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Published online 3 August 2006
Published in Soil Sci Soc Am J 70:1547-1555 (2006)
DOI: 10.2136/sssaj2005.0340
© 2006 Soil Science Society of America
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Soil & Water Management & Conservation

Acid Polysaccharide Coatings on Microporous Goethites

Controls of Slow Phosphate Sorption

Christian Mikutta*, Jaane Krüger, 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
 
Organic coatings on Fe oxides can decrease the accessibility of intraparticle pores for oxyanions like phosphate. We hypothesized that the slow sorption of phosphate to goethite coated with polygalacturonate (PGA) is controlled by the accessibility of external goethite surfaces to phosphate rather than by diffusion of phosphate into micropores (Ø < 2 nm). We studied the phosphate sorption kinetics of pure and PGA-coated goethites that differed in their microporosity (N2 at 77 K, 46 vs. 31 mm3 g–1). Because drying may affect the structure or surface coverage of PGA, we also tested the effect of freeze-drying on the slow phosphate sorption. The samples were examined by gas adsorption (N2, CO2) and electrophoretic mobility measurements. Phosphate sorption and PGA-C desorption were studied in batch experiments for 3 wk at pH 5. In PGA-coated samples, the slow phosphate sorption was independent of micropore volume. Phosphate displaced on average 57% of PGA-C within 3 wk. Similar to phosphate sorption, the PGA-C desorption comprised a rapid initial desorption, which was followed by a slow C desorption. Sorption competition between phosphate and presorbed PGA depended on the <10-nm porosity and the C loading of the adsorbent. The efficacy of phosphate to desorb PGA generally increased after freeze-drying. We conclude for PGA-coated goethites that (i) freeze-drying biased the slow phosphate sorption by changing the structure/surface coverage of PGA, and (ii) within the time frame studied, micropores did not limit the rate of the slow phosphate sorption. Rather, the slow, gradual desorption of PGA and/or the diffusion of phosphate through PGA coatings controlled the slow phosphate sorption to PGA-coated goethite.

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


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN SOILS AND SEDIMENTS, minerals are partially covered with organic matter (Ransom et al., 1997; Yuan et al., 1998; Mayer and Xing, 2001; Gerin et al., 2003). This coverage may drastically change the physicochemical properties of the mineral phases, such as surface charge (Heil and Sposito, 1993a; Kaiser and Zech, 1999; Mikutta et al., 2004) or colloidal stability (Heil and Sposito, 1993b; Kretzschmar et al., 1997). As a consequence, the presence of organic coatings on soil minerals may affect the sorption of nutritional or environmentally hazardous elements.

In the rhizosphere, organic coatings on mineral surfaces may be dominated by organic compounds released by plant roots and microorganisms. Root apices of most plant species are covered by granular or fibrillar gelatinous materials (mucilage) (Greaves and Darbyshire, 1972; Knee et al., 2001). Mucilage exuded by plant's root cap or epidermal cells (e.g., Vermeer and McCully, 1982) is confined to the soil–root interface because mucilage components are supposed to diffuse very slowly into the soil (Rovira, 1969; Sealey et al., 1995). Mucilage components consist mainly of polysaccharides with a notable proportion of polygalacturonic acid. For example, mucilage of maize consists of 90 to 95% polysaccharides with about 20 to 35% of uronic acids (Cortez and Billes, 1982; Morel et al., 1986). The effect of mucilage sorbed to Fe or Al oxides on the immobilization of oxyanions like phosphate is poorly understood.

