بررسی روش‌های حذف یون نیترات از آب آشامیدنی

Water is the source of life and the main building block of the universe, which is also the most consumed beverage in the world. Water contains various ions, such as nitrate, nitrite, fluoride, sulfate, chloride, calcium, magnesium, etc. Further, the type and amount of species in water might affect water quality and human health. Today, the high amount of nitrate in drinking water is one of the main public concerns in the health section. Therefore, World Health Organization (WHO) and National Standard Organization of Iran have set the permissible nitrate levels in drinking water at 45 and 50 mg/L, respectively. Nitrate removal from drinking water is necessary to ensure the supply of safe water and prevent the spread of various diseases affected by high amounts of nitrate. In the human body, nitrate has reduced to nitrites that are combined with hemoglobin, which then leads to the making of methemoglobin, which is deadly for babies. Long exposure to nitrate and nitrite leads to the formation of the carcinogenic compound nitrosamine. On the other hand, nitrate affects the size and function of the thyroid gland. Based on the initial quality of water, the level of application scale, the conditions of access to materials and equipment, the cost of the method, and the Type of application of water consumption, different methods are used for nitrification.

To investigate the methods used to reduce or remove nitrate ions from water, a comprehensive study was conducted on the basis of reports and research of various scientists in the last two decades. The collected reports were categorized and the characteristics of each method were precisely evaluated separately. Then, the advantages and disadvantages of each method were compared with other methods presented. The most important methods for nitrate reduction are as follows: Ion exchange, Reverse osmosis, Adsorption, Electrodialysis, Chemical, and Biological method. Each of these techniques has its own advantages and disadvantages and different factors, such as economic explanation, industrial application, efficiency, selectivity, and lack of polluting the environment determine the selective method.

Among the various methods, the ion exchange method is an effective, simple, relatively low-cost method to reduce the amount of nitrate in water that has good efficiency, especially for low volume water sources. This method involves passing water containing nitrate through a resin bed consisting of anion exchange resins in which the nitrate ion is exchanged with anions such as chloride and has a removal efficiency of more than 90%. In addition to salts and ions, the reverse osmosis method can remove suspended solids, organic matter, colloids, bacteria, and viruses. Generally, reverse osmosis units in the pressure range of 300 to 1500 psi might remove nitrate ions from water.

In addition to the mentioned method, ease of execution and uncomplicated design have made adsorption a cost-effective method for nitrate removal. Furthermore, various chemical processes, including electrochemical reduction methods, are used to remove nitrate ions. In these methods, common reducing agents such as active metals, ammonia, borohydride, formate, hydrazine, hydroxylamine, hydrogen, and iron are used.

The electrodialysis method purifies water by selectively removing undesirable ions through a semipermeable membrane. In this method, the reduction of nitrate concentration strongly depends on the applied current. The results showed that the optimal current of 20 volts and a concentration of 500 mM electrolyte leads to a significant increase in nitrate removal.

The biological method of nitrate reduction is based on the use of nitrate-free microbes, which uses nitrate as the final electron receptor, organic and inorganic materials as electron donors, and energy sources as a way to maintain microbial growth. Bio nitrification is of two types: heterotrophic and autotrophic. The heterotrophic nitrification method is more economical than the autotrophic method on a larger scale and has the possibility of reducing nitrate to nitrogen gas with higher selectivity.

The need to reduce or eliminate nitrate from drinking water to prevent various diseases in humans is an important factor in studying and updating nitrate removal techniques. In this study, an attempt was made to offer a variety of methods for nitrate removal, their mechanism, examples of studies, reflecting the properties, and finally, a brief comparison of methods, to provide a basis for studies to introduce and innovate in nitrate reduction techniques. An appropriate understanding of the features of the methods used will save time and cost, improve results, and observe the standards of the World Health Organization. After reviewing and comparing the studied methods, it would be stated that Ion exchange, Reverse osmosis and Electrodialysis are more efficient, compared with other methods, to remove nitrate from drinking water. High efficiency, being user-friendly, environmental considerations, and using at industrial scale are among the most favorable and common properties of these methods.