Effect of crystal alignment on the remanence of sintered NdFeB magnets
NdI5.4Fe77.8B6.8 magnets of various degrees of crystal alignment have been prepared by the conventional powder metaHurgy techni.que. The alignment of these magnets have been determined by x-ray diffraction and fitting the standard deviation of a Gaussian distribution for the relative intensity versus the angle between the normals of (hkl) and the tetragonal c axis. The standard deviation is a good indicator for crystal alignment. An aligning field of 8 kOe is found to be essential to obtain a well-aligned NdFeB magnet. The remanence of sintered magnets is directly affected by the crystal alignment. Furthermore, the effect of crystal alignment on the remanence follows the theoretical prediction of the Stoner-Wohlfarth model. Below the spin reorientation temperature, the effect of crystal alignment on the shape of hysteresis loop becomes more significant. The remanences extrapolated from first and second quadrant of the hysteresis loops have been found to be consistent with the prediction of Stoner-Wohlfarth model.
Permanent address: Magnetic and Electronic Materials Research Laboratory, Hitachi Metals, Ltd., Kumagaya, Saitama 360, Japan.
INTRODUCTION
Crystal alignment plays a significant role in affecting the magnetic properties of sintered NdFeB magnets. Several methods have already been proposed to evaluate the crystal alignment of these magnets using x-ray diffraction patternso Forinstance, Tenaud, Chamberbord, and Vanoni l assumed that the crystal alignment obeys the Gaussian distribution, and then fitted the ratio of the x-ray diffraction intensities of the aligned magnet to the corresponding peak intensity of the random sample to the angle between the c axis and the normal of (hkl) to obtain the standard deviation. Zhou, Zhou, and Graham 2 correlated the crystal alignment to the half-width of the rocking curve of a certain (001) plane. Chang, Wu, and Liu 3 studied the crystal alignment using pole-figure analysis. All these experiments were performed at room temperature. It is known that NdFeB magnets exhibit an axial to conical spin reorientation (SR) when cooled to below 135 K.4 It is of interest to see whether the crystal alignment determined by these methods could be further extended to describe its effect on the remanence in conjunction with SR phenomena. In this paper, we report the effect of crystal alignment on the remanence M, of Nd 15 .4 Fens B6 . 8 sintered magnets using the method described in Refo 1 and compare M, with the theoretical calculation based on the Stoner-Wohlfarth model. 5 This model is also extended to describe the effect of crystal alignment on the remanence when SR occurs.
(1)
The magnets used in this study were prepared by the conventional powder metallurgy technique. Nd I5 .4 Fe77.8 B6.8 magnets with various degrees of alignment were prepared by varying the applied magnetic field from 0 to 955 kAim (12 kOe) during compaction in a die press. An isotropic magnet, pressed in a nonmagnetic die and without applying a magnetic field, was used as the reference for comparing the x-ray diffraction intensities. A Rigaku diffractroa)
Effect of crystal alignment on the remanence of sintered NdFeB magnets
T. Kawai,a) B. M. Ma, S. G. Sankar, and Wo Eo Wallace
Mellon Institute and MEMS Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 1saturation magnetization of the magnet, (J is the angle between the tetragonal c axis and the applied magnetic field, and ec is the cone angle when SR occurs. Cone angles of 0·, 26·, and 30· were used for temperatures of 295, 77, and 4.2 K, respectively. The ( + ) and ( - ) signs used in Eq. ( 1 ) stand for the angles between the applied magnetic field and the magnetization direction of a crystal before and after spin flippingo Since the saturation magnetization Ms is difficult to determine precisely, we assume that all Fe in the magnet exists in the Nd2 Fe 14B phase and calculate Ms by fonowing method:
EXPERIMENT
meter of Co radiation was used in measuring the diffraction intensities. The relative intensity was determined by measuring the ratio of peak heights of the anisotropic magnets to those of the corresponding isotropic reference. This relative intensity was then plotted against the angle 8, the angle between the normal of (hkl) and the tetragonal c axis. This data were then fitted for a Gaussian distribution to obtain a standard deviation a, which then acts as an indicator for the crystal alignment. The My at room temperature was determined by tracing the M-H curve on a IO-mm-cubic sample with a peak applied field of 1590 kA/m (20 kOe). The Mr in the temperature ranging from 4.2 to 273 K were measured on spherical samples of diameter of 3 mm with a VSM-superconducting magnetometer with a peak field of 9 T (90 kOe). A demagnetizing factor Oq1T was applied to obtain the corrected Mr. The specific density of magnets was measured by Archimedes's method. The theoretical M" based on the Stoner-Wohlfarth model, was calculated using the following expression:
光的衍射英文作文
光的衍射英文作文Light DiffractionLight is a fundamental aspect of our physical world, and its behavior has been the subject of intense study and fascination for centuries. One of the most intriguing and complex phenomena associated with light is diffraction, which refers to the bending and spreading of light waves as they encounter obstacles or apertures. This phenomenon has profound implications in various fields, from optics and quantum mechanics to biology and technology.At its core, diffraction is a wave-like property of light, where the interaction between light and the physical structures it encounters leads to the interference and redistribution of the light waves. This process is governed by the principles of wave interference, where the constructive and destructive interference of light waves result in patterns of light and dark regions, known as diffraction patterns.The fundamental principles of diffraction can be understood by considering the wave nature of light. Light, like other forms of electromagnetic radiation, can be described as a wave, with a specific wavelength and frequency. When light encounters an obstacle or anaperture, the waves are forced to bend and spread out, creating a diffraction pattern. The specific characteristics of this pattern are determined by factors such as the size and shape of the obstacle or aperture, as well as the wavelength of the light.One of the most well-known examples of diffraction is the phenomenon of single-slit diffraction. When light passes through a narrow slit, the resulting diffraction pattern consists of a central bright region, known as the central maximum, surrounded by alternating bright and dark regions, known as diffraction fringes. The spacing and intensity of these fringes are directly related to the wavelength of the light and the width of the slit.Another important aspect of diffraction is the concept of the Fraunhofer diffraction, which describes the diffraction pattern observed at large distances from the aperture or obstacle. In this case, the diffraction pattern is characterized by a series of bright and dark spots, known as the Fraunhofer diffraction pattern. This pattern is particularly useful in applications such as optical imaging, spectroscopy, and the design of diffraction-based optical devices.Diffraction also plays a crucial role in the behavior of light in various natural and man-made systems. For example, the diffraction of light through small apertures or slits is responsible for the characteristic patterns observed in the interference of light, such as those seen