Restructuring of polygalacturonate on alumina upon hydration—Effect on phosphate sorption kinetics

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Abstract

Hydration of organic coatings in soils is expected to affect the sorption of oxyanions onto hydrous Fe and Al oxides. We hypothesized that the hydration of polygalacturonate (PGA) coatings on alumina (Al2O3) increases their permeability for phosphate. Pure and PGA-coated alumina were equilibrated in deionized water for 2 and 170 h at pH 5 and 20 °C before studying (i) their porosity with N2 gas adsorption and 1H NMR relaxometry, (ii) structural changes of PGA-coatings with differential scanning calorimetry (DSC), and (iii) the kinetics of phosphate sorption and PGA desorption in batch experiments. Scanning electron micrographs revealed that PGA molecules formed three-dimensional networks with pores ranging in size from <10 to several hundred nanometers. Our NMR results showed that the water content of intraparticle alumina pores decreased upon PGA sorption, indicating a displacement of pore water by PGA. The amount of water in interparticle alumina pores increased strongly after PGA addition, however, and was attributed to water in pores of PGA and/or in pores at the PGA-alumina interface. The flexibility of PGA molecules and the fraction of a PGA gel phase increased within one week of hydration, implying restructuring of PGA. Hydration of PGA coatings increased the amount of phosphate defined as instantaneously sorbed by 84%, showing that restructuring of PGA enhanced the accessibility of phosphate to external alumina surfaces. Despite the fact that the efficacy of phosphate to displace PGA was higher after 170 h than after 2 h, a higher phosphate surface loading was required after 170 h to set off PGA desorption. Our findings imply that the number of PGA chain segments directly attached to the alumina surface decreased with time. We conclude that hydration/dehydration of polymeric surface coatings affects the sorption kinetics of oxyanions, and may thus control the sorption and transport of solutes in soils.

Introduction

In soils and sediments minerals are partially covered with organic matter (Heil and Sposito, 1995, Ransom et al., 1997, Mayer and Xing, 2001, Gerin et al., 2003, Kaiser and Guggenberger, 2003). Under field and laboratory conditions, organic matter is subjected to moisture fluctuations that may change its physico-chemical properties due to interaction with water molecules (LeBoeuf and Weber, 2000, Schaumann et al., 2000, Schaumann, 2005, Schaumann and LeBoeuf, 2005). Hydration-induced changes in the macromolecules’ mobility (Schaumann and LeBoeuf, 2005) may affect the retention of nutrients and pollutants by minerals coated with organic matter. The ability of soils and soil organic matter to sorb or release organic pollutants has been shown to depend on the state of hydration, hydration time, wetting and drying cycles, and the water content of the samples (Gaillardon, 1996, Altfelder et al., 1999, Johnson et al., 1999, Schaumann et al., 2004). In addition, the structure of organic matter can be affected by the dehydration technique applied in the laboratory, e.g., prior to sorption experiments (Altfelder et al., 1999). For example, structural changes of organic matter upon freeze-drying have been reported (Wedlock et al., 1983, Jouppila and Roos, 1997, Allison et al., 1998, Souillac et al., 2002). The hydration/dehydration-induced change of molecular structures of organic matter is therefore expected to affect the transport of solutes like hydrophobic pollutants through organic matter of soils and sediments (Brusseau and Rao, 1989, Pignatello and Xing, 1996, Cornelissen et al., 1998).

It has been suggested that soil organic matter (SOM) consists of rubbery (more flexible) and glassy (less flexible) domains (LeBoeuf and Weber, 1997) as known for synthetic polymers. The glass transition temperature, Tg, marks the temperature at which a glassy matrix becomes rubbery (Young and Lovell, 1991), and is a function of the side chain mobility in macromolecules. Usually, the incorporation of water molecules into the polymeric framework of isolated humic substances, soil and peat samples upon hydration reduces Tg, i.e., plasticizes polymer matrices (LeBoeuf and Weber, 1997, Schaumann and Antelmann, 2000, Schaumann and LeBoeuf, 2005).

