Ionic Sieving Properties of Graphene Oxide (GO) Membranes

Ionic Sieving Properties of Graphene Oxide (GO) Membranes ABSTRACT: We characterized the ionic sieving properties of graphene oxide (GO) membranes by performing classical molecular dynamics (MD) simulations. The Lerf-Klinowski model is used for GO nanosheets structure. The Optimized Potentials for Liquid Simulations for all atoms (OPLS-AA) force field is used for GO potential. The SPC/E model is used for water molecules. We show that GO membranes can act as reverse osmosis (RO) membranes, although the water flow in GO membranes is hundred times faster than RO membranes. In this work two important factors in ionic sieving process are studied. First the GO layers separation and second the pressure of water. Each simulation runs until at least half of the water molecules are desalinated. The water flux, permeability, salt rejection, potential of mean force (PMF), and radial distribution function (RDF) are measured. We show that the GO membranes can be the appropriate choice for desalination of seawater in future due to the simplicity in produc tion, low cost, fast water flow, and great ion rejection ability. By 2030 nearly half the global population could be facing water scarcity, with demand outstripping supply by 40 percent, said United Nations Secretary General Ban Ki-Moon.Over 97% of the water on the Earth is saline water and only three percent is fresh water and about two thirds of this fresh water is frozen.So in the near future the only way to provide fresh water is desalination of seawater. There are common ways to desalinate seawater like reverse osmosis (RO) or methods based on distillation. In the RO method an applied pressure is used to overcome natural osmotic pressure so water passes through a semi-permeable membrane leaving salt behind. In the Distillation methods seawater is evaporated and then condensed to produce freshwater. Both methods require a lot of energy and are very costly. Recently nanotube-based membranes and graphene-based membranes have attracted many interests for their potential in water desalination due to their high permeability and great ion rejection. Although these membranes have a great theoretical advantages, the problem of synthesis and fabrication is a major challenge for producing cost effective membranes. Graphene oxide (GO) is a chemical derivative of graphene with several functional groups such as epoxide and hydroxyl that is produced from graphite by the Hummers method. GO has been synthesized and fabricated in the forms of papers and films in the industrial-scale. Functional groups and layers separation of GO membranes can optimized simply during synthesis process to achieve best performance for desalination. In the GO membranes, water molecules permeate through the nanochannels between oxidized regions (pristine regions), which are provided by the hydrophobicity of functional groups. Particles that have a smaller size than the GO nanochannels can permeate in the GO membrane with speed orders of magnitude greater than common membranes. Dry GO membranes have a layers separation of ~5 ±1 angstroms which only lets water vapor molecules permeate through the nanochannels. When a GO membrane is immersed in water, it is swelled so the layers separation is increased to ~12 ±1 angstroms. Na+ is the smallest ion in the saline water which has a hydrated diameter of à ¯Ã‚ Ã‚ ¾9 Ã…. Therefore after swelling of the membrane, small ions such as Na+ can permeate easier which leads to reduction of ion rejection. Several methods have been tried to prevent swelling of GO membranes, such as physical confinement, and crosslinking of nanosheets In this paper we present a next generation of ultrathin membranes which have remarkable abilities like high permeability, good ion rejection, and great resistance to blockage. Furthermore the simple and cheap methods for synthesis of GO membranes make them energy efficient. We performed Classical molecular dynamics (MD) simulations using the large-scale atomic molecular massively parallel simulator (LAMMPS).The VMD and OVITO were used for analysis and visualization. All simulations were carried out in NVT ensemble with a Nosà ©-Hoover thermostat and a damping constant of 10 femtoseconds. The equations of motion were integrated with a time step of 1 femtosecond using the velocity-verlet algorithm. The periodic boundary conditions (PBC) were applied for all three directions. The all-atom optimized potential for liquid simulations (OPLS-AA) is used for graphene oxide (GO) and salt ions.This potential contains many-body terms, including bond