Porosity and tortuosity relations as revealed by a mathematical model of biofilm structure

Diffusion and sorption of organic micropollutants in biofilms with varying thicknesses

Solid-liquid partitioning is one of the main fate processes determining the removal of micropollutants in wastewater. Little is known on the sorption of micropollutants in biofilms, where molecular diffusion may significantly influence partitioning kinetics. In this study, the diffusion and the sorption of 23 micropollutants were investigated in novel moving bed biofilm reactor (MBBR) carriers with controlled biofilm thickness (50, 200 and 500 μm) using targeted batch experiments (initial concentration = 1 μg L^-1, for X-ray contrast media 15 μg L^-1) and mathematical modelling. We assessed the influence of biofilm thickness and density on the dimensionless effective diffusivity coefficient f (equal to the biofilm-to-aqueous diffusivity ratio) and the distribution coefficient K_d,eq (L g^-1). Sorption was significant only for eight positively charged micropollutants (atenolol, metoprolol, propranolol, citalopram, venlafaxine, erythromycin, clarithromycin and roxithromycin), revealing the importance of electrostatic interactions with solids. Sorption equilibria were likely not reached within the duration of batch experiments (4 h), particularly for the thickest biofilm, requiring the calculation of the distribution coefficient K_d,eq based on the approximation of the asymptotic equilibrium concentration (t > 4 h). K_d,eq values increased with increasing biofilm thickness for all sorptive micropollutants (except atenolol), possibly due to higher porosity and accessible surface area in the thickest biofilm. Positive correlations between K_d,eq and micropollutant properties (polarity and molecular size descriptors) were identified but not for all biofilm thicknesses, thus confirming the challenge of improving predictive sorption models for positively charged compounds. A diffusion-sorption model was developed and calibrated against experimental data, and estimated f values also increased with increasing biofilm thickness. This indicates that diffusion in thin biofilms may be strongly limited (f ≪ 0.1) by the high biomass density (reduced porosity).

Nanoparticle deposition onto biofilms

We develop a mathematical model of nanoparticles depositing onto and penetrating into a biofilm grown in a parallel-plate flow cell. We carry out deposition experiments in a flow cell to support the modeling. The modeling and the experiments are motivated by the potential use of polymer nanoparticles as part of a treatment strategy for killing biofilms infecting the deep passages in the lungs. In the experiments and model, a fluid carrying polymer nanoparticles is injected into a parallel-plate flow cell in which a biofilm has grown over the bottom plate. The model consists of a system of transport equations describing the deposition and diffusion of nanoparticles. Standard asymptotic techniques that exploit the aspect ratio of the flow cell are applied to reduce the model to two coupled partial differential equations. We perform numerical simulations using the reduced model. We compare the experimental observations with the simulation results to estimate the nanoparticle sticking coefficient and the diffusion coefficient of the nanoparticles in the biofilm. The distributions of nanoparticles through the thickness of the biofilm are consistent with diffusive transport, and uniform distributions through the thickness are achieved in about four hours. Nanoparticle deposition does not appear to be strongly influenced by the flow rate in the cell for the low flow rates considered.

Biological degradation of simazine by mixed-microbial cultures immobilized on sepiolite and tepojal beads

Simazine degradation by mixed microbial cultures was carried out in biological reactors with tepojal and sepiolite beads. The inoculum used is derived from a biotechnological product applied to plant roots, which contains mixed microbial cultures. This inoculum presented a stable adherence to the microorganism support throughout the experiment. In this research, the supports were evaluated in relation to both biofilm formation and simazine removal. For this study, hydraulic and mass starting-up parameters were established for simazine degradation and for the use of these reactors in the two types of supports. Tepojal had never been used before as a microbial support in any previous research paper. Tepojal demonstrated to be more efficient than sepiolite. Statistical analysis was done for the relationship among the parameters of chemical oxygen demand, colony formation units, total suspended solids, and volatile suspended solids.

Parameters affecting drug release from inert matrices. 1: Monte Carlo simulation

This study investigates the use of Monte Carlo simulation for the determination of release properties from cubic inert matrices. Specifically, the study has focused on factors including porosity, surface area and tortuosity. The release platform was formed by simulating matrices with different ratios of drug and excipient, which undergo drug release in a uni-directional (two-face) or omni-directional (six-face) process. Upon completion of each simulation the matrix 'carcass' was examined and porosity and tortuosity of the medium evaluated. The tortuosity of the medium was evaluated directly by a blind random walk algorithm. These parameters as well as the release profile were then studied with respect to common mathematical models describing drug diffusion (the square-root, power and Weibull models). It was found that, depending on their composition, the matrices systems were either homogeneous or heterogeneous in nature. Furthermore, it was found that the physical parameters could be successfully fitted to the a and b constants of the Weibull model. This approach allows the prediction of drug release from an inert matrix system with the knowledge of a few physical parameters.