inYoung's double-slit experiment. Additionally, the diffraction of light around the edges of objects or through small openings is responsible for the phenomena of diffraction fringes, which can be observed in various optical devices and natural phenomena, such as the colorful patterns seen in the wings of some insects or the halos and glories observed around the Sun or Moon.The study of diffraction has also led to the development of numerous applications in science and technology. In optics, diffraction is used in the design of various optical devices, such as diffraction gratings, which are used in spectroscopy and other analytical techniques. In the field of quantum mechanics, the wave-like nature of particles, as described by the de Broglie hypothesis, has led to the observation of diffraction patterns in the behavior of subatomic particles, such as electrons and neutrons.Furthermore, the understanding of diffraction has been instrumental in the development of modern imaging techniques, such as X-ray crystallography, where the diffraction of X-rays by the atoms in a crystal is used to determine the arrangement and structure of the atoms within the crystal. Similarly, the diffraction of light by various biological structures, such as the compound eyes of insects or the structures found in the wings of some butterflies, has inspired the development of biomimetic materials and devices.In conclusion, the phenomenon of light diffraction is a fundamental and fascinating aspect of our physical world. It is a testament to the wave-like nature of light and the complex interplay between light and the physical structures it encounters. The study of diffraction has led to numerous insights and advancements in various fields, and its continued exploration promises to yield further discoveries and innovations that will shape our understanding of the universe and the technology we use to interact with it.。
你相信安联奇迹吗英语作文
你相信安联奇迹吗英语作文Do You Believe in the Miracle of Allianz Arena?The term "Miracle of Allianz Arena" often evokes a sense of awe and wonder in the world of football. But do we truly believe in such miracles?For many football enthusiasts, the idea of a miracle is not just a stroke of luck or a fluke. It's a bination of determination, teamwork, and an unwavering spirit that defies the odds. In the case of the Allianz Arena, there have been moments when teams have staged remarkable ebacks, turning the tide of a seemingly lost game. These instances have left fans on the edge of their seats, hearts pounding with excitement and disbelief.Believing in the Miracle of Allianz Arena means having fth in the power of human potential and the unpredictable nature of sports. It's about acknowledging that even when the situation looks dire, there's always a glimmer of hope. The players' never-say-die attitude and their ability to rise above challenges can inspire us to believe that miracles can happen.However, some might argue that these so-called miracles are just rare occurrences, the result of a perfect alignment of circumstances rather than a supernatural force. They might point to the statistical improbability and the role of pure chance in determining the oute of a game.In my opinion, whether one believes in the Miracle of Allianz Arena or not depends on one's perspective. If you see football as more than just a game, but a platform for showcasing human resilience and the pursuit of the impossible, then it's not hard to believe in miracles. But if you approach it from a purely analytical and rational standpoint, miracles might seem like an exaggeration.In the end, the Miracle of Allianz Arena remns a topic that stirs passionate debates among football lovers. It serves as a reminder of the magic and drama that the sport can bring, and why we continue to hold our breath and hope for the unexpected every time the whistle blows.。
晶格常数的英文
晶格常数的英文English:The lattice constant, also known as the lattice parameter or lattice spacing, is the physical constant that describes the distance between unit cells in a crystal lattice. It is a fundamental property of crystals and is often represented by the symbol "a." The lattice constant can be determined experimentally using techniques such as X-ray diffraction or electron diffraction, and it plays a critical role in understanding the properties of materials and predicting their behavior under different conditions. The lattice constant also influences various physical and mechanical properties of materials, including thermal expansion, electrical conductivity, and mechanical strength.中文翻译:晶格常数，也称为晶格参数或晶格间距，是描述晶格中单位晶胞之间距离的物理常数。
这是晶体的一项基本性质，通常用符号“a”表示。
晶格常数可以通过X射线衍射或电子衍射等技术实验确定，它在理解材料性质和预测在不同条件下行为方面起着至关重要的作用。
晶体生长过程视频
up a crystal growing attempt
Solubility Profile
Solubility
Nucleation – fewer nucleation sites are better. Too many nucleation sites (i.e. dust, hairs, etc.) lower the average crystal size
Mechanics – mechanical disturbances are bad.
Peak positions for true atomic positions
Limiting the Resolution of the Data
Limited qmax = 11.54° Resolution = 2.5 Å
Set-up simultaneous crystal growing experiments
Factors Affecting Crystallization
Solvent – moderate solubility is best. Supersaturation leads to sudden precipitation and smaller crystal size
positions are still resolvable
Limiting the Resolution of the Data
Limited qmax = 14.48° Resolution = 2.0 Å
增强型场效应晶体管英文
增强型场效应晶体管英文Enhanced Field-Effect Transistors, or EFETs, are really cool components in electronics. They're like the superheroes of the semiconductor world, able to handle more current and voltage than regular FETs. And the way they do it is just fascinating.Talking about EFETs, I gotta mention their awesome gate structure. It's designed to amplify the electric field, allowing for better control over the flow of electrons. That's why they're called "enhanced" – they really take performance to the next level.One thing I love about EFETs is their versatility. They can be used in a wide range of applications, from high-speed computers to power electronics. And with their improved performance, they're helping to push the boundaries of what's possible in electronics.EFETs are also pretty efficient. They minimize powerloss and heat generation, which is crucial for maintaining stable operation and extending the lifespan of electronic devices. So not only are they powerful, but they're also energy-smart.In my opinion, EFETs are the future of electronics. As technology continues to advance, we're going to see more and more devices utilizing these enhanced transistors to achieve incredible levels of performance and efficiency. Who knows, maybe one day we'll have computers that run entirely on EFETs!。
单色光频率与折射率的关系英语
