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Published in Soil Sci. Soc. Am. J. 68:1853-1862 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

Division S-2—Soil Chemistry

Soil Organic Matter Clogs Mineral Pores

Evidence from 1H-NMR and N2 Adsorption

Christian Mikutta*, Friederike Lang and Martin Kaupenjohann

Dep. of Soil Science, Institute of Ecology, Berlin Univ. of Technology, Salzufer 11-12, D-10587 Berlin, Germany

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


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Recent N2 adsorption studies have suggested a ‘pore clogging’ effect on mineral soil phases caused by organic matter coatings. For methodological reasons, this pore clogging effect has been studied only after drying. Our hypothesis was that pore clogging is affected by drying of organic coatings. In our study, we used AlOOH, which has been equilibrated with dissolved organic matter (DOM) and polygalacturonic acid [PGA; (C6H8O6)n]. To test our hypothesis, we determined the porosity of moist and freeze-dried AlOOH samples. Freeze-dried samples were analyzed by N2 adsorption, moist samples by 1H-nuclear magnetic resonance (NMR). In addition, the samples were characterized by environmental scanning electron microscopy—energy dispersive x-ray spectroscopy (ESEM-EDX). Both, DOM and PGA significantly reduced specific surface area (SSABET) of AlOOH by 34 m2 g–1 (15%) and 77 m2 g–1 (36%). The reduction in SSABET normalized to the amount of C sorbed was 1.0 m2 mg–1 DOM-C and 5.9 m2 mg–1 PGA-C. Dissolved OM reduced the pore volume of micro- and small mesopores <3 nm whereas PGA also reduced the volume of larger pores. The 1H-NMR results of moist samples showed that PGA sorption reduced the amount of water in pores <4 nm. In addition, the pore size maximum of AlOOH increased by 150%. Polygalacturonic acid coatings created new interparticle pores of about 10- to 70-nm size that are not stable upon freeze-drying. Porosity changes upon DOM-treatment were not commensurable by 1H-NMR. Our results indicate that clogging of micro- and small mesopores is not an artifact of freeze-drying. Polygalacturonic acid seems not only to cover the mouth of AlOOH-nanometer pores but also to fill them.

Abbreviations: BET, Brunauer, Emmett, and Teller • DOM, dissolved organic matter • ESEM-EDX, environmental scanning electron microscopy–energy dispersive x-ray spectroscopy • OM, organic matter • PGA, polygalacturonic acid • SSABET, specific surface area determined after Brunauer, Emmett, and Teller (1938a) • SSAMP, micropore surface area • TOC, total organic carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL MINERALS have been shown to be covered by organic matter (Courchesne et al., 1996; Yuan et al., 1998). Organic coatings may consist of humic substances (Hedges and Keil, 1995) or of polysugars (Schmidt et al., 2000; Wattel-Koekkoek et al., 2001).

Several authors have reported that adding organic C will reduce micropore (<2 nm) surface area and volume of soils and well-crystallized soil minerals (Lang and Kaupenjohann, 2002; Kaiser and Guggenberger, 2003). Eliminating organic C, on the other hand, increased surface area (Pennell et al., 1995; Mayer and Xing, 2001). However, little information is available on surface area and porosity changes induced by OM coatings on poorly crystalline Al- and Fe phases (Kaiser and Guggenberger, 2003). In addition, differences in the ability to change porosity and surface area of mineral phases between coatings, which consist of different types of organic matter, have not been assessed.

Organic matter coatings might affect the diffusion of anions into the pores of Fe oxides (Gaume et al., 2000; Grimal et al., 2001). From N2 adsorption studies Lang and Kaupenjohann (2002) concluded that OM coatings decrease the accessibility of intraparticle pores, thus reducing molybdate immobilization by Fe oxides. We found that the mesopore volume (10–50 nm) was reduced by a factor of 2 at high organic C loadings (0.77 mg C m–2). At lower loadings (0.12 mg C m–2) no effect was measured using N2 adsorption, albeit there was a marked decrease in molybdate diffusion. Since the structure of soil OM is not rigid, it can be argued that OM coatings are affected by hydration and dehydration (de Jonge and Mittelmeijer-Hazeleger, 1996). Shrinking of thin OM coatings along with freeze-drying might cause a reduction of coverage leaving these pores accessible to N2 diffusion at 77 K. On the other hand OM coatings might become more dense upon freeze-drying, which might increase the pore clogging.

