Linking Adsorption Strength to Chemical Fate of Organic Pollutants in Aqueous Media
Abstract
Motivation and Scientific Objectives: Adsorption of organics to surfaces in the environment is a key process in the dissolution of minerals and ultimately the fate and chemical transport of organic species. However, the relationship between adsorption and oxidation kinetics of organics is not well understood in the aqueous phase because of the interference of the solvent. Compared with the gas phase, the solvent environment can change the coverage of organics on surfaces, stabilize intermediates via hydrogen bonding, or even participate directly in the reaction. To predict the chemical fate of species in aqueous environments with high fidelity, knowledge of how geochemical environmental parameters affect adsorption and kinetics is needed. Herein, we propose to disentangle how common environmental parameters influence adsorption energies of common phenolic compounds and link these adsorption energies to the oxidation kinetics of these molecules. Phenolic compounds will be studied because they are a common class of pollutants in water that cause adverse environmental impact. We will test our hypothesis that the effect on kinetics with varying environmental parameters can be explained through the adsorption energies. We hypothesize the apparent adsorption energy will dictate the organic coverage on surfaces, but the intrinsic adsorption energy of the organic will link to the moleculeÕs reactivity. The intrinsic adsorption energy is the organic/surface interaction strength, and we hypothesize intrinsic adsorption will be significantly less affected by the solvent environment compared to the apparent adsorption. This high-risk, high payoff work will: (1) clarify how adsorption thermodynamics and oxidation kinetics of phenolic compounds are related to environmental parameters and the phenolic functional group; and (2) elucidate adsorption-kinetic relationships of phenolic compounds in aqueous solution. Methods Employed: We will study adsorption and oxidation of phenolic compounds (i.e., phenol, p-cresol, catechol, and guaiacol) on platinum and iron oxide surfaces under varying solvent conditions (i.e., water/acetic acid concentration, temperature, pH, chloride concentration). Our research goals will be achieved via two tasks. In Task 1, we will experimentally and computationally study the effect of varying solvent conditions on apparent and intrinsic adsorption energies of phenolic compounds. This task will be accomplished using density functional theory (DFT) modeling, molecular simulation, and experimental adsorption isotherms. In Task 2, we will determine how the solvent environment affects the electrochemical oxidation kinetics of these species. We will measure the oxidation kinetics using electrochemical techniques and conduct DFT modeling studies of the oxidation kinetics for comparison with experiment. We will then test our model of how kinetics correlate to the apparent and intrinsic adsorption energies of the phenolic compounds. Intellectual Merit: This research will develop a model that links the molecular structures, adsorption strengths, and oxidation kinetics of organics in the aqueous phase, which thus far has remained elusive. Although we study phenolics, the general insights we find regarding apparent and intrinsic adsorption energies and their link to kinetics as a function of environmental parameters should extend to many other organics in the environment. For the first time, this work will compare the impact of the intrinsic interaction of the molecule with the surface (intrinsic adsorption energy) with the competitive interaction between the reacting molecule and the solvent at the surface (apparent adsorption energy) on oxidation kinetics. These findings will be extendable to clarify the adsorption and fate of chemical pollutants, corrosion, and any systems involving adsorption and reaction at surfaces in condensed phase.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 25, 2021
- Source ID
- W911NF2110149
Entities
People
- Bryan R. Goldsmith
Organizations
- Army Contracting Command
- United States Army
- University of Michigan