单色光频率与折射率的关系英语Refractive Index and Frequency of Monochromatic Light.In the realm of optics, the relationship between the frequency of monochromatic light and the refractive index of a medium plays a pivotal role in understanding the phenomena of refraction and dispersion. This intricate relationship governs the bending of light as it traverses the boundary between two dissimilar media and the subsequent separation of light into its constituent colors.Refractive Index: A Measure of Light's Velocity.The refractive index (n) of a medium is a dimensionless quantity that characterizes the medium's ability to impede the propagation of light. It is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v):n = c / v.A higher refractive index indicates that light travels slower in that medium. The refractive index is dependent on the wavelength of light, exhibiting a phenomenon known as dispersion.Dispersion: Wavelength Dependence of Refractive Index.Dispersion arises due to the fact that different wavelengths of light interact with the charged particles within a medium to varying degrees. As a result, the refractive index of a medium changes with the wavelength of light. This variation is typically observed as a gradual increase in the refractive index with decreasing wavelength.For a given medium, the refractive index is generally higher for shorter wavelengths (higher frequencies) oflight compared to longer wavelengths (lower frequencies). This means that blue light, with its shorter wavelength, experiences a greater refractive index than red light,which has a longer wavelength.Refraction: Bending of Light at an Interface.When light encounters the boundary between two media with different refractive indices, it undergoes refraction. This phenomenon is characterized by a change in the direction of light as it crosses the interface. The angle of refraction (r) is related to the angle of incidence (i) and the refractive indices of the two media (n1 and n2) by Snell's law:n1 sin(i) = n2 sin(r)。
光与物质相互作用英文
光与物质相互作用英文Light-Matter Interaction.Light and matter interact in a variety of ways,including absorption, emission, scattering, and reflection. These interactions are essential for many natural phenomena, such as photosynthesis, vision, and the colors of objects. They are also used in a wide range of technologies, such as lasers, solar cells, and optical fibers.Absorption.When light is absorbed by matter, the energy of thelight is transferred to the matter. This can cause the matter to become excited, which can lead to a change in its chemical or physical properties. For example, absorption of light can cause a molecule to dissociate, or it can causean electron to be promoted to a higher energy level.Emission.When matter emits light, the energy of the light comes from the matter itself. This can happen when an excited atom or molecule returns to its ground state, or it can happen when an electron recombines with a hole. Emission of light is the basis for many light sources, such as lasers and LEDs.Scattering.When light is scattered by matter, the direction of the light is changed. This can happen when light interacts with particles that are smaller than the wavelength of light, or it can happen when light interacts with rough surfaces. Scattering of light is responsible for the blue color of the sky and the white color of clouds.Reflection.When light is reflected by matter, the direction of the light is changed, but the wavelength of the light remains the same. This can happen when light interacts with asmooth surface, such as a mirror, or it can happen when light interacts with a transparent material, such as glass. Reflection of light is used in a variety of applications, such as mirrors, lenses, and optical fibers.The interaction of light with matter is a complex and fascinating topic. It is essential for understanding a wide range of natural phenomena and technologies.Here are some additional details about each of the four types of light-matter interactions:Absorption.When light is absorbed by matter, the energy of the light is transferred to the matter. This can cause the matter to become excited, which can lead to a change in its chemical or physical properties. For example, absorption of light can cause a molecule to dissociate, or it can cause an electron to be promoted to a higher energy level.The amount of light that is absorbed by matter dependson the wavelength of the light and the properties of the matter. Some materials, such as metals, are very good at absorbing light, while other materials, such as glass, are very poor at absorbing light.Emission.When matter emits light, the energy of the light comes from the matter itself. This can happen when an excited atom or molecule returns to its ground state, or it can happen when an electron recombines with a hole.The wavelength of the light that is emitted by matter depends on the energy difference between the two states involved in the transition. For example, when an electron recombines with a hole in a semiconductor, the energy difference between the two states is typically in the visible range, so the emitted light is visible light.Scattering.When light is scattered by matter, the direction of thelight is changed. This can happen when light interacts with particles that are smaller than the wavelength of light, or it can happen when light interacts with rough surfaces.The amount of light that is scattered by matter depends on the size and shape of the scattering particles and the wavelength of the light. For example, small particles scatter light more effectively than large particles, and short-wavelength light is scattered more effectively than long-wavelength light.Reflection.When light is reflected by matter, the direction of the light is changed, but the wavelength of the light remains the same. This can happen when light interacts with a smooth surface, such as a mirror, or it can happen when light interacts with a transparent material, such as glass.The amount of light that is reflected by matter depends on the refractive index of the material. The refractive index is a measure of how much light is bent when it passesfrom one material to another. Materials with a high refractive index, such as glass, reflect more light than materials with a low refractive index, such as air.。
光学纯对映体 英文
光学纯对映体英文## Enantiomers and Optical Purity.In the realm of chemistry, chirality refers to the property of a molecule that lacks mirror symmetry, muchlike our left and right hands. Chiral molecules exist in two distinct forms known as enantiomers, which are mirror images of each other but cannot be superimposed. Enantiomers are like two non-identical twins, sharing the same molecular formula and connectivity but differing in their spatial arrangement.Optical purity, a crucial concept in stereochemistry, quantifies the enantiomeric excess of a chiral compound. It measures the proportion of one enantiomer relative to the other in a mixture. A mixture containing equal amounts of both enantiomers is considered racemic and has an optical purity of 0%. Conversely, a mixture containing only one enantiomer is optically pure and has an optical purity of 100%.### Separation of Enantiomers.The separation of enantiomers is a challenging yet essential task in many fields, including pharmaceuticals, agrochemicals, and fragrances. Various techniques can be employed to achieve this, including:Chiral chromatography: This technique utilizes achiral stationary phase that interacts differently with different enantiomers, allowing for their separation.Chiral resolution: This involves converting a racemic mixture into a pair of diastereomers, which can then be separated by conventional methods.Enzymatic resolution: Enzymes, being chiral themselves, can selectively catalyze reactions with one enantiomer over the other, leading to the formation of optically pure products.### Optical Purity Measurement.Optical purity can be determined using various methods, such as:Polarimetry: This technique measures the rotation of plane-polarized light as it passes through a chiral sample. The magnitude and direction of rotation depend on the enantiomeric composition of the sample.NMR spectroscopy: Chiral solvents or chiral shift reagents can be used in NMR spectroscopy to differentiate between enantiomers based on their different chemical shifts.Chromatographic methods: Chiral chromatography or capillary electrophoresis can be used to separate enantiomers and determine their relative abundance.