There are various research studies carried out on the synthesis method that has a significant impact on the Nano-particles that are being produced. The characteristics, as well as the different synthesis methods to produce nanoscale zero valent iron are studied here in detail. Theoretically, the nitrate reduction by zero-valent iron ions is favorable from electron potential perspective and free energy. With an optimal pH range of nitrate reduction, ammonia is produced as the end product rather than nitrogen gas. The nitrite is a limitation in the reaction according to recent studies. The characteristic and usage of the different synthesis methods to produce nanoscale zero valent iron is studied in detail. the synthesis method has a significant impact on the nanoparticles being produced. The literature review discusses the various methods in depth the merits and demerits of using NZVI.
Sodium borohydride is used as a reduction method for synthesis of NZVI. It is used as a result of its merits in high active surface and specific surface area, it can be easily scattered, which brings the increased denitrification rate of nitrate (Shima Ziajahromi, 2012). The resultant black precipitates can be observed in the mixed solution with vigorous stirring. Phosphate removal experiments conducted with the specific time interval was collected and studied. With the aid of phosphate analysis and reactor, a blue complex was formed after reducing the phosphomolybdic acid. Batch experiments were conducted under different temperature conditions to investigate the effect of temperature changes on phosphate removal by NZVI (bezbarauah, 2012). Once the specific temperature is reached, the NZVI particles are added into the phosphate solution. Within a short period of time, the pH increased rapidly and remains between 9 to 10 regardless of the amount of iron throughout the reaction. Other researchers have mentioned previously that nitrate reduction with scaled iron powder does not materialize in the alkaline solution above pH 9 or at least the reactivity decreased considerably (KyoungHee Sohn, 2006).
There are various methods of denitrification and the NZVI is a Nano material that is broadly studied and applied. This is because the metal is in abundance and it is not costly. Next, little maintenance is needed for the reduction process and the metal is also freely existing. The increased need for Nano-metals for subsurface remediation of chlorinated compound and sites contaminated by heavy metal has received significant attention in part because of the capability of Nano-metals to quickly convert impurities in controlled laboratory experimentations. Examples of Nano-metals used for this purpose are iron and zinc. However, the most commonly used is the NZVI. (O’Carroll et al, 2012). There are many methods that can be used to synthesize the NZVI particles namely the borohydride reduction (liquid phase reduction), gas phase reduction using hydrogen gas, micro emulsion, chemical vapour condensation, thermal reduction as well as physical methods such as peening. The most common of them all is the sodium borohydride reduction method. The purpose of this paper is to discuss the effects of synthesized NZVI on the nitrate reduction of water under different parameters. The parameters are such as reusability and ethanol to water ratio will be varied accordingly to determine which gives the most impact to the reduction. Other than that, the solid phase analysis will also be carried out using the scanning electron microscopy (SEM) machine. It aids in the study fo the characterization of the synthesized nanoparticles.
Nano-Zero-Valent Iron (NZVI)
In the recent years, nanotechnology has been expanding vastly and has been very promising in the contribution to knowledge and science. It has been used numerously for many purposes and one of them is for the remediation of contaminated areas. (Mueller, 2010). Nano particles are very small in sizes and have a dimension of less than 100nm. There are a wide range of applications and uses where nano-particles play an important role. Nano particles can be used in fields such as environment, engineering, electronics, medicine, and others. (li X, Elliot DW, Zhang W, 2006) Iron, magnesium, palladium as well as silver are generally used. (Vyjayanthi JP, 2012). The most well known to be used is the Nano zero valent Iron as recorded by Kober R 2002. The first ever field try of NZVI was recorded back in 2000 in New Jersey, USA where NZVI was used to treat ground water that was contaminated by trichloroethylene. (Bardos P et al, 2014)
NZVI has many beneficial properties that contribute to the onsite remediation of ground water and drinking water. These properties are such as large surface area, high reactivity, fast kinetics as well as magnetic properties (Wang X et al, 2014). These characteristics assist in the separation and recovery of tiny unwanted materials from ground water. Although the size is fixed to be around the range of 100nm, there are previous studies that suggest the size to be roughly 60nm while the critical diameter is at 30nm. (sun YP et.al., 2006). The advantage the NZVI has over the granular ZVI is the particle size as well as the surface area. The NZVI has a larger surface area which in turn increases the reactivity of the particle almost a thousand times more. The ability for absorption is much greater than for the granular ZVI. Due to this, the NZVI is said to be more cost efficient as only a small amount is needed for a great effect in the betterment of ground water contamination. (Ponder SM, et.al., 2001) The contaminant degrades at a faster pace and thus saving time in the process. NZVI has the capability to treat even the chemical compounds that are present in aquatic environment such as chlorinated organic compounds, and nitrates. It can also wholly eliminate the formation of toxic intermediate compounds. (Vyjayanthi JP, 2012)
Attributes of NZVI
The properties of NZVI are such that they have high specific surface area and high active surface. It is easily scattered and as a result, it increases the denitrification rate of nitrate. Amongst the various water treatment methods, NZVI has a high probability in the removal of nitrate. The study of NZVI has become a very vast area. It has been used in halogenated organics, azoaromatics, nitro aromatics as well as the handling of diverse kinds of compounds for example inorganic compounds. The study of NZVI in heavy metals have also been done. In this study, researchers have determined that the complete reduction of nitrate can be accomplished through metallic iron under anaerobic and aerobic conditions. (Ziajahromi, S, et al. 2013).
