In water treatment processes, pH is one of the key factors affecting coagulation/flocculation efficiency. For cationic organic coagulants (also known as cationic polymer flocculants/coagulants, such as Cationic Polyacrylamide (CPAM), Polydimethyldiallylammonium Chloride (PDMDAAC), and Dicyandiamide-Formaldehyde condensates), different pH conditions not only affect the charge density of positive functional groups and the molecular conformation on their chains but also alter the surface charge and existing forms of colloids and pollutants, thereby significantly impacting overall treatment performance.

Note: This article focuses on cationic organic polymer coagulants/flocculants and does not include inorganic metal salt coagulants (such as PAC, PFS, etc.). The pH influence mechanisms for the latter differ.

This paper systematically analyzes the performance differences of cationic organic coagulants under various pH conditions based on their action mechanisms and provides reference suggestions for practical engineering applications.

1. Impact of pH on the Action Mechanism of Cationic Organic Coagulants

1.1 Changes in Charge Neutralization Capacity

Cationic organic coagulants primarily neutralize the zeta potential of negatively charged colloids through positively charged groups on the molecular chain (such as —NH₃⁺ or quaternary ammonium salts —N⁺(CH₃)₃), destabilizing them. pH variations affect:

  • Surface charge state of pollutants: Different colloids have different isoelectric points (IEP); generally, increasing pH enhances their negative charge.
  • Effective charge density of coagulants: Polymers containing primary/secondary/tertiary amine groups undergo protonation under acidic conditions (—NH₂ + H⁺ → —NH₃⁺), increasing charge density; they undergo deprotonation under alkaline conditions, decreasing charge density.
  • Quaternary ammonium salt polymers possess permanent positive charges, so their susceptibility to pH is relatively low. However, their electro-neutralization efficiency may still be affected in high-alkalinity or high-ionic-strength systems.

Therefore, charge neutralization efficiency varies significantly under different pH conditions.

1.2 Molecular Conformation and Dispersion Solubility

Cationic polymers may undergo chain conformational changes under different pH conditions:

  • When charge density is high, intra-chain electrostatic repulsion increases, and the molecular chain tends to stretch.
  • When charge density is low, the molecular chain tends to curl.
  • pH affects the degree of ionization of functional groups, thereby altering the solubility and dispersion state of the polymer in water.

These changes directly impact their bridging capacity, floc formation rate, and structural characteristics.

1.3 Transformation of Pollutant Morphology

pH simultaneously influences the existing forms of pollutants, such as:

  • The dissociation degree of organic acids and humic substances;
  • The charge state of dye molecules (e.g., anionic dyes exhibit weakened negativity under acidic conditions);
  • The hydrolysis and complexation forms of certain metal ions.

These changes affect the interaction mode between pollutants and cationic coagulants and their removal efficiency.

2. Performance Characteristics Across Different pH Ranges

2.1 Acidic Conditions (pH < 6, typically favorable at pH 4–6)

Characteristics:

  • High protonation degree of amine-containing polymers, resulting in high positive charge density;
  • Weakened charge of some anionic pollutants, favoring electrical neutralization.

Performance:

  • Strong charge neutralization capability; decolorization effects are generally good for high-chroma wastewater (e.g., reactive dyes, direct dyes).

Potential Issues:

  • At excessively low pH (e.g., < 4), the charge of some colloidal particles decreases, potentially hindering effective floc formation;
  • Flocs may be small with average settling performance;
  • Strongly acidic environments may affect system stability or equipment durability.

2.2 Neutral Conditions (pH 6–8)

Characteristics:

  • The conventional operating range for most water treatment systems;
  • Pollutants and coagulants remain in a relatively stable state.

Performance:

  • Charge neutralization and adsorption bridging are relatively balanced;
  • Formed flocs are large and structurally compact;
  • Good settling performance and optimal comprehensive turbidity removal.

Engineering Evaluation: Typically the preferred operating pH range for cationic organic coagulants.

2.3 Alkaline Conditions (pH > 8)

Characteristics:

  • Amine-containing polymers undergo deprotonation, reducing positive charge density;
  • Negativity of colloids and pollutants increases.

Performance:

  • Difficulty in charge neutralization increases, usually requiring higher dosage;
  • Adsorption bridging may weaken.

Potential Issues:

  • Loose floc structure and decreased settling performance;
  • Greater fluctuation in treatment results.

3. Adaptability Differences Among Various Cationic Product Types

3.1 High Ionicity (High Charge Density) Products

  • E.g., High cationicity CPAM or Quaternary ammonium salt polymers;
  • Maintain strong electro-neutralization capacity over a wide pH range;
  • Suitable for wastewater rich in anions (e.g., dyeing, papermaking);
  • Flocs are relatively fine but structurally tight.

3.2 High Molecular Weight – Moderate Ionicity Products

  • Rely more on adsorption bridging;
  • Perform stably under neutral conditions (pH 6–8);
  • Suitable for solid-liquid separation, sludge thickening, and dewatering scenarios.

3.3 Modified/Salt-Tolerant Products

  • Improved adaptability by introducing structures like quaternary ammonium salts;
  • More stable performance in high-salt, high-alkali, or complex organic systems.

4. Selection and Control Recommendations for Engineering Applications

4.1 Control Reasonable pH Range

The recommended operating range is pH 6–8. If raw water pH deviates significantly, pre-adjustment should be performed.

4.2 Select Based on Treatment Targets

  • For decolorization or removing anionic pollutants: Prioritize high charge density products;
  • For solid-liquid separation or sludge treatment: Prioritize high molecular weight products.

4.3 Prefer Modified Products for Complex Water Quality

For high-alkali, high-salt, or high organic load wastewater, it is recommended to use salt/alkali-resistant products or combine them with inorganic coagulants.

4.4 Beaker Tests are Essential

Due to significant water quality variations, beaker tests must be conducted to determine the optimal pH, dosage, and combination schemes.

5. Conclusion

pH significantly impacts the performance of cationic organic coagulants, with its core lying in the comprehensive regulation of charge density, molecular conformation, and pollutant morphology.

  • Neutral conditions (pH 6–8): Optimal comprehensive effect;
  • Weakly acidic conditions (pH 4–6): Strong electro-neutralization capability, suitable for decolorization;
  • Alkaline conditions (pH > 8): Treatment effectiveness generally declines.

In practical applications, stable and efficient treatment results should be achieved by combining water quality characteristics with pH control, product selection, and experimental optimization.

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