Thus the objective of our study was to investigate the effect of drying on the clogging of mineral pores by organic matter. We used 1H-NMR logging for the determination of the porosity of moist samples and N2 adsorption to determine the porosity of freeze-dried samples. We compared the efficiency of PGA with DOM to plug the pores of a microporous hydrous Al oxide. We used PGA as a model substance for macromolecular pectin-like polysaccharide associations (mucilages) of the rhizosphere (Grimal et al., 2001; Gessa and Deiana, 1992) and DOM extracted from a O horizon of a forest floor soil sample (Haplorthod) to simulate OM coatings of the bulk soil.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Hydrous Aluminum Oxide
Hydrous Al oxide was synthesized according to Goldberg et al. (2001). Aluminum chloride hexahydrate (0.408 M) was reacted for 15 min with 1.088 M NaOH at a molar AlCl3/NaOH ratio of 0.375. The precipitate was then washed 16 times with doubly deionized water until electric conductivity was below 37 dS m–1 (pH 4.7), frozen with liquid N2 and freeze-dried at 476.0 Pa vacuum (Christ, {alpha} 2-4 freeze drier, Osterode, Germany). Powder x-ray diffraction patterns of the samples were obtained using a Siemens-D 5005 instrument (Siemens AG, Germany) (40 kV, 30 mA) with CuK{alpha}–radiation of wavelength 0.15406 nm. Measurement ranged from 4 to 70° (2{theta}), step size was 0.05° (2{theta}) and step time was 10 s.

Dissolved Organic Carbon and Polygalacturonic Acid
Dissolved organic matter solution was obtained by mixing 1 kg field-moist organic forest floor sample from the O horizon of a Haplorthod with 2 L doubly deionized water for 24 h. To extract sufficient C, which is needed to cover microporous materials, the pH of the suspension was adjusted to 7.0 using 0.1 M NaOH. The suspension was stirred periodically, centrifuged at 20000 x g for 20 min and successively filtered through a cellulose filter, which was rinsed before use with doubly deionized water and a 0.45-µm cellulose acetate membrane filter, respectively. Contamination by cellulose-C was <0.01% of the total C content measured. The concentration of C in the filtrate was (1064 ± 41) mg C L–1. Dissolved C measurements were performed by a Shimadzu TOC-5050A Autoanalyzer (Shimadzu Corp., Tokyo, Japan).

Polygalacturonic acid with a purity of 86% (dry basis) was purchased from Sigma. The molecular weight approximates 4000 to 6000 g mol–1 (Aldrich). The C content was (374 ± 4) mg g–1 on a dry matter basis measured with a Carlo Erba C/N NA 1500N Analyzer. 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). Polygalacturonic acid was dispersed in doubly deionized water with 7.5 µL of 1 M NaOH mg–1 PGA. The organic C concentration of PGA solution was (956 ± 11) mg L–1. The pH values of the PGA and DOM solutions were adjusted to 4.7 with 0.1 M NaOH and 0.1 M HNO3 before sorption.

Sorption of Dissolved Organic Matter and Polygalacturonic Acid
One gram of freeze-dried hydrous Al oxide was equilibrated with 40 mL DOM (1064 mg C L–1) and PGA solution (956 mg C L–1) in a 50-mL polypropylene tube for 24 h on a rotary shaker at 8 cycles min–1 and at 298 K in the dark. The pH of the supernatants was 5.5 (AlOOH/DOM) and 6.0 (AlOOH/PGA). The DOM-AlOOH suspension was filtered through a 0.45-µm cellulose acetate filter, which was rinsed with doubly deionized water before C measurement. Because the PGA-AlOOH suspension could not be filtered, a 10-mL aliquot was centrifuged at 10000 x g for 30 min, and total organic C (TOC) measured in the supernatant. Amounts of DOM and PGA sorbed were calculated from the difference between initial concentration and concentration in the supernatant. One part of the hydrous Al oxides, pretreated in different ways (DOM, PGA, no C), was freeze-dried at 476.0 Pa, the other part was stored moist at 281 K before 1H-NMR and ESEM experiments. All analyses were done in triplicate or quadruplicate.

