Model-based evaluation of tetracycline hydrochloride removal and mineralization in an intimately coupled photocatalysis and biodegradation reactor

Intimately coupled photocatalysis and biodegradation (ICPB) shows great potential for treatment of refractory pollutants; however, no kinetics for modeling ICPB performance has been developed and the major challenge is to determine the relationship between photocatalysis and biodegradation. In this work, we developed a simplified kinetic model to predict removal and mineralization of a target pollutant (tetracycline hydrochloride; TCH) by hypothesizing that all of the biodegradable photocatalysis products are immediately bio-utilized. Combined with a second-order photocatalytic kinetic model and Monod-type biodegradation model, we observed the interactions between photocatalysis and biodegradation in ICPB. Parameters in the kinetic equations were estimated using the First Optimization software to fit the experimental data to the proposed model with nonlinear regression. Our experimental results showed that TCH and chemical oxygen demand (COD) removal were as high as 94% and 70% within 8 h, respectively. TCH was transformed to non-toxic intermediates in only 4 h. Significantly, the kinetic models could satisfactorily predict the TCH and COD removal, and agreed well with the experimental data with an R2 > 0.92. The models confirmed that biodegradation in ICPB played a major role in accelerating TCH and its intermediates removal and mineralization, as the kinetic coefficient k1 of ICPB was 10% greater than that of photocatalysis alone. The developed models accurately predicted the ICPB efficiencies, and revealed the mechanisms of ICPB operation.