Nanomaterials in soils Geoderma

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Jul 11, 2008 - Cradwick, C.P.G., Farmer, V.C., Russell, J.D., Masson, C.R., Wada, K., Yoshinaga, N., 1972. ... Farmer, V.C., Adams, M.J., Fraser, A.R., Palmieri, F., 1983. ... Lee, B.D., Sears, S.K., Graham, R.C., Amrhein, C., Vali, H., 2003.
Geoderma 146 (2008) 291–302

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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a

Nanomaterials in soils Michael A. Wilson a,b,⁎, Nguyen H. Tran b,c, Adrian S. Milev b,c, G.S. Kamali Kannangara b,c, Herbert Volk a, G.Q. Max Lu b a b c

CSIRO Division of Petroleum Resources, PO Box 136, North Ryde NSW 1670, Australia ARC Centre of Excellence for Functional Nanomaterials University of Queensland, Brisbane, Qld 4072, Australia College of Heath and Science, University of Western Sydney, Locked Bag 1797, Penrith South DC 1797, Australia

A R T I C L E

I N F O

Article history: Received 21 October 2007 Received in revised form 7 May 2008 Accepted 4 June 2008 Available online 11 July 2008 Keywords: Soils Nanotechnology Humic substances Nanomaterials Phyllosillicates Imogolite

A B S T R A C T Soils are complex mixtures of solids from millimeter to nanometer in particle size, which may contain moisture. It is now possible to understand these structures using techniques developed for nanotechnology such as Transmission Electron Microscopy and Atomic Force Microscopy. These techniques illustrate the organisation of colloidal material in soils such as phyllosilicates, and humic acids and the discovery of new particles such as nanoparticles of iron oxides. They show that a micellular host guest supramolecular complex is the correct description for humic material. They show how humic materials restructure and rearrange in response to environmental change such as pH and ionic strength. Nanotechnology offers further potential in identifying single cells, individual DNA molecules, proteins, genes and other biological structures in soils. © 2008 Elsevier B.V. All rights reserved.

1. Introduction From time to time areas of science attract attention particularly when new developments occur between fields and new subject areas are born. Such processes are important, since they allow focussing of different disciplines on one area and this itself leads to new advancement. Over the last twenty years and particularly over the last ten, nanotechnology has evolved as an interdisciplinary area, which has attracted great interest. While nanotechnology is not new, it is not chemistry, physics, biology or engineering but a unique blend of all. The Greek and dictionary definition of nano is dwarf, but nanotechnology refers normally to science at the 10− 9 metre level, which is not outside the realms of biochemistry, macromolecular chemistry, or surface science. However, what characterises nanotechnology is the capacity of us now to see structures of this size with the microscope. The rise of transmission scanning electron microscopy and atomic force microscopy and the observation of structures in the nanometer range more than anything has led to the definition of a new discipline. For the authors, the observation of benzene as a hexagon when static and as a donut when spinning (Gimzewski et al., 1998) (Fig. 1) is the single most clear glimpse of what is possible. However it will be a long time before we can follow all reactions by microscopic examination in solution or in soils. Soil science is concerned with the science of all those materials we find in soils. For organisms this matrix can provide nutrition. In ⁎ Corresponding author. Tel.: +61 2 9490 8697. E-mail address: [email protected] (M.A. Wilson). 0016-7061/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2008.06.004

addition it is concerned with those microflauna and fauna that aid this process. This is a complex mix of chemicals and organisms some of which are organised at the nanolevel and some of which are not. The definition of nanotechnology has expanded from the initial discoveries of the capacity to move and locate atoms singularly to something much larger. Today we recognise: 1) atomic nanotechnology (often called molecular nanotechnology), 2) nanomaterials, 3) bionanomaterials 4) molecular mimics, and 5) nanoelectronics (Wilson et al., 2002a,b; Milev et al., 2005). Clearly the area of nanomaterial science is relevant to the analysis of soil structure and composition. However nanobiology relevant to soil biology and instruments for detecting nano quantities of substances relevant to soil processes are also important as are various aspects of nanotechnology applications in environmental science. This review concentrates on the materials aspect of soils, both inorganic and organic structures since this has already developed into a new field. First, the structure of inorganic matter is reviewed and then the structures in soil solution and the organic matter present. Finally some areas, which are developing, are discussed. It is the purpose of this review to stimulate the field rather than be all encompassing. 2. Soil inorganic structure and nanotechnology While transmission electron microscopy (TEM) has been with us for some time, high resolution transmission electron microscopy (HRTEM) is a unique and important technique, which provides information on local atomic structures by direct visualization of atom columns parallel to the electron beam. This can be useful for

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Fig. 1. Static (A) and rotating (B) benzene ring as observed by high resolution transmission electron microscopy (from Gimzewski et al., 1998).

understanding inorganic materials in soils. It has long been known that soils contain material of less than 2 μm in particle size, which are loosely called clays. The most common clays are phyllosilicates which are layered materials with a polymeric silicate base. They may have one octahedral sheet and one tetrahedral sheet (1:1 phyllosilicates) or one octahedral sheet and two tetrahedral sheets (2:1 phyllosilicates), see below. These structures are nanodimensional in one plane. Nanotechnology has allowed atomic packing of phyllosilicates to be observed so that there is already a wealth of date on high-resolution transmission electron microscopy of different phyllosilicates which has been reviewed recently elsewhere (Yuan, 2005; Annabi-Bergaya, 2008), but here we shall consider only that work done on soils. The 2:1 phyllosilicate pyrophyllite has been studied in soils. It is called a 2:1 phyllosilicate because it has one octahedral sheet aluminium and two tetrahedral silicate sheets. Nanotechnology now allows us to see these octahedral and tetrahedral sheets. Fig. 2 modified from Kogure

Fig. 2. HRTEM images of packing in pyrophyllite a simple 2:1 phyllosilicate. The octahedral layers can be seen sandwiched between the tetrahedral layers (Kogure et al., 2006).