Iron stands out as a robust, shiny and malleable metal that can be found in plenty within the earth’s crust. It can also be found in the periodic table in group 8 and period 4. NZVI is a derivation of a sub micrometer particle of the iron metal. The reactivity of iron is high as it can react well with oxygen as well as water. At a nanoscale size, the reactivity tends to increase greatly hence shows the significance of NZVI. There are some obvious difference between the properties of bulk iron particles and the NZVI particles. The various factors such as the samples’ history as well as the conduct and the processing of the NZVI that will impact the size, structure, and the composition of the NZVI can lead to the differences mentioned earlier. The attributes being measured can be altered by the packing of NZVI for measurement purposes. (Baer DR., et al 2007)
Many characterization tools such as scanning electron microscopy, optical spectroscopy, transmission electron microscopy, and the x-ray diffraction have been established to analyze the characteristics and properties of Nano-particles in details. The attributes of the NZVI that can be studied through the named tools are such as the size, shape, surface area as well as the chemical state. The core shell structure of NZVI have been revealed through these tools as well. It can be depicted that the NZVI exhibits sorption properties through its iron oxide and also metallic iron as an electron source. Besides that, it is seen that the mixed valence iron oxide shell is largely insoluble under neutral pH conditions which in turn can prevent the core of the zero valent iron from oxidizing quickly. (Baer DR., et al 2007). The NZVI has shown to own magnetic characteristic that depends on various factors such as the synthesis method, size, shape, chemical compositions, oxidation of the surface as well as the dimensions. (Wang C, et.al, 2009). Bare NZVI tends to form chain like structures that may result in accumulation. Magnetic separation is a common separation technique that has been used in various recovery processes such as to separate the coated NZVI particles from algae solution. (Krajewski M, et al, 2015). The NZVI has catalysis properties as well that can contribute in the various reduction reactions for instance by being used as a catalyst in the hydrogenation reaction of many substituted aromatic stability, reusability, ecological friendliness are reasons why transition metals play a significant role in catalysis as well as substitutes of platinum based catalysts. It has better effects when the metal particle is reduced to the nanoscale through the modification of its physical structure such as surface area to volume ratio. Such factors are advantageous in improving the efficiency of the catalyst. (Parimala L and Santhanalakshmi. J, 2014)
Morphology and size of NZVI
Under the SEM and TEM, the appearance of the Nano particles are usually accumulated together in a spherical shape. The accumulation is caused by the chemical as well as magnetic reactions. The particle is spherical and has a size range of 60nm to 120 nm in diameter. Through the various studies that have been conducted, it can be observed that the morphology of the nanoparticles are dependent on the method of synthesis. A comparison study of the morphology and crystallinity of NZVI produced by the two different methods namely the reduction of NaBH4 and reduction of goethite by hydrogen gas were done by Nurmi et al (2015). The NZVI synthesized via NaBH4 reduction is more spherical in shape and forms chain like aggregate. This is because the medium used was in a solution phase. On the other hand, for the hydrogen gas reduction method, the NZVI has more of an irregular shape as well