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Published online 27 February 2006
Published in Soil Sci Soc Am J 70:541-549 (2006)
DOI: 10.2136/sssaj2005.0250
© 2006 Soil Science Society of America
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Soil Chemistry

Kinetics of Phosphate Sorption to Polygalacturonate-coated Goethite

Christian Mikutta*, Friederike Lang and Martin Kaupenjohann

Dep. of Soil Science, Institute of Ecology, Berlin Univ. of Technology, Salzufer 12, D-10587 Berlin, Germany

* Corresponding author (christian.mikutta{at}tu-berlin.de)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Biogenetic polysugars may affect the sorption characteristics of soil mineral particles in the rhizosphere. We hypothesized that polygalacturonate [PGA, (C6H7O6)n–] coatings on goethite reduce the diffusion of phosphate into the pores of the adsorbent. Goethite was preloaded with PGA (0–10 mg C g–1). The samples were characterized by N2 and CO2 adsorption, electrophoretic mobility measurements, and scanning electron microscopy/energy dispersive X-ray analysis (SEM-EDX). The phosphate sorption kinetics was studied with batch experiments over 2 wk at pH 5 and an initial phosphate concentration of 250 µM. Pore volume and specific surface area of the goethite samples declined after PGA addition. The PGA coatings reduced the {zeta}-potential of goethite from 42.3 to –39.6 mV at the highest C loading. With increasing PGA-C content and decreasing {zeta}-potential the amount of phosphate sorbed after 2 wk decreased linearly (P < 0.001). Sorption of phosphate to pure and PGA-coated goethite showed an initial fast sorption followed by a slow sorption reaction. At the smallest C loading (5.5 mg C g–1) the portion of phosphate retained by the slow reaction was smaller than for the treatment without any PGA, while at higher C loadings the fraction of slowly immobilized phosphate increased. Our results suggest that at low C-loadings PGA impaired the intraparticle diffusion of phosphate. In contrast, the slow step-by-step desorption of PGA (<52% within 2 wk) or the diffusion of phosphate through PGA coatings or both are rate limiting for the slow phosphate reaction at C loadings > 5.5 mg C g–1.

Abbreviations: PGA, polygalacturonic acid/polygalacturonate • SSABET, specific surface area


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ORGANIC COATINGS have been identified and characterized on mineral surfaces of various soils (Courchesne et al., 1996; Yuan et al., 1998; Mayer and Xing, 2001; Amelung et al., 2002). Recently, X-ray photoelectron spectroscopy (XPS) data have shown that the surfaces of soil particles are coated with organic substances, even at low bulk organic C contents (<0.1 g kg–1, Gerin et al., 2003). Carbohydrates, which are important constituents of soil organic matter (5–25%, Stevenson, 1994), are dominated by polysaccharides that can contribute to organic coatings in soils (Miltner and Zech, 1998; Schmidt et al., 2000; Wattel-Koekkoek et al., 2001; Gerin et al., 2003). Many studies on the effect of low molecular weight organic acids like malic, citric, or oxalic acid on phosphate sorption to soils and minerals are available (Jones, 1998; Jones and Darrah, 1994; Liu et al., 1999), but scant attention has been paid to high molecular weight biomolecules released by plants and/or microorganisms. Root apices of many plant species are covered by granular or fibrillar gelatinous materials (Greaves and Darbyshire, 1972). These high molecular weight materials (mucilages) exuded by plant's root cap or epidermal cells (Mollenhauer et al., 1961; Vermeer and McCully, 1982) consist mainly of polysaccharides (Paull and Jones, 1975; Moody et al., 1988). For example, mucilage of corn comprised 95% polysaccharides with about 30% of polyuronic acids (Cortez and Billes, 1982). The actual amount of mucilage produced in soils still remains unknown (Nguyen, 2003). Reported polysaccharide-C contents (neutral sugar-C + galacturonic acid-C) of arable soils range from 0.22 to 3.83 mg C g–1 (Kiem and Kögel-Knabner, 2003). Studying organic coatings of soils with X-ray photoelectron spectroscopy (XPS), Gerin et al. (2003) found that particle surfaces were strongly enriched in organic C with surface concentrations in the range 50 to 500 mg C g–1. Therefore, it seems reasonable to assume that mineral surfaces adjacent to plant's root caps have at least C-loadings in the range reported by Gerin et al. (2003). As the macromolecular root exudates are supposed to not be diffusible in soils, or if so very slowly (Rovira, 1969; Sealey et al., 1995), their spatial distribution in soils is primarily confined to the soil–root interface. Cross-linked polysaccharide chains of exocellular slimes produced by plants or microbes act to bind soil or sediment minerals into microaggregates (Chenu, 1993; Ransom et al., 1997, 1999; Grimal et al., 2001). Organic coatings on hydrous Fe or Al oxide particles or their microaggregation by sorbed acid polysaccharides may decrease the immobilization of phosphate and hence increase its bioavailability.