It has been found by differential scanning calorimetry (DSC) and 1H NMR-relaxometry analyses that peat or humus-rich soil samples exhibit first-order swelling kinetics upon hydration with time constants (reciprocal rate constants) of up to 6 days (Schaumann et al., 2004, Schaumann et al., 2005, Schaumann and LeBoeuf, 2005). Changes in proton relaxation times upon swelling of organic matter depend on the pore size distribution initially present in organic samples and on the quality of the organic material studied (Schaumann et al., 2004): while swelling of starch led to an increase in proton relaxation times, swelling of semolina or organic matter in peat and soil samples generally reduced the relaxation times (Schaumann et al., 2004, Schaumann et al., 2005). These effects were interpreted as an increase in intraparticle pore size and a decrease in interparticle pore size of organic matter upon water absorption (Schaumann et al., 2004, Schaumann et al., 2005).

Swelling of mineral-associated polymers through the incorporation of water molecules into the polymer structure might affect the sorption of oxyanions like phosphate or arsenate to Fe and Al oxides. An increase in intraparticle pore size of organic matter voids upon swelling of organic matter in conjunction with an increased mobility of polymer chains upon hydration might facilitate the Brownian motion and Fickian diffusion through more flexible (rubbery) polymer domains and hence favor the fast sorption of oxyanions. However, a decrease in interparticle pore size of sorbed organic matter upon swelling might reduce the accessibility of mineral surfaces to oxyanions.

The objective of this study was therefore to test whether the slow swelling of polymers sorbed to Fe and Al oxides affects phosphate sorption kinetics. Specifically, we hypothesized that the hydration of polygalacturonate (PGA) coatings on alumina (Al2O3) increases their permeability for phosphate. We used PGA as a well-defined model substance for the gelatinous mucilage covering the root apices of many plant species (Knee et al., 2001). Mucilage exuded by plant’s root caps is confined to the soil–root interface because mucilage components diffuse very slowly into the soil (Rovira, 1969, Sealey et al., 1995). Mucilage of maize plants consists of 90–95% polysaccharides with about 20–35% of uronic acids (Cortez and Billes, 1982, Morel et al., 1986), and is susceptible to swelling due to water absorption (e.g., Guinel and McCully, 1986). At the soil–root interface the cycling of nutrients is therefore likely to be influenced by the state of hydration of organic coatings made up of macromolecular root exudates.

Section snippets

General approach

We used alumina as a non-paramagnetic high-surface-area model adsorbent that could be used for 1H NMR measurements. Pure and PGA-coated alumina samples were saturated in doubly deionized water at pH 5 for 2 and 170 h. After each equilibration time, phosphate sorption experiments were performed. Similarly, changes in pore size distribution were then monitored with 1H NMR relaxometry and N2 gas adsorption at 77 K. Differential scanning calorimetry (DSC) was used to identify changes in the molecular

SEM analysis

Fig. 1 depicts SEM images of pure and PGA-coated alumina. Particles of the pure oxide possessed feather-edged structures that, when further resolved, showed a cauliflower-like surface microtopography with pore entrances of about 5 nm (image 5). The cauliflower-like surface structure shown in the high-resolution SEM image 5 (Fig. 1) is likely due to Au isles formed during sputtering. Similar structures have been observed on surfaces of layer silicates like pyrophyllite or vermiculite (not shown).

Summary and conclusions

Porosity studies with 1H NMR and N2 gas adsorption of moist and freeze-dried PGA-coated alumina, which had been equilibrated in water for 2 and 170 h, respectively, revealed no swelling-induced change in pore size. Our NMR measurements showed that water held in intraparticle pores of alumina was partially displaced by sorbed PGA. Additionally, the hydration of PGA networks on external alumina surfaces increased the amount of water held in interparticle pores of alumina–PGA associations,

Acknowledgments

We thank Fabian Jaeger for useful discussions on the 1H NMR results. We also thank Jeannette Regnery who kindly supported us in the DSC analysis. In addition, we thank three anonymous reviewers for their encouraging statements. This research was funded by the German Research Fund (DFG, KA 1139/8).

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