stretching, bond angle bending, van der Waals, and electrostatic interactions. In addition, OPLS uses a geometric combining rule for the Lennard-Jones coefficients. The extended simple point charge model (SPC/E) is used for water molecules, following previous studies on similar systems. The force field parameters are given in the table S1 to table S4 completely (see supporting information). The SHAKE algorithm is applied for water molecules to reduce high frequency vibrations that require shorter time steps. The interaction between water and GO includes both van der Waals and electrostatic terms. The van der Waals forces are truncated at 1.0 nm, and the long-range Coulomb interactions are computed by using the particle-particle particle-mesh (PPPM) algorithm. As it is seen in the figure S1 (see supporting information), in our model of GO, both hydroxyl and epoxide groups are considered, following the Lerfà ¢Ã‹â€ Ã¢â‚¬â„¢Klinowski model that is the most well-known model for GO. The structure of the single sheet of GO was considered as 1.5ÃÆ'-3 nm2 containing 18 epoxide and 25 hydroxyl groups. The oxygen functional groups were distributed on both sides of GO sheet. The single sheet of GO contains 206 carbon atoms and 43 oxygen atoms. Therefore, the ratio of C/O is about 4.8 which is in consistent with the Lerfà ¢Ã‹â€ Ã¢â‚¬â„¢Klinowski model. The size of simulation box in the x, y and z directions were about 17, 37 and 11 nm respectively. For preventing the membrane from movement, carbon atoms in the edges of the sheets were fixed. In the first step, a membrane was designed with 13 GO sheets and two layers according to the GO membranes structure proposed in previous studies. Distance between the edges was considered 2 nm. Figure S2 shows the designed membrane (see supporting information). Simulations were carried out for multiple values of layers separation from 7 to 8.5 angstroms with increment of 0.5 angstroms. For each choice of layers separation, three simulations were run for different nominal water pressures of 500 atm, 1000 atm, and 2000 atm. These numbers are nominal pressures but in the feed side of simulation box using voronoi atom volume estimation, feed pressure determined as 600 atm, 980 atm, and 1600 atm. Water pressure on the feed side of the membrane was enforced by applying specified and uniform forces in the z-direction to the piston atoms, thus ensuring that the water pressure was kept constant. Figure S3 shows the membrane with the layers separation of 8.5 angstroms, water, salt ions, and the piston (see supporting information). In the Figure S3a after 0.1 ns water molecules are in the pressure of 2000 atm and in the Figure S3b after 14 ns, we have 94 percent salt rejection and more than half of water molecules purified. In our simulations, saltwater was generated on the feed side of the membrane, consisted of 4800 water molecules and 52 Na+/Clà ¢Ã‹â€ Ã¢â‚¬â„¢ pairs, corresponding to a salt concentration of 35.5 g/L, which is close to the normal salinity of seawater (~35 g/L). Figure 1a shows the flux of water (volume per unit of time per area) passing through the membrane as a function of applied pressure and layers separation. In our simulations, we had to use high pressures in compare to typical pressures that is needed for desalination, because we have a time scale limit in molecular dynamics. We can solve this problem with calculating permeability (volume per unit of time per area per pressure) of membrane that is shown in figure 1b. Another possible method is extrapolating the graphs in figure 1a to low pressures like 10 atm, so we can reach to appropriate flux due to approximately linear relation (R2=0.99). In figure 1b it is obvious that with increasing the layers separation, the membrane permeability increases linearly (R2=0.98). As it is expected the numbers for membrane permeability are in consistent with other reports.Figure 1c shows salt rejection for the membranes with different layers separation and different water pressures. Salt passage wa s calculated from proportion of filtered salt ions number at time t (t is the time that half of the water molecules passed from membrane) to initial salt ions number in the feed side. So we have salt rejection = (1 salt passage). As it is seen in the figure 1c, with increasing the pressure or layers separation, salt rejection reduces which is expected. It is clear that with using lower water pressures like 10 atm, we can achieve higher ion rejection. Figure 2a shows the number of water molecules versus time in the membrane part. For each value of separation there is a limit for number