Seawater is fluid, and is a very complex multicomponent solution. All kinds of elements in seawater exist in certain physical and chemical forms. The existence form of copper in seawater is more complex and most of it is in the form of organic complex. Only a small fraction of the free ions exists in the form of bivalent positive ions and most of them in the form of anion complexes. So free copper ions are only a small part of the total dissolved copper. Seawater is extremely rich in sodium, but its chemistry is so simple that it's almost entirely in the form of Na+ ions.
Dissolved organic matter in seawater is very complex, the main material is called "Marine humus", its nature is similar with humus and fulvic acids produced by the decomposition of vegetation in soil. The molecular structure of Marine humus has not been completely determined, but it can form strong complexes with metals.
The composition of seawater can be divided into five categories
1. Main ingredients (mass, major elements), refer to the concentration of component in seawater is greater than 1×106mg/kg. In this category has five cations, they are Na+, K+, Ca2+, Mg2+and Sr2+；and five anions, they are SO42¯ Br¯ HCO3¯(CO32¯)F¯ also has H3BO3, which exists in molecular form, all together make up 99.9% of the salt in seawater, therefore, we called them the main ingredients.
Due to the large content of these components in seawater, the concentration ratio of each component is approximately constant. The biological activities and total salinity changes have little effect on it, so it is called conservative element.
The content of Si in seawater is sometimes greater than 1mg/kg, but due to the great influence of biological activities on its concentration and unstable nature, it belongs to the non-conservative element, so it is not included in the discussion of the main components.
2.The components of gas dissolved in seawater, such as oxygen, nitrogen and inert gases and etc.
3.Nutrient elements (nutrient salts, source elements): mainly mean the elements related to the growth of Marine plants, usually refer to N, P and Si. The amount of these elements in seawater is often affected by plant activity. when the content is very low, it will limit the normal growth of plants, so these elements are important for living things.
4.Trace elements: low in seawater, but not a nutrient element.
5.Organic substances in seawater, such as amino acids, humus, chlorophyll, etc.
The main composition of seawater
There are various kinds of salt dissolved in seawater. The cause of salt in seawater is a complex problem, which is related to the origin of the earth, the formation and evolution of the oceans. It is generally believed that the salt mainly comes from the weathering products of crustal rocks and volcanic ejecta. In addition, rivers around the world send 5.5×1015 grams of dissolved salt to ocean each year, which is also a source of salt in seawater. From the source, it seems that seawater should contain all the elements on earth. The most important chemical form and the concentration of dissolved elements in seawater, the main elements known are Li 174 Μ g, Fe 55. Ng, B 4.5 mg, Ni0.50 μg, C27.6 mg, Cu 0.25 μg, N420μg, Zn0.40 μg, F1.3 mg,As1.7μg,Na10.77g,Br67mg,Mg 1.29 g,Rb120μg,Al540. Ng, Sr 7.9 mg,Si2.8 mg, Cd80ng ，P70μg， I50ng, S0.904 g, Cs0.29μ g, Cl19.354 g, Ba14 μg, K0.399 g, Hg1ng, Ca0.412 g, Pb2ng, Mn14ng, U3.3μg, etc..
The high concentration components in the table basically represent their average concentration in seawater, while some low abundance components cannot represent their average concentration due to the difficulty in determination and the few samples tested. Many of the metals which we interested are extremely low abundance in seawater and can only be determined by using sensitive testing instruments and techniques and avoiding contamination during sample collection and analysis.
Seawater pollution
Sewage, waste residue, waste oil and chemicals keep flowing into the sea. Dumping oily sewage is illegal in many seas, but it still happens, and real oil disasters are that supertankers spill or sink. Now we try to remove the oil by depositing it in water with chemicals.
Dumping chemical and radioactive wastes into the sea has been a practice for many years. The container will corrode away one day and the harmful substances will enter the sea. We don't know much about the deep and surface water cycles, and they may be circulating faster than we previously thought. So harmful substances can spread into the water layer of biological activity.
The role of energy in desalination technology
There are two ways to break the bonds in seawater: the heat lift method and the membrane permeation method. Heat lift needs thermal energy to boil the sea water, and the water evaporates, leaving the salt, then the water vapor is cooled to produce liquid fresh water.
The most common method of membrane penetration is called reverse osmosis. A semi-permeable membrane is used to separate the water from the salt in seawater. Because it's cheaper than lifting, most plants use it. For example, the plant in Tampa uses this method to desalinate seawater.
