The interaction between pollutants and sediment particles often occurs on the particle surface, so surface properties directly affect surface reaction. The physical and chemical processes occurring on sediment particle surfaces are microscopic processes and as such need to be studied from a microscopic perspective. In this study, field emission scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) were adopted to observe and analyze the pore structure and element distribution of sediment particles. In particular, a special method of sample preparation was used to achieve the corresponding cross-sectional information of sediment particles. Clear images of a particle profile and pore microstructure were obtained by high-resolution SEM, while element distribution maps of sediment particles were obtained by EDS. The results provide an intuitive understanding of the internal microenvironment and external behavior of sediment particles, in addition to revealing a significant role of pore microstructure in the adsorption and desorption of pollutants. Thus, a combination of different experimental instruments and observation methods can provide real images and information on microscopic pore structure and element distribution of sediment particles. These results should help to improve our understanding of sediment dynamics and its environmental effects.
Environmental problems and ecological contradictions are becoming increasingly prominent in the world, and the adsorption and desorption of contaminants onto and from sediment particles have become hot topics in the study of water environment [
Since the physical and chemical processes occurring on the sediment particle surface are microscopic processes, they need to be studied at a microscopic scale. The range of fine sediment size is in microns and millimeters, so the needs of sediment research are difficult to meet by merely observing the fine sediment surface with naked eyes. Thus, surface information of sediment particles should be obtained through experiments and observations. The surface pores of sediment particles significantly influence their physical and chemical properties but are difficult to observe because of their small size. In recent years, more than 60 types of surface analysis methods and analytical instruments have been introduced. Some of the most commonly used microscopic equipment includes scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), environmental scanning electron microscope (ESEM), energy dispersive X-ray spectrometer (EDS), energy loss spectrometer (ELS), X-ray diffraction (XRD), and inductively coupled plasma-mass spectrometry (ICP-MS) [
Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectrometry (EDS) were adopted in this study to observe and analyze the pore microstructure and element distribution of sediment particles. In particular, a special method of sample preparation was used to obtain the corresponding cross-sectional information. Clear images of particle cross-sectional profiles and pore morphology of sediment particles were provided by high-resolution SEM, and the element distribution maps of sediment particles were obtained by EDS.
The main equipment used for microscopic observation was FESEM. The signals most commonly used by SEM are secondary and backscattered electron signals. The former is generally used to display surface topography, and the latter is used to show the atomic number contrast. An S-5500 SEM (JEOL Ltd., Tokyo, Japan) was used in the current study. SEMs with different resolution have different requirements for sample preparation. S-5500 SEM has a high resolution with magnification of 60-2,000,000, and its secondary electron resolution ranges from 0.4 nm (30 kV) to 1.6 nm (1.0 kV). Specific parameters of the electron microscope were selected based on the purpose of the experiment and need for measurement accuracy.
SEM is equipped with EDS. EDS uses a focused electron beam acting on a small area of the observed sample to excite the characteristic X-ray of the elements contained in the sample. It then captures, processes, and analyzes the information to obtain qualitative or quantitative results of the sample elements. Figure
Working principle of EDS.
The sediment used in the experiment was obtained from Guanting Reservoir of Beijing, China. After sampling, it was cleaned, dried, and cleared of impurities and coarse sediments through a 2.0 mm sieve. This sample contained mostly fine particles with particles less than 0.02 mm in size comprising 94.4% of the total. The mineral composition of sediment samples in Guanting Reservoir was basically the same. Figure
Powder XRD image of sediment.
S-5500 SEM is an ultra-high-resolution SEM with high requirements for special sample preparation. Its sample holder is a chamfer box of 1.5 cm × 0.5 cm × 0.4 cm. The thickness of the sample on the sample holder was kept below 2 mm. Given that the sediments were granular materials that failed to meet the requirements of sample preparation, they were mosaicked and the final sample size was ensured not to exceed 3 mm × 3 m × 2 mm. The mixture of sediment and phenolic resin was placed into a mosaic machine, melted under 140°C, applied with high pressure, and recooled. The sediment particles were then embedded in resin and shaped into cylindrical workpieces. The workpieces were ground and polished to smoothen fractures for further analysis. After polishing, the workpiece was cut precisely into 2 mm thick sheet samples by lathe, and the samples were ground into blocks of 3 mm × 3 mm using sandpaper. After ultrasonic cleaning and drying, the sample was glued on the sample holder and then sent into the coater for film coating.
