwater aerationrocess (a material chemically similar to MDR), even though thepplied concentration of P was less than one-tenth the concentra-ion in our study.

The potential of a material to retain P through sorption ontoetal hydroxide surfaces may eventually be evaluated according

o its oxalate-extractable Fe and Al contents (Stoner et al., 2012).ig. 6b shows the P retentions by Sachtofer W3 and W4, MDR andand in relation to the sum of their molar concentrations of oxalate-xtractable Al and Fe. The concentration of oxalate-extractable Fend Al shows no significant correlation with the calculated cumu-ative P retention or with the Smax estimates. Considering linearegression models, the goodness of the fits (R2) and p-values fore + Alox vs. P retention equal 0.5 and 0.29, respectively, and fore + Alox vs. Smax they equal 0.49 and 0.29, respectively. Neitherid Makris et al. (2005), in a long-term study (80 days) of the Petention capacities of Fe- and Al-rich drinking water treatmentesiduals, find a significant correlation with the sums of molarxalate-extractable Fe and Al concentrations. In their materials,he sum of oxalate-extractable Fe and Al varied between 1.1 and.5 mmol/g.

In our experiments pH of the equilibrium solutions variedetween the materials, from highly acidic for the MDR to neutralnes for the sand and Sachtofer, which might have influenced the

retention. Additionally, the materials were crushed before thexalate extraction, thus more reactive sites were revealed than inhe P retention tests. In batch tests with industrial by-products,enn et al. (2011) noted that, for example, Ca-containing minerals

lswm

eering 68 (2014) 143–154 153

ay possibly coat amorphous Fe/Al hydroxides, resulting inmpeded P retention. Such findings suggest that a single corre-ation may not explicitly reveal the complexity of the retention

echanisms.

.3. Phosphorus release from the Ca- and Fe-rich materials

Calcium-phosphates dissolve when the equilibrium concen-rations of Ca2+, phosphate species and/or OH− ions in solutionecrease. The extended water extraction allowed the dissolution ofa-phosphates (and the release of P from other retention compo-ents) due to the wide material-to-solution ratio, and a significantelative release of P was registered for Filtralite. The relatively smalleleases of P from Filtra P and slag may stem from the fact that somef the precipitates were excluded from the water extraction as theyere discarded while changing the clogged filters in the retention

ests.In field conditions the release of P is likely to occur when

he solution pH within a Ca-rich filter decreases, for example,ue to reduced delivery of hydroxide ions from the material (inged filters) or during increased water flows. Thus, the stability ofa-phosphates within a filter would depend on the pH-bufferingapacity of the material. Diaz et al. (1994) noted that the extent ofhe dissolution of newly formed Ca-P precipitates ranged between0 and 90% as they decreased the pH of the ambient solution to 7.

Significant desorption of P from the Fe-rich materials did notccur during the extended water extraction (see Table 3). Similarly,akris et al. (2005) reported a small release of P (up to 8.3%) from

n Fe-rich drinking water treatment residual when using a 5 mMxalate solution as an extractant. Chardon et al. (2012) noted a Pelease of 37% from a P-saturated Fe-rich sludge after percolatinghe material with a P-free solution. However, in laboratory tests,he nature of the extractant as well as the experimental set-upstrongly influence the release of P. Sorption of P onto Fe/Al hydrox-de surfaces is an equilibrium reaction which is partly reversible

hen a phosphate group replaces a hydroxyl group on the metalydroxide surface (i.e., a monodentate complex). If the binding isssociated with the release of two hydroxyls groups (a bidentateomplex), the phosphate group becomes an integral part of theetal hydroxide particle, which strongly reduces the possibility of

uture release (Brady and Weil, 2008).Uusitalo et al. (2012) suggested that a molar P/Fe ratio of about

.1 may be close to the maximum saturation of Fe-rich materi-ls in field conditions. In that study, the P/Fe ratio in P-saturatedachtofer was 0.16, but decreased to 0.12 after immersing theaterial in an oligotrophic lake. However, we found no close rela-

ionships between the P/Fe ratio and the degrees of saturation ofhe materials (not shown). At a P saturation of 64%, for example,he P/Fe molar ratio in the MDR reached 0.05, while the respectivealue in the sand was 0.25 at the point of complete saturation. Dueo the negligible release of P during the extended water extraction,he P/Fe molar ratio in the former material remained unaffectednd declined to 0.22 in the latter one.

. Conclusions

Preserving the ecological status of sensitive bodies of water mayequire substantial reductions in the external P loading. In catch-ents with P-enriched soils, P-filters could be used to rapidly curb

concentrations in runoff. Potential P-filters must be first tested in

aboratory with appropriate protocols that take into account pos-ible changes in material characteristics and ambient conditionsith time. Weathering of fresh materials, extractions of P-saturatedaterials in solutions with variable pH and, especially if applying

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54 A. Klimeski et al. / Ecologica

igh P feed concentrations, extraction of P-saturated materials witharge volumes of water are useful ways for improving the laboratorycreening of potential P-retaining materials. The test scheme usedn this study allowed us to identify the P retention mechanisms,

hich for some materials may not be directly recognized accord-ng to their total elemental or water/oxalate extractable metalmounts. Filtra P, slag and Filtralite efficiently retained phosphatess long as they supplied Ca2+ and OH− ions, and the existence ofa-phosphate precipitates was further confirmed during the pH-anipulation experiments. In these experiments, the delivery of

from the P-saturated materials to the solutions increased withecreasing pH. Metal hydroxides controlled the P retention by theDR and the sand, as supported by the lower release of P from the

-saturated materials in acidic than in alkaline solutions. Sachtoferhowed interesting properties, with an initial supply of Ca2+ andH− ions, inducing Ca-phosphate formation, but as the pool of

hese species shrinked, the material efficiently retained P via sorp-ion onto Fe-hydroxide surfaces, behavior that was only revealedfter the weathering.

cknowledgments

We acknowledge Maj and Tor Nessling foundationMAAE/2011235), Maa- ja vesitekniikan tuki (27245) and theinnish Ministry of Agriculture and Forestry (2638/312/2009)or providing funding for this research. We also thank Helena

erkkiniemi and Mirva Ceder for their help during laboratoryork, and the anonymous reviewers for their comments. Finally,

ur thanks go to Stephen Stalter for reviewing the language.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.ecoleng.014.03.054.

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