Mass Fractal Dimension Research Articles

Agglomeration characteristics of nano-size TiO2 particles using analytical solution

We developed equations for nano-sized titanium dioxide (TiO2) particles self preserving time (SPT) lag that combines with agglomerate key parameters such as primary particle size (PPS), geometric standard deviation (GSD) and mass fractal dimension (MFD). A statistical formula has been developed that relies on SPT lag as the key parameter of agglomerates. Finally, this research presents the first analytical solution by integrating these key parameters into one formula, which can be utilized as a handy tool to calculate the time for reaching the asymptotic state.

Investigation of ambient aerosol effective density with and without using a catalytic stripper

Size-resolved effective densities of ambient aerosols in Riverside, CA were determined over 4 periods during 2015–2016. A DMA-CPMA-CPC technique was used to measure effective density for particles with selected diameters of 50, 70, 101 and 152 nm. A catalytic stripper (CS) was used alternately to remove the volatile fraction of aerosol before density measurements. Aerosol non-refractory composition measurement was conducted in June 2016 campaign to understand the effect of chemical composition on particle density. The average densities for particles over all the measurement campaigns over BP mode (i.e. bypassing the CS) were 1.17 g/cm3 at 50 nm and 1.25–1.28 g/cm3 at 70, 101 and 152 nm. The average density after CS conditioning (CS mode) showed a decreasing trend from 1.22 g/cm3 to 1.04 g/cm3, with increase in the selected size, and a mass fractal dimension (Df) of 2.85. Both the BP and CS mode particles showed the lowest effective density at 6–9 am and highest density at 11 a.m.-3 pm. The diurnal variation of density for both modes became more intensive as particle size increased. The variation was also more intense for the CS mode compared to the BP mode. Organic aerosol and ammonium nitrate mass in the size range of density measurements correlated well positively (R2 = 0.78) and negatively (R2 = 0.62), respectively with BP mode effective density. The study provides an update to the aerosol density profiles of a well-known receptor site (Riverside, CA) and investigates the transformation of particles in different seasons. The effective density profiles will be used in a follow-up study to better estimate the respiratory-deposited ambient aerosol mass.

Lateral hydraulic performance of subsurface drip irrigation based on spatial variability of soil: experiment

Under the zero slope conditions of surface drip irrigation the emitter discharge along the lateral is also decreased for non-compensating emitters. However, in subsurface drip irrigation the lateral emitter is buried in the soil and the pressure of the emitter outlets is affected by soil physical properties. Because of the spatial variability of soil physical properties, the emitter discharge along the lateral is complex; the traditional discharge equation could not show the influence of soil properties on emitter discharge. A new emitter discharge formula is established that has soil initial water content, soil bulk density, mass fractal dimension, and head pressure as factors, and a nonlinear mathematical model of lateral hydraulics based on the emitter discharge formula is created. Experiments were then conducted to: verify the model, compare the HEPING emitter (with an inside diameter of 12 mm and outside diameter of 13 mm) and PLASSIM emitter (with an inside diameter of 14 mm and outside diameter of 15.2 mm), and assess the relative contribution of various factors to emitter discharge. The results of the experiments indicate that: ① the emitter discharge along the lateral head in the direction of flow followed a decline law in homogeneous soil, and decreased in a fluctuating manner in heterogeneous soil; ② the larger standard deviation of soil initial water content, bulk density, mass fractal dimension when each factor remained the same, the more disperse the emitter discharge along lateral.