ESEM-EDX and SEM Analysis
We analyzed the surfaces of the samples and surface changes induced by drying using a Quanta 200 Scanning Electron Microscope (FEI, Eindhoven, the Netherlands) in the environmental scanning electron microscopy (ESEM) mode with a gaseous scanning electron detector (GSED). For studying the effects of drying, we reduced the relative pressure from 860 Pa (>96% relative humidity) down to 100 Pa (approximately 1% relative humidity). Additional drying was achieved in the low vacuum mode at 30 Pa using a large field detector (LFD) and in the high vacuum mode at <3.1 x 10–2 Pa for which a secondary electron detector (SED) was operated. The elemental composition of the particles was estimated using an energy dispersive x-ray (EDX) detector. Data reduction was performed with EDX control and mapping software version 1.0 (EDAX Inc., Mahwah, NJ). Additionally, SEM images were recorded of Au-sputtered freeze-dried DOM-coated and pure hydrous Al oxide samples.

Surface Area and Fractional Organic Matter Coverage
Specific surface area (SSABET) was determined with a Quantachrome Autosorb-1-C Automated Gas Sorption System (Quantachrome, Syosset, NY) using N2 as an adsorbate. A mass of about 0.1500 g sample was degassed until the pressure increase due to vapor was well below 7 Pa min–1 within an 1-min test interval. Helium was used as a backfill gas. We used multipoint Brunauer-Emmett-Teller (BET) N2 adsorption isotherms from 0.01 to 0.995 P/P0. For multipoint BET surface of microporous materials only linear portions of the middle partial pressure region 0.01 through 0.30 P/P0 were used. The BET equation (Gregg and Sing, 1982) gives the number of moles, n, adsorbed on 1 g of adsorbent at a special partial pressure:

[1]
where nm is the calculated number of moles adsorbed as a monolayer on 1 g of adsorbent, P is the gas pressure of N2, P0 is the ambient pressure, and C is the BET constant related to the enthalpy of gas adsorption. The parameters nm and C were determined from Eq.[1], and the specific surface area was calculated by multiplying nm by the cross-sectional area of N2 molecules (16.2 x 10–20 m2).

We used two alternate approaches to calculate OM coverages of minerals. First, the fractional OM coverage of the Al oxide, fcov-a (m2 m–2), was calculated using the approach of Mayer (1999). The model relates the N2 gas adsorption energetics to the fraction of surface coated with organic model substances. The algorithm uses the fact that physisorption of N2 gas involves higher enthalpies onto naked than onto organically coated oxides surfaces:

[2]
where {Delta}Hnaked and {Delta}Hcoated (kJ mol–1) is the enthalpy of gas adsorption of pure and of coated minerals, respectively, {Delta}Hxs is the difference between {Delta}Hads, which is the adsorption enthalpy of the gas directly on the surface and {Delta}Hcond, which is the enthalpy of gas condensation considered to be equivalent to multiplayer adsorption (Steele, 1974). The enthalpy of N2 adsorption was calculated on the C constant of the BET transform (Brunauer et al., 1938a):

[3]

Here M is the preexponential term assumed to equal 1 (Gregg and Sing, 1982), R is the gas constant, and T is the absolute temperature. Equation [2] was derived for non-microporous adsorbents. Because micropores can affect N2 adsorption energetics (Mayer and Xing, 2001) we might overestimate fcov-a.

Second, we calculated the fraction of total mineral surface covered by organic matter, fcov-b, by the relation

[4]
where SSABET/naked and SSABET/coated is the BET surface area of uncoated and coated Al oxides, respectively (Mayer and Xing, 2001). Equation [4] assumes that the difference in SSABET between coated and uncoated samples represents surface area that is occluded by OM, that is, inaccessible to N2 diffusion at 77 K(Mayer and Xing, 2001).

Analysis of Pores
According to the International Union of Pure and Applied Chemistry (Rouquerol et al., 1994), pores can be designated as micropores (0–2 nm), mesopores (2–50 nm), and macropores (>50 nm). Micropore analysis was done according to the t-V method of de Boer et al. (1966). In addition, we used the {alpha}S–plot method (Sing, 1970). The micropore surface area, SSAMP, is computed as the difference between the BET surface area and the external surface area from the t-plot:

[5]

Pore size of mesopores were computed using the BJH method (Barrett et al., 1951). The average pore size, Dp, was calculated from

[6]
where Vliq is the liquid volume N2 contained in the pores at 0.995 P/P0 and SSABET is the BET surface area. Since the hysteresis of the hydrous Al oxide falls within the type E category according to de Boer (1958), typical of "ink-bottle" pores, porosity was calculated on the adsorption branch. Total pore volume was taken at 0.995 P/Po To overcome the influence of aging recorded for metastable Al hydroxide phases (Dandashli et al., 2002; see micropore surface area in Table 2), N2 adsorption and 1H-NMR studies of OM-covered and OM-free replicates were performed within a 14-d period. Isotherms of OM-covered samples and respective references were recorded in quadruplicate and triplicate.


