Traditional Methods of Removing Nitrates
Traditional methods of removing nitrates from water include two main groups of treatment processes: biological and physicochemical. Various methods have been reported for the removal of nitrate from water and wastewater [5, 25, 26], including biological denitrification [27], chemical reduction [28], reverse osmosis [29], electrodialysis [30], adsorption [31], etc. Among these methods, adsorption is generally considered to be a simple and efficient method of nitrate removal [32,33].
Biological denitrification is an environmentally friendly and cost-effective albeit slow method, the process in which denitrification is realized by facultative anaerobic bacteria which reduce nitrate or nitrite in water in the absence of oxygen. However, biological denitrification processes are difficult to apply to inorganic wastewater treatment, as additional organic substrates to serve as electron donors are needed. Biological denitrification is important in particular in the treatment of industrial waste-water containing high nitrate concentrations at low temperatures.
One of the alternatives to this technology is the biological denitrification using systems with fixed or fluidized bed. To increase the rate of denitrification reaction various chemicals such as methanol, ethanol, acetic acid or sucrose are usually added as an organic carbon source, [34], but their use is quite costly.
In the practice of water purification, biological methods are widely used for removing mineral nitrogen-containing substances. Although these methods are particularly suitable for wastewater, there are settings that apply to clean up drinking water. However, the biological denitrification of drinking water is not widely used since it raises some yet unresolved issues such as selectivity and safety of bacteria as well as the use of an electron donor substrate non-toxic for drinking water. The use of biological denitrification of drinking water has also a problem of a complete biomass removal, which requires overtime for denitrification. Another disadvantage is the need to follow a mandatory chemical treatment of water to remove harmful bacteria.
Biological denitrification processes are sensitive to the contaminant concentration and pH and require additional purification of water from biological additives. In addition, there is a threat of greenhouse gas emissions (such as NOx and N20). The most acceptable method from an environmental viewpoint is catalytic removal of nitrate and nitrite ions by their reduction to molecular nitrogen. As an alternative to the classical catalytic denitrification, can be considered photocatalytic reduction of nitrate ions c using titanium dioxide can be considered [35]. Increasing the Ti02 photocatalytic activity of Ti02 is achieved by modifying the its surface of with the dye molecules, nanoparticles of noble metals, nitrogen, sulfur, carbon, and or transition metal ions, that leads to high photocatalytic activity of water denitrification reaction [36-38]. Tatsuki Ueda offers using of denitrification using sugar wastes, namely using molasses [39].
The World Health Organization [12] provides recommends a method for the biological denitrification as a method for removing nitrates. Nevertheless, the conventional techniques used to remove NO3 from water from N03 are expensive, have their limitations, and do not solve the problems associated with excessive N 03 in the environment.
It was proposed to use zero-valent iron (Fe0+) for its ability to reduce the level of pollution of various chemicals, including NO3 in groundwater [40]. However, Cheng et al. found that the major drawback of using zero-valent iron for reduction of NO3 in water is the presence of ammonium salts, and the need to control the pH. These shortcomings, in the opinion of the authors, are a major barrier to using this method [40,41].
Zotov et al. [42, 43] considered the use of semi-permeable barriers and creating reactive absorbent reaction zones by injection of zero-valent iron nanoparticles to remove contaminants from groundwater as extremely effective. Semi-permeable reactive barrier (PRB) is a new technology for treating contaminated groundwater. A successful treatment of contaminated groundwater by this method requires the transformation of contaminants into more harmless compounds or their immobilization during the passage through the treatment zone in situ. The construction of a semi-permeable reactive geochemical barrier that intercepts and converts spot pollution into the non-toxic form is attractive for several reasons. First, this technology does not require the construction of elevated water treatment devices, that can be the economic profitable. Second, there is no need for expensive above-ground processing of large amounts of water, its storage, transportation, or disposal. Thirdly, the maintenance costs are small. It is recommended to build PRB by digging a long narrow trench in the path of contaminated groundwater. The trench fill material in this case is zero-valent iron nanoparticles that can remove hazardous chemicals. The reaction materials may be mixed with sand in order to facilitate the passage of water through the wall of the barrier. Similarly, absorbing reaction zones (ARZ) can be created, into which the zero-valent iron nanoparticles would be injected. The use of PRB or ARZ allows reducing the concentration of pollutants in the stream below to the maximum permitted level; however, there is little information about the applicability of this approach to reducing the nitrate level. Although the authors initially designed this system to test on the model experiments with waste energy facilities, they suggest the use of nanoscale materials as a new concept of environmental treatment to remove any pollutant.
