Journal of Environmental Management 143 (2014) 26e33

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Phosphate sorption by three potential filter materials as assessed by isothermal titration calorimetry Gry Lyngsie a, *, Chad J. Penn b, Hans C.B. Hansen a, Ole K. Borggaard a a b

University of Copenhagen, Department of Plant and Environmental Sciences, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark Oklahoma State University, Department of Plant and Soil Sciences, 368 Agricultural Hall, Stillwater, OK 74078-6028, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 October 2013 Received in revised form 11 April 2014 Accepted 16 April 2014 Available online

Phosphorus eutrophication of lakes and streams, coming from drained farmlands, is a serious problem in areas with intensive agriculture. Installation of phosphate (P) sorbing filters at drain outlets may be a solution. The aim of this study was to improve the understanding of reactions involved in P sorption by three commercial P sorbing materials, i.e. Ca/Mg oxide-based Filtralite-P, Fe oxide-based CFH-12 and Limestone in two particle sizes (2e1 mm and 1e0.5 mm), by means of isothermal titration calorimetry (ITC), sorption isotherms, sequential extractions and SEM-EDS. The results indicate that P retention by CFH is due to surface complexation by rapid formation of strong FeeP bonds. In contrast, retention of P by Filtralite-P and Limestone strongly depends on pH and time and is interpreted due to formation of calcium phosphate precipitate(s). Consequently, CFH can unambiguously be recommended as P retention filter material in drain outlets, whereas the use of Filtralite-P and Limestone has certain (serious) limitations. Thus, Filtralite-P has high capacity to retain P but only at alkaline pH (pH
10) and P retention by Limestone requires long-time contact and a high ratio between sorbent and sorbate. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Calcium phosphates CFH-12 Fe-oxides Filtralite-P Limestone P sorbing reactions

1. Introduction Surface and subsurface transport of phosphate (P) from fertilized agricultural fields to open waters may lead to eutrophication, reduced biodiversity and fish kills in lakes and streams (Ballantine and Tanner, 2010; Delgado and Scalenghe, 2008). To mitigate eutrophic waters is difficult (Sharpley et al., 2003) but reduction of agricultural P leaching by means of filter structures intercepting drains and ditches with P sorbing materials may be one way to improve the water quality (Ballantine and Tanner, 2010; Penn et al., 2007). Several materials have been proposed as P sorbing materials (PSMs) (Ballantine and Tanner, 2010; Cucarella and Renman, 2009; Vohla et al., 2011; Westholm, 2006) for use in landscape P filters (Penn et al., 2007; Reinhardt et al., 2005). To handle the high water flow and relatively high P concentrations during peak flows found in farmland drains and ditches, PSMs must react fast and have a high affinity for P in order to effectively remove it. The P removal efficiency of most PSMs is typically related to pH and the presence of various Al, Ca and Fe compounds (Ballantine and Tanner, 2010; Cucarella and Renman, 2009; Vohla

* Corresponding author. E-mail addresses: [email protected], [email protected] (G. Lyngsie). http://dx.doi.org/10.1016/j.jenvman.2014.04.010 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.

et al., 2011; Westholm, 2006). Additionally, the particle size and hence specific surface area (SSA) of the sorbent particles increases reactivity and sorption capacity (Nair et al., 1984). Besides high P affinity and fast kinetics, PSM filters must also have good hydraulic conductivity to handle the high water flow seen in connection with rain-storms but at the same time also allow P-rich water to come into contact with the materials. Despite the abundance of literature on sorption to various PSMs such as Al and Fe oxides, limestone, shell-sand and various by-products from industry (Ballantine and Tanner, 2010; Cucarella and Renman, 2009; Vohla et al., 2011; Westholm, 2006), doubt still exists about detailed sorption reactions of these materials, and hence safe use and optimal management of these filters is not yet ensured. Sorption isotherms are commonly used in P sorption studies of PSMs (Ballantine and Tanner, 2010; Klimeski et al., 2012; Westholm, 2006). Although useful for assessment of P sorption capacity and affinity, isotherms are not suited for determination of the precise sorption reactions (Veith and Sposito, 1977). Isothermal titration calorimetry (ITC) provides a sensitive and direct quantitative measure of heat of a reaction and can be used as a complementary technique for establishing P sorption reactions. For instance, Penn and Zhang (2010) found that FeCl3 titrated with NaH2PO4 changed from exothermic to endothermic as the titration proceeded and interpreted this to a change of the P sorption

