Effect of surface functional group on adsorption of organic pollutants on hexagonal mesoporous silicate

Hexagonal mesoporous silicates (HMSs) are synthetic silicate materials that have uniform mesopores, comparatively large surface areas, and uniform surface functional groups, which lead to higher adsorption selectivity. Selective adsorption characteristics of HMSs for six types of organic pollutants (2,4-d, mecoprop, 4-chlorophenol, toluene, dichloroacetic acid, and thioflavin T) from synthetic wastewater were investigated. Five different types of HMSs were synthesized by surfactant-templating methods, and three of them were subsequently grafted with organic surface functional groups, i,e. n-octyldimethyl-, 3-aminopropyltriethoxy-, and 3-mercaptopropyl-groups. Titanium-substituted HMS was also made in the same way as pristine HMS. Increasing hydrophobicity of HMSs did not always enhance adsorption of hydrophobic adsorbates, such as toluene. Grafted organic functional groups changed surface charge, which enhanced electrostatic force between HMSs and ionic pollutants. Negatively charged contaminants, i.e. 2,4-d, mecoprop and dichloroacetic acid, were more readily adsorbed on positively charged AM-HMS by electrostatic interaction. Hydrogen bonding and van der Waals interaction between adsorbents and adsorbates, as well as combination of these forces, also enhanced the adsorption capacities of HMSs. In addition to the electrostatic interaction, a cationic dye, thioflavin T, was adsorbed on the surfaces due to hydrogen bonding and van der Waals interaction for hydrophilic surfaces and hydrophobic surfaces, respectively.