The excellent performance of poly(dimethyldiallylammonium chloride) (PDADMAC) in water purification lies in its three key functions: flocculation, adsorption, and sterilization, driven by the combined effects of its cationic properties and macromolecular structure.
1. Flocculation: The Core Means to Remove Suspended Particles and Colloidal Impurities
Flocculation is PDADMAC is the primary mechanism for eliminating suspended particles and colloidal impurities in water. Most suspended particles (such as sediment and clay) and colloidal impurities (such as humus and microbial cells) in water carry negative charges. Due to the mutual repulsion of like charges, these particles remain stably dispersed in water for long periods, which causes turbidity.
However, the positively charged groups on PDADMAC’s molecular chains generate a strong electrostatic attraction to negatively charged particles. This attraction neutralizes the negative charges on the surfaces of the particles, disrupting their stability. Particles that lose their repulsive force collide and aggregate with one another. Meanwhile, the long macromolecular chains of PDMDAAC act as “bridging” agents, connecting small particles into larger, denser flocs (commonly known as “alum flowers”).These flocs have excellent sedimentation properties—they sink rapidly under gravity and can be separated from water through sedimentation or filtration, thereby clarifying the water.
For example, when treating municipal drinking water, adding an appropriate amount of PDMDAAC effectively removes turbidity-causing substances, thus ensuring that the turbidity of the treated water meets the standards.
2. Adsorption: Targeting Dissolved Organic Pollutants and Heavy Metal Ions
Adsorption addresses dissolved organic pollutants and heavy metal ions in water—substances that are highly harmful and difficult to remove, such as phenolic compounds, dye molecules, and heavy metal ions (e.g., Pb²⁺, Cd²⁺, and Cr³⁺).
PDMDAAC’s macromolecular chains possess a large specific surface area and numerous adsorption sites. Its positively charged groups can bind to negatively charged organic pollutant molecules (e.g., anionic dyes) through electrostatic attraction. Additionally, these groups can form stable complexes with heavy metal ions. Furthermore, the nitrogen atoms in the molecular structure of PDMDAAC possess strong coordination capabilities, enabling them to undergo coordination reactions with heavy metal ions, thereby further enhancing the adsorption effect.
In practical applications:
- When treating printing and dyeing wastewater, PDMDAAC immobilizes dye molecules on flocs for simultaneous removal, significantly reducing the chromaticity of the wastewater.
- When treating industrial wastewater containing heavy metals (e.g., electroplating wastewater), it effectively adsorbs heavy metal ions, ensuring that the treated water meets discharge standards.
3. Sterilization: A Critical Guarantee for Improving Water Sanitation
Sterilization is essential for PDADMAC to enhance water sanitation. Domestic sewage and aquaculture wastewater contain large quantities of microorganisms (e.g., bacteria and fungi). Excessive reproduction of these microorganisms causes the water to become odorous, deteriorate, and even spread diseases.
The cationic groups of PDADMAC interact electrostatically with the negative charges on the surfaces of microbial cell membranes.This interaction damages the integrity of the cell membranes, increases their permeability, and leads to the leakage of intracellular nutrients. At the same time, PDADMAC molecules can penetrate into microbial cells, interfering with their metabolism and the replication of genetic material, ultimately resulting in microbial cell death.
Studies have shown that PDADMAC exhibits excellent inhibitory and killing effects on common pathogenic microorganisms, including Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. In aquaculture water purification, adding an appropriate amount of PDADMAC not only removes pollutants but also inhibits the reproduction of harmful microorganisms, improves the aquaculture environment, and reduces fish diseases.
It is the synergy of these three core roles that makes PDADMAC a highly efficient water purifier, providing reliable technical support for water treatment in diverse scenarios.