Phosphate sorption to Fe oxides usually comprises two stages. A rapid initial sorption to external surfaces is generally followed by a slow sorption that can last for days or weeks (Barrow et al., 1981; Torrent et al., 1990). The slow phosphate sorption has been attributed to the diffusion of phosphate into microporous imperfections of the crystals, micro- and mesopores between the crystal domains (Torrent, 1991; Barrow et al., 1993; Strauss et al., 1997; Makris et al., 2004), or the diffusion into aggregates of particles (Anderson et al., 1985; Willet et al., 1988). The sorption of high-molecular-weight biomolecules to porous Fe oxides may impair the diffusion of phosphate into intraparticle pores of these adsorbents. In a previous study, we observed that polygalacturonate (PGA) coatings impaired the diffusion of phosphate into pores of goethite ({alpha}-FeOOH) at a low C loading of 6.3 µmol m–2 (Mikutta et al., 2006). Phosphate, however, was highly competitive at higher C loadings, being able to desorb up to 52% of the polysaccharide C within 2 wk (Mikutta et al., 2006). Our results implied that processes other than micropore diffusion could control the slow phosphate immobilization of PGA-coated goethites. The diffusion of phosphate to external goethite surfaces and/or the desorption of organic matter by phosphate might be possible controls of the slow phosphate sorption. Both processes are expected to be influenced by the state of hydration of organic coatings. In the presence of free water, maize mucilage is able to hydrate extensively. Fully hydrated root-cap mucilage can have water contents of up to 100 000 wt% of its dry weight (Guinel and McCully, 1986). Reversible structural changes of pectin-like biomolecules on hydration/dehydration or irreversible structural changes through physicochemical alterations of the molecular framework on drying (Wedlock et al., 1983; Jouppila and Roos, 1997; Allison et al., 1998; Souillac et al., 2002) may change the coverage of mineral surfaces by organic matter and/or the desorbability of organic matter by phosphate. Porosity measurements by 1H-NMR relaxometry and N2 adsorption have indicated that labile interparticle pores created by PGA coatings may be destroyed during freeze-drying (Mikutta et al., 2004), thus possibly reducing the effectiveness of organic coatings as diffusion barriers for phosphate and/or changing the sorption competition between phosphate and PGA sorbed to Fe oxides.

The objective of this study was to elucidate whether micropores of PGA-coated goethite are responsible for the slow sorption reaction of phosphate. We hypothesized that the slow phosphate sorption to PGA-coated goethite is not controlled by the diffusion of phosphate into micropores but by the accessibility of external goethite surfaces to phosphate. The accessibility of external goethite surfaces to phosphate should be directly influenced by the structural arrangement of PGA at the surface. To test our hypothesis, we coated two goethites differing in their micro- and mesoporosity (<10 nm) with PGA and conducted phosphate sorption experiments using freeze-dried and nondried samples. Polygalacturonate was used as a simplified model substance for macromolecular root exudates (Morel et al., 1987; Gessa and Deiana, 1992). The experiment was conducted at pH 5 (i) to resemble pH conditions observed for soybean plants fertilized with NH4–N (Riley and Barber, 1971) and P-starved tomato, chickpea, and lupin plants fertilized with NO3–N (Neumann and Römheld, 1999) and (ii) to minimize interference with bicarbonate.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Goethites
Microporous goethite (G1) was synthesized by oxidative hydrolysis of Fe(II) (FeSO4·7H2O, extra pure; Merck, Darmstadt, Germany) at pH 7 using H2O2 as an oxidant. The precipitate was washed until the electric conductivity was below 10 µS cm–1, freeze-dried, softly ground, and sieved to a particle size < 200 µm. The oxalate-soluble Fe content determined according to Blakemore et al. (1987) was 4.9%. Goethite G2 was obtained by autoclaving G1 for 2 h (nine times) and 4 h (eight times) at 1 bar and 120°C. After autoclaving, the goethite was put into a microwave oven (Mars XPress; CEM, Kamp-Lintfort, Germany) for 2 h (four times) at 2.8 bar and 150°C. After each run, the goethite was oven-dried at 50°C.

The goethites were characterized by X-ray diffraction analysis (D5005; Siemens, Berlin-München, Germany) and scanning electron microscopy (S-4000; Hitachi, Tokyo, Japan). Potentiometric titrations of the goethites (~0.01 g L–1) in 0.01 M KNO3 using a Zetasizer 2000 connected with a MPT-1 autotitrator (Malvern Instruments, Herrenberg, Germany) were performed to determine the charge characteristics of both adsorbents. During titration, the {zeta}-potential was analyzed in triplicate at each target pH, and the average value was recorded.