Grimal et al. (2001) and Gaume et al. (2000) showed that polysugar molecules decreased the phosphate sorption capacity of goethite and ferrihydrite. In addition, phosphate mobilization from ferrihydrite increased in the presence of maize mucilage (Zea mays L.) and PGA. This has been explained—but not yet proven—by the competition for sorption sites and the decrease in oxide surface charge by PGA (Grimal et al., 2001). Lang and Kaupenjohann (2003) recognized that adsorbed dissolved organic matter affected the sorption of inorganic anions by clogging the pores of goethite. Yet, this mechanism has not been proven for mucilage components. Generally, polysaccharide coatings may decrease the sorption of phosphate to mineral surfaces by direct blocking of adsorption sites for phosphate, or by decreasing the accessibility of external or intraparticle sorption sites for phosphate.

We tested the hypothesis that acid polysugar coatings prevent phosphate from diffusion into intraparticle pores of goethite. We used synthetic goethite because it represents the most widespread Fe oxide in the soil environments (Cornell and Schwertmann, 2003). Polygalacturonic acid was taken as a model substance for macromolecular, pectin-like polysaccharides in the rhizospheric soil (Morel et al., 1987; Gessa and Deiana, 1992). The experiment was conducted at pH 5 to resemble pH conditions of the soil rhizosphere and the bulk of acid soils. The relevance of our study is confined to conditions where the pH of soil solution is lower than the isoelectric point (pHiep) of hydrous Fe or Al oxides (typically pHiep > 7), and hence the availability of phosphate to plants is strongly reduced because of its sorption to positively charged hydrous Fe and Al oxide surfaces.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Goethite
Goethite was synthesized by ageing of ferrihydrite, which precipitated after mixing Fe(NO3)3 · 9H2O and KOH solutions at a molar Fe/OH ratio of 0.05 (Schwertmann and Cornell, 1991). The solutions were aged at 333 ± 1 K for up to 16 d, dialyzed against deionized water until electric conductivity was below 10 µS cm–1, dried at 333 K, softly ground, sieved <200 µm and stored in PE-bottles. Powder X-ray diffraction patterns of the samples were obtained using a Siemens-D 5005 instrument (Siemens AG, Germany) with CuK{alpha}-radiation of wavelength 0.15406 nm (40 kV, 30 mA). The measurement ranged from 5 to 50° 2{theta}, step size was 0.05° 2{theta} and step time was 30 s. The goethite was mixed with 25% SiO2 as an internal standard. The scans indicated pure goethite with no detectable contamination (XRD spectra not shown). Oxalate soluble Fe of the goethite according to Blakemore et al. (1987) was 9.9 mg g–1 and total Fe according to Schulze (1984) was 619 mg g–1.

Polygalacturonic Acid
Polygalacturonic acid, (C6H8O6)n, with a purity of 86% (dry matter base) was purchased from Sigma (P-3889). Total acidity of PGA estimated from the structure is 5.7 molc kg–1 provided all acidity comes from COOH groups. The pKa of PGA is reported to be 3.5 (Grimal et al., 2001) or 3.9 (Kwok-Keung et al., 1998). The molecular weight approximates 4000 to 6000 g mol–1 (Aldrich). The PGA did not contain other sugars. The C content was 374 ± 4 mg g–1 on a dry matter basis measured with a Carlo Erba C/N NA 1500N Analyzer. The most prominent multivalent cation in the PGA determined after acid digestion in conc. HNO3 was Ca with 12 mmol kg–1 PGA. This content was too low to cause precipitation of Ca phosphates in the phosphate sorption experiment as calculated with MINTEQ (Allison et al., 1991).