of water molecules that can be in the membrane. In the simulations with higher pressures, the membrane gets filled faster as it is shown in figure 2b. Furthermore in longer times (about 15 ns) the separation value controls the number of water molecules in the membrane. Therefore, without attention to the water pressure, anyway the membrane is filled with water completely. Figure 3 indicates number of filtered water molecules against time. The graphs are plotted at the time that half of the water molecules are desalinated. According to the figure 3b, it is obvious that after about 5 ns the membrane is filled approximately. So we can see a stable flow due to linear relationship between filtered molecules and time. Figure 3a shows water flow for different layers separation and figure 3b shows water flow for different pressures in constant separation value. Figure 4 is the 3D color map for potential of mean force (PMF) for a particle passing through two sheets of GO. The PMF was calculated from steered molecular dynamics (SMD). We used harmonic potential U = K(x à ¢Ã‹â€ Ã¢â‚¬â„¢ x0)2/2, where K is 20 Kcal/mole-angstrom2 and end of spring moving with velocity of 0.00005 angstrom/femtosecond that is enough for reversible pulling. For checking the reversible pulling, the SMD was performed in X direction and -X direction at same width, but the results were same. Also using umbrella sampling and weighted histogram analysis method (WHAM) give us the same results as SMD for PMF calculation. For creating each PMF map, 30 simulations were performed to cover all of the GO layers width. We have done these simulations for 3 different layers separation. So we have a PMF map that shows barriers and valleys of energy all over the GO layers completely. In figure 4a, 4b, and 4c the PMF are plotted for Cl ion that passing from one side of GO layers to another side. In each path, Cl ion sees many barriers that prevent from movement of the ions. Also the ions can stuck in the valleys of energy between the barriers. Figure 4d, 4e, and 4f show PMF map for Na ion. In comparison to Cl ion, the barriers are shorter and valleys have a higher depth. So the Na ions in the valleys can move out with lower energy than Cl ions. PMF for H2O molecule in figure 4g, 4h, and 4i are shown. Flat surfaces indicate easy movement of H2O molecules across GO layers without encountering any barriers or valleys. As we can see in all of the plots, with increasing the layers separation, height of barriers and depth of valleys are reduced so the ions and water molecules move easier. Figure 5 shows salt concentration in the three part of feed, membrane, and filtered against time. In figure 5a the simulation is selected with layers separation of 8 angstroms and pressure of 2000 atmosphere. At the first of all simulations the salt concentration is 35.5 g/lit in the feed part which is same as sea water salinity. Salt concentration of feed part is slightly increased until reach to 90 g/lit at the time that half of the water molecules are desalinated. In the filtered part there are some peaks showing passage of ions through membrane. After the each peak, the salt concentration is reduced until the next peak because of passing water molecules from membrane into filtered part. Salt concentration in the membrane part fluctuates around the mean value of 17 g/lit until the end of simulation. So this fluctuation is enough to ensure that the membrane blockage does not occur even in higher salt concentrations like 90 g/lit. In figure 5b the layers separation is 8.5 angstroms with the water pressure of 2000 atmosphere. As we can see the behavior of plots is similar to figure 5a except number of peaks in the filtered part. Figure S4 indicates radial distribution function (RDF) for water and functional groups in GO layers (see supporting information). Figure S4a shows correlation between oxygen and hydrogen in water. Figure S4b presents RDF between oxygen in water and hydrogen in hydroxyl groups. Figure S4c shows RDF between hydrogen in water and oxygen in hydroxyl groups. Figure S4d shows correlation between hydrogen in water and oxygen of epoxide groups. The first peak in all of the plots in figure S4 shows length of hydrogen bond. As we can see in the figure the longest hydrogen bond is belong to hydrogen of water and oxygen of epoxide. We show that nanometer-scale pores in single-layer freestanding graphene can effectively filter NaCl salt from water. Using classical molecular dynamics, we report the desalination performance of such membranes as a function of pore size, chemical functionalization, and applied pressure.