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  		Table 2. Brunauer, Emmett, and Teller (BET) surface area, micropore area, pore volume, and average pore size of pure, DOM-treated and PGA-treated AlOOH. The number of measurements was n = 4 for PGA- and DOM-treated AlOOH, n = 3 for AlOOH, n = 2 for PGA.

 
1H-Nuclear Magnetic Resonance Spectroscopy Measurements
We used NMR spectroscopy to determine the pore-size distribution of moist samples. 1H-NMR logging is routinely applied in petrophysics to obtain oil and gas reservoir properties of geologic formations (Kenyon et al., 1989; Kleinberg, 1996; Kenyon, 1997) to identify different stages of water binding and porosity domains in soils and aquifer materials (Hinedi et al., 1997, 1993; Culligan et al., 2001) and to estimate rock permeabilities (Kenyon et al., 1988; Bilardo et al., 1991). It involves the measurement of the transversal (T2) or longitudinal relaxation times (T1) of water derived H atoms. Because of interactions with pore walls, H atoms of water being in pores relax much faster than H atoms of bulk water. Thus, 1H-NMR T2 relaxation time distributions of porous media are directly related to pore size.

Hydrous Al oxide samples were saturated to field capacity with doubly deionized water in 50-mL polypropylene tubes for 48 h and analyzed by 1H-NMR. After equilibration of AlOOH with PGA and DOM solutions as described above the supernatant was decanted and the saturated sediment analyzed. The water content of the samples was about 60% (w/w). Transversal relaxation measurements were performed on a Maran Ultra 2 MHz NMR spectrometer (Resonance Instruments Ltd., UK) at 288 K. The magnetic flux density was 0.047 T, and the proton Larmor frequency was 2 MHz. We used the CPMG (90°-{tau}-180°) pulse sequence with 4096 recorded echoes, an 150-µs echo spacing {tau} and a 1.2-s delay time to obtain the transversal relaxation time constant T2.

For transverse relaxation T2 three mechanisms contribute

[7]
where T2s is surface relaxation, T2b is bulk relaxation, and T2DG is the relaxation because of diffusion in magnetic gradient (Kenyon, 1997). While T2 and T2b are measured, T2DG is assumed to be negligible. For proton Larmor frequencies <5 MHz and for Carr-Purcell echo spacings, {tau}, less than about 1 ms, the enhancement in T2 decay coming from diffusion in the inhomogeneous local fields is negligible compared with the surface relaxation mechanism (Kleinberg and Horsfield, 1990). Thus, T2s can be calculated from Eq. [7]. Pore sizes can be derived from the relaxation times: Provided that diffusion of proton spins is much faster than the surface relaxation (‘fast diffusion limit’, Brownstein and Tarr, 1979) the surface relaxation in a pore can be described by a single exponential with a relaxation time constant T2s given by

[8]
where m is the shape factor, which is 4 assuming cylindrical pore geometry (Hinedi et al., 1997), Dp is the pore size, {rho}2 is the transversal surface relaxivity that parameterizes the strength of the surface relaxation and has dimensions of length/time, SAp is the surface area, and Vp is the volume of a pore (Loren and Robinson, 1970; Brownstein and Tarr, 1979; Kleinberg, 1996).

The exponential decay of the net magnetization M(t) of H nuclei in a pore is governed by a mean time constant 1/T2 of transversal magnetization decay:

[9]
where M(t) and M0 is the magnetization at time t and t0, respectively. The pore-size distribution corresponds to the distribution of T2 values (Kenyon, 1997). Since there is not a single typical pore size but rather a distribution, the wall relaxation Ms(t) is then the sum of the decaying signals for all pore sizes:

[10]
where Ai is the number of protons in pores of the ith size, and Ti is the corresponding decay time constant of transverse relaxation.

We used the Tikhonov regularization (Tikhonov and Arsenin, 1977) implemented in WinDXP software package (Resonance Instruments Ltd., UK) to compute robust T2 distributions. The T2 time constants were obtained by fitting exponentials to the net magnetization decay curves using WinFit software (Resonance Instruments Ltd., UK). The relaxivity parameter {rho}2 was derived from Eq. [8] as was proposed by Kleinberg (1996)(p.765) where the fluid imbedded was set as Vp and SAp is the BET surface area. Based on {rho}2, the pore sizes were calculated. All analyses were done in quadruplicate for DOM- and PGA-coated AlOOH, and triplicate for uncoated AlOOH.