et al., 2006, shows that both octahedral (O) and tetrahedral structures (T) in pyrophyllite can be readily observed by HRTEM. The octahedral structures (O) can be seen at the centre of the boundary of the tetrahedral layers (T). The combination of two HRTEM images at the same area but along different recording directions can enable threedimensional determination of the stacking to be seen. Defects in packing in phyllosilicates packing can also be studied. For example chlorite which has a general formula is: (Mg,Fe)3(Si, Al)4O10(OH)2·(Mg,Fe)3(OH)6 can be studied in soils. Chlorite has a 2:1 sandwich structure like pyrophyllite but unlike other 2:1 clay minerals, the interlayer space between each 2:1 sandwich filled contains cations composed of either magnesium ions (called a brucite layer), ferric ions or both as (Mg2+, Fe3+)(OH)6. Fig. 3 shows how layers pack in a defect in chlorite. As the layers are forced together in the defect, they taper like a wedge as some of the layers are squeezed out. These are points where the mineral will cleave if put under pressure and where weathering will occur. 2.1. Weathering of phyllosilicates Indeed snapshots of the weathering process can be observed. Vermiculite is a soil mineral like chlorite but the Mg2+ and Fe2+ cations are not in clearly defined sheets. Fig. 4 shows HRTEM images of chlorite (C, Fig. 4) expanding to vermiculite (V, Fig. 4) by loss of the brucite (Mg2+) layer (Lee et al., 2003). In Fig. 4 at the point C/V the transformations can be seen where the sheets are actually coming apart with larger interlayer spaces. It is clear that the expansion process does not involve destruction of the aluminosilicate sheets. Chlorite can however, weather in a different way. In Fig. 5 HRTEM images of interstratified chlorite are shown (Wang and Huifang, 2006) and can be seen as a series of chlorite layers (C) interdispersed with pyrophyllite (P). It can be seen that the transformation is through amorphous zone (called chaos by the authors). The process appears to be dissolution and precipitation. Growth of a mixed-layer mineral presumably occurs sheet by sheet, and each sheet starts with the formation of a base (Si) tetrahedron, whose structural configuration dimensions determines the type of new layer that forms. The sequence of sheet stacking can be described by kinetic modeling (Xu et al., 1996;

Fig. 3. HRTEM images of defects in packing in chlorite.

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Fig. 4. HRTEM images of Chlorite C expanding to vermiculite V by loss of the brucite layer (from Lee et al., 2003). At C/V the transformation can be seen as the sheets are actually coming apart.

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(Xu et al., 1996) since it is hard to conceive how these got there without precipitation. There are other inorganic nanomaterials in soils. One well known precipitated nanomaterial which is not a phyllosilicate is the tubular aluminosilicate imogolite and its non-tubular precursor called protoimogolite (Cradwick et al., 1972; Wada, 1978; Barron et al., 1982; Farmer et al., 1983). Imogolite is a tube consisting of an aluminium hydroxide sheet with isolated silicon tetrahedra (Fig. 6A). The tube consists of a curved single sheet of modified gibbsite. Whereas gibbsite consists of a flat sheet of octahedral alumina, in imogolite the hydrogens on the hydroxyls on one side of the gibbsite octahedral sheets are replaced by a silica tetrahedra such that the silanol groups are facing towards the inside of the tube. As the Si– O bonds are shorter than Al– O bonds, this substitution causes the gibbsite sheet to curve. The chemical composition of this mineral is given by (OH)3Al2O3SiOH, illustrating the layers of atoms as one passes from the exterior to interior of the tube. The external tube diameter has been shown to be approximately 2.5 nm by TEM (Bursill et al., 2000) consistent with its adsorption properties (Ackerman et al., 1993; Wilson et al., 2002a,b). However atomic force microscopy indicates much wider diameters between 40.2 and 95.5 nm (error +/− 0.1 nm) (Tani et al., 2004). These

Wang and Huifang, 2006), which accounts for two competing factors: (1) the affinity of each end-member structural component for attaching to the surface of the preceding sheet, and (2) the strain energy created by stacking next to each other two silicate sheets with different structural configurations. When the contacting solution becomes slightly supersaturated with respect to both structural components it precipitates amorphous regions which may then later dissolve. Despite these results, the origins of mixed-layer minerals do, however, still remain controversial and there is not general agreement that processes can be by dissolution rather than solid state reactions. There is at least some other cases where the evidence for aqueous solutions playing an important role in the formation of interstratified minerals is strong. For example, phyllosilicates present as a pore filling in recent massive sulfide deposits from a seamount near the East Pacific Rise must have precipitated directly from hydrothermal fluids (Alt and Jiang, 1991). Phyllosilicate mixed layers as a vein filling in hydrothermally alterated minerals must have formed by precipitation

Fig. 5. HRTEM images of interstratified chlorite: It can be seen there are areas of amorphism as they transform (modified from Wang and Huifang, 2006).

Fig. 6. Imogolite A naturally occurring aluminosilicate nanotube. Pores within tubes between aligned tubes and in voids where tubes are not aligned A) structure B) Transmission Electron Micrograph.

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enzymatic activity. That is they had no measured effect on activity and were non toxic. Fullerene C60 could hardly represent all nanoparticles and there is considerable scope in studying the effects of other nanoparticles. Nanoparticles of char from forest or other ecosystem fires may be relevant as are iron, silica and other weathered mineral components. Nanoparticles of soot are almost certainly present in soils since there is a significant literature on carbon in soils (Forbes et al., 2006). Soot particles in soils have been examined by high resolution transmission electron microscopy and show typical nano scaled concentric spheres (Quénéa et al., 2006). 2.3. Fertiliser coatings Fig. 7. Ferric oxide nanoparticles in soils (from Allard et al., 2004).