Classification and fractal characteristics of coal rock fragments under uniaxial cyclic loading conditions

Rock burst is a serious geological hazard in deep underground mines affecting progress of mining operations. Although rock burst is a complex process, a distribution law of fractal characteristics can explain the rock failure mechanism. Using a servo-controlled testing system, uniaxial cyclic loading tests on coal rock specimens were conducted to investigate the fractal characteristics of the fragments under different loading rates. To comprehensively characterize the coal fragments of different sizes, samples were divided into four groups of different size: particles, fine, medium-size, and coarse fragments. The distribution of the fragments under uniaxial cyclic loading conditions was then investigated based on the theory of fractal geometry, and the relationships between fractal dimensions and loading rates. Under uniaxial cyclic loading and unloading conditions, most of the fragments are irregular wedges and bulks, exhibiting obvious shape characteristics. Under various loading rates, the length-quantity fractal dimensions of the coal fragments ranged from 0.74 to 1.44, the width-quantity fractal dimensions range from 0.44 to 1.65, and the thickness-cumulative mass fractal dimensions range from 1.0 to 1.33. The coal rock’s crushing size-mass fractal dimensions under different loading rates were 2.27, 2.30, 2.32, and 2.35, respectively. Under a small loading rate, the dimension-quantity fractal dimensions are relatively small, suggesting that the coal rock was less crushed, with large fragments differing greatly in length, width, and thickness. The results show that the coal rock fragments exhibit certain shape characteristics after the cyclic loading, like irregular shapes and wedges. Under a larger loading rate, the fragments showed greater fractal dimensions of both size and mass; the coal samples crushed more thoroughly with more uniform fragments in length, width, thickness and mass. The conclusions obtained in this study confirm the classification and fractal characteristics of coal rock fragments by uniaxial cyclic loading conditions in laboratory test and provide the basis for further study on the mechanism of rock burst. This study is helpful for us to make a thorough inquiry the danger degree of rock burst in coal mine by using fractal geometry, understand the effects of methane to coal and the evolution mechanism of cracks, and it can be applied to the research on occurrence mechanism and early warning of rock burst.

Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms

This work investigates emulsion templating to synthesize hexadecane oil/geopolymer composites. In a system with hexadecane as the internal (dispersed) phase and an alkali activated continuous phase without added surfactant, adding aluminosilicate clay particles does not increase resistance against creaming or coalescence, while adding a surfactant (L35 or CTAB) stabilizes the solid-liquid interface. Infrared studies and rheological studies of the associated geopolymerization determined that the presence of the organic phase or surfactant has no significant effect on the geopolymerization kinetics, as determined by the change in time of the Si-O-T IR stretching frequency and the rheological moduli involved during the process. The stabilization of the organic template is reminiscent of Pickering emulsion even though we employ a much greater amount of inorganic material for geopolymer formation. Although the addition of surfactant has a significant effect on the behavior of the paste, the percolation of the network remains unmodified, highlighting the fact that the phenomenon is not dependent on viscosity. Finally, rheological measurements were used to obtain the mass fractal dimension of the as-made gel network, which is able to differentiate the interfacial effect between surfactant molecules with a slightly denser interphase when a cationic surfactant is used.

Three-Dimensional Lattice Boltzmann Simulations of Single-Phase Permeability in Random Fractal Porous Media with Rough Pore–Solid Interface

Single-phase permeability k has intensively been investigated over the past several decades by means of experiments, theories and simulations. Although the effect of surface roughness on fluid flow and permeability in single pores and fractures as well as networks of fractures was studied previously, its influence on permeability in a random mass fractal porous medium constructed of pores of different sizes remained as an open question. In this study, we, therefore, address the effect of pore–solid interface roughness on single-phase flow in random fractal porous media. For this purpose, we apply a mass fractal model to construct porous media with a priori known mass fractal dimensions $$2.579 \le D_{\mathrm{m}} \le 2.893$$ characterizing both solid matrix and pore space. The pore–solid interface of the media is accordingly roughened using the Weierstrass–Mandelbrot approach and two parameters, i.e., surface fractal dimension $$D_{\mathrm{s}}$$ and root-mean-square (rms) roughness height. A single-relaxation-time lattice Boltzmann method is applied to simulate single-phase permeability in the corresponding porous media. Results indicate that permeability decreases sharply with increasing $$D_{\mathrm{s}}$$ from 1 to 1.1 regardless of $$D_{\mathrm{m}}$$ value, while k may slightly increase or decrease, depending on $$D_{\mathrm{m}}$$ , as $$D_{\mathrm{s}}$$ increases from 1.1 to 1.6.

