Ferrate (VI) is a strong oxidizer with the chemical formula of FeO4−2. It is reported that in acidic conditions, the oxidation potential of ferrate(VI) exceeds all currently used oxidizer including ozone in water and wastewater treatment. Ferrate (VI) has a high solubility in water (approximately 15 g/l). In addition, during the oxidation and disinfection processes, this compound is converted into trivalent iron or ferric hydroxide (Fe(OH)3) which is a strong coagulant. Thus, by injecting ferrate (VI) the following three objectives could be achieved in a single unit: (1) the oxidation of pollutants in water, (2) water disinfection and (3) the elimination of suspended and colloidal particles. Utilizing ferrate (VI) as a multifunctional compound (chemical oxidizer and coagulant) offers several advantages, i.e. reducing treatment cost due to acting as both oxidizer and coagulant in a single unit and not producing by-product.
Applications of ferrate (VI) in water and wastewater treatment
All wastewater chemical treatment processes can consists of three primary sections, i.e. oxidation, coagulation and disinfection. There is a wide range of chemicals which can be utilized for these purposes; however, Ferrate (VI) is the sole potential candidate which can be employed for oxidation, coagulation and disinfection simultaneously.
In acidic environments (pH of less than 2), ferrate (VI) is stronger than any other oxidant including ozone. However, it does not mean that ferrate (VI) does not have ability of organic compounds oxidization in a higher levels of pH. Although, in the neutral or semi-alkaline conditions ferrate(VI) is not better than ozone or chlorine to oxidize pollutants or disinfect water and wastewater, it has enough ability to remove major part of the pollutants or microorganisms.
Chlorine is one of the most popular substances used in various wastewater treatment plants for disinfection. Since employing chlorine as a disinfection substance, a great deal of attention has been paid to address the issue of chlorination by-products. Chlorine in water is hydrolyzed to hypochlorous acid (HOCl) and hypochlorite (OCl−). It is discovered that hypochlorous acid react with naturally occurring organic matter to create many water disinfection by-products, including the four primary trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane and bromoform). All four the above mentioned compounds are absolutely carcinogen and their production during chlorination process must be controlled. So far, the emphasis has been given to minimize the concentration of natural/synthetic organic materials adequately before chlorination process to remedy the situation. Also, it has been attempted to replace chlorine by other substances such as bromine, iodine, chlorine dioxide and ozone; however, the main disadvantages of this remedy are the production of other by-products being harmful for human health and aquatic organisms and increase in the cost of disinfection process. Ferrate (VI) salt can disinfect treated wastewater and oxidize organic and inorganic pollution concurrently. Ferrate (VI) can kill the microorganisms in the two ways. (a) Since ferrate (VI) is a strong oxidizer compound, it can oxidize cell wall, protoplasm, DNA and other vital microorganism organs which kills them immediately. (b) Ferrate (VI) is gradually converted to Fe (III) which is a strong coagulant, therefore, colloidal particles including the microorganisms are coagulated and removed from the water. Small quantities of ferrate (VI) can coagulate colloidal particles and oxidize soluble pollutants simultaneously. These characteristics make ferrate salts as a proper substitute for other traditional disinfectants.
In the processes of oxidation and disinfection, ferrate (VI) is reduced to Fe (III), a powerful coagulant. Therefore, coagulation and flocculation are also occurred while using ferrate (VI) for oxidation and disinfection of pollutants in water. In the course of turbidity elimination, the amount of sludge produced using sodium ferrate (Na2FeO4) is much less than that while using ferrous sulfate (FeSO4−·7H2O), ferric nitrate (Fe(NO3)3), ferric chloride (FeCl3) and aluminum sulfate (Al2(SO4)3). It results in lower operation expense consumed for sedimentation disposal which can be considered as one of the advantages of using ferrate as a coagulant. Another advantage is destabilizing the colloidal particles in less than 1 min while ferric and ferrous salts require about 30 min to produce the same result. This feature of ferrate salt makes it as an efficient coagulator.
Removal of odor by ferrate (VI)
Malodorous compounds such as hydrogen sulfide (H2S), mercaptans, ammonia, etc. are formed during microbial activities under anaerobic condition in wastewater collection and treatment systems. Among different malodorous compounds, hydrogen sulfide is the most important one, esp. because of its adverse effect on both short-term and long-term human health. Production of hydrogen sulfide in wastewater not only makes malodorous environment but also causes corrosion in wastewater collection pipes. This corrosion occurs as a result of sulfuric acid produced by different aerobic bacteria such as Thiobacillus thiooxidans. One of the best strategies to control concrete pipe corrosion is the removal of hydrogen sulfide from wastewater. Among different methods which have been suggested for this purpose, using of oxidizers is the best choice. Several oxidizers such as oxygen, hydrogen peroxide, hypochlorites, chlorine and potassium permanganate have been suggested to oxidize hydrogen sulfide. Ferrate (VI) not only reacts with hydrogen sulfide but also degrades a diversity of compounds such as metallic or organic compounds which are considered as pollutants in groundwater.