### Significance of Optical Purity.Optical purity is of paramount importance in several areas:Pharmacology: Many drugs are chiral, and their enantiomers can have different pharmacological properties, including efficacy, toxicity, and metabolism. Enantiopure drugs offer advantages in terms of safety and effectiveness.Agrochemicals: Herbicides and pesticides can be chiral, and their enantiomers may differ in their selectivity and environmental impact. Optical purity ensures the targeted control of pests and weeds.Fragrances and flavors: The fragrance and flavor of chiral compounds can depend on their enantiomeric composition. Optical purity control allows for the creation of specific scents and tastes.### Applications of Chiral Compounds.Chiral compounds find widespread applications invarious industries:Pharmaceuticals: Enantiopure drugs include ibuprofen,naproxen, and thalidomide.Agrochemicals: Herbicides such as glyphosate and pesticides like cypermethrin are chiral.Fragrances and flavors: Enantiopure compounds like menthol, camphor, and limonene contribute to thedistinctive scents and tastes of products.Materials science: Chiral polymers, liquid crystals, and self-assembling systems have unique properties and applications in optics, electronics, and nanotechnology.### Conclusion.The concept of enantiomers and optical purity is crucial for understanding the stereochemistry of chiral compounds. The ability to separate and determine the optical purity of enantiomers is essential in numerous fields, including pharmaceuticals, agrochemicals, and fragrances. The significance of optical purity lies in itsimplications for the safety, efficacy, and properties of chiral compounds in various applications.。
物理学外文翻译
Effect of Quantum Confinement on Electrons and Phonons in Semiconductors We have studied the Gunn effect as an example of negative differential resistance(NDR).This effect is observed in semiconductors,such as GaAs,whose conduction band structure satisfies a special condition,namely,the existence of higher conduction minima separated from the band edge by about 0.2-0.4eV..As a way of achieving this condition in any semiconductor,Esaki and Tsu proposed in 1970 [9.1]the fabrication of an artificial periodic structure consisting of alternate layers of two dissimilar semiconductors with layer superlattice.They suggested that the artificial periodicity would fold the Brillouin zone into smaller Brillouin zones or “mini-zones”and therefore create higher conduction band minima with the requisite energies for Gunn oscillations.iWith the development of sophisticated growth techniques such as molecular beam epitaxy（MBE）and metal-organic chemical vapor deposition(MOCVD)discussed in Sect.1.2,it is now possible to fabricate the superlattices(to be abbreviated as SLs)envisioned by Esaki and Tsu[9.1].In fact,many other kinds of nanometer scale semiconductor structures(often abbreviated as nanostructures)have since been grown besides the SLs.A SL is only one example of a planar or two-dimensional nanostructure .Another example is the quantum well (often shortened to QW).These terms were introduced ｉｎＳｅｃｔｓ．１．２ａｎｄ７．１５ｂｕｔｈａｖｅｎｏｔｙｅｔｂｅｅｎｄｉｓｃｕｓｓｅｄｉｎｄｅｔｉａｌ．Ｔｈｅｐｒｏｐｏｓｅof this chapter is to study the electronic and vibrational properties of these two-dimensional nanostructures.Structures with even lower dimension than two have also been fabricated successfully and studied. For example,one-dimensional nanostructures are referred to as quantum wires.In the same spirit,nanometer-size crystallites are known as quantum dots.There are so many different kinds of nanostructures and ways to fabricate them that it is impossible to review them all in this introductory book. In some nanostructures strain may be introduced as a result of lattice mismatch between a substrate and its overlayer,giving rise to a so-called strained-layer superlattice.In this chapter we shall consider only the best-study nanostructures.Our purpose is to introduce readers to this fast growing field.One reason why nanostructures are of great interest is that their electronic and vibrational properties are modified as a result of their lower dimensions and symmetries.Thus nanostructures provide an excellent opportunity for applying the knowledge gained in the previous chapters to understand these new developments in the field of semiconductors physics.Due to limitations of space we shall consider in this chapter only the effects of spatial confinement on the electronic and vibrational properties of nanostructures and some related changers in their optical and transport properties.Our main emphasis will be on QWs,since at present they can be fabricated with much higher degrees of precision and perfection than all other structures.We shall start by defining the concept of quantum confinement and discuss its effect on the electrons and phonons in a crystal.This will be followed by a discussion of the interaction between confined electrons and phonons.Finally we shall conclude with a study of a device(known as a resonant tunneling device)based on confined electrons and the quantum Hall effect(QHE)in a two-dimensional electron gas.The latter phenomenon was discoveredby Klaus von Klitzing and coworkers in 1980 and its significance marked by the award of the 1985 Nobel Prize in physics to ｖｏｎ Klitzing for this discovery.Together with the fractional quantum Hall effect it is probably the most important development in semiconductor physics within the last two decades.Quantum Confinement and Density of StatesIn this book we have so far studied the properties of electrons ,phonons and excitons in either an infinite crystal or one with a periodic boundary condition(the cases of surface and interface states )In the absence of defects, these particles or excitations are described in terms of Bloch waves,which can propagate freely throughout the crystal.Suppose the crystal is finite and there are now two infinite barriers,separated by a distance L,which can reflect the Bloch waves along the z direction.These waves are then said to be spatially confined.A classical example of waves confined in one dimension by two impenetrable barriers is a vibrating string held fixed at two ends.It is well-known that the normal vibration modes of this string are standing waves whose wavelength λ takes on the discrete values given by Another classical example is a Fabry-Perot interferometer (which has been mentioned already in Set.7.2.6 in connection with Brillouin scattering).As a result of multiple reflections at the two end mirrors forming the cavity ，electromagnetic waves show maxima and minima in transmission through the interferometer at discrete wavelengths.If the space inside the cavity is filled with air,the condition for constructive interference is given by (9.1).At a transmission minimum the wave can be considered as “confined ”inside the interferometer.n λ=2L/n, n=1,2,3… .(9.1)For a free particle with effective mass *m confined in a crystal by impenetrablebarriers(i.e.,infinite potential energy)in the z direction,the allowed wavevectors z k of the Bloch waves are given byzn κ=2∏/n λ=n ∏/L, n=1,2,3… (9.2)And its ground state energy is increased by the amount E relative to the unconfined case:))(2(2222212Lm m k E z ∏==∆** (9.3)This increase in energy is referred to as the confinement energy of the particle.It is a consequence of the uncertainty principle in quantum mechanics. When the particle is confined within a distance L in space(along the z direction in this case)the uncertainty in the z component of its momentum increases by an amount of the order of /L.The corresponding increase in the particle ’s kinetic energy is then givenby(9.3).Hence this effect is known also as quantum confinement.In addition to increasing the minimum energy of the particle,confinement also causes its excited state energies to become quantized.We shall show later that for an infinite one-dimensional”square well”potential the excited state energies are given by n E∆2,where n=1,2,3…as in (9.2).It is important to make a distinction between confinement by barriers and localization via scattering with imperfections。