The adsorption process is generally considered to be the best for water treatment because of its convenience, ease of use and simplicity of design. Furthermore, adsorption can remove or minimize the concentration of a broad range of organic and inorganic contaminants from water or wastewater and thus has a wide scope for application as a method of combating water pollution. Adsorption techniques showed good results in the removal from water of various types of inorganic anions, such as fluoride, nitrate, bromate and perchlorate using different materials as adsorbents.
This advanced technology is reliable but rather energy-intensive and costly.
An electrochemical method to remove nitrite from water has been reported. In this method, from nitrites, the water passes through an electrolytic cell, where it is in contact with a cathode having a large surface area. Copper, iron, zinc, and other metals were used as the cathode material and graphite as the anode [44].
An interesting catalytic method of purifying water from nitrates and nitrites was suggested in [45]. Molecular hydrogen was used to reduce nitrite and nitrate on a composite catalyst consisting of a porous carrier impregnated with a metal component (palladium, rhodium, or copper group metals].
The electrochemical and catalytic methods were combined in [46]. The water containing nitrates and nitrites formed hydrogen in the electrolyzer cathode compartment, which reduced nitrates and nitrites to ammonia and intermediate nitrogen oxides, and then water was fed to the catalyst compartment where the redox reaction to form molecular nitrogen was completed.
A major disadvantage of this method is the use of expensive catalyst metals, which is not affordable for cleaning large volumes of water for the needs of centralized drinking water supply. Another drawback of the electrochemical reduction methods is the formation of ammonia and nitrogen oxides.
It was suggested in [47] to modify the nitrate reduction method by using ferrous ions bound to a solid matrix. Ferrous ions Fe2+ are introduced in the water as FeS04 and bound via ion exchange mechanism by a sulfonated carbon which has strongly acidic -S03H groups and acts as a cation exchanger. The sulfonated carbon impregnated with Fe2+ cations plays the role of a redox polymer which facilitates reduction of nitrate and nitrite to molecular nitrogen by ferrous ions which, in their turn, oxidize to ferric ions, Fe3+ which are firmly held by the sulfonated carbon. It has been suggested that the chemical reduction could be combined with electrolytic reduction by supplying water to the anode compartment of the electrolyzer in the anode compartment Fe2+ ions are generated by the dissolution of the iron-based electrode.
This process may be a cost-effective alternative to the catalytic reduction based on the noble metals described above.
A highly active electrocatalytic electrode for nitrate reduction was prepared by the electro-deposition of palladium onto a copper electrode. The capacity of nitrate reduction by a Pd/Cu electrode was studied using cyclic voltammetry (CV). The reduction peak at -0.605 V versus saturated calomel electrode in 0.1M sodium nitrate + 0.1M perchloric acid solution (pH = 0.86) was detected. The influence of solution properties, such as pH, nitrate concentration, and other anions in solution, on nitrate reduction was studied in detail. The results showed that nitrate reduction was suppressed in alkaline solution, while acidic or neutral solution was beneficial for nitrate reduction. At nitrate concentrations between 0.01 and 0.5 M, nitrate reduction current increased with increasing nitrate concentration, but it was hindered by sulfate. At high nitrate concentrations (1 to 5 M), no significant difference in removal of nitrate was observed. Compared with other electrodes prepared in this work (copper, titanium, and palladium- modified titanium electrodes), the palladium-modified copper electrode showed the highest electrocatalytic capacity and stability in the nitrate-reduction process [48].
