The Role and Mechanism of KDF Filter Media in Water Treatment

KDF (Kinetic Dioxide Dispersant) is a high-purity copper/zinc alloy granule that treats water through a micro-electrochemical redox reaction. Upon contact with water, the two metals in the alloy form countless tiny galvanic cell systems at a submicroscopic scale. This material exhibits strong reactivity and extremely fast reaction speed in water, capable of removing up to 99% of chlorine and dissolved metal ions and compounds such as lead, mercury, nickel, and chromium. It is highly effective in inhibiting the growth of bacteria, fungi, dirt, and algae. It is used in pretreatment, primary treatment, and wastewater treatment equipment. KDF improves upon or replaces existing technologies, significantly extending system lifespan, reducing heavy metals, microorganisms, and dirt, lowering overall costs, and simplifying system maintenance.

(1) Removal of Strong Oxidizing Agents (Residual Chlorine)

KDF has a strong reducing ability, effectively removing various strong oxidizing agents from water, and is particularly effective against residual chlorine. KDF (KindFeB) is composed of two different metals, copper and zinc. When in contact with water, the copper, with its positive potential, becomes the cathode, while the zinc, with its negative potential, becomes the anode, forming a galvanic cell. The zinc anode loses electrons in the reaction, generating zinc ions that enter the solution. A reduction reaction of free chlorine occurs at the copper cathode, without dissolving metallic copper. Water and residual chlorine become the final electron acceptors, simultaneously generating hydrogen ions, hydroxide ions, and chloride ions. The overall reaction is as follows: Zn + HOCl + H₂O + 2e⁻ → Zn²⁺ + Cl⁻ + H⁺ + 2OH⁻. Other oxidants in water, such as ozone, bromine, and iodine, can also undergo similar redox reactions upon contact with KDF.

(2) Removal of Heavy Metals

KDF treatment media can remove various heavy metal ions from water, such as lead, mercury, copper, nickel, cadmium, arsenic, antimony, aluminum, and many other soluble heavy metal ions. Their removal is accomplished through displacement reactions and physical and chemical adsorption reactions. The mechanism by which KDF removes heavy metal ions is as follows: Metal ions are adsorbed onto the surface of the KDF treatment medium and undergo a displacement reaction with zinc in the KDF medium. The resulting metal is either adsorbed on the KDF surface or enters the KDF crystal lattice, thus binding toxic heavy metal pollutants to the KDF. For example, dissolved lead ions in water are reduced to insoluble lead atoms, which are adsorbed onto the surface of the KDF medium. Mercury ions undergo a similar reaction with KDF, and X-ray diffraction studies have shown that mercury removal occurs through the formation of a copper-mercury alloy. The chemical reaction formulas for KDF treatment of heavy metal ions are as follows:

Zn/Cu/Zn+Pb2+ →Zn/Cu/Pb+Zn2+

Zn/Cu/Zn+Hg2+ →Zn/Cu/Hg+Zn2+ Metal ions also hydrolyze to form metal hydroxide precipitates when the pH of the water increases, which also removes metal ions. (3) Removal of Hydrogen Sulfide

When using membrane technology for water treatment, if groundwater is selected as the water source, hydrogen sulfide may be present in the water. If hydrogen sulfide is oxidized into sulfur, it will contaminate the surface of the filter membrane. KDF filter media has the function of removing hydrogen sulfide. The generated copper sulfide is insoluble in water and can be removed during the backwashing of KDF media. The chemical reaction formula is as follows:

Cu/Zn + H2S → Cu/Zn + CuS + H2

2H2 + O2 → 2H2O

(4) Reduction of Suspended Solids

The average particle size of KDF treatment media is approximately 60 mesh, and the smallest particles are approximately 110 mesh. It can also play a role in physical filtration to remove suspended matter. Generally, KDF filter media can effectively remove particles with a diameter of less than 50 μm. When water pipes made of steel corrode, iron oxidizes to form FeO colloid. FeO can also undergo a redox reaction with KDF (KindFeBest Chemicals), ultimately forming Fe2O3 solid precipitate on the KDF surface. This precipitate can be removed by backwashing. The chemical reaction formulas are as follows:

Zn + FeO ＝ ZnO + Fe

2Fe + 3O2＝2Fe2O3

(5) Reducing Mineral Scale

KDF treatment media has two effects on calcium carbonate scale.