Polygalacturonic acid was dispersed in doubly deionized water by adding 10 µL of 1 M KOH per milligram PGA. Six stock solutions containing 0, 20, 40, 80, 160, and 320 mg C L–1 were prepared. The pH value of the PGA solutions was adjusted to 5.0 with 0.1 M HNO3 before sorption experiments. Because of pH adjustment the ionic strength in the stock solutions increased to ~0.005 M. The size of PGA in the stock solutions was measured by dynamic light scattering using a high performance particle sizer (HPPS, Malvern, U.K.). The average diameter of the PGA ranged from 560 ± 12 nm at 160 mg C L–1 to 1287 ± 14 nm at 320 mg C L–1, but about 88% of the PGA in each treatment was smaller than 450 nm as determined after membrane filtration.

Sorption of PGA to Goethite
Goethite (1.30 g) was placed in a 2-L glass volumetric flask. Then 1000 mL of 20 mM KNO3 solution were added, and the pH was adjusted to pH 5.0 using 0.1 M HNO3. The suspensions were sonicated for 20 min and shaken on a reciprocating shaker at 140 rpm for 24 h to ensure aggregate dispersion and hydration of adsorption sites. The goethite suspensions were added to 1000 mL of PGA solution in a 2-L PE bottle to yield an ionic strength of background electrolyte of I = 0.01 M and C concentrations between 0 and 160 mg L–1. The suspensions were shaken on an end-over-end shaker at 20 rpm and at 293 ± 2 K. The pH was maintained at 5 ± 0.2 using 0.1 M HNO3 or 0.1 M KOH. After 45 h, the goethite suspensions were filtered through a 0.45-µm cellulose nitrate membrane filter (Sartorious, Germany). The filter residue was rinsed with 800 mL of 0.01 M KNO3 solution (pH 5.0), freeze-dried (Christ, {alpha} 2–4 freeze drier), and C contents of the samples were measured with a Carlo Erba C/N NA 1500N Analyzer. PGA-C contents of the samples are given in Table 1. For convenience the different C treatments are termed according to the rounded C loading, that is, G6 and G8 represent goethite with 5.5 and 7.6 mg C g–1. To measure dissolved Fe concentrations after 45 h of PGA sorption, three 5-mL aliquots were taken from each PE-bottle and ultracentrifuged at 300 000 x g for 1 h and Fe concentrations in the supernatant were determined with atomic absorption spectrometry (PerkinElmer 1100B).


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  		Table 1. PGA-C content (n = 3), fractional coverage, fcov (n = 2), and {zeta}-potential (n = 10) of pure and PGA-coated goethite. The fractional coverage calculated from Eq. [1] represents the fraction of total surface area that is not accessible by N2 adsorption at 77 K. Values in parentheses represent mean range for the fractional coverage and standard error for C contents and {zeta}-potentials, respectively. {zeta}-Potentials followed by the same letter are not statistically different at P < 0.05 (unpaired t-test).

 
We calculated the fraction of total mineral surface covered by organic matter, fcov, by the relation

Formula 1[1]
where SSABET/naked and SSABET/coated are the BET surface areas of uncoated and coated goethite, respectively (Mayer and Xing, 2001). Equation [1] assumes that the difference in SSABET between coated and uncoated samples represents surface area that is occluded by organic matter. This mechanism might impair the diffusion of N2 at 77 K into inter- and intraparticle pore space (Mayer and Xing, 2001).

Scanning Electron Microscopy Analysis
Freeze-dried samples were analyzed with scanning electron microscopy (Hitachi S-2700) to identify organic coatings and structural changes induced by PGA. The specimens were placed on conductive carbon tape, surface-sputtered with Au and measured in the secondary electron detection mode (Evenhart-Thornley detector). The elemental composition of PGA-coated surfaces was estimated by an energy dispersive X-ray detector (EDX) fitted to the microscope.

Phosphate Sorption Kinetics
Phosphate was provided as KH2PO4 p.a. (Merck, Germany). Triplicate samples of uncoated and PGA coated goethite (20 mg) were weighed into 50-mL polypropylene centrifuge tubes (Nalgene, USA), which contained an agate ball of 10-mm size to ensure good mixture. Subsequently, 40 mL of 0.01 M KNO3 solution with a phosphate concentration of 250 µM (pH 5.0) were added. At pH 5, the predominant chemical species of phosphate present is H2PO4–. Fifty microliters of 0.05 M AgNO3 were added per liter phosphate solution to inhibit microbial activity.