measurements are on bundles and reflect curvature at the side of the bundle rather than the tube real diameter. However they are useful for analysing the shapes of the bundles. The aluminogermanate analogue of imogolite has also been characterized (Mukherjee et al., 2007) and imogolite has been made into novel nanofibre hybrids with methacrylate and enzymes (Yamamoto et al., 2007). Electron micrographs have shown that the tubes exist in different degrees of order. The number and dispersion of bundles appears to be pH dependent (Karube, 1998). Fig. 6B shows large pores between individual tubes and some tubes bundled together. The consequence of this packing is that the imogolite structure consists of three types of pores. These are (a) intra-tube pores (Type A) (b) inter-tube spaces; which are the spaces between three aligned tubes in a regular packing (Type B), and (c) meso-pores which are the empty spaces which exist between bundles of tubes (Type C). Simple inspection of models shows that the pore size relationship is C N A N B. B pores are so small (0.3–0.4 nm) that they can be ignored in any discussion of adsorption. 2.2. Other weathered nanoparticles In general soil scientists have not usually looked for nanoparticles in soils other than phyllosilicates and imogolite, but if there is intense erosion nanoparticles of iron oxide and aluminium oxide should be present (Fig. 7). The factors affecting deposition and the geochemical reactivity of nanoparticulate iron oxide has been reviewed focusing on the way they could remove toxins (Waychunas et al., 2005). Nanoparticlate iron oxides as colloidal phases of ferrihydrite of 2– 5 nm length have been observed (Fig. 7) and found associated with organic matter in river-borne material (Allard et al., 2004). Zerovalent iron structures have practical use in removing (Giasuddin et al., (2007) humic material. Yang and Watts (2005) have shown that nanoparticles of alumina (aluminum oxide) slowed the growth of roots in five species of plants — corn, cucumber, cabbage, carrot and soybean. It is well known that aluminium causes toxicity in soils to some plants and this can be reduced by chelating agents, however Yang and Watts are the first to show effects of nanoparticular alumina. The smaller the particle, the larger is the total amount of surface area per unit weight and this will affect the particles solubility and capacity to adsorb water and ions. Others have studied non-naturally occurring nanoparticles which might enter soils. Tong et al. (2007) have studied the impact of fullerene C60 on soil microbial communities. They were trying to use this as a test compound for biological activity. They argue that this is an ideal model material for understanding the role of particle size since C60 has some harmful and some neutral biological consequences (Scrivens et al., 1994; DaRos et al., 1996; Moussa et al., 1996; Tusuchiya et al., 1996; Bosi et al., 2000; Tsao et al., 2001; Sayes et al., 2004; Jia et al., 2005; Lyon et al., 2005). They found little differences however in carbon dioxide released with and without fullerene. They also found little differences in phospholipid levels, DNA profiles or soil

The most obvious use for nanoparticles is as slow release fertilisers. Because of the high surface tension they will hold material more strongly from the plant than conventional surfaces. Moreover, nanocoatings can also provide surface protection for larger particles. Direct application of large amounts of fertiliser, in the form of ammonium salts, urea, nitrate or phosphate compounds, may produce extremely high local concentrations which are harmful. Much of the fertiliser may be dissolved in run off water and cause adverse effects such as pollution and will not be available to the plants of interest. Inappropriate short term growth may also occur and the plants become weakened. Ineffective concentrations for long term growth may occur resulting in delayed plant maturity and susceptibility to fungal diseases and pests. Similar arguments apply to trace metals, which may be added as fertilisers. Sulphur coated fertilisers are the most attractive of the slow release fertilisers because the sulphur content may be beneficial, especially for soils low in sulphur (Swanson et al., 1986; Germida and Janzen, 1993; Dana et al., 1994; Blair et al., 1994; Lefroy et al., 1994; Lefroy and Blair, 1995; Singh and Chaudhari, 1995; Santoso et al., 1995; Baker and Gawish, 1998; Brady and Weil, 1999). Other nanomaterials may be useful including delaminated kaolins and those which can trap enzyme inhibitor formulations. Fig. 8 shows a slow release sulphur coated fertiliser analysed in our laboratory. The coating can be from a few nanometers to 100 nm so there is room for improvement of uniformity. The stability of the coating dictates the rate of dissolution, which in turn is determined by the chemical phases, thickness, porosity and surface area. A successful fertiliser coated assembly can be made which consists of a well-adhered layer of elemental sulphur, and strategically released urea and phosphate to meet the soil and crop demands. Indeed, there is the potential for designing slow release fertilisers, which can release a specific nutrient solution suitable for an individual crop. With genetic engineering both crop and nanofertiliser may be matched with precision.

Fig. 8. Scanning electron microscopy of sulphur coated phosphate slow released fertiliser.

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3. Soil solution

Table 1 Characterisation of soil solution colloids (from Citeau et al., 2006)

Recently techniques for analysis of natural organic colloids in solution suspension have recently been reviewed (Lead and Wilkinson, 2006). It is well known that soil solutions contain colloids. The colloids with high specific surface areas are of great environmental importance, because the suspended colloids can carry many sorbed materials in the environment and facilitate transport of pollutants and other materials. Citeau et al., 2006 used two colloidal suspensions isolated from soil solutions collected in situ from the plough layers of two different soils and compared colloidal particle properties including morphology, size distribution, specific surface area, electrophoretic mobility, and surface site concentration before and after oxidation of the organic matter of the colloidal suspensions by hydrogen peroxide treatment. Not surprisingly TEM (Fig. 9) showed that the particles are mainly inorganic and phyllosilicates especially the phyllosilicate illite. The majority of the particle sizes range between 80 nm and 200 nm for the two samples, but for both samples the colloids are thinner than 20 nm. From analysis of images, the basal length and thickness of individual colloids can be compiled. Numberaveraged values are calculated to yield a mean size and a mean height of the colloids present (Table 1). Significant amounts of organic matter in the form of amorphous domains around coating the mineral matter were observed but also some material, which had the structure of fibrils. Fibrils were very thin as shown by the transact profile which can be measure by atomic force

Characteristic

Sample 1

Sample 2

Size (nm) Height (nm) Specific surface area (m2 g− 1) with ρ = 2.0 g cm− 3 Specific surface area (m2 g− 1) after H2O2 treatment with ρ = 2.0 g cm− 3

169 ± 64 9.9 ± 8.2 203 188

172 ± 76 6.5 ± 4.6 260 273

Fig. 9. TEM of a colloidal soil solution (from Citeau et al., 2006). Above — phyllosilicate particles are coated with organic matter. There is also some discrete organic nanoparticles at (i) and (ii) among the bulky illite and other phyllosilicate particles. Below — distances across the transects i) and ii).