Preparation of Polygalacturonate Coatings
Polygalacturonic acid was purchased from Fluka (Buchs, Switzerland) (P81325, (C5H5O2(OH)2COOH)n, > 95%, M = 25–50 kDa) and comprised 37.2% C and 0.05% N as determined with a Vario EIII C/N/S analyzer (Elementar; Hanau, Germany). To achieve high- and low-organic-C surface loadings on goethite, solutions with 1010 and 50.5 mg C L–1 were prepared. Polygalacturonic acid was dissolved in 1 L 0.01 M KNO3 solution after adding 10 µL 1 M KOH mg–1 PGA to enhance PGA solubility. One hundred microliters of 0.05 M AgNO3 solution were added to eliminate microbial activity. The PGA solutions were titrated back to pH 5 using 1 M HNO3 without any visible flocculation occurring. The final ionic strength of the solutions was ≤ 0.02 M.

Five grams of goethite were placed into 1-L centrifuge PE-bottles, and 10 mL 0.01 M KNO3 solution (pH 5) were added. To ensure particle disaggregation and hydration of adsorption sites, the goethites were shaken on a reciprocating shaker at 85 rev min–1 for 48 h, and pH was readjusted to 5 with dilute HNO3 or KOH. Polygalacturonate solutions (990 mL) were added to achieve C concentrations of 50 or 1000 mg C L–1 in 0.01 M KNO3 background electrolyte. The bottles were transferred onto a rotary shaker running at 20 rev min–1. The pH was manually kept within 5 ± 0.2 using dilute HNO3. After 24 h, the goethite suspensions were repetitively centrifuged at 5500 x g for 20 min (RC-3B Refrigerated Centrifuge; Sorvall Instruments, Langenselbold, Germany) and washed with 500 mL doubly deionized water until the total organic C concentration in supernatant solutions was < 5 mg C L–1 (TOC-5050A Autoanalyzer; Shimadzu, Duisburg, Germany). The goethite residue was freeze-dried, softly homogenized in an agate mortar, and stored in the dark until use or instantaneously used in the phosphate sorption experiment without any freeze-drying. Freeze-drying was accomplished after freezing the PGA-coated goethites at –80°C in an Christ {alpha} 2–4 freeze drier (Osterode, Germany).

Phosphate Sorption Kinetics
The phosphate sorption was conducted in batch systems in 0.01 M KNO3 solution at pH 5. Phosphate was used in the form of KH2PO4 p.a. (Merck). At pH 5, the predominant phosphate species is H2PO4– (99%). Triplicate 0.625-g samples of pure and PGA-coated goethites (moist or freeze-dried) were weighed into 2-L HD-PE bottles (Nalgene Nunc, Rochester, NY), which were coated with Al-foil to exclude light. Then 250 mL of background electrolyte with pH 5 were added, and the bottles were shaken on a reciprocating shaker at 150 rev min–1 for 1 h to facilitate dispersion and hydration. Afterward, 1 L background electrolyte solution (pH 5) containing 500 µM phosphate was added to achieve a phosphate concentration of 400 µM and a solid concentration of 0.5 g L–1. Additionally, 50 µL of 0.1 M AgNO3 solution was added to reduce microbial activity. The bottles were rotary shaken at 20 rev min–1 and at 298 ± 2 K. The pH was maintained manually at 5 ± 0.2 using dilute HNO3 or KOH. After 0.5, 1, 2, 4, 8, 24, 48, 168, 336, and 504 h, a 10-mL aliquot was removed, 0.45-µm membrane-filtered (polyethersulfone, Supor-450; Pall Life Science, Dreieich, Germany), and total organic C was measured in the filtrate. A 2.5-mL aliquot of the 0.45-µm filtrate was ultracentrifuged at 440 000 x g for 1 h, and phosphate was measured photometrically in the supernatant by the ascorbic-molybdenum blue method of Murphy and Riley (1962) at 710 nm. The amount of phosphate sorbed was calculated from its loss in solution. Adsorption of phosphate on container walls could be ruled out by checking blank solutions for dissolved phosphate. The analytical precision of the photometric determination of phosphate was <1%. Subsample variability was generally <1.5%. Preliminary tests showed that matrix interferences of phosphate with polyvalent cations bound in the PGA structure did not occur during ultracentrifugation (i.e., phosphate concentrations in solution did not decrease due to sedimentation of PGA during ultracentrifugation).

