Adsorption of Natural Organic Matter Surrogates from Aqueous Solution by Multiwalled Carbon Nanotubes

Adsorption of two natural organic matter (NOM) surrogates (tannic acid and gallic acid) by powdered activated carbon (PAC) and four kinds of multiwalled carbon nanotubes (MWCNTs) with different diameters was investigated in aqueous solution. The tannic acid (TA) adsorption isotherm fit the Langmuir model best, while adsorption of gallic acid (GA) fit the Freundlich model. PAC has a higher adsorption capacity for GA due to the pore-filling effect, while TA adsorption on PAC was lower than that on MWCNTs because of the molecule sieving effect. Adsorption of GA and TA on MWCNTs decreased with increasing MWCNT diameter, but neither surface area nor total pore volume alone can fully explain adsorption of the two NOM surrogates on MWCNTs. Adsorption mechanisms including electrostatic interaction, hydrophobic interaction, hydrogen bond, and pi-pi interaction play roles in the adsorption process. This study implies that NOM properties have a great effect on their adsorption on MWCNTs, and these properties should be further considered when evaluating the risks of NOM-coated nanomaterials.