Elsevier

Geochimica et Cosmochimica Acta

Volume 144, 1 November 2014, Pages 258-276
Geochimica et Cosmochimica Acta

Properties and reactivity of Fe-organic matter associations formed by coprecipitation versus adsorption: Clues from arsenate batch adsorption

https://doi.org/10.1016/j.gca.2014.08.026Get rights and content

Abstract

Ferric oxyhydroxides play an important role in controlling the bioavailability of oxyanions such as arsenate and phosphate in soil. Despite this, little is known about the properties and reactivity of Fe(III)-organic matter phases derived from adsorption (reaction of organic matter (OM) to post-synthesis Fe oxide) versus coprecipitation (formation of Fe oxides in presence of OM). Coprecipitates and adsorption complexes were synthesized at pH 4 using two natural organic matter (NOM) types extracted from forest floor layers (Oi and Oa horizon) of a Haplic Podzol. Iron(III) coprecipitates were formed at initial molar metal-to-carbon (M/C) ratios of 1.0 and 0.1 and an aluminum (Al)-to-Fe(III) ratio of 0.2. Sample properties were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), N2 gas adsorption, dynamic light scattering, and electrophoretic mobility measurements. Arsenic [As(V)] adsorption to Fe-OM phases was studied in batch experiments (168 h, pH 4, 100 μM As). The organic carbon (OC) contents of the coprecipitates (82–339 mg g−1) were higher than those of adsorption complexes (31 and 36 mg g−1), leading to pronounced variations in specific surface area (9–300 m2 g−1), average pore radii (1–9 nm), and total pore volumes (11–374 mm3 g−1) but being independent of the NOM type or the presence of Al. The occlusion of Fe solids by OM (XPS surface concentrations: 60–82 atom% C) caused comparable pHPZC (1.5–2) of adsorption complexes and coprecipitates. The synthesis conditions resulted in different Fe-OM association modes: Fe oxide particles in ‘M/C 0.1’ coprecipitates covered to a larger extent the outermost aggregate surfaces, for some ‘M/C 1.0’ coprecipitates OM effectively enveloped the Fe oxides, while OM in the adsorption complexes primarily covered the outer aggregate surfaces. Despite of their larger OC contents, adsorption of As(V) was fastest to coprecipitates formed at low Fe availability (M/C 0.1) and facilitated by desorption of weakly bonded OC and disaggregation. In contrast, ‘M/C 1.0’ coprecipitates showed a comparable rate of As uptake as the adsorption complexes. While small mesopores (2–10 nm) promoted the fast As uptake particularly to ‘M/C 0.1’ coprecipitates, the presence of micropores (<2 nm) appeared to impair As desorption. This study shows that the environmental reactivity of poorly crystalline Fe(III) oxides in terrestrial and aquatic systems can largely vary depending on the formation conditions. Carbon-rich Fe phases precipitated at low M/C ratios may play a more important role in oxyanion immobilization and Fe and C cycling than phases formed at higher M/C ratios or respective adsorption complexes.

Introduction

It is well established that adsorption processes to iron (Fe) oxyhydroxides control the retention, bioavailability, and bioaccumulation of many oxyanions such as arsenate and phosphate in natural ecosystems (Yuan and Lavkulich, 1994, Meharg and Rahman, 2003, Wang and Mulligan, 2006). Because of their large surface area and small particle size, poorly crystalline hydrous Fe(III) oxides contribute mostly to the (im)mobilization of anions (Gräfe et al., 2002, Bauer and Blodau, 2009, Heiberg et al., 2010) and play a significant role in organic matter (OM) cycling in tropical to arctic soils (Torn et al., 1997, Lipson et al., 2010).

