(633e) Tackling Water Pollution Via Sustainable Sorbents: Pb(II) Adsorption By Bio-Based Carbons Via First Principles Computational Models

Accessing clean water―one of the critical components of life and irreplaceable in the agriculture and energy sectors―is currently a global crisis that is growing in severity as the effects of climate change worsen. Bio-based carbons produced from the upgrading of lignocellulosic biomass waste present an opportunity to develop highly efficient and inexpensive sorbents for extraction of heavy metals from watercourses. Adsorption studies have demonstrated that these sustainable materials are three times as effective at removing copper, lead, zinc, cadmium, and cobalt than a more traditional sorbent, i.e. activated carbon. However, the complex, amorphous nature of bio-based carbons makes it nearly impossible using experiments alone to determine the fundamental, atomic-scale chemical structures, phenomena, and mechanisms that lead to efficient binding of heavy metals. Thus, there is a significant challenge here-in that the design and optimization of highly efficient bio-based carbons is limited by a lack of fundamental chemical insights into the chemical structures present on bio-based carbons and their subsequent ability to interact with and bind heavy metals. We address this challenge here by combining atomic-scale computational models (i.e. density functional theory) with fundamental, statistical mechanics to (1) determine the dominant functionalities present on bio-based carbon sorbents and (2) probe the Pb(II) adsorption mechanism onto these dominant functionalities.

First, to determine the dominant functionalities on bio-based carbons, a database of 225 distinct carbon, hydrogen, and oxygen containing defects, with the latter including hydroxyl, carboxylic, and ether functionalities, was created in a model graphene surface. The formation free energies for each defect structure were calculated at a range of temperatures (i.e. 200-2000 K) and gas phase oxygen and hydrogen chemical potentials, resulting in a series of phase diagrams that show the most thermodynamically stable―i.e. dominant―defects across a range of environmental conditions. For example, at 1000 K (Figure 1A), only four of the 225 tested defects are thermodynamically accessible, with the dominant defect structures being the Stone-Wales defect under highly reducing environments and a double carbon vacancy with four ether groups under highly oxidizing environments. Thus, our rigorous scan of the bio-based carbon defect space enables a physics-guided reduction of possible chemical structures to a database of those structures most likely to be present on bio-based carbons.

Second, to probe the Pb(II) adsorption mechanism onto the dominant functionalities, a Pb(II) complex was adsorbed to each dominant defect. On the defects containing only carbon, the Pb(II) complex adsorbs weakly (~-15 kJ mol^-1) and predominantly via van der Waals forces. The addition of oxygen to the adsorption site significantly increases the Pb(II) adsorption strength (~-86 kJ mol^-1). As shown in Figure 1B, the presence of oxygen defects in the model bio-based carbon creates a localized, negatively charged site unto which the positively charged Pb(II) complex bind. Thus, the structure and number of defects, whose formation can be directed using the atomic-scale insight from our phase diagrams, controls the sorption capacity of the bio-based carbons. Overall, the linkages elucidated here between bio-based carbon sorbent structure and Pb(II) adsorption capacity establish the necessary fundamental chemical design rules to facilitate the rational design of highly efficient bio-based carbon sorbents for water purification.