The suspensions were reacted in the dark at room temperature (293 ± 2) K on a rotary shaker at 22 rpm for 0.5, 1, 2, 4, 8, 16, 48, 168, and 336 h. After each reaction period, the pH was recorded and 10-mL aliquots were membrane filtered (0.45 µm), ultracentrifuged at 300 000 x g for 1 h and phosphate and Fe concentrations were measured in the supernatant. The filter residue was washed with 40 mL of doubly deionized water to remove excess phosphate and freeze-dried. The phosphate concentration was determined photometrically by the method of Murphy and Riley (1962) using a Specord 200 spectralphotometer (Analytik Jena AG). The accuracy of this method was tested to be <1.5%; precision of the measurements was <1%. Subsample variability was generally <2%. We checked the possibility that PGA is precipitated during ultracentrifugation, which would decrease phosphate concentration in solution if phosphate was bound to polyvalent cations associated with the carboxylic groups of PGA. We found no statistical indication of a matrix interference by PGA. The amount of phosphate sorbed was calculated as the difference between phosphate in solution prior and after each reaction time interval. Iron concentrations were measured by furnace atomic absorption spectrometry (PerkinElmer AAnalyst 700). The Fe concentrations were <3 µM, and hence goethite dissolution by PGA desorption was negligible. The amount of PGA-C desorbed was calculated from the initial PGA-C content in the sample and the total organic C concentration measured in the 0.45-µm filtrate using a Shimadzu TOC-5050A Autoanalyzer.

Modeling of Phosphate Sorption Kinetics
Two kinetic models were used to describe the phosphate sorption data. The fitting was performed with SigmaPlot for Windows (SPSS Inc., Chicago, IL).

  1. Combined Model. We combined a first-order model and the parabolic diffusion model (Crank, 1976) to account for fast sorption to external sorption sites and diffusion limited slow sorption of phosphate to goethite (Lang and Kaupenjohann, 2003). In PGA-coated samples a portion of phosphate reacted instantaneously. For this reason we permitted a positive intercept:

    Formula 2[2]
    where qt is the amount of phosphate sorbed at time t (µmol m–2), cm (µmol m–2)is the maximum amount of phosphate sorbed by the fast reaction, (cm – a0) is the amount of phosphate sorbed instantaneously (faster than could be quantified by the batch approach in µmol m–2), k is the rate constant of the initial fast phosphate sorption (h–1), b is the apparent rate constant of the slow reaction (µmol m–2 h–0.5), and t is time (h). The parameters cm, a0, k, and b were determined by fitting Eq. [2] to the sorption data. We used q336 h corrected for the total amount of phosphate rapidly sorbed (cm) as an approximation for the fraction of phosphate slowly immobilized (Fraction Pslowly).

  2. Diffusion in Heterogeneous Medium. Differentiation of the parabolic diffusion equation explicitly expressed as the reciprocal of the rate of diffusion in a heterogeneous medium yields (Aharoni et al., 1991):

    Formula 3[3]
    where {tau} = r2/D with D = diffusion coefficient and r = length of diffusion, {tau}i = smallest r and {tau}m = largest r. Equation [3] yields S-shaped plots of (dq/dt)–1 vs. t which are concave to the t axis at small times, convex at large times and linear in between. For diffusion in heterogeneous medium, the linear part of the Z-plots is most prominent, that is, for {tau}i and {tau}m there is a large range of t at which the two negative terms in Eq. [3] become negligible. Hence, Eq. [3] can be reduced to (Aharoni et al., 1991):

    Formula 4[4]
    The ratio {tau}m/{tau}i, is taken as a measure of the heterogeneity of diffusion pathways (Aharoni and Sparks, 1991).

Model Evaluation
The models applied to kinetic data were judged on the basis of the coefficient of determination, r2, and the standard error statistics. Model parameters were evaluated by their standard errors using the t-statistics, which tests the null hypothesis that the parameter is zero by comparing the parameter value with its standard error. Standard errors of derived parameters were calculated according to the rules of error propagation.

Surface Area and Pore Analysis
Specific surface area (SSABET) and pore volume were determined with a Quantachrome Autosorb-1 automated gas sorption system (Quantachrome, Syosset, NY) using N2 as an adsorbate. Approximately 0.100 g of sample were degassed until the rate of pressure increase by vapor evolution was below about 1.3 Pa min–1 within a 1-min interval. Helium was used as a backfill gas. We used 67-point N2 adsorption and desorption isotherms from 1.0 x 10–5 to 0.995 P/P0. Specific surface area was calculated from the BET equation (Gregg and Sing, 1982).

