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1811-5209/08/0004-0395$2.50 DOI: 10.2113/gselements.4.6.395

Nanoparticles in the Soil Environment

INTRODUCTIONIn soil science, the “clay fraction” conventionally denotes a class of materials whose particles are smaller than 2 micrometers (µm) in equivalent spherical diameter (e.s.d.). Soil chemists and mineralogists, therefore, have taken it for granted that this fraction of soil contains particles of colloidal size (1–1000 nm e.s.d.), including nanoscale particles (<100 nm). In this context, the concept of nanoscale and nanosize (~10–9 m) has been known since the advent of colloid science in the 1860s. The term “nano-particles”, however, did not come into general usage until about two decades ago, previous to which such particles were commonly referred to as “fi ne-grained”, “submicron”, or “ultrafi ne”. The prefi x “nano” (as in nanocomposite, nanomaterial, nanoscience, and nanotechnology) has now become part of the scientifi c literature, including that of soil science. Navrotsky (2003), for example, refers to rock weathering as “nanoscale corrosion” and to soil as “a most complex nanomaterial–mixed organic, inorganic, biological, and different sizes” where “much of the transport of nutrients, pollutants, organics, heavy metals, takes place at the nanoscale.”

An important feature of nano particles is that their surface properties can deviate markedly from those shown by their macroscopic (bulk) counterparts. Mineral solubility, for one thing, is expected to increase steeply as particle size

decreases below ~10 nm (Banfi eld and Zhang 2001; Hochella 2002). A similar relationship holds between surface area and particle size. Thus, the surface area of allophane, a soil clay comprising 3.5–5.0 nm spheroidal particles, can be as high as 900 m2/g, depending on the method of measurement (Wada 1989). Furthermore, a large proportion of the structural atoms and ions of nanoparticles are exposed on surface sites where chemical bonds are broken and coordination requirements are unsatisfi ed. The excess energy associated with such surface defects and dislocations lies behind the reactivity of mineral nanoparticles towards external solutes, such as nutrient ions and organic species (Hochella

et al. 2008). By the same token, the tiny size of clay particles relates to their having a high density of structural defects, as a result of which nucleation is favoured over crystal growth (Meunier 2006). Another example from soil of a size-dependent surface energy effect is the alteration of muscovite mica into nanoscale illite, which involves the loss of interlayer K+ ions and a decrease in layer charge. Similarly, the conversion of 2M1 mica into the 1Md polytype in soil is mediated by particle nanodivision (Wilson 1999).

Because of space limitations, only the important fi ndings and conclusions regarding a limited range of soil nanoparticles (clay minerals, metal (hydr)oxides, humic substances) will be described in the present article. As far as possible, we will also refer to review articles, books, and chapters in books, rather than individual papers. Special attention is directed to volcanic soils (Andisols), which are widespread in New Zealand (Lowe and Palmer 2005). This is because Andisols have rather unique mineralogical properties inasmuch as their clay fraction is rich in nanosize minerals of short-range order (allophane, imogolite, ferrihydrite) and (Al,Fe)-humic complexes (Parfi tt 1990; Dahlgren et al. 2004).

Soil is also a repository of many “engineered” nanoparticles that are produced annually in substantial quantities for industrial and environmental applications (cosmetics, electronics, pharmaceuticals, soil remediation, water treatment) or formed as by-products of human activity (biomass burning, fossil fuel combustion, waste incineration). The increasing entry into soil of engineered and anthropogenic nanoparticles (e.g. carbon nanotubes, fullerenes, photocatalysts, semiconductors, soot, black

Soils contain many kinds of inorganic and organic particles with at least one dimension in the nanoscale or colloidal range (<100 nm). Well-known examples are clay minerals, metal (hydr)oxides, and humic substances,

while allophane and imogolite are abundant in volcanic soils. Apparently, only a small proportion of nanoparticles in soil occur as discrete entities. Organic colloids in soil, for example, are largely associated with their inorganic counterparts or form coatings over mineral surfaces. For this reason, individual nanoparticles are diffi cult to separate and collect from the bulk soil, and extraction yields are generally low. By the same token, the characterization of soil nanoparticles often requires advanced analytical and spectroscopic techniques. Because of their large surface area and the presence of surface defects and dislocations, nanoparticles in soil are very reactive towards external solute molecules. The focus of research in recent years has been on the interactions of nanoparticles with environmental pollutants and on their impact on the movement, fate, and bioavailability of contaminants.