G. Lyngsie et al. / Journal of Environmental Management 143 (2014) 26e33

process from adsorption to precipitation. ITC can be used for estimation of enthalpy for mineralesolution interactions in connection with traditional sorption measurements as the reaction proceeds (Appel et al., 2013; Kabengi et al., 2006; Penn and Warren, 2009; Penn and Zhang, 2010; Rhue et al., 2002). Briefly, ITC measures changes in heat emitted (exotherm) or absorbed (endotherm) along product formation when a solute is added stepwise to a solution or solid suspension. For each solute addition (injection), the heat q (J s1) released or absorbed is given by Eq. (1):


 q ¼ V DH D np

(1)

Where D[np] (mol L1 s1) is the change in product concentration, DH is the enthalpy of the reaction (J mol1 product), and V (L) is the volume of reaction mixture. Because q is directly proportional to the increase in mass of product formed at each injection, its magnitude will gradually decrease as the reaction approaches saturation (equilibrium) of the system. The time integrated heat Q (J) released or absorbed is directly proportional to the energy of interaction:

Q ¼ V DH

X   D np

(2)

For a full introduction to this method see Steinberg (1981) and Freire et al. (1990). ITC is not a “stand alone” technique, as correct interpretation of the data requires additional knowledge of the system being studied (Penn and Zhang, 2010; Rhue et al., 2002). Furthermore, the calculated thermodynamic properties (q and Q) can only be meaningfully interpreted for pure systems with one (or very few) reactions that can be identified. In order to know which type of PSM to choose for landscape filters, it is central to understand the particular type of sorption reaction, e.g. whether it is a precipitation, surface complexation or ion exchange reaction. Thus, by means of ITC, sorption isotherms, sequential extractions and PSM characteristics, the objective of this study was to improve understanding of the reactions involved in P sorption by three potential commercial PSMs including CaCO3-based Limestone, Ca/Mg oxide-based Filtralite-P and Fe oxide-based CFH-12 at two different particle sizes. These three materials were chosen based on a prior screening (Lyngsie et al., 2014). 2. Materials and methods 2.1. Materials Limestone for this study consists of a mixture of bryozo and coral chalk from the Danian formation at Faxe in Denmark. The dried product was provided by Faxe Kalk A/S, Denmark. Filtralite-P produced by Weber, Norway is a Light Expanded Clay Aggregate (LECA)-resembling material calcinated at 1200  C consisting of granules of Ca/Mg oxides in a collapsed clay matrix. CFH-12 (CFH) produced by Kemira Oyj, Finland, consists of dried iron oxide. The particles of the three PSMs were separated by sieving into a 2e1 mm fraction and a 1e0.5 mm fraction. 2.2. Methods All chemicals were of pro analysis or better quality and the water was double deionized (DI). All glass and plastic wares were acid-washed prior to use. 2.2.1. PSM characteristics The mineralogy of the materials was assessed by powder X-ray diffraction analysis (XRD) on unoriented specimens using a