Mobility and settling rate of agglomerates of polydisperse nanoparticles.

Agglomerate settling impacts nanotoxicology and nanomedicine as well as the stability of engineered nanofluids. Here, the mobility of nanostructured fractal-like SiO2 agglomerates in water is investigated and their settling rate in infinitely dilute suspensions is calculated by a Brownian dynamics algorithm tracking the agglomerate translational and rotational motion. The corresponding friction matrices are obtained using the HYDRO++ algorithm [J. G. de la Torre, G. del Rio Echenique, and A. Ortega, J. Phys. Chem. B 111, 955 (2007)] from the Kirkwood-Riseman theory accounting for hydrodynamic interactions of primary particles (PPs) through the Rotne-Prager-Yamakawa tensor, properly modified for polydisperse PPs. Agglomerates are generated by an event-driven method and have constant mass fractal dimension but varying PP size distribution, mass, and relative shape anisotropy. The calculated diffusion coefficient from HYDRO++ is used to obtain the agglomerate mobility diameter dm and is compared with that from scaling laws for fractal-like agglomerates. The ratio dm/dg of the mobility diameter to the gyration diameter of the agglomerate decreases with increasing relative shape anisotropy. For constant dm and mean dp, the agglomerate settling rate, us, increases with increasing PP geometric standard deviation σp,g (polydispersity). A linear relationship between us and agglomerate mass to dm ratio, m/dm, is revealed and attributed to the fast Brownian rotation of such small and light nanoparticle agglomerates. An analytical expression for the us of agglomerates consisting of polydisperse PPs is then derived, us=1-ρfρpg3πμmdm (ρf is the density of the fluid, ρp is the density of PPs, μ is the viscosity of the fluid, and g is the acceleration of gravity), valid for agglomerates for which the characteristic rotational time is considerably shorter than their settling time. Our calculations demonstrate that the commonly made assumption of monodisperse PPs underestimates us by a fraction depending on σp,g and agglomerate mass mobility exponent. Simulations are in excellent agreement with deposition rate measurements of fumed SiO2 agglomerates in water.

The Effects of Timing of Inundation on Soil Physical Quality in the Water-Level Fluctuation Zone of the Three Gorges Reservoir Region, China

Core Ideas The soil pore network was analyzed by micro-CT images of intact soil core samples. The VESS method using the mass fractal dimension was applied to evaluate soil structural quality. Inundation decreases the number of soil pores and total porosity. Drainable micropores shift to non-drainable micropores after inundation in the reservoir zone. With the completion of the Three Gorges Dam, soils between the elevations of 145 and 175 m are exposed or submerged seasonally. Soil hydrological and physical properties are changed. A detailed investigation was performed to examine the influence of the timing of inundation on soil physical quality. Samples were collected in the water level fluctuation (WLF) zone and non-WLF zone in Zhongxian County, Chongqing. Soil samples were taken from depths of 0 to 10, 10 to 20, 20 to 30, and 30 to 40 cm. Soil structural quality was assessed by the visual evaluation of soil structure (VESS) method in situ; mean weight diameter (MWD) and mass fractal dimension (Dm) were measured to represent soil aggregate stability; and the soil water retention curve and computed microtomography (micro-CT) images were used to show soil pore networks. Our findings show a deterioration of soil structure indicated by a high structural quality (Sq) score in VESS (the higher the Sq score, the worse the soil structural quality) and a decline in soil aggregate stability indicated by low MWD and low Dm in the WLF zone. The data from both the soil water retention curve and micro-CT images demonstrate a deterioration of soil physical quality through decreased porosity and a shift of drainable micropores (0.1 < radii < 125 μm) to non-drainable micropores (radii < 0.1 μm) in the WLF zone of the Three Gorges Reservoir. The potential of environmental hazards of soil in WLF zone was also investigated.

