Production, characterization and application of Ferrate(VI) in water and wastewater treatments

This paper aimed at reviewing different research work done on the synthesis of ferrate(VI) salts of potassium and or sodium, their applications in industrial wastewater, municipal sewage and water treatment. In this review, it was found that ferrate(VI) salt can exhibit more than one function in water and wastewater treatment as this chemical can take the roles of coagulants, flocculants, antioxidant, bactericide or disinfectant, and oxidant. Despite these properties, its availability on the market in a solid state is still a big problem due to its high cost and difficulties during its production as well as its chemical instability. Furthermore, suitable methods or procedures for manufacturing pure and stable ferrate(VI) salts were established in the past decades but are too expensive to produce sufficient quantities required for a large-scale water and wastewater treatment. Current ferrate synthesis methods include wet chemical oxidation, dry and electrochemical techniques. Among them, the wet oxidation method is the most applicable and safe to generate ferrate(VI) as dry and electrochemical methods can provoke detonation due to elevated temperatures and high concentration of electrolytes used, respectively. Some analytical techniques used to characterise and to quantify the ferrate(VI) products are scanning electron microscope, X-ray diffraction, infrared spectrometry, Mössbauer, ultraviolet-visible spectroscopy, thermogravimetric analysis, and volumetric methods. Several studies have been conducted to evaluate ferrate stability and its effective application in water and wastewater treatment. However, these studies showed that ferrate(VI) can oxidise or degrade organic pollutants such as pharmaceuticals, illicit drugs, and can also destroy or eliminate suspended particulate organic matter in a single mixing and dosing unit procedure. The stability of ferrate(VI) was found to depend on concentration, pH, and temperature of the solution, and coexistence of ions in the solution. Lately, most researchers stated that ferrate(VI) can also be applied to treat different emerging micro-pollutants, viz., personal care products, industrial organic chemicals, endocrine disrupting chemicals, heavy metals, metal-complexed species, and others in water decontamination processes. Studies also showed that ferrate(VI) salt can be classified as a green chemical which can replace different disinfectants and oxidants producing toxic disinfectant by-products generated by the use of chemicals such as chlorine and chlorine dioxide. Mining industry is another sector, which still has a problem of high energy consumption during acid mine drainage (AMD) treatment. This is due to extensive stirring and aeration which required to facilitate the oxidation of Fe(II) ions. However, the authors recommend the use of ferrate(VI) salt for AMD treatment as it can work as a powerful oxidant converting Fe(II) to Fe(III) and act as a coagulant in a single treating unit, hence, thus reducing energy consumption and environmental pollution problems.