J DXV N QC R9�nanoparticles, colloid, soil, clay minerals, allophane, imogolite, metal (hydr)oxides, humic substances, contaminants

Benny K. G. Theng and Guodong Yuan*

* Landcare ResearchPrivate Bag 11052, Palmerston North 4442, New ZealandE-mail: ThengB@LandcareResearch.co.nz;YuanG@LandcareResearch.co.nz

ELEMENTS DECEMBER 2008396

carbon, zero-valent iron/nickel and vehicle exhaust emissions) has raised concerns about their potential adverse effects on animal and human health (Nowack and Bucheli 2007). Nevertheless, a discussion of the nature, properties, and environmental impacts of synthetic nanoparticles is beyond the scope of the present article.

NANOPARTICLES IN SOILWell-known examples of natural nanoparticles in soil are aluminosilicate (clay) minerals; oxides and hydroxides of Al, Fe, and Mn; enzymes; humic substances; viruses; and mobile colloids (Kretzschmar and Schäfer 2005). Organic nanoparticles in soil are mostly associated with their inorganic counterparts or occur as coatings on mineral surfaces (Oades 1989; Chorover et al. 2007). The interactions of enzymes, humic substances, and viruses with clay minerals have been summarized by Theng (2008). By their nature and surface properties, nanoparticles in soil participate in essential ecological services, ranging from regulating water storage and element cycling, through sorbing and transporting chemical and biological contaminants, to serving as a source or sink of organic carbon and plant nutrients.

The formation (and transformation) of nanoparticles in soil may be effected through either an abiotic or a biological pathway, or a combination of both. Clay minerals, for example, are largely formed via an abiotic pathway. On the other hand, humic substances are clearly biogenic since they represent the decomposition products of plant materials (biopolymers), while the formation of some nanosize iron and manganese minerals in soil is effected through a combination of abiotic and biological pathways.

Clay MineralsClay minerals, comprising a family of layer silicates, are the most ubiquitous nanoscale minerals in soil. They are commonly formed by way of an abiotic pathway involving three distinct (weathering) processes: (1) inheritance (from pre-existing parent rocks and other weathered materials), (2) transformation (where the overall layer structure is retained but the interlayer region is markedly altered), and (3) neoformation (by precipitation or crystallization from solution or a gel precursor). Nanoscale micaceous minerals in soil, for example, are for the most part inherited from the parent rock. On the other hand, soil smectites are formed by all three processes, while kaolinite is primarily a product of neoformation (Wilson 1999). Bacteria also play an important role in the formation of mineral nanoparticles, including layer silicates. Indeed, bacteria are well suited for mediating mineral formation in that they have a large surface area/volume ratio, while their cell walls are negatively charged. Accordingly, bacterial cells are effi cient accumulators of metal cations, which then combine with anions (carbonate, phosphate, silicate) from the surrounding medium to form a variety of nanosize minerals. Bacteria can also oxidize or reduce metals, causing precipitation (see Bargar et al. 2008 this issue). The formation of various clay minerals by bacteria in biofi lms and microbial communities, associated with ponds, hot springs, and deep-sea-fl oor vents, is well documented (Tazaki 2006). An example of this is shown in FIGURE 1. Relatively little information, however, is available on the microbially mediated formation and alteration of minerals in soil, except for such special environments as acid mine drainage systems and acid sulfate soils (Douglas and Beveridge 1998; Banfi eld and Zhang 2001).

Short-Range-Order Minerals in Volcanic SoilsThe abundance of short-range-order minerals in volcanic soils is related to the rapid weathering of volcanic ash (“tephra”), as a result of which nucleation and precipitation (of such minerals) are kinetically favoured over crystal growth. The principal nanoscale materials are imogolite, allophane, ferrihydrite, and Al,Fe-humic complexes.