27

Siemens D5000 instrument equipped with Co-Ka radiation and a diffracted beam monochromator. Diffractograms were recorded from 5 to 80 2q using 0.02 2q steps and a step speed of 10 s. Diffraction peak positions were used to calculate d-values for mineral identification. pH of the PSMs was measured potentiometrically in 0.01 M CaCl2 solution using a material:solution ratio of 1:2.5. Total contents of 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) and were measured by inductive coupled plasma mass spectroscopy (ICP-MS) on an Agilent 7500C instrument. Oxalate-extractable iron (FeOx) was determined by extraction with 0.2 M ammonium oxalate for 2 h at pH 3 in the dark (Schwertmann, 1964) with quantification of Fe by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Spectro Ciros CCD instrument. Reactive Ca and Mg in Filtralite-P and Limestone were determined by titration of 0.25 g PSM suspended in 7.5 mL water with 0.005 M HNO3 while stirring to the same final pH as after the 25 titrations with 0.01 M NaH2PO4 described below. The suspension was filtered using a 0.22 mm Millipore filter and the Mg and Ca concentrations were determined in the clear filtrate by atomic absorption spectroscopy (AAS) using a Perkin Elmer 3300 instrument. Total inorganic P (Ptotal) was determined by extraction with 6 M H2SO4 for 10 min at 70  C (Mehta et al., 1954). Plant available phosphorus (POlsen) was determined through 3 combined sequential extractions with 0.5 M NaHCO3 adjusted to pH 8.5 (Olsen and Sommers, 1982). The concentration of P in the extracts was determined by the molybdenumblue method (Murphy and Riley, 1962). All analyses were carried out in triplicates. The specific surface area (SSA) of the different fractions was determined by applying the BET equation to N2 adsorption data obtained by means of a Micromeritic Gemini VII 2390a instrument (Brunauer et al., 1938) after the sample has been outgassed for 12 h at room temperature. The reported SSAs are the average of five separate measurements. 2.2.2. Isothermal titration calorimetry All ITC experiments were conducted on a CSC 4200 Isothermal Titration Calorimeter (CSC Inc., Lindon, UT) at 25  C. The ITC has a sensitivity of 0.418 mJ detectable heat effect and a “noise level” of 0.0418 mJ s1 (deconvoluted signal). The ITC investigations were carried in two ways, i.e. as single-point and as multiple-point titrations. For single-point P sorption titrations, 100 mg of the material was placed in a 1.3 mL reaction vessel and suspended in 750 mL of DI water, 250 mL 0.01 M NaH2PO4 was added in one injection and the heat production monitored for the following 5 h. For the mulitiple-point P sorption titrations, 25 mg of the material was placed in the reaction vessel and suspended in 750 mL of water. Under continuous stirring, the suspension was titrated with 0.01 M NaH2PO4 by adding a total of 250 mL in 25 increments of 10 mL with 5 min between each injection. In order to compensate for heat of dilution, a blank was run where P solution was stepwise added in the same manner to water as to the PSM suspension. In addition, the heat of neutralization of Filtralite-P was determined by titration of 25 mg Filtralite-P in 750 mL of water with 0.005 M HNO3 to pH 7.4, which was the final pH after titration with 0.01 M NaH2PO4. All ITC experiments were run as duplicates. 2.2.3. P sorption isotherms Supplementary batch sorption isotherms were conducted in the same manner and at the same solid:solution ratio as the corresponding ITC titration, but scaled up 10 times. Accordingly, 0.25 g of PSM was suspended in 7.5 mL water and stepwise added 0.1 mL of 0.01 M NaH2PO4 with 5 min between each addition. Only the first 8

