The Miracle of Water Purification: Where Do the Heavy Metal Ions in Water Go?

The Miracle of Water Purification: Where Do the Heavy Metal Ions in Water Go?

The cats in the town staggered as if drunk, and then jumped into the sea like crazy, as if trying to put out the invisible flames on their bodies. Soon after, the residents of the town began to speak slurredly, staggered on the road, and had dementia on their faces... There were even mentally ill residents who gradually died. What happened to this town...

1. Minamata Disease

In 1953, a strange disease was discovered in Minamata Town, Kumamoto Prefecture, Japan. The disease first appeared in cats, and some cats showed abnormal behaviors, such as strange cries, unstable gait, and jumping into the sea to commit suicide, and were called "suicidal cats." Later, the disease began to appear in humans, with mild cases of slurred speech, staggering, facial dementia, and vision loss, and severe cases of mental disorders and even death. At that time, the disease was called "Minamata disease" because the cause was unknown.

Figure 1 Minamata disease

Minamata disease is a chronic mercury poisoning disease caused by long-term intake of aquatic products containing organic mercury. The central nervous system of the human body is damaged, mainly manifested as sensory impairment, movement disorders, muscle atrophy, visual and auditory impairment, etc. In severe cases, it can lead to death. Minamata disease not only affects adults, but can also be passed on to fetuses through pregnant women, causing congenital Minamata disease.

So where does the mercury in water come from?

2. Heavy Metal Ions in Water

The New Nippon Cyanotype Minamata Plant discharged wastewater containing a large amount of mercury directly into Minamata Bay without treatment. The mercury in the wastewater was converted into more toxic methylmercury by microorganisms in the sediment. It entered the animal or human body through bioaccumulation and caused great damage to their central nervous system.

In addition to mercury, common toxic metal ions in water bodies include lead, cadmium, chromium, arsenic, etc. These elements are difficult to be biodegraded and easily accumulate. At a certain concentration, they will have toxic effects on ecosystems and aquatic products and pose a threat to human health. This type of pollution mainly comes from industrial wastewater, mineral mining, pesticide use and metal smelting, and has strong chemical activity.

3. Heavy metal ion treatment technology in water

The treatment of heavy metal ions in water is an important branch of industrial wastewater treatment. Efficient, economical and environmentally friendly water treatment technology is of great significance to environmental protection and human health. According to the research results of Ma Linfeng and other scholars, Na₂S modified biochar has the ability to efficiently adsorb heavy metal ions in water, providing a very effective method for treating heavy metal contaminated water.

1. Preparation process:

(1) Raw material preparation: First, biochar is needed. Biochar is a carbon-rich material produced by high-temperature pyrolysis of biomass (such as plant straw, wood, etc.) in an oxygen-deficient environment. It has a rich pore structure and a large specific surface area, like a "microscopic apartment" with many small rooms, which provides space for the adsorption of heavy metal ions.

Figure 2 Biochar

(2) Adding modifiers: Na₂S is then added to the biochar. Na₂S undergoes hydrolysis in water to produce S²⁻ ions. These S²⁻ ions are like "little hooks" that can tightly bind to heavy metal ions.

(3) Mixing and reaction: The solution containing Na₂S is fully mixed with biochar to allow Na₂S to be evenly distributed on the surface and pores of the biochar. Under certain conditions (such as appropriate temperature, reaction time, etc.), Na₂S interacts with biochar to modify the biochar. After modification, the chemical properties and structure of the surface of the biochar change, making it more conducive to the adsorption of heavy metal ions.

2. Adsorption mechanism:

(1) Ion exchange: Biochar itself contains some cations, such as potassium ions (K⁺), sodium ions (Na⁺), etc. When a solution containing heavy metal ions comes into contact with Na₂S-modified biochar, the heavy metal ions will exchange with these cations on the surface of the biochar. It is like an "ion exchange station" where the heavy metal ions replace the original cations and are adsorbed onto the biochar.

(2) Chemical precipitation: The S²⁻ ions produced by the hydrolysis of Na₂S have a strong affinity with heavy metal ions, and they combine to form insoluble sulfide precipitates. These precipitates will adhere to the surface and pores of biochar, thereby achieving the adsorption of heavy metal ions. For example, copper ions (Cu²⁺) and mercury ions (Hg²⁺) can be adsorbed in this way.

(3) Surface complexation: The surface of modified biochar has abundant functional groups, such as hydroxyl (-OH) and carboxyl (-COOH). These functional groups can react with heavy metal ions to form stable complexes. It is like using "chemical bonds" to firmly fix heavy metal ions on biochar, thus enhancing the stability of adsorption.

(4) Physical adsorption: The pore structure of biochar provides a large specific surface area, allowing heavy metal ions to adhere to the surface of biochar by physical adsorption. This adsorption method mainly relies on the attraction between molecules. Although it is relatively weak, it also contributes to the overall adsorption effect.

Na₂S modified biochar can efficiently adsorb heavy metal ions in water through the synergistic effect of the above-mentioned multiple adsorption mechanisms. This method has the advantages of simple operation, low cost and good adsorption effect, and has broad application prospects in the fields of environmental protection and water pollution control.