In contrast to well studied pure Fe(III) oxide-OM systems, little is known about the interactions of oxyanions with so called Fe(III)-OM coprecipitates. Coprecipitation of Fe with solute components (OM, trace metals) due to changes in pH, redox potential, or ionic strength is a common process in many environments. For example, in soils, aquifers, and surface waters affected by acid mine drainages (Lee et al., 2002, Mitsunobu et al., 2008, Cheng et al., 2009) or temporarily waterlogged soils such as paddies (Kögel-Knabner et al., 2010), Fe2+ is rapidly oxidized upon aeration, followed by Fe3+ hydrolysis and the formation of Fe solids. Precipitation of Fe(III) phases and immediate adsorption of natural OM (NOM) to the newly built hydrous oxides and precipitation of NOM by monomeric or polymeric Fe species are parallel processes, summarized as ‘coprecipitation’. Even in well drained organic-rich soils, Fe can precipitate with dissolved NOM (Dolfing et al., 1999) which might also include compounds of bacteriogenic origin (Rancourt et al., 2005, Muehe et al., 2013). For acidic mineral soils (pH 3.5–4.5), Nierop et al. (2002) showed that more than 80% of dissolved NOM precipitated with Fe3+ at a Fe/C ratio of 0.1. This accords well with high OC/Fe mass ratios (>0.22) observed particularly in acidic Spodosols, which cannot be explained by adsorption reactions between NOM and Fe oxides (Wagai and Mayer, 2007). Iron-organic coprecipitation has been shown to slow down the decomposition of soil-derived OM (Eusterhues et al., 2014a) and was also postulated as a potential mechanism that preserves marine OM from biodegradation (Lalonde et al., 2012).

Unlike pure adsorption complexes where NOM adsorbs to already existing Fe oxides, coprecipitates may contain a variable mixture of insoluble Fe-organic complexes as well as pure and NOM-rich Fe oxyhydroxides such as ferrihydrite (Schwertmann et al., 2005, Mikutta et al., 2008). The interaction of inorganic (Rancourt et al., 2001, Cismasu et al., 2011) and organic components (Schwertmann et al., 2005, Eusterhues et al., 2008, Mikutta, 2011) with Fe during hydrolysis is well known to alter the particle size and structural order of the newly forming Fe oxyhydroxides. While both adsorption and precipitation of NOM is frequently selective to hydrophobic and aromatic compounds (Kaiser et al., 1996, Dolfing et al., 1999, Sharpless and McGown, 1999, Scheel et al., 2007), distinct differences in OM composition can arise with respect to sugar and lignin components depending on the adsorption and coprecipitation genesis of Fe-OM associations (Eusterhues et al., 2011). Interaction of OM during Fe precipitation can thus directly affect the environmental reactivity of the Fe oxyhydroxides in terms of their participation in adsorption (Liu and Huang, 2003), dissolution (Mikutta and Kretzschmar, 2008), or reduction reactions (Shimizu et al., 2013, Eusterhues et al., 2014b). Despite this, the effect of different associations caused by NOM adsorption versus coprecipitation on the properties and reactivity of these Fe(III)-OM phases are still poorly understood. We hypothesize that Fe oxides formed by coprecipitation are more reactive sorbents for oxyanions than the respective adsorption complexes irrespective of their larger C contents. This might be because coprecipitated OM causes larger aggregate sizes, hence provides larger nanometer-sized pores and thus diffusion pathways of sorbats to Fe sorption sites (Mikutta et al., 2012). On the other hand, one would generally expect that the interaction of oxyanions with Fe-OM coprecipitates depends on the initial OC content as the incorporated or surface-attached NOM blocks reactive sites. Moreover, we assume that Fe-OM coprecipitates themselves are heterogeneous with respect to their physicochemical properties and reactivities, depending on the soil solution composition such as the Fe/C ratio or the presence of aluminum. The latter is ubiquitously present in the soil solution and may be sorbed to or incorporated into freshly formed Fe oxides (Masue et al., 2007, Cismasu et al., 2012, Hofmann et al., 2013) or becoming complexed by NOM (Gerke, 1994), thus, potentially altering the reactivity of the newly formed Fe-OM coprecipitates.