After sampling, the 0.45-µm filter residue was washed with 20 mL doubly deionized water, freeze-dried, and stored in the dark in a desiccator until use for electrophoretic mobility measurements. The amount of phosphate sorbed was corrected for the water content of the samples (13 ± 1 wt%), which was determined by outgassing the samples in an Autosorb-1 gas sorption system (Quantachrome, Syosset, NY) until the rate of pressure increase by vapor evolution was below about 1.3 Pa min–1 within a 0.5-min test interval. Due to possible damage to PGA coatings, outgassing was not performed at elevated temperatures.

The phosphate sorption data were fitted with a linear combination of a modified first-order rate equation and the parabolic rate law (Crank, 1976) to account for the fast and the slow sorption of phosphate to goethite, respectively (Lang and Kaupenjohann, 2003):

Formula 1[1]
where qt is the amount of phosphate sorbed at time t (µmol g–1), cm is the maximum amount of phosphate sorbed by the fast reaction (µmol g–1), (cm – a0) is the amount of phosphate operationally defined as "sorbed instantaneously" (faster than could be quantified by the batch approach, µmol g–1), k is the rate constant of the initial fast phosphate sorption (h–1), t is time (h), and b is the apparent rate constant of the slow sorption (µmol g–1 h–0.5). The parameters cm, a0, k, and b were determined by minimizing the sum of the squared differences between the observed and predicted values of the phosphate sorption data using the Marquardt-Levenberg algorithm implemented in SigmaPlot for Windows (SPSS Inc.). In most cases, parameters were significant at the P = 0.05 level, which was tested with the t-statistics implemented in SigmaPlot.

The rate constant of the slow phosphate sorption, b, is related to the apparent diffusion constant (D/r2)app (h–1) (Lang and Kaupenjohann, 2003):

Formula 2[2]
where q{infty} is the amount of phosphate diffused at infinite time (µmol g–1), D is the apparent diffusion coefficient (m2 h–1), and r is the radius of diffusion (m). Unlike Lang and Kaupenjohann (2003), we accounted for cylindrical pore geometry by using a factor of 4 instead of 6 in Eq. [2]. We used the total amount of phosphate present at t = 0 h (µmol g–1) corrected for the total amount of phosphate sorbed to external surfaces (cm) as an approximation for q{infty} in Eq. [2] to calculate the apparent diffusion constant (D/r2)app. This calculation accounts for differing phosphate concentration gradients in the samples after the fast sorption of phosphate to external goethite surfaces but may lead to a systematic underestimation of (D/r2)app.

Surface Area and Porosity Measurements
Specific surface area (SSA) and pore volume were determined with a Quantachrome Autosorb-1 automated gas sorption system (Quantachrome, Syosset, NY) using N2 as an adsorbate. Approximately 80 mg of pure and PGA-coated goethite were degassed until the rate of pressure increase by vapor evolution was below about 1.3 Pa min–1 within a 0.5-min test interval. Helium was used as a backfill gas. We analyzed N2 adsorption and desorption at 79 points in the partial pressure range 1.0 x 10–5 – 0.995 P/P0. Specific surface area was calculated from the BET equation (Brunauer et al., 1938).

Micropore (<2 nm) volume and average micropore diameter were determined according to the Dubinin-Radushkevic method (Gregg and Sing, 1982). The mesopore (2–50 nm) size distribution was calculated on the desorption leg using the BJH method (Barrett et al., 1951). Separation between small (2–5 nm), medium (5–10 nm), and large mesopores (10–50 nm) was achieved by linear interpolation of the BJH desorption data. Total pore volume was taken at 0.995 P/P0, and the average pore diameter was calculated as Dp = 4Vliq/SSA, where Vliq is the volume of liquid N2 contained in pores at 0.995 P/P0, and SSA is the BET surface area. We also performed 16-point CO2 adsorption measurements from 1.0 x 10–3 to 3.0 x 10–2 P/P0 at 273 K to obtain the CO2 micropore volume and average micropore diameter according to the Dubinin-Radushkevic method (Gregg and Sing, 1982). All isotherms were recorded in triplicate.

