Assessment of the limiting step of mass transfer in n-hexadecane biodegradation in a bubble column reactor

A mathematical model was developed to assess limiting step of mass transfer in the n-hexadecane (HXD) biodegradation by a microbial consortium. A double Monod kinetic (oxygen and HXD) for biomass production was successfully used to describe the experimental data. Good fitting (r²=0.92) between the model solution and experimental data was obtained. The overall mass transfer coefficients for HXD, k(L)a(HXD), and oxygen, k(L)a(O₂), were experimentally determined and biosurfactant production was indirectly determined through surface tension measurements in the aqueous phase. It was observed that a surface tension reduction from 65 (0 h of culture) to 47 mN m⁻¹ (240 h of culture) was related to a decrease of 52% in the HXD droplet diameter and to an increase of 63% in k(L)a(HXD), respect the initial values. Conversely, k(L)a(O₂) was repressed up to 37% compared to the initial value. The maximum rate analysis based on the mathematical model showed that HXD transfer was up to 5-fold lower than its consumption. On the contrary, oxygen transfer was always higher than its consumption. Thus, the limiting step under the working conditions was the HXD transfer to the aqueous phase. However, slight reductions in k(L)a(O₂) could result in oxygen transfer limitations during the last 60 h of the cultures.