What are the differences between Aluminum sulfate and polyaluminum chloride as coagulants in water treatment?
Aluminum sulfate (Al₂(SO₄)₃) and polyaluminum chloride (PAC, [Al₂(OH)ₙCl₆₋ₙ]ₘ) are commonly used coagulants in water treatment. They exhibit significant differences in chemical properties, effectiveness, and application conditions:
1. Chemical Composition
Aluminum Sulfate: An inorganic salt with the chemical formula Al₂(SO₄)₃. When dissolved in water, it generates Al³⁺ and SO₄²⁻ ions.
Polyaluminum Chloride: An inorganic polymer with the chemical formula [Al₂(OH)ₙCl₆₋ₙ]ₘ. It contains various aluminum polymers combined with hydroxyl and chloride ions.
2. Coagulation Mechanism
Aluminum Sulfate: Dissolves in water to form Al(OH)₃ colloids, which aggregate suspended particles through adsorption and charge neutralization, leading to sedimentation.
Polyaluminum Chloride: Hydrolyzes to generate multiple charged polymers, which aggregate particles through charge neutralization and adsorption bridging, forming larger flocs.
3. Coagulation Effectiveness
Aluminum Sulfate: Suitable for low-turbidity water but less effective for high-turbidity water. Its performance is significantly influenced by pH.
Polyaluminum Chloride: Effective for water of various turbidity levels, particularly high-turbidity water. It forms larger flocs that settle quickly and is less affected by pH.
4. pH Adaptability
Aluminum Sulfate: Optimal pH range is 6.0-7.5. Outside this range, its effectiveness decreases, and it lowers the water's pH, requiring alkali adjustment.
Polyaluminum Chloride: Has a broader applicable pH range (5.0-9.0), with minimal impact on the original water pH, making it more adaptable.
5. Dosage
Aluminum Sulfate: Requires a higher dosage, resulting in relatively higher costs.
Polyaluminum Chloride: Requires a lower dosage, making it more cost-effective.
6. Residue Concerns
Aluminum Sulfate: May lead to aluminum residue, necessitating strict dosage control.
Polyaluminum Chloride: Produces less aluminum residue, offering higher safety.
7. Application Scenarios
Aluminum Sulfate: Often used in small water treatment plants or under specific water quality conditions.
Polyaluminum Chloride: Widely used in medium to large water treatment plants, especially for high-turbidity water.
Summary
Aluminum Sulfate: Higher cost, narrow pH range suitability, higher dosage requirements, and suitable for low-turbidity water.
Polyaluminum Chloride: Lower cost, broad pH range suitability, lower dosage requirements, and suitable for water of various turbidity levels with minimal residue.
The choice of coagulant should be based on water quality and treatment requirements.