沙子吸附铅
Journal of Hazardous Materials B137(2006)384–395Removal of copper(II)and lead(II)from aqueoussolution by manganese oxide coated sand I.Characterization and kinetic studyRunping Han a ,∗,Weihua Zou a ,Zongpei Zhang a ,Jie Shi a ,Jiujun Yang baDepartment of Chemistry,Zhengzhou University,No.75of Daxue North Road,Zhengzhou 450052,PR ChinabCollege of Material Science and Engineering,Zhengzhou University,No.75of Daxue North Road,Zhengzhou 450052,PR ChinaReceived 8November 2005;received in revised form 25December 2005;accepted 13February 2006Available online 28February 2006AbstractThe preparation,characterization,and sorption properties for Cu(II)and Pb(II)of manganese oxide coated sand (MOCS)were investigated.A scanning electron microscope (SEM),X-ray diffraction spectrum (XRD)and BET analyses were used to observe the surface properties of the coated layer.An energy dispersive analysis of X-ray (EDAX)and X-ray photoelectron spectroscopy (XPS)were used for characterizing metal adsorption sites on the surface of MOCS.The quantity of manganese on MOCS was determined by means of acid digestion analysis.The adsorption experiments were carried out as a function of solution pH,adsorbent dose,ionic strength,contact time and temperature.Binding of Cu(II)and Pb(II)ions with MOCS was highly pH dependent with an increase in the extent of adsorption with the pH of the media inves-tigated.After the Cu(II)and Pb(II)adsorption by MOCS,the pH in solution was decreased.Cu(II)and Pb(II)uptake were found to increase with the temperature.Further,the removal efficiency of Cu(II)and Pb(II)increased with increasing adsorbent dose and decreased with ionic strength.The pseudo-first-order kinetic model,pseudo-second-order kinetic model,intraparticle diffusion model and Elovich equation model were used to describe the kinetic data and the data constants were evaluated.The pseudo-second-order model was the best choice among all the kinetic models to describe the adsorption behavior of Cu(II)and Pb(II)onto MOCS,suggesting that the adsorption mechanism might be a chemisorption process.The activation energy of adsorption (E a )was determined as Cu(II)4.98kJ mol −1and Pb(II)2.10kJ mol −1,respectively.The low value of E a shows that Cu(II)and Pb(II)adsorption process by MOCS may involve a non-activated chemical adsorption and a physical sorption.©2006Elsevier B.V .All rights reserved.Keywords:Manganese oxide coated sand (MOCS);Cu(II);Pb(II);Adsorption kinetic1.IntroductionThe presence ofheavy metals in the aquatic environment is a major concern due to their extreme toxicity towards aquatic life,human beings,and the environment.Heavy metal ions from wastewaters are commonly removed by chemical precipitation,ion-exchange,reverse osmosis processes,and adsorption by activated carbon.Over the last few decades,adsorption has gained importance as an effective purification and separation technique used in wastewater treatment,and the removal of heavy metals from metal-laden tap or wastewater∗Corresponding author.Tel.:+8637167763707;fax:+8637167763220.E-mail address:rphan67@ (R.Han).is considered an important application of adsorption processes using a suitable adsorbent [1,2].In recent years,many researchers have applied metal oxides to adsorption of heavy metals from metal-laden tap or wastewa-ter [3].Adsorption can remove metals over a wider pH range and lower concentrations than precipitation [4].Iron,aluminum,and manganese oxides are typically thought to be the most important scavengers of heavy metals in aqueous solution or wastewater due to their relatively high surface area,microporous structure,and possess OH functional groups capable of reacting with met-als,phosphate and other specifically sorbing ions [5].However,most metal oxides are available only as fine powders or are gener-ated in aqueous suspension as hydroxide floc or gel.Under such conditions,solid/liquid separation is fairly difficult.In addition,metal oxides along are not suitable as a filter medium because of0304-3894/$–see front matter ©2006Elsevier B.V .All rights reserved.doi:10.1016/j.jhazmat.2006.02.021R.Han et al./Journal of Hazardous Materials B137(2006)384–395385their low hydraulic conductivity.Recently,several researchers have developed techniques for coating metal oxides onto the surface of sand to overcome the problem of using metal oxides powers in water treatment.Many reports have shown the impor-tance of these surface coatings in controlling metal distribution in soils and sediments[3,6,7].In recent years,coated minerals have been studied because of their potential application as effective sorbents[3,8,9].Iron oxide coated meterials for heavy metal removal have been proved successful for the enhancement of treatment capacity and efficiency when compared with uncoatedfilter media,such as sil-ica sand[10–14],granular activated carbon[15]and polymeric media[16,17].For example,Edwards and Benjamin[7]found that coated media have similar properties to unattached coating materials in removing metals over a wide pH range,and that Fe oxide coated sand was more effective than uncoated sand.Bai-ley et al.[18]used iron oxide coated sand to remove hexavalent chromium from a synthetic waste stream.The influent contained 20mg l−1Cr(VI)and better than99%removal was achieved. Satpathi and Chaudhuri[19]and Viraraghavan et al.[20]have recently reported on the ability of this medium to adsorb metals from electroplating rinse waters and arsenic from drinking water sources,respectively.Green-Pedersen and Pind reported that a ferrihydrite-coated montmorillonite surface had a larger specific surface area and an increased sorption capacity for Ni(II)com-pared to the pure systems[21].Meng and Letterman[22]found that the adsorption properties of oxide mixtures are determined by the relative amount of the components.They also found that ion adsorption on aluminum oxide-coated silica was better mod-eled assuming uniform coverage of the oxide rather than using two distinct surfaces[23].Lo and Chen[8]determined the effect of Al oxide mineralogy,amount of oxide coating,and acid-and alkali-resistance on the removal of selenium from water.Bran-dao and Galembecket reported that the impregnation of cellulose acetates with manganese dioxide resulted in high removal effi-cient of Cu(II),Pb(II),and Zn(II)from aqueous solutions[24]. Al-Degs and Khraisheh[25]also reported that diatomite and manganese oxide modified diatomite are effective adsorbents for removing Pb2+,Cu2+,and Cd2+ions.The sorption capac-ity of Mn-diatomite was considerably increased compared to the original material for removing the studied metals.Filtration quality of diatomite is significantly increased after modification with Mn-oxides.Merkle et al.