Micropore (<2 nm) porosity and average micropore diameter were determined according to the Dubinin-Radushkevic method (DR, Gregg and Sing, 1982). Because samples showed a large adsorption–desorption hysteresis, which suggests network effects during desorption that cause overestimation of surface area (Lowell and Shields, 1984), the mesopore-size distribution (2–50 nm) was calculated on the adsorption leg using the BJH method (Barrett et al., 1951). Separation between small (2–10 nm) and large mesopores (10–50 nm) was achieved by linear interpolation of BJH adsorption data. Total pore volume was taken at 0.995 P/P0. We also determined the micropore volume using CO2 as an adsorbate at 273 K with a NOVA gas sorption system (Quantachrome, Syosset, NY). A 25-point adsorption was performed from 1.0 x 10–3 to 3.0 x 10–2 P/P0 and analyzed using the Dubinin-Radushkevic equation (Gregg and Sing, 1982). All isotherms were recorded in duplicate.

Electrophoretic Mobility Measurements
Sorption of anionic polyelectrolytes like PGA to goethite may alter its surface charge and thus affect the kinetics of phosphate immobilization. Therefore, we determined the initial {zeta}-potential of the pure and PGA-coated goethites in 0.01 M KNO3 solution at pH 5.0. The changes in {zeta}-potential during phosphate sorption were monitored after resuspending about 200 µg of freeze-dried 0.45-µm filter residue into 4 mL of phosphate equilibrium solution of a respective point in time. Preliminary tests showed no statistically significant difference between {zeta}-potentials obtained from freeze-dried and non-dried pure and PGA-coated goethites (unpaired t-test, P < 0.05). The electrophoretic mobility was determined at 298 K with a Malvern Zetasizer 2000 (Malvern Instruments, UK). Before starting the measurements the instrument was calibrated with a {zeta}-potential transfer reference, which is referenced to the NIST goethite standard SRM1980 (Malvern Instruments, UK). Ten measurements were performed within <8 min and the average value was recorded. The {zeta}-potential was calculated from the electrophoretic mobility using the Smoluchowski approximation (Hunter, 1988). It is generally assumed that the {zeta}-potential represents the potential at a shear plane located in the diffuse layer close to the Stern layer (Hunter, 1988).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Fractional PGA Coverage and Surface Loadings
Fractional coverage values of our goethite samples indicate that about one third of the goethite surface area is lost due to polysaccharide coatings regardless of the amount of PGA addition (Table 1).

A negative correlation was observed between the amount of PGA-C sorbed and the coating-efficiency of PGA (i.e., loss of surface area per milligram PGA-C sorbed, r2 = 0.93, P < 0.01). The coating efficiency decreased from 4.42 ± 0.31 m2 mg–1 PGA-C for G6 to 2.68 ± 0.16 m2 mg–1 PGA-C for G10 (mean ± standard deviation). At similar C loadings per unit mass, PGA decreased the SSABET of goethite more effectively than sorbed dissolved organic matter (approximately a factor 2; Fig. 5a in Kaiser and Guggenberger, 2003). Kaiser and Guggenberger (2003) explained the increasing coating efficiency with decreasing C-loading of surfaces by varying surface arrangements of organic molecules (see also Theng, 1979, p. 42; Saito et al., 2004), organic multilayer formation or preferential sorption at specific reaction sites, that is, micropores.


Figure 5
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  		Fig. 5. Amount of phosphate slowly immobilized versus fractional PGA-C release after 2 wk. The amount of phosphate slowly immobilized was calculated as the difference between the total amount of phosphate sorbed after 2 wk and the total amount sorbed fast (cm of Eq. [2]). Figures after ‘G’ refer to the rounded C content of the sample in milligram C per gram. Error bars represent standard error.

 
Scanning electron microscopy images of pure goethite show the elongated acicular crystals with up to 2-µm length and 0.2-µm width. Large fibrous multidomain crystallites are well visible (Fig. 1a ). The images of PGA treated goethite samples reveal the occlusion of the goethite needles by organic matter (Fig. 1b, c). Cotton-wool like agglomerations dominate besides larger areas where no coatings can be inferred, possibly because of insufficient coating thickness. Energy dispersive X-ray analysis indicated that even surfaces where no coating was visible contained appreciable amounts of PGA-C (not shown).


