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K e y w o r d s : Potassium ferrate(VI), K2FeO4, EDCs, SPCs, PCPs, Pharmaceuticals. 1/2020 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 129 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 1/2020 d o i : 1 0 . 2 1 3 0 7 / A C E E 2 0 2 0 0 1 1 S . K l i ś , K . B a r b u s i ń s k i , M . T h o m a s , A . M o c h n a c k a 130 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 1/2020 properties. Only the fluorine and the •OH (hydroxyl radical) have a higher value of the redox potential. Under appropriate conditions, Fe(VI) ions are even able to oxidize water and the reaction products are hydrogen protons and oxygen [4]. They are also characterized by the ability to oxidize compounds containing, e.g. sulfur, nitrogen, in particular phenols, amines, aniline and alcohols [5, 6]. Generally, all pollutants are degraded to water and carbon dioxide or intermediates, which are less toxic than initial substrates, which makes Fe(VI) salts environmentally friendly and therefore potassium ferrate(VI) is often called “environmentally friendly oxidant” [7]. High oxidation capacity and non-toxic decomposition products allow to use K2FeO4 for the degradation of various organic and inorganic compounds. During the reduction of Fe(VI) to Fe(III) emerge a coagulation and adsorption of contaminants on the formation of flocs of iron(III) hydroxide. Potassium ferrate(VI) does not react with bromide ions, which avoids the formation of carcinogenic bromates. Among many methods of obtaining K2FeO4 it can distinguish three most common and proven methods of synthesis (wet, dry and electrochemical synthesis) [6]. • Wet synthesis consists of the preparation of sodium ferrate(VI) (Na2FeO4), in the reaction of FeCl3 with NaClO3 (at a concentration greater than 12%), in the presence of NaOH. Then KOH is added to the solution until precipitation of K2FeO4 [7]. However, this method requires many subsequent purification operations, such as: crystallization with 3 mol/L KOH, washing with anhydrous CH3OH to obtain a solid K2FeO4, with purity > 90%. The above method, without using mentioned purification processes of the synthesis products, makes it possible to obtain a technical K2FeO4, containing only 10–15% pure compound. • Dry synthesis is the process of oxidation of iron compounds at high temperatures and involves the heating of iron oxides, for examples Fe2O3, with potassium and sodium hydroxides or oxides and potassium nitrate and sodium nitrate at a temperature of 420–600°C. Depending on the molar ratios of the mixture components and temperature, the process proceeds in different efficiency and enables to obtain the final product with a purity of up to 99.9%. • Electrochemical synthesis consists in dissolving an anode made of metallic iron in an aqueous soluTable 1. Redox potentials of selected oxidants [3, modified] Element/Compound Chemical reaction E0, V Chlorine Cl2(g) + 2e-⇆ 2ClClO+ H2O + 2e-⇆Cl+ 2OH1.358 0.841 Chlorate(I) HClO + H + 2e-⇆Cl+ H2O 1.482 Chlorine(IV) oxide ClO2(aq) + 2e-⇆ClO20.954 Chlorate(VII) ClO4 + 8H + 8e-⇆Cl+ 4H2O 1.389 Fluorine F2(g) + 2e-⇆ 2F2.87 Hydroxyl radical OH• + H2O2 ⇆H2O + HO2• 2.76 Ozone O3 + 2H + 2e-⇆O2 + 2H2O 2.076 Hydrogen peroxide H2O2 +2H + 2e-⇆ 2H2O 1.776 Oxygen O2(g) + 4H + 4e-⇆ 2H2O 1.229 Permanganate MnO4 + 4H + 3e-⇆MnO2 + 2H2O MnO4 + 8H + 5e-⇆Mn2+ + 4H2O 1.679 1.507 Dichromate Cr2O7 + 14H + 6e⇆ 2Cr3 + 7H2O 1.33 Ferrate(VI) FeO4 + 8H + 3e⇆ Fe3 + 4H2O FeO4 + 8H2O + 3e⇆ Fe(OH)3 +8H2O 2.2 0.7 Figure 1. Structural formula of K2FeO4 [2] APPLICATION OF POTASSIUM FERRATE(VI) IN THE TREATMENT OF SELECTED WATER AND WASTEWATER POLLUTANTS – SHORT REVIEW E N V I R O N M E N T 1 /2020 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 131 tion, which is associated with very low efficiency of the process and obtaining a low purity of the product. Only the use of a solution of concentrated hydroxides (KOH, NaOH) instead of water results in K2FeO4 of high purity [8, 9]. Various research investigations have been carried out on the application of K2FeO4 as an oxidizer in industrial wastewater treatment, because this method is characterized by high efficiency of pollutant degradation. Selected applications of K2FeO4 for the degradation of various pollutants in water and wastewater, such as EDCs, PCPs, SPCs, pharmaceuticals are presented below. 