In this study we tested the reactivity of Fe(III) oxide-OM adsorption complexes versus coprecipitates in batch experiments by using arsenate [As(V)] as an environmentally relevant model compound. Pentavalent As is the most common species in soils and sediments (Bowell, 1994) and binds to hydrous Fe oxides mainly through inner-sphere surface complexes (Jain et al., 1999, Goldberg and Johnston, 2001, Sherman and Randall, 2003). As a negatively charged polyelectrolyte, NOM typically decreases the adsorption of As to Fe oxides by charge repulsion, direct site blocking, or microaggregation of the oxide particles (Gräfe et al., 2002, Bauer and Blodau, 2009). The sorption of As to Fe oxides usually comprises a fast and a slow reaction (Zhang and Stanforth, 2005, Luengo et al., 2007). While the initial fast reaction during oxyanion sorption is attributed to the adsorption to external surfaces (Cornell and Schwertmann, 2003), the slow reaction is caused by diffusion into micro- and mesopore domains (Strauss et al., 1997). Hence, possible differences in the properties of adsorption complexes and coprecipitates will be manifested in a variable As adsorption kinetics as well as in the extent of As remobilization. The main objectives of this study were therefore to (i) characterize and compare the physicochemical properties of ferrihydrite-OM adsorption complexes with those of Fe-OM coprecipitates and (ii) test the impact of these properties on the As(V) adsorption kinetics and sorption reversibility. We focused on a range of Fe-OM coprecipitates in order to investigate (iii) structural controls which determine their respective reactivity. This was achieved by varying the NOM composition, initial molar M/C ratios (1.0 and 0.1), and by adding Al as common soil solution constituent at a molar Al/Fe ratio of 0.2.

Section snippets

Extraction and characterization of natural organic matter

Natural OM from a litter (Oi) and Oa forest floor horizon of a Haplic Podzol under Norway spruce (Picea abies (L.) Karst.) was extracted by 18 MΩ doubly deionized water at a 1/10 (w/v) ratio for litter and a 1/5 ratio for the Oa material. All suspensions were shaken horizontally (90 rpm) for 16 h at 298 K, centrifuged at 3000×g for 10 min, and pressure-filtered through 0.7-μm glass fiber filters (GF 92, Whatman). Afterwards, the prefiltered suspensions were centrifuged at 3000×g for 15 min and

Element contents of ferrihydrite, adsorption complexes, and coprecipitates

The Fe content of the pure Fh was 547.6 mg g−1 (Fh/total Fe = 1.83). The coprecipitates contained between 130 and 438 mg g−1 Fe and 87 and 344 mg g−1 OC, respectively (Table 1). In presence of 20 mol% Al, the ‘Oi 0.1+Al’, ‘Oa 0.1+Al’, and ‘Oa 1.0+Al’ coprecipitates contained 5, 10, and 16 mg g−1 Al, corresponding to final molar Al/Fe ratios of 0.07, 0.16, and 0.10, respectively. The OC content of the ‘Oa 1.0’ coprecipitate formed from more decomposed aromatic NOM was approximately 38% higher than those of

Implications

The results show that the molar Fe/C ratio and to a lesser extent the NOM type or the presence of dissolved Al controls the OC content and hence the physicochemical properties of the coprecipitates with the ‘M/C 0.1’ samples having a larger particle size and being less aggregated due to their larger OM content. Noteworthy, under conditions of low Fe availability and high dissolved OC concentrations even ‘high-SSA’ Fe-OM phases may precipitate, thus, contrasting the perception that sorption of

Acknowledgments

We are grateful to the German Research Foundation (DFG project MI 1377/3-1) for financial support, Hilal Alemdar and Roger-Michael Klatt for laboratory assistance, and the editor and reviewers for their helpful comments on the manuscript.

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