Figure 1
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  		Fig. 1. Scanning electron microscopy images of (a) pure goethite, and goethite coated with polygalacturonate of different C loadings: (b) 5.5 mg C g–1, (c) 7.6 mg C g–1, and (d) 10 mg C g–1. Multidomainic goethite crystals are visible in Fig. 1a; Fig. 1b shows in more detail the clustering of goethite crystals induced by PGA at low PGA-C content; Fig. 1c and 1d give overviews of PGA-goethite clusters on differently sized aggregates of goethite.

 
Porosity Changes by PGA
Polygalacturonate coatings reduced the total pore volume by 85 mm3 g–1 (24%) on average. The N2 micropore- and small mesopore volumes declined to 65 and 51% of the initial values, respectively (Table 2). Our CO2 adsorption study showed that pores <0.5 nm, into which N2 diffusion at 77 K is kinetically restricted, existed in samples with intermediate and high PGA loadings (Table 2, G7, G8, G10). Upon PGA sorption, the SSABET declined on average by 35% independent of the C-loading (Table 2).


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  		Table 2. Specific surface area and porosity data of pure and PGA-coated goethite obtained by N2 adsorption at 77 K and CO2 adsorption at 273 K (n = 2). Figures after ‘G’ refer to the rounded C content of the sample in mg C g–1. Values in parentheses are given as mean range.

 
Phosphate Sorption
Various phosphate adsorption studies on Al oxides (Chen et al., 1973), Fe oxides (Madrid and Arambarri, 1985; Strauss et al., 1997), or soils (Torrent, 1987; Sanyal et al., 1993; Freese et al., 1995) show an initial rapid sorption, which is followed by a slow sorption. The rapid sorption has been attributed to the adsorption of phosphate to outer surfaces of the adsorbents, while the slow immobilization of phosphate has been shown to be diffusion of phosphate into particle pores (e.g., Strauss et al., 1997), which has also been confirmed for drinking-water treatment residuals that comprise amorphous Fe and Al oxides (Makris et al., 2004). This typical sorption pattern was also observed in our study (Fig. 2 ). Phosphate sorption onto G0, G6, and G10 attained apparent equilibrium within 2 wk. This finding is in agreement with Strauss et al. (1997) who found that phosphate sorption onto pure goethite was complete within 2 wk. However, sorption of phosphate onto goethite samples with intermediate PGA loadings (G7–9) continued and did not reach apparent equilibrium within 2 wk (Fig. 2).


Figure 2
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  		Fig. 2. Changes in phosphate sorption with time of PGA-coated and pure goethite. The solid concentration was 0.5 g L–1. Subsample variability was typically <2%. Figures after ‘G’ refer to the rounded C content of the sample in milligram C per gram.

 
The combined model provided a reasonable fit of the data with r2 values between 0.94 to 1.00 (Table 3), which is in agreement with the conceptual model of diffusion limited slow sorption. An exception was treatment G6, where no slowly continuing phosphate sorption could be observed. Sorption was nearly completed after 48 h, pointing out that diffusion was greatly reduced as indicated by a strong decrease in the rate constant of the slow phosphate reaction (b, Table 3). We will discuss the reason for this observation below.


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  		Table 3. Fit parameters of the regression of phosphate sorption vs. time of pure and PGA-coated goethite using the combined model, Eq. [2], and the diffusion in heterogeneous medium model, Eq. [4]. Also given are the slope parameters (1/{rho}) and heterogeneity indices ({tau}m/{tau}i) obtained from the heterogeneous diffusion model. Figures after ‘G’ refer to the rounded C content of the sample in milligram C per gram. Values in parentheses represent standard error (SE).

 
In the presence of PGA, the amount of phosphate sorbed via the fast reaction decreased by up to 50% (cm, Table 3). However, the rate constants of the fast reaction varied greatly (k, Table 3). Sensitivity analyses, however, showed that the amount of phosphate sorbed was rather insensitive to changes in k. The rate constant of the slow reaction increased in the treatments G7–9 compared with pure goethite (b, Table 3).

