(614a) Understanding the Removal of Pharmaceuticals from Water Effluents By Adsorption in Activated Carbons – a Molecular Simulation Approach

There is an increased concern in recent years on the removal of pharmaceuticals from water effluents. Many of these compounds are at concentrations of mg/L or below, are hardly biodegradable even at the outlet of wastewater treatment plants and much on their potential impact on environment and human health are still not completely known.Among the main removal technologies reported in the literature, removal by adsorption is one of the most promising techniques because of its convenience when applied in current water treatment processes. Adsorption using activated carbon is ideal for removing organic compounds due to the availability of high surface area and combination of well-developed pore structure and surface functional group properties [1]. Hence, the identification of the mechanisms governing the adsorption phenomena of pharmaceuticals in aqueous solutions is of fundamental importance.

The main objectives of this study was to evaluate, by molecular simulations, the competitive adsorption of pharmaceutical compounds as pollutants contented in water on activated carbons of varied characteristics, and to compare the removal capacities from equilibrium simulations on single component and aqueous mixtures. Molecular simulations using the Grand Canonical Monte Carlo (GCMC) method have been performed in order to understand the interaction of pharmaceutical molecules (ibuprofen, diclofenac, naproxen, paracetamol and amoxicillin, all with similar dimensions, except amoxicillin) on activated carbons in both, pure and aqueous mixtures. “Realistic” nanoporous carbon models based on units of polyaromatic molecules with different number of rings, defects and polar-oxygenated sites have been employed for this purpose[2].

The modeling approach we have used for the activated carbons, inspired by the early work of Gubbins and co-authors (see, for instance, reference [3]) represents a middle ground between the slit pore model and the fully atomistic models of carbon; it is an extension of the model packing idealized structures first developed by Segarra and Glandt (SG) [4]. We have modeled graphene planes as platelets, but including polar groups and defects, not explicitly considered in the SG model, and in a similar fashion of the extended Liu-Monson model [5]. Furthermore, in order to create imperfections on the surface, increased surface roughness were created by deleting up to 10–20% of the atoms in the inner carbon layers. Details on the calculations can be found in [2].

Our methodology allowed us to obtain a successful prediction of the drug adsorption properties. Comparison of the simulated capacities with experimental reported values is highly consistent. The model is sufficiently flexible to tune its parameters in a systematic way and introduce additional features to reflect specific properties of the material of interest, i.e., effects of platelet sizes and geometries, surface area, concentration and nature of surface groups, among others. Results confirm the role of surface sites in the enhancement of water adsorption and reduction of drug adsorption as the oxygenated sites exceeds 10%wt. Reduction of the surface functionality gives a higher adsorption capacity of pharmaceuticals in aqueous mixture, since there is not competition with water molecules.

This work is a further step in order predict macroscopic performance of activated carbons in drug removal applications, providing useful molecular-level insights into the drug/water adsorption mechanism and allowing finding the most favorable material.

References:

1. L. Nielsen, T. J. Bandosz, Analysis of the competitive adsorption of pharmaceuticals on waste

derived materials, Chemical Engineering Journal 287 (2016) 139–147.

2. D. Bahamon, L. Carro, S. Guri, L.F. Vega, Computational study of ibuprofen removal from water by adsorption in realistic activated carbons, Journal of Colloid and Interface Science, 498 (2017), 323–334.

3. J. Pikunic, C. Clinard, N. Cohaut, K.E. Gubbins, J.M. Guet, R.J.M. Pellenq, I. Rannou, J.-N. Rouzaud, Structural modeling of porous carbons: constrained reverse Monte Carlo method, Langmuir 19 (2003) 8565–8582.

4. E.I. Segarra, E.D. Glandt, Model microporous carbons: microstructure, surface polarity and gas adsorption, Chem. Eng. Sci. 49 (1994) 2953–2965.

5. J.C. Liu, P.A. Monson, Monte Carlo simulation study of water simulation study of water adsorption in activated carbon, Ind. Eng. Chem. Res. 45 (2006) 5649–5656.