Several Reasons For The Pollution Of Water Environmental Sciences Essay

There are several grounds for the pollution of H2O – metal, organic merchandises, every bit good as municipal, industrial and agricultural. ( Burande, Causes of Water Pollution ) May be the causes of H2O pollution caused by pollution beginnings of direct and indirect. Is the exchange of the former distillation and waste intervention workss, and workss. Emitted from the fluid of the different quality of H2O supply in urban countries. In the United States and some other states, and is controlled in these methods. However, the pollutants can be found still in the H2O organic structures. The latter is the proviso of H2O from the dirt, groundwater systems / by fertilisers, pesticides and industrial waste. This is besides through atmosphere such as bakeshops and mills and behavior vehicle emanations. It can besides be divided into non-organic pollutants, organic, and base / acid and radioactive substances.( Burande, Causes of Water Pollution )Causes of H2O pollutionThe chief beginnings of H2O pollution are as described below. Disposal of contaminated and / or hot H2O that was used for industrial intents. And overflow that contains a leak of crude oil merchandises. And overflow from building sites, farms, or other imperviable surfaces. Improper disposal of solid waste such as waste disposal on the range and compiled. In add-on, inordinate foods from overflow incorporating detergents or fertilisers besides called eutrophication. Geology of the groundwater extracted in footings of groundwater. Maltreatment of effluent discharged into the incorrect manner. The pattern of cut and burn agribusiness is switching cultivation agricultural systems. Radioactive stuffs from atomic power Stationss and industry, and the usage of medical, scientific and besides contribute. Uranium and Th excavation and refinement are some illustrations. Temperature is the chief ground, as it leads to the decease of many aquatic beings. And discharge of chilling H2O by mills and power workss reduces the temperature of H2O organic structures. Oil pollution is really harmful to the coastal wildlife. Oil spread over huge countries in the signifier of oil spills. If there are te sts of the dumping of oil or chemically treated, you may be marine ecosystems and beach vacations once more.( Burande, Causes of Water Pollution )Categorization of the causes of H2O pollutionMunicipal, industrial, agricultural and assorted classs of the causes of H2O pollution. The causes of the municipal H2O intervention of places and concerns. The chief aim was to turn to municipal effluent to cut down harmful bacteriums and substances that require O, and inorganic compounds assorted and suspended solids. The grounds for industrial vary harmonizing to demand biochemistry, and suspended solids, organic stuffs and inorganic. The grounds include farm animal, agricultural, commercial and breeding domestic fowl. These lead to relentless organic and inorganic in surface H2O and groundwater. ( Burande, Causes of Water Pollution )Effectss of Water PollutionSeen the effects of H2O pollution in the cool, heavy metals, agricultural countries, due to fertilisers, chemicals, oil and groundwate r taint. ( Burande )Effectss of Run-off PollutionChoose to rain on clay and soil and carries it to the H2O. If the soil and silt settee in the organic structure of H2O, and so forestall these sedimentations sunlight from making aquatic workss. If it is possible that the Sun does non make the workss, and this dice. These sedimentations can choke off the gills of fish and besides stifle the beings that live on the lower portion of the organic structure of H2O. ( Burande )Effectss of Oil Pollution and AntifreezeIf spilled oil in the H2O, and impacts on the ecosystem and harmful constituents. And can oppress many of the animate beings in instance of soaking up of oil. Oil has contaminated quarries can be a cause of decease for many. If the oil coats plumes, and these may decease. Oil, stop deading makes H2O an unpleasant olfactory property, and there is a gluey movie on the surface of the H2O, which kills animate beings. Oil is the most harmful pollutants in H2O. ( Burande )Contaminated Ground Water EffectsIf contaminated H2O enters the land, there may be serious effects. Peoples may go really ill and there is a possibility for the development of liver or kidney jobs, malignant neoplastic disease or other diseases. ( Burande )Fertilizers and other chemicalsNitrate in imbibing H2O lead to diseases of kids that may take to their deceases. Cadmium is a metal in the sludge derived fertilisers. Can be absorbed by these harvests. When people absorb this, they may do diarrheal upsets, liver and kidneys. Inorganic stuffs such as quicksilver, arsenic and lead are the causes of pollution. Other chemicals can besides take to jobs related to gustatory sensation and odor and colour of the H2O. Pesticides, PCBs and PCPs are toxic to all life. And pesticides are used in agribusiness, places and woods. Was found Cl and insularity in electrical transformers old. PCPs have been found in merchandises such as wood preservatives ( Burande )Effectss of Agricultural Water PollutionRain and irrigation H2O off the Bankss of cultivated land that has been fertilized and treated with pesticides, and the commixture of N with extra toxins in the H2O supply. This of toxic pesticides, H2O pollution, in a different place. Reason for the growing of aquatic workss take O from the H2O and the devastation of vegetations and zoologies of the watercourse and lakes and rivers. Fertilizers promote the growing of bacteriums in the H2O and increased concentration of bacteriums to unsafe degrees.