Efficacy of convective-diffusion models to study the transient behaviour of a sewage-sludge-filled packed column for aqueous phase adsorption of fluoroquinolones: Consideration of pseudo-kinetics driven depletion of species

Ciprofloxacin and ofloxacin belong to a class of antibiotics called Fluoroquinolones (FQs), which have a wide anti-bacterial activity against Gram-positive and Gram-negative bacteria. Since the recent Covid-19 pandemic witnessed a magnanimous rise in the use of antibiotics to prevent secondary bacterial infections, it led to vast production and use of such antibiotics. Ultimately the antibiotics get discharged into the municipal sewer pipes, thereby killing the useful microbial colony. In order to prevent environmental degradation a commercial scale-up of the adsorption of these antibiotics using raw sewage sludge is an absolute necessity. In this study, a continuous adsorption operation is conducted in a packed bed of semi-dried raw sewage sludge to remove the FQs from wastewater. Two transient convective-diffusion models are developed including pseudo-first and second-order kinetics driven depletion terms. The models are optimised using the data collected under various dynamic conditions in order to analyse the performance of the packed bed in terms of bed height, flow rate and initial concentration of the FQs. Damköhler numbers of the FQs are estimated to predict the breakthrough times of both the FQs. The ratios of Damköhler numbers of ciprofloxacin and ofloxacin do not change much with flow rate. In all the experiments, Das << 1 for both the FQs, indicating a faster diffusion process with respect to the rate of pseudo-reaction. Diffusion reaches an ‘equilibrium' well before the reaction achieves pseudo-chemical equilibrium. Ratios of the Damköhler numbers, meant to represent the first-order and second-order convective-diffusion models for ciprofloxacin to ofloxacin is < 1. © 2023 Elsevier Ltd

The Diffusion Behavior of CO2 Adsorption from a CO2/N2 Gas Mixture on Zeolite 5A in a Fixed-Bed Column

The objective of this research was to investigate the behavior and conditions for CO2 adsorption using a mixture of CO2/N2 over a fixed-bed column of zeolite 5A. The study was performed with a variation in gas composition of CO2/N2 as a 20/80, 50/50, and 80/20 volume %, the adsorption temperatures as 298, 333, and 373 K and the total feed flow rates as 1, 2, and 4 L/h under 100 kPa pressure. The Bohart–Adams, Yoon–Nelson, and Thomas models were used to predict the breakthrough behavior of CO2 adsorption in a fixed column. Furthermore, the adsorption mechanism has been investigated using the kinetics adsorption of pseudo-first-order, pseudo-second-order, Boyd model, and intraparticle model. Increasing the CO2 composition of a gas mixture resulted in a high CO2 adsorption capacity because of the high partial pressure of CO2. The capacity of CO2 adsorption was decreased with increasing temperature because of physical adsorption with an exothermic reaction. The CO2 adsorption capacity was also decreased with increasing feed flow rates with inadequate time for CO2 adsorbates diffusion into the pores of the adsorbent before exiting the packed bed. The CO2 adsorption by zeolite 5A confirmed that the physical adsorption with intraparticle diffusion was the rate-controlling step of the whole process.