Adsorption of dialkyl phthalate esters on carbon nanotubes

Dialkyl phthalate esters (DPEs), with endocrine disrupting functions, are widely used and categorized as priority pollutants. Carbon nanotubes (CNTs) as strong adsorbents could influence the fate, transport, and bioavailability of DPEs in the environment. Understanding adsorptive interactions between CNTs and DPEs is critical to the environmental applications of CNTs. Adsorption of DPEs by one type of single-walled CNTs (SWCNT) and three multiwalled CNTs (MWCNT) was evaluated. For a given CNT, the adsorptive affinity correlated well with hydrophobicity of DPEs with an order of dimethyl phthalate (DMP) < diethyl phthalate (DEP) < dibutyl phthalate (DBP). The normalized adsorption coefficient (K/K(HW)) of DPEs indicates that π-π electron-donor-acceptor (EDA) interaction was also important for adsorption of DPEs (π-acceptor) on CNTs (π-donor). A π-π charge-transfer band of EDA complexes (mixture of pyrene (PYR) as π-donor and DPEs as π-acceptor) showed their interaction strength in the order of DMP-PYR > DEP-PYR > DBP-PYR. Calculated monolayer adsorption capacities (log Q) were bigger (for DMP and DEP) than or approximately equal to (for DBP) the estimated adsorption capacities (log Q(0)), implying that the DPEs were adsorbed on the surface area of CNTs. For a given DPE, the adsorptive capacities decreased with the increasing outer diameters of CNTs in the order of SWCNT > MWCNT10 > MWCNT20 > MWCNT40.