Statistics and Error Analysis
The experiment was arranged in a complete-block design with two variants (freeze-dried vs. moist) at three treatments (PGA, DOM, no C). For comparisons, we applied the unpaired t-test. Statistical analyses were performed using SigmaPlot version 7.0 (SPSS Inc.). We used the Q-test to determine possible outlier in the data (Miller and Miller, 1988). Errors are presented as standard deviation and treated according to the rules of error propagation.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
X-Ray Diffraction Analysis
The synthesized material showed five broadened x-ray diffraction peaks at 14.05, 28.05, 38.80, 49.50, and 65.35° (2{theta}) indicative of poorly crystalline boehmite/pseudoboehmite (Berry, 1974; Fig. 1) . Peak broadening is caused by appreciable amounts of bound H2O and a small crystal size (Hsu and Bates, 1964; Tettenhorst and Hofmann, 1980; Okada et al., 2002). There is no evidence of phase transformation of AlOOH because of PGA and DOM sorption (Fig. 1). Upon sorption of PGA small carbohydrate peaks became noticeable (Fig. 1).



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  		Fig. 1. X-ray diffractograms of pure PGA- and DOM-coated AlOOH.

 
ESEM-EDX and Scanning Electron Microscopy Analysis
The ESEM micrographs of AlOOH showed a ‘feathered’ texture (Fig. 2a) . The Al/O atomic ratio of about 0.5 derived from EDX analysis is in agreement with AlOOH being the main product of the synthesis. Several authors have reported an enhancement of formation and stability of microaggregates after application of soluble organic substances to soils (Piccolo et al., 1994; Six et al., 2000). Gaume et al. (2000) reported a reduction in aggregates <15 µm, when ferrihydrite was coated with PGA (1488 mg C L–1). The ESEM images of pure and PGA-coated AlOOH are presented in Fig. 2a,b. No change in aggregation or structure of that poorly crystalline material was visible on a micrometer scale. In addition, there was no change in structure between freeze-dried DOM-free and DOM-treated AlOOH samples analyzed with SEM (Fig. 2c,d). The reduction of relative pressure from 100 to 30 Pa down to 3.1 x 10–3 Pa intensified the drying process. No structural change of external surface topography was visible upon pressure decrease of uncoated AlOOH (Fig. 2a). Thus, on a micrometer scale, the interparticle pore system of the hydrous Al oxide seems to be hardly affected by drying.



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  		Fig. 2. Environmental scanning electron micrographs of (a) uncoated AlOOH at 100 Pa, (b) PGA-coated AlOOH at 100 Pa, and scanning electron micrographs of (c) freeze-dried AlOOH, and (d) freeze-dried DOM-coated AlOOH.

 
Analysis of Freeze-dried Samples: Organic Matter-Coverage, Specific Surface Area, and Porosity
The AlOOH sorbed only half the amount of PGA-C than DOM-C (Table 1). Dissolved OM concentrations on AlOOH agree with those reported by Chorover and Amistadi (2001) for goethite. However, Kaiser and Guggenberger (2003) found considerably lower values for DOM sorption onto amorphous Al(OH)3 at pH 4. Also, the DOM extraction at pH 7 may have caused a more hydrophobic DOM composition. The high affinity of hydrophobic DOM for hydrous oxides (Kaiser and Zech, 1999; Guggenberger and Kaiser, 2003) might have caused the higher OM surface concentration in our samples compared with Al(OH)3 used by Kaiser and Guggenberger (2003).


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  		Table 1. Brunauer, Emmett, and Teller (BET) C constant from Eq. [1], {Delta}Hxs (difference between the adsorption enthalpy of the gas directly on the surface and the enthalpy of gas condensation; Steele, 1974), C concentration and fractional organic matter (OM) coverages calculated with Eq. [2] and [4]. The number of measurements was n = 4 for PGA- and DOM-treated AlOOH, n = 3 for AlOOH, n = 2 for PGA.

 
Boehmite sorbed 0.06 mg m–2 PGA-C (Table 1). This is nearly twice the sorption of PGA calculated for ferrihydrite with 0.03 mg C m–2 at pH 6.1 (Gaume et al., 2000) but less than values reported for goethite with 0.30 mg C m–2 at pH 5.5 (Grimal et al., 2001). However, in both studies high molecular weight PGA (25000–50000 g mol–1) and minerals with differing SSABET were used.