Electrophoretic Mobility Measurements
The electrophoretic mobility was determined at the start of the phosphate sorption experiment and over the entire phosphate sorption run. After each reaction time, about 200 µg of freeze-dried 0.45-µm filter residue were resuspended into 4 mL of 0.01 M KNO3 at pH 5. To facilitate sample handling, we used dried solids for electrophoretic mobility measurements. Preliminary tests revealed that during phosphate sorption for 1 wk, electrophoretic mobilities of pure and PGA-coated goethites in aqueous suspensions (0.01 M KNO3, pH 5) did not significantly differ from those obtained from samples that were freeze-dried after 0.45-µm membrane filtration and resuspended in background electrolyte for electrophoretic measurements (t-test, P < 0.05). The electrophoretic mobility was determined at 298 K with a Zetasizer 2000 (Malvern Instruments). Before the measurements, the accuracy of the measurements was checked with a transfer standard, which is referenced to the NIST goethite standard SRM1980 (Malvern Instruments). Ten measurements were performed, and the average value was recorded. The {zeta}-potential was calculated from the electrophoretic mobility using the Smoluchowski equation (Hunter, 1988).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Effects of Hydrothermal Treatment on Goethite Properties
Powder X-ray diffraction analysis of G1 goethite showed typical reflexes of goethite without any detectable contamination. In addition, differential X-ray analysis after oxalate treatment according to Schwertmann (1964) did not indicate the presence of ferrihydrite. Powder X-ray diffraction analysis of G2 goethite showed that traces of hematite appeared after hydrothermal treatment of G1. The [hematite/(hematite + goethite)] X-ray diffraction intensity ratio calculated from the ratio of areas under the 110 reflection of goethite and the 102 reflection of hematite according to Ruan and Gilkes (1995) was 0.05. Scanning electron microscope images obtained on a Hitachi S-4000 microscope at high resolution (x150 000) showed no visible difference in the crystal habit between G1 and G2 (not shown).

Potentiometric titrations of the goethites indicated that at the pH chosen for this study, their {zeta}-potentials were essentially identical (~ 30 mV). However, above pH 5, the {zeta}-potential of G2 was ~ 5 mV lower than that of G1. Hence, a slight decrease in the isoelectric point (pHiep) from 7.6 to 7.2 was noticed after hydrothermal treatment of G1. The pHiep of G1 was within the range of pHieps and points of zero charge reported for goethites (Kosmulski et al., 2003). The shift in the pHiep of G2 might be due to the presence of traces of hematite because published points of zero charge of synthetic hematites are on average lower than those of goethites (Kosmulski et al., 2003).

Hydrothermal treatment of G1 mainly affected pores < 10 nm. The N2 micropore volume decreased by 33%, and the mesopore volumes of <10-nm pores decreased by up to 46% (Table 1). The loss in micro- and mesoporosity was accompanied by a considerable drop in SSA (31%). In addition, the average pore size increased by 34% (Table 1). Micropore volumes of pure goethite samples determined with CO2 adsorption at 273 K were about 30% higher than micropore volumes determined with N2 adsorption at 77 K. De Jonge and Mittelmeijer-Hazeleger (1996) showed that CO2 is capable of penetrating into pores of soil organic matter < 0.5 nm at 273 K, whereas pores of this size remain inaccessible to N2 at 77 K. Therefore, it might be concluded that in both goethite samples approximately one fourth of micropores have diameters < 0.5 nm.


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  		Table 1. Carbon loadings of freeze-dried goethite samples, their specific surface area, and porosity properties obtained from N2 and CO2 adsorption measurements.

