Modeling speciation effects on biodegradation in mixed metal/chelate systems

A new model, CCBATCH, comprehensively couples microbially catalyzed reactio ns to aqueous geochemistry. The effect of aqueous speciation on biodegradat ion reactions and the effect of biological reactions on the concentration o f chemical species (e.g. H2CO3, NH4+, O-2) are explicitly included in CCBAT CH, allowing systematic investigation of kinetically controlled biological reactions. Bulk-phase chemical speciation reactions including acid/base and complexation are modeled as thermodynamically controlled, while biological reactions are modeled as kinetically controlled. A dual-Monod kinetic form ulation for biological degradation reactions is coupled with stoichiometry for the degradation reaction to predict the rate of change of all biologica l and chemical species affected by the biological reactions. The capability of CCBATCH to capture pH and speciation effects on biological reactions is demonstrated by a series of modeling examples for the citrate/Fe(III) syst em. pH controls the concentration of potentially biologically available for ms of citrate. When the percentage of the degradable substrate is low due t o complexation or acid/base speciation, degradation rates may be slow despi te high concentrations of substrate Complexation reactions that sequester s ubstrate in non-degradable forms may prevent degradation or stop degradatio n reactions prior to complete substrate utilization. The capability of CCBA TCH to couple aqueous speciation changes to biodegradation reaction kinetic s and stoichiometry allows prediction of these key behaviors in mixed metal /chelate systems.