Environ. Sci. Technol. 2011, 45, 527–533
Fractionation of Organic Matter Due to Reaction with Ferrihydrite: Coprecipitation versus Adsorption K A R I N E U S T E R H U E S , * ,† THILO RENNERT,† HEIKE KNICKER,‡ ¨ GEL-KNABNER,§ INGRID KO KAI U. TOTSCHE,† AND UDO SCHWERTMANN§ Institut fu ¨ r Geowissenschaften, Friedrich-Schiller-Universita¨t Jena, 07749 Jena, Germany, Instituto de Recursos Naturales y Agrobiologı´a de Sevilla (IRNAS-CSIC), 41012 Sevilla, Spain, and Lehrstuhl fu ¨ r Bodenkunde, Technische Universita¨t Mu ¨ nchen, 85350 Freising, Germany
Received July 14, 2010. Revised manuscript received November 12, 2010. Accepted November 13, 2010.
In soil and water, ferrihydrite frequently forms in the presence of dissolved organic matter. This disturbs crystal growth and gives rise to coprecipitation of ferrihydrite and organic matter. To compare the chemical fractionation of organic matter during coprecipitation with the fractionation involved in adsorption onto pristine ferrihydrite surfaces we prepared ferrihydrite-organic matter associations by adsorption and coprecipitation using (i) a forest-floor extract or (ii) a sulfonated lignin. The reaction products were studied by 13C CPMAS NMR, FTIR, and analysis of hydrolyzable neutral polysaccharides. Relative to the original forest-floor extract, the ferrihydriteassociated organic matter was enriched in polysaccharides, especially when adsorption took place. Moreover, mannose and glucose were bound preferentially to ferrihydrite, while fucose, arabinose, xylose, and galactose accumulated in the supernatant. This fractionation of sugar monomers was more pronounced during coprecipitation and led to an enhanced ratio of (galactose + mannose)/(arabinose + xylose). Experiments with lignin revealed that the ferrihydrite-associated material was enriched in its aromatic components but had a lower ratio of phenolic C to aromatic C than the original lignin. A compositional difference between the adsorbed and coprecipitated lignin is obvious from a higher contribution of methoxy C in the coprecipitated material. Coprecipitated organic matter may thus differ in amount and composition from adsorbed organic matter.
Introduction Ferrihydrite, a poorly crystalline Fe(III) oxyhydroxide, is known to be highly reactive toward natural organic matter (1). Because of its ubiquitous occurrence in the environment and its high surface area, ferrihydrite may have an important control on transport and fixation of organic matter (OM) in soils and sediments (2-4). This association is all the more important as the Fe-oxide surface stabilizes OM against * Corresponding author phone: +49 3641 948642; fax: + 49 3641 948622; e-mail:
[email protected]. † Friedrich-Schiller-Universita¨t Jena. ‡ Instituto de Recursos Naturales y Agrobiologı´a de Sevilla. § Technische Universita¨t Mu ¨ nchen. 10.1021/es1023898
2011 American Chemical Society
Published on Web 12/02/2010
microbial degradation (5-8) and, thus, for C storage in soils. To investigate such processes, ferrihydrite-OM associations are usually produced by adsorption of the organics onto synthetic ferrihydrite. In natural environments, ferrihydrite often forms in the presence of dissolved OM, which leads to coprecipitation of OM with ferrihydrite. Coprecipitation is defined as the carrying down of a normally soluble substance as the consequence of another substance’s precipitation by inclusion, occlusion, or adsorption. While in the case of ferrihydrite inclusion of large OM molecules on lattice positions of the Fe oxide can be excluded, other interactions together with poisoning of crystal growth (9, 10) must be taken into account. Mikutta et al. (11) precipitated ferrihydrite in the presence of synthetic acid polysaccharides. They observed a stronger aggregation of ferrihydrite particles, a slightly lower magnetic hyperfine field, but no significant change in crystallinity or in the local coordination of Fe. Other coprecipitation studies found that even small concentrations of complex natural soil OM have a clear impact on the structure of the solid products (12, 13). With increasing C/Fe of the initial solution, ferrihydrite coprecipitates displayed larger lattice spacings, fewer crystal planes, smaller particles, and a magnetic hyperfine splitting that disappears at lower temperatures. Coprecipitation with OM can therefore be expected as a likely process in nature, which may significantly change the reactivity of ferrihydrite. In this study, we compare two adsorption and coprecipitation experiments performed with (i) water-soluble OM from a Podzol forest-floor layer and (ii) a sulfonated lignin obtained from paper production. The former represents easily degradable, polysaccharide-rich dissolved OM entering the mineral soil as leachate. The latter is used as an analogue of the aromatic end-member composition of the lignin component in soil solution. Lignin is assumed to be highly reactive toward Fe oxides but is difficult to investigate in a natural extract because of its low concentration. The objectives of this study were (i) to quantify the OM loading of the solid reaction products and (ii) to characterize the chemical fractionation of the two OM types during coprecipitation and adsorption. Coprecipitated OM may differ from surfaceadsorbed OM because of the smaller particle size and changed crystal structure of coprecipitates (13) and the possibility of OM occlusion during coprecipitation. Since we regard coprecipitation a very likely process in nature, such investigations will add to our understanding of location and stabilization of different compound classes is soil.