Also, the heterogeneous diffusion model provided an adequate fit of the data with r2 values ranging from 0.91 to 0.98 (Table 3). Aharoni and Sparks (1991) predicted that a slope <0.24 for the relationship d(q/q{infty}) vs. ln t is indicative of heterogeneous diffusion. Using Eq. [4], we obtained slopes (1/{rho}) between 0.072 and 0.126, suggesting heterogeneous diffusion (Table 3). The ratio {tau}m/{tau}i differed by three orders of magnitude: 106 (G0) – 103 (G7 and G8), indicating that the heterogeneity, that is, the range of reciprocal apparent diffusion constants, (D/r2)app, of goethite decreased by PGA coatings (Table 3). For those samples where equilibrium was not reached after 2 wk, only a minimum value of {tau}m/{tau}i can be estimated from q/qmax (Aharoni et al., 1991).

Electrophoretic Mobility Measurements
Phosphate sorption to pure goethite reversed its {zeta}-potential from positive to negative values (Fig. 3 ). After about 16 h of phosphate sorption, the {zeta}-potential of the goethite increased again by approximately 20 mV. The increase in {zeta}-potential of goethite with time has been documented in other phosphate sorption studies using lower and higher phosphate concentrations compared with this study (Ler and Stanforth, 2003, Mikutta et al., unpublished data, 2005). There are several possible explanations including the surface precipitation of Fe phosphates or the formation of ternary surface complexes with dissolved Fe. The dissolution of goethite in the presence of phosphate increased the dissolved Fe concentrations in G0 samples up to 2.7 µM. The increase in {zeta}-potential observed (Fig. 3) might reflect the increase in the total dissolved Fe concentrations after 16 h and hence indicate the formation of ternary surface complexes as proposed by Ler and Stanforth (2003). However, no Fe phosphates were observed by XANES in a study by Khare et al. (2005) who used a much higher concentration than applied in our study (0.01 M phosphate). Also, no Fe phosphate precipitates on natural goethite were observed after 90 d at elevated phosphate concentrations (0.001 M, pH 4.5; Martin et al., 1988). Thus, the surface precipitation of Fe phosphates seems unrealistic.


Figure 3
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  		Fig. 3. Changes in {zeta}-potential of pure and PGA-coated goethite during phosphate sorption (I = 0.01 M KNO3, pH = 5). Note that x-axis is log scale. Error bars indicating the standard error of 10 replicate measurements are within the symbol size. Initial {zeta}-potentials of the samples (no phosphate contact) are presented in Table 1. Figures after ‘G’ refer to the rounded C content of the sample in milligram C per gram.

 
With increasing PGA loadings the {zeta}-potential decreased to negative values (Table 1). At a PGA loading of 7.2 mg C g–1 the {zeta}-potential dropped markedly from 42.3 to –37.6 mV; any additional amount of PGA altered the {zeta}-potential only slightly (Table 1). This result may be explained in terms of multilayer sorption of polyprotic PGA molecules, which can also be inferred from similar fractional surface coverages (Table 1), our SEM observations (Fig. 1), and the presence of multivalent cations in the PGA (see Material and Methods).

In all cases except those with no and small PGA content (G0, G6) the {zeta}-potential was independent of phosphate sorption, staying constant around –39 mV after contact with phosphate solution (Fig. 3). The most likely explanation is that the negative charge of phosphate ions conveyed to the surface was counterbalanced by a release of PGA into solution. This assumption is supported by the increasing C concentrations in solution with increasing phosphate sorption (Fig. 4 ). Up to 52% of PGA-C (G7) was desorbed by phosphate indicating the high competitiveness of phosphate for sorption sites (Fig. 5 ).


Figure 4
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  		Fig. 4. Kinetics of phosphate sorption and PGA-C desorption in samples with low (G6) and intermediate PGA-C content (G7) at an initial phosphate concentration of 250 µM in 0.01 M KNO3 at pH 5 with a solid concentration of 0.5 g L–1. Figures after ‘G’ refer to the rounded C content of the sample in milligram C per gram.

