Henry's Law & Phase Equilibrium for Air Quality

Quick Answer: Henry's law predicts equilibrium partitioning of VOCs between water and air. High Henry's constant compounds strip easily and contribute to air emissions and inhalation risk.

Volatile organic compounds move between aqueous and gas phases in treatment plants, groundwater, open tanks, and soil pores. Henry's law quantifies that equilibrium — essential for FE Environmental items linking water treatment to air emissions and fugitive VOC estimates.

Henry's Law Statement

At equilibrium and dilute solutions, partial pressure of a solute in the gas phase is proportional to its aqueous concentration:

[ P = H \times C_w ]

Alternate forms use dimensionless Henry's constant (H'):

[ H' = \frac{C_{air}}{C_{water}} ]

at equilibrium, where concentrations are in consistent mass/volume units. The handbook may present (K_H) forms — match the equation given before substituting.

Interpreting Magnitude

High (H') → equilibrium favors gas phase → greater emission potential from aeration basins, clarifiers, and cooling towers.

Temperature Effects

Increasing temperature generally decreases gas solubility in water for many VOCs, increasing volatilization driving force. Exam stems noting "warmer wastewater" may imply higher stripper off-gas VOC loading — check whether control (carbon on off-gas) is needed for Title V/HAP compliance.

Air Stripping Tower Basics

Air stripping transfers VOCs from water to air using high air-to-water ratios and packing media. Design depends on Henry's constant, temperature, influent concentration, and target effluent. Stripped VOCs exit in off-gas — an air-quality problem. Without control, strippers become major VOC point sources requiring permits and possibly oxidizers or carbon.

Fugitive Emissions from Water Surfaces

Open tanks, clarifiers, and lagoons release VOCs by mass transfer across the interface. Covered tanks reduce fugitive air emissions — often a compliance strategy for HAP rules.

Relation to Risk and Odor

Partitioning affects inhalation exposure near treatment works. Compounds with high (H') may produce detectable odor at low aqueous concentrations.

Worked Partitioning Sketch

If (H' = 0.2) (dimensionless) and aqueous benzene is 0.5 mg/L, equilibrium gas-phase concentration (same units basis) is:

[ C_{air} = H' \times C_{water} = 0.1 \text{ mg/L air} ]

Compare to ambient standards or occupational limits separately.

Multiphase Context on FE Exam

Henry's law complements Raoult's law for mixtures in tanks, phase equilibrium in GAC desorption during regeneration, and wastewater aeration increasing VOC emissions to atmosphere.

Exam Traps

1. Using mol fraction form vs. concentration form without conversion
2. Assuming stripping removes 100% regardless of Henry's constant
3. Ignoring off-gas treatment after water pollution control
4. Applying Henry's law to ionized species without noting non-volatile ion fraction

FE Exam Focus

Identify when a water-quality process creates an air-quality compliance obligation. Locate Henry's law forms in the handbook, watch units, and connect to AP-42 VOC factors for controlled vs. uncontrolled stripper off-gas.

Henry's Law Governing Equation Forms

At dilute equilibrium, partial pressure and aqueous concentration relate through Henry's constant. The FE Handbook may express H as atm·m³/mol, Pa·m³/mol, or dimensionless H' = C_air/C_water. Before calculating, identify which form is given and convert units consistently.

Worked partitioning: If C_water = 0.5 mg/L benzene and H' = 0.2 (dimensionless, gas/water same mass basis), then C_air = 0.1 mg/L in the gas phase at equilibrium — illustrative only; exam stems supply constants.

Open Tank and Clarifier Emissions

Quiescent surfaces allow VOCs to approach equilibrium with headspace air. Wind speed and turbulence increase mass-transfer rate beyond equilibrium predictions — empirical models add a mass-transfer coefficient K_L a. Covered tanks and floating roofs are air-quality control measures for storage vessels, not only safety devices.

Mass Transfer and Stripping Design (Conceptual)

Overall mass transfer depends on Henry's constant, interfacial area, and liquid-film/gas-film resistance. Packed towers increase area for VOC stripping. Required air-to-water ratios rise as target effluent concentration drops. At FE level, know that unfavorable Henry's constants (low H') make stripping impractical — choose GAC or advanced oxidation instead.

Aeration Basin VOC Emissions

Mechanical aeration in wastewater creates enormous interfacial area. Strippable VOCs in influent (industrial pretreatment) volatilize to atmosphere — potentially exceeding HAP major source thresholds at large plants. Covers, off-gas treatment, and source reduction at industrial pretreatment are engineering responses.

Raoult's Law Connection

For ideal mixtures in storage tanks, partial pressure of component i equals mole fraction times vapor pressure: ( P_i = x_i P_i^* ). Combined with Henry's law for dilute aqueous phases, FE items may ask which compound dominates headspace vapor — the most volatile (highest vapor pressure) fraction drives fire and inhalation hazards in tank headspaces.

Dimensionless Henry Constant

[ H' = \frac{C_{air}}{C_{water}} ]

Benzene H' high → partitions to vapor — stripping tower removes from groundwater.

Raoult's Law Link

Ideal dilute solution: partial pressure (P_i = x_i H_i). Environmental FE uses tabulated Henry's law constants at 25°C unless temperature correction given.

Air Stripping Tower

NTU and HTU concept: (Z = HTU \times NTU). Higher H' → easier stripping at same air:water ratio.

Aeration Basin Emissions

Volatilization of VOCs from surface aeration may require air permit — Henry's law predicts driving force.

Worked Example

C_water = 0.1 mg/L, H' = 0.25 → C_air equilibrium ≈ 0.025 mg/L in gas phase (illustrative units per stem).