 
Porosity and Surface Area Changes on PGA Sorption
Carbon loadings of the goethites are presented in Table 1. At low C loadings, no or only tiny porosity and SSA changes were noticed for both goethites, with the total pore volume being reduced most effectively (Table 1). At higher C loadings, however, SSA and micropore volume decreased significantly. The effect was stronger for the more microporous G1 compared with its less porous analog. Although average micropore diameters remained constant at low C loadings, they increased at high C loadings independent of the adsorbate used (Table 1). Contrary to the stronger decrease in micropore volumes for G1 than for G2, the relative reduction in 5- to 10-nm pore volume was five-fold greater for G2 (Table 1). The decrease in pore volume of < 10-nm pores with increasing C loading suggests a preferential sorption of PGA in or at these small pores. A pore filling mechanism by organic matter has been advocated by several researchers (Kaiser and Guggenberger, 2003; Mikutta et al., 2004; Zimmerman et al., 2004).

Controls of the Slow Phosphate Sorption in PGA-Coated Samples
The phosphate sorption kinetics of freeze-dried and nondried PGA-coated goethites are shown in Fig. 1 . Equilibrium was not reached within 3 wk in all samples. Increasing amounts of sorbed PGA decreased the total amount of phosphate being rapidly immobilized (Table 2, cm), indicating sorption competition of PGA and phosphate at external goethite surfaces. In all PGA-coated samples with high C loading, the rate constant of the slow phosphate sorption was higher compared with the C-free control treatment, irrespective of whether the samples were predried (Table 2b). Can these high rate constants of the slow phosphate sorption be ascribed to micropore clogging by PGA? Micropores being not detectable by CO2 at 273 K are likely not accessible to phosphate because of the smaller molecular size of CO2 as compared with phosphate (0.28 vs. 0.45 nm). Hence, a decreased accessibility of CO2 to micropores due to PGA sorption in goethite pores should be reflected in a decreased accessibility of micropores to phosphate. Accordingly, if microporosity limits the rate of the slow phosphate sorption to PGA-coated goethites, one would expect decreased (D/r2)app values with decreasing microporosity (i.e., with increasing diffusion resistance for phosphate ions). Values presented in Fig. 2 are inconsistent with this idea because (i) (D/r2)app values of PGA-coated goethites with the lowest CO2 micropore volume were higher than values for uncoated goethites, and (ii) (D/r2)app values were independent of the CO2 micropore volume of PGA-coated G2 samples (Fig. 2). In contrast to our initial reasoning, higher (D/r2)app values for PGA-coated than for pure goethites might be explained by a preferential clogging of small pores by PGA because phosphate diffusion would then be confined to the remaining larger pores. As a consequence, equilibrium would be reached faster in samples with high C loading than in C-free samples because the diffusion of phosphate into pores occupied by PGA would be impaired. This reasoning, however, disagrees with Fig. 1, which shows that at high C loadings, phosphate sorption proceeded at a rate similar to or higher than in the C-free controls. Therefore, we conclude that in PGA-coated goethite samples, micropore diffusion of phosphate does not control the slow phosphate sorption.


Figure 1
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  		Fig. 1. Phosphate sorption kinetics of freeze-dried and nondried pure and PGA-coated goethites. Solid lines show the predicted values using the combined model of Eq. [1]. Values in parentheses refer to the initial C content in mmol C g–1.

 

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  		Table 2. Kinetic parameters obtained by fitting the combined model to the phosphate sorption data of freeze-dried and nondried pure and polygalacturonate (PGA)-coated goethites.{dagger}

 

Figure 2
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  		Fig. 2. Apparent diffusion constants (D/r2)app of freeze-dried pure and PGA-coated goethites versus the CO2 micropore volume present before phosphate sorption. Bidirectional error bars indicate SE. Values in parentheses indicate the C content in mmol C g–1.

 
Up to 87% of C was displaced by phosphate within 3 wk, showing the high competitiveness of phosphate (Table 2). Similar to the phosphate sorption kinetics, the C desorption kinetics was biphasic; an initial rapid C desorption was followed by a slow C desorption (Fig. 3 ). Approximately 50% of the total desorbed C was desorbed after 24 h (Fig. 3). The increasing molar ratios of phosphate sorbed and PGA-C desorbed with increasing time (Fig. 4 ) indicate that phosphate was more effective in triggering PGA desorption at longer sorption times by direct ligand-exchange or by decreasing the surface potential of PGA-coated goethites. Off-sets in molar Cdes/Psorb ratios between freeze-dried and nondried samples are due primarily to higher C loadings of nondried goethites (Table 2). The plateaus in Fig. 4 that were reached after about 1 wk indicate that every phosphate desorbed on average two thirds of a carboxyl group when we assume that the amount of esterified carboxyls is low (one carboxyl-C per six C atoms in the structure of PGA).


