A computational and experimental investigation of anionic dyes adsorption onto hydrophilic macroporous polymers: Insights from morphological characterization and molecular simulations

Abstract

The World Health Organization considers removal of hazardous dyes as an emergency problem in the water treatment process. In this investigation, the adsorption behavior of two anionic dyes (Orange II and RB-19) from aqueous solution by a polymeric (polyHIPE-METAC) adsorbent using a combined experimental and computational modeling analysis is reported. The polymeric adsorbent bearing ammonium groups was synthesized via o/w (oil in water) emulsion polymerization. Scanning Electronic Microscopy (SEM) unveiled the macroporous structure of the prepared polyHIPE-METAC adsorbent while X-ray photoelectron spectroscopy (XPS) study suggested that dye adsorption is promoted through electrostatic interactions of the positively charged nitrogen of ammonium functionalized adsorbent with dye molecules. A two-step adsorption mechanism revealed from statistical physics analysis. This theoretical model was used to predict the adsorption energy distribution (AED). AED demonstrated that dye molecules adopt an end-on configuration on the adsorbent surface in both steps and adsorption strongly depends on surface chemistry and porosity in step 1 and step 2, respectively. For more details, frontier molecular orbitals and reactivity descriptors were calculated via molecular simulations revealing that RB-19 dye has a higher reactivity than orange II. Furthermore, the dominant role of electrostatic interactions during the adsorption process for both dyes was confirmed, while the higher electrostatic interaction energy value found for polyHIPE-METAC/RB-19 system, supports the stronger interactions developed between polyHIPE-METAC and RB-19 dye molecules. These results contribute on expanding the knowledge on the adsorption mechanism of dye molecules using a macroporous, positively charged polymeric adsorbent.