Changes in enthalpy of N2 adsorption, {Delta}Hxs, upon sorption of DOM and PGA were small and statistically not significant (Table 1). In contrast, the BET C constant of AlOOH decreased markedly upon sorption of PGA (P < 0.05) corresponding with a high fractional OM-coverage. For the AlOOH samples the enthalpies of N2 adsorption (Table 1) are in the range of values reported for acid soil samples (Mayer and Xing, 2001) but higher than values reported for minerals (approximately 10–12 kJ mol–1) and organic matter (7.5–12 kJ mol–1; Mayer, 1999). This may be because of a high proportion of micropores (Gregg and Sing, 1982) since we found a positive correlation between {Delta}Hxs and the micropore surface area, SSAMP (P < 0.05).

Only up to 0.07 m2 m–2 (fcov-a) of AlOOH surface were covered by DOM and PGA (Table 1). The value is in agreement with those obtained from acid soil A, B, and C horizons (Mayer and Xing, 2001). The calculation of the fractional coverage fcov-b using the alternative approach of Eq. [4] led to values of 0.16 for the DOM treatment, and 0.36 for the PGA treatment (Table 1). These values comply with fcov-b values reported for B and C horizons at pH 4 to 6 (Mayer and Xing, 2001). Our finding supports Mayer and Xing (2001) who also observed a discrepancy between fcov-a and fcov-b values, which they said was due to the occlusion of mineral surfaces by OM coatings. The high variability of fcov-a and fcov-b values suggests an inhomogeneous OM surface coating. Carbon contents obtained by ESEM-EDX are in agreement with those reported in Table 1. Spot measurements revealed variable surface concentrations of PGA-C between 0.8 and 2 wt%, which is in agreement with variable fractional coverage values (Table 1).

Figure 3 shows the adsorption and desorption isotherms of OM-covered AlOOH and a reference. Adsorption isotherms resemble a mixture of Type I and II isotherms according to Brunauer et al. (1938b), typical of micro- and mesoporous materials. The increase in volume of N2 adsorbed at >0.90 P/P0 was because of mesopores >10.4 nm (Lowell and Shields, 1991, p.62). The desorption branch exhibits a small slope at high relative pressures and a large slope where the wide part of the pores empties. This is explained by the presence of ‘ink-bottle’ pores resulting in a Type E hysteresis (de Boer, 1958). For all samples hysteresis commenced around 0.45 P/P0 indicating pores >1.85 nm (Lowell and Shields, 1991, p.62). Sorption of DOM and PGA significantly decreased the volume of N2 adsorbed and also hysteresis in the case of PGA (Fig. 3).



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  		Fig. 3. Nitrogen adsorption and desorption isotherms of freeze-dried pure, DOM-, and PGA-treated AlOOH samples.

 
The SSABET of AlOOH decreased by 15% upon sorption of DOM (P < 0.05, Table 2). The loss of SSAMP was nearly twice that of SSABET upon DOM treatment. This might indicate a preferential sorption of DOM close to micropores (Lang and Kaupenjohann, 2002; Kaiser and Guggenberger, 2003), which would reduce the diffusion of N2 into the pores. Figure 4a gives the cumulative SSABET of the DOM-treated replicates and their references and shows that most of the significant SSABET changes are related to pores <10 nm. The PGA sorption was more effective than the DOM treatment at reducing the SSABET and SSAMP of AlOOH samples (Table 2).



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  		Fig. 4. Cumulative pore-size distribution of surface area (SSABET) and pore volume of DOM + reference (a,c), and PGA + reference (b,d). The insets give the cumulative pore-size distribution of pores <5 nm. Error bars represent standard deviation.

 
While the total pore volume remained constant upon DOM sorption the micropore volume, VMP, decreased by 26% (t-V method) (P < 0.05, Table 2). Figure 4c illustrates the DOM induced pore volume changes and shows that only pores <3 nm are affected by DOM coatings. In agreement with the higher OM coverage PGA not only reduced VMP more effectively than DOM (Table 2) but also reduced the total pore volume by 27% (P < 0.001). Contrary to DOM sorption, the cumulative SSABET and pore volume of AlOOH were markedly reduced by PGA coatings across the entire pore diameter range (10–200 nm) analyzed (Fig. 4b,d).