Figure 3
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  		Fig. 3. Polygalacturonate-C desorption from goethites during phosphate sorption for 3 wk. Solid lines indicate the fit of Eq. [1] to the C desorption data of goethites with high C loadings. Coefficients of determination were >0.97. Average SE of total organic C measurements was 27 µmol g–1; the maximum SE recorded was 78 µmol g–1 (n = 80).

 

Figure 4
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  		Fig. 4. Changes in the molar ratio of PGA-C desorbed and phosphate sorbed of freeze-dried and nondried PGA-coated goethites with high C loadings during phosphate sorption for 3 wk. The mean SE of the molar Cdes/Psorb ratios was 0.2. The x axis is in logarithmic scale.

 
As a consequence of the ion exchange at the goethite surface, the {zeta}-potential of PGA-coated goethites remained relatively constant during the phosphate sorption run (Fig. 5 ). The {zeta}-potential of pure goethites increased again after about 24 h of phosphate sorption (Fig. 5). This observation was reported before (Ler and Stanforth, 2003; Mikutta et al., 2006) and is explained by the formation of ternary phosphate surface complexes (Ler and Stanforth, 2003).


Figure 5
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  		Fig. 5. {zeta}-Potential changes during phosphate sorption of freeze-dried uncoated and PGA-coated goethites at the highest PGA-level. The solid lines show linear regressions. Error bars are SE. Values in parentheses represent the C loading in mmol C g–1. Initial {zeta}-potentials (mV) at pH 5 in 0.01 M KNO3 were: G1: 29.8 ± 3.5, G2: 29.1 ± 0.5, G1 (1.76): –29.0 ± 3.6, G2 (1.43): –28.5 ± 1.2. The x axis is in logarithmic scale.

 
Figure 6 shows the relationship between the amount of phosphate sorbed and C desorbed. Although these relations were not strictly linear, we fitted the data with a linear function to obtain information on the average desorbability of PGA by phosphate. Slopes of nondried samples with low C loadings were not statistically different from zero at the 0.05 probability level and are therefore not presented in Fig. 6b. The slope of regressions presented in Fig. 6 can be taken as a measure of the average competitiveness of phosphate with presorbed PGA. Accordingly, in freeze-dried samples with low C loading, phosphate was less able to displace PGA from the more micro- and mesoporous G1 than from G2 (Fig. 6a). On the contrary, at higher C loadings, the reduced desorbability of PGA by phosphate in the more nanoporous G1 samples diminished (Fig. 6a). In addition, Fig. 6 indicates that the desorbability of C by phosphate was larger at high compared with low C loadings, indicating that at higher C loadings, polymers were less intimately associated with mineral surfaces (Theng, 1979; Kaiser and Guggenberger, 2003; Saito et al., 2004).


Figure 6
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  		Fig. 6. Plots of phosphate sorbed versus PGA-C desorbed for (a) freeze-dried and (b) nondried PGA-coated goethites. Values in parentheses refer to the amount of PGA-C initially present in the samples in mmol C g–1.

 
In summary, we found that (i) the apparent diffusion constant of PGA-coated samples was independent of the CO2 micropore volume and (ii) the C desorption showed a kinetic pattern similar to the phosphate sorption. These findings imply that the slow gradual desorption of PGA and/or the transport of phosphate to external goethite surfaces controlled the slow phosphate sorption to PGA-coated goethites. The ability of phosphate to diffuse through PGA networks at pH ≤ 4.5 and a phosphate concentration of 150 µM has recently been demonstrated by Gessa et al. (2005).