( Burande )Effectss of Thermal Water PollutionIs cooled machines in industries with H2O from lakes and rivers. This H2O is up to the river in the instance of hot. This H2O reduces the system ‘s ability to go on to H2O and O is the growing of warm H2O species. ( Burande )Effectss of Heavy Metal Water PollutionHeavy metals such as lead, quicksilver, Fe, Cd, aluminium and Mg found in H2O beginnings. If these metals are found in deposits, and this up to the nutrient concatenation through wor kss and aquatic animate beings. This causes heavy metal poisoning in the instance of the H2O degree is really high. ( Burande )Some other effects of H2O pollutionIn rivers and oceans and seas and H2O pollution effects workss and animate beings at that place. Furthermore, birds and animate beings that consume contaminated nutrient supplies can decease. Blood diseases and upsets of the nervous system and bosom disease are some of the effects of H2O pollution. Many of the toxins in contaminated H2O leads to malignant neoplastic disease. Rarely, the organic structure can alter the construction of chromosomes. Some of the less powerful of the tegument lesions, purging, and diarrhoea.( Burande )Wayss to Prevent Water PollutionWater pollution is a major job we face today. Here are some ways to forestall it. ( Putatunda ) Although they cover more than 70 per centum of the Earth ‘s surface, H2O is one of the most cherished natural resources of our planet. The ground is that approximately 97 per centum of it is salty, and hence unfit for imbibing, has been locked more than 2 per centum in glaciers and polar ice caps, go forthing merely approximately 1 % utile for imbibing and cookery. Apart from clean imbibing H2O, and we besides need to conserve H2O in the oceans, rivers, lakes, non-polluting because otherwise harms the planet and we are really survival. With the human population is turning quickly that it led to us all the pollution of H2O resources of our planet, to the point, and objects alone and cherished ecosystems are being harmed and even decease at an dismaying rate. ( Putatunda )How is Water Pollution Caused?Despite the fact that some natural procedures may do some taint of H2O, but that human activity is the biggest cause of our seas, rivers and lakes contaminated happen. We need to ut ilize the H2O daily in both our industries, every bit good as our places. Get this H2O from groundwater beginnings, rivers and lakes, and after usage, and pollution in most instances, most of this H2O back to rivers, lakes, and oceans. ( Putatunda ) Water used for agricultural patterns, industrial and family uses the creative activity of effluent, besides referred to the effluent. If this flow is allowed once more to H2O systems, without being treated, it may do pollution, which result in injury to both human and carnal life. Contaminated H2O, as happens when there is storm H2O overflow from industrial, agricultural, and urban countries, which flow straight through storm drains in H2O systems without any intervention. ( Putatunda ) Disposal of sewerage a major job in developing states where there is no equal sanitation in big countries, and therefore transport the disease doing bacteriums and viruses in H2O beginnings. In states that are developed, people are frequently the flow of pharmaceutical merchandises and chemicals in the lavatory of their ain. ( Putatunda ) Some other causes of pollution, oil spills, ocean dumping, and dumping of refuse in the watercourses and rivers, oceans, such as composition board, newspapers, froth, rosin, plastic packaging, aluminium, glass, and so forth. Some of these take a really long clip to degrade, for illustration, can take 400 old ages, plastic packaging, Styrofoam takes 80 old ages, the froth takes 50 old ages, and aluminium takes 200 old ages. ( Putatunda ) Nuclear waste, and deposit in the ambiance and escape of belowground storage are some of the other causes of H2O pollution. ( Putatunda )

Labor Force