microscopy (AFM) (around 1 nm or less, Fig. 9). Fibrils have also been observed by AFM and TEM in freshwaters and were attributed to polysaccharides (Wilkinson et al., 1999). Analysis of the coating of minerals by organic matter could be useful. Conventionally the surface areas of soils have been measured by nitrogen adsorption but there are problems. Organic matter adsorbs relatively little nitrogen and hence the measurements are organic matter dependent and low. In some soils TEM shows samples in which almost all mineral surfaces have thin coatings, consistent with highly sorptive metal oxide surfaces. Other soils, show occlusion of organics on minor amounts of surface area while leaving sufficient mineral surfaces uncovered to dominate the overall surface properties. Nanotechnology makes visual measurements to determine coverage, and hence can be used to clarify surface area measurements by nitrogen adsorption. Thick layers covering small areas would not cause large changes in observed values by nitrogen adsorption. While the area is in infancy, specific surface area of individual colloids can be calculated on the base of their basal length and the thickness and then averaged for the whole suspension. This was achieved by Citeau et al., 2006. The mean calculated values of specific surface area are quite high for the two samples discussed above and are 203 m2/g and 260 m2/g respectively. Table 1 shows that after peroxide oxidation to remove organic matter the surface areas of the samples studied by Citeau et al. 2006 do not change very much, suggesting organic matter on the surfaces is evenly coated in these examples. Soil solution colloids also have a significant role in ion exchange. However differentiating the role of organics which coat the particles, as well as the individual organic particles from mineral materials and their degradation products is difficult but TEM may play a role in making a distinction. Normally the cation exchange capacity of soils, a very important parameter in measuring holding capacity for cations in soils and hence soil fertility, is controlled by organic matter and phyllosilicate content and sometimes by other surfaces capable of holding positive charge e.g. metal oxyhydroxides. While this can be measured by simple wet chemical techniques, the measurement of individual components is usually done by difference. Organic matter may be removed from the soils by strong oxidative chemical interaction, however this can alter the mineral matter. Demineralising mineral matter can be removed but this alters the organic matter. Rather a direct TEM measurement is better, although much needs to be done to determine the cation exchange capacity of different inorganic components. One final cautionary note on using TEM for looking at soil colloids that were in solution. It should not be forgotten that in water solution these materials are hydrated but when examined by transmission electron microscopy they are not. TEM will not give information on hydrated forms. Environmental scanning electron microscopy although not as valuable for high resolution can give detailed information on hydrated material. There are significant differences between hydrated colloids and dried colloids revealed by scanning electron microscopy (Doucet et al., 2005). 4. Soil organic matter Organic matter in soils can be grouped into those compounds also found in living organisms such as carbohydrates and proteins and

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humic materials. Much is known of the former, but little of its nanoscale distribution. Despite the difficulty of investigation and less knowledge of humic materials more attention has been paid to its nanostructure. 4.1. Supramolecular structure Our understanding of humic soil organic matter has largely progressed by understanding its bulk functional group properties. Gel permeation chromatography, (Swift, 1989, 1999), NMR (Wilson, 1987, 1989, 1991), exchange (Tipping, 2002) and pyrolysis gas chromatography mass spectrometry (Saiz-Jimenez and de Leeuw, 1986) have been invaluable. Much has been summarised by Clapp & Hayes, (1999). However despite early suggestions of a super-molecular aggregate structure (Wershaw, 1999) it is only recently that a clearer description has evolved. An association of highly polar macromolecules grouped into micelles which act as host guest complexes explains many of the properties (Conte and Piccolo, 1999; Piccolo, 2001; Cozzolino et al., 2001, Smeulders et al., 2001; Whelan et al., 2003, 2005). More recently others using mass spectrometry agree that humic material is micellular (Sutton and Sposito, 2005; Diallo et al., 2005; Redwood et al., 2005), although they call them fractal aggregates. 4.2. Size of soil organic matter particles Without knowing conformation it is not possible to directly relate molecular weight to size. However, broad generalisations can be made. Assuming a lower limit for a nanometer stretch of humic material is about 5 atoms and about 60Da in weight then a molecular weight of 15,000Da Da is about 250 nm for a linear structure or just over 60 nm if more globular. In early work Thurman et al. (1982) determined a radius of gyration of 1.3 nm for aquatic humic substances using small-angle X-ray scattering which is much smaller than any values obtained on molecular weight. This could be due to internal packing but it could be that Thurman et al. were examining a building unit of a micelle. Schimpf and Petteys (1997) also reported that the hydrodynamic diameter of humic substances was around 1.0 nm. Larger sizes have been observed. Fluorescence correlation spectroscopy (FCS) gives a direct measurement of the values of diffusion coefficients, which can be related to molecular size, and molecular diameters were estimated to be 1.6–2.0 nm by Lead et al. (1999) using this technique. Fluka humic acid and Aldrich humic acid (Manning et al., 2000; Baalousha et al., 2005) are much larger, The mean square radius of