as large faceted plates which most likely the iron oxides. These two methods present two different characteristics in terms of their crystallinity. The NZVI via sodium borohydride reduction has an electron diffraction pattern of diffuse rings which shos that the metal phase is bcc polycrystalline iron with individual grain sizes of less than 1.5 nm. At this state, the difference between amorphous and polycrystalline iron is uncertain. The oxide phase tends to be amorphous and disoriented. This can be depicted through the lack of lattice fringe in a HR-TEM micrograph. On the other hand, for the hydrogen gas reduction method, the NZVI exists as a singular crystal particle having a very high crystalline iron oxide phase. This can be seen through the faceted surfaces as well as the presence of the periodic lattice fringes. (Nurmi et. al, 2005). The X-ray diffraction studies present similar observations as the TEM. The wide peak equivalent to bcc for NZVI synthesized by the borohydricde reduction shows a highly disordered iron oxide core. (Liu et al, 2014). No peaks equivalent to iron oxife remained observed in the XRD scale, constant with its vague nature. To examine the fine structure of the oxide layer, Mossbauer spectroscopy has been carried out on the NZVI samples whereby the results depicts the occurrence of mixed ferrous oxide and super paramagnetic ferric oxide. (Kanel et al., 2016). It is very important to determine the effective surface area as well as the relative amount of iron and oxide present in the nanoparticles as through this, the sum of surface reactive sites and reactivity of the particles can be influenced. These effects on the contrary are due the important parameters which are the size and also the size distribution of the NZVI particles. This is done through the geometric correlation between the surface area as well as the size of the NZVI. (Li. S et al 2009).
Water Treatment Types
The demand for safe drinking water is on the rise. Water treatment industries have a vital role to play in the removal of water pollutants such as nitrate from the water, the primary causes of the nitrate contamination of ground and surface water are the anthropogenic sources. The key sources of such pollutants are nitrogen fertilizer, nitrogen pesticides, and the industrial waste water discharge (Vodyanitskii Yu., et al., .2015). Human health is at risk due to the increase of nitrate contaminations is drinking water supplies. When NO3 is reduced to NO2 in newborn babies, methemoglobin will be generated when combined with hemoglobin in the blood. This in turn leads to cyanosis in the babies. Next, bladder and ovarian cancer can be caused by the consumption of the tap water that has high content of nitrate. (Ziajahromi, S, et al., 2013)
Ion Exchange Denitrification
This is the removal of nitrate from water by sending the water through a bed of synthetic resin bead. This causes the exchange of chloride to take place. the volume of resin achieved will be reformed through a sodium chloride solution. This is commonly referred to as brine. As a result, the resin becomes Cl- form. The water bed is rinsed with fresh and clean water for use in another denitrification process. The regenerant that has been consumed has in it a large concentration of NaCl combined with nitrate and sulphate anions that are removed from the bed of resin. The manufacture of high purity water and produce concentrated brine that requires additional treatment is the main reason why the ion exchange and absorption processes have been established mainly for the frequently concerning adsorption resins. Their capacity is swiftly reached by the produced concentrates hence the need of repeated replacement or regeneration (Naik, 2012).