Analysis of Moist Samples: 1H-Nuclear Magnetic Resonance Porosity
In general, the discussion of absolute porosity changes assessed with 1H-NMR and N2 adsorption is hampered by the fact that the exact pore geometry of AlOOH is unknown. Hence, a cylindrical pore geometry is assumed. Scaling T2s relaxation time constants to pore size according to Eq. [8] is based on the assumption that the surface relaxation T2s is much longer than the time it takes for a proton to diffuse across the pore, Td = r2/D, where r is the radius and D is the self-diffusion coefficient (Kenyon, 1997). For our samples the assumption seems plausible because (i) {rho}2 values are reasonably small (see Table 3), (ii) r is small for microporous materials and (iii) most of the fluid is in pores of small meso- and micropore size.


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  		Table 3. Transversal surface relaxivity {rho}2 of pure and PGA-treated AlOOH, transversal relaxation time constants T2, relative 1H-NMR amplitudes, and coefficients of determination as obtained from mono- and multiexponential fits to the magnetization decay curves M(t) of pure, DOM- and PGA-treated AlOOH.

 
Table 3 presents the result of the exponential curve fitting with WinFit software. The obtained T2 time constants and the corresponding amplitudes show a high variability, implying pore-size heterogeneities or an influence of varying gravimetric water contents in pore domains. The surface relaxivity parameters {rho}2 (Table 3) are in the range of {rho}1 values reported for soil and aquifer materials (0.2–0.6 nm ms–1, Hinedi et al., 1993). For comparisons {rho}1 is about 2/3 {rho}2 (Kleinberg et al., 1993). Errors of the surface relaxivity {rho}2 were estimated to be 9% (PGA/AlOOH) and 10% (AlOOHPGA-reference), which is attributable mainly to the variance in the SSABET values.

The 1H-NMR analysis of AlOOH revealed two distinct ‘pore classes’ or ‘states of water binding’ (Table 3, T2-1 vs. T2-2) that cannot be resolved into two peaks with Tikhonov-regularization (Fig. 5a) . In Fig. 5b, the relaxation time constants are scaled to their corresponding pore size via the surface relaxivity parameter {rho}2 from Table 3. The pore-size peak at 6 nm obtained from 1H-NMR is in good agreement with pore sizes obtained from N2 adsorption (Table 2). Polygalacturonic acid sorption onto AlOOH increased mean T2 time constants obtained from the monoexponential fit (Table 3). Multiexponential fit results clearly show a change in amplitudes; those belonging to T2 times of approximately 35 ms increased upon PGA sorption whereas those of about 9 ms decreased (Table 3). This finding suggests that PGA treatment decreased the amount of water in smaller pores. Figure 5a and b depict the shift in T2 time constants and pore sizes. Similar to N2 adsorption results, the pore system of AlOOH is changed by PGA sorption under moist conditions in two ways: (i) the pore sizes shifted to larger values and (ii) small nanometer pores disappeared (Fig. 5b). In contrast to PGA sorption, the DOM treatment did not significantly affect 1H-NMR results of AlOOH (Table 3). The reduction in pore volume induced by DOM (Fig. 4c, Table 2) might be below the detection limit of 1H-NMR.



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  		Fig. 5. (a) T2 relaxation time constant distribution f(T2) of pure and PGA-treated AlOOH, and (b) calculated pore-size distribution f(P) of pure and PGA-treated AlOOH.

 
Effect of Drying
To assess effects of drying one has to consider that 1H-NMR and N2 adsorption comparisons refer not only to moist and dry samples but also mirror two separate methodologies. While 1H-NMR gives the amount of H2O held in pores of various size, adsorption of N2 reflects the accessibility of pores for N2 at 77 K. At 77 K, organic matter sieves the N2 molecules. This is because 95 to 99% of SSABET of soil organic matter is contained in pores <0.5 nm (De Jonge and Mittelmeijer-Hazeleger, 1996). If pores were simply covered by OM that is not permeable for N2 at 77 K, we would detect a decrease in N2 adsorption but would not detect any effects by 1H-NMR logging. Nuclear magnetic resonance time constant spectra will show an effect of OM coatings only if the amount of H2O in pores is reduced, that is, when organic molecules displace water in pores.

Analyses of both, moist and freeze-dried PGA-treated AlOOH samples indicate a loss in micro- and small mesopores (Fig. 4, 5). Scaling the relaxation time constants of Fig. 5a to pore size yields the pore-size distribution of water-filled pores (Fig. 5b), which shows a reduction in water volume in the pores smaller than 4 nm of PGA-treated AlOOH samples. Thus, the 1H-NMR results suggest that PGA coatings not only decrease the accessibility of micro- and small mesopores for N2 but also for H2O (Fig. 5b). Water in the pores of C-free AlOOH must have been either displaced by PGA or driven out by a hydrophobization of surfaces.