1.1. Degradation of Endocrine Disrupting Chemicals (EDCs) EDCs (Endocrine Disrupting Chemicals) are chemical compounds that disrupt the hormonal balance of the body. They are common in everyday life, because they are used for the production of plastic packaging, they are found in cleaning agents and detergents and also a part of medicines or plant protection products. EDCs also include natural origin compounds that, despite their origin, also destabilize the functioning of the endocrine system. In this group, hormonal compounds can also be distinguished, which after entering the body may negatively affect its homeostasis [10]. Table 2 presents examples of compounds obtained by chemical synthesis and natural compounds, classified as EDCs. EDCs are a serious threat to human and animal health, especially if they enter the aquatic environment in a direct way (plant protection products, animal faeces) and uncontrolled. The biggest problem is the lack of adaptation of technologies used in water and wastewater treatment plants for the removal of these compounds. For the first time this problem was noticed and described in 1993 by Colbrom et al. [12]. During several years of research in the Great Lakes (USA), where mainly the chemical and heavy industry is located, several generations of animals inhabiting this area were examined. As a result of research, it was found that the next generations are physically weaker and thus the incidence of various types of diseases, increases with subsequent generations. Colbrom et al. [12] determined that substances escaping from industrial plants into the environment are permanently accumulated in animal organisms and destabilize their hormonal system. She also proved that these substances cause changes in the embryonic phase of animals and on this basis, the conclusion is that these substances also threaten people [12]. Therefore, the search for methods allowing effective removal of EDCs from the natural environment was started. However, the applied physical processes, such as membrane methods, were characterized by different effectiveness of removal of individual compounds, as shown in Table 3. The very strong oxidizing properties of potassium ferrate(VI) cause, that this compound is a place of interest to many scientists around the world, including endocrine disrupting compounds (EDCs). Conventional methods of purification are not effective, due to the huge amount and variety of pollutants. Most of the chemical compounds used in wastewater treatment processes have a low or selective effect, and the use of K2FeO4 under appropriate process conditions (pH, temperature, time, dose) allows for complete degradation or significant EDCs reduction in wastewater [13, 14]. At the Hong Kong University, research was carried out on various methods of removing EDCs by using K2FeO4. Two Bisphenol A compounds (BPA) were tested – used in the production process of plastic packaging, paper, cosmetics and cleaning products, and 17–α–ethynylestradiol (EE2) – a component of contraceptives [15]. First of all, the strong oxidizing properties of K2FeO4 were used to degrade the tested compounds. During tests, the effect of pH and reaction time on the oxidation efficiency of the tested compounds were determined and in the case of BPA a schematic sequence of changes taking place e Table 2. EDCs division depending on the origin [11] Table 3. Percentage removal of EDCs in membrane processes [11] EDCs obtained by synthesis Natural EDCs Bisfenol A Progesterone Bisfenol F Testosterone 17–α–ethynylestradiol (EE2) Androsterone Genistein 15–β–estradiol (E2) Biochanin A Estrone (E1) EDCs Effectiveness, % Bisfenol A (BPA) 18–99 Testosterone 30–60 17–α–ethynylestradiol (EE2) 34–60 15–β–estradiol (E2) 8–90

Application of Potassium Ferrate(VI) in the Treatment of Selected Water and Wastewater Pollutants – Short Review