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injections and the 25th were recorded. P sorption experiments resembling the single-point ITC titrations were also conducted, where 1 g PSM was suspended in 7.5 mL water and 2.5 mL of 0.01 M NaH2PO4 was added and allowed to react for 5 h. The suspensions were filtered using 0.22 mm Millipore filters. In the clear filtrate, pH was measured by a combination electrode and the P concentration determined spectrophotometrically by the molybdenum blue method (Murphy and Riley, 1962). All sorption experiments were done in triplicates. 2.2.4. Sequential P fractionation Materials from the single-point P sorption experiment were sequentially extracted with a modified Hedley sequence (Hedley et al., 1982). 0.5 g of previously P-treated material from the singlepoint P sorption experiments was extracted with 30 mL water by overnight shaking, centrifugation at 3000 g for 15 min, and filtered through a 0.22 mm Millipore filter. The sample was further sequentially extracted with 0.5 M NaHCO3, 0.1 M NaOH and 1 M HCl in the same manner as described for the water extraction. The P concentrations in the extracts were determined as described above and referred to as PH2 O , PNaHCO3 , PNaOH and PHCl pools depending on the treatment. All sequential experiments were done in duplicates. 2.2.5. Surface examination Samples from the multiple-point titration experiments were examined by scanning electron microscopy (SEM, FEI Quanta 600FE) and energy dispersive X-ray spectroscopy (EDS, EVEX Nanoanalysis Quantum Dot Detector using EVEX Nanoanalysis Software (EVEX, Princeton, New Jersey)). Samples were prepared for SEM-EDS by depositing on a C stub mount and coating with C to prevent charging. The samples were also examined by XRD in the same manner as described in Section 2.2.1. 3. Results and discussion 3.1. Filter characteristics CFH consists of amorphous (oxalate-extractable) Fe oxides, 2line ferrihydrite according to XRD, and minor contents of Al, Ca and Mg compounds (Supported Information (SI), Table S1). SSAs of the two particle size fractions are the same, 32 m2 g1. This is rather low compared to SSA of other Fe oxide materials used for P sorption (Chardon et al., 2012; Willett et al., 1988). The very high content of amorphous Fe oxides strongly suggests CFH as a strong P bonding sorbent (Lyngsie et al., 2014; Klimeski et al., 2012). Accordingly, although CFH contained substantial amount of P (indigenous P), it is unavailable as shown by undetectable POlsen. Filtralite-P contains little Fe and it is dominated by Al, Ca and Mg compounds (SI, Table S1). According to XRD, Filtralite-P consists of silicates, calcite and Ca/Mg oxides. No Al oxides were detected by XRD and the content of oxalate-extractable Al was negligible. Al oxides, which have good P retention properties (Vohla et al., 2011), may therefore be considered of very limited importance as P sorbents in Filtralite-P. Due to the high content of Ca/Mg oxides, an aqueous suspension of Filtralite-P has a pH of nearly 11. Under aquatic conditions the Ca/Mg oxides will hydrate and form Ca/Mg hydroxides, which together with the calcite may cause P sorption. Indigenous P amounted to 3 mmol kg1 that was partly available (POlsen ¼ 0.2 mmol kg1). The SSA of both fractions are considerably higher than expected from their particle sizes, indicating that both fractions have active/exposed sites on external as well as internal surfaces of the particles. Limestone is dominated by Ca, with minor contents of Mg, Al and Fe (SI, Table S1). XRD showed the material to be strongly dominated by crystalline calcite, but also aragonite and dolomite

were identified. Limestone contained 1 mmol kg1 of indigenous P, of which one fifth was available as POlsen. CaCO3 in different forms, e.g. sea-shells, egg-shells and limestone is often used as a P removal material because it is relatively cheap and widely available (Ballantine and Tanner, 2010). 3.2. P sorption to CFH Both CFH particle size fractions readily sorbed added P (Fig. 1a), which was expected due to the high content of amorphous Fe oxides (Klimeski et al., 2012). After addition of 25 injections of 0.01 mL of 10 mM P per 0.025 g CFH corresponding to 100 mmol kg1 in the multiple-point titration, a maximum of 55 mmol kg1 or about half of the added P was sorbed. However, as indicated by the almost linear sorption curve, neither of the particle size fractions reached the maximum sorption capacity (Pmax). Using the pedotransfer function by Borggaard et al. (2004) and assuming that Feox is the only P sorbing agent, a rough estimate of Pmax can be calculated as 0.12  FeOx resulting in Pmax ¼ 890 mmol kg1 for both CFH size fractions. This confirms that the Pmax for both CFH size fractions is far from reached. In the single-point P sorption, CFH sorbed all (25 mmol kg1) of the added P (Table 1). The sequential extraction (Fig. 2b) showed that less than 5% of the extracted P was in the labile pool (PH2 O þ PNaHCO3 ). The PNaOH, which is the pool associated to P bound to Al and Fe, amounted between 14 and 18% (5e 7 mmol kg1) of the sorbed P. PHCl is by far the largest pool (28e 29 mmol kg1) and is usually considered to comprise P associated with Ca as in Ca phosphates. Considering the large PHCl and the low Ca content in CFH (SI, Table S1) together with the modest PNaOH pool it is likely that the PHCl pool also includes Fe bound P. Nonetheless both the PNaOH and PHCl pools are considered non-labile emphasizing that the P is strongly sorbed to CFH and little sorbed P is expected to be released to the aqueous phase from the CFH. The EDS (SI, Fig. S1a) showed that P is present on the surface (6%) and randomly distributed on the surface of the particle (overlay image not shown). The SEM image (SI, Fig. S1a) did not show formation of any secondary minerals nor did the XRD analysis. The sorption of P by CFH is an exothermic reaction as shown by the ITC thermograms (Fig. 1c and d). Evolved heat decreases gradually with successive titration and after around 10 injections a steady-state heat evolution at each increment seems achieved. However, heat evolution does not cease even after addition of 100 mmol P kg1 (all 25 injections). This might be attributed to sorbed P being considerably less than the estimated Pmax (see above) corresponding to low P saturation and hence almost constant bonding energies. The rapid rise of the evolved heat peaks after each injection (Fig. 1c) indicates fast reaction between sorbent and sorbate (Penn and Warren, 2009; Steinberg, 1981). After this rapid heat evolution, the peaks for the first 6 injections (Fig. 1c) became somewhat broadened (tailing). This is even more clearly seen on the single-point thermogram (Fig. 2a), where the fast initial heat evolution is followed by a slower heat release shown by the rather long tail. The peak morphology indicates P sorption in two steps including a fast initial sorption followed by a slower reaction. This may be attributed to fast bonding of P to Fe oxide sorption sites at outer particles surfaces followed by diffusion into interior sorption sites as demonstrated for P sorption by ferrihydrite (Willett et al., 1988). The CFH single-point thermogram (Fig. 2a) indicates that exothermic heat peaked 210 s after P injection, and then the heat of reaction gradually decreased, and returns to baseline after approx. 1 h. Harvey and Rhue (2008) and Appel et al. (2013) examined P sorption by amorphous Fe and Al oxides using flow calorimetry and also found that the reaction between the oxides and P were exothermic and proceeded within about 1 h. The molar reaction