[26–28]reported that manganese dioxide coated sand was effective for removal of arsenic from ground water in column experiments.Merkle et al.developed a manganese oxide coating method on anthracite to improve the removal of Mn2+from drinking water and hazardous waste effluent.They generated afilter media with an increased surface area after coating with manganese oxides and found manganese oxide coated media have the ability to adsorb and coprecipi-tate a variety of inorganic species.Stahl and James[29]found their manganese oxide coated sands generated a larger surface area and increased adsorption capability with increasing pH as compared to uncoated silica sand.Additional researchers have been investigated to evalu-ate coating characteristics.X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FTIR),transmission electron microscopy(TEM), and scanning electron microscopy(SEM)have been used as well to identify components,distribution,and structure of surface oxide coating[7,9,30,31].An energy dispersive X-ray (EDAX)technique of analysis has been used to characterize metal adsorption sites on the sorbent surface.Typically,oxide was non-uniform over the surface as both the oxide and substratum had been observed[7].The research described here was designed to investigate characteristics of manganese oxide coated sand(MOCS)and test the properties of MOCS as an adsorbent for removing copper(II)and lead(II)from synthetic solutions in batch system.SEM/EDAX,XRD,XPS and BET analysis were employed to examine the properties of adsorption reactions for Cu(II)and Pb(II)ions on MOCS in water.The system variables studied include pH,MOCS dose,ionic strength,contact time and temperature.The kinetic parameters,such as E a,k1,k2, have been calculated to determine rate constants and adsorption mechanism.1.1.Kinetic parameters of adsorptionThe models of adsorption kinetics were correlated with the solution uptake rate,hence these models are important in water treatment process design.In order to analyze the adsorption kinetics of MOCS,four kinetic models including the pseudo-first-order equation[32],the pseudo-second-order equation[33], Elovich equation[34],and intraparticle diffusion model[35] were applied to experimental data obtained from batch metal removal experiments.A pseudo-first-order kinetic model of Lagergen is given as log(q e−q t)=log q e−K1t2.303(1)A pseudo-second-order kinetic model istq t=1(K2q2e)+tq e(2) andh=K2q2e(3) an intraparticle diffusion model isq t=K t t1/2+C(4) and an Elovich equation model is shown asq t=ln(αβ)β+ln tβ(5) where q e and q t are the amount of solute adsorbed per unit adsorbent at equilibrium and any time,respectively(mmol g−1), k1the pseudo-first-order rate constant for the adsorption process (min−1),k2the rate constant of pseudo-second-order adsorption (g mmol−1min−1),K t the intraparticle diffusion rate constant (mmol g−1min−1),h the initial sorption rate of pseudo-second-order adsorption(mmol g−1min−1),C the intercept,αthe initial sorption rate of Elovich equation(mmol g−1min−1),and386R.Han et al./Journal of Hazardous Materials B137(2006)384–395the parameter βis related to the extent of surface coverage and activation energy for chemisorption (g mmol −1).A straight line of log(q e −q t )versus t ,t /q t versus t ,q t versus ln t ,or q t versus t 1/2suggests the applicability of this kinetic model and kinetic parameters can be determined from the slope and intercept of the plot.1.2.Determination of thermodynamic parametersThe activation energy for metal ions adsorption was calcu-lated by the Arrhenius equation [36]:k =k 0exp −E aRT (6)where k 0is the temperature independent factor ing mmol −1min −1,E a the activation energy of the reaction of adsorption in kJ mol −1,R the gas constant (8.314J mol −1K −1)and T is the adsorption absolute temperature (K).The linear form is:ln k =−E aRT+ln k 0(7)when ln k is plotted versus 1/T ,a straight line with slope –E a /R is obtained.2.Materials and methods 2.1.AdsorbentThe quartz sand was provided from Zhengzhou’s Company of tap water in China.The diameter of the sand was ranged in size from 0.99to 0.67mm.The sand was soaked in 0.1mol l −1hydrochloric acid solution for 24h,rinsed with distilled water and dried at 373K in the oven in preparation for surface coating.Manganese oxide coated sand was accomplished by utilizing a reductive procedure modified to precipitate colloids of man-ganese oxide on the media surface.A boiling solution containing potassium permanganate was poured over dried sand placed in a beaker,and hydrochloric acid (37.5%,W HCl /W H 2O )solution was added dropwise into the solution.After stirring for 1h,the media was filtered,washed to pH 7.0using distilled water,dried at room temperature,and stored in polypropylene bottle for future use.2.2.Metal solutionsAll chemicals and reagents used for experiments and anal-yses were of analytical grade.Stock solutions of 2000mg l −1Pb(II)and Cu(II)were prepared from Cu(NO 3)2and Pb(NO 3)2in distilled,deionized water containing a few drops of concen-trated HNO 3to prevent the precipitation of Cu(II)and Pb(II)by hydrolysis.The initial pH of the working solution was adjusted by addition of HNO 3or NaOH solution.2.3.Mineral identificationThe mineralogy of the sample was characterized by X-ray diffraction (XRD)(Tokyo Shibaura Model ADG-01E).Pho-tomicrography of the exterior surface of uncoated sand and man-ganese oxide coated sand was obtained by SEM (JEOL6335F-SEM,Japan).The distribution of elemental concentrations for the solid sample can be analyzed using the mapping analysis of SEM/EDAX (JEOL SEM (JSM-6301)/OXFORD EDX,Japan).The existence of Cu(II)and Pb(II)ions on the surface of manganese oxide coated sand was also confirmed by using EDAX.Samples for EDAX analysis were coated with thin carbon film in order to avoid the influence of any charge effect during the SEM operation.The samples of MOCS and MOCS adsorbed with copper/lead ions were also analyzed by X-ray photoelectron spectroscopy (XPS)(ESCA3600Shimduz).2.4.Specific surface area and pore size distribution analysesAnalyses of physical characteristics of MOCS included spe-cific surface area,and pore size distributions.The specific sur-face area of MOCS and pore volumes were test using the nitrogen adsorption method with NOV A 1000High-Speed,Automated Surface Area and Pore Size Analyer (Quantachrome Corpora-tion,America)at 77K,and the BET adsorption model was used in the calculation.Calculation of pore size followed the method of BJH according to implemented software routines.2.5.Methods of adsorption studiesBatch adsorption studies were conducted by shaking the flasks at 120rpm for a period of time using a water bath cum mechanical shaker.Following a systematic work on the sorp-tion uptake capacity of Cu(II)and Pb(II)in batch systems were studied in the present work.The experimental process was as following:put a certain quantity of MOCS into conical flasks,then,added the solute of metals of copper or lead in single component system,vibrated sometime at a constant speed of 120rpm in a shaking water bath,when reached the sorption equilibrium after 180min,took out the conical flasks,filtrated to separate MOCS and the solution.No other solutions were provided for additional ionic strength expect for the effect of ionic strength.The concentration of the free metal ions in the filtrate was analyzed using flame atomic absorption spectrometer (AAS)(Aanalyst 300,Perkin Elmer).The uptake of the metal ions was calculated by the difference in their initial and final concentrations.Effect of pH (1.4–6.5),quantity of MOCS,contact time,temperature (288–318K)was studied.The pH of the solutions at the beginning and end of experiments was measured.Each experiment was repeated three times and the results given were the average values.2.5.1.Effect of contact time and temperature on Cu(II)and Pb(II)adsorptionA 2.0g l −1sample of MOCS was added to each 20ml of Cu(II)or Pb(II)solutions with initial concentration of Cu(II)0.315mmol l −1and Pb(II)0.579mmol l −1,respectively.The temperature was controlled with a water bath at the temperature ranged from 294to 318K for the studies.Adsorbent of