CO 2 partial pressure – pH relationships in the medium and relevance to CO 2 mass balance in outdoor open thin-layer Arthrospira ( Spirulina ) cultures

Abstract

A mathematical formula was derived for pCO 2 (partial pressure of dissolved carbon dioxide) in culture medium as a function of pH. The formula was verified in laboratory experiments with desorption of CO 2 from the mediuminto air which had been bubbled through a medium commonly used (ZARROUK'S recipe) for cultivation of the microalga Arthrospira (Spirulina). Concentration of sodium bicarbonate in the medium was 9 and 16.8 g l - 1 , respectively, at temperatures of 30 and 40 °C. The overall volumetric mass transfer coefficient K L a/V for CO 2 transfer from the medium into the gas phase increased about 2 % per 1°C increase of temperature and was not influenced by bicarbonate concentration in the medium. Mass balance of CO 2 in a suspension of Spirulina flowing in a layer of 8 mm thickness down an inclined cultivation surface was formulated and investigated theoretically, taking into account CO 2 consumption by the alga and CO 2 mass transfer from the suspension into the atmosphere. Calculations suggested that dissolved free (not chemically bound) carbon dioxide and concentration of bicarbonate ions in the suspension are practically in mutual chemical equilibrium. Mass flux of carbon dioxide from the suspension into the atmosphere is enhanced at increased suspension pH as a result of CO 2 formation from bicarbonate ions.