The electrocatalytic reduction of nitrate was investigated on Pt, Pd, and Pt+Pd electrodes covered with a submonolayer of germanium, Ge. Pt+Pd electrode was prepared by electroless deposition of submonolayers of Pd on Pt via exchange of PdCl2 for preadsorbed copper, that deposited Ge strongly enhanced the reduction rate of nitrate. The reduction of nitrite was enhanced to a lesser extent, whereas germanium was inactive in NO and hydroxylamine reduction. Further studies using cyclic voltammetry showed that the well-known inhibition of the nitrate reduction at low potentials was absent for germanium-modified electrodes. Amperometric measurements showed that the current densities for nitrate reduction at 0.1 V depended strongly on the composition of the electrode surface. The electrode activity increased in the order Pd < Pt < Pt+Pd and all electrodes displayed a proportional relationship between the activity and the germanium electrode surface coverage. These results confirmed that Ge is involved in the rate determining step, which is the reduction of nitrate to nitrite and its role is to bind the oxygen atom of nitrate. The higher activity of Pt+Pd electrode can be understood in terms of changes in the electronic structure of the metals as a result of alloying. Selectivity measurements with a rotating ring-disk electrode have shown for all electrodes that the hydroxylamine reducing selectivity increases with increasing germanium coverage. Pd displays higher hydroxylamine selectivity than Pt and Pt+Pd electrodes. No gaseous products were observed for Pt, whereas for Pt+Pd and Pd electrodes N20 selectivity up to 8% was detected [49].
From the review of various methods suggested for cleaning up water from nitrate, it is clear that so far universal methods of water purification from nitrates do not exist. Each method has its advantages and disadvantages. Among other methods, the electrochemical method seems to have some advantages because it is reagentless, can be easily automated, and produces a minimal amount of sludge. Its cost efficiency, however, remains to be proved
Reverse osmosis, RO can be a feasible option for the removal of nitrates [50-52]. The first commercial application of RO for drinking water was carried out in Coalinga, California, in 1965 [53]. Reverse osmosis can be used for the treatment of several pollutants simultaneously, including ions (e.g., nitrates, arsenic, sodium, chloride and fluoride], particles (e.g., asbestos, protozoan cysts] and the organic components (for example, certain pesticides] [54] from the water passing under pressure through a semi-permeable membrane which retains the impurities. The required operational pressure is dependent on the solute concentration in the water. The collected concentrate with high content of nitrates and other salts has to be disposed of. The degree of purification of water from sodium chloride and sodium nitrate may reach 98% and 93%, respectively [55]
The ion exchange process for the removal of nitrates is both simple and effective. It operates in the same manner as a common water softener and can easily remove well over 90% of the nitrates [56]. The chloride ion of the salt is utilized by the resin. The sodium ion passes right through the resin bed and does not affect the process [57]. This method of removal requires several steps for successful decontamination. Essentially, the process relies on the fact that water solutions must be electronically neutral, and therefore by inserting a negative ion, another negative ion can be removed from the water. Besides the negative nitrate ion (NO3), common anions include sulfate, chloride, bisulfate, bicarbonate and carbonate ions [56].
Analysis of literature showed that using existing traditional methods does not achieve the regulatory water quality without the use of additional methods of deep water purification from impurities of natural and anthropogenic origin. Many surface water sources are contaminated with anthropogenic pollutants in respect of which the barrier role of the existing wastewater treatment plants is extremely low. Groundwater rather than surface water has greater security and stability of water quality. However, since the pollution of underground water sources with mineral nitrogen compounds in recent years has increased and they contain excessive amounts of nitrate, which are several times higher than the maximum permitted concentration for drinking water (45 mg/L) [12].
Removal of nitrate at concentrations >45 mg /L is a very complicated problem, as they do not form precipitates or complex compounds.