The unit particle of imogolite is a slender hollow tubule with outer and inner diameters of about 2 and 0.7 nm, respectively (FIG. 2A). Transmission electron microscopy images show tubules forming 10–30 nm thick bundles

EHF T QD 1 Formation of clay minerals in biofi lms mediated by microorganisms. (A) Development of biofi lms over

glass slide, beaker wall, and upper surface of a dam sediment after “culturing” for up to two years at pH 6.0–7.4. (B) Optical micrograph of microorganisms, made visible by staining with 4’,6-diamidino-2-phenylindole (DAPI). (C) Microbially associated spherical clay particles (Tazaki 2006)

B

A

C

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several micrometers in length. X-ray diffractometry indi-cates that imogolite has long-range order along the tubule length. The unit formula of imogolite is usually written as (OH)3Al2O3SiOH; this formula gives an Al/Si ratio of 2.0 and denotes the sequence of ions from the periphery to the centre of the tubule where the orthosilicate group shares three oxygens with aluminium. Unlike imogolite, allophane has no fi xed chemical composition in that its Al/Si ratio may vary between 1.0 and 2.0. Irrespective of composition and origin, however, the unit particle of allo-phane is a hollow spherule with an outer diameter of 3.5–5.0 nm and a wall thickness of 0.7–1.0 nm. Defects in the wall structure give rise to perforations of ~0.3 nm in diameter (FIG. 2B) (Wada 1989; Parfi tt 1990; Abidin et al. 2007). Ferrihydrite (Fe1.55O1.66OH1.34; 6-line) is an important nanosize (2–5 nm) constituent of young volcanic soils. Ferrihydrite can transform into hematite through a solid-state reaction, and into goethite by dissolution and precipi-tation (Schwertmann 2008).

The ability of allophanic Andisols to accumulate organic matter – more precisely humic substances (or “humics” for short) – is well known. In some instances, soil burial by repeated additions of volcanic ash is responsible for this accumulation. In general, however, the stability of humics in volcanic soils is due to their strong and intimate associa-tion with allophane. The reactivity of allophane, in this respect, may be ascribed to its large surface area and to the exposure at wall perforations of (OH)Al(OH2) groups capable of forming inner-sphere complexes with the carboxyl groups of humics (Parfi tt 1990; Theng et al. 2005).

Allophane has a strong propensity for sorbing phosphate and arsenate, offering potential for the development of an environmentally friendly method of sequestering arsenic in drinking water and phosphorus in effl uent, as well as for the remediation of As-contaminated soils (Violante and Pigna 2002; Arai et al. 2005; Yuan and Wu 2007). The feasibility of using allophane for water treatment and soil remediation, however, has not been fully assessed. This might be because there are few sites in the world, apart from New Zealand, where allophane-rich materials can be mined.

Non-allophanic Andisols are known to accumulate Al-humic complexes that form when Al3+ ions, released from volcanic glass and primary minerals, react with the carboxyl functional groups of humics at pH <5. At higher

pH, Al3+ undergoes hydrolysis and polymerization, following which the polymerized aluminium species combines with silica to form allophane and/or imogolite structures. Non-allophanic Andisols also contain Fe-humic complexes, but these are generally less abundant than their aluminium counterparts because iron tends to form stable Fe-(hydr)oxides (Dahlgren et al. 2004). We might add here that allophane can apparently discriminate between left- and right-handed forms of amino acids (Hashizume et al. 2002).

Metal (Hydr)oxidesMost soils also contain various “(hydr)oxides” (using this as an umbrella term for oxides, hydroxides, and oxyhydroxides) of Al, Fe, and Mn, formed by weathering of primary and secondary silicate minerals or through microbial pathways.

Among the many species of Al-(hydr)oxides in the soil environment, gibbsite [γ-Al(OH)3] is the most widespread, whereas boehmite [AlO(OH)] is less common. Their formation involves the release (from primary aluminosilicate minerals) of Al, followed by its hydrolysis and precipitation. In many highly weathered tropical soils, gibbsite is formed by desilication of kaolinite. The majority of colloidal Al-(hydr)oxides in temperate soils are of short-range order (poorly crystalline) and occur as coatings over mineral surfaces (Kämpf et al. 2000).