In any industry or company service workers do raise some complaints due to the way their employers treat them or due to the condition of working place.   These are some of the complaints most of workers do raise.   Service workers at times complain about low pay, no benefits and lack of effective communication from their employers.   Other issues such as workers being treated like slave laborers with no breaks and sexual harassment by he employers.   Problems such as political, height and weight discrimination are also faced by workers.   Some workers are mocked either because they are too short, tall or overweight.   Last problem is failure by employer to provide safety gadgets to its employees. In order to solve the above issues raised by workers, unions have been found to be of help.   Unions always try to analyze workers problems and try to protect them against threats from their employers.  Ã‚   In a case where safety gadgets are to be provided, unions do assist them in obtaining these safety measures.   Just like in oil mining fields gumboots and helmets are necessary because of accidents. Most workers tend to run away from jobs because there is no safety at the work place (Gus, 1995). Some of the reasons why new collar workers would not want the same benefits and clout that Blue collar workers were able to get by unionization are:   New collar workers get more benefits while blue collar workers get less benefits.   This makes collective bargaining become very hard because of the difference.   The more benefits you receive the stronger the bargaining power.   Blue collar workers are paid less and retirement age is always a bit early. Since blue collar workers are influenced by their employers they are always afraid especially in presenting their grievances. They fear threats of being sacked by their employers, which may make them loose their jobs (Gani, 1996). New collar workers may be hesitant to join a union especially when they realize that there are no benefits in what the union has to offer.   Normally workers need benefits that collective bargaining can bring but if the union is found to have no teeth to protect their interest then unionization is seen as useless.   Another reason why new collar workers may be reluctant to join unions does come from the influence of employers. Some employers use divide and rule tactics where they hold meetings with individual employee.   In this case they send messages of threats in the workforce cautioning workers that they risk suspension.   New collar workers also become reluctant to join unions because of lack of patience.   Most of them lack that time to wait long when it comes to fighting to achieve what they want (Michael, 2003). The future prospects of union in service industry according to my feeling are that it may end up fading with time.   This is because most of these unions do not commit to their core objectives why they were formed.   Workers are withdrawing from the unions because they feel they are cheated where by the system that is suppose to help them protect there is doing no job. References Gani, A  Ã‚   (1996). International Journal Manpower: Who Joins the Unions and Why (pp54 – 55) Mc B. up Ltd. Gus, T. (1995).   Look for the Union Label. Me Sharpe Publishers. New York Michael, D. (2003).   Why Unions Makers.   Amazon.com Books Publishers. New York.   

How Current Project Teams Within The Organization Can Work...

Introduction This briefing paper will address the question of how current project teams within the organization can work more collaboratively and effectively. Due to the number of recent failed projects, the organization is currently looking into how project teams are set up and managed. The issue, given the circumstances, revolves around how to equip managers, team leaders, and members with the skills and support to progress from project teams to high performing project teams in order to deliver the much-needed results in their respective units. This is an interesting issue in light of the fact several departments already have teams set up to carry out special projects without any formal process or best practice in place. This brief†¦show more content†¦In many cases these individuals have their own ideas, goals and objectives oftentimes rendering it a challenge to get everyone on one accord. So how can they work together effectively to drive favorable results? Both the team leader and the team members have very important roles in ensuring the effectiveness of the team. These teams, while carefully built and very effective in what they do, require constant leadership attention. This perhaps is the most important issue that could affect a high performing team. A weak leader, who is unable to set good examples or enforce good behavior and or performance accountability, could ultimately result in a broad range of team performance issues. A high performing team requires a leader who communicates clear visions and goals, motivates team members, and capitalizes on each individual’s strengths. However leading a project team comes with its own issues. These could include among others: a) dealing with diversity in the group b) ensuring quality project performance c) communicating and managing virtual teams d) setting clear goals and ensuring clear communicating with the team e) resolving conflict within the team f) staying on top of group d eadlines and g) keeping team members motivated. Effective team members are willing to communicate respectfully and effectively with others. In