Adsorption potential of polymeric porous crystalline materials (MOFs) for the removal of Indigo carmine, Congo red, and Malachite green from water

This review discusses different conventional and non-conventional synthetic methods available to obtain polymeric crystalline materials and MOF composites. Through which is possible to produce materials with different crystallinity, sizes, and shapes, directly associated with their adsorption capacity. From this revision, it is clear that, 1) any material synthesized with improved crystallinity and smaller size will adsorb higher quantities of the adsorbate compared with materials with less crystallinity and bigger particle size. 2) most of the organic linkers employed for the synthesis of MOFs are aromatic linkers, some of which include hetero atoms in their structures all having a high electron density environment, ready to interact via pi-pi stacking with the aromatic systems of the colourants here reviewed, i.e. Indigo carmine (IC) and Congo red (CG) (anionic dyes) and Malachite green (MG), the later one being considered a cationic dye. The well-ordered polymeric materials, according to their structure allow contact with the dyes structures, through three principal kinds of interactions: electrostatic interactions, hydrogen bonding, and pi-pi stacking. Kinetic studies allow us to know and understand the adsorption process followed in most experiments through pseudo-second-order mechanisms. Moreover, very detailed information about investigations to understand electrostatic interactions, hydrogen bonding and pi-pi stacking interactions are explained with many experiments such as pH adsorption studies to understand the influence of pH on the adsorption process; infrared (IR) spectroscopy to identify and assess the change of functional groups; X-ray diffraction (XRD) to examine the crystallinity and arrangement of atoms in materials; scanning electron microscopy (SEM) to visualize the morphology and surface characteristics of polymeric materials; transmission electron microscopy (TEM) to obtain information about particles size, shape, phases and structural characteristics; zeta potential analysis to determine the surface charge of the polymeric materials; thermogravimetric analysis (TGA) to measure weight loss as a function of temperature and thermal stability; among other experiments.