Pyro-Catalytic Degradation of Pyrene by Bentonite-Supported Transition Metals: Mechanistic Insights and Trade-Offs with Low Pyrolysis Temperature

Abstract

Transition metal catalysts can significantly enhance the pyrolytic remediation of soils contaminated with polynuclear aromatic hydrocarbons (PAHs). Significantly higher pyrene removal efficiency was observed after the pyrolytic treatment of Fe-enriched bentonite (1.8% wt ion-exchanged content) relative to natural bentonite or soil (i.e., 93% vs 48% and 4%) at the unprecedentedly low temperature of 150 °C with only 15 min treatment time. DFT calculations showed that bentonite surfaces with Fe3+ or Cu2+ adsorb pyrene stronger than surfaces with Zn2+ or Na+. Enhanced pyrene adsorption results from increased charge transfer from its aromatic π-bonds to the cation site, which destabilizes pyrene allowing for faster degradation at lower temperatures. UV–vis and GC–MS analyses revealed pyrene decomposition products in extracts of samples treated at 150 °C, including small aromatic compounds. As the pyrolysis temperature increased above 200 °C, product distribution shifted from extractable compounds to char coating the residue particles. No extractable byproducts were detected after treatment at 400 °C, indicating that char was the final product of pyrene decomposition. Tests with human lung cells showed that extracts of samples pyrolyzed at 150 °C were toxic; thus, high removal efficiency by pyrolytic treatment does not guarantee detoxification. No cytotoxicity was observed for extracts from Fe-bentonite samples treated at 300 °C, inferring that char is an appropriate treatment end point. Overall, we demonstrate that transition metals in clay can catalyze pyrolytic reactions at relatively low temperatures to decrease the energy and contact times required to meet cleanup standards. However, mitigating residual toxicity may require higher pyrolysis temperatures.

Mechanistic Implications of the Varying Susceptibility of PAHs to Pyro-Catalytic Treatment as a Function of Their Ionization Potential and Hydrophobicity

“…15 Recently, we showed that ionexchanged Fe(III)-bentonite (i.e., Fe-bentonite) can be used as a catalyst to decrease the pyrolysis treatment temperature needed for high PAH removal. 19,20 We saw significantly higher pyrene removal efficiency using Fe-bentonite (93%) than natural bentonite (48%) or unamended contaminated soil (4%) at the unprecedentedly low temperature of 150 °C with only 15 min treatment time. 19 Density functional theory (DFT) shows that pyrene has stronger adsorption to clays with reducible Fe(III) sites than surfaces with less-reducible cationic transition metals (Cu(II) or Zn(II)), 19 and suggests that PAH pyro-catalytic treatment may be facilitated by direct electron transfer from the π-bonds of PAHs to cationic Fe(III) sites within the bentonite structure.…”

“…19,20 We saw significantly higher pyrene removal efficiency using Fe-bentonite (93%) than natural bentonite (48%) or unamended contaminated soil (4%) at the unprecedentedly low temperature of 150 °C with only 15 min treatment time. 19 Density functional theory (DFT) shows that pyrene has stronger adsorption to clays with reducible Fe(III) sites than surfaces with less-reducible cationic transition metals (Cu(II) or Zn(II)), 19 and suggests that PAH pyro-catalytic treatment may be facilitated by direct electron transfer from the π-bonds of PAHs to cationic Fe(III) sites within the bentonite structure. 21 Such electron transfer would destabilize the PAH molecule by forming an aromatic radical, leading to subsequent polymerization and char formation.…”

“…Fe-bentonite was characterized for metal type and content in our previous work. 19 The predominant oxidation state of Fe-bentonite is Fe(III), and the exchanged iron content in the Fe-bentonite sample was approximately 1.8%wt.…”

“…All chemicals were used as received. The background soil (43% clay and 1.57% OM) used in our previous work, 19 collected from Rice University in Houston, TX, was also used for this study. The soil was analyzed for agronomic properties by the Texas A&M Soil, Water, and Forage Testing Laboratory (Table S1).…”

“…The soil was analyzed for agronomic properties by the Texas A&M Soil, Water, and Forage Testing Laboratory (Table S1). After collection, the soil was dried at 60 °C for 72 h, 19 homogenized, sieved, and stored in closed glass containers before use.…”