Nano-enhanced Dialytic Fluid Purification System: Applications and Computational Fluid Dynamics Modeling of a Nanoadsorbent Slurry

Abstract

Global water scarcity has necessitated the development of new technologies to provide clean and reliable water for the future. Current global infrastructure is insufficient to meet projected demand, and technologies that can provide efficient and low-cost water are urgently needed. Among the various water treatment methods developed over the years, adsorption has become a cost effective, easy to operate, and reliable method for water treatment. Nanoadsorption has emerged recently as an extension of traditional adsorption by combining the tried-and-true adsorption principles with unique material properties such as fast kinetics, large surface areas, and contaminant selectivity that can be used to remove a variety of contaminants. Unfortunately, these new adsorbents cannot be used in traditional adsorption settings such as columns and flow through systems because they cause high pressure drops, have poor mechanical strength, and are difficult to separate from water. In application, nanoadsorbents generally have been dispersed in water or embedded in macroscale hierarchical structures, but the risk of releasing contaminant containing nanoparticles into treated water necessitates a recovery or retention system. Nano enhanced dialytic purification involves utilizing a dialytic purification system that employs a membrane to separate a suspension of continuously recirculated nanomaterials from a stagnant solution or counterflowing stream of contaminated water. Contaminants diffuse down their concentration gradient, through the membrane, and into the nanomaterial suspension to be adsorbed. The nanomaterials are retained behind the membrane and act as a continual sink for the contaminants. This process was first exemplified in proof-of-concept experiments using a dialyzer with a single tubular membrane. Stagnant solutions of arsenic or lead were added into the lumen of the membrane and dialyzed using a flowing stream of ferrihydrite or hexagonal birnessite nanoparticles, respectively, over a three hour period. A greater than 90% removal was obtained from both experiments at sufficiently high adsorbent loading. The dialytic experiment between arsenic and ferrihydrite was compared to batch adsorption studies with a 94% removal efficiency at similar adsorbent loading. The dialytic experiment between lead and hexagonal birnessite was developed into a 2D axisymmetric CFD model using COMSOL Multiphysics® to study the process mathematically, and computational results were in good agreement with experimental data. The computational model was then extended to feature (for the first time) a 3D dialyzer with multiple working hollow fibers, counterflowing contaminant and adsorbent streams, and two treatment modalities – single-pass and multi-pass. These mass exchangers feature a larger surface area and improved diffusions rates over the single-fiber tubular membrane dialyzer. Methylene blue (MB) and powder activated carbon were used as the model contaminant and adsorbent. The computational model explored key parameters of the dialytic purification process and provided insight into the impact of parameter values on the overall removal of MB. Through these efforts, the dialytic purification process was successfully described mathematically, and the model can be used to explore real batch water treatment or pump and treat remediation applications to provide clean water for the future. The idea of utilizing nanoadsorbents retained behind a membrane to facilitate contaminant removal in a mass exchanger can also benefit analogous fields utilizing similar dialytic processes such as hemodialysis in the medical field and carbon dioxide removal in the petrochemical industry.