 
Rate-limiting Processes of the Slow Phosphate Sorption
Polysaccharide coatings on goethite reduced the amount of phosphate sorbed and also affected the rate at which equilibrium with phosphate solution was attained. With increasing PGA-C content the amount of phosphate sorbed after 2 wk decreased linearly (r2 = 0.98, P < 0.001). One reason might be the decreasing accessibility of intraparticle pores caused by the PGA coating as reflected by porosity measurements (Table 2). Phosphate sorption after 2 wk was positively related to the amount of micropore (r2 = 0.90, P < 0.01) and small mesopore volume (r2 = 0.97, P < 0.001, Fig. 6 ). The statistical relationship for the latter pores persisted when the G0 sample was removed from the data set (P < 0.05). Figure 6 reveals that a portion of the surface area belonging to pores < 10 nm was either inaccessible or hardly accessible to phosphate because of PGA coatings. However, no relationship exists between the pore volumes of <10-nm pores and the amount of phosphate slowly immobilized (P > 0.73). The finding indicates that the slow phosphate immobilization by PGA-coated goethites was not primarily controlled by the diffusion of phosphate into intraparticle pores. In addition, the amount of phosphate sorbed after 2 wk was positively related to the initial {zeta}-potential (r2 = 0.97, P < 0.001), suggesting that the initial surface charge is a determinant of the amount of phosphate sorbed after 2 wk.


Figure 6
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  		Fig. 6. Relationship between the amount of phosphate sorbed after 2 wk and the micro- (<2 nm) and small mesopore volume (2–10 nm) of the samples analyzed with N2 adsorption at 77 K. Horizontal error bars indicate standard error, vertical error bars indicate mean range.

 
The applicability of diffusion-based models to our data indicates diffusion-limited phosphate sorption. The samples differed significantly in the fractions of phosphate slowly immobilized after 2 wk (Fraction Pslowly, Table 3), and the rate constants of the slow reaction (b, Table 3). For pure goethite intraparticle diffusion is rate limiting because phosphate diffuses into the micropores of goethite located between the crystal's domains (Strauss et al., 1997). At the lowest C loading (G6) the continuing phosphate reaction stopped after ~48 h (Fig. 2), and the rate constant of the slow phosphate reaction strongly decreased in comparison with the control treatment (b, Table 3). Thus, at low C loading, PGA seems to act as an intraparticle diffusion barrier preventing phosphate ions from penetrating into micro- and small mesopores because of a preferential sorption of PGA to micropores and small mesopores (Kaiser and Guggenberger, 2003). This interpretation is in line with Scheinost et al. (2001) who suggest that fulvic acid acts as a diffusion barrier for Cu and Pb between the solution and sorption sites of ferrihydrite.

Contrary to our expectation, the fraction of slowly immobilized phosphate at higher C loadings exceeded that of pure goethite (Fraction Pslowly, Table 3). Figure 5 shows that the amount of phosphate slowly immobilized was related to the fractional PGA-C release after 2 wk. Additionally, the rate constants, b, of both the phosphate sorption and the PGA-C desorption obtained from fitting Eq. [2] to both data sets were significantly correlated (P < 0.01, n = 5). The findings support the idea that sorption competition between phosphate and PGA and hence the step-by-step desorption of PGA from external goethite surfaces governed the rate of the slow phosphate sorption. Unfortunately, no data are available in the literature on the kinetics of the exchange between oxyanions and high molecular weight biopolymers bound at the Fe oxide interface via polynuclear surface complexes. Therefore, we cannot rule out the possibility that the rate of the slow phosphate sorption by PGA-coated goethite was limited by the diffusion of phosphate to external goethite surfaces. If a diffusion limitation of phosphate by sorbed PGA existed, it is less likely due to electrostatic but rather sterical interactions between PGA and phosphate because the slow phosphate sorption was independent of the {zeta}-potential (Table 3, Fig. 3).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our results showed that naturally ubiquitous acid polysaccharides coatings on Fe oxides might increase the bioavailability of phosphate in natural systems. The increase in bioavailability of phosphate possibly results from a combination of several processes including (i) the decrease in surface charge of the adsorbent upon PGA sorption, (ii) clogging of pores < 10-nm at low C loading (5.5 mg C g–1) with a subsequent decrease in intraparticle diffusion of phosphate, and (iii) sorption competition between phosphate and presorbed PGA or the diffusion of phosphate to external goethite surfaces or both at C loadings > 5.5 mg C g–1. As PGA is slowly displaced by phosphate due to sorption competition, the increase in the bioavailability of phosphate to plants following the exudation of acid polysaccharides may only be transient.


   ACKNOWLEDGMENTS
 
This study was funded by a grant of the German Research Fund (DFG, KA 1139/8). We are greatly indebted to Robert Mikutta (Martin-Luther University Halle-Wittenberg).

Received for publication July 28, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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