Materials and Methods Organic Matter. Water-soluble soil OM was extracted from the forest-floor layer (Oi and Oa horizon) of a Podzol developed from gravelly sand of Tertiary Molasse under spruce near Freising, Germany. Samples were air dried and passed through a 2 mm sieve to remove coarse plant remains. Aliquots of 150 g of forest floor and 700 mL of deionized H2O were end-over-end shaken for 16 h at room temperature and then centrifuged for 1 h at 4650g. The supernatant was pressure filtered through polyvinylidene fluoride (membranes Durapore; 0.45 µm pore width), concentrated in lowtemperature rotary evaporators, and freeze dried. The sulfonated lignin was provided by a paper mill near Kehlheim, Germany. A more detailed characterization of the material is given in ref (14). Synthesis of Ferrihydrite and Ferrihydrite-OM Associations. Two-line ferrihydrite was produced by titrating a 0.01 M Fe(NO3)3 solution with 0.1 M KOH to pH 7 (15). Coprecipitation experiments were carried out by dissolving VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Fe(NO3)3 in a solution of deionized H2O and either the forestfloor extract or the sulfonated lignin. Then, the pH was raised to 7 by adding 0.1 M KOH. Products with different OM loadings were gained by varying the C/Fe ratio of the initial solution between 0.1 and 2.3 for experiments with the forestfloor extract and between 0.1 and 3.8 for experiments with sulfonated lignin. The precipitates were separated by centrifugation and pressure filtration, washed twice with deionized H2O, and freeze dried. The supernatant material was concentrated in low-temperature rotary evaporators and freeze dried. Adsorption experiments were performed by mixing OM solutions of different concentrations (adjusted with HCl to pH 3.5-3.6) with neutral suspensions of freshly precipitated 2-line ferrihydrite, which led to pH 4.2-4.6. The solid-tosolution ratio was approximately 1:4500. After 12 h of stirring at room temperature, the suspensions were centrifuged. The supernatant was pressure filtered, concentrated in lowtemperature rotary evaporators, and freeze dried, while the solids were washed twice with deionized H2O and freeze dried. By this, adsorption of the forest-floor extract took place at the pH at which most ferrihydrite OM coprecipitates formed. This was shown by a sudden increase in turbidity at pH 4.5 (Figure SI-1 of the Supporting Information). In the case of lignin, we assume that most coprecipitates formed at a slightly higher pH of 5. The haze observed at pH 2.5-4 of coprecipitation experiments with lignin was supposedly induced by formation of Fe-OM complexes. Analytical Methods. Total C content of solids and dried supernatants was measured with a CN analyzer (Vario EL, Elementar Analysensysteme, Hanau, Germany). The solids of adsorption and coprecipitation experiments were analyzed by transmission FTIR spectroscopy (Nicolet iS10, Thermo Fisher Scientific, Dreieich, Germany) on pellets of 2 mg of sample diluted with 200 mg of KBr between 4000 and 400 cm-1, accumulating 32 scans at a resolution of 4 cm-1. Spectra were baseline corrected by subtracting a straight line running between the two minima of each spectrum and normalized by dividing each data point by the spectrum’s maximum. To obtain spectra of associated OM, the baseline-corrected and normalized spectrum of pure ferrihydrite was subtracted from the spectra of ferrihydrite-OM associates. Before calculating the second derivative by centered numerical differences, spectra were smoothed using the Savitzky-Golay algorithm over 11 points. The supernatant material of all adsorption and coprecipitation experiments was characterized by solidstate 13C NMR with a Bruker DSX-200 NMR spectrometer (Bruker BioSpin, Karlsruhe, Germany), applying