Aldrich humic acid has been shown to be 436.0 ± 35.6nm for example. However, like needs to be compared with like. These commercial materials resembles brown coal humic extracts since their NMR spectra are similar and they are much more lignin like and less degraded by oxidation. The actual geometry is by no means established. The molecular conformations of sphere, sponge, sheets (Namjesnik-Dejanovic and Maurice, 1997), globule (Balnois and Wilkinson, 1999), ring (Maurice and Namjesnik-Dejanovic, 1999) and cone (Liu et al., 2000) have been reported using Atomic Force Microscopy as observation tool. Others using electrophoretic methods suggest humic material to be spheres (Duval et al., 2005; Warwick et al., 2001). Representative images of River Tame and tributaries sampled from the UK (Lead et al., 2005) are presented in Fig. 10 which shows supporting evidence that humic material consists of both fibrils of 1–10 nm in height (taken as a proxy for diameter) and greater than 100 nm in length, and globular material primarily in the range of 1–10 nm (in height), although in some cases up to 60 nm and a depth of ca. 10 nm. 4.2.1. pH and size The knowledge that pH can affect precipitation of a humic solution has been the primary way humic material structure has been investigated by fractionation into humic and fulvic acids solutions. However it is less known that higher pH values of 10 and above produces differently physically precipitated material than at lower pH. At pH 10 (Fig. 11) surface films occur containing pits but not at lower pH. The pH will affect the dissociation of functional groups and thereby, develop differences in amount of negative charge on the humic material. This may affect conformation as the groups repel or align in a different way. For example, Swannee River humic substance, (SRHA) (from the International Humic Substances Society) contains 9.59 and 4.24 mol kg− 1 C of carboxylic and phenolic groups respectively (Baalousha et al., 2006), which dissociate at pH values of 4.42 and 9.68. At pH 4.5 and 7.5, only carboxylic groups will be dissociated, whereas at pH 9.3 both carboxylic and phenolic groups will be dissociated. This results in low surface charge at pH 4.5, intermediate surface charge at pH 7.5 and high surface charge at pH 9.3. At pH 4, there is less dissociation and hence there may not be the same driving force to align because of electrostatic repulsive forces. At neutral pH humic material must reduce electrostatic forces by aligning hydrophobic structures together in a core and aligning charged functional groups in the molecule towards solvating water molecules. At higher pH, there will be more dissociation but it will be energetically more favourable for the hydrophobic groups to align on a surface.

Fig. 10. 0 a) Colloidal River material from the Thame River A) globular material (round objects) and B) Close up of fibrils (long objects) seen between globular material (Lead et al., 2005). AFM images were obtained using a Digital Instruments Dimension 3100 in tapping mode, using a Si3N4 tip with a spring constant of 49 N m− 1.

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Table 3 Effect of time on average humic substance size in calcium chloride solution (modified from Baalousha et al., 2006) pH

Concentration (M)

Average size (nm) at 10 mina

Average size (nm) at 20 ha

4.5 7.5 9.3 4.5 7.5 9.3 4.5 7.5 9.3 4.5 7.5 9.3

0.01 0.01 0.01 0.05 0.05 0.05 0.10 0.10 0.10 0.50 0.50 0.50

50 500 1000 700 750 1600 500 800 1500 250 1300 1600

100 900 800 500 1100 1000 700 1100 750 300 900 800

a

Fig. 11. AFM of a sample from the River Tame at pH 10. This is a surface film. The black dots represent pits on a mica surface (from Lead et al., 2005).

Cozzolino et al. (2001) and Piccolo (2001) added acids to increase the pH of humic solution from 2 to 9.2. They report this increases molecular size conformation. That is, size and shape is pH dependent. These results can be explained by a change of micellular arrangement or macromolecular conformation or both. However, Baalousha et al. (2006) used photocorrelation spectroscopy to measure the size of Swannee River humic material. At constant ionic strength, size increased from 10 nm at pH 4.5 to 22 nm at pH 7.5, and similar changes were observed at different ionic strengths (Table 2). The change in size from 10 nm at low pH to so much larger a size at higher ph does suggest that it is composed of small basic units of less than or equal to 10 nm, which interconnect together and result in larger sizes at higher NaCl and CaCl2 concentrations. It could be argued that this is just a conformational change but it is difficult to see how a four fold increase in size might be achieved. There is considerable scope with current computing techniques in relating actual size with possible supramolecular conformations using lowest free energy calculations. 4.2.2. Ionic strength and size It is not surprising if pH affects size, that the ionic strength of a solution also alters size. The ionic strength alters the observed long range charge and thus conformation. The effect of ionic strength on size has been shown for Suwannee River humic Acid (SRHA) and Adour estuary samples humic material from South West France. (Baalousha et al., 2006; Gibson et al., 2007). The average size of Suwannee River humic acid in NaCl and CaCl2 solutions at different concentrations of 0.001, 0.01, 0.05, 0.1 M, were determined by the photon correlation spectroscopy. Table 3 displays the mean size of the SRHA as a function of the electrolyte (CaCl2) concentration respectively at the same times of dissolution and at different times. For a given time and pH, the size of the Suwannee River humic acid increases with the concentration of the CaCl2. In contrast (last

Table 2 Effect of ionic strength on average size of Swannee River humic material after 10 min dissolution in sodium chloride solution (modified from Baalousha et al., 2006) pH

Concentration (M)

Size (nm)a

4.5 4.5 7.5 7.5

0.001 0.1 0.001 0.1

10 22 22 42

a

Errors are approximately 20%.

Errors are approximately 20%.

column, Table 3), after 20h of sample preparation at a given CaCl2 concentration, the size of the Suwannee River humic acid increases with pH change from 4.5 to 7.5 and decreases again as pH increases to 9.3. Additionally for a given pH, the SRHA mean size increases with CaCl2 up to 0.05 M and then decreases again up to 0.5 M. The comparison in Table 3 shows that the SRHA size decreases with time at pH 9.3, whereas it shows a slight increase at pH 7.5 and shows no significant changes at pH 4.5. It is not shown here, but a big increase in humic size occurs for small changes in CaCl2 concentrations in the range b 0.02 M CaCl2 and a small increase occurs for higher concentrations in the range N 0.02 M CaCl2. It appears that there is a critical ionic strength at which the humic material is at maximum size. Moreover comparison of Tables 2 and 3 show that the cation is also important, not just ionic strength. TEM micrographs also show the increase of the SRHA network size with increasing CaCl2 concentration. TEM micrographics of five samples from the Adour estuary at different salinities show the same effects. They show an open branched network of organic matter at low values of salinity and this network becomes more connected with increasing salinity and has larger size with salinity increase. Baalousha et al. (2006) suggest a similar explanation to the pH effect. At low pH (4.5), the functional groups are weakly dissociated, implying low negative charges on the humic substance, and thus the ionic concentration will not affect size dynamics very much. In contrast, at high pH (9.4) most of the functional groups are dissociated, implying a high negative charge on the HA surface, and consequently, there is high effect of Ca2+ concentration and the humic materials may need time to find an equilibrium structure. There is other evidence of size effects of pH and ionic strength. Wang et al. (2001) showed that the diffusivity of SRHA increases with decreasing pH and increasing calcium concentration from 0.0041 to 0.00548 M Ca, and they explained this result by the compaction of humic acid molecules at low pH and increasing ionic strength. It is clear that the aggregation process kinetically takes hours rather than seconds. This is probably because the entropy of activation of such rearrangements are large negative numbers since they would involve a high degree or reordering. Baalousha et al. (2006) studied size after 10 min solution preparation and 20h solution preparation. Results show no significant changes in the SRHA size with time at pH 4.5, a small increase at pH of 7.5, and a significant decrease in size at pH 9.3. They suggest that this decrease in the size of the SRHA network (aggregates) might be related to the intra-aggregate contraction in which the entire aggregate can be compacted by the addition of cations because after a longer period of time some inaccessible negative charges become more accessible. However it could be that the first formed large aggregates are not the thermodynamically most stable ones and that a second time period is needed for rearrangement. It is clear that humic aggregation process takes place over hours, days, and

