Engineering zeolite pellets for the adsorption of organic micropollutants

Abstract

Our use of pharmaceuticals, pesticides and personal care products leads to an increase of organic micropollutants (OMPs) into the aquatic environment. Conventional wastewater treatment plants are not designed for the removal of OMPs and need to be upgraded to reduce OMP contamination. The adsorption of OMPs to zeolites is proposed as an alternative treatment method. Zeolites are synthesized as powders, however they need to be shaped into pellets to be used in water treatment practice. This research focussed on engineering zeolite pellets and which properties of these pellets are important for the adsorption of OMPs.
First the influence of the calcination temperature and binder content on the mechanical stability and porosity of the pellets was analysed. Second, the influence of two different preparation techniques (extrusion and high-shear granulation) and the introduction of a polymer on the mechanical stability, porosity and adsorption kinetics of the pellets was assessed. Third, the effect of the porosity on the breakthrough for an empty bed contact time (EBCT) of 20, 5 and 1 minute(s) was determined according to the linear driving force (LDF) model. It was found that an increasing binder content and calcination temperature increased the wear resistance and porosity of the pellets. However, the effect on the porosity is minimal. The introduction of the polymer had opposite effect on extruded and granulated pellets. For extruded pellets, the introduction of the polymer increased the porosity. However, the granulated pellets showed a decrease in porosity. A relation was found between the porosity and the wear resistance and between the porosity and adsorption kinetics. A larger porosity decreased the wear resistance of the pellets and increased the kinetic rate constant. The porosity was of great importance in relation to the breakthrough. First, the porosity in the zeolite pellets determined to a great extent the bulk density of the filter bed. A higher bulk density resulted in a later breakthrough point. The bulk density was also influenced by the shape of the pellet. Spherically-shaped (granulated) pellets had a higher bulk density than rod-shaped (extruded) pellets. Second, the porosity was related to the kinetic rate constant. At lower EBCTs (5 and 1 minute), the kinetic rate constant had an influence on the breakthrough point. For these EBCTs, a higher kinetic rate constant led to a later breakthrough point. It was recommended to optimize the zeolite pellets by making the extruded pellets in a more spherically-shaped form and to start column experiments to validate the LDF-model.