crosspolarization with magic angle spinning (CP MAS) at a spinning frequency of 6.8 kHz and a contact time of 1 ms. A ramped 1H pulse was used during contact time to circumvent spin modulation of Hartmann-Hahn conditions. Pulse delays between 200 and 2000 ms were chosen. Direct NMR measurements of the solid products were not possible because of their high contents of paramagnetic ferrihydrite. Spectra of the forest-floor extract were divided into 4 regions, alkyl C (0-45 ppm), O-alkyl C (45-110 ppm), aryl C (110-160 ppm), and carbonyl C (160-220 ppm), while spectra of the lignin were divided into six regions, methylene C (0-45 ppm), methoxy C (45-60 ppm), O-alkyl C (60-90 ppm), aromatic C (90-140 ppm), phenolic C (140-160 ppm), and carbonyl C (160-220 ppm). Noncellulosic neutral polysaccharides were analyzed in duplicate by gas chromatography (GC Agilent 6890; Agilent Technologies, Waldbronn, Germany) using a 60 m fused silica capillary column of type BPX 70 (SGE, Griesheim, Germany) after hydrolysis with trifluoroacetic acid, reduction of sugar monomers to the corresponding alcohols, and derivatization with acetic anhydride (16). Myo-Inositol (GC response factor) and methylglucose 528
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FIGURE 1. 13C CP MAS NMR spectra of the original forest-floor extract and the original lignin. (recovery) were added as internal standards. Recovery of methylglucose was between 72% and 107%.
Results and Discussion Carbon Loadings. Adsorption isotherms at equilibrium for the forest-floor extract and for lignin (Figure SI-2 of the Supporting Information) are best described by the Langmuir model. This model presumes a homogeneous surface, a single adsorption mechanism, no interactions between the adsorbing molecules, and development of a monomolecular adsorbate layer. In our case, a theoretical monolayer loading of 196 mg C g-1 and a value of 0.14 mL mg-1 for the constant related to the adsorbate and the adsorbent provided the best fit for the adsorption of the forest-floor extract. For lignin adsorption, we found a monolayer loading of 193 mg C g-1 and a constant of 0.19 mL mg-1. The highest loadings of ca. 195 mg C g-1 ferrihydrite achieved for both OM types correspond to ca. 525 mg OM g-1 ferrihydrite or to 1.4 mg OM m-2 ferrihydrite, when related to the specific surface area of 386 m2 g-1 of the control ferrihydrite. In comparison to maximum loadings from sorption experiments with watersoluble soil OM on ferrihydrite (1.1 mg C m-2 ferrihydrite (17)) and estimated sorption maxima on natural pedogenic Fe oxides from acid forest soils (1.2 mg C m-2 Fe oxide (4)), the maximum loadings obtained in our adsorption experiments are only slightly higher. Coprecipitation of the forestfloor extract with ferrihydrite also yielded a similar maximum loading of 170 mg C g-1 ferrihydrite, whereas coprecipitation with lignin led to a higher maximum loading of 360 mg g-1. We conclude that occlusion and smaller particle size, as associated with coprecipitation, does not necessarily cause higher amounts of mineral-bound OM and may depend on OM composition. In the case of lignin, the probable formation of Fe-OM complexes before the onset of coprecipitation may be related to the higher OM loading of coprecipitates.
Organic Matter Composition Solid-State 13C NMR. The freeze-dried original forest-floor extract contains 371 mg g-1 C and 13.7 mg g-1 N. Its solidstate 13C NMR spectrum ((12), Figure 1) is dominated by two peaks at 71 and 101 ppm, assigned to O-alkyl C groups in carbohydrates. Both peaks explain ca. 80% of TOC. The signal of carboxyl C, carbonyl C, and amide C accounts for 7% of the total C and is centered at 175 ppm, typical of esters. The total aromatic C content is estimated to be