MOCS and metal solution were separated at pre-determined time inter-R.Han et al./Journal of Hazardous Materials B137(2006)384–395387 vals,filtered and analyzed for residual Cu(II)and Pb(II)ionconcentrations.2.5.2.Effect of pH on the sorption of Cu(II)and Pb(II)byMOCSThe effect of pH on the adsorption capacity of MOCSwas investigated using solutions of0.157mmol l−1Cu(II)and0.393mmol l−1Pb(II)for a pH range of1.4–6.5at293K.A20g l−1of MOCS was added to20ml of Cu(II)and Pb(II)solu-tions.Experiments could not be performed at higher pH valuesdue to low solubility of metal ions.2.5.3.Effect of MOCS doseIt was tested by the addition of sodium nitrate and calciumnitrate to the solution of Cu(II)and Pb(II),respectively.The doseof adsorbents were varied from10to80g l−1keeping initial con-centration of copper0.157mmol l−1and lead0.393mmol l−1,respectively,and contact time was180min at the temperature of293K.2.5.4.Effect of ionic strength on Cu(II)and Pb(II)adsorptionThe concentration of NaNO3and Ca(NO3)2used rangedfrom0to0.2mol l−1.The dose of adsorbents were20g l−1,the initial concentration of copper0.157mmol l−1and lead0.393mmol l−1,respectively,and contact time was180min atthe temperature of293K.The data obtained in batch model studies was used to calculatethe equilibrium metal uptake capacity.It was calculated for eachsample of copper by using the following expression:q t=v(C0−C t)m(8)where q t is the amount of metal ions adsorbed on the MOCS at time t(mmol g−1),C0and C t the initial and liquid-phase concentrations of metal ions at time t(mmol l−1),v the volume of the aqueous phase(l)and m is the dry weight of the adsorbent(g).3.Results and discussion3.1.Mineralogy of manganese oxide coated sandThe samples of sand coated with manganese oxide were dark colored(brown–black)precipitates,indicating the presence of manganese in the form of insoluble oxides.The X-ray diffrac-tion spectrum(XRD)of the samples(data not shown)revealed that the manganese oxide were totally amorphous,as there was not any peak detected,indicative of a specific crystalline phase. SEM photographs in Fig.1were taken at10,000×magnifi-cations to observe the surface morphology of uncoated sand and manganese oxide coated sand,respectively.SEM images of acid-washed uncoated quartz sand in Fig.1(a)showed very ordered silica crystals at the surface.The virgin sand had a rela-tively uniform and smooth surface and small cracks,micropores or light roughness could be found on the sand -paring the images of virgin(Fig.1(a))and manganeseoxide Fig.1.SEM micrograph of sample:(a)sand;(b)manganese oxide coated sand. coated sand(Fig.1(b)),MOCS had a significantly rougher sur-face than plain sand and the coated sand surfaces were apparently occupied by newborn manganese oxides,which were formed during the coating process.Fig.1(b)also showed manganese oxides,formed in clusters,apparently on occupied surfaces.At the micron scale,the synthetic coating was composed of small particles on top of a more consolidated coating.In most regions individual particles of manganese oxide(diameter=2–3m) appeared to be growing in clumps in surface depressions and coating cracks.The amount of manganese on the surface of the MOCS,measured through acid digestion analysis,was approx-imately5.46mg Mn/g-sand.3.2.SEM/EDAX analysisThe elements indicated as being associated with manganese oxide coated were detected by the energy dispersive X-ray spec-trometer system(EDAX)using a standardless qualitative EDAX analytical technique.The peak heights in the EDAX spectra are proportional to the metallic elements concentration.The quali-tative EDAX spectra for MOCS(Fig.2(a))indicated that Mn,O,388R.Han et al./Journal of Hazardous Materials B137(2006)384–395Fig.2.EDAX spectrum of MOCS under:(a)adsorbed without copper and lead ion;(b)adsorbed copper ions;(c)adsorbed lead ions.Si,and K are the main constituents.These had been known as the principal elements of MOCS.EDAX analysis yielded indirect evidence for the mechanism of manganese oxide on the surface of MOCS.The peak of Si occurred in EDAX showed that man-ganese oxides do not covered a full surface of the MOCS.If the solid sample of MOCS caused a change of elemental con-stitution through adsorption reaction,it could be inferred that manganese oxide has already brought about chemical interac-tion with adsorbate.The EDAX spectrum for copper and lead system was illustrated in Fig.2(b and c).It could be seen that copper and lead ion became one element of solid sample in this spectrum.The reason was that copper and lead ions were chemisorbed on the surface of MOCS.Dot mapping can provide an indication of the qualitative abundance of mapping elements.The elemental distribution mapping of EDAX for the sample of MOCS and MOCS adsorbed copper or lead ions is illustrated in Fig.3.The bright points represented the single of the element from the solid sam-ple.A laryer of manganese oxide coating is clearly shown in the dot map for Mn in Fig.3(a),and a high density of white dots indicates manganese is the most abundant element.Results indicated that manganese oxide was spread over the surface of MOCS,and was a constituent part of the solid sample.The ele-ment distribution mapping of EDAX for the sample ofMOCS Fig.3.EDAX results of MOCS(white images in mapping represent the cor-responding element):(a)adsorbed without copper and lead ion;(b)adsorbed copper ions;(c)adsorbed lead ions.R.Han et al./Journal of Hazardous Materials B137(2006)384–395389Fig.4.XPS wide scan of the manganese oxide coated sand. reacting with copper and lead ions is illustrated in Fig.3(b and c).Copper or lead ions were spread over the surfaces of MOCS. Results indicated that manganese oxide produces chemical bond with copper or lead ions.Thus,copper or lead element was a constituent part of the solid sample.3.3.Surface characterization using the X-ray photoelectron spectroscopy(XPS)XPS analyses were performed on samples of MOCS alone and reacting with copper or lead ions.The wide scan of MOCS is presented in Fig.4.It can be noticed that the major elements constituent are manganese,oxygen,and silicon.Detailed spectra of the peaks are shown in Fig.5.Manganese oxides are generally expressed with the chemical formula of MnO x,due to the multiple valence states exhibited by Mn.Therefore,it is reasonable to measure the average oxidation state for a manganese mineral[37].The observation of the Mn 2p3/2peak at641.9eV and the separation between this and the Mn2p1/2peak of11.4eV indicates the manganese exhibited oxidation between Mn3+and Mn4+as shown from the auger plot,but it can be seen to show Mn4+predominantly from the Mn2p3/2peaks[38].The large peak in Fig.5(b)is a sum of the two peaks at 529.3and533.1eV,which can be assigned to O1s;a low bind-ing energy at529.7eV,which is generally accepted as lattice oxygen in the form of O2−(metal oxygen bond).This peak is characteristic of the oxygen in manganese oxides.The second peak at533.4eV can be assigned to surface adsorbed oxygen in the form of OH−[38].As seen the XPS spectra of the sample of MOCS reacting with copper,Fig.6(a)shows the binding energies of the observed photoelectron peaks of Cu2p3/2,2p1/2.The binding energy of the Cu2p3/2peak at a value of933.9eV shows the presence of copper(+2).The XPS spectra obtained after Pb(II)adsorption on MOCS is presented in Fig.6(b).Fig.6(b)shows that doublets charac-teristic of lead appear,respectively,at138.3eV(assigned to Pb 4f7/2)and at143.8eV(assigned to Pb4f5/2)after loadingMOCSFig.5.XPS detailed spectra of MOCS:(a)Mn2p3/2;(b)O1s.with Pb(II)solution.The peak observed at138.3eV agrees with the138.0eV value reported for PbO[39].This