Iron (hydr)oxides occur in nearly all soils, many of which are strongly coloured. Goethite (α-FeOOH), for example, imparts a brown to yellowish brown colour to soils, although this hue is masked when red hematite (α-Fe2O3) is present. Because of their high thermodynamic stability, goethite and hematite are the most common species in soil, while magnetite (Fe3O4), maghemite (γ-Fe2O3), lepidocrocite (γ-FeOOH) and ferrihydrite have also been identifi ed. The formation of iron (hydr)oxides generally involves the release

EHF T QD 2 (A) Atomic structure of a hollow imogolite nanotube with an outer diameter of 2 nm and an inner diameter

of about 0.7 nm. The tube wall consists of a curved Al(OH)3 (gibbsitic) sheet to which orthosilicate groups are attached on the inside. (B) Atomic structure of an aluminium-rich allophane nanoball with an outer diameter of 3.5–5.0 nm. The wall has a thickness of 0.7–1.0 nm, and its composition is similar to that of imogolite. Defects in the wall structure give rise to perforations (white areas). A @MC B QDOQNC T BDC�V HSG�ODQL HRRHNM�EQN L �ROQHMF DQ�’@AHCHM�DS�@K-�1/ / 6(

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ELEMENTS DECEMBER 2008398

of structural Fe(II) from primary minerals (by protolysis), oxidation of the liberated Fe(II) to Fe(III), and precipitation of the resulting, sparingly soluble Fe(III)-(hydr)oxides. Like their Al counterparts, Fe-(hydr)oxides in soil occur as particles of short-range order, varying in size from 5 to 100 nm. Since both Al- and Fe-(hydr)oxides have a point of zero charge (PZC) at pH >8, they are positively charged in the pH range (4.5–7.5) of most soils. Accordingly, these nanominerals play an important role in the adsorption and retention of nutrient anions (e.g. phosphate) by electrostatic interactions and ligand exchange, and they actively promote clay fl occulation and soil aggregate stabilization (McBride 1994; Schwertmann 2008).

Like their iron counterparts, the (hydr)oxides of Mn(III), Mn(IV), and Mn(III,IV) are formed by oxidation of Mn(II) released through weathering of Mn-containing rocks. The oxidation of Mn(II) to Mn(III,IV)-(hydr)oxides can also be mediated by bacteria and fungi. Indeed, most Mn(III,IV)-(hydr)oxides in the environment may be of biological origin. Birnessite and vernadite, the most common species in soil, occur as poorly crystalline, nanosize (20–100 nm) particles, forming coatings on other mineral surfaces or nodules in association with Fe-(hydr)oxides and other soil constituents. In contrast with Al- and Fe-(hydr)oxides, most Mn-(hydr)oxides have a low PZC (pH <4). Since their surface charge is negative in the pH range of most soils, Mn-(hydr)oxides are effi cient sorbents and scavengers of heavy-metal cations (McKenzie 1989; Tebo et al. 2004).

Humic SubstancesHumic substances (“humics”) are the most important class of organic materials in the environment in terms of quan-tity, stability, and ubiquity. Operationally, humics are divided into three fractions according to their solubility in acid or alkali: (1) humic acid (alkali-soluble), (2) fulvic acid (alkali- and acid-soluble), and (3) humin (alkali- and acid-insoluble). The conventional fractionation scheme, therefore, consists of adding dilute alkali to the soil sample, centrifuging (to separate humin), and acidifying the super-natant solution (to precipitate humic acid). Despite two centuries of investigation, we are still unable to assign a molecular structure to humics because of their intrinsic complexity and heterogeneity. A long-held view is that humics are a mixture of linear polymers formed by “heteropolycondensation” of various biological precursors, including phenolic acids, peptides, oligosaccharides, and fatty acids. The polymers so formed are of high molecular weight (>10,000 Da) and adopt a random coil conformation in solution. The macromolecular concept of humics, however, is being displaced by one in which they essentially comprise aggregates of biologically derived molecules of relatively low molecular weight (200–3000 Da) bound by weak dispersive forces and forming micelles in solution (Sutton and Sposito 2005). Because of their structural complexity, humics are chemically recalcitrant. Nevertheless, their biostability in soil is due more to interaction with minerals or even to physical protection within micropores of clay domains than to molecular recalcitrance (Lützow et al. 2006). Only the rare combination of surface interaction and intercalation into expanding clay minerals can effectively protect humics against microbial decomposition (Theng et al. 2005).