Seawater is drinkable after being desalinated. There are dozens of methods for seawater desalination, the main ones are distillation, electroosmosis, freezing and membrane separation. Distillation is the most widely used method at present. This method is to heat and boil water at first to make the sea water produce steam, and then condense the steam down to make distilled water.
The common methods of desalination
Distillation
Although distillation is an ancient method, it still dominates till now due to the continuous improvement and development of technology. The essence of the distillation and desalination process is the formation of water vapor, its principle is like the seawater under heat evaporation to form clouds, then clouds form rain in a certain condition, but rain is not salty. According to equipment distillation, steam compression distillation, multi-stage flash distillation, etc. In addition, the combination of the above methods is becoming more and more attention and concern.
Electrodialysis is also a membrane exchange electrodialysis method
The key technology of this method is the development of new ion exchange membrane. Ion-exchange membrane is a functional membrane with a thickness of 0.5-1.0mm. According to its selected transmittance, it is divided into positive ion exchange membrane (positive membrane) and negative ion exchange membrane (negative membrane).Electrodialysis is the alternating arrangement of the positive and negative membranes with selective permeability, forming a number of independent compartments where seawater is desalinated, and adjacent compartments where seawater is concentrated, so that fresh and concentrated water can be separated. Electrodialysis can not only desalinate seawater, but also contribute to the reuse of sewage. In addition, this method is increasingly used in concentration, separation and purification of chemical, pharmaceutical, food and other industries.
Reverse osmosis
It is a kind of membrane separation desalination method that has been used since 1953.The method is to separate seawater from fresh water by using a semi-permeable membrane which allows only solvent but not solute to permeate. Under normal circumstances, fresh water diffuses through the semi-permeable membrane to the seawater, so that the liquid level of the seawater increases gradually until a certain height then stops. This process is called infiltration. At this time, the higher static pressure of the water column on one side of the seawater is called osmotic pressure. If an external pressure greater than the osmotic pressure is applied to one side of the seawater, the pure water from the seawater will reverse osmosis into the fresh water. The greatest advantage of reverse osmosis is its energy saving. It uses only half as much energy as electrodialysis and a quarter as much as distillation. Therefore, the United States, Japan and other developed countries have turned the development of reverse osmosis method more than the filter method since 1974.
With the rapid development of science and technology, pressure-driven reverse osmosis membrane separation technology (RO) has made great innovation and improvement in membrane, membrane module, equipment and process, etc. What’s more, people are increasingly aware of the limitations of RO technology in the field of energy conservation and environmental protection. In terms of desalination, RO technology can be considered as close to the peak of development. Therefore, in recent years, the abroad has carried out "forward osmosis membrane separation technology (FO)" of the related research, and obtained certain achievements, which has been applied in seawater desalination, sewage treatment, food processing, medicine and other fields, especially that using pressure Delayed Osmosis (FRO) seawater to generate power is a very promising clean renewable energy development technology . However, Although we developed the creative invention of "unpressurized adsorption and osmosis seawater desalination" in the 1990s, there is little attention paid to the technology of forward osmotic membrane separation in China, and there are also not many relevant studies and papers.
Forward osmotic separation has been used for a long time. Salt has been used for long-term food storage long time ago, this is because most bacteria, molds, and pathogens die of dehydration or temporary inactivation by osmotic action in salty environment. At present, people have begun to use forward osmotic membrane separation technology for seawater desalination, industrial wastewater treatment, waste percolation treatment and the other research. The food industry concentrates beverages in the laboratory using forward osmotic membrane separation; The life support system for emergency rescue uses forward osmosis membrane separation technology to produce fresh water. Recently, with the development of materials science, forward osmosis technology has been applied to controlled drug release in human body.
Non pressurized adsorption osmosis desalination method, or called "forward osmosis," lets the water go through the porous membrane and penetrate into a super absorbent of adsorbent or salt solution or solids concentration even more than the sea. Special salt solution "extract" can evaporate easily and divided into solid salt, liquid salt without outside pressure and adding too much heat (heat and reverse osmosis pressure ?). Solid salt uses less energy for desorption.
Alleviate water shortages by desalinating more seawater
Desalination requires a lot of energy. Salt thaws easily in water but it is very difficult to break its chemical bonds. Both energy and desalination technologies are very expensive, which means desalination will be expensive.
Desalination is used to get drinking water, and places like Tampa, Florida, have decided to build the largest desalination plant in the country
