Selective adsorption of cationic/anionic tritoluene dyes on functionalized amorphous silica: A mechanistic correlation between the precursor, modifier and adsorbate

Abstract

Selective adsorption of synthetic and functionalized amorphous silica (silica nanosheets and silica nanoparticles (SiNSs and SiNPs) modified by hexadecyl trimethyl ammonium bromide (HDTAB) and hexadecyl trimethoxysilane (HDTMS)) towards dyes is investigated to obtain a mechanistic correlation between the silica precursor, modifier and adsorbate. Structures of the parent and functionalized counterparts are revealed by series of characterizations. Modifier has a clogging effect in the interlayer space of 2D SiNSs, resulting in the decrease of thermostability and surface area. 3D SiNPs with higher cationic exchange capacity (CEC) and greater activity is more prone to surface functionalization and higher modifier availability. Adsorption performances are tested by malachite green (MG), crystal violet (CV) and bromophenol blue (BPB) through combined batch experiment and density functional theory (DFT) calculation. Selective adsorption of HDTAB-SiNSs/SiNPs towards MG (287.36 and 289.86 mg/g), HDTAB/HDTMS-SiNSs for CV (196.91 and 260.64 mg/g) and BPB on HDTAB/HDTMS-SiNPs (211.54 and 149.94 mg/g) are obtained. Pseudo-second-order and Freundlich models are agreeable for the adsorption processes, holding for significant role of π-π stacking. The inherent mechanistic correlation is further concluded: (i) Electrostatic interaction play a dominant role for the adsorption of cationic MG/CV on silica, whereas the adsorption of anionic BPB is mainly through partition process. (ii) Affecting strength on the adsorption capacity: Adsorbate charge >functionalization manner >steric hindrance >additional interactions. (iii) Organic modification with flexible alkyl chain and tunable space is preferential for the accommodation of smaller dyes. Dye with greater steric hindrance is favorable for particle diffusion. (iv) Multidimensional structured silica and modifiers are inductive to stronger interactions towards target adsorbate, which deserve further explorations in the future.