FRACTIONATION AND CHARACTERIZATION OF SOIL NANOPARTICLESBecause of their tiny size and tendency to aggregate and interact with each other or form coatings over mineral surfaces, nanoparticles are diffi cult to separate and isolate from the bulk soil, so that yields are generally low (<1%

w/w). Yields may be enhanced by repeated wetting–drying and freezing–thawing, prolonged shaking, ultrasonication, and chemical pretreatment (e.g. with hydrogen peroxide). Allophane and imogolite are notable exceptions in that large amounts (up to 30% w/w) can be separated from volcanic soils. The separated nanoparticles may be characterized by conventional analytical methods, such as X-ray diffractometry, total organic carbon analysis, Fourier-transform infrared and nuclear magnetic resonance spectroscopies combined with atomic force, scanning electron, and transmission electron microscopies. A number of advanced instrumental techniques have also been used to characterize soil nanoparticles. These include X-ray photoelectron, X-ray absorption, and X-ray absorption fi ne structure spectroscopies as well as scanning transmission X-ray microscopy (Burleson et al. 2004; Kretzschmar and Schäfer 2005).

Tang et al. (2009) used a combination of sieving, sedimen-tation, centrifugation, and cross-fl ow fi ltration to separate nanoparticles (<100 nm) from three Chinese soils. Since no chemicals were used at any stage of the procedure, the creation of artefacts was minimized. The size distribution of the nanocolloids was assessed by laser diffraction granu-lometry, while their elemental composition was determined by inductively coupled plasma–atomic emission spectrom-etry (ICP–AES). Transmission electron microscopy of the nanocolloids separated from one of the soils (DX-9) shows spheroidal particles with a diameter of ~20 nm and forming globular aggregates of ~250 nm in size. The aggregates are associated with, and form a coat over, platy particles with a diameter between 50 and 100 nm (FIG. 3). Elemental analysis suggests that the spheroidal particles consist of iron (hydr)oxides embedded in amorphous colloidal silica (Fe/Si = 0.529), while the large platy particles represent kaolinite (Al/Si = 0.931).

CONCLUDING REMARKSAs already mentioned, much of the organic colloids in soil are associated with mineral surfaces that may also be coated with a metal (hydr)oxide. As a result, the sorptive properties of the mineral component can be greatly altered. The

EHF T QD 3 Transmission electron micrograph of nanocolloids separated from a Chinese soil. The spherical particles

(short arrows), with a diameter of about 20 nm and an Fe/Si ratio of 0.529, are assigned to Fe-(hydr)oxide associated with amorphous silica. The large, platy particles (long arrows), with an Al/Si ratio of 0.931, are composed of kaolinite. BNT QSDRX�NE�YX�S@MF

ELEMENTS DECEMBER 2008399

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formation of complexes between humic acid and allophane, for example, leads to a marked increase in Cu and Cd sorption (Yuan et al. 2002). That is why much of the research into the reactivity of soil nanoparticles (e.g. towards contaminants) has been done using laboratory-synthesized materials. Even then, the results are not always easy to interpret because many variables, such as pH, ionic strength, and contaminant loading, influence the nanoparticle–contaminant interaction (Chorover et al. 2007). For example, the affi nity of divalent metal cations for soil organic matter and clay minerals is broadly related to the electronegativity and hydrolysis constant of the ions. No consistent rule of metal selectivity is applicable, however, because of the involvement of the above-mentioned variables (McBride 1994; Churchman et al. 2006). Another important environmental factor is contact time. Thus, the sorption of heavy metals, metalloids, and

organic contaminants by soil nanoparticles shows a rapid initial phase (milliseconds), followed by a slow phase (days) due to diffusion into micropores, structural rearrangement, and precipitation. Likewise, the rate of desorption is biphasic (Sparks 1999). As a result, the bioavailability of contaminants decreases with contact time (“aging”) (Chorover et al. 2007). Nevertheless, trace metals, metalloids, and radionuclides can move within the soil column, and across to adjacent water bodies, by hitching onto mobile colloidal particles (Kretzschmar and Schäfer 2005; Hochella et al. 2008). Because of the intrinsic complexity of soil physicochemical processes, a multidisciplinary approach and the application of modern high-resolution instrumental techniques are required to improve our understanding of sorption phenomena and nanoparticle interactions.

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