Potassium ferrate is an inorganic substance with the chemical formula K2FeO4. It is a new type of non-chlorine green disinfectant with high efficiency and multifunctionality. It is mainly used for drinking water treatment.
Pure potassium ferrate is a dark purple shiny powder. It is stable in dry air below 198°C. It is very soluble in water to form a light purple-red solution. After standing, it will decompose and release oxygen, and precipitate hydrated iron oxide. The alkalinity of the solution increases with decomposition. It is quite stable in strong alkaline solutions and is an excellent oxidant. It has a highly efficient disinfection effect. It has stronger oxidizing properties than potassium permanganate. The iron in potassium ferrate is +6 valent iron, which is different from the +2 or +3 valent state of general iron elements. It has strong oxidizing activity in the entire pH range, and the oxidation potential can reach 2.20V. It can directly oxidize ammonia to nitrate, oxidize hydrogen sulfide to sulfate, and oxidize organic matter to carbon dioxide and water. It can both deodorize and detoxify. Since Fe(VI) in high iron does not directly transform into Fe3+ when it decomposes in water, but undergoes the evolution of intermediate forms from hexavalent to trivalent ions with different charges, positive hydrolysis products will be produced during the transformation process. These products have a large mesh structure, compressing and electrically neutralizing the colloidal impurity diffusion layer in the water, thus showing a unique flocculation effect. As a highly water-soluble bactericide, potassium ferrate can destroy the cell wall, cell membrane and some substances in the cell structure (such as enzymes) of microorganisms through strong oxidation, inhibit the synthesis of proteins and nucleic acids, hinder the growth and reproduction of microorganisms, and play a role in killing algae and bacteria.
Iodine X-ray contrast agents (ICMs) are intravascular drugs that can be introduced into the water environment through the human excretion system. In recent years, high concentrations of ICMs have been detected in surface water, groundwater and drinking water. Potassium ferrate, as its name implies, has the highest valence state of iron - hexavalent. It is a dark purple shiny powder, which turns light purple when dissolved in water. It has strong oxidizing ability and is an emerging green water treatment agent with oxidation-adsorption-flocculation synergistic effect in removing pollutants. It has good application potential in treating ICMs in water environment. Some researchers have studied the effect and mechanism of potassium ferrate on the degradation of iohexol.
The strong oxidizing effect of potassium ferrate on iohexol caused the concentration of iohexol to drop sharply in the first 2 minutes. With the consumption and self-decomposition of potassium ferrate, the removal efficiency tended to stabilize after 2 minutes. When potassium ferrate increased from 5 mg/L to 150 mg/L, the removal efficiency of iohexol increased from 56% to 72%. The removal rate tended to stabilize when adding more than 50 mg/L, which may be due to the exhaustion of iohexol by excessive high iron or the instability of concentrated potassium ferrate solution. When pH is less than 7, potassium ferrate has poor stability and self-decomposes. As pH increases, the stability gradually increases. When pH=8, the removal rate is as high as 90%. When pH>9, potassium ferrate is too stable and its oxidation performance decreases, resulting in a decrease in removal rate.
Humic acid can compete with iohexol, and its functional groups inhibit the free radicals from playing a role. At the same time, it will reduce the coagulation effect of potassium ferrate on iohexol and weaken the reaction between ferrate and iohexol. When the humic acid dosage reaches 50 mg/L, the removal rate of iohexol is only 45.4%; as [HCO3-] increases from 0 to 100 mg/L, the removal efficiency decreases from 77.3% to 69.6%, because HCO3- consumes free radicals, and hydrolysis increases pH and reduces the redox potential of potassium ferrate. Potassium ferrate self-decomposes to increase pH to 9.0-9.5; PO43- has a great influence on pH and removal effect. Its hydrolysis produces a large amount of OH-, which reduces the oxidation ability of potassium ferrate, and the generated Fe3+ is also consumed by complexing with PO43-; SO42- has little effect on the reaction and pH.
When the dosage of potassium ferrate in the actual water body is 5-30 mg/L, the removal rate of iohexol is 50%, which is lower than that of distilled water. It is speculated that there are substances that compete with iohexol in the actual water body; when potassium ferrate reaches 100 mg/L, the excess potassium ferrate consumes other competitors, and the removal efficiency reaches 76.3%, which is similar to distilled water.
The reaction mechanism may be that potassium ferrate first oxidizes the side chain hydroxyl of iohexol to cause dehydrogenation, and then attacks the N or O atoms on the side chain to induce amide hydrolysis reaction. Under alkaline conditions, part of it continues to undergo amide hydrolysis reaction and degradation reaction, and the other part undergoes deiodination reaction under the induction of potassium ferrate, and finally hydroxyl addition reaction occurs, and finally complete mineralization is achieved.
