Kinetics of carbon dioxide transfer across the air/water interface

Abstract

The transport rate of CO₂ into aqueous solution may be augmented by a chemically enhanced flux caused by the hydration of CO₂. The importance of hydration reactions in controlling CO₂ transport depends upon the solution composition and temperature as well as the hydrodynamic conditions at the air/water interface. The transfer has been examined under conditions approaching laminar flow at low Reynolds number. Factors considered to be important in determining the transport rate under these conditions include temperature, pH, alkalinity, solution flow rate and the gas-phase composition.

The transfer rate is expressed in the form V d Σ[CO₂]//_dt=K_LA([CO₂]_s–[CO₂]_b) where A is the surface area, V is the solution volume and Σ[CO₂] is the total inorganic carbon concentration. The subscripts s and b refer to the surface and bulk solution concentrations, respectively, and K_L is the gas transfer velocity. The CO₂ concentration at the solution surface is determined by assuming equilibrium between the gas-phase CO₂ and the solution. In the absence of chemical enhancement, the transfer velocity is independent of the pH and solution alkalinity and depends only on the temperature and hydrodynamic conditions at the interface. However, at high pH values the hydration reactions between OH^– and CO₂ cause deviations from the physical transfer model and K_L may be predicted from the solution of a second-order non-linear differential equation subject to appropriate boundary conditions and electroneutrality at all points within the diffusion layer. A numerical model is presented which permits K_L to be calculated for various degrees of chemical enhancement.

Experimental results for a range of bicarbonate concentrations [(2–40)× 10^–3 mol dm^–3] and CO₂ gas-phase compositions are given. The effects of temperature (20–35 °C) and flow rate (100–500 cm³ min^–1) have also been investigated. The carbonate alkalinity ([HCO^–₃]+ 2[CO^2–₃]) has a pronounced effect on K_L at high pH. The results are interpreted using the numerical model and compared with calculations based on the assumption of instantaneous hydration equilibrium. The effect of carbonic anhydrase on catalysing the hydration reactions is also briefly discussed.