The biological water treatment techniques are implemented to remove nitrate from waste water and to prevent eutrophication. The biological denitrification is another method of removing nitrate from water. In this process, a specific natural bacterium is analyzed as nitrate is used for the respiration process which occurs under anaerobic conditions. To achieve a successful biological denitrification on water, the heterotrophic and autotrophic bacteria is used. The bacteria used needs a source of food to enhance its growth in the heterotrophic denitrification. Therefore, the carbon substrate such as methanol and acetic acid can be used. On the other hand, for autotrophic denitrification, the bacteria growth is in need for energy source. The inorganic energy sources namely Sulphur, thiosulphate, and even hydrogen can be used. Bicarbonate found in the water itself can be used as a carbon source for the growth of the bacteria. Various substrates together with reactors are examples of unit processes that is required in this process for the growth of bacteria. To compare these two methods, heterotrophic denitrification systems are used much widely than the autotrophic processes. The waste product of this reaction is mainly biological solids which are not very harmful hence a good idea of water treatment. Industrial waste waters can be treated via this method whether it is domestic waste water or even a complex one. (Naik, 2012)
Membrane separation Techniques
Reverse osmosis and electro dialysis are common methods of membrane separations and can be applied in the process of nitrate removal. A major demerit to this is that it is up to eight times more expensive to operate with compared to the ion exchange process. The method is not economy friendly as it requires a very large capital as well as extensive operational costs. (Reddy and Lin, 2000). There have been a number of tests regarding the membrane technology that has been carried out. For instance, a chemical process is described by Murphy (1991) whereby aluminum powder reduces small amounts of nitrates in water to ammonia, nitrogen gas, and nitrite. The examples of reactions can be demonstrated by the equations shown below:-
The membrane technology is a focused technique that requires a post-treatment of the brine before discharging it out of the process. The chemical treatment alone is not very effective in the treatment of low nitrate concentrations of waste waters. A large amount of chemicals will be needed for this process. (Naik, 2012)
The catalysts used in the catalytic reduction which is the process of removal of nitrates form ground water are
- Rhodium on carbon (5-10%)
According to a research conducted by Reddy and Lin (2000), it is inferred that rhodium is the most effective catalyst among the list. The process is not expensive and thus can be used for treatment of water in small areas. The coating of rhodium catalyst on to the fiber glass mesh is done and using a photovoltaic cell, a desired redox potential is developed. The operating cost can be reduced by the usage of sunlight as the energy source for the catalyst activation.
The liquid phase hydrogenation using a Pd-Cu bimetallic catalyst can assist in reducing nitrate in drinking water. Basing from a study done by Lecloux (1999), Palladium alone is not effective in decomposing nitrates but can detach nitrites while copper is found to be unsuitable in the decomposition of nitrate. Copper to palladium in the ration of 5:1 was considered to be ideal for the selective reduction of nitrate to nitrite. In the process of reduction, nitrite, nitric oxide and atomic nitrogen formation is done so that the production of atomic oxygen formed can be absorbed strongly onto the surface of the catalyst. This in turn will cause the blockage of active sites and reduction of efficiency. In order to enhance the cleaning of catalytic surface and the formation of nitrogen, the partial pressure of hydrogen can be increased. Through the hydrogenation process, the electro neutrality of the solution is maintained through the formation of hydroxide ions. The byproduct of this reaction is hydroxide ions and it is undesired. The carbon dioxide can be used to handle the issue if necessary. (Fanning, 2000)
Among the various ways of reducing or removing nitrates, electrochemical techniques have also been widely established to serve that purpose on a marketable basis. Cathodes such as nickel, lead, zinc, and iron are used to achieve intermediate formation of nitrite based on Li et al. (1988). Ammonia is left as the final product. The zinc and lead cathodes successfully reduced 90% of the nitrate through the electrolysis process which takes a duration of about an hour. Chew and Zhang (1998) proposed that to treat the contaminated soil, electrokinetics that is coupled with the zero valent iron can be used. The transformation of nitrate to nitrogen was 84-88% when the iron oxide was well located beside the anode and the current was turned on. It is shown that there is probable use of the electrokinetics or iron wall in the treatment of ground water that is contaminated with nitrate.
The photochemical method has been studied and it is proven that it is possible to reduce nitrate for the treatment of waste water. Light is used for the activation of nitrate ion directly or indirectly in the presence of a catalyst and a reducing agent. Although it is a plausible method, it is a tough energy process thus it is not recommended for water treatment in large scales. (Fanning, 2000)/
Synthesis of NZVI
The nanoparticles in the presence of air, water soil or even sediments can occur naturally. They can also be produced synthetically for particular processes. Other than that, it can appear as a by-product from other processes (Mueller NC and Nowack B, 2010). Technically, micro particles are less costly compared to reactive nanoparticles due to the materials needed to synthesize them. (Hoch LB et al. 2008) There are many physical and chemical methods to produce NZVI for instance grinding, abrasion, lithography, annealing at high temperatures as well as most common one which is using reducing agents such as sodium borohydride. There are two approaches to the method of synthesis namely bottom-up as well as the top-down. The bottom up is basically the bringing together of individual atoms and molecules to form Nano sized structures. In this case, reducing agents such as sodium borohydride are used. The top-down approach on the other hand is the crushing or grinding of bulk particles into fine Nano sized particles chemically as well as mechanically. These types of methods can affect the shapes and sizes of the nanoparticles produced. Borohydride reduction or the liquid phase reduction, gas phase reduction, micro emulsion, chemical vapor condensation, thermal reduction as well as electrolysis are examples of chemically synthesized methods. Physical methods on the contrary are such as precision milling, inert gas condensation, polyphenol plant extract as well as ultrasound shot peening.