The Effects of Timing of Inundation on Soil Physical Quality in the Water‐Level Fluctuation Zone of the Three Gorges Reservoir Region, China

Core Ideas The soil pore network was analyzed by micro‐CT images of intact soil core samples. The VESS method using the mass fractal dimension was applied to evaluate soil structural quality. Inundation decreases the number of soil pores and total porosity. Drainable micropores shift to non‐drainable micropores after inundation in the reservoir zone. With the completion of the Three Gorges Dam, soils between the elevations of 145 and 175 m are exposed or submerged seasonally. Soil hydrological and physical properties are changed. A detailed investigation was performed to examine the influence of the timing of inundation on soil physical quality. Samples were collected in the water level fluctuation (WLF) zone and non‐WLF zone in Zhongxian County, Chongqing. Soil samples were taken from depths of 0 to 10, 10 to 20, 20 to 30, and 30 to 40 cm. Soil structural quality was assessed by the visual evaluation of soil structure (VESS) method in situ; mean weight diameter (MWD) and mass fractal dimension (Dm) were measured to represent soil aggregate stability; and the soil water retention curve and computed microtomography (micro‐CT) images were used to show soil pore networks. Our findings show a deterioration of soil structure indicated by a high structural quality (Sq) score in VESS (the higher the Sq score, the worse the soil structural quality) and a decline in soil aggregate stability indicated by low MWD and low Dm in the WLF zone. The data from both the soil water retention curve and micro‐CT images demonstrate a deterioration of soil physical quality through decreased porosity and a shift of drainable micropores (0.1 &lt; radii &lt; 125 μm) to non‐drainable micropores (radii &lt; 0.1 μm) in the WLF zone of the Three Gorges Reservoir. The potential of environmental hazards of soil in WLF zone was also investigated.

Particle and cluster analyses of silica powders via small angle neutron scattering

Small angle neutron scattering (SANS) responses of particulate silica as disordered and ordered porous and non-porous powders were obtained. Each classification provided distinct scattering patterns which were fitted and analysed using Guinier-Porod and mass-surface fractal (msf) models. The resulting particle dimensions showed good agreement with the same from nitrogen gas adsorption analyses. Additionally, the excellent fit of the msf model to the non-porous silica scattering response resulted in an agreement between the mass and surface fractal dimension values for it and similar analyses and results published previously for fumed silica. The msf fitting gave additional insight into particle and cluster scattering dimensions with particle dimensions being consistent with those from the G-P analyses.

Strength for Modified Polyurethane with Modified Sand

The Polyurethane grout is an effective method used for improving the soils stabilization without disturbing the original soil structure, which is workable for construction and enhances the performance of soil compressive strength. In order to study the effect temperature, acetone rate, sodium silicate rate, mass fractal dimension of particle, and particle shape on the compressive strength. In order to lighten the workload considerably, the central composite design under response surface methodology was utilized. In this study, different content percentages of PU were injected into the modified sand and the compressive strength of specimens from the modified sand injected was determined by conducting the unconfined compression test. The results indicated that the existence of temperature, beyond which the strength has no change with the increase of temperature. And the strength of specimen increases with the increase of acetone mass ratio, over a critical value, it decreases. And that the strength of the specimen increases with sodium silicate ratio until a critical value (6%), beyond which it decreases slightly. And also the comprehensive strength of specimen is well correlated to the fractal dimension and shape of particles.

