Adsorption of Organic Compounds by Biomass Chars: Direct Role of Aromatic Condensation (Ring Cluster Size) Revealed by Experimental and Theoretical Studies

Biomass chars are a major component of the soil environmental black carbon pool and prepared forms are a potentially useful tool in remediation. A function critical to the roles of both environmental and prepared chars is sorption of organic compounds. Char properties known to control sorption include surface area, porosity, functional group composition, and percent aromatic carbon. Here, we show that sorption affinity (but not maximum capacity) of organic compounds is directly related to the degree of condensation of the aromatic fraction. The Dubinin-Ashtakov characteristic sorption energy (E-DA, kJ mol(-1)) of 22 compounds on a thermoseries of bamboo chars correlates strongly with the DP/MAS-C-13 NMR-determined bridgehead aromatic carbon fraction (chi(b)), which relates to the mean ring cluster size. Density functional theory-computed binding energy (E-bd) for five of the compounds on a representative series of polybenzenoid hydrocarbon open-face sheets also correlates positively with chi(b), leveling off for rings larger than similar to C-55. The E-bd, in turn, correlates strongly with E-DA. An increase in E-bd with cluster size is also found for sorption, both monolayer and bilayer, between parallel sheets representing slit micropores. The increasing sorption energy with cluster size is shown to be due to increasing cluster polarizability, which strengthens dispersion forces with the sorbate. The findings underscore a previously overlooked explicit role of aromatic condensation in sorption energy, and illustrate the utility of E-DA-E-bd comparison for predicting sorption.

Automated summary

The sorption affinity of organic compounds is directly related to the degree of condensation of the aromatic fraction, while the sorption energy increases with cluster size. This underscores the important role of aromatic condensation in sorption energy.