The most popular used method is the borohydride of ferrous as it is a simple method that requires no special equipment or materials. It can even be done in a batch study ad a laboratory scale. (Tao NR, et al 1999). The NZVI produced via this structure is very reactive and has a uniform structure. The cost to produce NZVI at a laboratory scale is cheaper compared to other methods. Other methods have possibility of not being feasible when producing on a large scale. (Li S, et al 2009). NZVI can be synthesized by using an iron salt such as ferric chloride in the presence of a reducing agent such as sodium borohydride. The solution is slowly added into the iron salt while being continuously stirred under anaerobic conditions. As the borohydride solutions is being poured drop by drop, black particles begin to form the iron sal solution. The resulting black substances form is separated via vacuum filtration or a magnet and is then washed with ethanol to prevent oxidation and later dried. (Sun YP, et al., 2007).
To use bimetallic iron by coating NZVI with Tin
There are various alternatives for improving the aqueous mobility of NZVI. These are surfactants, polyelectrolyte coatings, and the use of remediation of non-aqueous phase liquids. The particle mobility and stabilization may be enhanced by using of surfactants and plyometric surface coatings. The steric deterrents offset the electrical and dipolar attractions between particles and promote colloidal stability. This can only be achieved when an adequate mass of coating substantial or surfactant is present to form a complete micelle, resulting in its limited applicability. For organic environments, the NZVI can be coated with high molecular weight polymers. The irreversible process provides a more appropriate method for increasing the amount of ions in the solution.
The tin-iron alloy is a suitable component in electroplating. Tin is soft and is usually applied in thickness of about 0.0005” equivalent to 12.7micrometres. This avoids some of the issues with interference fits, the machining, or grinding some of the especially difficult to plate heavy deposits of copper, nickel, and chromium on parts that are not well designed. The element that is mixed with Tin to obtain an alloy aims at preventing the rust or corrosion of other metals and alloys that it coats. It is remarkably resistant to corrosion especially under atmospheric conditions. When tin or its alloys are reflowed, some tin will alloy with either ferrous basis metals. This kind of alloy has been reportedly seen to improve the protective value of such coatings. The protection is also obtained as a result of a reduction in the number of pores due to the reflowing as well as greater protection at the remaining pore sites. The relatively slow rate of attack of the atmosphere on tin and its cathodic reaction when combined with more active metals, permits tin to be used as either an undercoating or a top coat with zin, iron, or even cadmium. The purpose of such coating is to improve the benefits derived from the underlying basic metal. When used as a top coating it protects the underlying metal from atmospheric dynamics. It tends to improve the resistance of metals such as zinc from marine atmosphere.
From the study of materials, it is known that tin does not have a low value of contact resistance as compared to other metals. It can maintain this value for even longer periods. It is normally cathodic to iron and copper. Its value as a corrosion barrier depends upon the continuity of the coating. It is essential that all electrodeposits that are to be soldered have excellent adhesion. The deposits that are plated over oxidized or oiled surfaces cannot be successfully soldered. Using such kind of coating in water treatment equipment has its merits and drawbacks.
- Longevity and anti-corrosion properties;
- It offers a high surface hardness and displays excellent wear resistance
- Low surface friction resistance and excellent galling resistance, enabling high-precision tapping.
- It has a high heat resistance which enables high speed tapping
There are three paramount conditions necessary for corrosion namely water, oxygen, and an electrolyte. For water treatment practices, using the NZVI, the tin is used in electroplating. During the process, a thin layer of metal is deposited on the object being protected. The tin plating requires the object that is being protected to be the negative terminal. It needs to be surrounded by a solution containing tin chloride ions of the metal to be deposited on the object for coating. The iron cathode and anode becomes coated in tin atoms. This reaction can be demonstrated by the following equation :-
Tin as is used in many applications; it is usually base iron coated with tin. This item is referred to colloquially as tin cans. Tin is mainly cheap hence it is used to store items which are availed to consumers at retail stores. There are two different synthesis methods Sn/Fe nanoparticles will affect the reactivity. It showed simultaneous reduction method consists of higher reactivity compared to sequential reduction method. This is because on sequential reduction method only allow a part of Sn nanoparticles combine with Fe nanoparticles (Haiyan Kang, 2012).