Variations of moisture and organics in activated sludge during Fe0/S2O82− conditioning–horizontal electro-dewatering process

The feasibility of using Fe0/S2O82− conditioning (zero valence iron (ZVI)/persulfate (ps)) integrated with horizontal electro-dewatering (HED) to improve the dewaterability of activated sludge (AS) was evaluated. The removal and migration of free and bound water in the ZVI/ps–HED process were determined, along with the organic matter migration between the solid and liquid phases of AS biosolids. Results showed that the optimum conditioning dosages were determined as 0.35 g ZVI/g dry solids (DS) and 0.15 g ps/g DS on the basis of capillary suction time variation. The lowest final water content (WC) of 83.67% in AS occurred after the HED treatment at 40 V and 120 min as determined using the response surface methodology. Despite the disruption of AS flocs and microbial cells through SO4−· oxidation during the ZVI/ps conditioning, the particle size and mass fractal dimension of the AS flocs remained relatively stable as the zeta potential increased from −8.5 mV to −4.6 mV. The coagulation of Fe3+ from the oxidation of Fe2+ by ps contributed to the relatively stable condition, which favored the subsequent momentum of the electroosmotic flow. Moreover, the ZVI/ps conditioning decreased the bound water content (BWC, from 1.83 g/g DS to 1.38 g/g DS) and increased the free water content (FWC, from 60.27 g/g DS to 60.91 g/g DS), indicating a transformation from free water to bound water. After the ZVI/ps–HED process, the FWC and BWC were removed significantly with ratios of 96.32% and 79.78%, respectively. Along with water removal through the ZVI/ps–HED process, the organic matter in the supernatant/filtrate initially increased and further rose at the HED stage. The analysis of the extracellular polymeric substance (EPS) content showed that the proteins (PNs) and polysaccharides (PSs) at the liquid and solid phases decreased due to oxidative degradation. Furthermore, the ZVI/ps stage significantly reduced the PNs content of slime to 57.22%, the PSs content of EPS to 68.50%, and the PN-like substances in the slime and tightly bound EPS to 74.90% and 52.47%, respectively. In addition, the WC of AS correlated with the contents of PN and microbial by-product-like material in slime, as well as tryptophan in TB-EPS, and the low EPS content in AS indicated good dewaterability. Thus, ZVI/ps should be selected as a pretreatment prior to HED.

Interaction forces between goethite and polymeric flocculants and their effect on the flocculation of fine goethite particles

Goethite is a major component in many mineral processing operations. Its presence can have adverse effects on solid/liquid separations. To understand the interactions between goethite surfaces and their influence on particle flocculation, the anionic polymer flocculants ammonium polyacrylate (NHPA), hydrolysed polyacrylamide (PHPA), and hydroxamic polyacrylamide (HXPA) were studied using a combination of atomic force microscopy (AFM), floc structure analyses, and settling tests. The floc settling velocity related to the different flocculants had the following order: NHPA > HXPA > PHPA. The different floc sizes indicated many small and large sized flocs formed with NHPA and PHPA, while monomodal medium sized flocs formed with HXPA. The mass fractal dimension values showed that more compact flocs were formed with HXPA than the other flocculants. The direct force measurements without flocculants confirmed that the goethite surfaces strongly repel each other in alkaline solutions, which agrees with DLVO theory for similarly charged surfaces. The various interactions measured with different flocculants can be related to their molecular structures and molecular weights. The incubation of NHPA at various pH values resulted in long-range adhesion after surface contact with multiple elastic minima, indicating strong adsorption and an expanded molecular conformation for the adsorbed flocculant. The strong elastic minima and long-range adhesion for HXPA indicated a strong adsorption of this hydroxamic flocculant, consistent with the flocculation performance. The force interactions support the results of the settling tests and floc structures well, and AFM could be a good method for studying the relationship between surface interactions and particle flocculation in polymer flocculants.

