Modelling and optimization of a bioremediation system utilizing silica gel encapsulated whole-cell biocatalyst

In this study, a 3-step design and optimization method is presented for a bioremediation system utilizing silica gel encapsulated whole-cell biocatalyst. Characterization experiments were conducted to determine the parameters of a steady state reaction/diffusion model for encapsulated AtzA biocatalyst that was subsequently verified experimentally. Dimensionless numbers governing the reaction rate, the Thiele modulus (Φ) and effectiveness factor (η), were evaluated based on the design parameters of the material, ρ (cell loading density) and L_c (characteristic length). Mechanical properties of the gel were determined as a function of ρ. Optimal values for ρ and L_c were determined for a case study, based on biocatalytic performance, mechanical properties and cost. It was found that free biocatalyst reaction kinetics are first order with kfree'=7.38×10^-2s^-1/(gcells/mL) and diffusivity of atrazine in the gel increases with ρ, from 3.51×10^-4mm²/s up to 6.93×10^-4mm²/s. Modeling results showed that ~20% of activity was lost during encapsulation. Diffusion limitations became significant (Φ>1) when L_c exceeded 0.1-0.3mm. The optimal catalyst radius and cell loading density were determined to be L_c=0.2mm and ρ=0.11gcells/mL gel, for an atrazine bioremediation setup with a desired effluent <3ppb.