Properties and reactivity of Fe-organic matter associations formed by coprecipitation versus adsorption: Clues from arsenate batch adsorption
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.
References (92)
- et al.
Influence of humic acid imposed changes of ferrihydrite aggregation on microbial Fe(III) reduction
Geochim. Cosmochim. Acta
(2012) - et al.
Particle size, charge and colloidal stability of humic acids coprecipitated with ferrihydrite
Chemosphere
(2014) - et al.
Mobilization of arsenic by dissolved organic matter from iron oxides, soils and sediments
Sci. Total Environ.
(2006) - et al.
Arsenic distribution in the dissolved, colloidal and particulate size fraction of experimental solutions rich in dissolved organic matter and ferric iron
Geochim. Cosmochim. Acta
(2009) Sorption of arsenic by iron oxides and oxyhydroxides in soils
Appl. Geochem.
(1994)- et al.
Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces
Geochim. Cosmochim. Acta
(2001) - et al.
Geochemical processes controlling fate and transport of arsenic in acid mine drainage (AMD) and natural systems
J. Hazard. Mater.
(2009) - et al.
Properties of impurity-bearing ferrihydrite I. Effects of Al content and precipitation rate on the structure of 2-line ferrihydrite
Geochim. Cosmochim. Acta
(2012) - et al.
XPS studies on the electronic structure of bonding between solid and solutes: adsorption of arsenate, chromate, phosphate, Pb2+, and Zn2+ ions on amorphous black ferric oxyhydroxide
Geochim. Cosmochim. Acta
(2000) Aluminum complexation by humic substances and aluminum species in the soil solution
Geoderma
(1994)
Mechanisms of arsenic adsorption on amorphous oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface complexation modeling
J. Colloid Interface Sci.
Control of organic and iron colloids on arsenic partition and transport in the Hetao basin, Inner Mongolia
Appl. Geochem.
Effect of adsorbed and substituted Al on Fe(II)-induced mineralization pathways of ferrihydrite
Geochim. Cosmochim. Acta
A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents
Process Saf. Environ. Prot.
Interaction of Fe(III) and Al(III) during hydroxylation by forced hydrolysis: the nature of Al–Fe oxyhydroxy co-precipitates
J. Colloid Interface Sci.
Mobility of Fe(II), Fe(III) and Al in acidic forest soils mediated by dissolved organic matter: influence of solution pH and metal/organic carbon ratios
Geoderma
The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils
Org. Geochem.
Sorption of DOM and DOM fractions to forest soils
Geoderma
Biogeochemistry of paddy soils
Geoderma
Removal of trace metals by coprecipitation with Fe, Al and Mn from natural waters contaminated with acid mine drainage in the Ducktown Mining District, Tennessee
Appl. Geochem.
Complexation of arsenate with humic substance in water extract of compost
Chemosphere
Kinetics of lead adsorption by iron oxides formed under the influence of citrate
Geochim. Cosmochim. Acta
Adsorption kinetics of phosphate and arsenate on goethite. A comparative study
J. Colloid Interface Sci.
Alteration of ferrihydrite reductive dissolution and transformation by adsorbed As and structural Al: implications for As retention
Geochim. Cosmochim. Acta
Extent of coverage of mineral surfaces by organic matter in marine sediments
Geochim. Cosmochim. Acta
X-ray absorption spectroscopy study on the effect of hydroxybenzoic acids on the formation and structure of ferrihydrite
Geochim. Cosmochim. Acta
Effect of citrate on the local Fe coordination in ferrihydrite, arsenate binding, and ternary arsenate complex formation
Geochim. Cosmochim. Acta
Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part II: siderophore-promoted dissolution
Geochim. Cosmochim. Acta
Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part I: characterization
Geochim. Cosmochim. Acta
Stabilization of extracellular polymeric substances (Bacillus subtilis) by adsorption to and coprecipitation with Al forms
Geochim. Cosmochim. Acta
Characterization of Fe(III) (hydr)oxides in arsenic contaminated soil under various redox conditions by XAFS and Mössbauer spectroscopies
Appl. Geochem.
Dissolved organic matter, aluminium and iron interactions: precipitation induced by metal/carbon ratio, pH and competition
Sci. Total Environ.
Hydrous ferric oxide precipitation in the presence of nonmetabolizing bacteria: constraints on the mechanism of a biotic effect
Geochim. Cosmochim. Acta
Properties of organic matter precipitated from acidic forest soil solutions
Org. Geochem.
Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy
Geochim. Cosmochim. Acta
Changes in humic acid conformation during coagulation with ferric chloride: implications for drinking water treatment
Water Res.
The effects of adsorbed humic substances on the surface charge of goethite (α-FeOOH) in freshwaters
Geochim. Cosmochim. Acta
Sorptive stabilization of organic matter in soils by hydrous iron oxides
Geochim. Cosmochim. Acta
Occurrence of arsenic contamination in Canada: sources, behavior and distribution
Sci. Total Environ.
Arsenite and arsenate adsorption on coprecipitated bimetal oxide magnetic nanomaterials: MnFe2O4 and CoFe2O4
Chem. Eng. J.
Residence time effects on arsenate surface speciation at the aluminum oxide–water interface
Soil Sci.
Long-range Forecasting: From Crystal Ball to Computer
The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms
J. Am. Chem. Soc.
Adsorption of gases in multimolecular layers
J. Am. Chem. Soc.
Scavenging of As from acid mine drainage by schwertmannite and ferrihydrite: a comparison with synthetic analogues
Environ. Sci. Technol.
Composition and structural aspects of natural occurring ferrihydrite
C. R. Geosci.
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