Lateral hydraulic performance of subsurface drip irrigation based on spatial variability of soil: Simulation

Soil physical properties (initial water content, bulk density, and mass fractal dimension) have a major influence on subsurface drip capillary water systems. The spatial distribution of lateral hydraulic performance varies along with the spatial variability of soil and is thus highly complex. In this paper, initial soil water content, soil bulk density, and mass fractal dimension generated according to a Gaussian distribution were used as input variables to a nonlinear lateral hydraulic mathematical model of subsurface drip irrigation. The numerical simulation indicated the following. 1) The greater the initial moisture content, soil bulk density and mass fractal dimension, the smaller was the lateral distribution of emitter discharge, and the smaller was the deviation rate of lateral flow. 2) The greater the standard deviation of initial moisture content, soil bulk density, and mass fractal dimension, the higher was the deviation rate of lateral flow, and the more laterally dispersed was the emitter discharge. 3) The greater the initial moisture content, soil bulk density, mass fractal dimension, and inner lateral diameter, the greater was the uniformity of lateral flow; The higher the inlet lateral pressure and emitter spacing, the lower was the uniformity of lateral flow; The uniformity of lateral flow increased with slope in the range −0.0003 to 1 and decreased with the increase of slope in the range 0–0.0005. The improved lateral hydraulic model by taking into account the influence of soil physical properties on subsurface drip capillary water systems, which permits identification of the critical points of the irrigation lateral

Cement grout transport within sand with fractal characteristics considering filtration

Filtration effect exists widely in the process of turbid fluid seepage in porous media, which directly influence the whole diffusion and migration process of turbid fluid particle. To better understand the effects of grouting parameters on the transport rules (e.g. velocity, viscosity and effective distance) of cement-based particle considering filtration, laboratory tests were carried out on fractal characteristics’ sand, with different grouting pressure, confining pressure, water/cement and mass fractal dimension taken into account. In this study, the mechanism governing the transport of particles was investigated by analysing the variety of velocity and viscosity of slurry at the column outlet, as well as the porosity of specimens at different grouting parameters. The results indicated that (1)The viscosity of passed grouts and the effective range is increasing firstly and then decreasing with the grouting pressure; (2) The viscosity increases non-linearly when the water/cement ratio is less than 3.0, beyond which it decreases slowly; (3) The effective range presents non-linear increasing firstly and then decreasing with increase of water/cement ratio; (4) The viscosity increases non-linearly firstly, and then decreases rapidly with increase of mass fractal dimension; (5) The effective range decreases when the fractal dimension increases.

Polaron effects in cylindrical GaAs/AlxGa1–xAs core-shell nanowires**Project supported by the National Natural Science Foundation of China (Grant Nos. 10574011 and 10974017).

By the fractal dimension method, the polaron properties in cylindrical GaAs/AlxGa1–xAs core-shell nanowire are explored. In this study, the polaron effects in GaAs/AlxGa1–xAs core-shell nanowire at different values of shell width and aluminum concentration are discussed. The polaron binding energy, polaron mass shift and fractal dimension parameter are numerically worked out each as a function of core radius. The calculation results show that the binding energy and mass shift of the polaron first increase and then decrease as the core radius increases, forming their corresponding maximum values for different aluminum concentrations at a given shell width. Polaron problems in the cylindrical GaAs/AlxGa1–xAs core-shell nanowire are solved simply by using the fractal dimension method to avoid complex and lengthy calculations.

Mass transfer characteristics, rheological behavior and fractal dimension of anammox granules: The roles of upflow velocity and temperature

In this study, the mass transfer, rheological behavior and fractal dimension of anaerobic ammonium oxidation (anammox) granules in upflow anaerobic sludge blanket reactors at various temperatures (8.5–34.5°C) and upflow velocities (0.06, 0.18mh−1) were investigated. The results demonstrated that a lower temperature increased the external mass transfer coefficient and apparent viscosity and impaired the performance of anammox granules. The external mass transfer coefficient was decreased, but efficient nitrogen removal of up to 96% was achieved under high upflow velocity, which also decreased the apparent viscosity. Furthermore, a fractal dimension of up to 2.93 achieved at low temperature was higher than the previously reported values for mesophilic anammox granules. A higher upflow velocity was associated with the lower fractal dimension. Because of the disturbance in granule flaking, the effectiveness factor was less suitable than the external mass transfer coefficient for characterization of mass transfer resistance.

