Assuming that the dissolved concentration of tetrachloroethylene (C2Cl4) in a lake is 0.25 ppb.

Question

Assuming that the dissolved concentration of tetrachloroethylene (C2Cl4) in a lake is

0.25 ppb. Given a Henry’s law constant (H) of 0.081, and a measured gas exchange coefficient of 2.6 x 10-3 cm/s in water for propane (C3H8). Assuming tetrachloroethylene Ca is essentially negligible. What is the flux density of tetrachloroethylene from the lake? What are the two important environmental factors that can significantly affect the volatilization flux of dissolved concentration of tetrachloroethylene from the water to the atmosphere?

Answer 1

Step 1: Henry's Law and Concentration in Air

Henry's Law relates the concentration of a gas in the air to its concentration in water: Cair=H×CwaterCair​=H×Cwater​

Where:

1. CairCair​ is the concentration of the gas in the air (atm)
2. HH is Henry's law constant (atm·m³/mol)
3. CwaterCwater​ is the concentration of the gas in water (mol/m³)

Given:

1. Cwater=0.25 ppb=0.25×10−9 g/LCwater​=0.25ppb=0.25×10−9g/L

First, convert CwaterCwater​ from g/L to mol/m³: Molecular weight of C2Cl4=165.83 g/molMolecular weight of C2​Cl4​=165.83g/mol

Cwater=0.25×10−9 g/L165.83 g/molCwater​=165.83g/mol0.25×10−9g/L​ Cwater=1.507×10−12 mol/L=1.507×10−9 mol/m3Cwater​=1.507×10−12mol/L=1.507×10−9mol/m3

Using Henry's Law: Cair=H×CwaterCair​=H×Cwater​ H=0.081 atm\cdotpm3/molH=0.081atm\cdotpm3/mol

Cair=0.081×1.507×10−9Cair​=0.081×1.507×10−9 Cair=1.221×10−10 atmCair​=1.221×10−10atm

Step 2: Calculate the Flux Density

The volatilization flux (J) can be calculated using the gas exchange coefficient and the concentration difference between water and air: J=kL×(Cwater−Cair/H)J=kL​×(Cwater​−Cair​/H)

Given:

1. kL=2.6×10−3 cm/s=2.6×10−5 m/skL​=2.6×10−3cm/s=2.6×10−5m/s

Assuming CairCair​ is negligible: J=kL×CwaterJ=kL​×Cwater​ J=2.6×10−5 m/s×1.507×10−9 mol/m3J=2.6×10−5m/s×1.507×10−9mol/m3 J=3.92×10−14 mol/(m2s)J=3.92×10−14mol/(m2s)

Step 3: Convert the Flux Density to a More Convenient Unit

To convert to g/(m²·s): J=3.92×10−14 mol/(m2s)×165.83 g/molJ=3.92×10−14mol/(m2s)×165.83g/molJ=6.5×10−12 g/(m2s)J=6.5×10−12g/(m2s)

Important Environmental Factors

Two significant environmental factors that can affect the volatilization flux of tetrachloroethylene are:

1. Water Temperature: Higher water temperatures increase the kinetic energy of the molecules, which can enhance the volatilization rate. Additionally, Henry's law constant is temperature-dependent, typically increasing with temperature, leading to higher gas concentrations in the air above the water.
2. Wind Speed and Turbulence: Increased wind speed and surface water turbulence enhance the gas exchange rate by reducing the boundary layer resistance at the water-air interface. This increases the volatilization flux of tetrachloroethylene from the water to the atmosphere.