shows afixation of lead onto MOCS during the process.3.4.Specific surface area and pore size distribution analysesThe specific surface areas for sand and MOCS under un/adsorbed Pb(II)ions are summarized in Table1.Plain uncoated sand had a surface area of0.674m2g−1.A surface coating of manganese oxide increased the surface area of sand to0.712m2g−1,while average pore diameter decreased from 51.42to42.77˚A.This may be caused by the increase in both Table1Specific surface areas and average pore diameters for sand and various MOCSSurface area(m2g−1)Average pore diameter(˚A) Sand0.67451.42Unadsorbed a0.71242.77Adsorbed b0.55239.64Desorbed c0.70142.71a Without reacting with Pb(II)ions.b After reacting with Pb(II)ions.c After soaking with0.5mol l−1acid solution.390R.Han et al./Journal of Hazardous Materials B137(2006)384–395Fig.6.XPS detailed spectra of MOCS reacting with(a)copper;(b)lead. inner and surface porosity after adding the manganese oxides admixture.After reacting with Pb(II)ions,the pore size distribu-tion of MOCS had been changed,and parts of pores disappeared through the adsorption process.The results indicated the parts of pores were occupied with Pb(II)ions and average pore diameters decreased simultaneously,compared with unadsorbed MOCS, the surface area value of adsorbed MOCS is decreased,varying from of0.712to0.552m2g−1.Besides,pore size distribution of desorbed MOCS was similar to that of unadsorbed MOCS. The surface area of desorbed MOCS increased and average pore diameter also increased after regeneration with acid solution. The results indicated Pb(II)ions could be desorbed from the surface site of micropore and mesopores.3.5.Effect of contact time and temperature on Cu(II)andPb(II)adsorptionEffect of contact time and temperature on the adsorption of the copper(II)and lead(II)on MOCS was illustrated in Fig.7(a and b).The uptake equilibrium of Cu(II)and Pb(II) were achieved after180min and no remarkable changes were observed for higher reaction times(not shown in Fig.7).The shapes of the curves representing metal uptake versus time suggest that a two-step mechanism occurs.Thefirstportion Fig.7.Effect of contact time on Cu(II)and Pb(II)ions adsorption at pH4and various temperatures:(a)adsorption capacity vs.time;(b)adsorption percent vs.time(C0(Cu)=0.315mmol l−1,C0(Pb)=0.579mmol l−1).indicates that a rapid adsorption occurs during thefirst30min after which equilibrium is slowly achieved.Almost80%of total removal for both Cu(II)and Pb(II)occurred within60min.The equilibrium time required for maximum removal of Cu(II)and Pb(II)were90and120min at all the experimental temperatures, respectively.As a consequence,180min was chosen as the reac-tion time required to reaching pseudo-equilibrium in the present “equilibrium”adsorption experiments.Higher removal for cop-per and lead ions was also observed in the higher temperature range.This was due to the increasing tendency of adsorbate ions to adsorb from the interface to the solution with increasing temperature and it is suggested that the sorption of Cu(II)and Pb(II)by MOCS may involve not only physical but also chem-ical sorption.The metal uptake versus time curves at different temperatures are single,smooth and continuous leading to sat-uration,suggesting possible monolayer coverage of Cu(II)and Pb(II)on the surface of MOCS[40].3.6.Effect of pH on the sorption of Cu(II)and Pb(II)by MOCSIt is well known that the pH of the system is an important vari-able in the adsorption process.The charge of the adsorbate and the adsorbent often depends on the pH of the solution.The man-R.Han et al./Journal of Hazardous Materials B137(2006)384–395391 ganese oxide surface charge is also dependent on the solution pHdue to exchange of H+ions.The surface groups of manganeseoxide are amphoteric and can function as an acid or a base[41].The oxide surface can undergo protonation and deprotonationin response to changes in solution pH.As shown in Fig.8,the uptake of free ionic copper and leaddepends on pH,increasing with pH from1.4to5.1for Cu(II)and1.4to4.3for Pb(II).Above these pH levels,the adsorptioncurves increased very slightly or tended to level out.At low pH,Cu(II)and Pb(II)removal were inhibited possibly as result ofa competition between hydrogen and metal ions on the sorp-tion sites,with an apparent preponderance of hydrogen ions.Asthe pH increased,the negative charge density on MOCS sur-face increases due to deprotonation of the metal binding sitesand thus the adsorption of metal ions increased.The increase inadsorption with the decrease in H+ion concentration(high pH)indicates that ion exchange is one of major adsorption process.Above pH6.0,insoluble copper or lead hydroxide starts precip-itating from the solution,making true sorption studies impossi-ble.Therefore,at these pH values,both adsorption and precipita-tion are the effective mechanisms to remove the Cu(II)and Pb(II)in aqueous solution.At higher pH values,Cu(II)and Pb(II)inaqueous solution convert to different hydrolysis products.In order to understand the adsorption mechanism,the varia-tion of pH in a solution and the metal ions adsorbed on MOCSduring adsorption were measured,and the results are shown inFig.8.The pH of the solution at the end of experiments wasobserved to be decreased after adsorption by MOCS.Theseresults indicated that the mechanism by means of which Cu(II)and Pb(II)ion was adsorbed onto MOCS perhaps involved anexchange reaction of Cu2+or Pb2+with H+on the surface andsurface complex formation.According to the principle of ion-exchange,the more metalions that is adsorbed onto MOCS,the more hydrogen ions arereleased,thus the pH value was decreased.The complex reac-tions of Cu2+and Pb2+with manganese oxide may be writtenas follows(X=Cu,Pb and Y=Pb)[42]:MnOH+X2+ MnO−X2++H+(9)MnO−+X2+ MnO−X2+(10)Fig.8.Effect of pH on adsorption of Cu(II)and Pb(II)by MOCS.2(MnOH)+X2+ (MnO−)2X2++2H+(11)2(MnO−)+X2+ (MnO−)2X2+(12)MnOH+X2++H2O MnOXOH+2H+(13)MnOH+2Y2++H2O MnOY2OH2++2H+(14)Eqs.(9)–(14)showed the hydrogen ion concentration increasedwith an increasing amount of Cu(II)or Pb(II)ion adsorbed onthe MOCS surface.3.7.Effect of MOCS doseFig.9shows the adsorption of Cu(II)and Pb(II)as a functionof adsorbent dosage.It was observed that percent adsorptionof Cu(II)and Pb(II)increased from29to99%and19to99%with increasing adsorbent load from10to80g l−1,respectively.This was because of the availability of more and more bindingsites for complexation of Cu(II)ions.On the other hand,theplot of adsorption per unit of adsorbent versus adsorbent doserevealed that the unit adsorption capacity was high at low dosesand reduced at high dose.There are many factors,which can con-tribute to this adsorbent concentration effect.The most importantfactor is that adsorption sites remain unsaturated during theadsorption reaction.This is due to the fact that as the dosageof adsorbent is increased,there is less commensurate increasein adsorption resulting from the lower adsorptive capacity uti-lization of the adsorbent.It is readily understood that the numberof available adsorption sites increases by increasing the adsor-bent dose and it,therefore,results in the increase of the amountof adsorbed metal ions.The decrease in equilibrium uptake withincrease in the adsorbent dose is mainly because of unsaturationof adsorption sites through the adsorption process.The corre-sponding linear plots of the values of percentage removal(Γ)against dose(m s)were regressed to obtain expressions for thesevalues in terms of the m s parameters.This relationship is asfollows:for Cu(II):Γ=m s0.221+6.61×10−3m s(15)Fig.9.Effect of dosage of MOCS on Cu(II)and Pb(II)removal.。