Ultrasonically Enhanced Mass Transport and Degradation of Polycyclic Aromatic Hydrocarbons in Solid-Liquid Two Phase Partitioning Systems
Abstract
The remediation of soil contaminated with polycyclic aromatic hydrocarbons (PAHs) is endorsed by environmental protection agencies worldwide. Recent studies demonstrated the removal of these contaminants from soil utilizing polymer beads, with subsequent PAH release and degradation in solid-liquid two phase partitioning bioreactors (TPPBs). Although such a process was successful, significant mass transport limitations involving PAH release from the polymers hampered productivity. The current work examined the possibility of applying sonication in solid-liquid partitioning systems to enhance delivery and degradation of PAHs.
Small scale physical testing revealed delivery rates of PAHs from Desmopan, increased by 5 fold under intermittent sonication relative to non-sonicated conditions. Enhancements were also displayed as shifts to higher release equilibria under sonicated conditions, agreeing with sonochemistry concepts. Improvements were demonstrated across a range of polymers, suggesting that sonication could enhance PAH release with any polymers deemed feasible for environmental applications. A PAH-degrading microbial consortium was enriched, and it was demonstrated that sonication also improved the rate of phenanthrene degradation delivered from Desmopan by four times, confirming transport improvements while minimizing cellular inactivation effects.
A mass transport analysis showed that without sonication, delivery of PAHs was restricted by the external resistance at the solid-liquid interface. Ultrasound was shown to enhance both external and internal transport properties, allowing rates not achievable through increased liquid mixing. Diffusivities quantified with and without ultrasound decreased as a function of permeant molecular size. Additionally, partitioning coefficients under sonicated and non-sonicated conditions decreased with PAH molecular size. Finally, an examination of permeant property data demonstrated that polarizability was the best descriptor of thermodynamic and transport behaviour in solid-liquid systems.
The possibility of inducing equivalent improvements was investigated in a bench scale TPPB, in which sonic exposure improved degradation rates of phenanthrene by 2.7 fold when delivered from Desmopan. A window of on/off operation for ultrasonic cycling was also demonstrated, providing potential for optimizing sonication via rational selection of exposure times. DNA analysis also revealed that the consortium composition was maintained in the presence of sonication and also demonstrated that the consortium was comprised of bacteria belonging to the Pandoraea, Sphingobium, and Pseudoxanthomonas genera.