① On the one hand, based on the relationship between pH, carbon dioxide concentration, and calcium carbonate solubility, when carbon dioxide is removed from the solution, the pH value increases, thus reducing the solubility of calcium carbonate. KDF also increases the pH value of the water through an electrochemical reaction, reducing the solubility of calcium carbonate, resulting in easier precipitation of calcium carbonate scale. ② On the other hand, due to the dissolution of zinc ions in the KDF treatment medium, the zinc ion content in the water increases. The presence of zinc ions in the water can alter the crystal growth mechanism of scale, causing calcium carbonate scale in the water to precipitate in the form of aragonite crystals, forming soft scale on the container walls, rather than crystallizing into hard calcite scale. Previous studies have investigated the effect of impurities in water on calcite crystal growth, finding that even at very low zinc ion concentrations, calcite crystal formation can be prevented.

Further experiments demonstrate that the KDF treatment medium prevents the formation and accumulation of mineral scale primarily by inhibiting the crystallization of calcium carbonate in the calcite form. Crystallographic studies using scanning electron microscopy and X-ray diffraction have demonstrated that the scale formed in water untreated with KDF consists of relatively large, regularly shaped needle-like crystals of calcium and magnesium salts. These salts are hard, have low solubility, and possess a network structure, resembling glassy limestone scale. The scale formed in water treated with KDF fundamentally alters the morphology of calcium (magnesium) carbonate crystals, resulting in smaller, flatter, round, granular, and rod-shaped scale composed of non-hard, powdery components. These components do not adhere to metal, plastic, or ceramic surfaces and are easily removed by physical filtration.

(6) Inhibition of Microbial Growth

KDF treatment media controls microbial growth and reproduction not through a single mechanism, but through several mechanisms, inhibiting microorganisms through the individual or synergistic effects of each mechanism. The main mechanisms include: changes in redox potential, the formation of hydroxide ions and hydrogen peroxide, and the dissolution of zinc in the medium. Under normal circumstances, when KDF (KindFeB) treatment medium is used as a pretreatment method for reverse osmosis membranes, it can inhibit the reproduction of microorganisms such as bacteria and algae, thereby preventing microbial damage to the membrane.

① Changes in Oxidation-Reduction Potential

When water passes through KDF treatment medium, its oxidation-reduction potential changes from +200mV to -500mV. Under normal circumstances, various types of microorganisms can only grow under specific oxidation-reduction potentials. This significant change in potential can damage bacterial cells, thus controlling microbial growth. However, the change in the oxidation-reduction potential of water is very small. To control bacteria with KDF, the bacteria must be in direct contact with KDF. The inhibitory effect of KDF on bacteria mainly occurs at the contact surface between KDF and water. Therefore, changes in oxidation-reduction potential alone cannot completely control microorganisms.

② Hydroxide Ions and Hydrogen Peroxide

During the oxidation of ferrous iron to ferric iron in KDF (Kinetic Dioxide-Fe3O4) filtration, hydroxide ions and hydrogen peroxide are generated. This inhibits microorganisms that can survive at low oxidation potentials but are sensitive to hydrogen ions and hydrogen peroxide. However, hydroxide ions and hydrogen peroxide have short lifespans and are only highly reactive during the filtration process, resulting in a significant inhibitory effect on microorganisms, with relatively small residual effects in the effluent.

③ Zinc Ions and Their Effect on Microbial Control

The zinc released from the KDF treatment medium has a significant effect on microbial control. Zinc can inhibit enzyme synthesis, thereby affecting the normal growth of organisms and achieving the purpose of inhibiting microbial reproduction. In addition, KDF medium controls algal growth by inhibiting chlorophyll synthesis. The presence of zinc ions essentially reduces the ability of organisms to produce food through photosynthesis, which will significantly affect bacterial growth.