(638e) Investigation of Micropollutants Adsorption with Graphene Utilizing Interparticle Diffusion

Micropollutants (MPs) are emerging as persistent contaminants that are recalcitrant to biodegradation and oxidation and can permeate reverse osmosis. Therefore, novel adsorptive media for tertiary treatment are required for safe effluent discharge. Interactions such as Ï€-Ï€ and electron donor-acceptor with graphene can be a factor in the rapid kinetic removal of MPs. However, mechanistic insight is generalized, and solute characteristics are inadequately addressed. Here, we show the comparative adsorption of four aromatic MPs, caffeine, DEET, acetaminophen (APAP), and diethyl phthalate (DEP), with granular activated carbon (GAC) and graphene nanoplatelets (GnP). To understand the difference between the performance of GnP and GAC, the mechanism of removal was determined using experimental techniques, K_ow analysis, IPD kinetics, and NMR. Langmuir isotherm model correlation coefficients for GnP (R² 0.36-0.99) were compared to the Freundlich isotherm correlation coefficients of R² of 0.44 to 0.99. All MPs showed Langmuir adsorption behavior, indicating monolayer adsorption, except DEET treated with GnP, indicating multilayer adsorption. When comparing the performance of GnP to GAC, GAC performed slightly better in most cases except for DEP. The capacity ratio of qe,max for GnP/GAC, was 0.866 for caffeine, 0.719 for APAP, 0.560 for DEET, and 1.002 for DEP. A comparison of the capacities of the sorbents revealed that GAC capacity was less dependent on the adsorbate, with a capacity range of 17.7mg/g, relative to GnP, which had a difference of 82.1 mg/g between DEP and DEET. This suggests the role of molecular structure in GnP's sorption capacity. As expected, GnP showed significantly faster kinetics for all MPs with k₂ values 2-3 orders of magnitude higher than GAC. However, while GAC capacity exhibited insensitivity to molecular structure, GnP adsorption capacities were influenced by the solutes’ structure, with DEP > Caffeine > APAP > DEET in terms of capacity. For all intraparticle diffusion Kp, two values were higher for GAC except for DEP, which has the highest capacity for GnP. The data demonstrates that after the initial attraction, the MP must be able to diffuse into GnP’s mesopores to have high adsorption capacity. This phenomenon is confirmed by NMR, which indicates that Kp,2 dictates capacity. Therefore, micropore diffusion has a greater effect on the adsorption of MPs for GAC than for GnP. The external mass transfer will remain rapid, but the capacity will be limited to surface interactions if structural constraints such as steric hindrance cause resistance to solute entry into the graphene particle. Overall, the results show that the performance is molecule- and structure-dependent. Other molecular structure considerations such as heteroaromaticity and planarity play a role in limiting intraparticle pore diffusion, thereby limiting overall capacity for GnP compared to GAC. Choosing the correct molecular structure to interact with GnP is pivotal in designing effective and rapid treatment solutions for the environmental treatment of emerging contaminants like MPs. Using the selectivity available with GnP can lead to improvements in capacity for treatment opposed to GAC in complex water matrices.