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weeks, and thus, the time effect should be considered when reporting their size and/or their molecular mass. This work strongly supports the concept of a supramolecular structure in which SRHA is composed from units of about 10 nm and the supramolecular structure of the SRHA (network) results from the association of these basic building units. The main factors affecting this supramolecular structure and controlling the size of the humic substance network are the pH, the cation type and concentration, and residence time. 4.2.3. Concentration effects If ionic strength is important humic materials themselves may alter ionic strength since at high concentration individual humic molecules affect the ionic environment of others. Some workers have found concentration effects size. Prazas et al. (2003) have shown that in solutions of 1 g/L, fulvic acid at pH 2–3 and humic acid at pH 10, form spherical particles with maximum diameters of about 3.5 nm but single huge agglomerates with size up to 2 μm at higher concentration (10 g/L) were found. 4.2.4. Polar fractionations Natural organic matter (NOM) and its fractions from River Songhua have been concentrated and fractionated with reverse osmosis technology and ion exchange resins (Gua and Ma, 2006). The effect of solution properties (pH, Ca2+) on natural organic matter (NOM) microtopography were also studied in this work. Again, spherical conformation with relatively smaller diameters was observed in the acidic condition compared with the neutral and basic condition. In the presence of 0.1 mol L− 1 NaCl, aggregation phenomena were observed in all pH conditions. Reverse osmosis (RO) technology was conducted to concentrate NOM from the Songhua river. XAD ion exchange resins were used to fractionate it into the more hydrophobic fractions (HyO), more hydrophilic fractions (HyI), humic acid fractions (HAF) and fulvic acid fractions (FAF). After the NOM with its fractions were adsorbed on mica, tapping mode AFM in air was used to probe their morphology. The humic material was separated into various fractions. The characteristic comparison among the NOM and other fractions are listed in Table 4. The (UV254/ TOC × 100) called the SUVA value was also measured and represents an aromaticity parameter. As shown in Table 4, the lowest SUVA value was the more hydrophilic fractions (HyI) of NOM. The Mw followed the order HyI (4228) N HAF(3392)N HyO (3258)N NOM(3049) NFAF(2772). The polydispersity of the organic matter (P) decreased in the order HAFNHyINNOM≈HyON FAF. Dh, the hydrodynamic diameter was also measured assuming similar geometry. The calculated hydrodynamic diameters Dh (2Rh) assuming the same geometry were all around 3 nm which is in general agreement with measurements discussed above for unseparated humic material. Interestingly, the molecular aggregation phenomenon is thus proved to be ubiquitous in that the same size is produced for different fractions. This is supported by AFM data. Fig. 12 shows tapping mode AFM data of the isolated NOM adsorbed on mica. The height distributions

Table 4 Songhua River humic fraction characterisation (from Gua and Ma, 2006) Sample

SUVAa

Mw (RSD%)b

Mn (RSD%)b

Ρc (RSD%)b

Dhd (nm)

HAF FAF HyO HyI NOM

1.93 1.71 1.80 1.10 1.99

3392 (1.08) 2772 (1.04) 3258 (1.15) 4228 (1.87) 3049 (1.65)

1175 (1.12) 1549 (1.07) 1647 (1.09) 1776 (1.45) 1461 (1.37)

2.89 (2.37) 1.79 (2.01) 1.98 (1.97) 2.38 (2.26) 2.09 (2.25)

3.15 2.84 3.08 3.51 2.98

a b c d

The (UV254 / TOC × 100). Relative standard deviation see Zhou et al., 2000. Polydispersity Mw / Mn. Hydrodynamic diameter Dh = 2 Rh; Rh = 0.27√Mw.

Fig. 12. Tapping mode-AFM of isolated NOM adsorbed on mica (50 mg L− 1, 0.1 mol L− 1 NaCl, pH 7.3) (from Gua and Ma, 2006).

were measured for each of the polar fractions. A range of distributions were found for each of these and concentrations of different particles. The fulvic acid largest particle was 7 nm but greatest concentration was of 5 nm and the 5 nm was also the greatest concentration of HA, and HyI particles, although particles approximately twice this size were observed. It might be argued that these larger particles represent aggregates of particles all about the same size. However, the most significant result is that the polar fractions are all not that much different in size. 4.2.5. Chelation effects Ions in natural solution may play a role in affecting conformation by changing local electrostatic environments but they may also chelate (Tipping, 2002). Prazas et al. (2003) found in the presence of copper ions, disordered agglomerates and network structures are formed, depending on the Cu2+ concentration, thus showing Cu2+ may alter molecular weight by other methods. Baalousha et al. (2006) also report that there is initial decrease in Leonardite humic acid (a brown coal) aggregate size when adding small amount of metal ions (Na+, Mg2+, Sm3+). However, further addition of metal ions results in an increase in the aggregate size. They related the decrease of the humic substance size to the compaction of the individual humic acid molecules due to the intramolecular contraction through surface charge neutralization and cross-linking of the humic acid. Furthermore, they suggested that the addition of more metal ions leads to intermolecular associations, and thus, an increase in aggregate size. Some researchers have looked at the effects of ferric salts (Jung et al., 2005). Experiments were conducted with reconstituted water containing either synthetic or natural extracts of humic substances, and then with water from Moselle River (France). The characterization of the freeze-dried coagulated sediment by electron energy loss spectroscopy (EELS) in the 250–450eV range showed not surprisingly that Fe-coagulant species predominantly associate with the carboxylic groups of organic matter, and that this interaction is accompanied by a release of previously complexed calcium ions. The variation of Fe/C elemental ratio with iron concentration provides insightful information into the coagulation mechanism of humic substances. At acidic pH, Fe/C remains close to 3 over the whole range of iron concentrations investigated, while a much lower atomic ratio is expected from the value of optimal coagulant dosage. This suggests that both a charge neutralization and a complexation mechanism is responsible for the removal of the humic colloids, the aggregates being formed with both iron-coagulated and proton-neutralized organic compounds. At pH 8, the decrease in Fe/C around optimal coagulant concentration is interpreted as a bridging of stretched humic macromolecules by Fe-hydrolyzed species. Aggregation would then result from a competition between reconformation of humic chains