G. Lyngsie et al. / Journal of Environmental Management 143 (2014) 26e33

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Fig. 1. Sorption of P by the two particle size fractions of CFH. a) Showing the amount of sorbed P as a function of injection number for direct comparison with the multi-point P thermogram; b) pH resulting from the P additions shown in a), c) thermograms for multi-point titration of CFH with 0.01 M NaH2PO4, d) corresponding integration of the thermograms. Error bars are standard deviations. Exothermic upwards.

heat (DHr) for the P sorption in this study was w70 kJ mol1 and 25 to 30 kJ mol1 sorbed P for the single-point and multiple-point P titrations, respectively (Table 1). The difference in DHr can be attributed to about twice as much P was sorbed in the multi-point than in the single-point experiments (Table 1), as Q expressed per kg CFH (denoted Qnorm in Table 1) gave values not too different (1.2 to 1.7 kJ kg1 solid). The DHr obtained in the multipoint titration is considered the most reliable as in the single-point experiment all added P was sorbed. In good agreement with DHr obtained from the multi-point titration, Harvey and Rhue (2008) found DHr in the range 25 to 39 kJ mol1 for P sorption by a mixed Al/Fe oxide sorbent. Miltenburg and Golterman (1997) titrated Fe oxide with a P solution, resulting in a thermogram resembling those in Fig. 1c. Further, an initial exothermic reaction found by ITC of FeCl3 titrated with P was also interpreted as phosphate complexation to Fe(III), while further titration resulting in an endothermic reaction was considered due to an unspecified precipitation (Penn and Zhang, 2010). However, FeeP precipitates such as strengite (FePO4) only forms under acidic conditions (and elevated P concentrations) but

not at neutral to weakly alkaline pH, where Fe will precipitate as Fe oxides (Lindsay, 1979) that will sorb P through surface complexation. This also explains why in the present study at neutral pH, no Fe(III)-phosphate minerals were observed in the SEM images or in XRD despite detection of P on the CFH surface (SI, Fig. S1a). The results of the present study and numerous other investigations (Harvey and Rhue, 2008; Kwon and Kubicki, 2004; Penn and Zhang, 2010) have shown that P sorption by Fe oxides can be explained by surface complexation but doubt still exists about the precise surface complex(es) formed (Dideriksen and Stipp, 2003; Kwon and Kubicki, 2004). Thus, diprotonated, monoprotonated and deprotonated forms of bidentate, binuclear and monodentate, mononuclear complexes have been suggested (Arai and Sparks, 2001; Kwon and Kubicki, 2004; Persson et al., 1996). Probably the complex formed depends on various external conditions such as pH and P concentration. Thus, based on comprehensive quantum mechanical calculations, Kwon and Kubicki (2004) concluded that bidentate, binuclear complexes formed at acidic pH (4e6), whereas at neutral pH as in the present study monodentate (mononuclear) complexes are the most stable