In addition, wet and freeze-dried PGA coatings induced a shift in the pore-size distribution of AlOOH to larger pores. The average pore diameter increased by 14% under freeze-dried (Table 2) and 150% under moist conditions (Fig. 5b).

The change in average pore diameter expressed as the average pore-size ratio of coated to uncoated AlOOH obtained from 1H-NMR [rNMR(AlOOH/PGA)/rNMR(AlOOH)] was 2.5; that of freeze-dried samples analyzed with N2 adsorption [rBET(AlOOH/PGA)/rBET(AlOOH)] was 1.1. Hence, the PGA-induced pore size increase was more pronounced under moist conditions. The shift to larger pore sizes observed with both methodologies could be explained by PGA-induced aggregate formation on a submicron scale. The increase in amplitude of NMR peaks upon PGA sorption supports this suggestion (Fig. 5). Further, sorption of PGA onto AlOOH changed the interparticle pore system of AlOOH under moist conditions because the pore volume contained in approximately 10- to 70-nm pores increased (Fig. 5b). However, pores of this size have not been detected on freeze-dried samples using N2 adsorption. The destruction of labile interparticle pores—created by OM sorption—upon freeze-drying might explain these results. Therefore, N2 adsorption porosity data seem to be partially biased as a consequence of freeze-drying. However, results obtained from freeze-dried samples might reflect the accessibility of micro- and small mesopores within soil mineral particles. Thus, wet samples have to be analyzed if the interparticle pores of biphasic systems are addressed.

Coating Efficiency of Polygalacturonic Acid and Dissolved Organic Matter
The extent of micropore clogging of PGA and DOM was comparable. Normalized to SSABET, the loss in micropore area, {Delta}SSAMP, upon OM treatment was 0.24 ± 0.11 for DOM and 0.38 ± 0.08 for PGA (Table 2). The reduction in SSABET normalized to the amount of C sorbed was (1.0 ± 0.6) m2 mg–1 DOM-C and (5.9 ± 1.4) m2 mg–1 PGA-C. The result is consistent with Kaiser and Guggenberger (2003) who reported that at low C loadings organic matter occupies a larger portion of the available surface area than at higher C loadings. Based on the SSABET reduction normalized to the amount of C sorbed on AlOOH, PGA covered a six-fold greater surface area compared with DOM molecules. Hence, PGA was more effective at reducing N2 diffusion into meso- and macropores of AlOOH (Fig. 4). The stronger clogging effect of PGA relative to DOM was also revealed by 1H-NMR experiments. Here, PGA sorbates reduced the amount of water held in micro- and mesopores; DOM did not. The higher coating efficiency of polysugars is of special interest in the context of induced changes of soil properties (Chenu, 1993; Welch and Vandevivere, 1994; Chenu and Tessier, 1995; Czarnes et al., 2000; Traoré et al., 2000). Mucilaginous OM coatings on reactive soil mineral surfaces of for example hydrous Al and Fe oxides might control the accessibility of diffusion pathways for nutrients and pollutants at the soil–root interface. The results furthermore imply that C contents on a unit area and mass basis (Table 1) do not adequately reflect the ‘coating efficiency’ of different soil organic matter qualities. Instead, the approaches of Eq. [2] and [4] are better suited for this purpose.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Both DOM and PGA clog the pores of poorly crystalline AlOOH in a freeze-dried state. However, PGA seems to be more effective than DOM from the bulk soil at reducing surface area and porosity of poorly crystalline AlOOH. The ‘pore clogging’ of micro- and small mesopores by PGA is not an artifact of freeze-drying because we also found PGA clogging micro- and small mesopores under moist conditions. On the other hand, freeze-drying can destruct labile interparticle pores of C-coated mineral phases. Results of the 1H-NMR logging indicate that PGA not only covers the entrance of micro- and small mesopores but also penetrates into the pores.


   ACKNOWLEDGMENTS
 
This study was funded by a grant of the Deutsche Forschungsgemeinschaft (DFG). We are greatly indebted to Roger-Michael Klatt (Hannover University), Robert Mikutta (Martin Luther University Halle-Wittenberg) and Dr. Martin Müller (Berlin University of Technology) for their technical assistance.


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




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