M.A. Wilson et al. / Geoderma 146 (2008) 291–302 Table 5 Thickness of film and sorbed mass of humic material on cantilevers in AFM (from Lead et al., 2005) Resonance frequencya

Resonance frequencyb

Added mass 10− 10 (g) Thickness (nm)

18807 +/− 16 19191 +/− 6 19312 +/− 6

18,082+/−8 19,291+/−6 19,107+/−3

4.40 0.22 1.17

a b

102 5 27

In air before immersion. In air after immersion.

around coagulant species and collision of destabilized humic material. EELS also enabled a fingerprinting of natural organic substances contained in the iron-coagulated surface water, N/C elemental analyses revealing that humic colloids are removed prior to proteinic compounds. Some interesting results are presented by dos Santos et al. (2005). The shape of gold nanoparticles precipitated from solution is dependent on fulvic acid concentration and pH. What is particularly interesting is that depending on pH the shapes are hexagons, triangles or truncated triangles. The particle shape must be controlled by the structure and coordination of the fulvic acid. Hence this might be a unique way of analyzing structure of different fractions. 4.3. Surface films Organic matter may be deposited as surface films on minerals, especially with phyllosilicates, oxides and hydroxides, in river waters and soils and some researchers have appreciated that the organic matter has a more enhanced role in this physical state. Surface films control exchange and flow properties and therefore are important. A surface film of organic matter will have different properties since it is a three-dimensional solid in which one of its dimensional properties may be modified by the surface it is adsorbed on. Nanoscale surface films are known to develop on surfaces exposed to natural waters and have potential impacts on many environmental processes including blocking pathways by which petroleum may be released in rocks which petroleum may migrate through. Suwannee River humic acid was exposed to mica, and the surface film thickness as a function of pH and exposure time was measured (Gibson et al., 2007). Discrete and very small colloids in the range 1–5 nm were observed as expected. Low pH values of 2 gave rise to relatively thick surface films of about 3 nm, although these films were not continuous at higher pH values. At pH 4.8, the film thickness increased with exposure time up to about 5h and did not subsequently increase. The maximum film thickness measured was about 1 nm at that pH. Lead et al. (2005) studied a surface film, of at least several nanometers thickness, which coats mica surface within 30 min of exposure to river water. Coherent surface films only develop at high

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concentrations of (ca. 1000 mg L− 1) of extracted humic acid, while coverage is much lower at more realistic concentrations. AFM heights along a transect of a mica slide after exposure to distilled water and river water at various pH values have been studied. The mica surface exposed to distilled water is essentially flat on an atomic scale with variations in height of a few tenths of nanometers over several micrometers of length. However, after exposure to river water, a substantially increased height variability of several nanometers was observed. No variation of the surface layer thickness was observed as a function of pH, despite the difference in discrete colloids but at pH 10 pits occurred. Table 5 from Lead et al. 2005 shows values of added mass and surface thickness calculated from resonance frequency changes on immersion of cantilevers in samples of river water. After immersion in distilled water no material was detected sorbed to the cantilever. For waters containing material nominally less than 1 μm in size, good agreement was observed between replicates with an average film thickness of 29 nm. In the unfractionated water sample a surface film certainly developed, but poor agreement between replicates was obtained (5 and 102 nm). The reason for this is unclear to the authors, but the heterogeneity and polydispersity of natural aquatic colloids would certainly contribute. For instance, the 102 nm film may have been based on the sorption of a small number of relatively high mass particles. In addition, the small surface areas and absolute values of mass measured would contribute to this variability, and repetition of these initial experiments with cantilevers with larger surface areas and resonant frequencies will be explored in the future to give more detailed information. In particular, the results indicate that surface films developed are unlikely to be composed of monolayers of very fine colloids. 4.4. Molecular composition of the aggregates Recent advances in high pressure liquid chromatography (HPLC) have allowed the separation of many components of humic materials at a level hitherto impossible. With a normal 250 μL injection at 6 mg/ ml this represents 6 × 250/106g in a sample. With the average molecular weight of the extract about 50,000Da, 300 × 10− 10mol are observed when injected, and the resolution per chromatogram is 25 × 10 −14mol or about 15 × 109 molecules per peak. With this degree of separation the spectrum of high pressure liquid chromatographic humic material is not a continuum, but rather as clusters of peaks (Perminova et al., 1998; Piccolo and Conte, 2000; Whelan et al., 2005). These clusters represent some form of molecular aggregation based on their size and/or polarity and reflect the fact that only certain configurations of molecular structures are warranted. The reason(s) for formation of such aggregates and their corresponding molecular weights are important parameters to consider, as these will

Fig. 13. Two dimensional HPLC of humic substances (from Whelan et al., 2005). Three-dimensional surface representation of humic fractions cut from the first dimension at 3.32, 3.64, 3.80, 4.20, 4.68, 5.08, 5.48, 5.88, 6.28, 6.68, 7.08, 7.48 and 7.88 min that were subsequently separated in the second dimension.