Table 1 Final pH for the single-point P addition experiment, total amount of P sorbed for the single-point (Psingle) and the multiple P injection experiment (Pmulti), corresponding heat P released for the single-point (Q) and multiple ( Q) experiment, heat normalized to sorbent (Qnorm), and heat released per mol P sorbed (DH). CFH

pHsingle Psingle Pmulti Qsingle P Qmulti Q Psingle norm Qmulti norm DHsingle DHmulti

mmol kg1 mmol kg1 mJ mJ kJ kg1 solid kJ kg1 solid kJ mol1 P kJ mol1 P

Filtralite-P

Limestone

2e1 mm

1e0.5 mm

2e1 mm

1e0.5 mm

2e1 mm

1e0.5 mm

7.7  0.04 24  0.5 47  4.5 170 29 1.7 1.2 71 25

7.8  0.02 25  0.2 55  7.0 175 41 1.7 1.6 70 30

8.1  0.26 10  0.7 2.5  1.9 139 48 1.4 1.9 139 774

8.4  0.06 11  0.4 5.8  1.8 148 44 1.5 1.8 135 303

8.3  0.09 20  0.1 3.1  1.8 33 17 0.3 0.7 16 222

8.3  0.24 21  0.1 6.6  1.9 32 12 0.3 0.5 15 73

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G. Lyngsie et al. / Journal of Environmental Management 143 (2014) 26e33

complexes on outer surfaces followed by slow migration of P into interior sorption sites. 3.3. P sorption by Filtralite-P

Fig. 2. a) Thermograms from single-point P injection with 250 mL of 0.01 M NaH2PO4. Shown is the heat formation from 1 to 0.5 mm size fraction of CFH, Filtralite-P and Limestone; exothermic upwards. b) Sequentially extracted P on previously P-treated material from the single-point P titration experiment. The P was extracted with H2O, NaHCO3, HCl and NaOH. The amount of P extracted is given in the table in mmol kg1.

form, although occurrence of a deprotonated bidentate, binuclear complex cannot be completely ruled out. The results of the present investigation may therefore be interpreted as fast and strong P sorption by CFH under formation of probably monodentate

P sorption by the two Filtralite-P particle size fractions was very different (Fig. 3a). While the 2e1 mm fraction did not significantly sorb P during the first 8 injections, the 1e0.5 mm fraction sorbed almost the same amount of P as CFH up to the fifth injection, which is equivalent to about half (w10 mmol kg1) of the added P (20 mmol kg1). However, after further P addition, P sorption completely ceased and previously sorbed P was desorbed. This decrease in P sorption coincides with a sharp drop in pH from about 10 to nearly 8 at the 6th P injection (Fig. 3b). For the 2-1 mm fraction, the pH immediately dropped to about 8 after the first injection and no P sorbed. Evidently, P retention by Filtralite-P is strongly pH dependent. Previous studies have shown that P sorption by Ca-rich materials is dependent on both the Ca concentration and elevated pH (Claveau-Mallet et al., 2012; Klimeski et al., 2012; Stoner et al., 2012; Vohla et al., 2011). However, after 25 injections in the multi-point experiment, the 1e0.5 mm fraction sorbed about 6 mmol kg1 and the 2-1 mm fraction about 2.5 mmol kg1 (Table 1; Fig. 3a), while both size fractions sorbed about 40% (10 mmol kg1) in the single-point ITC investigation. The sequential extraction of Filtralite-P showed that 3e 4 mmol P kg1 was extractable by water and that the labile P pool (PH2 O þ PNaHCO3 ) accounted for about 65% of the extracted P (Fig. 2b), demonstrating that the sorbed P is very reactive. PHCl accounted for w30% (w5 mmol kg1) of the desorbed P, but about half of this most likely originated from indigenous P present in the Filtralite-P (SI, Table S1). SEM (SI, Fig. S1b) did not show formation of new minerals on the surface nor was any detected by XRD. The overlay EDS indicates that Ca is connected to the light (white) spots on the images of the surface and that no P was detected. EDS closeup on light spots showed that P is present on the surface but in small amounts