This is a redox reaction is a type of chemical reaction that transfers electrons from one element to another. It can be split into reduction and oxidation half reactions. The half reactions to make it easier to see the electron transfer. In this study, the synthesizing nanoparticles is also a type of redox reactions. In the synthesis reaction, there is an exchange of electrons.There are many advantages of the application of nanoparticles especially in the remediation of the environment. The benefits are such as the amount of potential dangerous substances are decreased, the cost and the time for big scale remediation are also reduced. It is very efficient in the removal of contaminants in water and soils. Many researches have been done in regards to the study of NZVI over the years. These studies are aimed to improve and innovate the current technology and techniques of the synthesis as well as boost the stability and mobility of the NZVI particles. There are possible threats that results from the application of various modified as well as non-modified NZVI.
Barrett, J. H., Parslow, R. C., McKinney, P. A., Law, G. R. and Forman, D. 1998. Nitrate in drinking water and the incidence of gastric, esophageal, and brain cancer in Yorkshire, England. Cancer Causes Contr., 9: 153–159
Chen, J., Xiu, Z., Lowry, G. V. and Alvarez, P. J. 2011. Effect of natural organic matter on toxicity and reactivity of nano-scale zero-valent iron. Water Res., 45: 1995–2001.
Cheng, I. F., Muftikian, R., Fernando, Q. and Korte, N. 1997. Reduction of nitrate to ammonia by zero-valent iron. Chemosphere, 35: 2689–2695.
Elliott, D. W. and Zhang, W. X. 2001. Field assessment of nanoscale biometallic particles for groundwater treatment. Environ. Sci. Technol., 35: 4922–4926.
Epron, F., Gauthard, F., Pineda, C. and Barbier, J. 2001. Catalytic reduction of nitrate and nitrite on Pt-Cu/Al2O3 catalysts in aqueous solution: Role of the interaction between copper and platinum in the reaction. J. Catal., 198: 309–318
Nolan, B. T., Ruddy, B. C., Hitt, K. J. and Helsel, D. R. 1997. Risk of nitrate in groundwaters of the United States – A national perspective. Environ. Sci. Technol., 31: 2229–2236.
Prusse, U. and Vorlop, K. D. 2001. Supported bimetallic palladium catalysts for water-phase nitrate reduction. J. Mol. Cataly. A: Chem., 173: 313–328
Ponder S. M., Darab J. G., & Mallouk T. E. 2000. Remediation of Cr(VI) e Pb(II) aqueous solutions using supported, nanoscale zero-valent iron. Environmental Science & Technology, 34(12), 2564–2569.
Sparis D., Mystrioti C., Xenidis A., Papassiopi N. 2013. Reduction of nitrate by copper-coated ZVI nanoparticles, Desalination and Water Treatment, 51, 2926–2933.
Vodyanitskii Yu. N. and Mineev V. G.2015. Degradation of Nitrates with the Participation of Fe(II) and Fe(0) in Groundwater: A Review, Eurasian Soil Science, 2015, 48, 139–147.
Zhang, W. X., Wang, C. B. and Lien, H. L. 1998. Treatment of chlorinated organic contaminants with nanoscale bimetallic particles. Catalysis Today, 40: 387–395
Zhou H., He Y., Lan Y., Mao J., Chen S., 2008. Influence of complex reagents on removal of chromium(VI) by zero-valent iron. Chemosphere 72, 870–874.
Zin M. T., Borja J., Hinode H., Kurniawan W.2013. Synthesis of Bimetallic Fe/Cu Nanoparticles with Different Copper Loading Ratios, International Journal of Chemical, Nuclear, Materials and Metallurgical Engineering, 7, 12, 687-690.
Xu Y., Zhao D., 2007, Reductive immobilization of chromate in water and soil using stabilized iron nanoparticles. Water Res., 41: 2101–2108.