Chemical Reaction and Stoichiometry

Sodium hydroxide, a strong base, reacts with H₂S in water to form sodium hydrosulfide (NaHS) or sodium sulfide (Na₂S), depending on the NaOH dosage and pH. These reactions neutralize H₂S, converting it into non-volatile, soluble compounds. The primary reactions are:

Reaction 1: Formation of Sodium Hydrosulfide

H₂S + NaOH → NaHS + H₂O

  • Stoichiometry: 1 mole of H₂S reacts with 1 mole of NaOH.

  • Molar masses:

    • H₂S: 34.08 g/mol

    • NaOH: 40.00 g/mol

  • Mass ratio: ~1:1.17 (1 g of H₂S requires ~1.17 g of NaOH; 34.08 g H₂S : 40.00 g NaOH).

  • Conditions: Occurs at moderate pH (7–10) with stoichiometric or slightly excess NaOH dosing.

Reaction 2: Formation of Sodium Sulfide

H₂S + 2NaOH → Na₂S + 2H₂O

  • Stoichiometry: 1 mole of H₂S reacts with 2 moles of NaOH.

  • Mass ratio: ~1:2.35 (34.08 g H₂S : 80.00 g NaOH).

  • Conditions: Favored at high pH (>10) or with excess NaOH, fully converting H₂S to Na₂S.

Key Stoichiometric Considerations:

  • The NaOH dose depends on the desired product (NaHS or Na₂S) and H₂S concentration.

  • In practice, a slight excess of NaOH (1.1–1.5 times stoichiometric) is used to ensure complete H₂S neutralization and account for competing reactions (e.g., with CO₂ or other acids).

  • Example: For 1 mg/L H₂S (0.0294 mmol/L):

    • For NaHS: ~1.17 mg/L NaOH.

    • For Na₂S: ~2.35 mg/L NaOH.

Reaction Kinetics

The kinetics of H₂S neutralization by NaOH are influenced by:

  • pH: The reaction is rapid at pH 7–10, where H₂S dissociates into HS⁻, which reacts readily with OH⁻. At pH > 10, Na₂S formation dominates. At pH < 7, undissociated H₂S slows the reaction.

  • Temperature: Higher temperatures slightly increase reaction rates, but the effect is minimal due to the fast acid-base nature of the reaction.

  • NaOH Concentration: Higher concentrations accelerate the reaction, but excessive dosing raises pH unnecessarily, increasing costs.

  • Rate Law: The reaction is generally first-order with respect to H₂S and OH⁻:

    • Rate = k[H₂S][OH⁻]

    • Typical k values: 10³–10⁵ M⁻¹s⁻¹ at 25°C, reflecting near-instantaneous reaction.

  • Reaction Time: Neutralization is extremely fast, typically completing within seconds under well-mixed conditions.

Practical Considerations:

  • NaHS formation is preferred for cost efficiency and moderate pH control, but NaHS remains partially odorous and toxic.

  • Na₂S formation requires more NaOH and results in higher pH, which may need adjustment before discharge.

  • The products (NaHS or Na₂S) are soluble, avoiding solids formation but requiring further treatment if sulfide removal is needed.

Typical Treatment Methods

NaOH is commonly used in wastewater treatment, industrial effluents, and odor control systems for H₂S removal, often as a preliminary step before oxidation or biological treatment. Common methods include:

a. Direct Injection

  • Process: NaOH (typically 20–50% w/w solution) is injected into water via metering pumps in pipelines, scrubbers, or tanks.

  • Conditions: pH maintained at 7–10 for NaHS formation, or >10 for Na₂S, with NaOH dosed at 1.1–1.5 times stoichiometric requirement.

  • Advantages: Simple, rapid, effective for H₂S levels of 0.1–50 mg/L, no solid byproducts.

  • Challenges: Increases pH, produces sulfide salts requiring further treatment, and may not fully eliminate sulfide odor.

b. Batch Treatment

  • Process: Water is treated in a reactor with NaOH addition, mixing, and minimal retention time (1–5 minutes).

  • Conditions: Used for small-scale or intermittent treatment, with pH monitoring to avoid over-dosing.

  • Advantages: Precise control, suitable for high H₂S concentrations.