Given that the sediment is nonconductive in a vacuum chamber, no clear image could be obtained from any sediment with no coating. Two kinds of coating apparatus were used in sample preparation, namely, a gold coater and a carbon coater. When we only sought to observe the surface morphology of the particles, the surfaces of the sample were coated with gold film. The obtained particle surface image was clear owing to the strong conductive property of gold film. Moreover, gold film does not affect surface morphology. When surface elements of the particles needed to be probed, the samples were coated with carbon film. The atomic number of gold is high, which means that the coverage of gold film is strong, which can interfere with detection of other elements. By contrast, carbon film meets the requirements of conductivity and does not affect detection of elements other than carbon.
Pore structure is a common feature of most particulate matter. Previous studies have shown that many pores exist on the surface of sediment particles, which are porous materials [
Pores of an individual sediment particle from Guanting Reservoir: (a) Particle 1 (×4000, size: 0.075 mm) and (b) Particle 2 (×8000, size: 0.042 mm).
To study the pore microstructure of sediment particles, a special mosaic method for sample preparation was used for observation using an ultra-high-resolution electron microscope. Figure
High-resolution SEM images of sediment particle profile and pore microstructure: (a) edge morphology of particle profile (size: 0.017 mm) and (b) amplified pore structure.
High-resolution SEM S-5500 with EDS can be used for line scanning of the material surface to analyze the distribution of elements of interest along the scanning line. In this study, the equipment did not scan the surface of sediment particles directly but only one cross section of a sediment particle. Through a special mosaic method for sample preparation, the cross section of a sediment particle embedded in the resin was cut and scanned. Figure
Line scanning results of sediment sample: (a) scanning strip of the sediment particle, (b) Al, (c) Ca, (d) Cl, (e) Fe, (f) K, (g) Mg, (h) Na, (i) O, (j) P, (k) S, and (l) Si.
As shown in Figure
Generally, pores can be classified based on absolute size into supermicropores, ultramicropores, mesopores, and macropores. In addition, they can also be classified by causes and chronological order into primary pores and secondary pores. The pores discussed in this paper are primary intragranular spaces within intact skeletal grains. These pores are generally several to tens of nanometers in size. We tried to observe the three-dimensional microstructure of these pores using high-resolution SEM. Although other tools such as TEM and SPM offer higher resolution and are therefore able to reveal the crystal or atomic structure [
Bulk analysis of porosity and pore size distributions in sediments is accomplished by several methods, and each has advantages and disadvantages [
Since pores in the range of 2 nm or smaller overlap with the size of spaces related to crystal site vacancies and other crystal defects, they are not considered to be part of porosity in general. However, this kind of pores is of great significance to the study of adsorption and desorption of pollutants on sediment particles. The pores of sediment particles increase the specific surface area, and large specific surface area greatly enhances the physical and chemical effects on the microinterface (solid-liquid interface). We previously reported that the adsorption of phosphorus onto sediments decreases the volume of pores smaller than 10 nm by filling these pores with contaminants [
Many researchers have reported that the oxides of surface metal elements, especially Al and Fe, are important for adsorption of phosphorus and other pollutants [
Line scanning energy spectrum of phosphorus: (a) P, (b) Fe, and (c) Al.
SEM and EDS were used to observe the micropore structure and elemental distribution of natural sediment samples after sampling and special treatment. Cross-sectional information on the pore microstructure characteristics and element distribution rule was analyzed in detail. The combination of a variety of experimental instruments and observation methods provided real images and information on the microscopic pore structure and element distribution of sediment particles. Our SEM images showed that complex pore microstructures are an important part of sediment particle morphology. The characteristics of pores have an important influence on the sediment particle microinterface, especially on the adsorption and desorption of pollutants by sediment particles. Our scanning results showed that the metal elements of Al and Fe existed on the surface of the particle, and elemental P exhibited consistency with elemental Fe and Al with correlation of 0.74 and 0.89, respectively. This underscores the importance of Al and Fe (hydr)oxide for P adsorption onto sediment particles. Our findings indicate that observation of the microscopic pore structure and element distribution can lead to a better understanding of the interaction between sediment and pollutants and ultimately of environmental effects of sediments.
The authors declare that they have no conflicts of interest.
This investigation was supported by the National Natural Science Foundation of China no. 51479213 and Chinese Universities Scientific Fund no. 2016TC006.