Quantification of branching in fumed silica

The fractal structure of ceramic aggregates has been widely studied in terms of the mass fractal dimension and primary particle size. However, there has not yet been a quantitative description of the branched topology in these structures. In this work, the highly ramified aggregate structure in six commercial grades of hydrophilic fumed silica is investigated. Ultra-small angle X-ray scattering was coupled to a hierarchical scattering model, the unified scattering function, to obtain topological parameters. These generic topological parameters were used to quantify branching and were then compared with simple aggregation simulations. Surprisingly, a single parameter, the sticking probability, can empirically account for complex topological differences in the materials studied. It is found that samples of higher specific surface area display a higher branch fraction. For grades of highest specific surface area, a hyper-branched structure is observed with extensive branch on branch aggregation. This approach can be used to quantify complex morphological differences in ceramic oxide and carbon black aggregates.

Effect of vegetation type on microstructure of soil aggregates on the Loess Plateau, China

Several types of vegetation restoration have been implemented on the Loess Plateau in China to control soil erosion and improve soil quality. Different revegetation types, however, have varying effects on soil structure; effects on the pore network of aggregates are especially not well understood. We used synchrotron-based high-resolution X-ray micro-computed tomography to quantify the microstructure of soil aggregates under four types of revegetation and an active cropland on the plateau. Five aggregates (3–5mm) collected from the topsoil at each site were scanned at a voxel resolution of 3.25μm, and the aggregate pore structure was visualized and quantified with ImageJ. Total porosities, >75μm porosities, fractions of elongated pores, 3D mass fractal dimensions, and connectivity were higher and the numbers of pores, <75μm porosities, the mean pore-shape factors, and the fractions of regular and irregular pores were lower in the revegetated sites than the control plot. Total porosities, macro-porosities, micro-porosities, fractions of regular pores, and 3D mass fractal dimensions differed significantly among the revegetated sites. We suggest that the fraction of elongated pores can be used as an important indicator for monitoring the recovery of soil structure. 3D mass fractal dimensions differed more than connectivity in the aggregates in the same samples, and thus could be a more sensitive indicator of changes in the pore network. Age and revegetation type both significantly affected the development of soil structure, but revegetation type was more important for the recovery of soil structure. We used a soil structural index (SSI) obtained by principal component analysis to assess the overall quality of soil structure. SSI values were higher in all revegetated sites than the cropland site and differed among the revegetated sites in the order: shrubland>grassland>woodland>pastureland. We recommend shrub plantation and natural grassland for the revegetation of degraded land on the Loess Plateau.

On the dissolution state of cellulose in cold alkali solutions

We have characterized the dissolved state of microcrystalline cellulose (MCC) in cold alkali [2.0 M NaOH(aq)] solutions using a combination of small angle X-ray (SAXS) and static light scattering (SLS), ^1H NMR, NMR self-diffusion, and rheology experiments. NMR and SAXS data demonstrate that the cellulose is fully molecularly dissolved. SLS, however, shows the presence of large concentration fluctuations in the solution, consistent with significant attractive cellulose-cellulose interactions. The scattering data are consistent with fractal cellulose aggregates of micrometre size having a mass fractal dimension Dsim 1.5. At 25,^{circ }mathrm {C} the solution structure remains unchanged on the time scale of weeks. However, upon heating the solutions above 35,^{circ }mathrm {C} additional aggregation occurs on the time scale of minutes. Decreasing or increasing the NaOH concentration away from the “optimum” 2 M also leads to additional aggregation. This is seen as an increase of the SAXS intensity at lower q values. Addition of urea (1.8 and 3.6 M, respectively) does not significantly influence the solution structure. With these examples, we will discuss how scattering methods can be used to assess the quality of solvents for cellulose.