  • Challenges: Labor-intensive, not ideal for continuous flow systems.

c. Combined Systems

  • Process: NaOH treatment is paired with subsequent oxidation (e.g., using H₂O₂ or NaOCl) or biological treatment to convert NaHS/Na₂S to sulfate or elemental sulfur.

  • Example: NaOH neutralization followed by aeration or chemical oxidation to fully remove sulfides.

Typical Treatment Rates

  • H₂S Concentrations: Effective for 0.1–50 mg/L H₂S (municipal wastewater: 0.1–5 mg/L; industrial: 5–50 mg/L).

  • NaOH Dosage:

    • For NaHS: 1–2 mg NaOH per mg H₂S.

    • For Na₂S: 2–4 mg NaOH per mg H₂S.

    • Practical dosing: 1.5–3 mg/L NaOH for low H₂S (0.1–1 mg/L); 50–150 mg/L for high H₂S (10–50 mg/L).

  • Contact Time: Seconds to 1 minute for NaHS; 1–5 minutes for Na₂S.

  • Flow Rates: Systems handle 10–50,000 m³/day, from small scrubbers to large wastewater plants.

  • pH Control: Post-treatment acid (e.g., HCl) may be needed to lower pH to 6–9 for discharge, with dosing rates of 10–100 mg/L depending on final pH.

  • Residual Sulfides: NaHS/Na₂S levels should be <0.1 mg/L as H₂S equivalents for odor control, often requiring secondary treatment.

Practical Considerations and Challenges

  • Byproducts:

    • NaHS and Na₂S are soluble, avoiding turbidity but remaining toxic and odorous, often requiring further oxidation to sulfate.

    • High pH from excess NaOH may cause scaling or corrosion in downstream equipment.

  • NaOH Stability: NaOH solutions are stable but absorb CO₂ from air, forming sodium carbonate, which reduces effectiveness. Solutions are stored in sealed containers.

  • Cost: NaOH is relatively inexpensive (~$0.3–0.5/kg for 50% solutions), but secondary treatment for sulfide removal increases costs.

  • Monitoring: H₂S, pH, and sulfide levels are tracked using colorimetric tests, ion-selective electrodes, or online sensors.

  • Safety: NaOH is highly caustic, requiring protective equipment, spill containment, and careful handling to avoid burns or environmental release.

Comparison with Hydrogen Peroxide

  • NaOH Advantages: Lower cost, extremely fast reaction, no solid byproducts, simple to implement.

  • NaOH Disadvantages: Does not fully remove sulfides, requires secondary treatment, increases pH significantly.

  • H₂O₂ Advantages: Converts H₂S to benign products (sulfur or sulfate), environmentally friendly, no residual sulfides.

  • H₂O₂ Disadvantages: Higher cost, slower reaction at low pH, potential for turbidity with sulfur formation.

Example Calculation

Scenario: Treat 1,000 m³/day of wastewater with 5 mg/L H₂S, targeting NaHS formation.

  • H₂S mass: 5 mg/L × 1,000 m³ × 1,000 L/m³ = 5,000 g/day H₂S.

  • NaOH requirement: 1.17:1 mass ratio → 5,000 g × 1.17 = 5,850 g/day NaOH (stoichiometric).

  • Practical dose: 1.5× stoichiometric = 8,775 g/day NaOH.

  • NaOH solution: Using 50% w/w NaOH (density ~1.52 g/mL):

    • Mass of solution: 8,775 g ÷ 0.50 = 17,550 g/day.

    • Volume: 17,550 g ÷ 1,520 g/L ≈ 11.5 L/day.

  • Cost estimate: At ~$0.4/kg for 50% NaOH, cost ≈ $7.02/day (excluding secondary treatment or pH adjustment).

Additional Notes

  • Regulatory Limits: Treated water must meet discharge standards (e.g., H₂S < 0.1 mg/L, total sulfides < 0.1–1 mg/L, pH 6–9). Secondary treatment is often needed to comply with sulfide limits.

  • Scale-Up: Pilot testing is advised for large systems to optimize NaOH dosing and secondary treatment needs.

  • Environmental Impact: NaHS/Na₂S discharge may contribute to oxygen demand or toxicity in receiving waters, requiring oxidation or dilution.