Adsorption plays a critical role in the mobility, biological availability and reactivity of organic pollutants in soil, sediment and atmospheric aerosols, and often forms the strategic basis for remediation technologies. Among the strongest of environmental adsorbents is the carbonaceous substance remaining after pyrolysis or incomplete burning of organic material known as black carbon. Closely related structurally to black carbon are a number of manufactured substances, such as biochar, activated carbon and carbon nanotubes prescribed to improve soil fertility or to assist in the sensing or removal of pollutants in water and stabilization of contaminated soil and sediment. Many contaminants of emerging concern are ionic or become ionic under conditions normally encountered in nature or in treatment systems. The molecular mechanisms by which ionic and ionizable compounds adsorb to black carbon are poorly understood, however. This project addresses adsorption of ionic and ionizable compounds selected among pharmaceuticals, personal care products, pesticides, industrial solvents and endocrine-disrupting compounds. It will explore novel bonding interactions with the polyaromatic surface, which is characteristic of environmental and manufactured carbons. The first type of interaction applies to weak organic acids which are postulated to form exceptionally strong H-bonds, known as negative charge-assisted hydrogen bonds (CAHB), with surface carboxyl and phenoxyl groups. The second type of interaction applies to positively-charged aromatic amines and heteroaromatic amines. Because these aromatic cations are electron-poor they are capable of undergoing pi-pi electron donor-acceptor interactions, assisted by cation-pi interactions, with the electron-rich polyaromatic surface of black carbon, a bond known as pi+-pi EDA. To enable formation of a CAHB or pi+-pi EDA it is postulated that a compound will undergo proton exchange with water, releasing hydroxide ion into solution, and resulting in a positive shift of its pKa on the surface relative to its pKa in solution. A number of thermodynamic and spectroscopic experiments will be carried out to test these hypotheses, determine reaction scope, and provide parameterization useful in constructing structure-property free energy relationships of adsorption.
A deep, molecular-level understanding of adsorption of pollutant molecules to the surfaces of environmental particles is a key to predicting the fate and movement of pollutants in the environment, contributes to the knowledge base underlying public health regulations, and is a prerequisite to technological control. This project will investigate novel bonding interactions of positively and negatively charged emerging contaminants with the surfaces of environmental black carbon and related commercial products that contact pollutants through their intended use. Such interactions have so far received little attention by environmental scientists. It will lead to scientific advances of both theoretical and practical importance for scientists interested in the fate and risk of such compounds. Other constituencies that will benefit from advances made in this project include regulatory agencies and the remediation, water purification, and agricultural industries and their stakeholders. The project will link smoothly with other projects now underway in the researcher?s laboratory and will foster collaboration with other institutions. The project will help train graduate students, postdoctoral researchers and visiting scholars, and will influence the course of high school, college and graduate-level educational outreach. The results will be disseminated in both public and scientific forums.