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provide valuable information on the chemical dynamics of humic substances. Two dimensional HPLC has promise which allows separation on two structural features of the molecules. The chromatograph shown in Figs. 13 illustrates the separation that was achieved in the first dimension on a BioSep-S2000 size-exclusion column and a Phenomenex Synergi polar-RP reversed phase column running a curved gradient using formic acid (0.1%) and acetonitrile. Hundreds of fractions can be collected and the data exhibited as a movie or “flipogram” (Whelan et al., 2005). In the chromatogram shown in Fig. 13 there is a single broad band eluting virtually throughout the entire exclusion range of the column, with some secondary retention phenomena obviously apparent. The broad elution profile indicates that there exists a substantial size distribution of the humic substances with the majority of the sample eluting around 3.80 min. The shaded regions in Fig. 13 indicate fractions (200 μL cuts) that were cut from the first dimension at and loaded onto the second dimension for further separation on the basis of polarity. Fig. 13 shows that it is possible to resolve uniform band profiles that show promise of containing essentially pure individual components. In the three fractions that have been cut from the first sizeexclusion dimension and 18 components were resolved to baseline in the second dimension, with many other peaks exhibiting less resolution. Analysis of the three bands collected from the second dimension fraction cut at 6.92 min by mass spectrometry clearly indicated that they contained some compounds that are of simple structure e.g. simple aromatic carboxylic acids and phenols (Whelan et al., 2005). This begs the question why are small molecular weight compounds present in molecular weight fractions which are supposed to be of larger molecular weight, and are they present as some sort of structure that when separated in the second dimension disaggregates? The results would suggest so. Indeed there are two possible explanations for these observed phenomena. Firstly small molecules were entrapped in a complex of larger molecules and were transported through the size-exclusion column, eluting entrapped with the larger molecules, which has been referred to as the “hidden host guest model”. Alternatively, small molecules were interbound to each other but could not be separated from the host complex during transportation through the sizeexclusion column, which has been referred to as the “micellular host guest model”. It is significant that the humic substances that were eluting closest to exclusion contained less of the discrete molecular material than eluting at longer times. This shows the complexes are different and not just larger entities with the same components. It also suggests that the lower molecular weight material separated in the second dimension is capable of entrapping the small guests more than the larger molecular weight material. Some of the prepared materials from these fractions have very distinct micellular structure under TEM not unlike those structures found for micellular proteins and much clearer and regular than those described for unfractionated material. They offer evidence that these may be individually characterised once small guests may be removed. It is certainly clear that this work agrees well with the work described in other parts of this section. The structure is micellular, but within the micelles small guests exist. Indeed the distinction between guests and host is blurred since the host may be a micellular collection of building units. 5. Future issues As we discover more inorganic nanoparticles in soils it is probable that we will realize their large surface areas are important in geocatalysis. These surface areas may play a significant role in holding soil enzymes. While the number of particles is small it is the total surface area that is important so that a component present in less than

0.1% concentration may still be process controlling. These particles are also the size of microorganisms and hence much needs to be discovered how they affect their health. Two areas where organic matter will be significant are in global warming and space exploration. It is probable that we are at a key point at understanding organic matter structure by looking at organic matter composition by TEM and there are a number of useful practical matters that could be explored. Soils contain 82% of terrestrial carbon (see for example Robertson et al., 2000). Enhancing the natural processes that remove carbon dioxide from the atmosphere could be the most cost-effective means of reducing atmospheric levels of carbon dioxide. Soil organic carbon is the largest reservoir in interaction with the atmosphere. According to United Nations Food & Agriculture Organisation data (Robertson et al., 2000) vegetation contributes 650Gt, atmosphere 750Gt Gtons, and soil 1500Gt. Storing carbon in agricultural soils presents an immediate option to reduce atmospheric carbon dioxide and slow global warming. Farmers who adopt practices that store carbon in soil may be able to “sell” the stored carbon to buyers seeking to offset greenhouse gas emissions. Before farmers can sell carbon credits, however, they need to be able to verify that changing soil management has increased the soil organic carbon (SOC) in their fields. Bricklemyer et al. (2007) estimated tillage effects on soil organic carbon content and soil organic carbon change using soil texture, weather, and farm management information. They measured carbon storage and soil texture in 10 paired fields under notill and conventional-till management. They estimated the increase in carbon stored under tillage adoption as the difference between carbon levels in no-till and till fields. This work while preliminary illustrates what could be achieved. The effects of surface area such as clay content, microbiological activity and other factors could be important under different climatic conditions and can assist in establishing the value of agriculture to carbon credits. Nevertheless it is clear a more detailed analysis of soils revealed by nanotechnology will allow better estimates of carbon uptake and carbon credits. Finally it is not unreasonable to suggest that nano is by no means as small as we can expect to go. El-Azhari et al. (2007) measure PcaH, a molecular marker for estimating the diversity of the protocatechuatedegrading bacterial community in the soil environment. It might constitute a suitable molecular marker to estimate the response of the protocatechuate-degrading bacterial community to agricultural practices. Smaller still is possible. Some researchers (Ilic et al., 2004) have been able to detect the mass of a single cell using submicroscopic devices. They have moved beyond the prefixes “nano” “pico (10− 12)” and “femto (10− 15)” to “atto (10− 18)”. They report using tiny oscillating cantilevers to detect masses as small as 6 attograms by noting the change an added mass produces in the frequency of vibration. The mass of a small virus, for example, is about 10 attograms and hence can be measured. We should therefore be able to eventually weigh viruses in soils. After testing various cantilever lengths and another type of oscillator suspended between two points, they calculated that the minimum resolvable mass would be 0.37 of an attogram. They report that with refinements, the devices could be extended to the zeptogram range, or one one-thousandth of an attogram. The sensitivity is such that the devices could be used to detect and identify DNA molecules, proteins and other single biological molecules by coating the cantilevers with appropriate antibodies or other materials that would bind to the targets. This will one day, but not now be applied in soil science. In humic substance chemistry, the search for order goes on which looks promising, as we now understand the micellular concept. Nevertheless the inputs to humic material are so complex and varied, and then the effects of oxidation on these dependent on molecular type as well as diversity in climate and microbial activity so varied as well that disorder is more expected than order. While it has been remarked in

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