Caffeine, riboflavin and curcumin amended clays for PFAS binding

Abstract

Per- and polyfluoroalkyl substances (PFAS) are usually found in mixtures with other toxic compounds. Therefore, the study and design of broad acting sorbents, such as clays, is an attractive sorption solution. We previously demonstrated that clays amended with choline and carnitine could enhance PFAS sorption properties. Here, we used computations to screen from a pool of chemical compounds, which are either supplements or generally recognized as safe, and identified particular supplements that can be amended to clay and potentially improve its sorbing capacity for PFAS in acidic conditions. Simulations were initially used as a tool to identify promising amendments to the clay, while subsequently, simulations evaluated which selected amendments could potentially bind PFAS. Our results showed that caffeine-, riboflavin- and curcumin-amended clays can, in particular instances, enhance the binding of different PFAS compared to parent clays. Experiments investigated the sorption properties of the designed systems. Notably, caffeine-amended clay significantly enhanced GenX binding when compared to parent clay, with its binding capacity being increased from 0.15 mol/kg to 1.17 mol/kg. Caffeine-amended clay also enhanced binding for PFOS by 125%, compared to the parent clay, and for PFOA to a lesser extent. Additionally, riboflavin-amended clay enhanced binding for GenX, PFOA and PFOS by 120%, 23%, and 70%, respectively, compared to the parent clay. Our studies provide atomistic details into their mechanisms of action. Both the novel computational library of chemical compound-amended clays and the approach utilized, combining computations and experiments, could enhance the future design of novel amended clays for other toxins.

Keywords: Caffeine; Curcumin; Molecular dynamics simulations; Montmorillonite clay; PFAS; Riboflavin.

Fig. 1.

Panels A-D show the percentage binding probability of GenX, PFOA, PFOS, and PFBS, respectively, to different chemical compound-amended clays from a single simulation at pH 2. Triplicate runs were performed only for parent clay as a control. Average values are shown for the control which were calculated from triplicate runs. Error bars denote standard deviation values calculated from triplicate runs.

Fig. 2.

Panels A-D show the average percentage binding probability of GenX, PFOA, PFOS, and PFBS, respectively, to different supplement-amended clays at pH 2. In each case, the average percentage binding probability of PFAS molecules to the parent clay (CM) is also shown. Direct, direct-assisted, direct-helped, and indirect-assisted interactions are shown in dark blue, orange, green, and cyan, respectively. The average values are calculated from triplicate runs. Error bars denote standard deviation values for the total binding probability calculated from triplicate runs.

Fig. 3.

Panels A-D show the adsorption isotherm of GenX, PFOA, PFOS, and PFBS, respectively, on parent and supplement-amended clays at pH 2. The solid lines represent the adsorption isotherm plots based on the Langmuir/Freundlich model, while the dashed lines represent the 95% confidence band of Langmuir/Freundlich model.

Fig. 4.

Panels A-C show the binding of caffeine, curcumin, and riboflavin molecules, respectively, directly to clay. These correspond to the last snapshots extracted from the simulations of the molecules in the absence of PFAS and to the initial structure of the supplement-amended clays when simulated with PFAS. Solid, dotted and dashed arrows are indicating molecules and/or binding modes discussed in the main text. Clay and amendments are shown in vdW representation. Atoms are colored by atom type. Hydrogen atoms of CM were omitted for clarity (Humphrey et al., 1996).

Fig. 5.

Panels A-D show GenX, PFOA, PFOS, and PFBS, respectively, directly binding to clay in the absence of amendments. These correspond to the last snapshots extracted from a particular simulation of the corresponding control system. Solid, dotted and dashed arrows are indicating molecules and/or binding modes discussed in the main text. Clay is shown in vdW representation, while PFAS are shown in licorice representation. Atoms are colored by atom type. Hydrogen atoms of CM were omitted for clarity (Humphrey et al., 1996).

Fig. 6.

Panels A-C show the interactions of GenX with caffeine-, curcumin-, and riboflavin-amended clay, respectively. These correspond to selected snapshots extracted from a particular simulation of the corresponding systems. Solid, dotted and dashed arrows and brackets are indicating molecules and/or binding modes discussed in the main text. Clay and amendments are shown in vdW representation, while GenX is shown in licorice representation. Atoms are colored by atom type. Hydrogen atoms of CM were omitted for clarity (Humphrey et al., 1996).

Fig. 7.

Panels A-C show the interactions of PFOS with caffeine-, curcumin-, and riboflavin-amended clay, respectively. These correspond to selected snapshots extracted from a particular simulation of the corresponding systems. Solid and dotted arrows are indicating molecules and/or binding modes discussed in the main text. Clay and amendments are shown in vdW representation, while PFOS is shown in licorice representation. Atoms are colored by atom type. Hydrogen atoms of CM were omitted for clarity (Humphrey et al., 1996).