Effects of Drying on the Phosphate Sorption Kinetics
The rate constant b of the slow phosphate sorption to C-free, freeze-dried G2 was significantly lower than for freeze-dried G1 (Table 2). This finding agrees with the diffusion of phosphate into pores of Fe oxides (Torrent, 1991; Barrow et al., 1993; Strauss et al., 1997; Makris et al., 2004) because the strong reduction in the pore volume of <10-nm pores on hydrothermal treatment of G1 has rendered fewer pores accessible to phosphate in G2 samples (Table 1). In contrast, we found equal rate constants for pure G1 and G2 in nondried systems (Table 2). Also, similar apparent diffusion constants, (D/r2)app, for nondried G1 and G2 samples indicate a similar diffusion resistance for phosphate in both samples (Table 2). It seems that freeze-drying has induced an aggregation of G2, which partly explains its loss in micro- and mesoporosity. The aggregation of G2 on freeze-drying probably led to an occlusion of mineral surfaces that were accessible to neither N2 and CO2 nor phosphate. In nondried systems, however, G2 samples were shaken in background electrolyte for 72 h before phosphate addition. This treatment likely caused a sufficient redispersion of G2 and hence a similar slow phosphate sorption in G1 and G2 samples (Table 2). Therefore, the observed decrease in the apparent diffusion constant of dried G2 samples with respect to dried G1 (Table 2) was most probably caused by a reduced intra-aggregate diffusion.

Freeze-drying PGA-coated goethites altered the phosphate sorption pattern especially at times <10 h (Fig. 1). The amount of phosphate instantaneously sorbed (Table 2, cm – a0) increased significantly after freeze-drying samples with low C content, which implies that the coatings impaired the sorption of phosphate to external surfaces less effectively than in nondried samples (Fig. 1).

At high C loading, the sorption kinetics of nondried G1 were similar to its freeze-dried counterpart (Table 2). On the contrary, the rate constant of the slow phosphate sorption to nondried G2 at high C loading increased anomalously (Table 2). The reason for this observation is unclear; possible explanations may include discontinuous PGA desorption and particle disaggregation.

Freeze-drying also changed the average desorbability of PGA by phosphate, as indicated by the slopes in Fig. 6. At low C loadings, PGA was more prone to desorption by phosphate in freeze-dried compared with nondried samples (Fig. 6). For example, 45% PGA-C were less desorbed within 3 wk in nondried compared with freeze-dried G2 samples (Table 2). At higher C loadings, freeze-drying only increased significantly the average desorbability of PGA by phosphate in G2 samples (Fig. 6). These results suggest that freeze-drying PGA-coated goethites alters the ability of phosphate to displace presorbed PGA. This finding may be attributed to physicochemical changes in the structure of sorbed PGA due to dehydration/hydration processes, which have been reported for pure organics, including proteins and polysaccharides (Wedlock et al., 1983; Jouppila and Roos, 1997; Allison et al., 1998; Souillac et al., 2002) and PGA coatings on {gamma}-AlOOH (Mikutta et al., 2004).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our results show that micropores of PGA-coated goethite do not significantly contribute to the slow and continuous phosphate sorption. Instead, sorption competition and/or the diffusion of phosphate through PGA coatings controlled the slow phosphate sorption to PGA-coated goethite. With increasing < 10-nm porosity, the ability of phosphate to displace PGA decreased for freeze-dried goethites with low C loading (0.30 and 0.37 mmol C g–1). However, the stabilization of PGA against desorption by phosphate exerted by nanoporous surfaces diminished at higher C loadings (1.43 and 1.76 mmol C g–1). In freeze-dried samples, PGA was less easily desorbed by phosphate at low C loadings compared with high C loadings, indicating a stronger attachment of PGA to goethites at low C loadings. Microaggregation of goethite on freeze-drying can affect the slow phosphate sorption. In addition, freeze-drying C-coated goethites can change the competition between phosphate and presorbed organic matter. Thus, freeze-drying may lead to errors in the interpretation of sorption studies when only freeze-dried pure and organic matter–coated Fe oxides are used.


   ACKNOWLEDGMENTS
 
We appreciate the support of Klaus Kaiser, Robert Mikutta, and Peter Dominik. This study was funded by the German Research Fund (DFG, KA 1139/8).

Received for publication October 8, 2005.


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





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