acid neutralization

Chemical significance of acid neutralization capacity (ANC)

Definition and mechanism

Acid neutralization capacity (ANC) characterizes the buffering strength of water bodies against pH decline, with the core mechanism being:

Carbonate equilibrium system:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻

Key component contributions:

Component Neutralization equivalent ratio pH buffering range

HCO₃⁻ 75-95% 6.3-8.3

CO₃²⁻ 5-20% ＞8.3

OH⁻/organic base ＜5% ＞9.0

Natural water body ANC classification

ANC value (μeq/L) acid-neutralizing capacity ecological risk

＞200 Strong buffer Acid rain events pH decrease < 0.2

50-200 Moderate buffer pH decrease 0.5-1.0 (fish can tolerate)

＜50 Fragile system pH drop > 2.0 (risk of biological extinction)

Engineering control standards

■ Water Safety:

ANC ≥ 100mg/L (as CaCO₃, ensuring pipeline corrosion control)

Buffer Efficiency: Every 50mg/L ANC neutralizes 30mm of pH 4.0 acid rain

■ Wastewater Treatment:

Biochemical system ANC > 150 mg/L (to prevent pH collapse during nitrification)

Sludge anaerobic digestion: ANC 2000-4000 mg/L (to maintain pH 6.8-7.2)

Enhancement measures

Mineral strengthening

Limestone filter bed: CaCO₃ dissolution increases ANC by 80-120 mg/L·h (contact time ≥ 15 min)

Calcium carbonate addition: Mg²⁺ co-buffering (40% higher unit increase in ANC compared to limestone)

Process optimization

Scenario Technical solution ANC improvement effect

Low alkalinity water source CO₂ stripping + lime precipitation +150-300mg/L

Acidic mine drainage Two-stage limestone fluidized bed ANC 转 positive ＞200mg/L

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